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[homepage.git] / reports / rfc / draft-ietf-http-v11-spec-03.txt

HTTP Working Group                                R. Fielding, UC Irvine
INTERNET-DRAFT                                       H. Frystyk, MIT/LCS
<draft-ietf-http-v11-spec-03.html>               T. Berners-Lee, MIT/LCS
                                                          J. Gettys, DEC
                                                        J. C. Mogul, DEC
Expires October 2, 1996                                      May 2, 1996

                Hypertext Transfer Protocol -- HTTP/1.1


1 Status of this Memo
This document is an Internet-Draft. Internet-Drafts are working
documents of the Internet Engineering Task Force (IETF), its areas, and
its working groups. Note that other groups may also distribute working
documents as Internet-Drafts.

Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or made obsolete by other documents at any
time. It is inappropriate to use Internet-Drafts as reference material
or to cite them other than as "work in progress".

To learn the current status of any Internet-Draft, please check the
"1id-abstracts.txt" listing contained in the Internet-Drafts Shadow
Directories on ftp.is.co.za (Africa), nic.nordu.net (Europe),
munnari.oz.au (Pacific Rim), ds.internic.net (US East Coast), or
ftp.isi.edu (US West Coast).

Distribution of this document is unlimited. Please send comments to the
HTTP working group at <http-wg@cuckoo.hpl.hp.com>. Discussions of the
working group are archived at
<URL:http://www.ics.uci.edu/pub/ietf/http/>. General discussions about
HTTP and the applications which use HTTP should take place on the <www-
talk@w3.org> mailing list.

  NOTE: This specification is for discussion purposes only. It is not
  claimed to represent the consensus of the HTTP working group, and
  contains a number of proposals that either have not been discussed
  or are controversial. The working group is discussing significant
  changes in many areas, including - support for caching, persistent
  connections, range retrieval, content negotiation, MIME
  compatibility, authentication, timing of the PUT operation.


2 Abstract
The Hypertext Transfer Protocol (HTTP) is an application-level protocol
for distributed, collaborative, hypermedia information systems. It is a
generic, stateless, object-oriented protocol which can be used for many
tasks, such as name servers and distributed object management systems,
through extension of its request methods (commands). A feature of HTTP
is the typing and negotiation of data representation, allowing systems
to be built independently of the data being transferred.


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HTTP has been in use by the World-Wide Web global information initiative
since 1990. This specification defines the protocol referred to as
"HTTP/1.1".

3 Note to Readers of This Document


We believe this draft to be very close to consensus of the working group
in terms of functionality for HTTP/1.1, and the text substantially
correct.  One final technical change NOT reflected in this draft is to
make persistent connections the default behavior for HTTP/1.1; editorial
changes to reflect this in the next, and we hope final draft, are being
circulated in the working group mailing list.

This draft has undergone extensive reorganization to improve
presentation.  Let us know if there are remaining problems.

The terminology used in this draft has changed to reduce confusion.
While we are converging on a shared set of terminology and definitions,
it is possible there will be a final set of terminology adopted in the
next draft.  Despite any terminology changes that may occur to improve
the presentation of the specification, we do not expect to change the
name of any header field or parameter name.

There are a very few remaining issues indicated by Editor's Note: in
bold font.




























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4 Table of Contents


HYPERTEXT TRANSFER PROTOCOL -- HTTP/1.1

1 Status of this Memo

2 Abstract

3 Note to Readers of This Document

4 Table of Contents

5 Introduction
 5.1 Purpose
 5.2 Requirements
 5.3 Terminology
 5.4 Overall Operation
 5.5 HTTP and MIME

6 Notational Conventions and Generic Grammar
 6.1 Augmented BNF
 6.2 Basic Rules

7 Protocol Parameters
 7.1 HTTP Version
 7.2 Uniform Resource Identifiers
 7.3 Date/Time Formats
 7.4 Character Sets
 7.5 Content Codings
 7.6 Transfer Codings
 7.7 Media Types
 7.8 Product Tokens
 7.9 Quality Values
 7.10 Language Tags
 7.11 Entity Tags
 7.12 Variant IDs
 7.13 Variant Sets
 7.14 Range Protocol Parameters

8 HTTP Message
 8.1 Message Types
 8.2 Message Headers
 8.3 General Header Fields

9 Request
 9.1 Request-Line
 9.2 The Resource Identified by a Request
 9.3 Request Header Fields

10 Response
 10.1 Status-Line
 10.2 Response Header Fields

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11 Entity
 11.1 Entity Header Fields
 11.2 Entity Body

12 Status Code Definitions
 12.1 Informational 1xx
 12.2 Successful 2xx
 12.3 Redirection 3xx
 12.4 Client Error 4xx
 12.5 Server Error 5xx

13 Method Definitions
 13.1 OPTIONS
 13.2 GET
 13.3 HEAD
 13.4 POST
 13.5 PUT
 13.6 DELETE
 13.7 TRACE

14 Access Authentication
 14.1 Basic Authentication Scheme
 14.2 Digest Authentication Scheme

15 Content Negotiation
 15.1 Negotiation Facilities Defined in this Specification

16 Caching in HTTP
 16.1 Semantic Transparency
 16.2 Expiration Model
 16.3 Validation Model
 16.4 Constructing Responses From Caches
 16.5 Caching and Generic Resources
 16.6 Shared and Non-Shared Caches
 16.7 Selecting a Cached Response
 16.8 Errors or Incomplete Response Cache Behavior
 16.9 Side Effects of GET and HEAD
 16.10 Invalidation After Updates or Deletions
 16.11 Write-Through Mandatory
 16.12 Generic Resources and HTTP/1.0 Proxy Caches
 16.13 Cache Replacement
 16.14 Caching of Negative Responses
 16.15 History Lists

17 Persistent Connections
 17.1 Purpose
 17.2 Overall Operation
 17.3 Proxy Servers
 17.4 Interaction with Security Protocols
 17.5 Practical Considerations

18 Header Field Definitions
 18.1 Accept
 18.2 Accept-Charset

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 18.3 Accept-Encoding
 18.4 Accept-Language
 18.5 Accept-Ranges
 18.6 Age
 18.7 Allow
 18.8 Alternates
 18.9 Authorization
 18.10 Cache-Control
 18.11 Connection
 18.12 Content-Base
 18.13 Content-Encoding
 18.14 Content-Language
 18.15 Content-Length
 18.16 Content-Location
 18.17 Content-MD5
 18.18 Content-Range
 18.19 Content-Type
 18.20 Date
 18.21 ETag
 18.22 Expires
 18.23 From
 18.24 Host
 18.25 If-Modified-Since
 18.26 If-Match
 18.27 If-NoneMatch
 18.28 If-Range
 18.29 If-Unmodified-Since
 18.30 Last-Modified
 18.31 Location
 18.32 Max-Forwards
 18.33 Persist
 18.34 Pragma
 18.35 Proxy-Authenticate
 18.36 Proxy-Authorization
 18.37 Public
 18.38 Range
 18.39 Referer
 18.40 Retry-After
 18.41 Server
 18.42 Title
 18.43 Transfer Encoding
 18.44 Upgrade
 18.45 User-Agent
 18.46 Vary
 18.47 Via
 18.48 Warning
 18.49 WWW-Authenticate

19 Security Considerations
 19.1 Authentication of Clients
 19.2 Safe Methods
 19.3 Abuse of Server Log Information
 19.4 Transfer of Sensitive Information
 19.5 Attacks Based On File and Path Names

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 19.6 Personal Information
 19.7 Privacy Issues Connected to Accept headers
 19.8 DNS Spoofing
 19.9 Location Headers and Spoofing

20 Acknowledgments

21 References

22 Authors' Addresses

23 Appendices
 23.1 Internet Media Type message/http
 23.2 Tolerant Applications
 23.3 Differences Between  HTTP Bodies and RFC 1521 Internet Message
 Bodies
 23.4 Changes from HTTP/1.0
 23.5 Additional Features





































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5 Introduction
5.1 Purpose
The Hypertext Transfer Protocol (HTTP) is an application-level protocol
for distributed, collaborative, hypermedia information systems. HTTP has
been in use by the World-Wide Web global information initiative since
1990. The first version of HTTP, referred to as HTTP/0.9, was a simple
protocol for raw data transfer across the Internet. HTTP/1.0, as defined
by RFC xxxx , improved the protocol by allowing messages to be in the
format of MIME-like entities, containing metainformation about the data
transferred and modifiers on the request/response semantics. However,
HTTP/1.0 does not sufficiently take into consideration the effect of
hierarchical proxies , caching, the need for persistent connections and
virtual hosts.. In addition, the proliferation of incompletely-
implemented applications calling themselves "HTTP/1.0" has necessitated
a protocol version change in order for two communicating applications to
determine each other's true capabilities.

This specification defines the protocol referred to as "HTTP/1.1". This
protocol is backwards-compatible with HTTP/1.0, but includes more
stringent requirements in order to ensure reliable implementation of its
features.

Practical information systems require more functionality than simple
retrieval, including search, front-end update, and annotation. HTTP
allows an open-ended set of methods that indicate the purpose of a
request. It builds on the discipline of reference provided by the
Uniform Resource Identifier (URI) , as a location (URL)  or name (URN)
,
for indicating the resource to which a method is to be applied. Messages
are passed in a format similar to that used by Internet Mail  and the
Multipurpose Internet Mail Extensions (MIME) .

HTTP is also used as a generic protocol for communication between user
agents and proxies/gateways to other Internet protocols, such as SMTP ,
NNTP , FTP , Gopher , and WAIS , allowing basic hypermedia access to
resources available from diverse applications and simplifying the
implementation of user agents.


5.2 Requirements
This specification uses the same words as RFC 1123  for defining the
significance of each particular requirement. These words are:


MUST
     This word or the adjective "required" means that the item is an
     absolute requirement of the specification.

SHOULD
     This word or the adjective "recommended" means that there may
exist
     valid reasons in particular circumstances to ignore this item, but
     the full implications should be understood and the case carefully
     weighed before choosing a different course.



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MAY
     This word or the adjective "optional" means that this item is
truly
     optional. One vendor may choose to include the item because a
     particular marketplace requires it or because it enhances the
     product, for example; another vendor may omit the same item.
An implementation is not compliant if it fails to satisfy one or more of
the MUST requirements for the protocols it implements. An implementation
that satisfies all the MUST and all the SHOULD requirements for its
protocols is said to be "unconditionally compliant"; one that satisfies
all the MUST requirements but not all the SHOULD requirements for its
protocols is said to be "conditionally compliant".


5.3 Terminology
This specification uses a number of terms to refer to the roles played
by participants in, and objects of, the HTTP communication.


connection
  A transport layer virtual circuit established between two programs
  for the purpose of communication.

message
  The basic unit of HTTP communication, consisting of a structured
  sequence of octets matching the syntax defined in section 8 and
  transmitted via the connection.

request
  An HTTP request message as defined in section 9.

response
  An HTTP response message as defined in section 10.

resource
  A network data object or service that can be identified by a URI
  (section 7.2).  At any point in time, a resource may be either a
  plain resource, which corresponds to only one possible
  representation, or a generic resource.

generic resource
  A resource that is a set of closely related representations of the
  same document, form, applet, etc. A generic resource is always
  identified by a URI. The individual representations may each be
  identified by a unique URI, or by the combination of the generic
  resource's URI and a variant-ID, or by the combination of the generic
  resource's URI and some "content-negotiation" mechanism.  In this
  case, other URIs may exist which identify a resource more
  specifically.

plain resource
  A resource that is not a generic resource.  A plain resource is
  always identified by a URI.



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entity
  The set of information transferred as the payload of a request or
  response  An entity consists of metainformation in the form of
  Entity-Header fields and content in the form of an Entity-Body, as
  described in section 11.

resource entity
  A specific representation, rendition, encoding, or presentation of a
  network data object or service, either a plain resource or a specific
  member of a generic resource.  A resource entity might be identified
  by a URI, or by the combination of a URI and a variant-ID, or by the
  combination of a URI and some other mechanism. An plain resource MUST
  be bound to a single resource entity at any instant in time.

variant
  A resource entity that is a member of at least one generic resource.
  Sometimes called a resource variant.  Note that the set of variants
  of a generic resource may change over time as well.

content negotiation
  The mechanism for selecting the appropriate variant of a generic
  resource when servicing a request, as described in section 15.

entity tag
  An opaque string associated with an entity and used to distinguish it
  from other entities of the requested resource .  A "strong entity
  tag" is one that may be shared by two entities of a resource only if
  they are equivalent by octet equality.  A "weak entity tag" is one
  that may be shared by two entities of a resource if they are
  equivalent and could be substituted for each other with no
  significant change in semantics.  A given entity tag value may be
  used for entities obtained by requests on different URIs without
  implying anything about the equivalence of these entities.

client
  An application program that establishes connections for the purpose
  of sending requests.

user agent
  The client which initiates a request. These are often browsers,
  editors, spiders (web-traversing robots), or other end user tools.

server
  An application program that accepts connections in order to service
  requests by sending back responses. Any given program MAY be capable
  of being both a client and a server; our use of these terms refers
  only to the role being performed by the program for a particular
  connection, rather than to the program's capabilities in general.
  Likewise, any server MAY act as an origin server, proxy, gateway, or
  tunnel, switching behavior based on the nature of each request.

origin server
  The server on which a given resource resides or is to be created.


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proxy
  An intermediary program which acts as both a server and a client for
  the purpose of making requests on behalf of other clients. Requests
  are serviced internally or by passing them on, with possible
  translation, to other servers. A proxy MUST interpret and, if
  necessary, rewrite a request message before forwarding it. Proxies
  are often used as client-side portals through network firewalls and
  as helper applications for handling requests via protocols not
  implemented by the user agent.

gateway
  A server which acts as an intermediary for some other server. Unlike
  a proxy, a gateway receives requests as if it were the origin server
  for the requested resource; the requesting client may not be aware
  that it is communicating with a gateway. Gateways are often used as
  server-side portals through network firewalls and as protocol
  translators for access to resources stored on non-HTTP systems.

tunnel
  An intermediary program which is acting as a blind relay between two
  connections. Once active, a tunnel is not considered a party to the
  HTTP communication, though the tunnel may have been initiated by an
  HTTP request. The tunnel ceases to exist when both ends of the
  relayed connections are closed. Tunnels are used when a portal is
  necessary and the intermediary cannot, or should not, interpret the
  relayed communication.

cache
  A program's local store of response messages and the subsystem that
  controls its message storage, retrieval, and deletion. A cache stores
  cachable responses in order to reduce the response time and network
  bandwidth consumption on future, equivalent requests. Any client or
  server MAY include a cache, though a cache cannot be used by a server
  that acts acting as a tunnel.

cachable
  A response is cachable if a cache is allowed to store a copy of the
  response message for use in answering subsequent requests. The rules
  for determining the cachability of HTTP responses are defined in
  Section 16.  Even if a resource is cachable, there may be additional
  constraints on when and if a cache can use the cached copy for a
  particular request.

firsthand
  A response is firsthand if it comes directly and without unnecessary
  delay from the origin server, perhaps via one or more proxies.  A
  response is also firsthand if its validity has just been checked
  directly with the origin server.

explicit expiration time
  The time at which the origin server intends that an entity should no
  longer be returned by a cache without further validation.



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heuristic expiration time
  An expiration time assigned by a cache when no explicit expiration
  time is available.

age
  The age of a response is the time since it was generated by, or
  successfully validated with, the origin server.

freshness lifetime
  The length of time between the generation of a response and its
  expiration time.

fresh
  A response is fresh if its age has not yet exceeded its freshness
  lifetime.

stale
  A response is stale if its age has passed its freshness lifetime. A
  cache may use a fresh response without validating it,  but "normally"
  may not use a stale response without first validating it.
  ("Normally" means "unless configured to provide better performance at
  the expense of transparency.")
  Therefore, what expires is the cache's authority to use a cached
  response, without validation, in its reply to a subsequent request.

semantically transparent
  Ideally, an HTTP/1.1 cache would be "semantically transparent." That
  is, use of the cache would not affect either the clients or the
  servers in any way except to improve performance. When a client makes
  a request via a semantically transparent cache, it receives exactly
  the same entity headers and entity body it would have received if it
  had made the same request to the origin server, at the same time.

validator
  An entity tag, or a Last-Modified time, which is used to find out
  whether a cache entry is a semantically transparent copy of a
  resource entity.  A cache entry is semantically transparent if its
  validator exactly matches the validator that the server would provide
  for current instance of that resource entity.

5.4 Overall Operation
The HTTP protocol is a request/response protocol. A client sends a
request to the server in the form of a request method, URI, and protocol
version, followed by a MIME-like message containing request modifiers,
client information, and possible body content over a connection with a
server. The server responds with a status line, including the message's
protocol version and a success or error code, followed by a MIME-like
message containing server information, entity metainformation, and
possible entity body content.

Most HTTP communication is initiated by a user agent and consists of a
request to be applied to a resource on some origin server. In the
simplest case, this may be accomplished via a single connection (v)
between the user agent (UA) and the origin server (O).

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          request chain ------------------------>
       UA -------------------v------------------- O
          <----------------------- response chain

A more complicated situation occurs when one or more intermediaries are
present in the request/response chain. There are three common forms of
intermediary: proxy, gateway, and tunnel. A proxy is a forwarding
agent,
receiving requests for a URI in its absolute form, rewriting all or part
of the message, and forwarding the reformatted request toward the server
identified by the URI. A gateway is a receiving agent, acting as a layer
above some other server(s) and, if necessary, translating the requests
to the underlying server's protocol. A tunnel acts as a relay point
between two connections without changing the messages; tunnels are used
when the communication needs to pass through an intermediary (such as a
firewall) even when the intermediary cannot understand the contents of
the messages.

          request chain -------------------------------------->
       UA -----v----- A -----v----- B -----v----- C -----v----- O
          <------------------------------------- response chain

The figure above shows three intermediaries (A, B, and C) between the
user agent and origin server. A request or response message that travels
the whole chain MUST pass through four separate connections. This
distinction is important because some HTTP communication options may
apply only to the connection with the nearest, non-tunnel neighbor, only
to the end-points of the chain, or to all connections along the chain.
Although the diagram is linear, each participant may be engaged in
multiple, simultaneous communications. For example, B may be receiving
requests from many clients other than A, and/or forwarding requests to
servers other than C, at the same time that it is handling A's request.

Any party to the communication which is not acting as a tunnel may
employ an internal cache for handling requests. The effect of a cache is
that the request/response chain is shortened if one of the participants
along the chain has a cached response applicable to that request. The
following illustrates the resulting chain if B has a cached copy of an
earlier response from O (via C) for a request which has not been cached
by UA or A.

          request chain ---------->
       UA -----v----- A -----v----- B - - - - - - C - - - - - - O
          <--------- response chain

Not all responses are cachable, and some requests may contain modifiers
which place special requirements on cache behavior. HTTP requirements
for cache behavior and cachable responses are defined in section 16.

HTTP communication usually takes place over TCP/IP connections. The
default port is TCP 80 , but other ports can be used. This does not
preclude HTTP from being implemented on top of any other protocol on the
Internet, or on other networks. HTTP only presumes a reliable
transport;
any protocol that provides such guarantees can be used; the mapping of
the HTTP/1.1 request and response structures onto the transport data

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units of the protocol in question is outside the scope of this
specification.

However, HTTP/1.1 implementations SHOULD implement persistent
connections (See section 17). Both clients and servers MUST be capable
of handling cases where either party closes the connection prematurely,
due to user action, automated time-out, or program failure. In any
case,
the closing of the connection by either or both parties always
terminates the current request, regardless of its status.


5.5 HTTP and MIME
HTTP/1.1 uses many of the constructs defined for MIME, as defined in RFC
1521 . Appendix 23.3 describes the ways in which the context of HTTP
allows for different use of Internet Media Types than is typically found
in Internet mail, and gives the rationale for those differences.


6 Notational Conventions and Generic Grammar

6.1 Augmented BNF
All of the mechanisms specified in this document are described in both
prose and an augmented Backus-Naur Form (BNF) similar to that used by
RFC 822 . Implementers will need to be familiar with the notation in
order to understand this specification. The augmented BNF includes the
following constructs:


name = definition
     The name of a rule is simply the name itself (without any enclosing
     "<" and ">") and is separated from its definition by the equal
     character "=". Whitespace is only significant in that indentation
     of continuation lines is used to indicate a rule definition that
     spans more than one line. Certain basic rules are in uppercase,
     such as SP, LWS, HT, CRLF, DIGIT, ALPHA, etc. Angle brackets are
     used within definitions whenever their presence will facilitate
     discerning the use of rule names.

"literal"
     Quotation marks surround literal text. Unless stated otherwise, the
     text is case-insensitive.

rule1 | rule2
     Elements separated by a bar ("I") are alternatives, e.g., "yes |
     no" will accept yes or no.

(rule1 rule2)
     Elements enclosed in parentheses are treated as a single element.
     Thus, "(elem (foo | bar) elem)" allows the token sequences "elem
     foo elem" and "elem bar elem".

*rule
     The character "*" preceding an element indicates repetition. The
     full form is "<n>*<m>element" indicating at least <n> and at most

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     <m> occurrences of element. Default values are 0 and infinity so
     that "*(element)" allows any number, including zero; "1*element"
     requires at least one; and "1*2element" allows one or two.

[rule]
     Square brackets enclose optional elements; "[foo bar]" is
     equivalent to "*1(foo bar)".

N rule
     Specific repetition: "<n>(element)" is equivalent to
     "<n>*<n>(element)"; that is, exactly <n> occurrences of (element).
     Thus 2DIGIT is a 2-digit number, and 3ALPHA is a string of three
     alphabetic characters.

#rule
     A construct "#" is defined, similar to "*", for defining lists of
     elements. The full form is "<n>#<m>element " indicating at least
     <n> and at most <m> elements, each separated by one or more commas
     (",") and optional linear whitespace (LWS). This makes the usual
     form of lists very easy; a rule such as "( *LWS element *( *LWS
","
     *LWS element )) "  can be shown as "1#element". Wherever this
     construct is used, null elements are allowed, but do not contribute
     to the count of elements present. That is, "(element), , (element)
     " is permitted, but counts as only two elements. Therefore, where
     at least one element is required, at least one non-null element
     MUST be present. Default values are 0 and infinity so that
     "#(element) " allows any number, including zero; "1#element"
     requires at least one; and "1#2element" allows one or two.

; comment
     A semi-colon, set off some distance to the right of rule text,
     starts a comment that continues to the end of line. This is a
     simple way of including useful notes in parallel with the
     specifications.

implied *LWS
     The grammar described by this specification is word-based. Except
     where noted otherwise, linear whitespace (LWS) can be included
     between any two adjacent words (token or quoted-string), and
     between adjacent tokens and delimiters (tspecials), without
     changing the interpretation of a field. At least one delimiter
     (tspecials) MUST exist between any two tokens, since they would
     otherwise be interpreted as a single token. However, applications
     SHOULD attempt to follow "common form" when generating HTTP
     constructs, since there exist some implementations that fail to
     accept anything beyond the common forms.

6.2 Basic Rules
The following rules are used throughout this specification to describe
basic parsing constructs. The US-ASCII coded character set is defined by.

       OCTET          = <any 8-bit sequence of data>
       CHAR           = <any US-ASCII character (octets 0 - 127)>

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       UPALPHA        = <any US-ASCII uppercase letter "A".."Z">
       LOALPHA        = <any US-ASCII lowercase letter "a".."z">
       ALPHA          = UPALPHA | LOALPHA
       DIGIT          = <any US-ASCII digit "0".."9">
       CTL            = <any US-ASCII control character
                        (octets 0 - 31) and DEL (127)>
       CR             = <US-ASCII CR, carriage return (13)>
       LF             = <US-ASCII LF, linefeed (10)>
       SP             = <US-ASCII SP, space (32)>
       HT             = <US-ASCII HT, horizontal-tab (9)>
       <">            = <US-ASCII double-quote mark (34)>

HTTP/1.1 defines the octet sequence CR LF as the end-of-line marker for
all protocol elements except the Entity-Body (see appendix 23.2 for
tolerant applications). The end-of-line marker within an Entity-Body is
defined by its associated media type, as described in section 7.7.

       CRLF           = CR LF

HTTP/1.1 headers can be folded onto multiple lines if the continuation
line begins with a space or horizontal tab. All linear whitespace,
including folding, has the same semantics as SP.

       LWS            = [CRLF] 1*( SP | HT )

The TEXT rule is only used for descriptive field contents and values
that are not intended to be interpreted by the message parser. Words of
*TEXT MAY contain octets from character sets other than US-ASCII only
when encoded according to the rules of RFC 1522 .

       TEXT           = <any OCTET except CTLs,
                        but including LWS>

Recipients of header field TEXT containing octets outside the US-ASCII
character set range MAY assume that they represent ISO-8859-1 characters
if there is no other encoding indicated by an RFC 1522 mechanism.

Hexadecimal numeric characters are used in several protocol elements.

       HEX            = "A" | "B" | "C" | "D" | "E" | "F"
                      | "a" | "b" | "c" | "d" | "e" | "f" | DIGIT

Many HTTP/1.1 header field values consist of words separated by LWS or
special characters. These special characters MUST be in a quoted string
to be used within a parameter value.

       word           = token | quoted-string

       token          = 1*<any CHAR except CTLs or tspecials>

       tspecials      = "(" | ")" | "<" | ">" | "@"
                      | "," | ";" | ":" | "\" | <">
                      | "/" | "[" | "]" | "?" | "="
                      | "{" | "}" | SP | HT

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Comments can be included in some HTTP header fields by surrounding the
comment text with parentheses. Comments are only allowed in fields
containing "comment" as part of their field value definition. In all
other fields, parentheses are considered part of the field value.

       comment        = "(" *( ctext | comment ) ")"
       ctext          = <any TEXT excluding "(" and ")">

A string of text is parsed as a single word if it is quoted using
double-quote marks.

       quoted-string  = ( <"> *(qdtext) <"> )


       qdtext         = <any CHAR except <"> and CTLs,
                        but including LWS>

The backslash character ("\") may be used as a single-character quoting
mechanism only within quoted-string and comment constructs.

       quoted-pair    = "\" CHAR


7 Protocol Parameters

7.1 HTTP Version
HTTP uses a "<major>.<minor>" numbering scheme to indicate versions of
the protocol. The protocol versioning policy is intended to allow the
sender to indicate the format of a message and its capacity for
understanding further HTTP communication, rather than the features
obtained via that communication. No change is made to the version number
for the addition of message components which do not affect communication
behavior or which only add to extensible field values. The <minor>
number is incremented when the changes made to the protocol add features
which do not change the general message parsing algorithm, but which may
add to the message semantics and imply additional capabilities of the
sender. The <major> number is incremented when the format of a message
within the protocol is changed.

The version of an HTTP message is indicated by an HTTP-Version field in
the first line of the message. If the protocol version is not specified,
the recipient MUST assume that the message is in the simple HTTP/0.9
format .

       HTTP-Version   = "HTTP" "/" 1*DIGIT "." 1*DIGIT

Note that the major and minor numbers SHOULD be treated as separate
integers and that each MAY be incremented higher than a single digit.
Thus, HTTP/2.4 is a lower version than HTTP/2.13, which in turn is lower
than HTTP/12.3. Leading zeros SHOULD be ignored by recipients and never
generated by senders.

Applications sending Full-Request or Full-Response messages, as defined
by this specification, MUST include an HTTP-Version of "HTTP/1.1". Use

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of this version number indicates that the sending application is at
least conditionally compliant with this specification.

Proxy and gateway applications MUST be careful in forwarding requests
that are received in a format different from that of the application's
native HTTP version. Since the protocol version indicates the protocol
capability of the sender, a proxy/gateway MUST never send a message with
a version indicator which is greater than its native version; if a
higher version request is received, the proxy/gateway MUST either
downgrade the request version, respond with an error, or switch to
tunnel behavior. Requests with a version lower than that of the
application's native format MAY be upgraded before being forwarded; the
proxy/gateway's response to that request MUST follow the server
requirements listed above.

  Note: Converting between versions of HTTP may involve addition or
  deletion of headers required or forbidden by the version involved.
  It is likely more involved than just changing the version
  indicator.


7.2 Uniform Resource Identifiers
URIs have been known by many names: WWW addresses, Universal Document
Identifiers, Universal Resource Identifiers , and finally the
combination of Uniform Resource Locators (URL)  and Names (URN) . As far
as HTTP is concerned, Uniform Resource Identifiers are simply formatted
strings which identify--via name, location, or any other characteristic--
a network resource.


7.2.1 General Syntax
URIs in HTTP can be represented in absolute form or relative to some
known base URI , depending upon the context of their use. The two forms
are differentiated by the fact that absolute URIs always begin with a
scheme name followed by a colon.

       URI            = ( absoluteURI | relativeURI ) [ "#" fragment ]

       absoluteURI    = scheme ":" *( uchar | reserved )

       relativeURI    = net_path | abs_path | rel_path

       net_path       = "//" net_loc [ abs_path ]       abs_path
                      = "/" rel_path
       rel_path       = [ path ] [ ";" params ] [ "?" query ]

       path           = fsegment *( "/" segment )
       fsegment       = 1*pchar
       segment        = *pchar

       params         = param *( ";" param )
       param          = *( pchar | "/" )



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       scheme         = 1*( ALPHA | DIGIT | "+" | "-" | "." )
       net_loc        = *( pchar | ";" | "?" )
       query          = *( uchar | reserved )
       fragment       = *( uchar | reserved )

       pchar          = uchar | ":" | "@" | "&amp;" | "=" | "+"
       uchar          = unreserved | escape
       unreserved     = ALPHA | DIGIT | safe | extra | national

       escape         = "%" HEX HEX
       reserved       = ";" | "/" | "?" | ":" | "@" | "&amp;" | "=" | "+"
       extra          = "!" | "*" | "'" | "(" | ")" | ","
       safe           = "$" | "-" | "_" | "."
       unsafe         = CTL | SP | <"> | "#" | "%" | "<" | ">"
       national       = <any OCTET excluding ALPHA, DIGIT,
                        reserved, extra, safe, and unsafe>

For definitive information on URL syntax and semantics, see RFC 1738
and RFC 1808 . The BNF above includes national characters not allowed in
valid URLs as specified by RFC 1738, since HTTP servers are not
restricted in the set of unreserved characters allowed to represent the
rel_path part of addresses, and HTTP proxies may receive requests for
URIs not defined by RFC 1738.

The HTTP protocol does not place any a priori limit on the length of a
URI.   Servers MUST be able to handle the URI of any resource they
serve,  and SHOULD be able to handle URIs of unbounded length if they
provide GET-based forms that could generate such URIs. A server SHOULD
return a status code of

      414 Request-URI Too Large

 if a URI is longer than the server can handle.  See section 12.4.1.15.

  Note: Servers should be cautious about depending on URI lengths
  above 255 bytes, because some older client or proxy implementations
  may not properly support these.

 All client and proxy implementations MUST be able to handle a URI of
any finite length.


7.2.2 http URL
The "http" scheme is used to locate network resources via the HTTP
protocol. This section defines the scheme-specific syntax and semantics
for http URLs.

       http_URL       = "http:" "//" host [ ":" port ] [ abs_path ]

       host           = <A legal Internet host domain name
                         or IP address (in dotted-decimal form),
                         as defined by Section 2.1 of RFC 1123>

       port           = *DIGIT

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If the port is empty or not given, port 80 is assumed. The semantics are
that the identified resource is located at the server listening for TCP
connections on that port of that host, and the Request-URI for the
resource is abs_path.  The use of IP addresses in URL's SHOULD be
avoided whenever possible.  See RFC 1900.  If the abs_path is not
present in the URL, it MUST be given as "/" when used as a Request-URI
for a resource (section 9.1.2).

  Note: Although the HTTP protocol is independent of the transport
  layer protocol, the http URL only identifies resources by their TCP
  location, and thus non-TCP resources MUST be identified by some
  other URI scheme.

The canonical form for "http" URLs is obtained by converting any UPALPHA
characters in host to their LOALPHA equivalent (hostnames are case-
insensitive), eliding the [ ":" port ] if the port is 80, and replacing
an empty abs_path with "/".


7.2.3 URI Canonicalization
A cache, when comparing two URIs to decide if they match or not, a cache
MUST use a case-sensitive octet-by-octet comparison of the entire URIs,
with these exceptions:

Following the rules from section 7.2.2:

  .  A port that is empty or not given is equivalent to port 80.
  .  Comparisons of host names MUST be case-insensitive.
  .  Comparisons of scheme names MUST be case-insensitive.
  .  An empty abs_path is equivalent to an abs_path of "/"
Characters except those in the reserved set and the unsafe set (see
section 7.2) are equivalent to their ""%" HEX HEX" encodings.

For example, the following three URIs are equivalent:

      http://abc.com:80/~smith/home.html
      http://ABC.com/%7Esmith/home.html
      http://ABC.com:/%7esmith/home.html




7.3 Date/Time Formats

7.3.1 Full Date
HTTP applications have historically allowed three different formats for
the representation of date/time stamps:

       Sun, 06 Nov 1994 08:49:37 GMT   ; RFC 822, updated by RFC 1123
       Sunday, 06-Nov-94 08:49:37 GMT  ; RFC 850, obsoleted by RFC 1036
       Sun Nov  6 08:49:37 1994        ; ANSI C's asctime() format

The first format is preferred as an Internet standard and represents a
fixed-length subset of that defined by RFC 1123  (an update to RFC 822).

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The second format is in common use, but is based on the obsolete RFC
850  date format and lacks a four-digit year. HTTP/1.1 clients and
servers that parse the date value MUST accept all three formats, though
they MUST generate only the RFC 1123 format for representing date/time
stamps in HTTP message fields.

  Note: Recipients of date values are encouraged to be robust in
  accepting date values that may have been generated by non-HTTP
  applications, as is sometimes the case when retrieving or posting
  messages via proxies/gateways to SMTP or NNTP.

All HTTP date/time stamps MUST be represented in Universal Time (UT),
also known as Greenwich Mean Time (GMT), without exception. This is
indicated in the first two formats by the inclusion of "GMT" as the
three-letter abbreviation for time zone, and SHOULD be assumed when
reading the asctime format.

       HTTP-date      = rfc1123-date | rfc850-date | asctime-date

       rfc1123-date   = wkday "," SP date1 SP time SP "GMT"
       rfc850-date    = weekday "," SP date2 SP time SP "GMT"
       asctime-date   = wkday SP date3 SP time SP 4DIGIT

       date1          = 2DIGIT SP month SP 4DIGIT
                        ; day month year (e.g., 02 Jun 1982)
       date2          = 2DIGIT "-" month "-" 2DIGIT
                        ; day-month-year (e.g., 02-Jun-82)
       date3          = month SP ( 2DIGIT | ( SP 1DIGIT ))
                        ; month day (e.g., Jun  2)

       time           = 2DIGIT ":" 2DIGIT ":" 2DIGIT
                        ; 00:00:00 - 23:59:59

       wkday          = "Mon" | "Tue" | "Wed"
                      | "Thu" | "Fri" | "Sat" | "Sun"

       weekday        = "Monday" | "Tuesday" | "Wednesday"
                      | "Thursday" | "Friday" | "Saturday" | "Sunday"

       month          = "Jan" | "Feb" | "Mar" | "Apr"
                      | "May" | "Jun" | "Jul" | "Aug"
                      | "Sep" | "Oct" | "Nov" | "Dec"

  Note: HTTP requirements for the date/time stamp format apply only
  to their usage within the protocol stream. Clients and servers are
  not required to use these formats for user presentation, request
  logging, etc.

Additional rules for requirements on parsing and representation of dates
and other potential problems with date representations include:

  .  HTTP/1.1 clients and caches should assume that an RFC-850 date
     which appears to be more than 50 years in the future is in fact in
     the past (this helps solve the "year 2000" problem).

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  .  An HTTP/1.1 implementation may internally represent a parsed
     Expires date as earlier than the proper value, but MUST NOT
     internally represent a parsed Expires date as later than the proper
     value.
  .  All expiration-related calculations must be done in Universal Time
     (GMT).  The local time zone MUST NOT influence the calculation or
     comparison of an age or expiration time.
  .  If an HTTP header incorrectly carries a date value with a time zone
     other than GMT, it must be converted into GMT using the most
     conservative possible conversion.




7.3.2 Delta Seconds
Some HTTP header fields allow a time value to be specified as an integer
number of seconds, represented in decimal, after the time that the
message was received. This format SHOULD only be used to represent short
time periods or periods that cannot start until receipt of the message.

       delta-seconds  = 1*DIGIT


7.4 Character Sets
HTTP uses the same definition of the term "character set" as that
described for MIME:

  The term "character set" is used in this document to refer to a
  method used with one or more tables to convert a sequence of octets
  into a sequence of characters. Note that unconditional conversion
  in the other direction is not required, in that not all characters
  may be available in a given character set and a character set may
  provide more than one sequence of octets to represent a particular
  character. This definition is intended to allow various kinds of
  character encodings, from simple single-table mappings such as US-
  ASCII to complex table switching methods such as those that use ISO
  2022's techniques. However, the definition associated with a MIME
  character set name MUST fully specify the mapping to be performed
  from octets to characters. In particular, use of external profiling
  information to determine the exact mapping is not permitted.

  Note: This use of the term "character set" is more commonly
  referred to as a "character encoding." However, since HTTP and MIME
  share the same registry, it is important that the terminology also
  be shared.

HTTP character sets are identified by case-insensitive tokens. The
complete set of tokens is defined by the IANA Character Set registry .
However, because that registry does not define a single, consistent
token for each character set, we define here the preferred names for
those character sets most likely to be used with HTTP entities. These
character sets include those registered by RFC 1521  -- the US-ASCII
and ISO-8859  character sets -- and other names specifically recommended
for use within MIME charset parameters.

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       charset = "US-ASCII"
               | "ISO-8859-1" | "ISO-8859-2" | "ISO-8859-3"
               | "ISO-8859-4" | "ISO-8859-5" | "ISO-8859-6"
               | "ISO-8859-7" | "ISO-8859-8" | "ISO-8859-9"
               | "ISO-2022-JP" | "ISO-2022-JP-2" | "ISO-2022-KR"
               | "UNICODE-1-1" | "UNICODE-1-1-UTF-7"
               | "UNICODE-1-1-UTF-8" | token

Although HTTP allows an arbitrary token to be used as a charset value,
any token that has a predefined value within the IANA Character Set
registry  MUST represent the character set defined by that registry.
Applications SHOULD limit their use of character sets to those defined
by the IANA registry.

The character set of an entity body SHOULD be labeled as the lowest
common denominator of the character codes used within that body, with
the exception that no label is preferred over the labels US-ASCII or
ISO-8859-1.


7.5 Content Codings
Content coding values indicate an encoding transformation that has been
or can be applied to a resource entity. Content codings are primarily
used to allow a document to be compressed or encrypted without losing
the identity of its underlying media type. Typically, the resource
entity is stored in this encoding and only decoded before rendering or
analogous usage.

       content-coding   = "gzip" | "x-gzip"
                        | "compress" | "x-compress" | token

  Note: For historical reasons, HTTP applications SHOULD consider "x-
  gzip" and "x-compress" to be equivalent to "gzip" and "compress",
  respectively.

All content-coding values are case-insensitive. HTTP/1.1 uses content-
coding values in the Accept-Encoding (section 18.3) and
Content-Encoding
(section 18.13) header fields. Although the value describes the
content-
coding, what is more important is that it indicates what decoding
mechanism will be required to remove the encoding. Note that a single
program MAY be capable of decoding multiple content-coding formats. Two
values are defined by this specification:


gzip
     An encoding format produced by the file compression program "gzip"
     (GNU zip) developed by Jean-loup Gailly. This format is typically a
     Lempel-Ziv coding (LZ77) with a 32 bit CRC.

compress
     The encoding format produced by the file compression program
     "compress". This format is an adaptive Lempel-Ziv-Welch coding
     (LZW).


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  Note: Use of program names for the identification of encoding
  formats is not desirable and should be discouraged for future
  encodings. Their use here is representative of historical practice,
  not good design.

HTTP defines a registration process which uses the Internet Assigned
Numbers Authority (IANA) as a central registry for content-coding value
tokens.  Additional content-coding value tokens beyond the four defined
in this document (gzip x-gzip compress x-compress)  SHOULD be registered
with the IANA. To allow interoperability between clients and servers,
specifications of the content coding algorithms used to implement a new
value SHOULD be publicly available and adequate for independent
implementation, and MUST conform to the purpose of content coding
defined in this section.


7.6 Transfer Codings
Transfer coding values are used to indicate an encoding transformation
that has been, can be, or may need to be applied to an Entity-Body in
order to ensure safe transport through the network. This differs from a
content coding in that the transfer coding is a property of the
message,
not of the original resource entity.

       transfer-coding         = "chunked" | transfer-extension

       transfer-extension      = token

All transfer-coding values are case-insensitive. HTTP/1.1 uses transfer
coding values in the Transfer-Encoding header field (section 18.43).

Transfer codings are analogous to the Content-Transfer-Encoding values
of MIME , which were designed to enable safe transport of binary data
over a 7-bit transport service. However, "safe transport" has a
different focus for an 8bit-clean transfer protocol. In HTTP, the only
unsafe characteristic of message bodies is the difficulty in determining
the exact body length (section 11.2.2), or the desire to encrypt data
over a shared transport.

All HTTP/1.1 applications MUST be able to receive and decode the
"chunked" transfer coding , and MUST ignore transfer coding extensions
they do not understand. A server which receives a an entity-body with a
transfer-coding it does not understand SHOULD return
501(Unimplemented),
and close the connection. A server MUST NOT send transfer-codings to a
client that were not defined in the version of HTTP used in the client's
request. Clients sending entity-bodies with transfer-codings SHOULD must
be prepared for the connection to be closed if the server doesn't
understand the transfer-coding.  The chunked encoding modifies the body
of a message in order to transfer it as a series of chunks, each with
its own size indicator, followed by an optional footer containing
entity-header fields. This allows dynamically-produced content to be
transferred along with the information necessary for the recipient to
verify that it has received the full message.



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       Chunked-Body   = *chunk
                        "0" CRLF
                        footer
                        CRLF

       chunk          = chunk-size [ chunk-ext ] CRLF
                        chunk-data CRLF

       chunk-size     = hex-no-zero *HEX
       chunk-ext      = *( ";" chunk-ext-name [ "=" chunk-ext-value ] )
       chunk-ext-name = token
       chunk-ext-val  = token | quoted-string
       chunk-data     = chunk-size(OCTET)

       footer         = *<<Content-MD5 and future headers that specify
                         they are allowed in footer>>

       hex-no-zero    = <HEX excluding "0">

Note that the chunks are ended by a zero-sized chunk, followed by the
footer and terminated by an empty line. An example process for decoding
a Chunked-Body is presented in appendix 23.3.6.


7.7 Media Types
HTTP uses Internet Media Types  in the Content-Type (section 18.19) and
Accept (section 18.1) header fields in order to provide open and
extensible data typing and type negotiation.

       media-type     = type "/" subtype *( ";" parameter )
       type           = token
       subtype        = token

Parameters may follow the type/subtype in the form of attribute/value
pairs.

       parameter      = attribute "=" value
       attribute      = token
       value          = token | quoted-string

The type, subtype, and parameter attribute names are case-insensitive.
Parameter values may or may not be case-sensitive, depending on the
semantics of the parameter name. LWS MUST NOT be generated between the
type and subtype, nor between an attribute and its value. Upon receipt
of a media type with an unrecognized parameter, a user agent SHOULD
treat the media type as if the unrecognized parameter and its value were
not present.

Some older HTTP applications do not recognize media type parameters.
HTTP/1.1 applications SHOULD only use media type parameters when they
are necessary to define the content of a message.




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Media-type values are registered with the Internet Assigned Number
Authority (IANA ). The media type registration process is outlined in
RFC 1590 . Use of non-registered media types is discouraged.


7.7.1 Canonicalization and Text Defaults
Internet media types are registered with a canonical form. In general,
an Entity-Body transferred via HTTP MUST be represented in the
appropriate canonical form prior to its transmission; the exception is
"text" types, as defined in the next paragraph..

when in canonical form , media subtypes of the "text" type use CRLF as
the text line break. However, HTTP allows the transport of text media
with plain CR or LF alone representing a line break when if it is done
consistently for an entire Entity-Body.. HTTP applications MUST accept
CRLF, bare CR, and bare LF as being representative of a line break in
text media received via HTTP.In addition, if the text media is
represented in a character set that does not use octets 13 and 10 for CR
and LF respectively, as is the case for some multi-byte character sets,
HTTP allows the use of whatever octet sequences are defined by that
character set to represent the equivalent of CR and LF for line breaks.
This flexibility regarding line breaks applies only to text media in the
Entity-Body; a bare CR or LF MUST NOT be substituted for CRLF within any
of the HTTP control structures (such as header fields and multipart
boundaries).

If an Entity-Body is encoded with a Content-Encoding, the underlying
data MUST be in a form defined above prior to being encoded.

The "charset" parameter is used with some media types to define the
character set (section 7.4) of the data. When no explicit charset
parameter is provided by the sender, media subtypes of the "text" type
are defined to have a default charset value of "ISO-8859-1" when
received via HTTP. Data in character sets other than "ISO-8859-1" or its
subsets MUST be labeled with an appropriate charset value in order to be
consistently interpreted by the recipient.

  Note: Many current HTTP servers provide data using charsets other
  than "ISO-8859-1" without proper labeling. This situation reduces
  interoperability and is not recommended. To compensate for this,
  some HTTP user agents provide a configuration option to allow the
  user to change the default interpretation of the media type
  character set when no charset parameter is given.




7.7.2 Multipart Types
MIME provides for a number of "multipart" types -- encapsulations of one
or more entities within a single message's Entity-Body. All multipart
types share a common syntax, as defined in section 7.2.1 of RFC 1521 ,
and MUST include a boundary parameter as part of the media type value.
The message body is itself a protocol element and MUST therefore use
only CRLF to represent line breaks between body-parts. Unlike in RFC

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1521, the epilogue of any multipart message MUST be empty; HTTP
applications MUST NOT transmit the epilogue even if the original
resource entity contains an epilogue.

In HTTP, multipart body-parts MAY contain header fields which are
significant to the meaning of that part.

In general, an HTTP user agent SHOULD follow the same or similar
behavior as a MIME user agent would upon receipt of a multipart type. If
an application receives an unrecognized multipart subtype, the
application MUST treat it as being equivalent to "multipart/mixed".

  Note: The "multipart/form-data" type has been specifically defined
  for carrying form data suitable for processing via the POST request
  method, as described in RFC 1867 .




7.8 Product Tokens
Product tokens are used to allow communicating applications to identify
themselves via a simple product token, with an optional slash and
version designator. Most fields using product tokens also allow sub-
products which form a significant part of the application to be listed,
separated by whitespace. By convention, the products are listed in order
of their significance for identifying the application.

       product         = token ["/" product-version]
       product-version = token

Examples:

       User-Agent: CERN-LineMode/2.15 libwww/2.17b3
       Server: Apache/0.8.4

Product tokens should be short and to the point -- use of them for
advertising or other non-essential information is explicitly forbidden.
Although any token character may appear in a product-version, this token
SHOULD only be used for a version identifier (i.e., successive versions
of the same product SHOULD only differ in the product-version portion of
the product value).


7.9 Quality Values
HTTP content negotiation (section 15) uses short "floating point"
numbers to indicate the relative importance ("weight") of various
negotiable parameters. The weights are normalized to a real number in
the range 0 through 1, where 0 is the minimum and 1 the maximum value.
In order to discourage misuse of this feature, HTTP/1.1 applications
MUST NOT generate more than three digits after the decimal point. User
configuration of these values SHOULD also be limited in this fashion.

       qvalue         = ( "0" [ "." 0*3DIGIT ] )
                      | ( "1" [ "." 0*3("0") ] )

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"Quality values" is a slight misnomer, since these values actually
measure relative degradation in perceived quality. Thus, a value of
"0.8" represents a 20% degradation from the optimum rather than a
statement of 80% quality.


7.10 Language Tags
A language tag identifies a natural language spoken, written, or
otherwise conveyed by human beings for communication of information to
other human beings. Computer languages are explicitly excluded. HTTP
uses language tags within the Accept-Language, and Content-Language
fields.

The syntax and registry of HTTP language tags is the same as that
defined by RFC 1766 . In summary, a language tag is composed of 1 or
more parts: A primary language tag and a possibly empty series of
subtags:

        language-tag  = primary-tag *( "-" subtag )

        primary-tag   = 1*8ALPHA
        subtag        = 1*8ALPHA

Whitespace is not allowed within the tag and all tags are case-
insensitive. The name space of language tags is administered by the
IANA. Example tags include:

       en, en-US, en-cockney, i-cherokee, x-pig-latin

where any two-letter primary-tag is an ISO 639 language abbreviation and
any two-letter initial subtag is an ISO 3166 country code.  (The last
three tags above are not registered tags; all but the last are examples
of tags which could be registered in future.)


7.11 Entity Tags
Entity tags are quoted strings whose internal structure is not visible
to clients or caches. Entity tags are used as cache validators in
HTTP/1.1.

      entity-tag = strong-entity-tag | weak-entity-tag
                              | null-entity-tag
      strong-entity-tag = quoted-string
      weak-entity-tag = quoted-string "/W"
      null-entity-tag = <"> <">

  Note that the "/W" tag is considered part of a weak entity tag; it
  MUST NOT be removed by any cache or client.

There are two comparison functions on  validators:

  .  The strong comparison function: in order to be considered equal,
     both validators must be identical in every way, and neither may be
     weak.

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  .  The weak comparison function: in order to be considered equal, both
     validators must be identical in every way, except for the presence
     or absence of a "weak" tag.
The weak comparison function MAY be used for simple (non-subrange) GET
requests. The strong comparison function MUST be used in all other
cases.

The null validator is a special value, defined as never matching the
current validator of an existing resource entity, and always matching
the "current" validator of a resource entity  that does not exist.


7.12 Variant IDs
A cache stores instances of resource entities, not instances of generic
resources per se. Therefore, the URI of a generic resource is not
sufficient for use as an identifier for a specific resource entity. In
certain interactions between a cache and an origin server, it is
convenient to encode that identifier using a more   compact
representation than the full set of selecting request headers (which may
not even be possible if the selection criteria are not known to the
cache).

For these reasons, the HTTP protocol provides an optional mechanism for
identifying a specific entity source of a generic resource,  called a
variant-ID.

Variant-IDs are used to identify specific variants of a generic
resource; see section 16.5.3 for how they are used.

      variant-id = quoted-string

Variant-IDs are compared using string octet-equality; case is
significant.

All responses from generic resources SHOULD include variant-IDs.  If
these are not present, the resource author can expect caches to
correctly handle requests on the generic resource, but cannot expect the
caching to be efficient.




7.13 Variant Sets
Validator sets are used for doing conditional retrievals on generic
resources; see section 16.5.3.

      variant-set = 1#variant-set-item
      variant-set-item = opaque-validator ";" variant-id


7.14 Range Protocol Parameters
This section defines certain HTTP protocol parameters used in range
requests and related responses.


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7.14.1 Range Units
A resource entity may be broken down into subranges according to various
structural units.

      range-unit       = bytes-unit | other-range-unit

      bytes-unit       = "bytes"
      other-range-unit = token

The only range unit defined by HTTP/1.1 is "bytes".  HTTP/1.1
implementations may ignore ranges specified using other units.


7.14.2 Byte Ranges
Since all HTTP entities are represented in HTTP messages as sequences of
bytes, the concept of a byte range is meaningful for any HTTP entity.
(However, not all clients and servers need to support byte-range
operations.)

Byte range specifications in HTTP apply to the sequence of bytes that
would be transferred by the protocol if no transfer-coding were being
applied.

  This means that if Content-coding is applied to the data, the byte
  range specification applies to the resulting content-encoded byte
  stream, not to the unencoded byte stream.  It also means that if
  the entity-body's media-type is a composite type (e.g., multipart/*
  and message/rfc822), then the composite's body-parts may have their
  own content-encoding and content-transfer-encoding, and the byte
  range applies to the result of the those encodings.

A byte range operation may specify a single range of bytes, or a set of
ranges within a single entity.

       ranges-specifier = byte-ranges-specifier

       byte-ranges-specifier = bytes-unit "=" byte-range-set

       byte-range-set = 1#( byte-range-spec | suffix-byte-range-spec )

       byte-range-spec = first-byte-pos "-" [last-byte-pos]

       first-byte-pos = 1*DIGIT

       last-byte-pos = 1*DIGIT

The first-byte-pos value in a byte-range-spec gives the byte-offset of
the first byte in a range.  The last-byte-pos value gives the byte-
offset of the last byte in the range; that is, the byte positions
specified are inclusive.  Byte offsets start at zero.

If the last-byte-pos value is present, it must be greater than or equal
to the first-byte-pos in that byte-range-spec, or the byte-range-spec is
invalid.  The recipient of an invalid byte-range-spec must ignore it.

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If the last-byte-pos value is absent, it is assumed to be equal to the
current length of the entity in bytes.

If the last-byte-pos value is larger than the current length of the
entity, it is assumed to be equal to the current length of the entity.

       suffix-byte-range-spec = "-" suffix-length

       suffix-length = 1*DIGIT

A suffix-byte-range-spec is used to specify the suffix of the entity, of
a length given by the suffix-length value.  (That is, this form
specifies the last N bytes of an entity.)  If the entity is shorter than
the specified suffix-length, the entire entity is used.

Examples of byte-ranges-specifier values (assuming an entity of length
10000):

  .  The first 500 bytes (byte offsets 0-499, inclusive):
       bytes=0-499

  .  The second 500 bytes (byte offsets 500-999, inclusive):
       bytes=500-999

  .  The final 500 bytes (byte offsets 9500-9999, inclusive):
       bytes=-500

  .  Or
       bytes=9500-

  .  The first and last bytes only (bytes 0 and 9999):
       bytes=0-0,-1

  .  Several legal but not canonical specifications of the second 500
     bytes (byte offsets 500-999, inclusive):
       bytes=500-600,601-999

       bytes=500-700,601-999


7.14.3 Content Ranges
When a server returns a partial response to a client, it must describe
both the extent of the range covered by the response, and the length of
the entire entity.

       content-range-spec      = byte-content-range-spec

       byte-content-range-spec = bytes-unit SP first-byte-pos "-"
                                      last-byte-pos "/" entity-length

       entity-length            = 1*DIGIT




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Unlike byte-ranges-specifier values, a byte-content-range-spec may only
specify one range, and must contain absolute byte positions for both the
first and last byte of the range.

A byte-content-range-spec whose last-byte-pos value is less than its
first-byte-pos value, or whose entity-length value is less than or equal
to its last-byte-pos value, is invalid.  The recipient of an invalid
byte-content-range-spec MUST ignore it and any content transferred along
with it.

Examples of byte-content-range-spec values, assuming that the entity
contains a total of 1234 bytes:

  .  The first 500 bytes:
       bytes 0-499/1234

  .  The second 500 bytes:
       bytes 500-999/1234

  .  All except for the first 500 bytes:
       bytes 500-1233/1234

  .  The last 500 bytes:
       bytes 734-1233/1234


8 HTTP Message

8.1 Message Types
HTTP messages consist of requests from client to server and responses
from server to client.

       HTTP-message   = Full-Request              ; HTTP/1.1 messages
                      | Full-Response

Full-Request and Full-Response use the generic message format of RFC 822
for transferring entities. Both messages may include optional header
fields (also known as "headers") and an entity body. The entity body is
separated from the headers by a null line (i.e., a line with nothing
preceding the CRLF).


8.2 Message Headers
HTTP header fields, which include General-Header (Section 8.3),
Request-
Header (Section 9.2), Response-Header (Section 10.2), and Entity-Header
(Section 11.1) fields, follow the same generic format as that given in
Section 3.1 of RFC 822 . Each header field consists of a name followed
by a colon (":") and the field value. Field names are case-insensitive.
The field value may be preceded by any amount of LWS, though a single SP
is preferred. Header fields can be extended over multiple lines by
preceding each extra line with at least one SP or HT.

       HTTP-header    = field-name ":" [ field-value ] CRLF


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       field-name     = token
       field-value    = *( field-content | LWS )

       field-content  = <the OCTETs making up the field-value
                        and consisting of either *TEXT or combinations
                        of token, tspecials, and quoted-string>

The order in which header fields with differing field names are received
is not significant. However, it is "good practice" to send General-
Header fields first, followed by Request-Header or Response-Header
fields, and ending with the Entity-Header fields.

Multiple HTTP-header fields with the same field-name may be present in a
message if and only if the entire field-value for that header field is
defined as a comma-separated list [i.e., #(values)]. It MUST be possible
to combine the multiple header fields into one "field-name:
field-value"
pair, without changing the semantics of the message, by appending each
subsequent field-value to the first, each separated by a comma.  Thus,
the order in which multiple header fields with the same field-name are
received may be significant to the interpretation of the combined
field-
value.


8.3 General Header Fields
There are a few header fields which have general applicability for both
request and response messages, but which do not apply to the entity
being transferred. These headers apply only to the message being
transmitted.

       General-Header = Cache-Control            ; Section 18.10
                      | Connection               ; Section 18.11
                      | Date                     ; Section 18.20
                      | Via                      ; Section 18.47
                      | Keep-Alive               ; Section 23.5.2.5.1
                      | Pragma                   ; Section 18.34
                      | Upgrade                  ; Section 18.44

General header field names can be extended reliably only in combination
with a change in the protocol version. However, new or experimental
header fields may be given the semantics of general header fields if all
parties in the communication recognize them to be general header
fields.
Unrecognized header fields are treated as Entity-Header fields.


9 Request
A request message from a client to a server includes, within the first
line of that message, the method to be applied to the resource, the
identifier of the resource, and the protocol version in use. For
backwards compatibility with the more limited HTTP/0.9 protocol, there
are two valid formats for an HTTP request:

       Request        = Full-Request



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       Full-Request   = Request-Line              ; Section 9.1
                        *( General-Header         ; Section 8.3
                         | Request-Header         ; Section 9.2
                         | Entity-Header )        ; Section 11.1
                        CRLF
                        [ Entity-Body ]           ; Section 11.2




9.1 Request-Line
The Request-Line begins with a method token, followed by the
Request-URI
and the protocol version, and ending with CRLF. The elements are
separated by SP characters. No CR or LF are allowed except in the final
CRLF sequence.

       Request-Line   =  CRLF | Method SP Request-URI SP HTTP-Version CRLF

In the interest of robustness, HTTP/1.1 servers SHOULD ignore null
request lines (ones that comprise just CRLF).  An HTTP/1.1 client MUST
NOT preface a request with CRLF.


9.1.1 Method
The Method token indicates the method to be performed on the resource
identified by the Request-URI. The method is case-sensitive.

       Method         = "OPTIONS"                ; Section 13.1
                      | "GET"                    ; Section 13.2
                      | "HEAD"                   ; Section 13.3
                      | "POST"                   ; Section 13.4
                      | "PUT"                    ; Section 13.5
                      | "DELETE"                 ; Section 13.6
                      | "TRACE"                  ; Section 13.7
                      | extension-method

       extension-method = token

The list of methods acceptable by a plain resource can be specified in
an Allow header field (section 18.7). However, the client is always
notified through the return code of the response whether a method is
currently allowed on a plain resource, as this can change dynamically.
Servers SHOULD return the status code 405 (method not allowed) if the
method is known by the server but not allowed for the requested
resource, and 501 (not implemented) if the method is unrecognized or not
implemented by the server. The list of methods known by a server can be
listed in a Public response header field (section 18.37).

The methods GET and HEAD MUST be supported by all general-purpose
servers. Servers which provide Last-Modified dates for resources MUST
also support the conditional GET method. All other methods are
optional;
however, if the above methods are implemented, they MUST be implemented
with the same semantics as those specified in section 13.

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9.1.2 Request-URI
The Request-URI is a Uniform Resource Identifier (section 7.2) and
identifies the resource upon which to apply the request.

       Request-URI    = "*" | absoluteURI | abs_path

The three options for Request-URI are dependent on the nature of the
request. The asterisk "*" means that the request does not apply to a
particular resource, but to the server itself, and is only allowed when
the Method used does not necessarily apply to a resource. One example
would be

       OPTIONS * HTTP/1.1

The absoluteURI form is required when the request is being made to a
proxy. The proxy is requested to forward the request or service it from
a valid cache, and return the response.. Note that the proxy MAY forward
the request on to another proxy or directly to the server specified by
the absoluteURI. In order to avoid request loops, a proxy MUST be able
to recognize all of its server names, including any aliases, local
variations, and the numeric IP address. An example Request-Line would
be:

       GET http://www.w3.org/pub/WWW/TheProject.html HTTP/1.1

To allow for transition to absoluteURIs in all requests in future
versions of HTTP, all HTTP/1.1 servers MUST accept the absoluteURI form
in requests, even though HTTP/1.1 clients will only generate them in
requests to proxies.  The Host request-header field MUST be ignored in
requests using an absoluteURL as the Request-URI.

The most common form of Request-URI is that used to identify a resource
on an origin server or gateway. In this case the absolute path of the
URI MUST be transmitted (see 7.2.1, abs_path) as the Request-URI, and
the network location of the URI (net_loc) MUST be transmitted in a Host
header field.. For example, a client wishing to retrieve the resource
above directly from the origin server would create a TCP connection to
port 80 of the host "www.w3.org" and send the lines:

       GET /pub/WWW/TheProject.html HTTP/1.1
       Host:www.w3.org

followed by the remainder of the Full-Request. Note that the absolute
path cannot be empty; if none is present in the original URI, it MUST be
given as "/" (the server root).

If a proxy receives a request without any path in the Request-URI and
the method specified is capable of supporting the asterisk form of
request, then the last proxy on the request chain MUST forward the
request with "*" as the final Request-URI. For example, the request

       OPTIONS http://www.ics.uci.edu:8001 HTTP/1.1

would be forwarded by the proxy as

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       OPTIONS * HTTP/1.1
       Host: www.ics.uci.edu:8001

after connecting to port 8001 of host "www.ics.uci.edu".

The Request-URI is transmitted as an encoded string, where some
characters may be escaped using the "% HEX HEX" encoding defined by RFC
1738 . The origin server MUST decode the Request-URI in order to
properly interpret the request.  In requests that they forward, proxies
MUST NOT rewrite the "abs_path" part of a Request-URI in any way except
as noted above to replace a null abs_path with "*". Illegal
Request-URIs
SHOULD be responded to with an appropriate status code. Proxies MAY
transform the Request-URI for internal processing purposes, but SHOULD
NOT send such a transformed Request-URI  in forwarded requests.

  The main reason for this rule is to make sure that the form of
  Request-URI is well specified, to enable future extensions without
  fear that they will break in the face of some rewritings. Another
  is that one consequence of rewriting the Request-URI is that
  integrity or authentication checks by the server may fail; since
  rewriting MUST be avoided in this case, it may as well be
  proscribed in general. Implementers should be aware that some pre-
  HTTP/1.1 proxies do some rewriting.


9.2 The Resource Identified by a Request
HTTP/1.1 origin servers SHOULD be aware that the exact resource
identified by an Internet request is determined by examining both the
Request-URI and the Host header field.  An origin server that does not
allow resources to differ by the requested host MAY ignore the Host
header field.  An origin server that does differentiate resources based
on the host requested (sometimes referred to as virtual hosts or vanity
hostnames) MUST use the following rules for determining the requested
resource on an HTTP/1.1 request:.

  1. If Request-URI is an absoluteURI, the host is included in the
     Request-URI.  Any Host header field in the request MUST be
ignored.
  2. If the Request-URI is not an absoluteURI, and the request includes
     a Host header field, the host is determined by the Host header
     field.
  3. If the request-URI is not an absoluteURI and no Host header field
     is present (or does not represent a valid host on that server),
     the response MUST be a 400 (Bad Request) error message.
Recipients of an HTTP/1.0 request lacking a Host header field MAY
attempt to use heuristics (e.g., examination of the URI path for
something unique to a particular host) in order to determine what exact
resource is being requested.


9.3 Request Header Fields
The request header fields allow the client to pass additional
information about the request, and about the client itself, to the
server. These fields act as request modifiers, with semantics
equivalent


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to the parameters on a programming language method (procedure)
invocation.

       Request-Header = Accept                   ; Section 18.1
                      | Accept-Charset           ; Section 18.2
                      | Accept-Encoding          ; Section 18.3
                      | Accept-Language          ; Section 18.4
                      | Authorization            ; Section 18.8
                      | From                     ; Section 18.23
                      | Host                     ; Section 18.24
                      | If-Modified-Since        ; Section 18.25
                      | If-Range                 ; Section 18.28
                      | Proxy-Authorization      ; Section 18.36
                      | Range                    ; Section 18.38
                      | Referer                  ; Section 18.39
                      | User-Agent               ; Section 18.45
                      | Max-Forwards             ; Section 18.32

Request-Header field names can be extended reliably only in combination
with a change in the protocol version. However, new or experimental
header fields MAY be given the semantics of request header fields if all
parties in the communication recognize them to be request header
fields.
Unrecognized header fields are treated as Entity-Header fields.


10 Response
After receiving and interpreting a request message, a server responds in
the form of an HTTP response message.

       Response        = Full-Response

       Full-Response   = Status-Line               ; Section 10.1
                         *( General-Header         ; Section 8.3
                          | Response-Header        ; Section 10.2
                          | Entity-Header )        ; Section 11.1
                         CRLF
                         [ Entity-Body ]           ; Section 11.2


10.1 Status-Line
The first line of a Full-Response message is the Status-Line, consisting
of the protocol version followed by a numeric status code and its
associated textual phrase, with each element separated by SP
characters.
No CR or LF is allowed except in the final CRLF sequence.

       Status-Line = HTTP-Version SP Status-Code SP Reason-Phrase CRLF


10.1.1 Status Code and Reason Phrase
The Status-Code element is a 3-digit integer result code of the attempt
to understand and satisfy the request. The Reason-Phrase is intended to
give a short textual description of the Status-Code. The Status-Code is
intended for use by automata and the Reason-Phrase is intended for the


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human user. The client is not required to examine or display the
Reason-
Phrase.

The first digit of the Status-Code defines the class of response. The
last two digits do not have any categorization role. There are 5 values
for the first digit:


  .  1xx: Informational - Request received, continuing process

  .  2xx: Success - The action was successfully received, understood,
     and accepted

  .  3xx: Redirection - Further action must be taken in order to
     complete the request

  .  4xx: Client Error - The request contains bad syntax or cannot be
     fulfilled

  .  5xx: Server Error - The server failed to fulfill an apparently
     valid request
The individual values of the numeric status codes defined for HTTP/1.1,
and an example set of corresponding Reason-Phrase's, are presented
below. The reason phrases listed here are only recommended -- they may
be replaced by local equivalents without affecting the protocol. These
codes are fully defined in section 12.

       Status-Code    = "100"   ; Continue
                      | "101"   ; Switching Protocols
                      | "200"   ; OK
                      | "201"   ; Created
                      | "202"   ; Accepted
                      | "203"   ; Non-Authoritative Information
                      | "204"   ; No Content
                      | "205"   ; Reset Content
                      | "206"   ; Partial Content
                      | "300"   ; Multiple Choices
                      | "301"   ; Moved Permanently
                      | "302"   ; Moved Temporarily
                      | "303"   ; See Other
                      | "304"   ; Not Modified
                      | "305"   ; Use Proxy
                      | "400"   ; Bad Request
                      | "401"   ; Unauthorized
                      | "402"   ; Payment Required
                      | "403"   ; Forbidden
                      | "404"   ; Not Found
                      | "405"   ; Method Not Allowed
                      | "406"   ; Not Acceptable
                      | "407"   ; Proxy Authentication Required
                      | "408"   ; Request Time-out
                      | "409"   ; Conflict
                      | "410"   ; Gone
                      | "411"   ; Length Required

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                      | "412"   ; Precondition Failed
                      | "413"   ; Request Entity Too Large
                      | "414"   ; Request URI Too Large
                      | "415"   ; Unsupported Media Type
                      | "500"   ; Internal Server Error
                      | "501"   ; Not Implemented
                      | "502"   ; Bad Gateway
                      | "503"   ; Service Unavailable
                      | "504"   ; Gateway Time-out
                      | "505"   ; HTTP Version not supported
                      | extension-code

       extension-code = 3DIGIT

       Reason-Phrase  = *<TEXT, excluding CR, LF>

HTTP status codes are extensible. HTTP applications are not required to
understand the meaning of all registered status codes, though such
understanding is obviously desirable. However, applications MUST
understand the class of any status code, as indicated by the first
digit, and treat any unrecognized response as being equivalent to the
x00 status code of that class, with the exception that an unrecognized
response MUST NOT be cached. For example, if an unrecognized status code
of 431 is received by the client, it can safely assume that there was
something wrong with its request and treat the response as if it had
received a 400 status code. In such cases, user agents SHOULD present to
the user the entity returned with the response, since that entity is
likely to include human-readable information which will explain the
unusual status.


10.2 Response Header Fields
The response header fields allow the server to pass additional
information about the response which cannot be placed in the Status-
Line. These header fields give information about the server and about
further access to the resource identified by the Request-URI.

       Response-Header = Location                ; Section 18.31
                       | Proxy-Authenticate      ; Section 18.35
                       | Public                  ; Section 18.37
                       | Retry-After             ; Section 18.40
                       | Server                  ; Section 18.41
                       | WWW-Authenticate        ; Section 18.46

Response-Header field names can be extended reliably only in combination
with a change in the protocol version. However, new or experimental
header fields MAY be given the semantics of response header fields if
all parties in the communication recognize them to be response header
fields. Unrecognized header fields are treated as Entity-Header fields.


11 Entity
Full-Request and Full-Response messages MAY transfer an entity within
some requests and responses. An entity consists of Entity-Header fields

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and (usually) an Entity-Body. In this section, both sender and recipient
refer to either the client or the server, depending on who sends and who
receives the entity.


11.1 Entity Header Fields
Entity-Header fields define optional metainformation about the Entity-
Body or, if no body is present, about the resource identified by the
request.

       Entity-Header  = Allow                    ; Section 18.7
                      | Content-Base             ; Section 18.12
                      | Content-Encoding         ; Section 18.3
                      | Content-Language         ; Section 18.14
                      | Content-Length           ; Section 18.15
                      | Content-Location         ; Section 18.16
                      | Content-MD5              ; Section 0
                      | Content-Range            ; Section 18.18
                      | Content-Type             ; Section 18.19
                      | Expires                  ; Section 18.22
                      | Last-Modified            ; Section 18.30
                      | Title                    ; Section 18.42
                      | Transfer-Encoding        ; Section 18.43
                      | extension-header

       extension-header = HTTP-header

The extension-header mechanism allows additional Entity-Header fields to
be defined without changing the protocol, but these fields cannot be
assumed to be recognizable by the recipient. Unrecognized header fields
SHOULD be ignored by the recipient and forwarded by proxies.


11.2 Entity Body
The entity body (if any) sent with an HTTP request or response is in a
format and encoding defined by the Entity-Header fields.

       Entity-Body    = *OCTET

An entity body MUST  ONLY be included with a request message when the
request method calls for one. The presence of an entity body in a
request is signaled by the inclusion of a Content-Length and/or
Content-
Type header field in the request message headers.

For response messages, whether or not an entity body is included with a
message is dependent on both the request method and the response code.
All responses to the HEAD request method MUST NOT include a body, even
though the presence of entity header fields may lead one to believe they
do. All 1xx (informational), 204 (no content), and 304 (not modified)
responses MUST NOT include a body. All other responses MUST include an
entity body or a Content-Length header field defined with a value of
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11.2.1 Type
When an entity body is included with a message, the data type of that
body is determined via the header fields Content-Type,
Content-Encoding,
and Transfer-Encoding. These define a three-layer, ordered encoding
model:

       entity-body :=
          Transfer-Encoding( Content-Encoding( Content-Type( data ) ) )

The default for both encodings is none (i.e., the identity function).
Content-Type specifies the media type of the underlying data. Content-
Encoding may be used to indicate any additional content codings applied
to the type, usually for the purpose of data compression, that are a
property of the resource entity  requested. Transfer-Encoding may be
used to indicate any additional transfer codings applied by an
application to ensure safe and proper transfer of the message. Note that
Transfer-Encoding is a property of the message, not of the resource
entity.

Any HTTP/1.1 message containing an entity body SHOULD include a
Content-
Type header field defining the media type of that body. If and only if
the media type is not given by a Content-Type header, the recipient may
attempt to guess the media type via inspection of its content and/or the
name extension(s) of the URL used to identify the resource. If the media
type remains unknown, the recipient SHOULD treat it as type
"application/octet-stream".


11.2.2 Length
When an entity body is included with a message, the length of that body
may be determined in one of several ways. If a Content-Length header
field is present, its value in bytes represents the length of the entity
body. Otherwise, the body length is determined by the Transfer-Encoding
(if the "chunked" transfer coding has been applied) or by the server
closing the connection.

  Note: Any response message which MUST NOT include an entity body
  (such as the 1xx, 204, and 304 responses and any response to a HEAD
  request) is always terminated by the first empty line after the
  header fields, regardless of the entity header fields present in
  the message.

Closing the connection cannot be used to indicate the end of a request
body, since it leaves no possibility for the server to send back a
response. For compatibility with HTTP/1.0 applications, HTTP/1.1
requests containing an entity body MUST include a valid Content-Length
header field unless the server is known to be HTTP/1.1 compliant.
HTTP/1.1 servers MUST accept the "chunked" transfer coding (section
7.6), thus allowing this mechanism to be used for a request when
Content-Length is unknown.

If a request contains an entity body and Content-Length is not
specified, the server SHOULD respond with 400 (bad request) if it cannot
determine the length of the request message's content, or with 411

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(length required) if it wishes to insist on receiving a valid Content-
Length.

Messages MUST NOT include both a Content-Length header field and the
"chunked" transfer coding. If both are received, the Content-Length MUST
be ignored.

When a Content-Length is given in a message where an entity body is
allowed, its field value MUST exactly match the number of OCTETs in the
entity body. HTTP/1.1 user agents MUST notify the user when an invalid
length is received and detected.


12 Status Code Definitions
Each Status-Code is described below, including a description of which
method(s) it can follow and any metainformation required in the
response.


12.1 Informational 1xx
This class of status code indicates a provisional response, consisting
only of the Status-Line and optional headers, and is terminated by an
empty line. Since HTTP/1.0 did not define any 1xx status codes, servers
MUST NOT send a 1xx response to an HTTP/1.0 client except under
experimental conditions.


12.1.1.1 100 Continue
The client may continue with its request. This interim response is used
to inform the client that the initial part of the request has been
received and has not yet been rejected by the server. The client SHOULD
continue by sending the remainder of the request or, if the request has
already been completed, ignore this response. The server MUST send a
final response after the request has been completed.


12.1.1.2 101 Switching Protocols
The server understands and is willing to comply with the client's
request, via the Upgrade message header field (section 18.44), for a
change in the application protocol being used on this connection. The
server will switch protocols to those defined by the response's Upgrade
header field immediately after the empty line which terminates the 101
response.

The protocol should only be switched when it is advantageous to do so.
For example, switching to a newer version of HTTP is advantageous over
older versions, and switching to a real-time, synchronous protocol may
be advantageous when delivering resources that use such features.


12.2 Successful 2xx
This class of status code indicates that the client's request was
successfully received, understood, and accepted.


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12.2.1.1 200 OK
The request has succeeded. The information returned with the response is
dependent on the method used in the request, as follows:


GET
  an entity corresponding to the requested resource is sent in the
  response;

HEAD
  the response MUST only contain the header information and no Entity-
  Body;

POST
  an entity describing or containing the result of the action;

TRACE
  an entity containing the request message as received by the end
  server;

otherwise,
  an entity describing the result of the action;
If the entity corresponds to a resource, the response MAY include a
Content-Location header field giving the actual location of that plain
resource for later reference.


12.2.1.2 201 Created
The request has been fulfilled and resulted in a new resource being
created. The newly created resource can be referenced by the URI(s)
returned in the entity of the response, with the most specific URL for
the resource given by a Location header field. The origin server SHOULD
create the resource before returning this status code. If the action
cannot be carried out immediately, the server MUST include in the
response body a description of when the resource will be available;
otherwise, the server SHOULD respond with 202 (Accepted).


12.2.1.3 202 Accepted
The request has been accepted for processing, but the processing has not
been completed. The request MAY or MAY NOT eventually be acted upon, as
it MAY be disallowed when processing actually takes place. There is no
facility for re-sending a status code from an asynchronous operation
such as this.

The 202 response is intentionally non-committal. Its purpose is to allow
a server to accept a request for some other process (perhaps a batch-
oriented process that is only run once per day) without requiring that
the user agent's connection to the server persist until the process is
completed. The entity returned with this response SHOULD include an
indication of the request's current status and either a pointer to a
status monitor or some estimate of when the user can expect the request
to be fulfilled.


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12.2.1.4 203 Non-Authoritative Information
The returned metainformation in the Entity-Header is not the definitive
set as available from the origin server, but is gathered from a local or
a third-party copy. The set presented MAY be a subset or superset of the
original version. For example, including local annotation information
about the resource MAY result in a superset of the metainformation known
by the origin server. Use of this response code is not required and is
only appropriate when the response would otherwise be 200 (OK).


12.2.1.5 204 No Content
The server has fulfilled the request but there is no new information to
send back. If the client is a user agent, it SHOULD NOT change its
document view from that which caused the request to be generated. This
response is primarily intended to allow input for actions to take place
without causing a change to the user agent's active document view. The
response MAY include new metainformation in the form of entity headers,
which SHOULD apply to the document currently in the user agent's active
view.

The 204 response MUST NOT include an entity body, and thus is always
terminated by the first empty line after the header fields.


12.2.1.6 205 Reset Content
The server has fulfilled the request and the user agent SHOULD reset the
document view which caused the request to be generated. This response is
primarily intended to allow input for actions to take place via user
input, followed by a clearing of the form in which the input is given so
that the user can easily initiate another input action. The response
MUST include a Content-Length with a value of zero (0) and no entity
body.


12.2.1.7 206 Partial Content
The server has fulfilled the partial GET request for the resource. The
request MUST have included a Range header field (section 18.38)
indicating the desired range. The response MUST include a Content-Range
header field (section 18.18) indicating the range included with this
response. All entity header fields in the response MUST describe the
partial entity transmitted rather than what would have been transmitted
in a full response. In particular, the Content-Length header field in
the response MUST match the actual number of OCTETs transmitted in the
entity body. It is assumed that the client already has the complete
entity's header field data.


12.3 Redirection 3xx
This class of status code indicates that further action needs to be
taken by the user agent in order to fulfill the request. The action
required MAY be carried out by the user agent without interaction with
the user if and only if the method used in the second request is GET or
HEAD. A user agent SHOULD NOT automatically redirect a request more than
5 times, since such redirections usually indicate an infinite loop.

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12.3.1.1 300 Multiple Choices
This status code is reserved for future use by a planned content
negotiation mechanism.  HTTP/1.1 user agents receiving a 300 response
which includes a Location header field can treat this response as they
would treat a 303 (See Other) response.  If no Location header field is
included, the appropriate action is to display the entity enclosed in
the response to the user.


12.3.1.2 301 Moved Permanently
The requested resource has been assigned a new permanent URI and any
future references to this resource SHOULD be done using one of the
returned URIs. Clients with link editing capabilities SHOULD
automatically re-link references to the Request-URI to one or more of
the new references returned by the server, where possible. This response
is cachable unless indicated otherwise.

If the new URI is a location, its URL MUST be given by the Location
field in the response. Unless it was a HEAD request, the Entity-Body of
the response SHOULD contain a short hypertext note with a hyperlink to
the new URI(s).

If the 301 status code is received in response to a request other than
GET or HEAD, the user agent MUST NOT automatically redirect the request
unless it can be confirmed by the user, since this might change the
conditions under which the request was issued.

  Note: When automatically redirecting a POST request after receiving
  a 301 status code, some existing HTTP/1.0 user agents will
  erroneously change it into a GET request.


12.3.1.3 302 Moved Temporarily
The requested resource resides temporarily under a different URI. Since
the redirection may be altered on occasion, the client SHOULD continue
to use the Request-URI for future requests. This response is only
cachable if indicated by a Cache-Control or Expires header field.

If the new URI is a location, its URL MUST be given by the Location
field in the response. Unless it was a HEAD request, the Entity-Body of
the response SHOULD contain a short hypertext note with a hyperlink to
the new URI(s).

If the 302 status code is received in response to a request other than
GET or HEAD, the user agent MUST NOT automatically redirect the request
unless it can be confirmed by the user, since this might change the
conditions under which the request was issued.

  Note: When automatically redirecting a POST request after receiving
  a 302 status code, some existing HTTP/1.0 user agents will
  erroneously change it into a GET request.




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12.3.1.4 303 See Other
The response to the request can be found under a different URI and
SHOULD be retrieved using a GET method on that resource. This method
exists primarily to allow the output of a POST-activated script to
redirect the user agent to a selected resource. The new resource is not
a update reference for the original Request-URI. The 303 response is not
cachable, but the response to the second request MAY be cachable.

If the new URI is a location, its URL MUST be given by the Location
field in the response. Unless it was a HEAD request, the Entity-Body of
the response SHOULD contain a short hypertext note with a hyperlink to
the new URI(s).


12.3.1.5 304 Not Modified
If the client has performed a conditional GET request and access is
allowed, but the document has not been modified since the date and time
specified in the If-Modified-Since field, the server MUST respond with
this status code and not send an Entity-Body to the client. Header
fields contained in the response SHOULD only include information which
is relevant to cache managers or which MAY have changed independently of
the entity's Last-Modified date. Examples of relevant header fields
include: Date, Server, Content-Length, Content-MD5, Content-Version,
Cache-Control and Expires.

A cache SHOULD update its cached entity to reflect any new field values
given in the 304 response. If the new field values indicate that the
cached entity differs from the current resource entity  (as would be
indicated by a change in Content-Length, Content-MD5, or Content-
Version), then the cache MUST disregard the 304 response and repeat the
request without an If-Modified-Since field.

The 304 response MUST NOT include an entity body, and thus is always
terminated by the first empty line after the header fields.


12.3.1.6 305 Use Proxy
The requested resource MUST be accessed through the proxy given by the
Location field in the response. In other words, this is a proxy
redirect.


12.4 Client Error 4xx
The 4xx class of status code is intended for cases in which the client
seems to have erred. If the client has not completed the request when a
4xx code is received, it SHOULD immediately cease sending data to the
server. Except when responding to a HEAD request, the server SHOULD
include an entity containing an explanation of the error situation, and
whether it is a temporary or permanent condition. These status codes are
applicable to any request method.

  Note: If the client is sending data, server implementations using
  TCP SHOULD be careful to ensure that the client acknowledges
  receipt of the packet(s) containing the response prior to closing

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  the input connection. If the client continues sending data to the
  server after the close, the server's controller will send a reset
  packet to the client, which may erase the client's unacknowledged
  input buffers before they can be read and interpreted by the HTTP
  application.


12.4.1.1 400 Bad Request
The request could not be understood by the server due to malformed
syntax. The client SHOULD NOT repeat the request without modifications.


12.4.1.2 401 Unauthorized
The request requires user authentication. The response MUST include a
WWW-Authenticate header field (section 18.46) containing a challenge
applicable to the requested resource. The client MAY repeat the request
with a suitable Authorization header field (section 18.8). If the
request already included Authorization credentials, then the 401
response indicates that authorization has been refused for those
credentials. If the 401 response contains the same challenge as the
prior response, and the user agent has already attempted authentication
at least once, then the user SHOULD be presented the entity that was
given in the response, since that entity MAY include relevant diagnostic
information. HTTP access authentication is explained in section 14.


12.4.1.3 402 Payment Required
This code is reserved for future use.


12.4.1.4 403 Forbidden
The server understood the request, but is refusing to fulfill it.
Authorization will not help and the request SHOULD not be repeated. If
the request method was not HEAD and the server wishes to make public why
the request has not been fulfilled, it SHOULD describe the reason for
the refusal in the entity body. This status code is commonly used when
the server does not wish to reveal exactly why the request has been
refused, or when no other response is applicable.


12.4.1.5 404 Not Found
The server has not found anything matching the Request-URI. No
indication is given of whether the condition is temporary or permanent.
If the server does not wish to make this information available to the
client, the status code 403 (Forbidden) can be used instead. The 410
(Gone) status code SHOULD be used if the server knows, through some
internally configurable mechanism, that an old resource is permanently
unavailable and has no forwarding address.


12.4.1.6 405 Method Not Allowed
The method specified in the Request-Line is not allowed for the resource
identified by the Request-URI. The response MUST include an Allow header
containing a list of valid methods for the requested resource.

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12.4.1.7 406 Not Acceptable
The resource identified by the request is only capable of generating
response entities which have content characteristics not acceptable
according to the accept headers sent in the request.

HTTP/1.1 servers are allowed to return responses which are not
acceptable according to the accept headers sent in the request. In some
cases, this may even be preferable to sending a 406 response.  User
agents are encouraged to inspect the headers of an incoming response to
determine if it is acceptable. If the response is not acceptable, user
agents SHOULD interrupt the receipt of the response if doing so would
save network resources.  If it is unknown whether an incoming response
would be acceptable, a user agent SHOULD temporarily stop receipt of
more data and query the user for a decision on furtheractions.


12.4.1.8 407 Proxy Authentication Required
This code is similar to 401 (Unauthorized), but indicates that the
client MUST first authenticate itself with the proxy. The proxy MUST
return a Proxy-Authenticate header field (section 18.35) containing a
challenge applicable to the proxy for the requested resource. The client
MAY repeat the request with a suitable Proxy-Authorization header field
(section 18.36). HTTP access authentication is explained in section 14.


12.4.1.9 408 Request Timeout
The client did not produce a request within the time that the server was
prepared to wait. The client MAY repeat the request without
modifications at any later time.


12.4.1.10 409 Conflict
The request could not be completed due to a conflict with the current
state of the resource. This code is only allowed in situations where it
is expected that the user MAY be able to resolve the conflict and
resubmit the request. The response body SHOULD include enough
information for the user to recognize the source of the conflict.
Ideally, the response entity would include enough information for the
user or user-agent to fix the problem; however, that MAY not be possible
and is not required.

Conflicts are most likely to occur in response to a PUT request. If
versioning is being used and the entity being PUT includes changes to a
resource which conflict with those made by an earlier (third-party)
request, the server MAY use the 409 response to indicate that it can't
complete the request. In this case, the response entity SHOULD contain a
list of the differences between the two versions in a format defined by
the response Content-Type.


12.4.1.11 410 Gone
The requested resource is no longer available at the server and no
forwarding address is known. This condition SHOULD be considered
permanent. Clients with link editing capabilities SHOULD delete

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references to the Request-URI after user approval. If the server does
not know, or has no facility to determine, whether or not the condition
is permanent, the status code 404 (Not Found) SHOULD be used instead.
This response is cachable unless indicated otherwise.

The 410 response is primarily intended to assist the task of web
maintenance by notifying the recipient that the resource is
intentionally unavailable and that the server owners desire that remote
links to that resource be removed. Such an event is common for limited-
time, promotional services and for resources belonging to individuals no
longer working at the server's site. It is not necessary to mark all
permanently unavailable resources as "gone" or to keep the mark for any
length of time -- that is left to the discretion of the server owner.


12.4.1.12 411 Length Required
The server refuses to accept the request without a defined Content-
Length. The client MAY repeat the request if it adds a valid Content-
Length header field containing the length of the entity body in the
request message.


12.4.1.13 412 Precondition Failed
The precondition given in one or more of the request header fields
evaluated to false when it was tested on the server. This response code
allows the client to place preconditions on the current resource
metainformation (header field data) and thus prevent the requested
method from being applied to a resource other than the one intended.


12.4.1.14 413 Request Entity Too Large
The server is refusing to process a request because it considers the
request entity to be larger than it is willing or able to process. The
server SHOULD close the connection if that is necessary to prevent the
client from continuing the request.

If the client manages to read the 413 response, it MUST honor it and
SHOULD reflect it to the user.

If this restriction is considered temporary, the server MAY include a
Retry-After header field to indicate that it is temporary and after what
time the client MAY try again.


12.4.1.15 414 Request-URI Too Long
The server is refusing to service the request because the Request-URI is
longer than the server is willing to interpret. This rare condition is
only likely to occur when a client has improperly converted a POST
request to a GET request with long query information, when the client
has descended into a URL "black hole" of redirection (e.g., a redirected
URL prefix that points to a suffix of itself), or when the server is
under attack by a client attempting to exploit security holes present in
some servers using  fixed-length buffers for reading or manipulating the
Request-URI.

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12.4.1.16 415 Unsupported Media Type
The server is refusing to service the request because the entity body of
the request is in a format not supported by the requested resource for
the requested method.


12.5 Server Error 5xx
Response status codes beginning with the digit "5" indicate cases in
which the server is aware that it has erred or is incapable of
performing the request. If the client has not completed the request when
a 5xx code is received, it SHOULD immediately cease sending data to the
server. Except when responding to a HEAD request, the server SHOULD
include an entity containing an explanation of the error situation, and
whether it is a temporary or permanent condition. These response codes
are applicable to any request method and there are no required header
fields.


12.5.1.1 500 Internal Server Error
The server encountered an unexpected condition which prevented it from
fulfilling the request.


12.5.1.2 501 Not Implemented
The server does not support the functionality required to fulfill the
request. This is the appropriate response when the server does not
recognize the request method and is not capable of supporting it for any
resource.


12.5.1.3 502 Bad Gateway
The server, while acting as a gateway or proxy, received an invalid
response from the upstream server it accessed in attempting to fulfill
the request.


12.5.1.4 503 Service Unavailable
The server is currently unable to handle the request due to a temporary
overloading or maintenance of the server. The implication is that this
is a temporary condition which will be alleviated after some delay. If
known, the length of the delay MAY be indicated in a Retry-After
header.
If no Retry-After is given, the client SHOULD handle the response as it
would for a 500 response.

  Note: The existence of the 503 status code does not imply that a
  server must use it when becoming overloaded. Some servers MAY wish
  to simply refuse the connection.


12.5.1.5 504 Gateway Timeout
The server, while acting as a gateway or proxy, did not receive a timely
response from the upstream server it accessed in attempting to complete
the request.


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12.5.1.6 505 HTTP Version Not Supported
The server does not support, or refuses to support, the HTTP protocol
version that was used in the request message.  The server is indicating
that it is unable or unwilling to complete the request using the same
major version as the client, as described in section 7.1, other than
with this error message.  The response SHOULD contain an entity
describing why that version is not supported and what other protocols
are supported by that server.


13 Method Definitions
The set of common methods for HTTP/1.1 is defined below. Although this
set can be expanded, additional methods cannot be assumed to share the
same semantics for separately extended clients and servers.

The Host request-header field (section 18.24) MUST accompany all
HTTP/1.1 requests.


13.1 OPTIONS
The OPTIONS method represents a request for information about the
communication options available on the request/response chain identified
by the Request-URI. This method allows the client to determine the
options and/or requirements associated with a resource, or the
capabilities of a server, without implying a resource action or
initiating a resource retrieval.

Unless the server's response is an error, the response MUST NOT include
entity information other than what can be considered as communication
options (e.g., Allow is appropriate, but Content-Type is not) and MUST
include a Content-Length with a value of zero (0). Responses to this
method are not cachable.

If the Request-URI is an asterisk ("*"), the OPTIONS request is intended
to apply to the server as a whole. A 200 response SHOULD include any
header fields which indicate optional features implemented by the server
(e.g., Public), including any extensions not defined by this
specification, in addition to any applicable general or response header
fields. As described in section 9.1.2, an "OPTIONS *" request can be
applied through a proxy by specifying the destination server in the
Request-URI without any path information.

If the Request-URI is not an asterisk, the OPTIONS request applies only
to the options that are available when communicating with that
resource.
A 200 response SHOULD include any header fields which indicate optional
features implemented by the server and applicable to that resource
(e.g., Allow), including any extensions not defined by this
specification, in addition to any applicable general or response header
fields. If the OPTIONS request passes through a proxy, the proxy MUST
edit the response to exclude those options known to be unavailable
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13.2 GET
The GET method means retrieve whatever information (in the form of an
entity) is identified by the Request-URI. If the Request-URI refers to a
data-producing process, it is the produced data which shall be returned
as the entity in the response and not the source text of the process,
unless that text happens to be the output of the process.

The semantics of the GET method change to a "conditional GET" if the
request message includes an If-Modified-Since header field. A
conditional GET method requests that the identified resource entity  be
transferred only if it has been modified since the date given by the
If-
Modified-Since header, as described in section 18.25. The conditional
GET method is intended to reduce unnecessary network usage by allowing
cached entities to be refreshed without requiring multiple requests or
transferring data already held by the client.

The semantics of the GET method change to a "partial GET" if the request
message includes a Range header field. A partial GET requests that only
part of the identified resource entity be transferred, as described in
section 18.38. The partial GET method is intended to reduce unnecessary
network usage by allowing partially-retrieved entities to be completed
without transferring data already held by the client.

The response to a GET request may be cachable if and only if it meets
the requirements for HTTP caching described in section 16.


13.3 HEAD
The HEAD method is identical to GET except that the server MUST NOT
return any Entity-Body in the response. The metainformation contained in
the HTTP headers in response to a HEAD request SHOULD be identical to
the information sent in response to a GET request. This method can be
used for obtaining metainformation about the resource entity identified
by the Request-URI without transferring the Entity-Body itself. This
method is often used for testing hypertext links for validity,
accessibility, and recent modification.

The response to a HEAD request may be cachable in the sense that the
information contained in the response may be used to update a previously
cached entity from that resource. If the new field values indicate that
the cached entity differs from the current resource entity (as would be
indicated by a change in Content-Length, Content-MD5, or Content-
Version), then the cache MUST mark the  cache entry stale.

There is no "conditional HEAD" or "partial HEAD" request analogous to
those associated with the GET method. If an If-Modified-Since and/or
Range header field is included with a HEAD request, they SHOULD be
ignored.


13.4 POST
The POST method is used to request that the destination server accept
the entity enclosed in the request as a new subordinate of the resource


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identified by the Request-URI in the Request-Line. POST is designed to
allow a uniform method to cover the following functions:


  .  Annotation of existing resources;

  .  Posting a message to a bulletin board, newsgroup, mailing list, or
     similar group of articles;

  .  Providing a block of data, such as the result of submitting a form
     , to a data-handling process;

  .  Extending a database through an append operation.
The actual function performed by the POST method is determined by the
server and is usually dependent on the Request-URI. The posted entity is
subordinate to that URI in the same way that a file is subordinate to a
directory containing it, a news article is subordinate to a newsgroup to
which it is posted, or a record is subordinate to a database.

For compatibility with HTTP/1.0 applications, all POST requests MUST
include a valid Content-Length header field unless the server is known
to be HTTP/1.1 compliant. When sending a POST request to an HTTP/1.1
server, a client MUST use a valid Content-Length or the "chunked"
Transfer-Encoding. The server SHOULD respond with a 400 (bad request)
message if it cannot determine the length of the request message's
content, or with 411 (length required) if it wishes to insist on
receiving a valid Content-Length.

A successful POST does not require that the entity be created as a
resource on the origin server or made accessible for future reference.
That is, the action performed by the POST method might not result in a
resource that can be identified by a URI. In this case, either 200 (OK)
or 204 (no content) is the appropriate response status, depending on
whether or not the response includes an entity that describes the
result.

If a resource has been created on the origin server, the response SHOULD
be 201 (Created) and contain an entity (preferably of type "text/html")
which describes the status of the request and refers to the new
resource.

Responses to this method are not cachable. However, the 303 (See Other)
response can be used to direct the user agent to retrieve a cachable
resource.

POST requests must obey the entity transmission requirements set out in
section 13.4.1.


13.4.1 SLUSHY: Entity Transmission Requirements
Editor's Note: The issues here around reliable transmission of large
entities to servers, particularly HTTP/1.0 servers, are complicated and
subtle, particularly since we'd like optimistic transmission to be the


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normal situation.  We would like it if we can redraft this section to be
simpler in the next draft

General requirements:

  .  HTTP/1.1 servers should maintain persistent connections and use
     TCP's flow control mechanisms to resolve temporary overloads,
     rather than terminating connections with the expectation that
     clients will retry.  The latter technique can exacerbate network
     congestion.
  .  An HTTP/1.1 (or later) client doing a PUT-like method SHOULD
     monitor the network connection for an error status while it is
     transmitting the request.  If  the client sees an error status, it
     should immediately cease transmitting the body.  If the body is
     being sent using a "Chunked" encoding, a zero length chunk is used
     to mark the end of the message.  If the body was preceded by a
     Content-length header, the client MUST close the connection.
  .  An HTTP/1.1 (or later) client MUST be prepared to accept a "100
     Continue" status followed by a regular response.
  .  An HTTP/1.1 (or later) server that receives a request from a
     HTTP/1.0 (or earlier) client MUST NOT transmit the 100 (continue)
     response; it SHOULD either wait for the request to be completed
     normally (thus avoiding an interrupted request) or close the
     connection prematurely.
Upon receiving a method subject to these requirements from an HTTP/1.1
(or later) client, an HTTP/1.1 (or later) server MUST either immediately
respondwith 100 (continue) and continue to read from the input stream,
or respond with an error status.  If it responds with an error status,
it MAY close the transport (TCP) connection or it MAY continue to read
and discard the rest of the request.  It MUST NOT perform the requested
method if it returns an error status.

If an HTTP/1.1 client has seen an HTTP/1.1 or later response from the
server (clients SHOULD remember the version number of at least the most
recently used server), and it sees the connection close before receiving
any status from the server, the client SHOULD retry the request.  If the
client does retry the request,

  .  it MUST first send the request headers,
  .  and then MUST wait for the server to respond with either a 100
     (continue) response, in which case the client should continue, or
     with an error status.
If an HTTP/1.1 client has not seen an HTTP/1.1 or later response from
the server, it should assume that the server implements HTTP/1.0 or
older and will not use the 100 (Continue) response. If in this case the
client sees the connection close before receiving any status from the
server, the client SHOULD retry the request.  If the client does retry
the request, it should use the following "binary exponential backoff"
algorithm to be assured of obtaining a reliable response:

  1.
     Initiate a new connection to the server
  2.
     Transmit the request headers
  3.
     Initialize a variable R to the estimated round-trip time to the
     server (e.g., based on the time it took to establish the

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     connection), or to a constant value of 5 seconds if the round-trip
     time is not available.
  4.
     Compute T = R * (2**N), where N is the number of previous retries
     of this request.
  5.
     Wait either for an error response from the server, or for T seconds
     (whichever comes first)
  6.
     If no error response is received, after T seconds transmit the body
     of the request.
  7.
     If client sees that the connection is closed prematurely, repeat
     from step 1 until the request is accepted, an error response is
     received, or the user becomes impatient.
No matter what the server version, if an error status is received,

  .  the client MUST NOT continue and
  .  MUST close the connection if it has not already completed sending
     the full request body including any encoding mechanism used to
     transmit the body.
An HTTP/1.1 (or later) client that sees the connection close after
receiving a 100 (continue) but before receiving any other status SHOULD
retry the request, and need not wait for 100 (continue) response (but
MAY do so if this simplifies the implementation).


13.5 PUT
The PUT method requests that the enclosed entity be stored under the
supplied Request-URI. If the Request-URI refers to an already existing
resource, the enclosed entity SHOULD be considered as a modified version
of the one residing on the origin server. If the Request-URI does not
point to an existing resource, and that URI is capable of being defined
as a new resource by the requesting user agent, the origin server can
create the resource with that URI. If a new resource is created, the
origin server MUST inform the user agent via the 201 (created)
response.
If an existing resource is modified, either the 200 (OK) or 204 (No
Content) response codes SHOULD be sent to indicate successful completion
of the request. If the resource could not be created or modified with
the Request-URI, an appropriate error response SHOULD be given that
reflects the nature of the problem.

If the request passes through a cache and the Request-URI identifies a
currently cached entity, that entity MUST be removed from the cache.
Responses to this method are not cachable.

The fundamental difference between the POST and PUT requests is
reflected in the different meaning of the Request-URI. The URI in a POST
request identifies the resource that will handle the enclosed entity as
an appendage. That resource may be a data-accepting process, a gateway
to some other protocol, or a separate entity that accepts annotations.
In contrast, the URI in a PUT request identifies the entity enclosed
with the request -- the user agent knows what URI is intended and the
server MUST NOT attempt to apply the request to some other resource. If
the server desires that the request be applied to a different URI, it
MUST send a 301 (Moved Permanently) response; the user agent MAY then
make its own decision regarding whether or not to redirect the request.


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A single resource MAY be identified by many different URIs. For
example,
an article may have a URI for identifying "the current version" which is
separate from the URI identifying each particular version. In this
case,
a PUT request on a general URI may result in several other URIs being
defined by the origin server.

For compatibility with HTTP/1.0 applications, all PUT requests MUST
include a valid Content-Length header field unless the server is known
to be HTTP/1.1 compliant. When sending a PUT request to an HTTP/1.1
server, a client MUST use a valid Content-Length or the "chunked"
Transfer-Encoding. The server SHOULD respond with a 400 (bad request)
message if it cannot determine the length of the request message's
content, or with 411 (length required) if it wishes to insist on
receiving a valid Content-Length.

The actual method for determining how the resource entity is placed, and
what happens to its predecessor, is defined entirely by the origin
server.

PUT requests must obey the entity transmission requirements set out in
section 13.4.1.


13.6 DELETE
The DELETE method requests that the origin server delete the resource
identified by the Request-URI. This method MAY be overridden by human
intervention (or other means) on the origin server. The client cannot be
guaranteed that the operation has been carried out, even if the status
code returned from the origin server indicates that the action has been
completed successfully. However, the server SHOULD not indicate success
unless, at the time the response is given, it intends to delete the
resource or move it to an inaccessible location.

A successful response SHOULD be 200 (OK) if the response includes an
entity describing the status, 202 (Accepted) if the action has not yet
been enacted, or 204 (No Content) if the response is OK but does not
include an entity.

If the request passes through a cache and the Request-URI identifies a
currently cached entity, that entity MUST be removed from the cache.
Responses to this method are not cachable.


13.7 TRACE
The TRACE method is used to invoke a remote, application-layer
loop-back
of the request message.  The final recipient of the request SHOULD
reflect the message received back to the client as the entity body of a
200 (OK) response.  The final recipient is either the origin server or
the first proxy or gateway to receive a Max-Forwards value of zero (0)
in the request (see section 18.32).  A TRACE request MUST NOT include an
entity.

TRACE allows the client to see what is being received at the other end
of the request chain and use that data for testing or diagnostic

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information.  The value of the Via header field (section 18.47) is of
particular interest, since it acts as a trace of the request chain.  Use
of the Max-Forwards header field allows the client to limit the length
of the request chain, which is useful for testing a chain of proxies
forwarding messages in an infinite loop.

If successful, the response SHOULD contain the entire request message in
the entity body, with a Content-Type of "message/http",
"application/http", or "text/plain".  Responses to this method MUST NOT
be cached.


14 Access Authentication
HTTP provides a simple challenge-response authentication mechanism which
MAY be used by a server to challenge a client request and by a client to
provide authentication information. It uses an extensible, case-
insensitive token to identify the authentication scheme, followed by a
comma-separated list of attribute-value pairs which carry the parameters
necessary for achieving authentication via that scheme.

       auth-scheme    = token

       auth-param     = token "=" quoted-string

The 401 (Unauthorized) response message is used by an origin server to
challenge the authorization of a user agent. This response MUST include
a WWW-Authenticate header field containing at least one challenge
applicable to the requested resource.

       challenge      = auth-scheme 1*SP realm *( "," auth-param )

       realm          = "realm" "=" realm-value
       realm-value    = quoted-string

The realm attribute (case-insensitive) is required for all
authentication schemes which issue a challenge. The realm value (case-
sensitive), in combination with the canonical root URL of the server
being accessed, defines the protection space. These realms allow the
protected resources on a server to be partitioned into a set of
protection spaces, each with its own authentication scheme and/or
authorization database. The realm value is a string, generally assigned
by the origin server, which may have additional semantics specific to
the authentication scheme.

A user agent that wishes to authenticate itself with a server--usually,
but not necessarily, after receiving a 401 or 411 response--MAY do so by
including an Authorization header field with the request. The
Authorization field value consists of credentials containing the
authentication information of the user agent for the realm of the
resource being requested.

       credentials    = basic-credentials
                      | auth-scheme 0#auth-param


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The domain over which credentials can be automatically applied by a user
agent is determined by the protection space. If a prior request has been
authorized, the same credentials MAY be reused for all other requests
within that protection space for a period of time determined by the
authentication scheme, parameters, and/or user preference. Unless
otherwise defined by the authentication scheme, a single protection
space cannot extend outside the scope of its server.

If the server does not wish to accept the credentials sent with a
request, it SHOULD return a 401 (Unauthorized) response. The response
MUST include a WWW-Authenticate header field containing the (possibly
new) challenge applicable to the requested resource and an entity
explaining the refusal.

The HTTP protocol does not restrict applications to this simple
challenge-response mechanism for access authentication. Additional
mechanisms MAY be used, such as encryption at the transport level or via
message encapsulation, and with additional header fields specifying
authentication information. However, these additional mechanisms are not
defined by this specification.

Proxies MUST be completely transparent regarding user agent
authentication. That is, they MUST forward the WWW-Authenticate and
Authorization headers untouched, and MUST NOT cache the response to a
request containing Authorization.

HTTP/1.1 allows a client to pass authentication information to and from
a proxy via the Proxy-Authenticate and Proxy-Authorization headers.


14.1 Basic Authentication Scheme
The "basic" authentication scheme is based on the model that the user
agent must authenticate itself with a user-ID and a password for each
realm. The realm value should be considered an opaque string which can
only be compared for equality with other realms on that server. The
server will service the request only if it can validate the user-ID and
password for the protection space of the Request-URI. There are no
optional authentication parameters.

Upon receipt of an unauthorized request for a URI within the protection
space, the server SHOULD respond with a challenge like the following:

       WWW-Authenticate: Basic realm="WallyWorld"

where "WallyWorld" is the string assigned by the server to identify the
protection space of the Request-URI.

To receive authorization, the client sends the user-ID and password,
separated by a single colon (":") character, within a base64  encoded
string in the credentials.

       basic-credentials = "Basic" SP basic-cookie



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       basic-cookie   = <base64 [7] encoding of user-pass,
                        except not limited to 76 char/line>

       user-pass = userid ":" password

       userid    = [ token ]

       password  = *TEXT

If the user agent wishes to send the user-ID "Aladdin" and password
"open sesame", it would use the following header field:

       Authorization: Basic QWxhZGRpbjpvcGVuIHNlc2FtZQ==

The basic authentication scheme is a non-secure method of filtering
unauthorized access to resources on an HTTP server. It is based on the
assumption that the connection between the client and the server can be
regarded as a trusted carrier. As this is not generally true on an open
network, the basic authentication scheme should be used accordingly. In
spite of this, clients SHOULD implement the scheme in order to
communicate with servers that use it.


14.2 Digest Authentication Scheme
The "digest" authentication scheme is [currently described in an expired
Internet-Draft, and this description will have to be improved to
reference a new draft or include the old one].


15 Content Negotiation
A generic resource has multiple entities associated with it, all of
which are representations of the content of the resource.  Content
negotiation is the process of selecting the best representation when a
GET or HEAD request is made on the generic resource.  HTTP/1.1 has
provisions for two kinds of content negotiation: opaque negotiation and
transparent negotiation.

With opaque negotiation, the selection of the best representation is
done by an algorithm located at the origin server, and unknown to the
proxies and user agents involved.  Selection is based on the contents of
particular header fields in the request message, or on other information
pertaining to the request, like the network address of the sending
client.  A typical example of opaque negotiation would be the selection
of a text/html response in a particular language based on the contents
of the Accept-Language request header field.  A disadvantage of opaque
negotiation is that the request headers may not always contain enough
information to allow for selection.  If the Accept header

        Accept: text/*: q=0.3, text/html, */*: q=0.5

is sent in a request on a generic resource which has a video/mpeg and a
video/quicktime representation, the selection algorithm in the origin
server will either have to make a default choice, or return an error
response which allows the user to decide on further actions.

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With transparent negotiation, the selection of the best representation
is done by a distributed algorithm which can perform computation steps
in the origin server, in proxies, or in the user agent.  Transparent
negotiation guarantees that, if the user agent supports the transparent
negotiation algorithm and is correctly configured, the request will
always correctly yield either the video/mpeg representation, the
video/quicktime representation, or an error message indicating that the
resource cannot be displayed by the user agent.


15.1  Negotiation Facilities Defined in this Specification
This specification defines all protocol facilities for opaque
negotiation, but does not define the distributed algorithm for
transparent negotiation.  This specification only defines the basic
facilities (Vary, Alternates, Accept) in the core protocol allowing
requests on transparently negotiated resources to be correctly handled
by HTTP/1.1 caches.  All other information about transparent content
negotiation is found in a separate document[29].

If a generic resource is opaquely negotiated, successful responses to
requests on the resource will always include a Vary header.  If a
generic resource is transparently negotiated, successful responses to
requests on the resource will always include an Alternates header.  If a
successful response contains an Alternates header, it will also always
contain a Content-Location header.  A future specification may allow a
combination of opaque and transparent negotiation that would lead to the
inclusion of both a Vary header and an Alternates header in a response.


16 Caching in HTTP
The World Wide Web is a distributed system, and so its performance can
be improved by the use of caches. These caches are typically placed at
proxies and in the clients themselves. The HTTP/1.1 protocol includes a
number of elements intended to make caching work as well as possible.
Because these elements are inextricable from other aspects of the
protocol, and because they interact with each other, it is useful to
describe the basic caching design of HTTP separately from the detailed
descriptions of methods, headers, response codes, etc.


16.1 Semantic Transparency
Requirements for performance, availability, and disconnected operation
require us to be able to relax the goal of semantic transparency. The
HTTP/1.1 protocol allows origin servers, caches, and clients to
explicitly reduce transparency when necessary. However, because non-
transparent operation may confuse non-expert users, and may be
incompatible with certain server applications (such as those for
ordering merchandise), the protocol requires that transparency may not
be relaxed

  .  without an explicit protocol-level request (when relaxed by client
     or origin server)
  .  without a means for warning the end user (when relaxed by cache or
     client)

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Therefore, the HTTP/1.1 protocol provides these important elements:

  1. Protocol features that provide full semantic transparency when this
     is desired by all parties.
  2. Protocol features that allow an origin server or end-user client to
     explicitly request and control non-transparent operation.
  3. Protocol features that allow a cache to attach warnings to
     responses that do not preserve semantic transparency.
A basic principle is that it must be possible for the clients to detect
any potential breakdown of semantic transparency.

Caching would be useless if it did not significantly improve
performance. The goal of caching in HTTP/1.1 is to eliminate the need to
send requests in many cases, and to eliminate the need to send full
responses in many other cases. The former reduces the number of network
round-trips required for many operations; we use an "expiration"
mechanism for this purpose (see section 16.1.2). The latter reduces
network bandwidth requirements; we use a "validation" mechanism for this
purpose (see section 13.3).

The server, cache, or client implementer may be faced with design
decisions not explicitly discussed in this specification. If a decision
may affect semantic transparency, the implementer ought to err on the
side of maintaining transparency unless a careful and complete analysis
shows significant benefits in breaking transparency.


16.1.1 Cache Correctness
If the cache can communicate with the origin-server, then a correct
cache MUST respond to a request with a response that meets all the
following conditions:

  1. its end-to-end headers (see section 16.4.1) and entity-body value
     are equivalent to what the server would have returned for that
     request if the resource had not been modified since the response
     was cached. This may be accomplished by revalidating the response
     with the origin server, if is not fresh.
  2. it is "fresh enough" (see section 16.1.2). In the default case,
     this means it meets the least restrictive freshness requirement of
     the client, server, and cache (see section 18.10); if the origin-
     server so specifies, it is the freshness requirement of the
origin-
     server alone.
  3. it includes a warning if the freshness demand of the client or the
     origin-server is violated (see section 16.1.5 and 18.48).
  4. it is the most up-to-date response appropriate to the request the
     cache has seen (see section 16.2.6, 16.2.8, and 16.13).
If the cache can not communicate with the origin server, then a correct
cache SHOULD respond as above if the response can be correctly served
from the cache; if not it MUST return an error or warning indicating
that there was a communication.





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16.1.2 Cache-control Mechanisms
The basic cache mechanisms in HTTP/1.1 (server-specified expiration
times and validators) are implicit directives to caches. In some cases,
a server or client may need to provide explicit directives to the HTTP
caches. We use the Cache-Control header for this purpose.

The Cache-Control header allows a client or server to transmit a variety
of directives in either requests or responses. These directives
typically override the default caching algorithms. As a general rule, if
there is any apparent conflict between header values, the most
restrictive interpretation should be applied (that is, the one that is
most likely to preserve semantic transparency). However, in some cases,
Cache-Control directives are explicitly specified as weakening semantic
transparency (for example, "max-stale" or "public").

The Cache-Control directives are described in detail in section 18.10.


16.1.3 Warnings
Whenever a cache returns a response that is not semantically
transparent, it must attach a warning to that effect, using a Warning
response header. This warning allows clients and user agents to take
appropriate action.

Warnings may be used for other purposes, both cache-related and
otherwise. The use of a warning, rather than an error status code,
distinguish these responses from true failures.

Warnings are always cachable, because they never weaken the transparency
of a response. This means that warnings can be passed to HTTP/1.0 caches
without danger; such caches will simply pass the warning along as a
entity header in the response.

Warnings are assigned numbers between 0 and 99. This specification
defines the code numbers and meanings of each warning, allowing a client
or cache to take automated action in some (but not all) cases.

Warnings also carry a warning message text in any appropriate natural
language (perhaps based on the client's Accept headers), and an optional
indication of what language and character set are used.

Multiple warning messages may be attached to a response (either by the
origin server or by a cache), including multiple warnings with the same
code number. For example, a server may provide the same warning with
texts in both English and Basque.

When multiple warnings are attached to a response, it may not be
practical or reasonable to display all of them to the user. This version
of HTTP does not specify strict priority rules for deciding which
warnings to display and in what order, but does suggest some
heuristics.

The Warning header and the currently defined warnings are described in
section 18.48.


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16.1.4 Explicit User Agent Warnings
Many user agents make it possible for users to override the basic
caching mechanisms. For example, the user agent may allow the user to
specify that cached entities (even explicitly stale ones) are never
validated. Or the user agent might habitually add "Cache-Control: max-
stale=3600" or "Cache-Control: reload" to every request. We recognize
that there may be situations which require such overrides, although user
agents SHOULD NOT default to any behavior contrary to the HTTP/1.1
specification. That is, the user should have to explicitly request
either non-transparent behavior, or behavior that results in abnormally
ineffective caching.

If the user has overridden the basic caching mechanisms, the user agent
should explicitly indicate to the user whenever this results in the
display of information that might not meet the server's transparency
requirements (in particular, if the displayed entity is known to be
stale). Since the protocol normally allows the user agent to determine
if responses are stale or not, this indication need only be displayed
when this actually happens. The indication need not be a dialog box; it
could be an icon (for example, a picture of a rotting fish) or some
other visual indicator.

If the user has overridden the caching mechanisms in a way that would
abnormally reduce the effectiveness of caches, the user agent should
continually display an indication (for example, a picture of currency in
flames) so that the user does not inadvertently consume excess resources
or suffer from excessive latency.


16.1.5 Exceptions to the Rules and Warnings
In some cases, the operator of a cache may choose to configure it to
return stale responses even when not requested by clients. This decision
not be made lightly, but may be necessary for reasons of availability or
performance, especially when the cache is poorly connected to the origin
server. Whenever a cache returns a stale response, it MUST mark it as
such (using a Warning header). This allows the client software to alert
the user that there may be a potential problem.

It also allows the user to take steps to obtain a firsthand or fresh
response, if the user so desires. For this reason, a cache MUST NOT
return a stale response if the client explicitly requests a first-hand
or fresh one, unless it is impossible to comply.


16.1.6 Client-controlled Behavior
While the origin server (and to a lesser extent, intermediate caches, by
their contribution to the age of a response) are the primary source of
expiration information, in some cases the client may need to control a
cache's decision about whether to return a cached response without
validating it. Clients do this using several directives of the Cache-
Control header.

A client's request may specify the maximum age it is willing to accept
for an unvalidated response; specifying a value of zero forces the

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cache(s) to revalidate all responses. A client may also specify the
minimum time remaining before a response expires. Both of these options
increase constraints on the behavior of caches, and so cannot decrease
semantic transparency.

A client may also specify that it will accept stale responses, up to
some maximum amount of staleness. This loosens the constraints on the
caches, and so may violate semantic transparency, but may be necessary
to support disconnected operation, or high availability in the face of
poor connectivity.


16.2 Expiration Model

16.2.1 Server-Specified Expiration
HTTP caching works best when caches can entirely avoid making requests
to the origin server. The primary mechanism for avoiding requests is for
an origin server to provide an explicit expiration time in the future,
indicating that a response may be used to satisfy subsequent requests.
In other words, a cache can return a fresh response without first
contacting the server.

Our expectation is that servers will assign future explicit expiration
times to responses in the belief that the entity is not likely to
change, in a semantically significant way, before the expiration time is
reached. This normally preserves semantic transparency, as long as the
server's expiration times are carefully chosen.

If an origin server wishes to force a semantically transparent cache to
validate every request, it may assign an explicit expiration time in the
past. This means that the response is always stale, and so the cache
SHOULD validate it before using it for subsequent requests.  (See
section 18.10.4 for a more restrictive way to force revalidation).

  Note that a firsthand response MUST always be returned to the
  requesting client, independent of its expiration time, unless the
  connection to the client is lost.

If an origin server wishes to force any HTTP/1.1 cache, no matter how it
is configured, to validate every request, it should use the "must-
revalidate" Cache-Control directive.  See section 18.10.

Servers specify explicit expiration times using either the Expires
header, or the max-age directive of the Cache-Control header.


16.2.2 Limitations on the Effect of Expiration Times
An expiration time cannot be used to force a user agent to refresh its
display or reload a resource entity; its semantics apply only to caching
mechanisms, and such mechanisms need only check a resource's expiration
status when a new request for that resource is initiated.

User agents often have history mechanisms, such as "Back" buttons and
history lists, which can be used to redisplay an entity retrieved

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earlier in a session. By default, an expiration time does not apply to
history mechanisms. If the entity is still in storage, a history
mechanism should display it even if the entity has expired, unless the
user has specifically configured the agent to refresh expired history
documents.


16.2.3 Heuristic Expiration
Since origin servers do not always provide explicit expiration times,
HTTP caches typically assign heuristic expiration times, employing
algorithms that use other header values (such as the Last-Modified
time)
to estimate a plausible expiration time. The HTTP/1.1 specification does
not provide specific algorithms, but does impose worst-case constraints
on their results. Since heuristic expiration times may compromise
semantic transparency, they should be used cautiously, and we encourage
origin servers to provide explicit expiration times as much as
possible.


16.2.4 Age Calculations
In order to know if a cached entry is fresh, a cache needs to know if
its age exceeds its freshness lifetime. We discuss how to calculate the
latter in section 0; this section describes how to calculate the age of
a response or cache entry.

In this discussion, we use the term "now" to mean "the current value of
the clock at the host performing the calculation." All HTTP
implementations, but especially origin servers and caches, should use
NTP [RFC1305] or some similar protocol to synchronize their clocks to a
globally accurate time standard.

Also note that HTTP/1.1 requires origin servers to send a Date header
with every response, giving the time at which the response was
generated. We use the term "date_value" to denote a representation of
the value of the Date header, in a form appropriate for arithmetic
operations.

HTTP/1.1 uses the "Age" response header to help convey age information
between caches. The Age header value is the sender's estimate of the
amount of time since the response was generated at the origin server. In
the case of a cached response that has been revalidated with the origin
server, the Age value is based on the time of revalidation, not of the
original response.

In essence, the Age value is the sum of the time that the response has
been resident in each of the caches along the path from the origin
server, plus the amount of time it has been in transit along network
paths.

We use the term "age_value" to denote a representation of the value of
the Age header, in a form appropriate for arithmetic operations.

An response's age can be calculated in two entirely independent ways:



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  1. now - date_value, if the local clock is reasonably well
     synchronized to the origin server's clock. If the result is
     negative, this is replaced by zero.
  2. age_value, if all of the caches along the response path implement
     HTTP/1.1.
Given that we have two independent ways to compute the age of a response
when it is received, we can combine these as

       corrected_received_age = max(now - date_value, age_value)

and as long as we have either nearly synchronized clocks or
all-HTTP/1.1
paths, one gets a reliable (conservative) result.

Note that this correction is applied at each HTTP/1.1 cache along the
path, so that if there is an HTTP/1.0 cache in the path, the correct
received age is computed as long as the receiving cache's clock is
nearly in sync. We don't need end-to-end clock synchronization
(although
it is good to have), and there is no explicit clock synchronization
step.

Because of network-imposed delays, some significant interval may pass
from the time that a server generates a response, and the time it is
received at the next outbound cache or client. If uncorrected, this
delay could result in improperly low ages.

Because the request that resulted in the returned Age value must have
been initiated prior to that Age value's generation, we can correct for
delays imposed by the network by recording the time at which the request
was initiated. Then, when an Age value is received, it MUST be
interpreted relative to the time the request was initiated, not the time
that the response was received. This algorithm results in conservative
behavior no matter how much delay is experienced. So, we compute:

      corrected_initial_age = corrected_received_age
                            + (now - request_time)

where "request_time" is the time (according to the local clock) when the
request that elicited this response was sent.

Summary of age calculation algorithm, when a cache receives a response:

      /*
       * age_value
       *      is the value of Age: header received by the cache with
       *              this response.
       * date_value
       *      is the value of the origin server's Date: header
       * request_time
       *      is the (local) time when the cache made the request
       *              that resulted in this cached response
       * response_time
       *      is the (local) time when the cache received the
       *              response
       * now

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       *      is the current (local) time
       */
      apparent_age = max(0, now - date_value);
      corrected_received_age = max(apparent_age, age_value);
      response_delay = now - request_time;
      corrected_initial_age = corrected_received_age + response_delay;
      resident_time = now - response_time;
      current_age   = corrected_initial_age + resident_time;

When a cache sends a response, it must add to the corrected_initial_age
the amount of time that the response was resident locally. It must then
transmit this total age, using the Age header, to the next recipient
cache.

  Note that a client can usually tell if a response is firsthand by
  comparing the Date to its local request-time, and hoping that the
  clocks are not badly skewed.




16.2.5 Expiration Calculations
In order to decide whether a response is fresh or stale, we need to
compare its freshness lifetime to its age. The age is calculated as
described in section 16.2.4; this section describes how to calculate the
freshness lifetime, and to determine if a response has expired.

We use the term "expires_value" to denote a representation of the value
of the Expires header, in a form appropriate for arithmetic operations.
We use the term "max_age_value" to denote an appropriate representation
of the number of seconds carried by the max-age directive of the Cache-
Control header in a response (see section 18.11).

The max-age directive takes priority over Expires, so if max-age is
present in a response, the calculation is simply:

      freshness_lifetime = max_age_value

Otherwise, if Expires is present in the response, the calculation is:

      freshness_lifetime = expires_value - date_value

Note that neither of these calculations is vulnerable to clock skew,
since all of the information comes from the origin server.

If neither Expires nor Cache-Control max-age appears in the response,
and the response does not include other restrictions on caching, the
cache MAY compute a freshness lifetime using a heuristic. This heuristic
is subject to certain limitations; the minimum value may be zero, and
the maximum value MUST be no more than 24 hours.

Also, if the response does have a Last-Modified time, the heuristic
expiration value SHOULD be no more than some fraction of the interval
since that time.  A typical setting of this fraction might be 10%.

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The calculation to determine if a response has expired is quite simple:

      response_is_fresh = (freshness_lifetime > current_age)


16.2.6 Scope of Expiration
HTTP/1.1's expiration model is that as soon as any variant of a URI
becomes stale, all variants becomes stale as well.  Thus, "freshness"
applies to all the variants of URI, rather than any particular variant.
Dates and expires etc. apply to any cached variant that a proxy might
have with a URI and not just the one particular entity.

Editor's note: This restriction may be dropped in the next draft; there
are still discussions about whether this restriction is needed.


16.2.7 Disambiguating Expiration Values
Because expiration values are assigned optimistically, it is possible
that two caches may contain fresh values for the same resource that are
different.

If a client performing a retrieval receives a non-firsthand response for
a resource entity  that was already fresh in its own cache, and the Date
header in its existing cache entry is newer than the Date on the new
response, then the client MAY ignore the response. If so, it MAY retry
the request with a "Cache-Control: max-age=0" directive (see section
18.10), to force a check with the origin server.

If a cache that is pooling cached responses from other caches sees two
fresh responses for the same resource entity with different validators,
it SHOULD use the one with the newer Date header.


16.2.8 Disambiguating Multiple Responses
Because a client may be receiving responses via multiple paths, so that
some responses flow through one set of caches and other responses flow
through a different set of caches, a client may receive responses in an
order different from that in which the origin server generated them. We
would like the client to use the most recently generated response, even
if older responses are still apparently fresh.

Neither the entity tag nor the expiration value can impose an ordering
on responses, since it is possible that a later response intentionally
carries an earlier expiration time. However, the HTTP/1.1 specification
requires the transmission of Date headers on every response, and the
Date values are ordered to a granularity of one second.

If a client performs a request for a resource entity that it already has
in its cache, and the response it receives contains a Date header that
appears to be older than the one it already has in its cache, then the
client SHOULD repeat the request unconditionally, and include

       Cache-Control: max-age=0


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to force any intermediate caches to validate their copies directly with
the origin server, or

      Cache-Control: no-cache

to force any intermediate caches to obtain a new copy from the origin
server. This prevents certain paradoxes arising from the use of multiple
caches.

If the Date values are equal, then the client may use either response
(or may, if it is being extremely prudent, request a new response).
Servers MUST NOT depend on clients being able to choose
deterministically between responses generated during the same second, if
their expiration times overlap.


16.3 Validation Model
When a cache has a stale entry  that it would like to use as a response
to a client's request, it first has to check with the origin server (or
possibly an intermediate cache with a fresh response) to see if its
cached entry is still usable. We call this "validating" the cache
entry.
Since we do not want to have to pay the overhead of retransmitting the
full response if the cached entry is good, and we do not want to pay the
overhead of an extra round trip if the cached entry is invalid, the
HTTP/1.1 protocol supports the use of conditional methods.

The key protocol features for supporting conditional methods are those
concerned with "cache validators." When an origin server generates a
full response, it attaches some sort of validator to it, which is kept
with the cache entry. When a client (end-user or cache) makes a
conditional request for a resource for which it has a cache entry, it
includes the associated validator in the request.

The server then checks that validator against the current validator for
the resource entity, and, if they match, it responds with a special
status code (usually, "304 Not Modified") and no entity body.
Otherwise,
it returns a full response (including entity body). Thus, we avoid
transmitting the full response if the validator matches, and we avoid an
extra round trip if it does not match.

  Note: the comparison functions used to decide if validators match
  are defined in section 16.3.3.

In HTTP/1.1, a conditional request looks exactly the same as a normal
request for the same resource, except that it carries a special header
(which includes the validator) that implicitly turns the method
(usually, GET) into a conditional.

The protocol includes both positive and negative senses of cache-
validating conditions. That is, it is possible to request either that a
method be performed if and only if the validators match, or if and only
if the validators do not match.



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  Note: a response that lacks a cache validator may still be cached,
  and served from cache until it expires, unless this is explicitly
  prohibited by a Cache-Control directive. However, a cache cannot do
  a conditional retrieval if it does not have a cache validator for
  the entity, which means it will not be refreshable after it
  expires.




16.3.1 Last-modified Dates
In HTTP/1.0, the only cache validator is the Last-Modified time carried
by a response. Clients validate entities using the If-Modified-Since
header. In simple terms, a cache entry is considered to be valid if the
actual resource entity has not been modified since the original response
was generated.


16.3.2 Entity Tags
HTTP/1.1 introduces the possibility of using an "opaque" validator,
called an "entity tag," for situations where the Last-Modified date is
not appropriate. This may include server implementations where it is not
convenient to store modification dates, or where the one-second
resolution of HTTP date values is insufficient, or where the origin
server wishes to avoid certain paradoxes that may arise from the use of
modification dates.

An entity tag is simply a string of octets whose internal structure is
not known to clients or caches. Caches store entity tags and return them
when making conditional requests. Also, when a cache receives a
conditional request for a resource for which it has a fresh cache
entry,
it may compare entity tags using strict octet-equality. Otherwise,
entity tags have no semantic value to clients or caches.

To preserve compatibility with HTTP/1.0 clients and caches, and because
the Last-Modified date may be useful for purposes other than cache
validation, HTTP/1.1 servers SHOULD send Last-Modified whenever
feasible.

The headers used to convey entity tags are described in sections Error!
Reference source not found., Error! Reference source not found., 18.26,
and 18.46.


16.3.3 Weak and Strong Validators
Since both origin servers and caches will compare two validator values
to decide if they represent the same or different resource entities, one
normally would expect that if the resource entity  (the entity body or
any entity headers) changes in any way, then the associated validator
would change as well. If this is true, then we call this validator a
"strong validator."

However, there may be cases when a server prefers to change the
validator only on semantically significant changes, and not when

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insignificant aspects of the resource entity change. A validator that
does not always change when the resource changes is a "weak validator."

One can think of a strong validator as one that changes whenever the
bits of an entity changes, while a weak value changes whenever the
meaning of an entity changes. Alternatively, one can think of a strong
validator as part of an identifier for a specific entity, while a weak
validator is part of an identifier for a set of semantically equivalent
entities.

  Note: One example of a strong validator is an integer that is
  incremented in stable storage every time an entity is changed.

  An entity's modification time, if represented with one-second
  resolution, could be a weak validator, since it is possible that
  the resource entity  may be modified twice during a single second.

Entity tags are normally "strong validators," but the protocol provides
a mechanism to tag an entity tag as "weak."

A "use" of a validator is either when a client generates a request and
includes the validator in a validating header field, or when a server
compares two validators.

Strong validators are usable in any context. Weak validators are only
usable in contexts that do not depend on exact equality of an entity.
For example, either kind is usable for a conditional GET of a full
entity. However, only a strong validator is usable for a sub-range
retrieval, since otherwise the client may end up with an internally
inconsistent entity body.

The only function that the HTTP/1.1 protocol defines on validators is
comparison. There are two validator comparison functions, depending on
whether the comparison context allows the use of weak validators or
not:

  .  The strong comparison function: in order to be considered equal,
     both validators must be identical in every way, and neither may be
     weak.
  .  The weak comparison function: in order to be considered equal, both
     validators must be identical in every way, but either or both of
     them may be tagged as "weak" without affecting the result.
The weak comparison function SHOULD be used for simple (non-subrange)
GET requests. The strong comparison function MUST be used in all other
cases.

An entity tag is strong unless it is explicitly tagged as weak. Section
16.3 gives the syntax for entity tags.

A Last-Modified time, when used as a validator in a request, is
implicitly weak unless it is possible to deduce that it is strong, using
the following rules:

  .  The validator is being compared by an origin server to the actual
     current validator for the entity and,

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  .  That origin server reliably knows that the associated entity did
     not change twice during the second covered by the presented
     validator. or

  .  The validator is about to be used by a client in an If-Modified-
     Since or If-Unmodified-Since header, because the client has a cache
     entry for the associated entity, and
  .  That cache entry includes a Date value, which gives the time when
     the origin server generated the original response, and
  .  The presented Last-Modified time is at least 60 seconds before the
     Date value. or

  .  The validator is being compared by an intermediate cache to the
     validator stored in its cache entry for the entity, and
  .  That cache entry includes a Date value, which gives the time when
     the origin server generated the original response, and
  .  The presented Last-Modified time is at least 60 seconds before the
     Date value.
This method relies on the fact that if two different responses were
generated by the origin server during the same second, but both had the
same Last-Modified time, then at least one of those responses would have
a Date value equal to its Last-Modified time. The arbitrary 60-second
limit guards against the possibility that the Date and Last-Modified
values are generated from different clocks, or at somewhat different
times during the preparation of the response. An implementation may use
a value larger than 60 seconds, if it is believed that 60 seconds is too
short.

If a client wishes to perform a sub-range retrieval on a value for which
it has only a Last-Modified time and no opaque validator, it may do this
only if the Last-Modified time is strong in the sense described here.

A cache or origin server receiving a cache-conditional request, other
than a full-body GET request, must use the strong comparison function to
evaluate the condition.

These rules allow HTTP/1.1 caches and clients to safely perform sub-
range retrievals on values that have been obtained from HTTP/1.0
servers.


16.3.4 Rules for When to Use Entity Tags and Last-modified Dates
We adopt a set of rules and recommendations for origin servers,
clients,
and caches regarding when various validator types should be used, and
for what purposes.

HTTP/1.1 origin servers:

  .  SHOULD send an entity tag validator unless performance
     considerations support the use of weak entity tags, or unless it is
     unfeasible to send a strong entity tag.
  .  MAY send a weak entity tag instead of a strong one.

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  .  MAY send no entity tag if it is not feasible to generate one.
  .  SHOULD send a Last-Modified value if it is feasible to send one,
     unless the risk of a breakdown in semantic transparency that could
     result from using this date in an If-Modified-Since header would
     lead to serious problems.
In other words, the preferred behavior for an HTTP/1.1 origin server is
to send both a strong entity tag and a Last-Modified value.

In order to be legal, a strong entity tag MUST change whenever the
associated entity value changes in any way. A weak entity tag SHOULD
change whenever the associated entity  changes in a semantically
significant way.

  Note: in order to provide semantically transparent caching, an
  origin server should avoid reusing a specific strong entity tag
  value for two different resource entities, or reusing a specific
  weak entity tag value for two semantically different instances of a
  resource entity.  Cache entries may persist for arbitrarily long
  periods, regardless of expiration times, so it may be inappropriate
  to expect that a cache will never again attempt to validate an
  entry using a validator that it obtained at some point in the past.

HTTP/1.1 clients:

  .  If an entity tag has been provided by the origin server, MUST use
     that entity tag in any cache-conditional request (using If-Match or
     If-NoneMatch).
  .  If only a Last-Modified value has been provided by the origin
     server, SHOULD use that value in non-subrange cache-conditional
     requests (using If-Modified-Since).
  .  If only a Last-Modified value has been provided by an HTTP/1.0
     origin server, MAY use that value in subrange cache-conditional
     requests (using If-Unmodified-Since:). The user agent should
     provide a way to disable this, in case of difficulty.
  .  If both an entity tag and a Last-Modified value have been provided
     by the origin server, SHOULD use both validators in cache-
     conditional requests. This allows both HTTP/1.0 and HTTP/1.1 caches
     to respond appropriately.
An HTTP/1.1 cache, upon receiving a request, MUST use the most
restrictive validator when deciding whether the client's cache entry
matches the cache's own cache entry. This is only an issue when the
request contains both an entity tag and a last-modified-date validator
(If-Modified-Since or If-Unmodified-Since).

  A note on rationale: The general principle behind these rules is
  that HTTP/1.1 servers and clients should transmit as much non-
  redundant information as is available in their responses and
  requests. HTTP/1.1 systems receiving this information will make the
  most conservative assumptions about the validators they receive.

  HTTP/1.0 clients and caches will ignore entity tags. Generally,
  last-modified values received or used by these systems will support
  transparent and efficient caching, and so HTTP/1.1 origin servers
  should provide Last-Modified values. In those rare cases where the

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  use of a Last-Modified value as a validator by an HTTP/1.0 system
  could result in a serious problem, then HTTP/1.1 origin servers
  should not provide one.


16.3.5 Non-validating Conditionals
The principle behind entity tags is that only the service author knows
the semantics of a resource well enough to select an appropriate cache
validation mechanism, and the specification of any validator comparison
function more complex than byte-equality would open up a can of worms.
Thus, comparisons of any other headers (except Last-Modified, for
compatibility with HTTP/1.0) are never used for purposes of validating a
cache entry.


16.4 Constructing Responses From Caches
The purpose of an HTTP cache is to store information received in
response to requests, for use in responding to future requests. In many
cases, a cache simply returns the appropriate parts of a response to the
requester. However, if the cache holds a cache entry based on a previous
response, it may have to combine parts of a new response with what is
held in the cache entry.


16.4.1 End-to-end and Hop-by-hop Headers
For the purpose of defining the behavior of caches and non-caching
proxies, we divide HTTP headers into two categories:

  .  End-to-end headers, which must be transmitted to the ultimate
     recipient of a request or response. End-to-end headers in responses
     must be stored as part of a cache entry and transmitted in any
     response formed from a cache entry.
  .  Hop-by-hop headers, which are meaningful only for a single
     transport-level connection, and are not stored by caches or
     forwarded by proxies.
The following HTTP/1.1 headers are hop-by-hop headers:

  .  Connection
  .  Keep-Alive
  .  Upgrade
  .  Public
  .  Proxy-Authenticate
  .  Transfer-Encoding
All other headers defined by HTTP/1.1 are end-to-end headers.

Hop-by-hop headers introduced in future versions of HTTP MUST be listed
in a Connection header, as described in section 18.11.


16.4.2 Non-modifiable Headers
Some features of the HTTP/1.1 protocol, such as Digest Authentication,
depend on the value of certain end-to-end headers. A cache or non-
caching proxy SHOULD NOT modify an end-to-end header unless the
definition of that header requires or specifically allows that.

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A cache or non-caching proxy MUST NOT modify any of the following fields
in a request or response, nor may it add any of these fields if not
already present:

  .  Content-Type
  .  Content-Encoding
  .  Content-Length
  .  Expires
  .  Last-Modified
  .  Content-Range
  .  Content-Location
  Warning: unnecessary modification of end-to-end headers may cause
  authentication failures if stronger authentication mechanisms are
  introduced in later versions of HTTP. Such authentication
  mechanisms may rely on the values of header fields not listed here.




16.4.3 Combining Headers
When a cache makes a validating request to a server, and the server
provides a 304 Not Modified response, the cache must construct a
response to send to the requesting client.  The cache uses the entity-
body stored in the cache entry as the entity-body of this outgoing
response. It uses the end-to-end headers from the incoming response, not
from the cache entry.  Unless it decides to remove the cache entry, it
must also replace the end-to-end headers stored with the cache entry
with those received in the incoming response.

In other words, the complete set of end-to-end headers received in the
incoming response overrides all end-to-end headers stored with the cache
entry. The cache may add Warning headers (see section 18.48) to this
set.

A cache MUST preserve the order of all headers as received in an
incoming response.

These rule allows an origin server to completely control the response
seen by the client of a cache when the cache revalidates an entry, and
may be necessary for preserving semantic transparency or for certain
kinds of security mechanisms or future extensions.


16.4.4 Combining Byte Ranges
A response may transfer only a subrange of the bytes of an entity,
either because the request included one or more Range specifications, or
because a connection was broken prematurely. After several such
transfers, a cache may have received several ranges of the same entity.

If a cache has a stored non-empty set of subranges for an entity, and an
incoming response transfers another subrange, the cache MAY combine the
new subrange with the existing set if both the following conditions are
met:


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  .  Both the incoming response and the cache entry must have a cache
     validator.
  .  The two cache validators must match using the strong comparison
     function (see section 16.3.3).
If either requirement is not meant, the cache must use only the most
recent partial response (based on the Date values transmitted with every
response, and using the incoming response if these values are equal or
missing), and must discard the other partial information.


16.5 Caching and Generic Resources
Generic resources interacts with caching in several ways:

  .  A generic resource (one subject to content negotiation) may be
     bound to more than one entity. Each of these entities is called a
     "variant" of the resource.
  .  The request-URI may be only one part of the cache key.

16.5.1 Vary Header Use
Origin servers may respond to requests for generic resources use the
Vary header (see section 18.46 for a full description) to inform the
cache which header fields of the request were used to select the variant
returned in the response. A cache can use that response to reply to a
subsequent request only if the two requests not only specify the same
URI, but also have the same value for all headers specified in the Vary
response-header.

The Vary header may also inform the cache that the variant was selected
using criteria not limited to the request headers; in this case, the
response MUST NOT be used in a reply to a subsequent request except if
the cache relays the new request to the origin server in a conditional
request, and the origin server responds with 304 (Not Modified) and
includes the same variant-ID (see 13.8.3).


16.5.2 Alternates Header Use
The Alternates header is present in the HTTP/1.1 to enable caching of
entities from the planned content negotiation facilities. If a cache
receives an Alternates header in a response from the origin server (and
implement these planned facilities), it should act as if the response
carried a "Vary:{accept-headers}" header.  This means that the response
may be returned in reply to a subsequent request with Accept-* headers
identical to those in the current request.


16.5.3 Variant-ID Use
If an origin server chooses to use the variant-ID mechanism, it assigns
a variant-ID (see section 7.12) to each distinct resource entity
(variant). This assignment can only be made by the origin server. It
then returns the appropriate variant-ID with each response that applies
to a specific resource entity (variant), using the ETag header (see
Error! Reference source not found.).



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When sending an entity derived from a particular variant in a response,
an origin server SHOULD include a variant-ID identifying the variant in
the ETag header (see section Error! Reference source not found.).  This
variant-ID can be used for cache replacement and in conditional requests
on the generic resource. When a cache receives a successful response
with a variant-ID, it SHOULD use this information to replace any
existing cache entries for the same variant of the corresponding URI.
That is, it forms a cache key using the URI of the request and the
variant-ID of the response. If this key matches the key of an existing
cache entry, it SHOULD replace the existing entry with the new response
(subject to all of the other rules on caching). See section Error!
Reference source not found. for more details on update.

When a cache performs a conditional request on a generic resource, and
it has one or more cache entries for the resource that include variant-
IDs, the cache MUST transmit the (cache-validator, variant-ID) tuples in
the conditional request, using the variant-set mechanism (see section
7.13). This tells the server which variants are currently in the
requester's cache.

  The client MAY choose to transmit only a subset of the (cache-
  validator, variant-ID) tuples corresponding to its cache entries
  for this resource.

When a server receives a conditional request that includes a variant-
set, and the server is able to reply with an appropriate variant
(either
because it is the origin server, or because it is an intermediate cache
that can properly implement the variant selection algorithm), once it
has selected the variant it should examine the elements of the supplied
variant-set. If one of these matches the variant-ID of the selected
variant, and if the cache validators match, the server SHOULD reply with
a 304 (Not Modified) response, including the variant-ID of the selected
variant. Otherwise, the server should reply as if the request were
unconditional.

The server may optionally use the variant-set information in its
selection algorithm. For example, if the selection algorithm yields
several variants with equal preference, and one of these is already in
the requester's cache, the server could select that variant and avoid an
extra data transfer. This is a performance optimization; otherwise, the
variant-selection mechanism is orthogonal to the variant-ID mechanism.


16.6 Shared and Non-Shared Caches
For reasons of security and privacy, it is necessary to make a
distinction between "shared" and "non-shared" caches. A non-shared cache
is one that is accessible only to a single user. Accessibility in this
case SHOULD be enforced by appropriate security mechanisms. All other
caches are considered to be "shared." Other sections of this
specification place certain constraints on the operation of shared
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16.7 Selecting a Cached Response
When a cache receives a request it tries to see if it has a cached
response appropriate for that request, using the matching rules in this
section. If such a response exists, then the cache can decide if it is
fresh enough (using the expiration model in section 16.1.2 and the
freshness requirements of client and origin-server expressed in the
Cache-Control headers of the request and cached response) to return in
reply to the request.
If on a cache lookup there are two or more fresh entries that appear to
match the request, then the one with the most recent Date value MUST be
used.
16.7.1 Plain Resources
If the cached response was for a plain resource (that is, the response
includes no Vary or Alternates headers), it matches if the Request-URI
of the request matches the Request-URI of the of the request that caused
the cached response to be stored. Request-URIs match if their canonical
forms (see section 7.2.3) are equal.

16.7.2 Generic Resources
If the cached response was for a generic resource (that is, the response
includes Vary, or Alternates headers), it matches if the Request-URI of
the request matches the Request-URI of the request that caused the
cached response to be stored, and the selecting request header field
values of the request match those of the request that caused the cached
response to be stored. (See section 18.46 on Vary, which defines the
canonical form for selecting request headers and the matching rules for
them.)
If the response contains "Vary: {other}", then the selecting request
header field values for its request are defined as never matching a set
of request headers.

16.8 Errors or Incomplete Response Cache Behavior
A cache that receives an incomplete response (for example, with fewer
bytes of data than specified in a Content-length: header) may store the
response. However, the cache MUST treat this as a partial response.
Partial responses may be combined as described in section 16.4.4; the
result might be a full response or might still be partial. A cache MUST
NOT return a partial response to a client without explicitly marking it
as such, using the 206 (Partial Content) status code. A cache MUST NOT
return a partial response using a status code of 200 (OK).

A cache that receives a response with a zero-length Entity-body and no
explicit indication that the correct length is zero (such as "Content-
Length: 0") MUST NOT  store the response. The same rule applies to a
response of any length received without an explicit length indication if
the transport connection was terminated in any unusual way.

If a cache receives a response carrying Retry-After header (see section
18.40), it may either forward this response to the requesting client, or
act as if the server failed to respond. In the latter case, it MAY
return a previously received response, although it MUST follow all of
the rules applying to stale responses. In particular, it MUST NOT
override the "must-revalidate" Cache-Control directive (see section
18.10).

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16.8.1 Caching and Status Codes
A response received with a status code of 200 or 206 may be stored by a
cache and used in reply to a subsequent request, subject to the
expiration mechanism, unless a Cache-control directive prohibits
caching.

A response received with any other status code MUST NOT be returned in a
reply to a subsequent request unless it carries at least one of the
following:

  .  an Expires header
  .  a max-age Cache-control directive
  .  a must-revalidate Cache-control directive
  .  a public Cache-control directive

16.8.2 Handling of Retry-After
If a cache receives a response carrying a Retry-After header (see
section 18.40), it may either forward this response to the requesting
client, or act as if the server failed to respond.  In the latter case,
it MAY return a previously received response, although it MUST follow
all of the rules applying to stale responses.  In particular, it MUST
NOT override the "must-revalidate" Cache-Control directive (see section
18.10).


16.9 Side Effects of GET and HEAD
Unless the origin server explicitly prohibits the caching of their
responses, the application of GET and HEAD methods to any resources
SHOULD NOT have side effects that would lead to erroneous behavior if
these responses are taken from a cache. They may still have side
effects, but a cache is not required to consider such side effects in
its caching decisions. Caches are always expected to observe an origin
server's explicit restrictions on caching.

We note one exception to this rule: since some applications have
traditionally used GETs and HEADs with query URLs (those containing a
"?" in the rel_path part) to perform operations with significant side
effects, caches MUST NOT treat responses to such URLs as fresh unless
the server provides an explicit expiration time.

This specifically means that responses from HTTP/1.0 servers for such
URIs should not be taken from a cache.

See section 19.2 for related information.


16.10 Invalidation After Updates or Deletions
The effect of certain methods at the origin server may cause one or more
existing cache entries to become non-transparently invalid. That is,
although they may continue to be "fresh," they do not accurately reflect
what the origin server would return for a new request.

There is no way for the HTTP protocol to guarantee that all such cache
entries are marked invalid.  For example, the request that caused the

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change at the origin server may not have gone through the proxy where a
cache entry is stored.  However, several rules help reduce the
likelihood of erroneous behavior.

In this section, the phrase "invalidate an entity" means that the cache
should either remove all instances of that entity from its storage, or
should mark these as "invalid" and in need of a mandatory revalidation
before they can be returned in response to a subsequent request.

Some HTTP methods invalidate a single entity.  This is either the entity
referred to by the Request-URI, or by the Location or Content-Location
response headers (if present).  These methods are:

  .  PUT
  .  DELETE
  .  POST
In order to prevent denial of service attacks, an invalidation based on
the URI in a Location or Content-Location header MUST only be performed
if the host part is the same as in the Request-URI.


16.11 Write-Through Mandatory
All methods that may be expected to cause modifications to the origin
server's resources MUST be written through to the origin server. This
currently includes all methods except for GET and HEAD. A cache MUST NOT
reply to such a request from a client before having transmitted the
request to the inbound server, and having received a corresponding
response from the inbound server.

The alternative (known as "write-back" or "copy-back" caching) is not
allowed in HTTP/1.1, due to the difficulty of providing consistent
updates and the problems arising from server, cache, or network failure
prior to write-back.


16.12  Generic Resources and HTTP/1.0 Proxy Caches
If the correct handling of responses from a generic resource (Section
15) by HTTP/1.0 proxy caches in the response chain is important,
HTTP/1.1 origin servers can include the following Expires (Section
18.22) response header in all responses from the generic resource:

     Expires: Thu, 01 Jan 1980 00:00:00 GMT

If this Expires header is included, the server should usually also
include a Cache-Control header for the benefit of HTTP/1.1 caches, for example

     Cache-Control: max-age=604800

which overrides the freshness lifetime of zero seconds specified by the
included Expires header.




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16.13 Cache Replacement
If a new cacheable response (see sections 18.10.2, 16.2.6, 16.2.8 and
16.8) is received from a plain resource while any existing responses for
the same resource are cached, the cache MUST NOT return any of those
older responses to any  future requests for the resource.

  Note: a new response that has an older Date header value than
  existing cached responses is not cacheable.

If a new cacheable response is received from a generic resource with a
certain variant-ID while any old responses with the same variant-ID for
the same resource are cached, the cache MUST NOT return any of those old
responses to any future requests for the resource.

  Note: In some cases, this may mean that the cache chooses to delete
  the old response(s) from cache storage to recover space. However,
  note that there will never be a new response to signal that a
  variant-ID is no longer in use. It is expected that the cache's
  update heuristics will eventually cause such old responses to be
  deleted.

The cache SHOULD use the new response to reply to the current request.
It may insert it into cache storage and may, if it meets all other
requirements, use it to respond to any future requests that would
previously have caused the old response to be returned. If it inserts
the new response into cache storage it should follow the rules in
section 16.4.3.


16.14 Caching of Negative Responses
Caching of negative responses has often been a significant performance
advantage in distributed systems.  In some future draft or specification
we may have more to say about negative caching.


16.15 History Lists
History lists as implemented in many user agents and caches are
different.  In particular history lists SHOULD NOT try to show a
semantically transparent view of the current state of a resource
entity.
Rather, a history list is meant to show exactly what the user saw at the
time when the resource was retrieved .

This should not be construed to prohibit the history mechanism from
telling the user that a view may be stale.


17 Persistent Connections

17.1 Purpose
HTTP's greatest strength and its greatest weakness has been its
simplicity.  Prior to persistent connections, a separate TCP connection
was established to fetch each URL, increasing the load on HTTP servers,
and causing congestion on the Internet.   The use of inline images and
other associated data often requires a client to make multiple requests

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of the same server in a short amount of time.   An excellent analysis of
these performance problems is available [30]; analysis and results from
a prototype implementation are in [33], [34].

 Persistent HTTP connections have a number of advantages:

  .  By opening and closing fewer TCP connections, CPU time is saved,
     and memory used for TCP protocol control blocks is also saved
  .  HTTP requests and responses can be pipe-lined on a connection.
     Pipe-lining allows a client to make multiple requests without
     waiting for each response, allowing a single TCP connection to be
     used much more efficiently, with much lower elapsed time.
  .  Network congestion is reduced by reducing the number of packets
     caused by TCP opens, and by allowing TCP sufficient time to
     determine the congestion state of the network.
  .  HTTP can evolve more gracefully; since errors can be reported
     without the penalty of closing the TCP connection. Clients using
     future versions of HTTP might optimistically try a new feature, but
     if communicating with an older server, retry with old semantics
     after an error is reported.
HTTP implementations SHOULD implement persistent connections.


17.2 Overall Operation
Persistent connections provides a mechanism by which a client and a
server can negotiate the use of a TCP connection for an extended
conversation. This negotiation takes place using the Connection and
Persist header fields. Once this option has been negotiated, the client
can make multiple HTTP requests over a single transport connection.


17.2.1 Negotiation
To request the use of persistent connections, a client sends a
Connection header with a connection-token "Persist". If the server
wishes to accept persistent connections, it will respond with the same
connection-token. Both the client and server MUST send this connection-
token with every request and response for the duration of the persistent
connection. If either the client or the server omits the Persist token
from the Connection header, that request becomes the last one for the
connection.

A server MUST NOT establish a persistent connection with an HTTP/1.0
client that uses the above form of the Persist header due to problems
with the interactions between HTTP/1.1 clients and HTTP/1.0 proxy
servers. (See section 23.5.2.5 for more information on backwards
compatibility with HTTP/1.0 clients.)


17.2.2 Pipe-lining
Clients and servers which support persistent connections MAY
"pipe-line"
their requests and responses. When pipe-lining, a client will send
multiple requests without waiting for the responses. The server MUST
then send all of the responses in the same order that the requests were
made.

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A client MAY assume that a server supports persistent connections if the
same server has accepted persistent connections within the past 24
hours. Clients which assume persistent connections and pipeline
immediately SHOULD be prepared to retry their connection if the first
pipe-lined attempt fails. If a client does such a retry, it MUST NOT
pipeline without first receiving an explicit Persist token from the
server. Clients MUST also be prepared to resend their requests if the
server closes the connection before sending all of the corresponding
responses.


17.2.3 Delimiting Entity-Bodies
When using persistent connections, both the client and the server MUST
mark the exact endings of transmitted entity-bodies using one of the
following three techniques:

  1. Send a Content-length field in the header with the exact number of
     bytes in the entity-body.
  2. Send the message using chunked Transfer Coding as described in
     section 7.6. Chunked Transfer Coding allows the server to transmit
     the data to the client a piece at a time while still communicating
     an exact ending of the entity-body.
  3. Close the transport connection after the entity body.
Sending the Content-length is the preferred technique. Chunked encoding
SHOULD be used when the size of the entity-body is not known before
beginning to transmit the entity-body.  Finally, the connection MAY be
closed and fall back to non-persistent connections, if neither 1 or 2
are possible.

Clients and servers that support persistent connections MUST correctly
support receiving via all three techniques.


17.3 Proxy Servers
It is especially important that proxies correctly implement the
properties of the Connection header field as specified in 14.2.1.

The proxy server MUST negotiate persistent connections separately with
its clients and the origin servers (or other proxy servers) that it
connects to.  Each persistent connection applies to only one transport
link.

A proxy server MUST NOT establish a persistent connection with an
HTTP/1.0 client.


17.4 Interaction with Security Protocols
It is expected that persistent connections will operate with both SHTTP
[31] and SSL [32]. When used in conjunction with SHTTP, the SHTTP
request is prepared normally and the persist connection-token is placed
in the outermost request block (the one containing the "Secure"
method).
When used in conjunction with SSL, a SSL session is started as normal
and the first HTTP request made using SSL contains the persistent
connection header.

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17.5 Practical Considerations
Servers will usually have some time-out value beyond which they will no
longer maintain an inactive connection. Proxy servers might make this a
higher value since it is likely that the client will be making more
connections through the same server. The use of persistent connections
places no requirements on the length of this time-out for either the
client or the server.

When a client or server wishes to time-out it SHOULD issue a graceful
close on the transport connection. Clients and servers SHOULD both
constantly watch for the other side of the transport close, and respond
to it as appropriate. If a client or server does not detect the other
side's close promptly it could cause unnecessary resource drain on the
network.

A client, server, or proxy MAY close the transport connection at any
time. For example, a client MAY have started to send a new request at
the same time that the server has decided to close the "idle"
connection. From the server's point of view, the connection is being
closed while it was idle, but from the client's point of view, a request
is in progress.

This means that clients, servers, and proxies MUST be able to recover
from asynchronous close events. Client software SHOULD reopen the
transport connection and retransmit the aborted request without user
interaction. However, this automatic retry SHOULD NOT be repeated if the
second request fails.

Servers SHOULD always respond to at least one request per connection, if
at all possible. Servers SHOULD NOT close a connection in the middle of
transmitting a response, unless a network or client failure is
suspected.

It is suggested that clients which use persistent connections SHOULD
limit the number of simultaneous connections that they maintain to a
given server. A single-user client SHOULD maintain AT MOST 2 connections
with any server of proxy. A proxy SHOULD use up to 2*N connections to
another server or proxy, where N is the number of simultaneously active
users. These guidelines are intended to improve HTTP response times and
avoid congestion of the Internet or other networks.


18 Header Field Definitions
This section defines the syntax and semantics of all standard HTTP/1.1
header fields. For Entity-Header fields, both sender and recipient refer
to either the client or the server, depending on who sends and who
receives the entity.


18.1 Accept
The Accept request-header field can be used to specify certain media
types which are acceptable for the response.  Accept headers can be used
to indicate that the request is specifically limited to a small set of
desired types, as in the case of a request for an in-line image.

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The field MAY be folded onto several lines and more than one occurrence
of the field is allowed, with the semantics being the same as if all the
entries had been in one field value.

       Accept         = "Accept" ":" #(
                             media-range
                             [ ( ":" | ";" )

                               range-parameter

                               *( ";" range-parameter ) ]

                            | extension-token )

       media-range    = ( "*/*"
                        | ( type "/" "*" )
                        | ( type "/" subtype )
                        ) *( ";" parameter )

       range-parameter = ( "q" "=" qvalue )
                       | extension-range-parameter

       extension-range-parameter = ( token "=" token )

       extension-token = token

The asterisk "*" character is used to group media types into ranges,
with "*/*" indicating all media types and "type/*" indicating all
subtypes of that type. The range-parameter q is used to indicate the
media type quality factor for the range, which represents the user's
preference for that range of media types.  The default value is q=1.  In
Accept headers generated by HTTP/1.1 clients, the character separating
media-ranges from range-parameters SHOULD be a ":".  HTTP/1.1 servers
SHOULD be tolerant of use of the ";" separator by HTTP/1.0 clients.

The example

       Accept: audio/*: q=0.2, audio/basic

SHOULD be interpreted as "I prefer audio/basic, but send me any audio
type if it is the best available after an 80% mark-down in quality."

If no Accept header is present, then it is assumed that the client
accepts all media types.  If Accept headers are present, and if the
server cannot send a response which is acceptable according to the
Accept headers, then the server SHOULD send an error response with the
406 (not acceptable) status code, though the sending of an unacceptable
response is also allowed.

A more elaborate example is

       Accept: text/plain: q=0.5, text/html,
               text/x-dvi: q=0.8, text/x-c


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Verbally, this would be interpreted as "text/html and text/x-c are the
preferred media types, but if they do not exist, then send the text/x-
dvi entity, and if that does not exist, send the text/plain entity."

Media ranges can be overridden by more specific media ranges or specific
media types. If more than one media range applies to a given type, the
most specific reference has precedence. For example,

       Accept: text/*, text/html, text/html;level=1, */*

have the following precedence:

       1) text/html;level=1
       2) text/html
       3) text/*
       4) */*

The media type quality factor associated with a given type is determined
by finding the media range with the highest precedence which matches
that type. For example,

       Accept: text/*:q=0.3, text/html:q=0.7, text/html;level=1,
               */*:q=0.5

would cause the following values to be associated:

       text/html;level=1         = 1
       text/html                 = 0.7
       text/plain                = 0.3
       image/jpeg                = 0.5
       text/html;level=3         = 0.7

  Note: A user agent MAY be provided with a default set of quality
  values for certain media ranges. However, unless the user agent is
  a closed system which cannot interact with other rendering agents,
  this default set SHOULD be configurable by the user.




18.2 Accept-Charset
The Accept-Charset request-header field can be used to indicate what
character sets are acceptable for the response. This field allows
clients capable of understanding more comprehensive or special-purpose
character sets to signal that capability to a server which is capable of
representing documents in those character sets. The ISO-8859-1 character
set can be assumed to be acceptable to all user agents.

       Accept-Charset = "Accept-Charset" ":"

                 1#( charset [ ";" "q" "=" qvalue ] )




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Character set values are described in section 7.4. Each charset  may be
given an associated quality value which represents the user's preference
for that charset.  The default value is q=1.  An example is

       Accept-Charset: iso-8859-5, unicode-1-1;q=0.8

If no Accept-Charset header is present, the default is that any
character set is acceptable. If an Accept-Charset header is present, and
if the server cannot send a response which is acceptable according to
the Accept-Charset header, then the server SHOULD send an error response
with the 406 (not acceptable) status code, though the sending of an
unacceptable response is also allowed.




18.3 Accept-Encoding
The Accept-Encoding request-header field is similar to Accept, but
restricts the content-coding values (18.13) which are acceptable in the
response.

       Accept-Encoding  = "Accept-Encoding" ":"
                                 #( content-coding )

An example of its use is

       Accept-Encoding: compress, gzip

If no Accept-Encoding header is present in a request, the server MAY
assume that the client will accept any content coding. If an Accept-
Encoding header is present, and if the server cannot send a response
which is acceptable according to the Accept-Encoding  header, then the
server SHOULD send an error response with the 406 (not acceptable)
status code.


18.4 Accept-Language
The Accept-Language request-header field is similar to Accept, but
restricts the set of natural languages that are preferred as a response
to the request.

       Accept-Language  = "Accept-Language" ":"
                          1#( language-range [ ";" "q" "=" qvalue ] )

       language-range   = ( ( 1*8ALPHA *( "-" 1*8ALPHA ) )
                        | "*" )

Each language-range MAY be given an associated quality value which
represents an estimate of the user's comprehension of the languages
specified by that range.  The quality value defaults to "q=1" (100%
comprehension).For example,

       Accept-Language: da, en-gb;q=0.8, en;q=0.7


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would mean: "I prefer Danish, but will accept British English (with 80%
comprehension) and other types of English(with 70% comprehension)."  A
language-range matches a language-tag if it exactly equals the tag, or
if it exactly equals a prefix (a sub-sequence starting at the first
character) of the tag such that the first tag character following the
prefix is "-".  The special range "*", if present in the
Accept-Language
field, matches every tag not matched by any other ranges present in the
Accept-Language field.

  Note: This use of a prefix matching rule does not imply that
  language tags are assigned to languages in such a way that it is
  always true that if a user understands a language with a certain
  tag, then this user will also understand all languages with tags
  for which this tag is a prefix.  The prefix rule simply allows the
  use of prefix tags if this is the case.

The language quality factor assigned to a language-tag by the Accept-
Language field is the quality value of the longest language-range in the
field that matches the language-range.  If no language-range in the
field matches the tag, the language quality factor assigned is 0. If no
Accept-Language header is present in the request, the server SHOULD
assume that all languages are equally acceptable.  If an
Accept-Language
header is present, then all languages which are assigned a quality
factor greater than 0 are acceptable.  If the server cannot generate a
response for an audience capable of understanding at least one
acceptable language, it can send a response that uses one or more un-
accepted languages.

It may be contrary to the privacy expectations of the user to send an
Accept-Language header with the complete linguistic preferences of the
user in every request.  For a discussion of this issue, see section
19.7.

  Note: As intelligibility is highly dependent on the individual
  user, it is recommended that client applications make the choice of
  linguistic preference available to the user. If the choice is not
  made available, then the Accept-Language header field MUST NOT be
  given in the request.




18.5 Accept-Ranges
In some cases, a client may want to know if the server accepts range
requests using a certain range unit. The server may indicate its
acceptance of range requests for a resource entity  by providing this
header in a response for that resource:

       Accept-Ranges     = "Accept-Ranges" ":" acceptable-ranges

       acceptable-ranges = 1#range-unit | "none"

Origin servers that accept byte-range requests MAY send


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       Accept-Ranges: bytes

but are not required to do so.  Clients MAY generate byte-range requests
without having received this header for the plain resource involved, but
the server MAY ignore such requests.

Origin servers that do not accept any kind of range request for a plain
resource MAY send

       Accept-Ranges: none

to advise the client not to attempt a range request.


18.6 Age
Caches transmit age values using:

        Age = "Age" ":" age-value

        age-value = delta-seconds

Age values are non-negative decimal integers, representing time in
seconds.

If a cache receives a value larger than the largest positive integer it
can represent, or if any of its age calculations overflows, it MUST
transmit an Age header with a value of 2147483648 (2^31).  Otherwise,
HTTP/1.1 caches MUST send an Age header in every response.  Caches
SHOULD use a representation with at least 31 bits of range..


18.7 Allow
The Allow entity-header field lists the set of methods supported by the
resource identified by the Request-URI. The purpose of this field is
strictly to inform the recipient of valid methods associated with the
resource. An Allow header field MUST be present in a 405 (method not
allowed) response. The Allow header field is not permitted in a request
using the POST method, and thus SHOULD be ignored if it is received as
part of a POST entity.

       Allow          = "Allow" ":" 1#method

Example of use:

       Allow: GET, HEAD, PUT

This field cannot prevent a client from trying other methods. However,
the indications given by the Allow header field value SHOULD be
followed. The actual set of allowed methods is defined by the origin
server at the time of each request.

The Allow header field MAY be provided with a PUT request to recommend
the methods to be supported by the new or modified resource. The server


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is not required to support these methods and SHOULD include an Allow
header in the response giving the actual supported methods.

A proxy MUST NOT modify the Allow header field even if it does not
understand all the methods specified, since the user agent MAY have
other means of communicating with the origin server.

The Allow header field does not indicate what methods are implemented at
the server level. Servers MAY use the Public response header field
(section 18.37) to describe what methods are implemented on the server
as a whole.


18.8 Alternates
The Alternates response-header field is used by origin servers to signal
that the resource identified by the current request has the capability
to send different responses depending on the accept headers in the
request message.  This has an important effect on cache management,
particularly for caching proxies which service a diverse set of user
agents.  This effect is covered in section 18.46.

       Alternates           = "Alternates" ":" opaque-field

       opaque-field         = field-value

The Alternates header is included into HTTP/1.1 to make HTTP/1.1 caches
compatible with a planned content negotiation mechanism.  HTTP/1.1
allows a future content negotiation standard to define the format of the
Alternates header field-value, as long as the defined format satisfies
the general rules in section 18.8.

To ensure compatibility with future experimental or standardized
software, caching HTTP/1.1 clients MUST treat all Alternates headers in
a response as synonymous to the following Vary header:

         Vary: {accept-headers}

and follow the caching rules associated with the presence of this Vary
header, as covered in Section 18.46.  HTTP/1.1 allows origin servers to
send Alternates headers under experimental conditions.


18.9 Authorization
A user agent that wishes to authenticate itself with a server--usually,
but not necessarily, after receiving a 401 response--MAY do so by
including an Authorization request-header field with the request. The
Authorization field value consists of credentials containing the
authentication information of the user agent for the realm of the
resource being requested.

       Authorization  = "Authorization" ":" credentials




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HTTP access authentication is described in section 14. If a request is
authenticated and a realm specified, the same credentials SHOULD be
valid for all other requests within this realm.

When a shared cache (see section 16.6) receives a request containing an
Authorization field, it MUST NOT return the corresponding response as a
reply to any other request, unless one of the following specific
exceptions holds:

  1. If the response includes the "proxy-revalidate" Cache-Control
     directive, the cache MAY use that response in replying to a
     subsequent request, but a proxy cache MUST first revalidate it with
     the origin server, using the request headers from the new request
     to allow the origin server to authenticate the new request.
  2. If the response includes the "must-revalidate" Cache-Control
     directive, the cache MAY use that response in replying to a
     subsequent request, but all caches MUST first revalidate it with
     the origin server, using the request headers from the new request
     to allow the origin server to authenticate the new request.
  3. If the response includes the "public" Cache-Control directive, it
     may be returned in reply to any subsequent request.

18.10 Cache-Control
The Cache-Control general-header field is used to specify directives
that MUST be obeyed by all caching mechanisms along the
request/response
chain. The directives specify behavior intended to prevent caches from
adversely interfering with the request or response. .  These directives
typically override the default caching algorithms.  Cache directives are
unidirectional in that the presence of a directive in a request does not
imply that the same directive should be given in the response.

Cache directives must be passed through by a proxy or gateway
application, regardless of their significance to that application, since
the directives may be applicable to all recipients along the
request/response chain. It is not possible to specify a cache-directive
for a specific cache.

       Cache-Control   = "Cache-Control" ":" 1#cache-directive

       cache-directive = "public"
                       | "private" [ "=" <"> 1#field-name <"> ]
                       | "no-cache" [ "=" <"> 1#field-name <"> ]
                       | "no-store"
                       | "no-transform"
                       | "must-revalidate"
                       | "proxy-revalidate"
                       | "only-if-cached"
                       | "max-age" "=" delta-seconds
                       | "max-stale" "=" delta-seconds
                       | "min-fresh" "=" delta-seconds
                       | "min-vers" "=" HTTP-Version

When a directive appears without any 1#field-name parameter, the
directive applies to the entire request or response. When such a

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directive appears with a 1#field-name parameter, it applies only to the
named field or fields, and not to the rest of the request or response.
This mechanism supports extensibility; implementations of future
versions of the HTTP protocol may apply these directives to header
fields not defined in HTTP/1.1.

The cache-control directives can be broken down into these general
categories:

  .  Restrictions on what is cachable; these may only be imposed by the
     origin server.
  .  Restrictions on what may be stored by a cache; these may be imposed
     by either the origin server or the end-user client.
  .  Modifications of the basic expiration mechanism; these may be
     imposed by either the origin server or the end-user client.
  .  Controls over cache revalidation and reload; these may only be
     imposed by an end-user client.
  .  Restrictions on the number of times a cache entry may be used, and
     related demographic reporting mechanisms.
  .  Miscellaneous restrictions
Caches never add or remove Cache-Control directives to requests or
responses.

Check: is this true?


18.10.1 Cache-Control Restrictions on What is Cachable
Unless specifically constrained by a Cache-Control directive, a caching
system may always store a successful response (see section 16.8) as a
cache entry, may return it without validation if it is fresh, and may
return it after successful validation. If there is neither a cache
validator nor an explicit expiration time associated with a response, we
do not expect it to be cached, but certain caches may violate this
expectation (for example, when little or no network connectivity is
available).  A client can usually detect that such a response was taken
from a cache by comparing the Date header to the current time.

  Note that some HTTP/1.0 caches are known to violate this
  expectation without providing any Warning.

However, in some cases it may be inappropriate for a cache to retain a
resource entity, or to return it in response to a subsequent request.
This may be because absolute semantic transparency is deemed necessary
by the service author, or because of security or privacy
considerations.
Certain Cache-Control directives are therefore provided so that the
server can indicate that certain resource entities, or portions
thereof,
may not be cached regardless of other considerations.

Note that section 18.8 normally prevents a shared cache from saving and
returning a response to a previous request if that request included an
Authorization header.

The following Cache-Control response directives add or remove
restrictions on what is cachable:

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public
  Overrides the restriction in section 18.8 that prevents a shared
  cache from saving and returning a response to a previous request if
  that request included an Authorization header. However, any other
  constraints on caching still apply.
 private
  Indicates that all or part of the response message is intended for a
  single user and MUST NOT be cached by a shared cache. This allows an
  origin server to state that the specified parts of the response are
  intended for only one user and are not a valid response for requests
  by other users. A private (non-shared) cache may ignore this
  directive.
  Note: This usage of the word "private" only controls where the
  response may be cached, and cannot ensure the privacy of the
  message content. Note in particular that HTTP/1.0 caches will not
  recognize or obey this directive.


no-cache
  indicates that all or partof the response message MUST NOT be cached
  anywhere. This allows an origin server to prevent caching even by
  caches that have been configured to return stale responses to client
  requests.
  Note: HTTP/1.0 caches will not recognize or obey this directive.

TBS: precedence relations between public, private, and no-cache.


18.10.2 What May be Stored by Caches
The "no-store" directive applies to the entire message, and may be sent
either in a response or in a request. If sent in a request, a cache MUST
NOT store any part of either this request or any response to it. If sent
in a response, a cache MUST NOT store any part of either this response
or the request that elicited it. This directive applies to both non-
shared and shared caches.

Even when this directive is associated with a response, users may
explicitly store such a response outside of the caching system (e.g.,
with a "Save As" dialog). History buffers may store such responses as
part of their normal operation.

The purpose of this directive is to meet the stated requirements of
certain users and service authors who are concerned about accidental
releases of information via unanticipated accesses to cache data
structures. While the use of this directive may improve privacy in some
cases, we caution that it is NOT in any way a reliable or sufficient
mechanism for ensuring privacy. In particular, HTTP/1.0 caches will not
recognize or obey this directive, malicious or compromised caches may
not recognize or obey this directive, and communications networks may be
vulnerable to eavesdropping.

The "min-vers" directive applies to the entire message, and may be sent
either in a response or in a request. If sent in a request, a cache

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whose HTTP version number is less than the specified version MUST NOT
store any part of either this request or any response to it. If sent in
a response, a cache whose HTTP version number is less than the specified
version MUST NOT store any part of either this response or the request
that elicited it, nor may any cache transmit a stored (non-firsthand)
copy of the response to any client with a lower HTTP version number.
This directive applies to both non-shared and shared caches, and is made
mandatory to allow for future protocol extensions that may affect
caching.

  Note that the lowest version that can be sensibly included in a
  "min-vers" directive is HTTP/1.1, since HTTP/1.0 caches do not obey
  it.


18.10.3 Modifications of the Basic Expiration Mechanism
The expiration time of a resource entity may be specified by the origin
server using the Expires header (see section 18.22). Alternatively, it
may be specified using the "max-age" directive in a response.

If a response includes both an Expires header and a max-age directive,
the max-age directive overrides the Expires header, even if the Expires
header is more restrictive. This rule allows an origin server to
provide, for a given response, a longer expiration time to an HTTP/1.1
(or later) cache than to an HTTP/1.0 cache. This may be useful if
certain HTTP/1.0 caches improperly calculate ages or expiration times,
perhaps due to synchronized clocks.

Other directives allow an end-user client to modify the basic expiration
mechanism, making it either stricter or looser. These directives may be
specified on a request:


max-age
  Indicates that the client is willing to accept a response whose age
  is no greater than the specified time in seconds. Unless "max-stale"
  is also included, the client is not willing to accept a stale
  response. This directive overrides any policy of the cache.

min-fresh
  Indicates that the client is willing to accept a response whose
  freshness lifetime is no less than its current age plus the specified
  time in seconds. That is, the client wants a that response will still
  be fresh for at least the specified number of seconds.

max-stale
  Indicates that the client is willing to accept a response that has
  exceeded its expiration time by no more than the specified number of
  seconds. If a cache returns a stale response in response to such a
  request, it MUST mark it as stale using the Warning header.
  Note that HTTP/1.0 caches will ignore these directives.

If a cache returns a stale response, either because of a max-stale
directive on a request, or because the cache is configured to override

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the expiration time of a response, the cache MUST attach a Warning
header to the stale response, using Warning 10 (Response is stale).


18.10.4 Cache Revalidation and Reload Controls
Sometimes an end-user client may want or need to insist that a cache
revalidate its cache entry with the origin server (and not just with the
next cache along the path to the origin server), or to reload its cache
entry from the origin server. End-to-end revalidation may be necessary
if either the cache or the origin server has overestimated the
expiration time of the cached response. End-to-end reload may be
necessary if the cache entryhas become corrupted for some reason, and
the fact that its validator is up-to-date is irrelevant.

End-to-end revalidation may be requested either when the client does not
have its own local cached copy, in which case we call it "unspecified
end-to-end revalidation", or when the client does have a local cached
copy, in which case we call it "specific end-to-end revalidation."

The client can specify these three kinds of action using Cache-Control
request directives:


End-to-end reload
  The request includes "Cache-Control: no-cache" or, for compatibility
  with HTTP/1.0 clients, "Pragma: no-cache". No field names may be
  included with the "no-cache" directive in a request. The server MUST
  NOT use a cached copy when responding to such a request.

Specific end-to-end revalidation
  The request includes "Cache-Control: max-age=0", which forces each
  cache along the path to the origin server to revalidate its own
  entry, if any, with the next cache or server. The initial request
  includes a cache-validating conditional with the client's current
  validator.

Unspecified end-to-end revalidation
  The request includes "Cache-Control: max-age=0", which forces each
  cache along the path to the origin server to revalidate its own
  entry, if any, with the next cache or server. The initial request
  does not include a cache-validating conditional; the first cache
  along the path (if any) that holds a cache entry for this resource
  includes a cache-validating conditional with its current validator.
  Note that HTTP/1.0 caches will ignore these directives, except
  perhaps for "Pragma: no-cache".

When an intermediate cache is forced, by means of a "max-age=0"
directive, to revalidate its own cache entry, and the client has
supplied its own validator in the request, the supplied validator may
differ from the validator currently stored with the cache entry. In this
case, the cache may use either validator in making its own request
without affecting semantic transparency.



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However, the choice of validator may affect performance. The best
approach is for the intermediate cache to use its own validator when
making its request. If the server replies with 304 (Not Modified), then
the cache should return its now validated copy to the client with a 200
(OK) response. If the server replies with a new Entity-body and cache
validator, however, the intermediate cache should compare the returned
validator with the one provided in the client's request, using the
strong comparison function. If the client's validator is equal to the
origin server's, then the intermediate cache simply returns 304 (Not
Modified). Otherwise, it returns the new Entity-body with a 200 (OK)
response.

If a request includes the "no-cache" directive, it should not include
"min-fresh", "max-stale", or "max-age".

In some cases, such as times of extremely poor network connectivity, a
client may want a cache to return only those responses that it currently
has stored, and not to reload or revalidate with the origin server. To
do this, the client may include the "only-if-cached" directive in a
request. If it receives this directive, a cache SHOULD either respond
using a cached entry that is consistent with the other constraints of
the request, or respond with a 504 (Gateway Timeout) status. However, if
a group of caches is being operated as a unified system with good
internal connectivity, such a request MAY be forwarded within that group
of caches.

Because a cache may be configured to ignore a server's specified
expiration time, and because a client request may include a max-stale
directive, which has a similar effect, the protocol also includes a
mechanism for the origin server to require revalidation of a cache entry
on any subsequent use. When the "must-revalidate" directive is present
in a response received by a cache, that cache MUST NOT use the entry
after it becomes stale to respond to a subsequent request without first
revalidating it with the origin server. (I.e., the cache must do an
end-
to-end revalidation every time, if, based solely on the origin server's
Expires or max-age value, the cached response is stale.)

The "must-revalidate" directive is necessary to support reliable
operation for certain protocol features. In all circumstances an
HTTP/1.1 cache MUST obey the "must-revalidate" directive; in
particular,
if the cache cannot reach the origin server for any reason, it MUST
generate a 504 (Gateway Timeout) response. Note that HTTP/1.0 caches
will ignore this directive.

Servers should send the "must-revalidate" directive if and only if
failure to revalidate a request on the entity could result in incorrect
operation, such as a silently unexecuted financial transaction.
Recipients MUST NOT take any automated action that violates this
directive, and MUST NOT automatically provide an unvalidated copy of the
entity if revalidation fails.

Although this is not recommended, user agents operating under severe
connectivity constraints may violate this directive but, if so, MUST
explicitly warn the user that an unvalidated response has been
provided.

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The warning MUST be provided on each unvalidated access, and SHOULD
require explicit user confirmation.

The "proxy-revalidate" directive has the same meaning as the "must-
revalidate" directive, except that it does not apply to user-agent
caches.


18.10.5 Miscellaneous Restrictions
In certain circumstances, an intermediate cache (proxy) may find it
useful to convert the encoding of an entity body. For example, a proxy
might use a compressed content-coding to transfer the body to a client
on a slow link.

Because end-to-end authentication of entity bodies and/or entity headers
relies on the specific encoding of these values, such transformations
may cause authentication failures. Therefore, an intermediate cache MUST
NOT change the encoding of an entity body if the response includes the
"no-transform" directive.

  Note: the use of hop-by-hop compression in conjunction with Range
  retrievals may require additional specification in a subsequent
  draft.


18.11 Connection
HTTP version 1.1 provides a new request and response header field called
"Connection". This header field allows the client and server to specify
options which should only exist over that particular connection and MUST
NOT be communicated by proxies over further connections. The connection
header field MAY have multiple tokens separated by commas (referred to
as connection-tokens).

HTTP version 1.1 proxies MUST parse the Connection header field and for
every connection-token in this field, remove a corresponding header
field from the request before the request is forwarded. The use of a
connection option is specified by the presence of a connection token in
the Connection header field, not by the corresponding additional header
field (which may not be present).

When a client wishes to establish a persistent connection it MUST send a
"Persist" connection-token:

       Connection: persist

The Connection header has the following grammar:

       Connection-header = "Connection" ":" 1#(connection-token)
       connection-token  = token






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18.12 Content-Base
The Content-Base entity-header field may be used to specify the base URI
for resolving relative URLs within the entity. This header field is
described as "Base" in RFC 1808 , which is expected to be revised soon.

       Content-Base           = "Content-Base" ":" absoluteURI

If no Content-Base field is present, the base URI of an entity is
defined either by its Content-Location or the URI used to initiate the
request, in that order of precedence. Note, however, that the base URI
of the contents within the entity body may be redefined within that
entity body.


18.13 Content-Encoding
The Content-Encoding entity-header field is used as a modifier to the
media-type. When present, its value indicates what additional content
codings have been applied to the resource entity, and thus what decoding
mechanisms MUST be applied in order to obtain the media-type referenced
by the Content-Type header field. Content-Encoding is primarily used to
allow a document to be compressed without losing the identity of its
underlying media type.

       Content-Encoding        = "Content-Encoding" ":"
1#content-coding

Content codings are defined in section 7.5. An example of its use is

       Content-Encoding: gzip

The Content-Encoding is a characteristic of the resource entity
identified by the Request-URI. Typically, the resource entity is stored
with this encoding and is only decoded before rendering or analogous
usage.

If multiple encodings have been applied to a resource entity, the
content codings MUST be listed in the order in which they were applied.
Additional information about the encoding parameters MAY be provided by
other Entity-Header fields not defined by this specification.


18.14 Content-Language
The Content-Language entity-header field describes the natural
language(s) of the intended audience for the enclosed entity. Note that
this may not be equivalent to all the languages used within the entity.

       Content-Language        = "Content-Language" ":" 1#language-tag

Language tags are defined in section 7.10. The primary purpose of
Content-Language is to allow a selective consumer to identify and
differentiate resource variants according to the consumer's own
preferred language. Thus, if the body content is intended only for a
Danish-literate audience, the appropriate field is

       Content-Language: dk

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If no Content-Language is specified, the default is that the content is
intended for all language audiences. This may mean that the sender does
not consider it to be specific to any natural language, or that the
sender does not know for which language it is intended.

Multiple languages MAY be listed for content that is intended for
multiple audiences. For example, a rendition of the "Treaty of
Waitangi," presented simultaneously in the original Maori and English
versions, would call for

       Content-Language: mi, en

However, just because multiple languages are present within an entity
does not mean that it is intended for multiple linguistic audiences. An
example would be a beginner's language primer, such as "A First Lesson
in Latin," which is clearly intended to be used by an English-literate
audience. In this case, the Content-Language should only include "en".

Content-Language MAY be applied to any media type -- it SHOULD not be
limited to textual documents.


18.15 Content-Length
The Content-Length entity-header field indicates the size of the
Entity-
Body, in decimal number of octets, sent to the recipient or, in the case
of the HEAD method, the size of the Entity-Body that would have been
sent had the request been a GET.

       Content-Length = "Content-Length" ":" 1*DIGIT

An example is

       Content-Length: 3495

Applications SHOULD use this field to indicate the size of the Entity-
Body to be transferred, regardless of the media type of the entity. It
must be possible for the recipient to reliably determine the end of a
HTTP/1.1 request method containing an entity body, e.g., because the
request has a valid Content-Length field, uses Transfer-Encoding:
chunked or a multipart body.

Any Content-Length greater than or equal to zero is a valid value.
Section 11.2.2 describes how to determine the length of an Entity-Body
if a Content-Length is not given.

  Note: The meaning of this field is significantly different from the
  corresponding definition in MIME, where it is an optional field
  used within the "message/external-body" content-type. In HTTP, it
  SHOULD be used whenever the entity's length can be determined prior
  to being transferred.





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18.16 Content-Location
The Content-Location entity-header field is used to define the location
of the plain resource associated with the entity enclosed in the
message. A server SHOULD provide a Content-Location if, when including
an entity in response to a GET request on a generic resource, the entity
corresponds to a specific, non-negotiated location which can be accessed
via the Content-Location URI. A server SHOULD provide a
Content-Location
with any 200 (OK) response which was internally (not visible to the
client) redirected to a resource other than the one identified by the
request and for which correct interpretation of that resource MAY
require knowledge of its actual location.

        Content-Location = "Content-Location" ":" absoluteURI

If no Content-Base header field is present, the value of Content-
Location also defines the base URL for the entity (see Section 18.12).

  Note that the Content-Location information is advisory, and that
  there is no guarantee that the URI of the Content-Location actually
  corresponds in any way to the original request URI. For example, a
  cache cannot reliably assume that the data returned as a result of
  the request can be returned from a new request on any URI other
  than the original request.  See section 19.9.


18.17 Content-MD5
The Content-MD5 entity-header field is an MD5 digest of the
entity-body,
as defined in RFC 1864 [], for the purpose of providing an end-to-end
message integrity check (MIC) of the entity-body. (Note: an MIC is good
for detecting accidental modification of the entity-body in transit, but
is not proof against malicious attacks.)

        ContentMD5      = "Content-MD5" ":" md5-digest

        md5-digest      = <base64 of 128 bit MD5 digest as per RFC
1864>

The Content-MD5 header may be generated by an origin server to function
as an integrity check of the entity-body. Only origin-servers may
generate the Content-MD5 header field; proxies and gateways MUST NOT
generate it, as this would defeat its value as an end-to-end integrity
check. Any recipient of the entity-body, including gateways and
proxies,
MAY check that the digest value in this header field matches that of the
entity-body as received.

The MD5 digest is computed based on the content of the entity body,
including any Content-Encoding that has been applied, but not including
any Transfer-Encoding.  If the entity is received with a Transfer-
Encoding, that encoding must be removed prior to checking the Content-
MD5 value against the received entity.

This has the result that the digest is computed on the octets of the
entity body exactly as, and in the order that, they would be sent if no
Transfer-Encoding were being applied.


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HTTP extends RFC 1864 to permit the digest to be computed for MIME
composite media-types (e.g., multipart/* and message/rfc822), but this
does not change how the digest is computed as defined in the preceding
paragraph.

  Note: There are several consequences of this. The entity-body for
  composite types may contain many body-parts, each with its own MIME
  and HTTP headers (including Content-MD5, Content-Transfer-Encoding,
  and Content-Encoding headers). If a body-part has a Content-
  Transfer-Encoding or Content-Encoding header, it is assumed that
  the content of the body-part has had the encoding applied, and the
  body-part is included in the Content-MD5 digest as is -- i.e.,
  after the application. Also, the HTTP Transfer-Encoding header
  makes no sense within body-parts; if it is present, it is ignored -
  - i.e. treated as ordinary text.

  Note: while the definition of Content-MD5 is exactly the same for
  HTTP as in RFC 1864 for MIME entity-bodies, there are several ways
  in which the application of Content-MD5 to HTTP entity-bodies
  differs from its application to MIME entity-bodies. One is that
  HTTP, unlike MIME, does not use Content-Transfer-Encoding, and does
  use Transfer-Encoding and Content-Encoding. Another is that HTTP
  more frequently uses binary content types than MIME, so it is worth
  noting that in such cases, the byte order used to compute the
  digest is the transmission byte order defined for the type. Lastly,
  HTTP allows transmission of text types with any of several line
  break conventions and not just the canonical form using CRLF.
  Conversion of all line breaks to CRLF should not be done before
  computing or checking the digest: the line break convention used in
  the text actually transmitted should be left unaltered when
  computing the digest.




18.18 Content-Range
The Content-Range header is sent with a partial entity body to specify
where in the full entity body the partial body should be inserted.  It
also indicates the total size of the entity.

       Content-Range = "Content-Range" ":" content-range-spec

When an HTTP message includes the content of a single range (for
example, a response to a request for a single range, or to request for a
set of ranges that overlap without any holes), this content is
transmitted with a Content-Range header, and a Content-Length header
showing the number of bytes actually transferred.  For example,

       HTTP/1.0 206 Partial content
       Date: Wed, 15 Nov 1995 06:25:24 GMT
       Last-modified: Wed, 15 Nov 1995 04:58:08 GMT
       Content-Range: 21010-47021/47022
       Content-Length: 26012
       Content-Type: image/gif

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18.18.1 MIME multipart/byteranges Content-type
When an HTTP message includes the content of multiple ranges (for

example, a response to a request for multiple non-overlapping ranges),
these are transmitted as a multipart MIME message.  The multipart MIME
content-type used for this purpose is defined in this specification to
be "multipart/byteranges".

The MIME multipart/byteranges content-type includes two or more parts,
each with its own Content-Type and Content-Range fields.  The parts are
separated using a MIME boundary parameter.

For example:

       HTTP/1.0 206 Partial content
       Date: Wed, 15 Nov 1995 06:25:24 GMT
       Last-modified: Wed, 15 Nov 1995 04:58:08 GMT
       Content-type: multipart/byteranges;
boundary=THIS_STRING_SEPARATES

       --THIS_STRING_SEPARATES
       Content-type: application/pdf
      Content-range: bytes 500-999/8000

       ...the first range...
       --THIS_STRING_SEPARATES
       Content-type: application/pdf
      Content-range: bytes 7000-7999/8000

       ...the second range
       --THIS_STRING_SEPARATES_


18.18.2 Additional Rules for Content-Range
A client that cannot decode a MIME multipart/byteranges message should
not ask for multiple byte-ranges in a single request.

When a client requests multiple byte-ranges in one request, the server
SHOULD return them in the order that they appeared in the request.

If the server ignores a byte-range-spec because it is invalid,  the
server should treat the request as if the invalid Range header field did
not exist (normally, this means return a 200 response containing the
full resource entity).  The reason is that the only time a client will
make such an invalid request is when the resource entity has changed
(shrunk) since the prior request.


18.19 Content-Type
The Content-Type entity-header field indicates the media type of the
Entity-Body sent to the recipient or, in the case of the HEAD method,
the media type that would have been sent had the request been a GET.

       Content-Type   = "Content-Type" ":" media-type


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Media types are defined in section 7.7. An example of the field is

       Content-Type: text/html; charset=ISO-8859-4

Further discussion of methods for identifying the media type of an
entity is provided in section 11.2.1.


18.20 Date
The Date general-header field represents the date and time at which the
message was originated, having the same semantics as orig-date in RFC
822. The field value is an HTTP-date, as described in section 7.3.1.

       Date           = "Date" ":" HTTP-date

An example is

       Date: Tue, 15 Nov 1994 08:12:31 GMT

If a message is received via direct connection with the user agent (in
the case of requests) or the origin server (in the case of responses),
then the date can be assumed to be the current date at the receiving
end. However, since the date--as it is believed by the origin--is
important for evaluating cached responses, origin servers SHOULD always
include a Date header. Clients SHOULD only send a Date header field in
messages that include an entity body, as in the case of the PUT and POST
requests, and even then it is optional. A received message which does
not have a Date header field SHOULD be assigned one by the recipient if
the message will be cached by that recipient or gatewayed via a protocol
which requires a Date.

In theory, the date SHOULD represent the moment just before the entity
is generated. In practice, the date can be generated at any time during
the message origination without affecting its semantic value.

  Note: An earlier version of this document incorrectly specified
  that this field SHOULD contain the creation date of the enclosed
  Entity-Body. This has been changed to reflect actual (and proper)
  usage.

Origin servers MUST send a Date field in every response.  However, if a
cache receives a response without a Date field, it SHOULD attach one
with the cache's best estimate of the time at which the response was
originally generated.

The format of the Date is an absolute date and time as defined by HTTP-
date in Section 7.3; it MUST be in RFC1123-date format.




18.21 ETag
The ETag header is used to transmit entity tags with variant id's in
HTTP/1.1 responses.

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      ETag = "ETag" ":" etag-info
      etag-info = entity-tag [ ";" variant-id ]

Examples:

      ETag: "xyzzy"
      ETag: "xyzzy"/W
      ETag: "xyzzy";"3"
      ETag: "xyzzy"/W;"3"
      ETag: ""

  Note that the variant-id is not part of the entity tag. The ETag
  field is used to transmit a variant-id simply as a matter of
  compact representation of responses.


18.22 Expires
The Expires entity-header field gives the date/time after which the
entity should be considered stale. A stale cache entry may not normally
be returned by a cache (either a proxy cache or an end-user cache)
unless it is first validated with the origin server (or with an
intermediate cache that has a fresh copy of the resource entity). See
section 16.1.2 for further discussion of the expiration model.

The presence of an Expires field does not imply that the original
resource will change or cease to exist at, before, or after that time.

The format is an absolute date and time as defined by HTTP-date in
section 7.3; it MUST be in rfc1123-date format:

      Expires = "Expires" ":" HTTP-date

An example of its use is

      Expires: Thu, 01 Dec 1994 16:00:00 GMT

  Note: if a response includes a Cache-Control field with the max-age
  directive, that directive overrides the Expires field.

HTTP/1.1 clients and caches MUST treat other invalid date formats,
especially including the value "0", as in the past (i.e., "already
expired").

To mark a response as "already expired," an origin server should use an
Expires date that is equal to the Date header value. (See the rules for
expiration calculations in section 0.)

To mark a response as "never expires," an origin server should use an
Expires date approximately one year from the time the response is
generated. HTTP/1.1 servers should not send Expires dates more than one
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18.23 From
The From request-header field, if given, SHOULD contain an Internet e-
mail address for the human user who controls the requesting user agent.
The address SHOULD be machine-usable, as defined by mailbox in RFC 822
(as updated by RFC 1123 ):

       From   = "From" ":" mailbox

An example is:

       From: webmaster@w3.org

This header field MAY be used for logging purposes and as a means for
identifying the source of invalid or unwanted requests. It SHOULD NOT be
used as an insecure form of access protection. The interpretation of
this field is that the request is being performed on behalf of the
person given, who accepts responsibility for the method performed. In
particular, robot agents SHOULD include this header so that the person
responsible for running the robot can be contacted if problems occur on
the receiving end.

The Internet e-mail address in this field MAY be separate from the
Internet host which issued the request. For example, when a request is
passed through a proxy the original issuer's address SHOULD be used.

  Note: The client SHOULD not send the From header field without the
  user's approval, as it may conflict with the user's privacy
  interests or their site's security policy. It is strongly
  recommended that the user be able to disable, enable, and modify
  the value of this field at any time prior to a request.


18.24 Host
The Host request-header field specifies the Internet host and port
number of the resource being requested, as obtained from the original
URL given by the user or referring resource (generally an HTTP URL, as
described in section 7.2.2).  The Host field value MUST represent  the
network location of the origin server or gateway given by the original
URL.  This allows the origin server or gateway to differentiate between
internally-ambiguous URLs, such as the root "/"  URL of a server for
multiple host names on a single IP address.

       Host = "Host" ":" host [ ":" port ]    ; Section 7.2.2

A "host" without any trailing port information implies the default port
for the service requested (e.g., "80" for an HTTP URL).  For example, a
request on the origin server for <http://www.w3.org/pub/WWW/> MUST
include:

       GET /pub/WWW/ HTTP/1.1
       Host: www.w3.org

The Host header field MUST be included in all HTTP/1.1 request messages
on the Internet (i.e., on any message corresponding to a request for a

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URL which includes an Internet host address for the service being
requested).  If the Host field is not already present, an HTTP/1.1 proxy
MUST add a Host field to the request message prior to forwarding it on
the Internet.  All Internet-based HTTP/1.1 servers MUST respond with a
400 status code to any HTTP/1.1 request message which lacks a Host
header field.


18.25 If-Modified-Since
The If-Modified-Since request-header field is used with the GET method
to make it conditional: if the requested resource entity has not been
modified since the time specified in this field, a copy of the resource
entity will not be returned from the server; instead, a 304 (not
modified) response will be returned without any Entity-Body.

       If-Modified-Since = "If-Modified-Since" ":" HTTP-date

An example of the field is:

       If-Modified-Since: Sat, 29 Oct 1994 19:43:31 GMT

A GET method with an If-Modified-Since header and no Range header
requests that the identified resource entity be transferred only if it
has been modified since the date given by the If-Modified-Since header.
The algorithm for determining this includes the following cases:


a)If the request would normally result in anything other than a 200
  (OK) status, or if the passed If-Modified-Since date is invalid, the
  response is exactly the same as for a normal GET. A date which is
  later than the server's current time is invalid.

b)If the resource entity has been modified since the If-Modified-Since
  date, the response is exactly the same as for a normal GET.

c)If the resource entity  has not been modified since a valid If-
  Modified-Since date, the server MUST return a 304 (not modified)
  response.
The purpose of this feature is to allow efficient updates of cached
information with a minimum amount of transaction overhead.

  Note that the Range request-header field modifies the meaning of
  If-Modified-Since; see section 18.38 for full details.

  Note that If-Modified-Since is ignored for generic resources.

  Note that If-Modified-Since times are interpreted by the server,
  whose clock may not be synchronized with the client.

  Note that if a client uses an arbitrary date in the If-Modified-
  Since header instead of a date taken from the Last-Modified header
  for the same request, the client should be aware of the fact that
  this date is interpreted in the server's understanding of time.
  The client should consider unsynchronized clocks and rounding

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  problems due to the different representations of time between the
  client and server.  This includes the possibility of race
  conditions if the document has changed between the time it was
  first requested and the If-Modified-Since date of a subsequent
  request, and the possibility of clock-skew-related problems if the
  If-Modified-Date date is derived from the client's clock without
  correction to the server's clock.  Corrections for different time
  bases between client and server are at best approximate due to
  network latency.




18.26 If-Match
The If-Match request-header field is used with a method to make it
conditional. A client that has a cache entry for the relevant entity
supplies the associated entity tag using the If-Match header; if this
entity tag matches the server's current entity tag for the entity, the
server SHOULD perform the requested operation as if the If-Match header
were not present.

If the entity tags do not match, the server MUST NOT perform the
requested operation, and MUST return a 412 (Precondition failed)
response with no Entity-Body. This behavior is most useful when the
client wants to prevent an updating method, such as PUT or POST, from
modifying a resource entity  that  has changed since the client last
checked it.

When the If-Match header is used, the server should use the strong
comparison function (see section 18.26) to compare entity tags.

If the If-Match header is used to make a conditional request on generic
resource, it may be used to pass a set of validators.  This is done
using the variant-set mechanism if the client has variant IDs for the
corresponding cache entries (see sections 16.5.3 and 7.13 ).  The server
selects the appropriate variant based on other request headers; if the
variant-ID for that resource entity is listed in the If-Match header,
and if the entity-tag associated with that variant-ID in the header
matches the current entity-tag of the resource entity, then the
requested operation SHOULD be performed.  Otherwise, it MUST NOT be
performed.

      If-Match = "If-Match" ":" if-match-rhs
      if-match-rhs = opaque-validator | variant-set

An updating request (e.g., a PUT or POST) on a generic  resource should
include only one variant-set-item, the one associated with the
particular variant whose value is being conditionally updated.

Examples of plain resource  form:

       If-Match: "xyzzy"
       If-Match: "xyzzy"/W


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Examples of generic resource  form:

       If-Match: "xyzzy";"4"
       If-Match: "xyzzy";"3", "r2d2xxxx";"5", "c3piozzzz";"7"
       If-Match: "xyzzy"/W; "3", "r2d2xxxx"/W; "5", "c3piozzzz"/W; "7"

If the request would, without the If-Match header, result in anything
other than a 2xx status, then the If-Match header is ignored.

The purpose of this feature is to allow efficient updates of cached
information with a minimum amount of transaction overhead. It is also
used, on updating requests, to prevent inadvertent modification of the
wrong variant of a resource.


18.27 If-NoneMatch
The If-NoneMatch request-header field is used with a method to make it
conditional. A client that has a cache entry for the relevant entity
supplies the associated entity tag using the If-NoneMatch header; if
this entity tag matches the server's current entity tag for the entity,
the server SHOULD return a 304 (Not Modified) response without any
Entity-Body.

If the entity tags do not match, the server should treat the request as
if the If-NoneMatch header was not present.

See section 18.26 for rules on how to determine if two entity tags
match.

If the If-NoneMatch header is used to make a conditional request on
generic resource, it may be used to pass a set of validators.  This is
done using the variant-set mechanism if the client has variant IDs for
the corresponding cache entries (see sections 16.5.3 and 7.13).  The
server selects the appropriate variant based on other request headers;
if the variant-ID for that resource entity is listed in the
If-NoneMatch
header, and if the entity-tag associated with that variant-ID in the
header matches the current entity-tag of the resource entity, then the
requested operation SHOULD NOT be performed.  Otherwise, it SHOULD be
performed.

      If-NoneMatch = "If-NoneMatch" ":" if-nonematch-rhs
      if-nonematch-rhs = opaque-validator | variant-set

Examples of plain resource form:

       If-NoneMatch: "xyzzy"
       If-NoneMatch: "xyzzy"/W

Examples of generic resource form:

       If-NoneMatch: "xyzzy";"4"
       If-NoneMatch: "xyzzy";"3", "r2d2xxxx";"5", "c3piozzzz";"7"
       If-NoneMatch: "xyzzy"/W; "3", "r2d2xxxx"/W; "5", "c3piozzzz"/W;7


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If the request would, without the If-NoneMatch header, result in
anything other than a 2xx status, then the If-NoneMatch header is
ignored.

The purpose of this feature is to allow efficient updates of cached
information with a minimum amount of transaction overhead.


18.28 If-Range
If a client has a partial copy of an entity in its cache, and wishes to
have an up-to-date copy of the entire entity in its cache, it could use
the Range request header with a conditional GET (using either or both of
If-Unmodified-Since and If-Match.)  However, if the condition fails
because the entity has been modified, the client would then have to make
a second request to obtain the entire current entity body.

The If-Range header allows a client to "short-circuit" the second
request.  Informally, its meaning is "if the entity is unchanged, send
me the part(s) that I am missing; otherwise, send me the entire new
entity.'"

        Range-If = "Range-If" ":" (if-valid-rhs | HTTP-date)

If the client has no entity tag for a plain resource, but does have a
Last-Modified date, it may use that date in a If-Range header.  (The
server can detect this because an HTTP-date, unlike any form of if-
valid-rhs, does not start with a `"' quotation mark.)  Dates may only be
used in If-Range for plain resources, not for generic resources.  The
If-Range header should only be used together with a Range header, and
must be ignored if the request does not include a Range header, or if
the server does not support the sub-range operation.

If the entity tag given in the If-Range header matches the current
entity tag for the entity, then the server should provide the specified
sub-range of the entity using a 206 (Partial content) response.  If the
entity tag does not match, then the server should return the entire
entity using a 200 (OK) response.


18.29 If-Unmodified-Since
The If-Unmodified-Since request-header field is used with a method to
make it conditional. If the requested resource entity has not been
modified since the time specified in this field, the server should
perform the requested operation as if the If-Unmodified-Since header
were not present.

If the requested resource entity has been modified since the specified
time, the server MUST NOT perform the requested operation, and MUST
return a 412 (Precondition Failed) response with no Entity-Body.

      If-Unmodified-Since = "If-Unmodified-Since" ":" HTTP-date

An example of the field is:


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       If-Unmodified-Since: Sat, 29 Oct 1994 19:43:31 GMT

If the request normally (i.e., without the If-Unmodified-Since header)
would result in anything other than a 2xx status, the If-Unmodified-
Since header should be ignored.

If the specified date is invalid, the header is ignored.


18.30 Last-Modified
The Last-Modified entity-header field indicates the date and time at
which the sender believes the resource entity was last modified. The
exact semantics of this field are defined in terms of how the recipient
SHOULD interpret it: if the recipient has a copy of this resource entity
which is older than the date given by the Last-Modified field, that copy
SHOULD be considered stale.

       Last-Modified  = "Last-Modified" ":" HTTP-date

An example of its use is

       Last-Modified: Tue, 15 Nov 1994 12:45:26 GMT

The exact meaning of this header field depends on the implementation of
the sender and the nature of the original resource. For files, it may be
just the file system last-modified time. For entities with dynamically
included parts, it may be the most recent of the set of last-modify
times for its component parts. For database gateways, it may be the
last-update time stamp of the record. For virtual objects, it may be the
last time the internal state changed.

An origin server MUST NOT send a Last-Modified date which is later than
the server's time of message origination. In such cases, where the
resource's last modification would indicate some time in the future, the
server MUST replace that date with the message origination date.

An origin server should obtain the Last-Modified value of the entity as
close as possible to the time that it generates the Date value of its
response. This allows a recipient to make an accurate assessment of the
entity's modification time, especially if the entity changes near the
time that the response is generated.


18.31 Location
The Location response-header field is used to redirect the recipient to
a location other than the Request-URI for completion of the request or
identification of a new resource. For 201 (created) responses, the
Location is that of the new resource which was created by the request.
For 3xx responses, the location SHOULD indicate the server's preferred
URL for automatic redirection to the resource. The field value consists
of a single absolute URL.

       Location       = "Location" ":" absoluteURI


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An example is

       Location: http://www.w3.org/pub/WWW/People.html

  Note: The Content-Location header field (section 18.16) differs
  from Location in that the Content-Location identifies the original
  location of the entity enclosed in the request.  It is therefore
  possible for a response to contain header fields for both Location
  and Content-Location.


18.32 Max-Forwards
[JG14]The Max-Forwards general-header field may be used with the TRACE
method (section 18.32) to limit the number of times that a proxy or
gateway can forward the request to the next inbound server.  This can be
useful when the client is attempting to trace a request chain which
appears to be failing or looping in mid-chain.

       Max-Forwards   = "Max-Forwards" ":" 1*DIGIT

The Max-Forwards value is a decimal integer indicating the remaining
number of times this request message may be forwarded.

Each proxy or gateway recipient of a TRACE request containing a Max-
Forwards header field SHOULD check and update its value prior to
forwarding the request.  If the received value is zero (0), the
recipient SHOULD NOT forward the request; instead, it SHOULD respond as
the final recipient with a 200 (OK) response containing the received
request message as the response entity body (as described in Section
13.7).  If the received Max-Forwards value is greater than zero, then
the forwarded message SHOULD contain an updated Max-Forwards field with
a value decremented by one (1).

The Max-Forwards header field SHOULD be ignored for all other methods
defined by this specification and for any extension methods for which it
is not explicitly referred to as part of that method definition.


18.33 Persist
When the Persist connection-token has been transmitted with a request or
a response a Persist header field MAY also be included. The Persist
header field takes the following form:

       Persist-header = "Persist" ":" 0#pers-param

       pers-param = param-name "=" word
       param-name = token

The Persist header itself is optional, and is used only if a parameter
is being sent. HTTP/1.1 does not define any parameters.

If the Persist header is sent, the corresponding connection token MUST
be transmitted. The Persist header MUST be ignored if received without
the connection token.

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18.34 Pragma
The Pragma general-header field is used to include implementation-
specific directives that may apply to any recipient along the
request/response chain. All pragma directives specify optional behavior
from the viewpoint of the protocol; however, some systems MAY require
that behavior be consistent with the directives.

       Pragma                  = "Pragma" ":" 1#pragma-directive

       pragma-directive        = "no-cache" | extension-pragma
       extension-pragma        = token [ "=" word ]

When the "no-cache" directive is present in a request message, an
application SHOULD forward the request toward the origin server even if
it has a cached copy of what is being requested. This pragma directive
has the same semantics as the "no-cache" cache-directive (see section
18.10) and is defined here for backwards compatibility with HTTP/1.0.
Clients SHOULD include both header fields when a "no-cache" request is
sent to a server not known to be HTTP/1.1 compliant.

Pragma directives MUST be passed through by a proxy or gateway
application, regardless of their significance to that application, since
the directives may be applicable to all recipients along the
request/response chain. It is not possible to specify a pragma for a
specific recipient; however, any pragma directive not relevant to a
recipient SHOULD be ignored by that recipient.

HTTP/1.1 clients SHOULD NOT send the Pragma request header. HTTP/1.1
caches SHOULD treat "Pragma: no-cache" as if the client had sent
"Cache-
control: no-cache".  No new Pragma directives will be defined in HTTP.


18.35 Proxy-Authenticate
The Proxy-Authenticate response-header field MUST be included as part of
a 407 (Proxy Authentication Required) response. The field value consists
of a challenge that indicates the authentication scheme and parameters
applicable to the proxy for this Request-URI.

       Proxy-Authentication    = "Proxy-Authentication" ":" challenge

The HTTP access authentication process is described in section 14.
Unlike WWW-Authenticate, the Proxy-Authenticate header field applies
only to the current connection and MUST NOT be passed on to downstream
clients.


18.36 Proxy-Authorization
The Proxy-Authorization request-header field allows the client to
identify itself (or its user) to a proxy which requires authentication.
The Proxy-Authorization field value consists of credentials containing
the authentication information of the user agent for the proxy and/or
realm of the resource being requested.

       Proxy-Authorization     = "Proxy-Authorization" ":" credentials

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The HTTP access authentication process is described in section 14.
Unlike Authorization, the Proxy-Authorization applies only to the
current connection and MUST NOT be passed on to upstream servers. If a
request is authenticated and a realm specified, the same credentials
SHOULD be valid for all other requests within this realm.


18.37 Public
The Public response-header field lists the set of non-standard methods
supported by the server. The purpose of this field is strictly to inform
the recipient of the capabilities of the server regarding unusual
methods. The methods listed may or may not be applicable to the
Request-
URI; the Allow header field (section 18.7) SHOULD be used to indicate
methods allowed for a particular URI. This does not prevent a client
from trying other methods. The field value SHOULD not include the
methods predefined for HTTP/1.1 in section 9.1.1.

       Public         = "Public" ":" 1#method

Example of use:

       Public: OPTIONS, MGET, MHEAD

This header field applies only to the server directly connected to the
client (i.e., the nearest neighbor in a chain of connections). If the
response passes through a proxy, the proxy MUST either remove the Public
header field or replace it with one applicable to its own capabilities.


18.38 Range
HTTP retrieval requests using conditional or unconditional GET methods
may request one or more sub-ranges of the entity, instead of the entire
entity.  This is done using the Range request header:

      Range = "Range" ":" ranges-specifier

A server MAY ignore the Range header.  However, HTTP/1.1 origin servers
and intermediate caches SHOULD support byte ranges whenever possible,
since this supports efficient recovery from partially failed transfers,
and it supports efficient partial retrieval of large entities.

If the server supports the Range header and the specified range or
ranges are appropriate for the entity:

  .  The presence of a Range header in an unconditional GET modifies
     what is returned if the GET is otherwise successful.  In other
     words, the response carries a status code of 206 (Partial Content)
     instead of 200 (OK).
  .  The presence of a Range header in a conditional GET (a request
     using one or both of If-Modified-Since and If-NoneMatch, or one or
     both of If-Unmodified-Since and If-Match) modifies what is returned
     if the GET is otherwise successful and the condition is true.  It
     does not affect the 304 (Not Modified) response returned if the
     conditional is false.

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In some cases, it may be more appropriate to use the If-Range header
(see section 18.28) in addition to the Range header.


18.39 Referer
The Referer[sic] request-header field allows the client to specify, for
the server's benefit, the address (URI) of the resource from which the
Request-URI was obtained. This allows a server to generate lists of
back-links to resources for interest, logging, optimized caching, etc.
It also allows obsolete or mistyped links to be traced for maintenance.
The Referer field MUST NOT be sent if the Request-URI was obtained from
a source that does not have its own URI, such as input from the user
keyboard.

       Referer        = "Referer" ":" ( absoluteURI | relativeURI )

Example:

       Referer: http://www.w3.org/hypertext/DataSources/Overview.html

If a partial URI is given, it SHOULD be interpreted relative to the
Request-URI. The URI MUST NOT include a fragment.

  Note: Because the source of a link may be private information or
  may reveal an otherwise private information source, it is strongly
  recommended that the user be able to select whether or not the
  Referer field is sent. For example, a browser client could have a
  toggle switch for browsing openly/anonymously, which would
  respectively enable/disable the sending of Referer and From
  information.


18.40 Retry-After
The Retry-After response-header field can be used with a 503 (Service
Unavailable) response to indicate how long the service is expected to be
unavailable to the requesting client. The value of this field can be
either an HTTP-date or an integer number of seconds (in decimal) after
the time of the response.

       Retry-After    = "Retry-After" ":" ( HTTP-date | delta-seconds )

Two examples of its use are

       Retry-After: Wed, 14 Dec 1994 18:22:54 GMT
       Retry-After: 120

In the latter example, the delay is 2 minutes.


18.41 Server
The Server response-header field contains information about the software
used by the origin server to handle the request. The field can contain
multiple product tokens (section 7.8) and comments identifying the
server and any significant subproducts. By convention, the product

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tokens are listed in order of their significance for identifying the
application.

       Server         = "Server" ":" 1*( product | comment )

Example:

       Server: CERN/3.0 libwww/2.17

If the response is being forwarded through a proxy, the proxy
application MUST NOT add its data to the product list. Instead, it
SHOULD include a Via field (as described in section 18.47).

  Note: Revealing the specific software version of the server may
  allow the server machine to become more vulnerable to attacks
  against software that is known to contain security holes. Server
  implementers are encouraged to make this field a configurable
  option.


18.42 Title
The Title entity-header field indicates the title of the entity

       Title          = "Title" ":" *TEXT

An example of the field is

       Title: Hypertext Transfer Protocol -- HTTP/1.1

This field is isomorphic with the <TITLE> element in HTML .


18.43 Transfer Encoding
The Transfer-Encoding general-header field indicates what (if any) type
of transformation has been applied to the message body in order to
safely transfer it between the sender and the recipient. This differs
from the Content-Encoding in that the transfer coding is a property of
the message, not of the original resource entity.

       Transfer-Encoding       = "Transfer-Encoding" ":" 1#transfer- coding

Transfer codings are defined in section 7.6. An example is:

       Transfer-Encoding: chunked

Many older HTTP/1.0 applications do not understand the
Transfer-Encoding
header.


18.44 Upgrade
The Upgrade general-header allows the client to specify what additional
communication protocols it supports and would like to use if the server
finds it appropriate to switch protocols. The server MUST use the

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Upgrade header field within a 101 (Switching Protocols) response to
indicate which protocol(s) are being switched.

       Upgrade        = "Upgrade" ":" 1#product

For example,

       Upgrade: HTTP/2.0, SHTTP/1.3, IRC/6.9, RTA/x11

The Upgrade header field is intended to provide a simple mechanism for
transition from HTTP/1.1 to some other, incompatible protocol.  It does
so by allowing the client to advertise its desire to use another
protocol, such as a later version of HTTP with a higher major version
number, even though the current request has been made using  HTTP/1.1.
This eases the difficult transition between incompatible protocols by
allowing the client to initiate a request in the more commonly supported
protocol while indicating to the server that it would like to use a
"better" protocol if available (where "better" is determined by the
server, possibly according to the nature of the method and/or resource
being requested).

The Upgrade header field only applies to switching application-layer
protocols upon the existing transport-layer connection.  Upgrade cannot
be used to insist on a protocol change; its acceptance and use by the
server is optional.  The capabilities and nature of the application-
layer communication after the protocol change is entirely dependent upon
the new protocol chosen, although the first action after changing the
protocol MUST be a response to the initial HTTP request containing the
Upgrade header field.

The Upgrade header field only applies to the immediate connection.
Therefore, the "upgrade" keyword MUST be supplied within a Connection
header field (section 18.11) whenever Upgrade is present in an HTTP/1.1
message.

The Upgrade header field cannot be used to indicate a switch to a
protocol on a different connection.  For that purpose, it is more
appropriate to use a 301, 302, 303, or 305 redirection response.

This specification only defines the protocol name "HTTP" for use by the
family of Hypertext Transfer Protocols, as defined by the HTTP version
rules of section 7.1 and future updates to this specification.  Any
token can be used as a protocol name; however, it will only be useful if
both the client and server associate the name with the same protocol.


18.45 User-Agent
The User-Agent request-header field contains information about the user
agent originating the request. This is for statistical purposes, the
tracing of protocol violations, and automated recognition of user agents
for the sake of tailoring responses to avoid particular user agent
limitations. Although it is not required, user agents SHOULD include
this field with requests. The field can contain multiple product tokens
(section 7.8) and comments identifying the agent and any subproducts

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which form a significant part of the user agent. By convention, the
product tokens are listed in order of their significance for identifying
the application.

       User-Agent     = "User-Agent" ":" 1*( product | comment )

Example:

       User-Agent: CERN-LineMode/2.15 libwww/2.17b3


18.46 Vary
The Vary response-header field is used by an origin server to signal
that the resource identified by the current request is a generic)
resource.  A generic resource has multiple entities associated with it,
all of which are representations of the content of the resource.  If a
GET or HEAD request on a generic resource is received, the origin server
will select one of the associated entities as the entity best matching
the request.  Selection of this entity is based on the contents of
particular header fields in the request message, or on other information
pertaining to the request, like the network address of the sending
client.

A resource being generic has an important effect on cache management,
particularly for caching proxies which service a diverse set of user
agents.  All 200 (OK) responses from generic resources MUST contain at
least one Vary header (section 18.46) or Alternates header (section
18.8) to signal variance.

If no Vary headers and no Alternates headers are present in a 200 (OK)
response, then caches may assume, as long as the response is fresh, that
the resource in question is plain, and has only one associated entity.
Note however that this entity can still change through time, as possibly
indicated by a Cache-Control response header (section 18.10).

After selection of the entity best matching the current request, the
origin server will usually generate a 200 (OK) response, but it can also
generate other responses like 206 (Partial Content) or 304 (Not
Modified) if headers which modify the semantics of the request, like
Range (section 18.38) or If-Match (section 18.26), are present.  An
origin server need not be capable of selecting an entity for every
possible incoming request on a generic resource; it can choose to
generate a 3xx (redirection) or 4xx (client error) type response for
some requests.

In a request message on a generic resource, the selecting request
headers are those request headers whose contents were used by the origin
server to select the entity best matching the request. The Vary header
field specifies the selecting request headers and any other selection
parameters that were used by the origin server.

       Vary                 = "Vary" ":" 1#selection-parameter



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       selection-parameter  = request-header-name
                            | "{accept-headers}"
                            | "{other}"
                            | "{" extension-parameter "}"

       request-header-name  = field-name

       extension-parameter  = token

The presence of a request-header-name signals that the request-header
field with this name is selecting.  Note that the name need not belong
to a request-header field defined in this specification, and that header
names are case-insensitive.  The presence of the "{accept-headers}"
parameter signals that all request headers whose names start with
"accept" are selecting.

The inclusion of the "{other}" parameter in a Vary field signals that
parameters other than the contents of request headers, for example the
network address of the sending party, play a role in the selection of
the response.

  Note: This specification allows the origin server to express that
  other parameters were used, but does not allow the origin server to
  specify the exact nature of these parameters.  This is left to
  future extensions.

If an extension-parameter unknown to the cache is present in a Vary
header, the cache MUST treat it as the "{other}" parameter. If multiple
Vary and Alternates header fields are present in a response, these MUST
be combined to give all selecting parameters.

The field name "Host" MUST never be included in a Vary header; clients
MUST ignore it if it is present.  The names of fields which change the
semantics of a GET request, like "Range" and "If-Match" MUST also never
be included, and MUST be ignored when present.

Servers which use access authentication are not obliged to send "Vary:
Authorization" headers in responses.  It MUST be assumed that requests
on authenticated resources can always produce different responses for
different users.  Note that servers can signal the absence of
authentication by including "Cache-Control: public" header in the
response.

A cache MAY store and refresh 200 (OK) responses from a generic resource
according to the rules in section 16.4.  The partial entities in 206
(Partial Content) responses from generic resources MAY also be used by
the cache.

When getting a request on a generic resource, a cache can only return a
cached 200 (OK) response to one of its clients in two particular cases.

First, if a cache gets a request on a generic resource for which it has
cached one or more responses with Vary or Alternates headers, it can
relay that request towards the origin server, adding an If-NoneMatch

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header listing the etag-info values in the ETag headers (section Error!
Reference source not found.) of the cached responses which have
variant-
IDs.  If it then gets back a 304 (Not Modified) response with the etag-
info of a cached 200 (OK) response in its ETag header, it can return
this cached 200 (OK) response to its client, after merging in any of the
304 response headers as specified in section 16.4.2.

Second, if a cache gets a request on a generic resource, it can return
to its client a cached, fresh 200 (OK) response which has Vary or
Alternates headers, provided that


  .  the Vary and Alternates headers of this fresh response specify that
     only request header fields are selecting parameters,

  .  the specified selecting request header fields of the current
     request match the specified selecting request header fields of a
     previous request on the resource relayed towards the origin
server,

  .  this previous request got a 200 (OK) or 304 (Not Modified) response
     which had the same etag-info value in its ETag header as the
     cached, fresh 200 (OK) response.
Two sequences of selecting request header fields match if and only if
the first sequence can be transformed into the second sequence by only
adding or removing whitespace at places in fields where this is allowed
according to the syntax rules in this specification.

If a cached 200 (OK) response MAY be returned to a request on a generic
resource which includes a Range request header, then a cache MAY also
use this 200 (OK) response to construct and return a 206 (Partial
Content) response with the requested range.

  Note: Implementation of support for the second case above is mainly
  interesting in user agent caches, as a user agent cache will
  generally have an easy way of determining whether the sequence of
  request header fields of the current request equals the sequence
  sent in an earlier request on the same resource.  Proxy caches
  supporting the second case would have to record diverse sequences
  of request header fields previously relayed; the implementation
  effort associated with this may not be balanced by a sufficient
  payoff in traffic savings.  A planned specification of a content
  negotiation mechanism will define additional cases in which proxy
  caches can return a cached 200 (OK) response without contacting the
  origin server.  The implementation effort associated with support
  for these additional cases is expected to have a much better
  cost/benefit ratio.


18.47 Via
The Via general-header field MUST be used by gateways and proxies to
indicate the intermediate protocols and recipients between the user
agent and the server on requests, and between the origin server and the
client on responses. It is analogous to the "Received" field of RFC 822
and is intended to be used for tracking message forwards, avoiding

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request loops, and identifying the protocol capabilities of all senders
along the request/response chain.

      Via   =   "Via" ":" 1#( received-protocol received-by [ comment ]
)

      received-protocol = [ protocol-name "/" ] protocol-version
      protocol-name     = token
      protocol-version  = token
      received-by       = ( host [ ":" port ] ) | pseudonym
      pseudonym         = token

The received-protocol indicates the protocol version of the message
received by the server or client along each segment of the
request/response chain.  The received-protocol version is appended to
the Via field value when the message is forwarded so that information
about the protocol capabilities of upstream applications remains visible
to all recipients.

The protocol-name is optional if and only if it would be "HTTP".  The
received-by field is normally the host and optional port number of a
recipient server or client that subsequently forwarded the message.
However, if the real host is considered to be sensitive information, it
MAY be replaced by a pseudonym.  If the port is not given, it MAY be
assumed to be the default port of the received-protocol.

Multiple Via field values represent each proxy or gateway that has
forwarded the message.  Each recipient MUST append its information such
that the end result is ordered according to the sequence of forwarding
applications.

Comments MAY be used in the Via header field to identify the software of
the recipient proxy or gateway, analogous to the User-Agent and  Server
header fields.  However, all comments in the Via field are optional and
MAY be removed by any recipient prior to forwarding the message.

For example, a request message could be sent from an HTTP/1.0 user agent
to an internal proxy code-named "fred", which uses HTTP/1.1 to forward
the request to a public proxy at nowhere.com, which completes the
request by forwarding it to the origin server at www.ics.uci.edu.  The
request received by www.ics.uci.edu would then have the following Via
header field:

       Via: 1.0 fred, 1.1 nowhere.com (Apache/1.1)

Proxies and gateways used as a portal through a network firewall SHOULD
NOT, by default, forward the names and ports of hosts within the
firewall region. This information SHOULD only be propagated if
explicitly enabled. If not enabled, the received-by host of any host
behind the firewall SHOULD be replaced by an appropriate pseudonym for
that host.

For organizations that have strong privacy requirements for hiding
internal structures, a proxy MAY combine an ordered subsequence of Via

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header field entries with identical received-protocol values into a
single such entry.  For example,

       Via: 1.0 ricky, 1.1 ethel, 1.1 fred, 1.0 lucy

   could be collapsed to

       Via: 1.0 ricky, 1.1 mertz, 1.0 lucy

Applications SHOULD NOT combine multiple entries unless they are all
under the same organizational control and the hosts have already been
replaced by pseudonyms.  Applications MUST NOT combine entries which
have different received-protocol values.

  Note: The Via header field replaces the Forwarded header field
  which was present in earlier drafts of this specification.


18.48 Warning
Warning headers are sent with responses using:

       Warning    = "Warning" ":" warn-code SP warn-agent SP warn-text
       warn-code  = 2DIGIT
       warn-agent = ( host [ ":" port ] ) | pseudonym
                       ; the name or pseudonym of the server adding
                       ; the Warning header, for use in debugging
       warn-text  = quoted-string

A response may carry more than one Warning header.

The warn-text should be in a natural language and character set that is
most likely to be intelligible to the human user receiving the
response.
This decision may be based on any available knowledge, such as the
location of the cache or user, the Accept-Language field in a request,
the Content-Language field in a response, etc. The default language is
English and the default character set is ISO-8599-1.

If a character set other than ISO-8599-1 is used, it must be encoded in
the warn-text using the method described in RFC 1522 [14].

Any server or cache may add Warning headers to a response. New Warning
headers should be added after any existing Warning headers. A cache MUST
NOT delete any Warning header that it received with a response.
However,
if a cache successfully validates a cache entry, it SHOULD remove any
Warning headers previously attached to that entry. It MUST then add any
Warning headers received in the validating response. In other words,
Warning headers are those that would be attached to the most recent
relevant response.

When multiple Warning headers are attached to a response, the user agent
SHOULD display as many of them as possible, in the order that they
appear in the response. If it is not possible to display all of the
warnings, the user agent should follow these heuristics:


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  .  Warnings that appear early in the response take priority over those
     appearing later in the response.
  .  Warnings in the user's preferred character set take priority over
     warnings in other character sets but with identical warn-codes and
     warn-agents.
Systems that generate multiple Warning headers should order them with
this user-agent behavior in mind.

This is a list of the currently-defined warn-codes, each with a
recommended warn-text in English, and a description of its meaning.


10 Response is stale
  MUST be included whenever the returned response is stale. A cache may
  add this warning to any response, but may never remove it until the
  response is known to be fresh.

11 Revalidation failed
  MUST be included if a cache returns a stale response because an
  attempt to revalidate the response failed, due to an inability to
  reach the server. A cache may add this warning to any response, but
  may never remove it until the response is successfully revalidated.

12 Disconnected operation
   SHOULD be included if the cache is intentionally disconnected from
  the rest of the network for a period of time.

99 Miscellaneous warning
  The warning text may include arbitrary information to be presented to
  a human user, or logged. A system receiving this warning MUST NOT
  take any automated action.



18.49 WWW-Authenticate
The WWW-Authenticate response-header field MUST be included in 401
(Unauthorized) response messages. The field value consists of at least
one challenge that indicates the authentication scheme(s) and parameters
applicable to the Request-URI.

       WWW-Authenticate        = "WWW-Authenticate" ":" 1#challenge

The HTTP access authentication process is described in section 14. User
agents MUST take special care in parsing the WWW-Authenticate field
value if it contains more than one challenge, or if more than one WWW-
Authenticate header field is provided, since the contents of a challenge
may itself contain a comma-separated list of authentication parameters.


19 Security Considerations
This section is meant to inform application developers, information
providers, and users of the security limitations in HTTP/1.1 as
described by this document. The discussion does not include definitive


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solutions to the problems revealed, though it does make some suggestions
for reducing security risks.


19.1 Authentication of Clients
As mentioned in section 14, the Basic authentication scheme is not a
secure method of user authentication, nor does it in any way protect the
Entity-Body, which is transmitted in clear text across the physical
network used as the carrier. HTTP does not prevent additional
authentication schemes and encryption mechanisms from being employed to
increase security or the addition of enhancements (such as schemes to
use one-time passwords) to Basic authentication.

The most serious flaw in Basic authentication is that it results in the
essentially clear text transmission of the user's password over the
physical network.  It is this problem which Digest Authentication
attempts to address.

Because Basic authentication involves the clear text transmission of
passwords it SHOULD never be used (without enhancements) to protect
sensitive or valuable information.

A common use of Basic authentication is for identification purposes --
requiring the user to provide a user name and password as a means of
identification, for example, for purposes of gathering accurate usage
statistics on a server.  When used in this way it is tempting to think
that there is no danger in its use if illicit access to the protected
documents is not a major concern.  This is only correct if the server
issues both user name and password to the users and in particular does
not allow the user to choose his or her own password.  The danger arises
because naive users frequently reuse a single password to avoid the task
of maintaining multiple passwords.

If a server permits users to select their own passwords, then the threat
is not only illicit access to documents on the server but also illicit
access to the accounts of all users who have chosen to use their account
password.  If users are allowed to choose their own password that also
means the server must maintain files containing the (presumably
encrypted) passwords.  Many of these may be the account passwords of
users perhaps at distant sites.  The owner or administrator of such a
system could conceivably incur liability if this information is not
maintained in a secure fashion.

Basic Authentication is also vulnerable to spoofing by counterfeit
servers.  If a user can be led to believe that he is connecting to a
host containing information protected by basic authentication when in
fact he is connecting to a hostile server or gateway then the attacker
can request a password, store it for later use, and feign an error.
This type of attack is not possible with Digest Authentication[26].
Server implementers SHOULD guard against the possibility of this sort of
counterfeiting by gateways or CGI scripts.  In particular it is very
dangerous for a server to simply turn over a connection to a gateway
since that gateway can then use the persistent connection mechanism to


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engage in multiple transactions with the client while impersonating the
original server in a way that is not detectable by the client.


19.2 Safe Methods
The writers of client software should be aware that the software
represents the user in their interactions over the Internet, and should
be careful to allow the user to be aware of any actions they may take
which may have an unexpected significance to themselves or others.

In particular, the convention has been established that the GET and HEAD
methods should never have the significance of taking an action other
than retrieval. These methods should be considered  "safe. " This allows
user agents to represent other methods, such as POST, PUT and DELETE, in
a special way, so that the user is made aware of the fact that a
possibly unsafe action is being requested.

Naturally, it is not possible to ensure that the server does not
generate side-effects as a result of performing a GET request; in fact,
some dynamic resources consider that a feature. The important
distinction here is that the user did not request the side-effects, so
therefore cannot be held accountable for them.


19.3 Abuse of Server Log Information
A server is in the position to save personal data about a user's
requests which may identify their reading patterns or subjects of
interest. This information is clearly confidential in nature and its
handling may be constrained by law in certain countries. People using
the HTTP protocol to provide data are responsible for ensuring that such
material is not distributed without the permission of any individuals
that are identifiable by the published results.


19.4 Transfer of Sensitive Information
Like any generic data transfer protocol, HTTP cannot regulate the
content of the data that is transferred, nor is there any a priori
method of determining the sensitivity of any particular piece of
information within the context of any given request. Therefore,
applications SHOULD supply as much control over this information as
possible to the provider of that information. Four header fields are
worth special mention in this context: Server, Via, Referer and From.

Revealing the specific software version of the server may allow the
server machine to become more vulnerable to attacks against software
that is known to contain security holes. Implementers SHOULD make the
Server header field a configurable option.

Proxies which serve as a portal through a network firewall SHOULD take
special precautions regarding the transfer of header information that
identifies the hosts behind the firewall. In particular, they SHOULD
remove, or replace with sanitized versions, any Via fields generated
behind the firewall.


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The Referer field allows reading patterns to be studied and reverse
links drawn. Although it can be very useful, its power can be abused if
user details are not separated from the information contained in the
Referer. Even when the personal information has been removed, the
Referer field may indicate a private document's URI whose publication
would be inappropriate.

The information sent in the From field might conflict with the user's
privacy interests or their site's security policy, and hence it SHOULD
NOT be transmitted without the user being able to disable, enable, and
modify the contents of the field. The user MUST be able to set the
contents of this field within a user preference or application defaults
configuration.

We suggest, though do not require, that a convenient toggle interface be
provided for the user to enable or disable the sending of From and
Referer information.


19.5 Attacks Based On File and Path Names
Implementations of HTTP origin servers SHOULD be careful to restrict the
documents returned by HTTP requests to be only those that were intended
by the server administrators. If an HTTP server translates HTTP URIs
directly into file system calls, the server MUST take special care not
to serve files that were not intended to be delivered to HTTP clients.
For example, UNIX, Microsoft Windows, and other operating systems use
".." as a path component to indicate a directory level above the current
one. On such a system, an HTTP server MUST disallow any such construct
in the Request-URI if it would otherwise allow access to a resource
outside those intended to be accessible via the HTTP server. Similarly,
files intended for reference only internally to the server (such as
access control files, configuration files, and script code) MUST be
protected from inappropriate retrieval, since they might contain
sensitive information. Experience has shown that minor bugs in such HTTP
server implementations have turned into security risks.


19.6 Personal Information
HTTP clients are often privy to large amounts of personal information
(e.g. the user's name, location, mail address, passwords, encryption
keys, etc.), and SHOULD be very careful to prevent unintentional leakage
of this information via the HTTP protocol to other sources.  We very
strongly recommend that a convenient interface be provided for the user
to control dissemination of such information, and that designers and
implementers be particularly careful in this area. History shows that
errors in this area are often both serious security and/or privacy
problems, and often generate very adverse publicity for the
implementer's company.


19.7 Privacy Issues Connected to Accept headers
Accept request headers can reveal information about the user to all
servers which are accessed.  The Accept-Language header in particular
can reveal information the user would consider to be of a private

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nature, because the understanding of particular languages is often
strongly correlated to the membership of a particular ethnic group.
User agents which offer the option to configure the contents of an
Accept-Language header to be sent in every request are strongly
encouraged to let the configuration process include a message which
makes the user aware of the loss of privacy involved.

An approach that limits the loss of privacy would be for a user agent to
omit the sending of  Accept-Language headers by default, and to ask the
user whether it should start sending Accept-Language headers to a server
if it detects, by looking for any Vary or Alternates response headers
generated by the server, that such sending could improve the quality of
service.

Elaborate user-customized accept header fields sent in every request, in
particular if these include quality values, can be used by servers as
relatively reliable and long-lived user identifiers. Such user
identifiers would allow content providers to do click-trail tracking,
and would allow collaborating content providers to match cross-server
click-trails or form submissions of individual users.  Note that for
many users not behind a proxy, the network address of the host running
the user agent will also serve as a long-lived user identifier.  In
environments where proxies are used to enhance privacy, user agents
should be conservative in offering accept header configuration options
to end users.  As an extreme privacy measure, proxies could filter the
accept headers in relayed requests.  General purpose user agents which
provide a high degree of header configurability should warn users about
the loss of privacy which can be involved.


19.8 DNS Spoofing
Clients using HTTP rely heavily on the Domain Name Service, and are thus
generally prone to security attacks based on the deliberate miss-
association of IP addresses and DNS names.  The deployment of DNSSEC
should help this situation.  In advance of this deployment, however,
clients need to be cautious in assuming the continuing validity of an IP
number/DNS name association.

In particular, HTTP clients SHOULD rely on their name resolver for
confirmation of an IP number/DNS name association, rather than caching
the result of previous host name lookups.  Many platforms already can
cache host name lookups locally when appropriate, and they SHOULD be
configured to do so.  These lookups should be cached, however, only when
the TTL (Time To Live) information reported by the name server makes it
likely that the cached information will remain useful.

If HTTP clients cache the results of host name lookups in order to
achieve a performance improvement, they MUST observe the TTL information
reported by DNS.

If HTTP clients do not observe this rule, they could be spoofed when a
previously-accessed server's IP address changes.  As renumbering is
expected to become increasingly common, the possibility of this form of


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attack will grow.  Observing this requirement thus reduces this
potential security vulnerability.

This requirement also improves the load-balancing behavior of clients
for replicated servers using the same DNS name and reduces the
likelihood of a user's experiencing failure in accessing sites which use
that strategy.


19.9 Location Headers and Spoofing
If a single server supports multiple organizations that do not trust one
another, then it must check the values of Location and Content-Location
headers in responses that are generated under control of said
organizations to make sure that they do not attempt to invalidate
resources over which they have no authority.


20 Acknowledgments
This specification makes heavy use of the augmented BNF and generic
constructs defined by David H. Crocker for RFC 822 . Similarly, it
reuses many of the definitions provided by Nathaniel Borenstein and Ned
Freed for MIME . We hope that their inclusion in this specification will
help reduce past confusion over the relationship between HTTP and
Internet mail message formats.

The HTTP protocol has evolved considerably over the past four years. It
has benefited from a large and active developer community--the many
people who have participated on the www-talk mailing list--and it is
that community which has been most responsible for the success of HTTP
and of the World-Wide Web in general. Marc Andreessen, Robert Cailliau,
Daniel W. Connolly, Bob Denny, John Franks, Jean-Francois Groff, Phillip
M. Hallam-Baker, Hakon W. Lie, Ari Luotonen, Rob McCool, Lou Montulli,
Dave Raggett, Tony Sanders, and Marc VanHeyningen deserve special
recognition for their efforts in defining early aspects of the
protocol.

This document has benefited greatly from the comments of all those
participating in the HTTP-WG. In addition to those already mentioned,
the following individuals have contributed to this specification:

       Gary Adams                         Harald Tveit Alvestrand
       Keith Ball                         Brian Behlendorf
       Paul Burchard                      Maurizio Codogno
       Mike Cowlishaw                     Roman Czyborra
       Michael A. Dolan                   Alan Freier
       Marc Hedlund                       Koen Holtman
       Alex Hopmann                       Bob Jernigan
       Shel Kaphan                        Rohit Khare
       Martijn Koster                     Alexei Kosut
       David M. Kristol                    Daniel LaLiberte
       Paul J. Leach                      Albert Lunde
       John C. Mallery                    Jean-Philippe Martin-Flatin
       Larry Masinter                     Mitra
       Gavin Nicol                        Scott Powers
       Bill Perry                         Jeffrey Perry

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       Owen Rees                          Luigi Rizzo
       David Robinson                     Marc Salomon
       Rich Salz                          Jim Seidman
       Chuck Shotton                      Eric W. Sink
       Simon E. Spero                     Richard N. Taylor
       Robert S. Thau                     Francois Yergeau
       Mary Ellen Zurko                   David Morris
       Greg Herlihy                       Bill (BearHeart) Weinman
       Allan M. Schiffman


Much of the content and presentation of the caching design is due to
suggestions and comments from individuals including: Shel Kaphan, Paul
Leach, Koen Holtman, David Morris, Larry Masinter, and Roy Fielding.

Most of the specification of ranges is based on work originally done by
Ari Luotonen and John Franks, with additional input from Steve Zilles
and Roy Fielding.

XXX need acks for subgroup work.




21 References

[1]     H. Alvestrand. "Tags for the identification of languages." RFC

  1766, UNINETT, March 1995.

[2]     F. Anklesaria, M. McCahill, P. Lindner, D. Johnson, D. Torrey,
  B. Alberti. "The Internet Gopher Protocol: (a distributed document

  search and retrieval protocol)", RFC 1436, University of Minnesota,
  March 1993.

[3]     T. Berners-Lee. "Universal Resource Identifiers in WWW" A

  Unifying Syntax for the Expression of Names and Addresses of Objects
  on the Network as used in the World-Wide Web." RFC 1630, CERN, June
  1994.

[4]     T. Berners-Lee, L. Masinter, M. McCahill.
  "Uniform Resource Locators (URL)." RFC 1738, CERN, Xerox PARC,

  University of Minnesota, December 1994.

[5]     T. Berners-Lee, D. Connolly.
  "HyperText Markup Language Specification - 2.0." RFC 1866, MIT/LCS,

  November 1995.




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[6]     T. Berners-Lee, R. Fielding, H. Frystyk.
  "Hypertext Transfer Protocol - HTTP/1.0." Work in Progress (draft-

  ietf-http-v10-spec-04.txt), MIT/LCS, UC Irvine, September 1995.

[7]     N. Borenstein, N. Freed.
  "MIME (Multipurpose Internet Mail Extensions) Part One: Mechanisms

  for Specifying and Describing the Format of Internet Message Bodies."
  RFC 1521, Bellcore, Innosoft, September 1993.

[8]     R. Braden.
  "Requirements for Internet hosts - application and support." STD 3,

  RFC 1123, IETF, October 1989.

[9]     D. H. Crocker.
  "Standard for the Format of ARPA Internet Text Messages." STD 11, RFC

  822, UDEL, August 1982.

[10]    F. Davis, B. Kahle, H. Morris, J. Salem, T. Shen, R. Wang, J.
  Sui, M. Grinbaum. "WAIS Interface Protocol Prototype Functional
  Specification." (v1.5), Thinking Machines Corporation, April 1990.

[11]    R. Fielding. "Relative Uniform Resource Locators." RFC 1808, UC

  Irvine, June 1995.

[12]    M. Horton, R. Adams.
  "Standard for interchange of USENET messages." RFC 1036 (Obsoletes

  RFC 850), AT&amp;T Bell Laboratories, Center for Seismic Studies,
  December 1987.

[13]    B. Kantor, P. Lapsley. "Network News Transfer Protocol A

  Proposed Standard for the Stream-Based Transmission of News." RFC
  977, UC San Diego, UC Berkeley, February 1986.

[14]    K. Moore. "MIME (Multipurpose Internet Mail Extensions) Part Two

  : Message Header Extensions for Non-ASCII Text." RFC 1522, University
  of Tennessee, September 1993.

[15]    E. Nebel, L. Masinter. "Form-based File Upload in HTML." RFC

  1867, Xerox Corporation, November 1995.

[16]    J. Postel. "Simple Mail Transfer Protocol." STD 10, RFC 821,

  USC/ISI, August 1982.



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[17]    J. Postel. "Media Type Registration Procedure." RFC 1590,

  USC/ISI, March 1994.

[18]    J. Postel, J. K. Reynolds. "File Transfer Protocol (FTP)" STD
9,

  RFC 959, USC/ISI, October 1985.

[19]    J. Reynolds, J. Postel. "Assigned Numbers." STD 2, RFC 1700,

  USC/ISI, October 1994.

[20]    K. Sollins, L. Masinter.
  "Functional Requirements for Uniform Resource Names." RFC 1737,

  MIT/LCS, Xerox Corporation, December 1994.

[21]    US-ASCII. Coded Character Set - 7-Bit American Standard Code for
  Information Interchange. Standard ANSI X3.4-1986, ANSI, 1986.

[22]    ISO-8859. International Standard -- Information Processing --
  8-bit Single-Byte Coded Graphic Character Sets --
  Part 1: Latin alphabet No. 1, ISO 8859-1:1987.
  Part 2: Latin alphabet No. 2, ISO 8859-2, 1987.
  Part 3: Latin alphabet No. 3, ISO 8859-3, 1988.
  Part 4: Latin alphabet No. 4, ISO 8859-4, 1988.
  Part 5: Latin/Cyrillic alphabet, ISO 8859-5, 1988.
  Part 6: Latin/Arabic alphabet, ISO 8859-6, 1987.
  Part 7: Latin/Greek alphabet, ISO 8859-7, 1987.
  Part 8: Latin/Hebrew alphabet, ISO 8859-8, 1988.
  Part 9: Latin alphabet No. 5, ISO 8859-9, 1990.

[23]    Meyers, M. Rose "The Content-MD5 Header Field." RFC 1864,

  Carnegie Mellon, Dover Beach Consulting, October, 1995.

[24]    B. Carpenter, Y. Rekhter, "Renumbering Needs Work". RFC 1900,

  IAB, February 1996.

[25]    Gzip is available from the GNU project at
  <URL:ftp://prep.ai.mit.edu/pub/gnu/>.  A more formal specification is

  currently a work in progress.

[26]    Work In Progress for Digest authentication of the IETF HTTP
  working group.

[27]    TBS, Work in progress (XXX should put RFC in here_ )

[28]    Mills, D, "Network Time Protocol, Version 3", Specification,

  Implementation and Analysis RFC 1305, University of Delaware, March,
  1992.

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[29]    Work in progress of the HTTP working group (XXX is this correct
  reference for incomplete work?).

[30]    S. Spero. "Analysis of HTTP Performance Problems"
  <URL:http://sunsite.unc.edu/mdma-release/http-prob.html>

[31]    E. Rescorla, A. Schiffman "The Secure HyperText Transfer
  Protocol" Internet-Draft (work in progress).

[32]    A. Freier, P Karlton, P. Kocher. "SSL Version 3.0" Internet-
  Draft" (work in progress).

[33]    Jeffrey C. Mogul. "The Case for Persistent-Connection HTTP".  In
  Proc.SIGCOMM '95 Symposium on Communications Architectures and
  Protocols, pages 299-313. Cambridge, MA, August, 1995.

[34]    Jeffrey C. Mogul. "The Case for Persistent-Connection HTTP".
  Research, Report 95/4, Digital Equipment Corporation Western Research
  Laboratory, May, 1995.,
  <URL
  :http://www.research.digital.com/wrl/techreports/abstracts/95.4.html>


[35]    Work in progress of the HTTP working group on state management.

22 Authors' Addresses

Roy T. Fielding

Department of Information and Computer Science
University of California
Irvine, CA 92717-3425, USA
Fax: +1 (714) 824-4056
Email: fielding@ics.uci.edu

Henrik Frystyk Nielsen

W3 Consortium
MIT Laboratory for Computer Science
545 Technology Square
Cambridge, MA 02139, USA
Fax: +1 (617) 258 8682
Email: frystyk@w3.org

Tim Berners-Lee

Director, W3 Consortium
MIT Laboratory for Computer Science
545 Technology Square
Cambridge, MA 02139, USA
Fax: +1 (617) 258 8682
Email: timbl@w3.org



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Jim Gettys

MIT Laboratory for Computer Science
545 Technology Square
Cambridge, MA 02139, USA
Fax: +1 (617) 258 8682
Email: jg@w3.org

Jeffrey C. Mogul

Western Research Laboratory
Digital Equipment Corporation
250 University Avenue
Palo Alto, California, 94305, U.S.A.
Email: mogul@wrl.dec.com




23 Appendices
These appendices are provided for informational reasons only -- they do
not form a part of the HTTP/1.1 specification.


23.1 Internet Media Type message/http
In addition to defining the HTTP/1.1 protocol, this document serves as
the specification for the Internet media type "message/http". The
following is to be registered with IANA .

       Media Type name:         message
       Media subtype name:      http
       Required parameters:     none
       Optional parameters:     version, msgtype

              version: The HTTP-Version number of the enclosed message
                       (e.g., "1.1"). If not present, the version can be
                       determined from the first line of the body.

              msgtype: The message type -- "request" or "response". If not
                       present, the type can be determined from the first
                       line of the body.

       Encoding considerations: only "7bit", "8bit", or "binary" are
                                permitted

       Security considerations: none


23.2 Tolerant Applications
Although this document specifies the requirements for the generation of
HTTP/1.1 messages, not all applications will be correct in their
implementation. We therefore recommend that operational applications be

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tolerant of deviations whenever those deviations can be interpreted
unambiguously.

Clients SHOULD be tolerant in parsing the Status-Line and servers
tolerant when parsing the Request-Line. In particular, they SHOULD
accept any amount of SP or HT characters between fields, even though
only a single SP is required.

The line terminator for HTTP-header fields is the sequence CRLF.
However, we recommend that applications, when parsing such headers,
recognize a single LF as a line terminator and ignore the leading CR.


23.3 Differences Between  HTTP Bodies and RFC 1521 Internet Message Bodies
HTTP/1.1 uses many of the constructs defined for Internet Mail (RFC 822
) and the Multipurpose Internet Mail Extensions (MIME ) to allow
entities to be transmitted in an open variety of representations and
with extensible mechanisms. However, RFC 1521 discusses mail, and HTTP
has a few features that are different than those described in RFC 1521.
These differences were carefully chosen to optimize performance over
binary connections, to allow greater freedom in the use of new media
types, to make date comparisons easier, and to acknowledge the practice
of some early HTTP servers and clients.

At the time of this writing, it is expected that RFC 1521 will be
revised. The revisions may include some of the practices found in
HTTP/1.1 but not in RFC 1521.

This appendix describes specific areas where HTTP differs from RFC
1521.
Proxies and gateways to strict MIME environments SHOULD be aware of
these differences and provide the appropriate conversions where
necessary. Proxies and gateways from MIME environments to HTTP also need
to be aware of the differences because some conversions may be
required.


23.3.1 Conversion to Canonical Form
RFC 1521 requires that an Internet mail entity be converted to canonical
form prior to being transferred, as described in Appendix G of RFC 1521
. Section 7.7.1 of this document describes the forms allowed for
subtypes of the "text" media type when transmitted over HTTP.  RFC 1521
requires that content with a  typeof "text" represent line breaks as
CRLF and forbids the use of CR or LF outside of line break sequences.
HTTP allows CRLF, bare CR, and bare LF to indicate a line break within
text content when a message is transmitted over HTTP.

Where it is possible, a proxy or gateway from HTTP to a strict RFC 1521
environment SHOULD translate all line breaks within the text media types
described in section 7.7.1 of this document to the RFC 1521 canonical
form of CRLF. Note, however, that this may be complicated by the
presence of a Content-Encoding and by the fact that HTTP allows the use
of some character sets which do not use octets 13 and 10 to represent CR
and LF, as is the case for some multi-byte character sets.


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23.3.2 Conversion of Date Formats
HTTP/1.1 uses a restricted set of date formats (section 7.3.1) to
simplify the process of date comparison. Proxies and gateways from other
protocols SHOULD ensure that any Date header field present in a message
conforms to one of the HTTP/1.1 formats and rewrite the date if
necessary.


23.3.3 Introduction of Content-Encoding
RFC 1521 does not include any concept equivalent to HTTP/1.1's Content-
Encoding header field. Since this acts as a modifier on the media type,
proxies and gateways from HTTP to MIME-compliant protocols MUST either
change the value of the Content-Type header field or decode the Entity-
Body before forwarding the message. (Some experimental applications of
Content-Type for Internet mail have used a media-type parameter of
";conversions=<content-coding>" to perform an equivalent function as
Content-Encoding. However, this parameter is not part of RFC 1521.)


23.3.4 No Content-Transfer-Encoding
HTTP does not use the Content-Transfer-Encoding (CTE) field of RFC
1521.
Proxies and gateways from MIME-compliant protocols to HTTP MUST remove
any non-identity CTE ("quoted-printable" or "base64") encoding prior to
delivering the response message to an HTTP client.

Proxies and gateways from HTTP to MIME-compliant protocols are
responsible for ensuring that the message is in the correct format and
encoding for safe transport on that protocol, where "safe transport" is
defined by the limitations of the protocol being used. Such a proxy or
gateway SHOULD label the data with an appropriate Content-Transfer-
Encoding if doing so will improve the likelihood of safe transport over
the destination protocol.


23.3.5 HTTP Header Fields in Multipart Body-Parts
In RFC 1521, most header fields in multipart body-parts are generally
ignored unless the field name begins with "Content-". In HTTP/1.1,
multipart body-parts may contain any HTTP header fields which are
significant to the meaning of that part.


23.3.6 Introduction of Transfer-Encoding
HTTP/1.1 introduces the Transfer-Encoding header field (section 18.43).
Proxies/gateways MUST remove any transfer coding prior to forwarding a
message via a MIME-compliant protocol. The process for decoding the
"chunked" transfer coding (section 7.6) can be represented in pseudo-
code as:

       length := 0
       read chunk-size and CRLF
       while (chunk-size > 0) {
          read chunk-data and CRLF
          append chunk-data to Entity-Body
          length := length + chunk-size

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          read chunk-size and CRLF
       }
       read entity-header
       while (entity-header not empty) {
          append entity-header to existing header fields
          read entity-header
       }
       Content-Length := length
       Remove "chunked" from Transfer-Encoding




23.3.7 MIME-Version
HTTP is not a MIME-compliant protocol (see Appendix 23.3). However,
HTTP/1.1 messages may include a single MIME-Version general-header field
to indicate what version of the MIME protocol was used to construct the
message. Use of the MIME-Version header field indicates that the message
is in full compliance with the MIME protocol (as defined in ).
Proxies/gateways are responsible for ensuring full compliance (where
possible) when exporting HTTP messages to strict MIME environments.

       MIME-Version   = "MIME-Version" ":" 1*DIGIT "." 1*DIGIT

MIME version "1.0" is the default for use in HTTP/1.1. However,
HTTP/1.1
message parsing and semantics are defined by this document and not the
MIME specification.


23.4 Changes from HTTP/1.0
This section will summarize major differences between versions HTTP/1.0
and HTTP/1.1.


23.4.1 Changes to Simplify Multi-homed Web Servers and Conserve IP Addresses
The requirements that clients and servers support the Host  request-
header, report an error if the Host request-header (section 18.24) is
missing from an HTTP/1.1 request,  and accept absolute URIs (Section
9.1.2) are among the most important changes from HTTP/1.0.

In HTTP/1.0 there is a one-to-one relationship of IP addresses and
servers. There is no other way to distinguish the intended server of a
request than the IP address to which that request is directed. The
HTTP/1.1 change will allow the Internet, once HTTP/1.0 clients and
servers are no longer common, to support multiple Web sites from a
single IP address, greatly simplifying large operational Web servers,
where allocation of many IP addresses to a single  host has created
serious problems.  The Internet will also be able to recover the IP
addresses that have been used for the sole purpose of allowing root-
level domain names to be used in HTTP URLs. Given the rate of growth of
the Web, and the number of servers already deployed, it is extremely
important that implementations of HTTP/1.1 correctly implement these new
requirements:

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  .  both clients and servers MUST support the Host request-header

  .  Host request-headers are required in HTTP/1.1 requests.

  .  servers MUST report an error if an HTTP/1.1 request does not
     include a Host request-header

  .  servers MUST accept absolute URIs

23.5 Additional Features
This appendix documents protocol elements used by some existing HTTP
implementations, but not consistently and correctly across most
HTTP/1.1
applications. Implementers should be aware of these features, but cannot
rely upon their presence in, or interoperability with, other HTTP/1.1
applications.  Some of these describe proposed experimental features,
and some describe features that experimental deployment found lacking
that are now addressed in the base HTTP/1.1 specification.


23.5.1 Additional Request Methods

23.5.1.1 PATCH
The PATCH method is similar to PUT except that the entity contains a
list of differences between the original version of the resource
identified by the Request-URI and the desired content of the resource
entity  after the PATCH action has been applied. The list of differences
is in a format defined by the media type of the entity (e.g.,
"application/diff") and MUST include sufficient information to allow the
server to recreate the changes necessary to convert the original version
of the resource entity to the desired version.

If the request passes through a cache and the Request-URI identifies a
currently cached entity, that entity MUST be removed from the cache.
Responses to this method are not cachable.

For compatibility with HTTP/1.0 applications, all PATCH requests MUST
include a valid Content-Length header field unless the server is known
to be HTTP/1.1 compliant. When sending a PATCH request to an HTTP/1.1
server, a client MUST use a valid Content-Length or the "chunked"
Transfer-Encoding. The server SHOULD respond with a 400 (Bad Request)
message if it cannot determine the length of the request message's
content, or with 411 (Length Required) if it wishes to insist on
receiving a valid Content-Length.

The actual method for determining how the patched resource is placed,
and what happens to its predecessor, is defined entirely by the origin
server. If the original version of the resource being patched included a
Content-Version header field, the request entity MUST include a
Derived-
From header field corresponding to the value of the original Content-
Version header field. Applications are encouraged to use these fields
for constructing versioning relationships and resolving version
conflicts.



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PATCH requests must obey the entity transmission requirements set out in
section 13.4.1.

Caches that implement PATCH should invalidate cached responses as
defined in section 16.10 for PUT.


23.5.1.2 LINK
The LINK method establishes one or more Link relationships between the
existing resource identified by the Request-URI and other existing
resources. The difference between LINK and other methods allowing links
to be established between resources is that the LINK method does not
allow any Entity-Body to be sent in the request and does not directly
result in the creation of new resources.

If the request passes through a cache and the Request-URI identifies a
currently cached entity, that entity MUST be removed from the cache.
Responses to this method are not cachable.

Caches that implement LINK should invalidate cached responses as defined
in section 16.10 for PUT.


23.5.1.3 UNLINK
The UNLINK method removes one or more Link relationships from the
existing resource identified by the Request-URI. These relationships may
have been established using the LINK method or by any other method
supporting the Link header. The removal of a link to a resource does not
imply that the resource ceases to exist or becomes inaccessible for
future references.

If the request passes through a cache and the Request-URI identifies a
currently cached entity, that entity MUST be removed from the cache.
Responses to this method are not cachable.

Caches that implement UNLINK should invalidate cached responses as
defined in section 16.10 for PUT.


23.5.1.4 PUT
To support the PATCH method, if the entity being PUT was derived from an
existing resource which included a Content-Version header field, the new
entity MUST include a Derived-From header field corresponding to the
value of the original Content-Version header field. Multiple Derived-
From values may be included if the entity was derived from multiple
resources with Content-Version information. Applications are encouraged
to use these fields for constructing versioning relationships and
resolving version conflicts.


23.5.2 Additional Header Field Definitions

23.5.2.1 Content-Version


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The Content-Version entity-header field defines the version tag
associated with a rendition of an evolving entity. Together with the
Derived-From field described in section 23.5.2.2, it allows a group of
people to work simultaneously on the creation of a work as an iterative
process. The field SHOULD be used to allow evolution of a particular
work along a single path. It SHOULD NOT be used to indicate derived
works or renditions in different representations. It MAY also me used as
an opaque value for comparing a cached entity's version with that of the
current resource entity.

       Content-Version = "Content-Version" ":" quoted-string

Examples of the Content-Version field include:

       Content-Version: "2.1.2"
       Content-Version: "Fred 19950116-12:26:48"
       Content-Version: "2.5a4-omega7"

The value of the Content-Version field SHOULD be considered opaque to
all parties but the origin server. A user agent MAY suggest a value for
the version of an entity transferred via a PUT request; however, only
the origin server can reliably assign that value.


23.5.2.2 Derived-From
The Derived-From entity-header field can be used to indicate the version
tag of the resource from which the enclosed entity was derived before
modifications were made by the sender. This field is used to help manage
the process of merging successive changes to a resource, particularly
when such changes are being made in parallel and from multiple sources.

       Derived-From   = "Derived-From" ":" quoted-string

An example use of the field is:

       Derived-From: "2.1.1"

The Derived-From field is required for PUT and PATCH requests if the
entity being sent was previously retrieved from the same URI and a
Content-Version header was included with the entity when it was last
retrieved.


23.5.2.3 Link
The Link entity-header field provides a means for describing a
relationship between two resources, generally between the requested
resource and some other resource. An entity MAY include multiple Link
values. Links at the metainformation level typically indicate
relationships like hierarchical structure and navigation paths. The Link
field is semantically equivalent to the <LINK> element in HTML .

       Link           = "Link" ":" #("<" URI ">" *( ";" link-param )



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       link-param     = ( ( "rel" "=" relationship )
                          | ( "rev" "=" relationship )
                          | ( "title" "=" quoted-string )
                          | ( "anchor" "=" <"> URI <"> )
                          | ( link-extension ) )

       link-extension = token [ "=" ( token | quoted-string ) ]

       relationship   = sgml-name
                      | ( <"> sgml-name *( SP sgml-name) <"> )

       sgml-name      = ALPHA *( ALPHA | DIGIT | "." | "-" )

Relationship values are case-insensitive and MAY be extended within the
constraints of the sgml-name syntax. The title parameter MAY be used to
label the destination of a link such that it can be used as
identification within a human-readable menu. The anchor parameter MAY be
used to indicate a source anchor other than the entire current
resource,
such as a fragment of this resource or a third resource.

Examples of usage include:

       Link: <http://www.cern.ch/TheBook/chapter2>; rel="Previous"

       Link: <mailto:timbl@w3.org>; rev="Made"; title="Tim Berners-Lee"

The first example indicates that chapter2 is previous to this resource
in a logical navigation path. The second indicates that the person
responsible for making the resource available is identified by the given
e-mail address.


23.5.2.4 URI
The URI header field has, in past versions of this specification, been
used as a combination of the existing Location, Content-Location, and
Alternates header fields. Its primary purpose has been to include a list
of additional URIs for the resource, including names and mirror
locations.  However, it has become clear that the combination of many
different functions within this single field has been a barrier to
consistently and correctly implementing any of those functions.
Furthermore, we believe that the identification of names and mirror
locations would be better performed via the Link header field. The URI
header field is therefore deprecated in favor of those other fields.

       URI-header    = "URI" ":" 1#( "<" URI ">" )


23.5.2.5 Compatibility with HTTP/1.0 Persistent Connections
Some clients and servers may wish to be compatible with some previous
implementations of persistent connections in HTTP/1.0 clients and
servers.  These implementations are faulty, and the new facilities in
HTTP/1.1 are designed to rectify these problems.   The fear was that
some existing 1.0 clients may be sending Keep-Alive to a proxy server
that doesn't understand Connection, which would then erroneously forward

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it to the next inbound server, which would establish the Keep-Alive
connection and result in a dead 1.0 proxy waiting for the close on the
response.  The result is that 1.0 clients must be prevented from using
Keep-Alive when talking to proxies.

However, talking to proxies is the most important use of persistent
connections, so that is clearly unacceptable.  Therefore, we need some
other mechanism for indicating a persistent connection is desired, which
is safe to use even when talking to an old proxy that ignores
Connection.  As it turns out, there are two ways to accomplish that:

1.
  Introduce a new keyword (persist) which is declared to be valid only
  when received from an HTTP/1.1 message.

2.
  Declare persistence to be the default for HTTP/1.1 messages and
  introduce a new keyword (close) for declaring non-persistence.

The following describes the original, buggy form of persistent
connections.

When connecting to an origin server an HTTP client MAY send the Keep-
Alive connection-token in addition to the Persist connection-token:

       Connection: Keep-Alive,Persist

An HTTP/1.0 server would then respond with the Keep-Alive connection
token and the client may proceed with an HTTP/1.0 (or Keep-Alive)
persistent connection.

An HTTP/1.1 server may also establish persistent connections with
HTTP/1.0 clients upon receipt of a Keep-Alive connection token.
However, a persistent connection with an HTTP/1.0 client cannot make use
of the chunked transfer-coding, and therefore MUST use a Content-Length
for marking the ending boundary of each Entity-Body.

A client MUST NOT send the Keep-Alive connection token to a proxy server
as HTTP/1.0 proxy servers do not obey the rules of HTTP/1.1 for parsing
the Connection header field.


23.5.2.5.1 The Keep-Alive Header
When the Keep-Alive connection-token has been transmitted with a request
or a response a Keep-Alive header field MAY also be included. The Keep-
Alive header field takes the following form:

       Keep-Alive-header = "Keep-Alive" ":" 0# keepalive-param

       keepalive-param = param-name "=" value

The Keep-Alive header itself is optional, and is used only if a
parameter is being sent. HTTP/1.1 does not define any parameters.




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INTERNET-DRAFT            HTTP/1.1      Friday, May 03, 1996


If the Keep-Alive header is sent, the corresponding connection token
MUST be transmitted. The Keep-Alive header MUST be ignored if received
without the connection token.


23.5.3 Compatibility with Previous Versions
It is beyond the scope of a protocol specification to mandate compliance
with previous versions.  HTTP/1.1 was deliberately designed, however, to
make supporting previous versions easy.  While we are contemplating a
separate document containing advice to implementers, we feel it worth
noting that at the time of composing this specification, we would expect
commercial HTTP/1.1 servers to:


  .  recognize the format of the Request-Line for HTTP/0.9, 1.0, and 1.1
     requests;

  .  understand any valid request in the format of HTTP/0.9, 1.0, or
     1.1;

  .  respond appropriately with a message in the same major version used
     by the client.
And  we would expect HTTP/1.1 clients to:


  .  recognize the format of the Status-Line for HTTP/1.0 and 1.1
     responses;

  .  understand any valid response in the format of HTTP/0.9, 1.0, or
     1.1.
For most implementations of HTTP/1.0, each connection is established by
the client prior to the request and closed by the server after sending
the response.  A few implementations implement the Keep-Alive version of
persistent connections described in section 23.5.2.5.1.





















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