source: draft-ietf-httpbis/18/draft-ietf-httpbis-p2-semantics-18.txt @ 2111

HTTPbis Working Group                                   R. Fielding, Ed.
Internet-Draft                                                     Adobe
Obsoletes: 2616 (if approved)                                  J. Gettys
Updates: 2817 (if approved)                               Alcatel-Lucent
Intended status: Standards Track                                J. Mogul
Expires: July 7, 2012                                                 HP
                                                              H. Frystyk
                                                               Microsoft
                                                             L. Masinter
                                                                   Adobe
                                                                P. Leach
                                                               Microsoft
                                                          T. Berners-Lee
                                                                 W3C/MIT
                                                           Y. Lafon, Ed.
                                                                     W3C
                                                         J. Reschke, Ed.
                                                              greenbytes
                                                         January 4, 2012


                  HTTP/1.1, part 2: Message Semantics
                   draft-ietf-httpbis-p2-semantics-18

Abstract

   The Hypertext Transfer Protocol (HTTP) is an application-level
   protocol for distributed, collaborative, hypertext information
   systems.  HTTP has been in use by the World Wide Web global
   information initiative since 1990.  This document is Part 2 of the
   seven-part specification that defines the protocol referred to as
   "HTTP/1.1" and, taken together, obsoletes RFC 2616.

   Part 2 defines the semantics of HTTP messages as expressed by request
   methods, request header fields, response status codes, and response
   header fields.

Editorial Note (To be removed by RFC Editor)

   Discussion of this draft should take place on the HTTPBIS working
   group mailing list (ietf-http-wg@w3.org), which is archived at
   <http://lists.w3.org/Archives/Public/ietf-http-wg/>.

   The current issues list is at
   <http://tools.ietf.org/wg/httpbis/trac/report/3> and related
   documents (including fancy diffs) can be found at
   <http://tools.ietf.org/wg/httpbis/>.




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   The changes in this draft are summarized in Appendix C.19.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
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   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
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   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on July 7, 2012.

Copyright Notice

   Copyright (c) 2012 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
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   described in the Simplified BSD License.

   This document may contain material from IETF Documents or IETF
   Contributions published or made publicly available before November
   10, 2008.  The person(s) controlling the copyright in some of this
   material may not have granted the IETF Trust the right to allow
   modifications of such material outside the IETF Standards Process.
   Without obtaining an adequate license from the person(s) controlling
   the copyright in such materials, this document may not be modified
   outside the IETF Standards Process, and derivative works of it may
   not be created outside the IETF Standards Process, except to format
   it for publication as an RFC or to translate it into languages other
   than English.

Table of Contents




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   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  6
     1.1.  Conformance and Error Handling . . . . . . . . . . . . . .  6
     1.2.  Syntax Notation  . . . . . . . . . . . . . . . . . . . . .  7
       1.2.1.  Core Rules . . . . . . . . . . . . . . . . . . . . . .  7
       1.2.2.  ABNF Rules defined in other Parts of the
               Specification  . . . . . . . . . . . . . . . . . . . .  7
   2.  Method . . . . . . . . . . . . . . . . . . . . . . . . . . . .  7
     2.1.  Overview of Methods  . . . . . . . . . . . . . . . . . . .  8
     2.2.  Method Registry  . . . . . . . . . . . . . . . . . . . . .  8
       2.2.1.  Considerations for New Methods . . . . . . . . . . . .  9
   3.  Header Fields  . . . . . . . . . . . . . . . . . . . . . . . .  9
     3.1.  Considerations for Creating Header Fields  . . . . . . . .  9
     3.2.  Request Header Fields  . . . . . . . . . . . . . . . . . . 11
     3.3.  Response Header Fields . . . . . . . . . . . . . . . . . . 12
   4.  Status Code and Reason Phrase  . . . . . . . . . . . . . . . . 13
     4.1.  Overview of Status Codes . . . . . . . . . . . . . . . . . 13
     4.2.  Status Code Registry . . . . . . . . . . . . . . . . . . . 15
       4.2.1.  Considerations for New Status Codes  . . . . . . . . . 15
   5.  Representation . . . . . . . . . . . . . . . . . . . . . . . . 16
     5.1.  Identifying the Resource Associated with a
           Representation . . . . . . . . . . . . . . . . . . . . . . 16
   6.  Method Definitions . . . . . . . . . . . . . . . . . . . . . . 17
     6.1.  Safe and Idempotent Methods  . . . . . . . . . . . . . . . 17
       6.1.1.  Safe Methods . . . . . . . . . . . . . . . . . . . . . 17
       6.1.2.  Idempotent Methods . . . . . . . . . . . . . . . . . . 17
     6.2.  OPTIONS  . . . . . . . . . . . . . . . . . . . . . . . . . 18
     6.3.  GET  . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
     6.4.  HEAD . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
     6.5.  POST . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
     6.6.  PUT  . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
     6.7.  DELETE . . . . . . . . . . . . . . . . . . . . . . . . . . 23
     6.8.  TRACE  . . . . . . . . . . . . . . . . . . . . . . . . . . 23
     6.9.  CONNECT  . . . . . . . . . . . . . . . . . . . . . . . . . 24
       6.9.1.  Establishing a Tunnel with CONNECT . . . . . . . . . . 25
   7.  Status Code Definitions  . . . . . . . . . . . . . . . . . . . 25
     7.1.  Informational 1xx  . . . . . . . . . . . . . . . . . . . . 26
       7.1.1.  100 Continue . . . . . . . . . . . . . . . . . . . . . 26
       7.1.2.  101 Switching Protocols  . . . . . . . . . . . . . . . 26
     7.2.  Successful 2xx . . . . . . . . . . . . . . . . . . . . . . 27
       7.2.1.  200 OK . . . . . . . . . . . . . . . . . . . . . . . . 27
       7.2.2.  201 Created  . . . . . . . . . . . . . . . . . . . . . 27
       7.2.3.  202 Accepted . . . . . . . . . . . . . . . . . . . . . 28
       7.2.4.  203 Non-Authoritative Information  . . . . . . . . . . 28
       7.2.5.  204 No Content . . . . . . . . . . . . . . . . . . . . 28
       7.2.6.  205 Reset Content  . . . . . . . . . . . . . . . . . . 29
       7.2.7.  206 Partial Content  . . . . . . . . . . . . . . . . . 29
     7.3.  Redirection 3xx  . . . . . . . . . . . . . . . . . . . . . 29
       7.3.1.  300 Multiple Choices . . . . . . . . . . . . . . . . . 30



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       7.3.2.  301 Moved Permanently  . . . . . . . . . . . . . . . . 31
       7.3.3.  302 Found  . . . . . . . . . . . . . . . . . . . . . . 32
       7.3.4.  303 See Other  . . . . . . . . . . . . . . . . . . . . 32
       7.3.5.  304 Not Modified . . . . . . . . . . . . . . . . . . . 33
       7.3.6.  305 Use Proxy  . . . . . . . . . . . . . . . . . . . . 33
       7.3.7.  306 (Unused) . . . . . . . . . . . . . . . . . . . . . 33
       7.3.8.  307 Temporary Redirect . . . . . . . . . . . . . . . . 33
     7.4.  Client Error 4xx . . . . . . . . . . . . . . . . . . . . . 34
       7.4.1.  400 Bad Request  . . . . . . . . . . . . . . . . . . . 34
       7.4.2.  401 Unauthorized . . . . . . . . . . . . . . . . . . . 34
       7.4.3.  402 Payment Required . . . . . . . . . . . . . . . . . 34
       7.4.4.  403 Forbidden  . . . . . . . . . . . . . . . . . . . . 34
       7.4.5.  404 Not Found  . . . . . . . . . . . . . . . . . . . . 35
       7.4.6.  405 Method Not Allowed . . . . . . . . . . . . . . . . 35
       7.4.7.  406 Not Acceptable . . . . . . . . . . . . . . . . . . 35
       7.4.8.  407 Proxy Authentication Required  . . . . . . . . . . 36
       7.4.9.  408 Request Timeout  . . . . . . . . . . . . . . . . . 36
       7.4.10. 409 Conflict . . . . . . . . . . . . . . . . . . . . . 36
       7.4.11. 410 Gone . . . . . . . . . . . . . . . . . . . . . . . 36
       7.4.12. 411 Length Required  . . . . . . . . . . . . . . . . . 37
       7.4.13. 412 Precondition Failed  . . . . . . . . . . . . . . . 37
       7.4.14. 413 Request Representation Too Large . . . . . . . . . 37
       7.4.15. 414 URI Too Long . . . . . . . . . . . . . . . . . . . 37
       7.4.16. 415 Unsupported Media Type . . . . . . . . . . . . . . 37
       7.4.17. 416 Requested Range Not Satisfiable  . . . . . . . . . 38
       7.4.18. 417 Expectation Failed . . . . . . . . . . . . . . . . 38
       7.4.19. 426 Upgrade Required . . . . . . . . . . . . . . . . . 38
     7.5.  Server Error 5xx . . . . . . . . . . . . . . . . . . . . . 38
       7.5.1.  500 Internal Server Error  . . . . . . . . . . . . . . 38
       7.5.2.  501 Not Implemented  . . . . . . . . . . . . . . . . . 39
       7.5.3.  502 Bad Gateway  . . . . . . . . . . . . . . . . . . . 39
       7.5.4.  503 Service Unavailable  . . . . . . . . . . . . . . . 39
       7.5.5.  504 Gateway Timeout  . . . . . . . . . . . . . . . . . 39
       7.5.6.  505 HTTP Version Not Supported . . . . . . . . . . . . 39
   8.  Date/Time Formats  . . . . . . . . . . . . . . . . . . . . . . 40
   9.  Header Field Definitions . . . . . . . . . . . . . . . . . . . 42
     9.1.  Allow  . . . . . . . . . . . . . . . . . . . . . . . . . . 42
     9.2.  Date . . . . . . . . . . . . . . . . . . . . . . . . . . . 43
     9.3.  Expect . . . . . . . . . . . . . . . . . . . . . . . . . . 44
     9.4.  From . . . . . . . . . . . . . . . . . . . . . . . . . . . 44
     9.5.  Location . . . . . . . . . . . . . . . . . . . . . . . . . 45
     9.6.  Max-Forwards . . . . . . . . . . . . . . . . . . . . . . . 46
     9.7.  Referer  . . . . . . . . . . . . . . . . . . . . . . . . . 47
     9.8.  Retry-After  . . . . . . . . . . . . . . . . . . . . . . . 47
     9.9.  Server . . . . . . . . . . . . . . . . . . . . . . . . . . 48
     9.10. User-Agent . . . . . . . . . . . . . . . . . . . . . . . . 48
   10. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 49
     10.1. Method Registry  . . . . . . . . . . . . . . . . . . . . . 49



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     10.2. Status Code Registry . . . . . . . . . . . . . . . . . . . 50
     10.3. Header Field Registration  . . . . . . . . . . . . . . . . 51
   11. Security Considerations  . . . . . . . . . . . . . . . . . . . 52
     11.1. Transfer of Sensitive Information  . . . . . . . . . . . . 52
     11.2. Encoding Sensitive Information in URIs . . . . . . . . . . 53
     11.3. Location Headers and Spoofing  . . . . . . . . . . . . . . 54
     11.4. Security Considerations for CONNECT  . . . . . . . . . . . 54
   12. Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 54
   13. References . . . . . . . . . . . . . . . . . . . . . . . . . . 54
     13.1. Normative References . . . . . . . . . . . . . . . . . . . 54
     13.2. Informative References . . . . . . . . . . . . . . . . . . 55
   Appendix A.  Changes from RFC 2616 . . . . . . . . . . . . . . . . 56
   Appendix B.  Collected ABNF  . . . . . . . . . . . . . . . . . . . 57
   Appendix C.  Change Log (to be removed by RFC Editor before
                publication)  . . . . . . . . . . . . . . . . . . . . 60
     C.1.  Since RFC 2616 . . . . . . . . . . . . . . . . . . . . . . 60
     C.2.  Since draft-ietf-httpbis-p2-semantics-00 . . . . . . . . . 60
     C.3.  Since draft-ietf-httpbis-p2-semantics-01 . . . . . . . . . 60
     C.4.  Since draft-ietf-httpbis-p2-semantics-02 . . . . . . . . . 61
     C.5.  Since draft-ietf-httpbis-p2-semantics-03 . . . . . . . . . 62
     C.6.  Since draft-ietf-httpbis-p2-semantics-04 . . . . . . . . . 62
     C.7.  Since draft-ietf-httpbis-p2-semantics-05 . . . . . . . . . 62
     C.8.  Since draft-ietf-httpbis-p2-semantics-06 . . . . . . . . . 63
     C.9.  Since draft-ietf-httpbis-p2-semantics-07 . . . . . . . . . 63
     C.10. Since draft-ietf-httpbis-p2-semantics-08 . . . . . . . . . 64
     C.11. Since draft-ietf-httpbis-p2-semantics-09 . . . . . . . . . 64
     C.12. Since draft-ietf-httpbis-p2-semantics-10 . . . . . . . . . 64
     C.13. Since draft-ietf-httpbis-p2-semantics-11 . . . . . . . . . 65
     C.14. Since draft-ietf-httpbis-p2-semantics-12 . . . . . . . . . 65
     C.15. Since draft-ietf-httpbis-p2-semantics-13 . . . . . . . . . 66
     C.16. Since draft-ietf-httpbis-p2-semantics-14 . . . . . . . . . 67
     C.17. Since draft-ietf-httpbis-p2-semantics-15 . . . . . . . . . 67
     C.18. Since draft-ietf-httpbis-p2-semantics-16 . . . . . . . . . 67
     C.19. Since draft-ietf-httpbis-p2-semantics-17 . . . . . . . . . 67
   Index  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68
















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1.  Introduction

   This document defines HTTP/1.1 request and response semantics.  Each
   HTTP message, as defined in [Part1], is in the form of either a
   request or a response.  An HTTP server listens on a connection for
   HTTP requests and responds to each request, in the order received on
   that connection, with one or more HTTP response messages.  This
   document defines the commonly agreed upon semantics of the HTTP
   uniform interface, the intentions defined by each request method, and
   the various response messages that might be expected as a result of
   applying that method to the target resource.

   This document is currently disorganized in order to minimize the
   changes between drafts and enable reviewers to see the smaller errata
   changes.  A future draft will reorganize the sections to better
   reflect the content.  In particular, the sections will be ordered
   according to the typical processing of an HTTP request message (after
   message parsing): resource mapping, methods, request modifying header
   fields, response status, status modifying header fields, and resource
   metadata.  The current mess reflects how widely dispersed these
   topics and associated requirements had become in [RFC2616].

1.1.  Conformance and Error Handling

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].

   This document defines conformance criteria for several roles in HTTP
   communication, including Senders, Recipients, Clients, Servers, User-
   Agents, Origin Servers, Intermediaries, Proxies and Gateways.  See
   Section 2 of [Part1] for definitions of these terms.

   An implementation is considered conformant if it complies with all of
   the requirements associated with its role(s).  Note that SHOULD-level
   requirements are relevant here, unless one of the documented
   exceptions is applicable.

   This document also uses ABNF to define valid protocol elements
   (Section 1.2).  In addition to the prose requirements placed upon
   them, Senders MUST NOT generate protocol elements that are invalid.

   Unless noted otherwise, Recipients MAY take steps to recover a usable
   protocol element from an invalid construct.  However, HTTP does not
   define specific error handling mechanisms, except in cases where it
   has direct impact on security.  This is because different uses of the
   protocol require different error handling strategies; for example, a
   Web browser may wish to transparently recover from a response where



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   the Location header field doesn't parse according to the ABNF,
   whereby in a systems control protocol using HTTP, this type of error
   recovery could lead to dangerous consequences.

1.2.  Syntax Notation

   This specification uses the ABNF syntax defined in Section 1.2 of
   [Part1] (which extends the syntax defined in [RFC5234] with a list
   rule).  Appendix B shows the collected ABNF, with the list rule
   expanded.

   The following core rules are included by reference, as defined in
   [RFC5234], Appendix B.1: ALPHA (letters), CR (carriage return), CRLF
   (CR LF), CTL (controls), DIGIT (decimal 0-9), DQUOTE (double quote),
   HEXDIG (hexadecimal 0-9/A-F/a-f), HTAB (horizontal tab), LF (line
   feed), OCTET (any 8-bit sequence of data), SP (space), and VCHAR (any
   visible US-ASCII character).

1.2.1.  Core Rules

   The core rules below are defined in [Part1]:

     BWS           = <BWS, defined in [Part1], Section 1.2.2>
     OWS           = <OWS, defined in [Part1], Section 1.2.2>
     RWS           = <RWS, defined in [Part1], Section 1.2.2>
     obs-text      = <obs-text, defined in [Part1], Section 1.2.2>
     quoted-string = <quoted-string, defined in [Part1], Section 3.2.3>
     token         = <token, defined in [Part1], Section 3.2.3>

1.2.2.  ABNF Rules defined in other Parts of the Specification

   The ABNF rules below are defined in other parts:

     absolute-URI  = <absolute-URI, defined in [Part1], Section 2.7>
     comment       = <comment, defined in [Part1], Section 3.2>
     partial-URI   = <partial-URI, defined in [Part1], Section 2.7>
     product       = <product, defined in [Part1], Section 5.2>
     URI-reference = <URI-reference, defined in [Part1], Section 2.7>

2.  Method

   The Method token indicates the request method to be performed on the
   target resource (Section 4.3 of [Part1]).  The method is case-
   sensitive.

     Method         = token

   The list of methods allowed by a resource can be specified in an



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   Allow header field (Section 9.1).  The status code of the response
   always notifies the client whether a method is currently allowed on a
   resource, since the set of allowed methods can change dynamically.
   An origin server SHOULD respond with the status code 405 (Method Not
   Allowed) if the method is known by the origin server but not allowed
   for the resource, and 501 (Not Implemented) if the method is
   unrecognized or not implemented by the origin server.  The methods
   GET and HEAD MUST be supported by all general-purpose servers.  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 6.

2.1.  Overview of Methods

   The methods listed below are defined in Section 6.

   +-------------+---------------+
   | Method Name | Defined in... |
   +-------------+---------------+
   | OPTIONS     | Section 6.2   |
   | GET         | Section 6.3   |
   | HEAD        | Section 6.4   |
   | POST        | Section 6.5   |
   | PUT         | Section 6.6   |
   | DELETE      | Section 6.7   |
   | TRACE       | Section 6.8   |
   | CONNECT     | Section 6.9   |
   +-------------+---------------+

   Note that this list is not exhaustive -- it does not include request
   methods defined in other specifications.

2.2.  Method Registry

   The HTTP Method Registry defines the name space for the Method token
   in the Request line of an HTTP request.

   Registrations MUST include the following fields:

   o  Method Name (see Section 2)

   o  Safe ("yes" or "no", see Section 6.1.1)

   o  Pointer to specification text

   Values to be added to this name space are subject to IETF review
   ([RFC5226], Section 4.1).




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   The registry itself is maintained at
   <http://www.iana.org/assignments/http-methods>.

2.2.1.  Considerations for New Methods

   When it is necessary to express new semantics for a HTTP request that
   aren't specific to a single application or media type, and currently
   defined methods are inadequate, it may be appropriate to register a
   new method.

   HTTP methods are generic; that is, they are potentially applicable to
   any resource, not just one particular media type, "type" of resource,
   or application.  As such, it is preferred that new HTTP methods be
   registered in a document that isn't specific to a single application,
   so that this is clear.

   Due to the parsing rules defined in Section 3.3 of [Part1],
   definitions of HTTP methods cannot prohibit the presence of a
   message-body on either the request or the response message (with
   responses to HEAD requests being the single exception).  Definitions
   of new methods cannot change this rule, but they can specify that
   only zero-length bodies (as opposed to absent bodies) are allowed.

   New method definitions need to indicate whether they are safe
   (Section 6.1.1), what semantics (if any) the request body has, and
   whether they are idempotent (Section 6.1.2).  They also need to state
   whether they can be cached ([Part6]); in particular what conditions a
   cache may store the response, and under what conditions such a stored
   response may be used to satisfy a subsequent request.

3.  Header Fields

   Header fields are key value pairs that can be used to communicate
   data about the message, its payload, the target resource, or about
   the connection itself (i.e., control data).  See Section 3.2 of
   [Part1] for a general definition of their syntax.

3.1.  Considerations for Creating Header Fields

   New header fields are registered using the procedures described in
   [RFC3864].

   The requirements for header field names are defined in Section 4.1 of
   [RFC3864].  Authors of specifications defining new fields are advised
   to keep the name as short as practical, and not to prefix them with
   "X-" if they are to be registered (either immediately or in the
   future).




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   New header field values typically have their syntax defined using
   ABNF ([RFC5234]), using the extensions defined in Section 1.2.1 of
   [Part1] as necessary, and are usually constrained to the range of
   ASCII characters.  Header fields needing a greater range of
   characters can use an encoding such as the one defined in [RFC5987].

   Because commas (",") are used as a generic delimiter between field-
   values, they need to be treated with care if they are allowed in the
   field-value's payload.  Typically, components that might contain a
   comma are protected with double-quotes using the quoted-string ABNF
   production (Section 3.2.3 of [Part1]).

   For example, a textual date and a URI (either of which might contain
   a comma) could be safely carried in field-values like these:

     Example-URI-Field: "http://example.com/a.html,foo",
                        "http://without-a-comma.example.com/"
     Example-Date-Field: "Sat, 04 May 1996", "Wed, 14 Sep 2005"

   Note that double quote delimiters almost always are used with the
   quoted-string production; using a different syntax inside double
   quotes will likely cause unnecessary confusion.

   Many header fields use a format including (case-insensitively) named
   parameters (for instance, Content-Type, defined in Section 6.8 of
   [Part3]).  Allowing both unquoted (token) and quoted (quoted-string)
   syntax for the parameter value enables recipients to use existing
   parser components.  When allowing both forms, the meaning of a
   parameter value ought to be independent of the syntax used for it
   (for an example, see the notes on parameter handling for media types
   in Section 2.3 of [Part3]).

   Authors of specifications defining new header fields are advised to
   consider documenting:

   o  Whether the field is a single value, or whether it can be a list
      (delimited by commas; see Section 3.2 of [Part1]).

      If it does not use the list syntax, document how to treat messages
      where the header field occurs multiple times (a sensible default
      would be to ignore the header field, but this might not always be
      the right choice).

      Note that intermediaries and software libraries might combine
      multiple header field instances into a single one, despite the
      header field not allowing this.  A robust format enables
      recipients to discover these situations (good example: "Content-
      Type", as the comma can only appear inside quoted strings; bad



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      example: "Location", as a comma can occur inside a URI).

   o  Under what conditions the header field can be used; e.g., only in
      responses or requests, in all messages, only on responses to a
      particular request method.

   o  Whether it is appropriate to list the field-name in the Connection
      header (i.e., if the header is to be hop-by-hop, see Section 8.1
      of [Part1]).

   o  Under what conditions intermediaries are allowed to modify the
      header field's value, insert or delete it.

   o  How the header might interact with caching (see [Part6]).

   o  Whether the header field is useful or allowable in trailers (see
      Section 5.1.1 of [Part1]).

   o  Whether the header field should be preserved across redirects.

3.2.  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 to the parameters on a programming language method
   invocation.
























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   +---------------------+------------------------+
   | Header Field Name   | Defined in...          |
   +---------------------+------------------------+
   | Accept              | Section 6.1 of [Part3] |
   | Accept-Charset      | Section 6.2 of [Part3] |
   | Accept-Encoding     | Section 6.3 of [Part3] |
   | Accept-Language     | Section 6.4 of [Part3] |
   | Authorization       | Section 4.1 of [Part7] |
   | Expect              | Section 9.3            |
   | From                | Section 9.4            |
   | Host                | Section 8.3 of [Part1] |
   | If-Match            | Section 3.1 of [Part4] |
   | If-Modified-Since   | Section 3.3 of [Part4] |
   | If-None-Match       | Section 3.2 of [Part4] |
   | If-Range            | Section 5.3 of [Part5] |
   | If-Unmodified-Since | Section 3.4 of [Part4] |
   | Max-Forwards        | Section 9.6            |
   | Proxy-Authorization | Section 4.3 of [Part7] |
   | Range               | Section 5.4 of [Part5] |
   | Referer             | Section 9.7            |
   | TE                  | Section 8.4 of [Part1] |
   | User-Agent          | Section 9.10           |
   +---------------------+------------------------+

3.3.  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 target resource (Section 4.3 of [Part1]).

   +--------------------+------------------------+
   | Header Field Name  | Defined in...          |
   +--------------------+------------------------+
   | Accept-Ranges      | Section 5.1 of [Part5] |
   | Age                | Section 3.1 of [Part6] |
   | Allow              | Section 9.1            |
   | Date               | Section 9.2            |
   | ETag               | Section 2.3 of [Part4] |
   | Location           | Section 9.5            |
   | Proxy-Authenticate | Section 4.2 of [Part7] |
   | Retry-After        | Section 9.8            |
   | Server             | Section 9.9            |
   | Vary               | Section 3.5 of [Part6] |
   | WWW-Authenticate   | Section 4.4 of [Part7] |
   +--------------------+------------------------+





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4.  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 and is intended for a human user.  The client does
   not need to examine or display the Reason-Phrase.

     Status-Code    = 3DIGIT
     Reason-Phrase  = *( HTAB / SP / VCHAR / obs-text )

   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 representation
   enclosed with the response, since that representation is likely to
   include human-readable information which will explain the unusual
   status.

4.1.  Overview of Status Codes

   The status codes listed below are defined in Section 7 of this
   specification, Section 4 of [Part4], Section 3 of [Part5], and
   Section 3 of [Part7].  The reason phrases listed here are only
   recommendations -- they can be replaced by local equivalents without
   affecting the protocol.

















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   +-------------+------------------------------+----------------------+
   | Status-Code | Reason-Phrase                | Defined in...        |
   +-------------+------------------------------+----------------------+
   | 100         | Continue                     | Section 7.1.1        |
   | 101         | Switching Protocols          | Section 7.1.2        |
   | 200         | OK                           | Section 7.2.1        |
   | 201         | Created                      | Section 7.2.2        |
   | 202         | Accepted                     | Section 7.2.3        |
   | 203         | Non-Authoritative            | Section 7.2.4        |
   |             | Information                  |                      |
   | 204         | No Content                   | Section 7.2.5        |
   | 205         | Reset Content                | Section 7.2.6        |
   | 206         | Partial Content              | Section 3.1 of       |
   |             |                              | [Part5]              |
   | 300         | Multiple Choices             | Section 7.3.1        |
   | 301         | Moved Permanently            | Section 7.3.2        |
   | 302         | Found                        | Section 7.3.3        |
   | 303         | See Other                    | Section 7.3.4        |
   | 304         | Not Modified                 | Section 4.1 of       |
   |             |                              | [Part4]              |
   | 305         | Use Proxy                    | Section 7.3.6        |
   | 307         | Temporary Redirect           | Section 7.3.8        |
   | 400         | Bad Request                  | Section 7.4.1        |
   | 401         | Unauthorized                 | Section 3.1 of       |
   |             |                              | [Part7]              |
   | 402         | Payment Required             | Section 7.4.3        |
   | 403         | Forbidden                    | Section 7.4.4        |
   | 404         | Not Found                    | Section 7.4.5        |
   | 405         | Method Not Allowed           | Section 7.4.6        |
   | 406         | Not Acceptable               | Section 7.4.7        |
   | 407         | Proxy Authentication         | Section 3.2 of       |
   |             | Required                     | [Part7]              |
   | 408         | Request Time-out             | Section 7.4.9        |
   | 409         | Conflict                     | Section 7.4.10       |
   | 410         | Gone                         | Section 7.4.11       |
   | 411         | Length Required              | Section 7.4.12       |
   | 412         | Precondition Failed          | Section 4.2 of       |
   |             |                              | [Part4]              |
   | 413         | Request Representation Too   | Section 7.4.14       |
   |             | Large                        |                      |
   | 414         | URI Too Long                 | Section 7.4.15       |
   | 415         | Unsupported Media Type       | Section 7.4.16       |
   | 416         | Requested range not          | Section 3.2 of       |
   |             | satisfiable                  | [Part5]              |
   | 417         | Expectation Failed           | Section 7.4.18       |
   | 426         | Upgrade Required             | Section 7.4.19       |
   | 500         | Internal Server Error        | Section 7.5.1        |
   | 501         | Not Implemented              | Section 7.5.2        |



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   | 502         | Bad Gateway                  | Section 7.5.3        |
   | 503         | Service Unavailable          | Section 7.5.4        |
   | 504         | Gateway Time-out             | Section 7.5.5        |
   | 505         | HTTP Version not supported   | Section 7.5.6        |
   +-------------+------------------------------+----------------------+

   Note that this list is not exhaustive -- it does not include
   extension status codes defined in other specifications.

4.2.  Status Code Registry

   The HTTP Status Code Registry defines the name space for the Status-
   Code token in the Status-Line of an HTTP response.

   Values to be added to this name space are subject to IETF review
   ([RFC5226], Section 4.1).

   The registry itself is maintained at
   <http://www.iana.org/assignments/http-status-codes>.

4.2.1.  Considerations for New Status Codes

   When it is necessary to express new semantics for a HTTP response
   that aren't specific to a single application or media type, and
   currently defined status codes are inadequate, a new status code can
   be registered.

   HTTP status codes are generic; that is, they are potentially
   applicable to any resource, not just one particular media type,
   "type" of resource, or application.  As such, it is preferred that
   new HTTP status codes be registered in a document that isn't specific
   to a single application, so that this is clear.

   Definitions of new HTTP status codes typically explain the request
   conditions that produce a response containing the status code (e.g.,
   combinations of request headers and/or method(s)), along with any
   interactions with response headers (e.g., those that are required,
   those that modify the semantics of the response).

   New HTTP status codes are required to fall under one of the
   categories defined in Section 7.  To allow existing parsers to
   properly handle them, new status codes cannot disallow a response
   body, although they can mandate a zero-length response body.  They
   can require the presence of one or more particular HTTP response
   header(s).

   Likewise, their definitions can specify that caches are allowed to
   use heuristics to determine their freshness (see [Part6]; by default,



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   it is not allowed), and can define how to determine the resource
   which they carry a representation for (see Section 5.1; by default,
   it is anonymous).

5.  Representation

   Request and Response messages MAY transfer a representation if not
   otherwise restricted by the request method or response status code.
   A representation consists of metadata (representation header fields)
   and data (representation body).  When a complete or partial
   representation is enclosed in an HTTP message, it is referred to as
   the payload of the message.  HTTP representations are defined in
   [Part3].

   A representation body is only present in a message when a message-
   body is present, as described in Section 3.3 of [Part1].  The
   representation body is obtained from the message-body by decoding any
   Transfer-Encoding that might have been applied to ensure safe and
   proper transfer of the message.

5.1.  Identifying the Resource Associated with a Representation

   It is sometimes necessary to determine an identifier for the resource
   associated with a representation.

   An HTTP request representation, when present, is always associated
   with an anonymous (i.e., unidentified) resource.

   In the common case, an HTTP response is a representation of the
   target resource (see Section 4.3 of [Part1]).  However, this is not
   always the case.  To determine the URI of the resource a response is
   associated with, the following rules are used (with the first
   applicable one being selected):

   1.  If the response status code is 200 or 203 and the request method
       was GET, the response payload is a representation of the target
       resource.

   2.  If the response status code is 204, 206, or 304 and the request
       method was GET or HEAD, the response payload is a partial
       representation of the target resource.

   3.  If the response has a Content-Location header field, and that URI
       is the same as the effective request URI, the response payload is
       a representation of the target resource.

   4.  If the response has a Content-Location header field, and that URI
       is not the same as the effective request URI, then the response



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       asserts that its payload is a representation of the resource
       identified by the Content-Location URI.  However, such an
       assertion cannot be trusted unless it can be verified by other
       means (not defined by HTTP).

   5.  Otherwise, the response is a representation of an anonymous
       (i.e., unidentified) resource.

   [[TODO-req-uri: The comparison function is going to have to be
   defined somewhere, because we already need to compare URIs for things
   like cache invalidation.]]

6.  Method Definitions

   The set of common request 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.

6.1.  Safe and Idempotent Methods

6.1.1.  Safe Methods

   Implementors need to be aware that the software represents the user
   in their interactions over the Internet, and need to allow the user
   to be aware of any actions they take which might have an unexpected
   significance to themselves or others.

   In particular, the convention has been established that the GET,
   HEAD, OPTIONS, and TRACE request methods SHOULD NOT have the
   significance of taking an action other than retrieval.  These request
   methods ought to 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.

6.1.2.  Idempotent Methods

   Request methods can also have the property of "idempotence" in that,
   aside from error or expiration issues, the intended effect of
   multiple identical requests is the same as for a single request.
   PUT, DELETE, and all safe request methods are idempotent.  It is



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   important to note that idempotence refers only to changes requested
   by the client: a server is free to change its state due to multiple
   requests for the purpose of tracking those requests, versioning of
   results, etc.

6.2.  OPTIONS

   The OPTIONS method requests information about the communication
   options available on the request/response chain identified by the
   effective request URI.  This method allows a 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.

   Responses to the OPTIONS method are not cacheable.

   If the OPTIONS request includes a message-body (as indicated by the
   presence of Content-Length or Transfer-Encoding), then the media type
   MUST be indicated by a Content-Type field.  Although this
   specification does not define any use for such a body, future
   extensions to HTTP might use the OPTIONS body to make more detailed
   queries on the server.

   If the request-target is an asterisk ("*"), the OPTIONS request is
   intended to apply to the server in general rather than to a specific
   resource.  Since a server's communication options typically depend on
   the resource, the "*" request is only useful as a "ping" or "no-op"
   type of method; it does nothing beyond allowing the client to test
   the capabilities of the server.  For example, this can be used to
   test a proxy for HTTP/1.1 compliance (or lack thereof).

   If the request-target 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 that indicate
   optional features implemented by the server and applicable to that
   resource (e.g., Allow), possibly including extensions not defined by
   this specification.  The response body, if any, SHOULD also include
   information about the communication options.  The format for such a
   body is not defined by this specification, but might be defined by
   future extensions to HTTP.  Content negotiation MAY be used to select
   the appropriate response format.  If no response body is included,
   the response MUST include a Content-Length field with a field-value
   of "0".

   The Max-Forwards header field MAY be used to target a specific proxy
   in the request chain (see Section 9.6).  If no Max-Forwards field is



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   present in the request, then the forwarded request MUST NOT include a
   Max-Forwards field.

6.3.  GET

   The GET method requests transfer of a current representation of the
   target resource.

   If the target resource is a data-producing process, it is the
   produced data which shall be returned as the representation 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, If-Unmodified-Since,
   If-Match, If-None-Match, or If-Range header field.  A conditional GET
   requests that the representation be transferred only under the
   circumstances described by the conditional header field(s).  The
   conditional GET request is intended to reduce unnecessary network
   usage by allowing cached representations 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 representation be transferred, as
   described in Section 5.4 of [Part5].  The partial GET request is
   intended to reduce unnecessary network usage by allowing partially-
   retrieved representations to be completed without transferring data
   already held by the client.

   Bodies on GET requests have no defined semantics.  Note that sending
   a body on a GET request might cause some existing implementations to
   reject the request.

   The response to a GET request is cacheable and MAY be used to satisfy
   subsequent GET and HEAD requests (see [Part6]).

   See Section 11.2 for security considerations when used for forms.

6.4.  HEAD

   The HEAD method is identical to GET except that the server MUST NOT
   return a message-body in the response.  The metadata contained in the
   HTTP header fields 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 metadata about the representation implied
   by the request without transferring the representation body.  This



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   method is often used for testing hypertext links for validity,
   accessibility, and recent modification.

   The response to a HEAD request is cacheable and MAY be used to
   satisfy a subsequent HEAD request; see [Part6].  It also MAY be used
   to update a previously cached representation from that resource; if
   the new field values indicate that the cached representation differs
   from the current representation (as would be indicated by a change in
   Content-Length, ETag or Last-Modified), then the cache MUST treat the
   cache entry as stale.

   Bodies on HEAD requests have no defined semantics.  Note that sending
   a body on a HEAD request might cause some existing implementations to
   reject the request.

6.5.  POST

   The POST method requests that the origin server accept the
   representation enclosed in the request as data to be processed by the
   target resource.  POST is designed to allow a uniform method to cover
   the following functions:

   o  Annotation of existing resources;

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

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

   o  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 effective request URI.

   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 code,
   depending on whether or not the response includes a representation
   that describes the result.

   If a resource has been created on the origin server, the response
   SHOULD be 201 (Created) and contain a representation which describes
   the status of the request and refers to the new resource, and a
   Location header field (see Section 9.5).

   Responses to POST requests are only cacheable when they include
   explicit freshness information (see Section 2.3.1 of [Part6]).  A



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   cached POST response with a Content-Location header field (see
   Section 6.7 of [Part3]) whose value is the effective Request URI MAY
   be used to satisfy subsequent GET and HEAD requests.

   Note that POST caching is not widely implemented.  However, the 303
   (See Other) response can be used to direct the user agent to retrieve
   a cacheable resource.

6.6.  PUT

   The PUT method requests that the state of the target resource be
   created or replaced with the state defined by the representation
   enclosed in the request message payload.  A successful PUT of a given
   representation would suggest that a subsequent GET on that same
   target resource will result in an equivalent representation being
   returned in a 200 (OK) response.  However, there is no guarantee that
   such a state change will be observable, since the target resource
   might be acted upon by other user agents in parallel, or might be
   subject to dynamic processing by the origin server, before any
   subsequent GET is received.  A successful response only implies that
   the user agent's intent was achieved at the time of its processing by
   the origin server.

   If the target resource does not have a current representation and the
   PUT successfully creates one, then the origin server MUST inform the
   user agent by sending a 201 (Created) response.  If the target
   resource does have a current representation and that representation
   is successfully modified in accordance with the state of the enclosed
   representation, then either a 200 (OK) or 204 (No Content) response
   SHOULD be sent to indicate successful completion of the request.

   Unrecognized header fields SHOULD be ignored (i.e., not saved as part
   of the resource state).

   An origin server SHOULD verify that the PUT representation is
   consistent with any constraints which the server has for the target
   resource that cannot or will not be changed by the PUT.  This is
   particularly important when the origin server uses internal
   configuration information related to the URI in order to set the
   values for representation metadata on GET responses.  When a PUT
   representation is inconsistent with the target resource, the origin
   server SHOULD either make them consistent, by transforming the
   representation or changing the resource configuration, or respond
   with an appropriate error message containing sufficient information
   to explain why the representation is unsuitable.  The 409 (Conflict)
   or 415 (Unsupported Media Type) status codes are suggested, with the
   latter being specific to constraints on Content-Type values.




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   For example, if the target resource is configured to always have a
   Content-Type of "text/html" and the representation being PUT has a
   Content-Type of "image/jpeg", then the origin server SHOULD do one
   of: (a) reconfigure the target resource to reflect the new media
   type; (b) transform the PUT representation to a format consistent
   with that of the resource before saving it as the new resource state;
   or, (c) reject the request with a 415 response indicating that the
   target resource is limited to "text/html", perhaps including a link
   to a different resource that would be a suitable target for the new
   representation.

   HTTP does not define exactly how a PUT method affects the state of an
   origin server beyond what can be expressed by the intent of the user
   agent request and the semantics of the origin server response.  It
   does not define what a resource might be, in any sense of that word,
   beyond the interface provided via HTTP.  It does not define how
   resource state is "stored", nor how such storage might change as a
   result of a change in resource state, nor how the origin server
   translates resource state into representations.  Generally speaking,
   all implementation details behind the resource interface are
   intentionally hidden by the server.

   The fundamental difference between the POST and PUT methods is
   highlighted by the different intent for the target resource.  The
   target resource in a POST request is intended to handle the enclosed
   representation as a data-accepting process, such as for a gateway to
   some other protocol or a document that accepts annotations.  In
   contrast, the target resource in a PUT request is intended to take
   the enclosed representation as a new or replacement value.  Hence,
   the intent of PUT is idempotent and visible to intermediaries, even
   though the exact effect is only known by the origin server.

   Proper interpretation of a PUT request presumes that the user agent
   knows what target resource is desired.  A service that is intended to
   select a proper URI on behalf of the client, after receiving a state-
   changing request, SHOULD be implemented using the POST method rather
   than PUT.  If the origin server will not make the requested PUT state
   change to the target resource and instead wishes to have it applied
   to a different resource, such as when the resource has been moved to
   a different URI, then the origin server MUST send a 301 (Moved
   Permanently) response; the user agent MAY then make its own decision
   regarding whether or not to redirect the request.

   A PUT request applied to the target resource MAY have side-effects on
   other resources.  For example, an article might have a URI for
   identifying "the current version" (a resource) which is separate from
   the URIs identifying each particular version (different resources
   that at one point shared the same state as the current version



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   resource).  A successful PUT request on "the current version" URI
   might therefore create a new version resource in addition to changing
   the state of the target resource, and might also cause links to be
   added between the related resources.

   An origin server SHOULD reject any PUT request that contains a
   Content-Range header field, since it might be misinterpreted as
   partial content (or might be partial content that is being mistakenly
   PUT as a full representation).  Partial content updates are possible
   by targeting a separately identified resource with state that
   overlaps a portion of the larger resource, or by using a different
   method that has been specifically defined for partial updates (for
   example, the PATCH method defined in [RFC5789]).

   Responses to the PUT method are not cacheable.  If a PUT request
   passes through a cache that has one or more stored responses for the
   effective request URI, those stored responses will be invalidated
   (see Section 2.5 of [Part6]).

6.7.  DELETE

   The DELETE method requests that the origin server delete the target
   resource.  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
   representation describing the status, 202 (Accepted) if the action
   has not yet been enacted, or 204 (No Content) if the action has been
   enacted but the response does not include a representation.

   Bodies on DELETE requests have no defined semantics.  Note that
   sending a body on a DELETE request might cause some existing
   implementations to reject the request.

   Responses to the DELETE method are not cacheable.  If a DELETE
   request passes through a cache that has one or more stored responses
   for the effective request URI, those stored responses will be
   invalidated (see Section 2.5 of [Part6]).

6.8.  TRACE

   The TRACE method requests a remote, application-layer loop-back of
   the request message.  The final recipient of the request SHOULD



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   reflect the message received back to the client as the message-body
   of a 200 (OK) response.  The final recipient is either the origin
   server or the first proxy to receive a Max-Forwards value of zero (0)
   in the request (see Section 9.6).  A TRACE request MUST NOT include a
   message-body.

   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
   information.  The value of the Via header field (Section 8.8 of
   [Part1]) 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 the request is valid, the response SHOULD have a Content-Type of
   "message/http" (see Section 9.3.1 of [Part1]) and contain a message-
   body that encloses a copy of the entire request message.  Responses
   to the TRACE method are not cacheable.

6.9.  CONNECT

   The CONNECT method requests that the proxy establish a tunnel to the
   request-target and then restrict its behavior to blind forwarding of
   packets until the connection is closed.

   When using CONNECT, the request-target MUST use the authority form
   (Section 3.1.1.2 of [Part1]); i.e., the request-target consists of
   only the host name and port number of the tunnel destination,
   separated by a colon.  For example,

     CONNECT server.example.com:80 HTTP/1.1
     Host: server.example.com:80


   Other HTTP mechanisms can be used normally with the CONNECT method --
   except end-to-end protocol Upgrade requests, since the tunnel must be
   established first.

   For example, proxy authentication might be used to establish the
   authority to create a tunnel:

     CONNECT server.example.com:80 HTTP/1.1
     Host: server.example.com:80
     Proxy-Authorization: basic aGVsbG86d29ybGQ=


   Bodies on CONNECT requests have no defined semantics.  Note that
   sending a body on a CONNECT request might cause some existing



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   implementations to reject the request.

   Like any other pipelined HTTP/1.1 request, data to be tunnel may be
   sent immediately after the blank line.  The usual caveats also apply:
   data may be discarded if the eventual response is negative, and the
   connection may be reset with no response if more than one TCP segment
   is outstanding.

6.9.1.  Establishing a Tunnel with CONNECT

   Any successful (2xx) response to a CONNECT request indicates that the
   proxy has established a connection to the requested host and port,
   and has switched to tunneling the current connection to that server
   connection.

   It may be the case that the proxy itself can only reach the requested
   origin server through another proxy.  In this case, the first proxy
   SHOULD make a CONNECT request of that next proxy, requesting a tunnel
   to the authority.  A proxy MUST NOT respond with any 2xx status code
   unless it has either a direct or tunnel connection established to the
   authority.

   An origin server which receives a CONNECT request for itself MAY
   respond with a 2xx status code to indicate that a connection is
   established.

   If at any point either one of the peers gets disconnected, any
   outstanding data that came from that peer will be passed to the other
   one, and after that also the other connection will be terminated by
   the proxy.  If there is outstanding data to that peer undelivered,
   that data will be discarded.

7.  Status Code Definitions

   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:

   o  1xx: Informational - Request received, continuing process

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

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

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



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   o  5xx: Server Error - The server failed to fulfill an apparently
      valid request

   Each Status-Code is described below, including any metadata required
   in the response.

7.1.  Informational 1xx

   This class of status code indicates a provisional response,
   consisting only of the Status-Line and optional header fields, and is
   terminated by an empty line.  There are no required header fields for
   this class of status code.  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.

   A client MUST be prepared to accept one or more 1xx status responses
   prior to a regular response, even if the client does not expect a 100
   (Continue) status message.  Unexpected 1xx status responses MAY be
   ignored by a user agent.

   Proxies MUST forward 1xx responses, unless the connection between the
   proxy and its client has been closed, or unless the proxy itself
   requested the generation of the 1xx response.  (For example, if a
   proxy adds a "Expect: 100-continue" field when it forwards a request,
   then it need not forward the corresponding 100 (Continue)
   response(s).)

7.1.1.  100 Continue

   The client SHOULD 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.  See Section 6.2.3 of [Part1] for detailed discussion of
   the use and handling of this status code.

7.1.2.  101 Switching Protocols

   The server understands and is willing to comply with the client's
   request, via the Upgrade message header field (Section 8.7 of
   [Part1]), 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 be switched only when it is advantageous to do



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   so.  For example, switching to a newer version of HTTP is
   advantageous over older versions, and switching to a real-time,
   synchronous protocol might be advantageous when delivering resources
   that use such features.

7.2.  Successful 2xx

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

7.2.1.  200 OK

   The request has succeeded.  The payload returned with the response is
   dependent on the method used in the request, for example:

   GET  a representation of the target resource is sent in the response;

   HEAD  the same representation as GET, except without the message-
      body;

   POST  a representation describing or containing the result of the
      action;

   TRACE  a representation containing the request message as received by
      the end server.

   Caches MAY use a heuristic (see Section 2.3.1.1 of [Part6]) to
   determine freshness for 200 responses.

7.2.2.  201 Created

   The request has been fulfilled and has resulted in a new resource
   being created.  The newly created resource can be referenced by the
   URI(s) returned in the payload of the response, with the most
   specific URI for the resource given by a Location header field.  The
   response SHOULD include a payload containing a list of resource
   characteristics and location(s) from which the user or user agent can
   choose the one most appropriate.  The payload format is specified by
   the media type given in the Content-Type header field.  The origin
   server MUST create the resource before returning the 201 status code.
   If the action cannot be carried out immediately, the server SHOULD
   respond with 202 (Accepted) response instead.

   A 201 response MAY contain an ETag response header field indicating
   the current value of the entity-tag for the representation of the
   resource just created (see Section 2.3 of [Part4]).





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7.2.3.  202 Accepted

   The request has been accepted for processing, but the processing has
   not been completed.  The request might or might not eventually be
   acted upon, as it might 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 representation 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.

7.2.4.  203 Non-Authoritative Information

   The representation in the response has been transformed or otherwise
   modified by a transforming proxy (Section 2.4 of [Part1]).  Note that
   the behaviour of transforming intermediaries is controlled by the no-
   transform Cache-Control directive (Section 3.2 of [Part6]).

   This status code is only appropriate when the response status code
   would have been 200 (OK) otherwise.  When the status code before
   transformation would have been different, the 214 Transformation
   Applied warn-code (Section 3.6 of [Part6]) is appropriate.

   Caches MAY use a heuristic (see Section 2.3.1.1 of [Part6]) to
   determine freshness for 203 responses.

7.2.5.  204 No Content

   The 204 (No Content) status code indicates that the server has
   successfully fulfilled the request and that there is no additional
   content to return in the response payload body.  Metadata in the
   response header fields refer to the target resource and its current
   representation after the requested action.

   For example, if a 204 status code is received in response to a PUT
   request and the response contains an ETag header field, then the PUT
   was successful and the ETag field-value contains the entity-tag for
   the new representation of that target resource.

   The 204 response allows a server to indicate that the action has been
   successfully applied to the target resource while implying that the
   user agent SHOULD NOT traverse away from its current "document view"



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   (if any).  The server assumes that the user agent will provide some
   indication of the success to its user, in accord with its own
   interface, and apply any new or updated metadata in the response to
   the active representation.

   For example, a 204 status code is commonly used with document editing
   interfaces corresponding to a "save" action, such that the document
   being saved remains available to the user for editing.  It is also
   frequently used with interfaces that expect automated data transfers
   to be prevalent, such as within distributed version control systems.

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

7.2.6.  205 Reset Content

   The server has fulfilled the request and the user agent SHOULD reset
   the document view which caused the request to be sent.  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 message-body included with the response MUST be empty.  Note that
   receivers still need to parse the response according to the algorithm
   defined in Section 3.3 of [Part1].

7.2.7.  206 Partial Content

   The server has fulfilled the partial GET request for the resource and
   the enclosed payload is a partial representation as defined in
   Section 3.1 of [Part5].

   Caches MAY use a heuristic (see Section 2.3.1.1 of [Part6]) to
   determine freshness for 206 responses.

7.3.  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.  If the
   required action involves a subsequent HTTP request, it 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 known to be "safe", as
   defined in Section 6.1.1.

   There are several types of redirects:

   1.  Redirects of the request to another URI, either temporarily or
       permanently.  The new URI is specified in the Location header



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       field.  In this specification, the status codes 301 (Moved
       Permanently), 302 (Found), and 307 (Temporary Redirect) fall
       under this category.

   2.  Redirection to a new location that represents an indirect
       response to the request, such as the result of a POST operation
       to be retrieved with a subsequent GET request.  This is status
       code 303 (See Other).

   3.  Redirection offering a choice of matching resources for use by
       agent-driven content negotiation (Section 5.2 of [Part3]).  This
       is status code 300 (Multiple Choices).

   4.  Other kinds of redirection, such as to a cached result (status
       code 304 (Not Modified)).

      Note: In HTTP/1.0, only the status codes 301 (Moved Permanently)
      and 302 (Found) were defined for the first type of redirect, and
      the second type did not exist at all ([RFC1945], Section 9.3).
      However it turned out that web forms using POST expected redirects
      to change the operation for the subsequent request to retrieval
      (GET).  To address this use case, HTTP/1.1 introduced the second
      type of redirect with the status code 303 (See Other) ([RFC2068],
      Section 10.3.4).  As user agents did not change their behavior to
      maintain backwards compatibility, the first revision of HTTP/1.1
      added yet another status code, 307 (Temporary Redirect), for which
      the backwards compatibility problems did not apply ([RFC2616],
      Section 10.3.8).  Over 10 years later, most user agents still do
      method rewriting for status codes 301 and 302, therefore this
      specification makes that behavior compliant in case the original
      request was POST.

   A Location header field on a 3xx response indicates that a client MAY
   automatically redirect to the URI provided; see Section 9.5.

   Clients SHOULD detect and intervene in cyclical redirections (i.e.,
   "infinite" redirection loops).

      Note: An earlier version of this specification recommended a
      maximum of five redirections ([RFC2068], Section 10.3).  Content
      developers need to be aware that some clients might implement such
      a fixed limitation.

7.3.1.  300 Multiple Choices

   The target resource has more than one representation, each with its
   own specific location, and agent-driven negotiation information
   (Section 5 of [Part3]) is being provided so that the user (or user



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   agent) can select a preferred representation by redirecting its
   request to that location.

   Unless it was a HEAD request, the response SHOULD include a
   representation containing a list of representation metadata and
   location(s) from which the user or user agent can choose the one most
   appropriate.  The data format is specified by the media type given in
   the Content-Type header field.  Depending upon the format and the
   capabilities of the user agent, selection of the most appropriate
   choice MAY be performed automatically.  However, this specification
   does not define any standard for such automatic selection.

   If the server has a preferred choice of representation, it SHOULD
   include the specific URI for that representation in the Location
   field; user agents MAY use the Location field value for automatic
   redirection.

   Caches MAY use a heuristic (see Section 2.3.1.1 of [Part6]) to
   determine freshness for 300 responses.

7.3.2.  301 Moved Permanently

   The target resource has been assigned a new permanent URI and any
   future references to this resource SHOULD use one of the returned
   URIs.  Clients with link editing capabilities ought to automatically
   re-link references to the effective request URI to one or more of the
   new references returned by the server, where possible.

   Caches MAY use a heuristic (see Section 2.3.1.1 of [Part6]) to
   determine freshness for 301 responses.

   The new permanent URI SHOULD be given by the Location field in the
   response.  Unless the request method was HEAD, the representation 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 method
   that is known to be "safe", as defined in Section 6.1.1, then the
   request MAY be automatically redirected by the user agent without
   confirmation.  Otherwise, 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: For historic reasons, user agents MAY change the request
      method from POST to GET for the subsequent request.  If this
      behavior is undesired, status code 307 (Temporary Redirect) can be
      used instead.




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7.3.3.  302 Found

   The target resource resides temporarily under a different URI.  Since
   the redirection might be altered on occasion, the client SHOULD
   continue to use the effective request URI for future requests.

   The temporary URI SHOULD be given by the Location field in the
   response.  Unless the request method was HEAD, the representation 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 method
   that is known to be "safe", as defined in Section 6.1.1, then the
   request MAY be automatically redirected by the user agent without
   confirmation.  Otherwise, 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: For historic reasons, user agents MAY change the request
      method from POST to GET for the subsequent request.  If this
      behavior is undesired, status code 307 (Temporary Redirect) can be
      used instead. [[issue312: but see
      <http://trac.tools.ietf.org/wg/httpbis/trac/ticket/312>]]

7.3.4.  303 See Other

   The 303 status code indicates that the server is redirecting the user
   agent to a different resource, as indicated by a URI in the Location
   header field, that is intended to provide an indirect response to the
   original request.  In order to satisfy the original request, a user
   agent SHOULD perform a retrieval request using the Location URI (a
   GET or HEAD request if using HTTP), which may itself be redirected
   further, and present the eventual result as an answer to the original
   request.  Note that the new URI in the Location header field is not
   considered equivalent to the effective request URI.

   This status code is generally applicable to any HTTP method.  It is
   primarily used to allow the output of a POST action to redirect the
   user agent to a selected resource, since doing so provides the
   information corresponding to the POST response in a form that can be
   separately identified, bookmarked, and cached independent of the
   original request.

   A 303 response to a GET request indicates that the requested resource
   does not have a representation of its own that can be transferred by
   the server over HTTP.  The Location URI indicates a resource that is
   descriptive of the target resource, such that the follow-on
   representation might be useful to recipients without implying that it



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   adequately represents the target resource.  Note that answers to the
   questions of what can be represented, what representations are
   adequate, and what might be a useful description are outside the
   scope of HTTP and thus entirely determined by the URI owner(s).

   Except for responses to a HEAD request, the representation of a 303
   response SHOULD contain a short hypertext note with a hyperlink to
   the Location URI.

7.3.5.  304 Not Modified

   The response to the request has not been modified since the
   conditions indicated by the client's conditional GET request, as
   defined in Section 4.1 of [Part4].

7.3.6.  305 Use Proxy

   The 305 status code was defined in a previous version of this
   specification (see Appendix A), and is now deprecated.

7.3.7.  306 (Unused)

   The 306 status code was used in a previous version of the
   specification, is no longer used, and the code is reserved.

7.3.8.  307 Temporary Redirect

   The target resource resides temporarily under a different URI.  Since
   the redirection can change over time, the client SHOULD continue to
   use the effective request URI for future requests.

   The temporary URI SHOULD be given by the Location field in the
   response.  Unless the request method was HEAD, the representation of
   the response SHOULD contain a short hypertext note with a hyperlink
   to the new URI(s), since many pre-HTTP/1.1 user agents do not
   understand the 307 status code.  Therefore, the note SHOULD contain
   the information necessary for a user to repeat the original request
   on the new URI.

   If the 307 status code is received in response to a request method
   that is known to be "safe", as defined in Section 6.1.1, then the
   request MAY be automatically redirected by the user agent without
   confirmation.  Otherwise, 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.






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      Note: This status code is similar to 302 Found, except that it
      does not allow rewriting the request method from POST to GET.
      This specification defines no equivalent counterpart for 301 Moved
      Permanently.

7.4.  Client Error 4xx

   The 4xx class of status code is intended for cases in which the
   client seems to have erred.  Except when responding to a HEAD
   request, the server SHOULD include a representation 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.  User agents SHOULD display any included
   representation to the user.

   If the client is sending data, a server implementation using TCP
   SHOULD be careful to ensure that the client acknowledges receipt of
   the packet(s) containing the response, before the server closes the
   input connection.  If the client continues sending data to the server
   after the close, the server's TCP stack will send a reset packet to
   the client, which might erase the client's unacknowledged input
   buffers before they can be read and interpreted by the HTTP
   application.

7.4.1.  400 Bad Request

   The server cannot or will not process the request, due to a client
   error (e.g., malformed syntax).

7.4.2.  401 Unauthorized

   The request requires user authentication (see Section 3.1 of
   [Part7]).

7.4.3.  402 Payment Required

   This code is reserved for future use.

7.4.4.  403 Forbidden

   The server understood the request, but refuses to authorize it.
   Providing different user authentication credentials might be
   successful, but any credentials that were provided in the request are
   insufficient.  The request SHOULD NOT be repeated with the same
   credentials.

   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



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   reason for the refusal in the representation.  If the server does not
   wish to make this information available to the client, the status
   code 404 (Not Found) MAY be used instead.

7.4.5.  404 Not Found

   The server has not found anything matching the effective request URI.
   No indication is given of whether the condition is temporary or
   permanent.  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.
   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.

7.4.6.  405 Method Not Allowed

   The method specified in the Request-Line is not allowed for the
   target resource.  The response MUST include an Allow header field
   containing a list of valid methods for the requested resource.

7.4.7.  406 Not Acceptable

   The resource identified by the request is only capable of generating
   response representations which have content characteristics not
   acceptable according to the Accept and Accept-* header fields sent in
   the request (see Section 6 of [Part3]).

   Unless it was a HEAD request, the response SHOULD include a
   representation containing a list of available representation
   characteristics and location(s) from which the user or user agent can
   choose the one most appropriate.  The data format is specified by the
   media type given in the Content-Type header field.  Depending upon
   the format and the capabilities of the user agent, selection of the
   most appropriate choice MAY be performed automatically.  However,
   this specification does not define any standard for such automatic
   selection.

      Note: HTTP/1.1 servers are allowed to return responses which are
      not acceptable according to the accept header fields sent in the
      request.  In some cases, this might even be preferable to sending
      a 406 response.  User agents are encouraged to inspect the header
      fields of an incoming response to determine if it is acceptable.

   If the response could be unacceptable, a user agent SHOULD
   temporarily stop receipt of more data and query the user for a
   decision on further actions.




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7.4.8.  407 Proxy Authentication Required

   This code is similar to 401 (Unauthorized), but indicates that the
   client must first authenticate itself with the proxy (see Section 3.2
   of [Part7]).

7.4.9.  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.

7.4.10.  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 might 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 representation would include enough information
   for the user or user agent to fix the problem; however, that might
   not be possible and is not required.

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

7.4.11.  410 Gone

   The target resource is no longer available at the server and no
   forwarding address is known.  This condition is expected to be
   considered permanent.  Clients with link editing capabilities SHOULD
   delete references to the effective 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.

   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



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   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.

   Caches MAY use a heuristic (see Section 2.3.1.1 of [Part6]) to
   determine freshness for 410 responses.

7.4.12.  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 message-body
   in the request message.

7.4.13.  412 Precondition Failed

   The precondition given in one or more of the header fields evaluated
   to false when it was tested on the server, as defined in Section 4.2
   of [Part4].

7.4.14.  413 Request Representation Too Large

   The server is refusing to process a request because the request
   representation is larger than the server is willing or able to
   process.  The server MAY close the connection to prevent the client
   from continuing the request.

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

7.4.15.  414 URI Too Long

   The server is refusing to service the request because the effective
   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 URI "black hole" of
   redirection (e.g., a redirected URI 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 effective request URI.

7.4.16.  415 Unsupported Media Type

   The server is refusing to service the request because the request
   payload is in a format not supported by this request method on the
   target resource.



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7.4.17.  416 Requested Range Not Satisfiable

   The request included a Range header field (Section 5.4 of [Part5])
   and none of the range-specifier values in this field overlap the
   current extent of the selected resource.  See Section 3.2 of [Part5].

7.4.18.  417 Expectation Failed

   The expectation given in an Expect header field (see Section 9.3)
   could not be met by this server, or, if the server is a proxy, the
   server has unambiguous evidence that the request could not be met by
   the next-hop server.

7.4.19.  426 Upgrade Required

   The request can not be completed without a prior protocol upgrade.
   This response MUST include an Upgrade header field (Section 8.7 of
   [Part1]) specifying the required protocols.

   Example:

     HTTP/1.1 426 Upgrade Required
     Upgrade: HTTP/2.0
     Connection: Upgrade


   The server SHOULD include a message body in the 426 response which
   indicates in human readable form the reason for the error and
   describes any alternative courses which may be available to the user.

7.5.  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.  Except when responding to a HEAD request,
   the server SHOULD include a representation containing an explanation
   of the error situation, and whether it is a temporary or permanent
   condition.  User agents SHOULD display any included representation to
   the user.  These response codes are applicable to any request method.

7.5.1.  500 Internal Server Error

   The server encountered an unexpected condition which prevented it
   from fulfilling the request.







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7.5.2.  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.

7.5.3.  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.

7.5.4.  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 field (Section 9.8).  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 might
      wish to simply refuse the connection.

7.5.5.  504 Gateway Timeout

   The server, while acting as a gateway or proxy, did not receive a
   timely response from the upstream server specified by the URI (e.g.,
   HTTP, FTP, LDAP) or some other auxiliary server (e.g., DNS) it needed
   to access in attempting to complete the request.

      Note to implementors: some deployed proxies are known to return
      400 or 500 when DNS lookups time out.

7.5.6.  505 HTTP Version Not Supported

   The server does not support, or refuses to support, the 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
   2.6 of [Part1], other than with this error message.  The response
   SHOULD contain a representation describing why that version is not
   supported and what other protocols are supported by that server.




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8.  Date/Time Formats

   HTTP applications have historically allowed three different formats
   for date/time stamps.  However, the preferred format is a fixed-
   length subset of that defined by [RFC1123]:

     Sun, 06 Nov 1994 08:49:37 GMT  ; RFC 1123

   The other formats are described here only for compatibility with
   obsolete implementations.

     Sunday, 06-Nov-94 08:49:37 GMT ; obsolete RFC 850 format
     Sun Nov  6 08:49:37 1994       ; ANSI C's asctime() format

   HTTP/1.1 clients and servers that parse a date value MUST accept all
   three formats (for compatibility with HTTP/1.0), though they MUST
   only generate the RFC 1123 format for representing HTTP-date values
   in header fields.

   All HTTP date/time stamps MUST be represented in Greenwich Mean Time
   (GMT), without exception.  For the purposes of HTTP, GMT is exactly
   equal to UTC (Coordinated Universal Time).  This is indicated in the
   first two formats by the inclusion of "GMT" as the three-letter
   abbreviation for time zone, and MUST be assumed when reading the
   asctime format.  HTTP-date is case sensitive and MUST NOT include
   additional whitespace beyond that specifically included as SP in the
   grammar.

     HTTP-date    = rfc1123-date / obs-date

   Preferred format:




















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     rfc1123-date = day-name "," SP date1 SP time-of-day SP GMT
     ; fixed length subset of the format defined in
     ; Section 5.2.14 of [RFC1123]

     day-name     = %x4D.6F.6E ; "Mon", case-sensitive
                  / %x54.75.65 ; "Tue", case-sensitive
                  / %x57.65.64 ; "Wed", case-sensitive
                  / %x54.68.75 ; "Thu", case-sensitive
                  / %x46.72.69 ; "Fri", case-sensitive
                  / %x53.61.74 ; "Sat", case-sensitive
                  / %x53.75.6E ; "Sun", case-sensitive

     date1        = day SP month SP year
                  ; e.g., 02 Jun 1982

     day          = 2DIGIT
     month        = %x4A.61.6E ; "Jan", case-sensitive
                  / %x46.65.62 ; "Feb", case-sensitive
                  / %x4D.61.72 ; "Mar", case-sensitive
                  / %x41.70.72 ; "Apr", case-sensitive
                  / %x4D.61.79 ; "May", case-sensitive
                  / %x4A.75.6E ; "Jun", case-sensitive
                  / %x4A.75.6C ; "Jul", case-sensitive
                  / %x41.75.67 ; "Aug", case-sensitive
                  / %x53.65.70 ; "Sep", case-sensitive
                  / %x4F.63.74 ; "Oct", case-sensitive
                  / %x4E.6F.76 ; "Nov", case-sensitive
                  / %x44.65.63 ; "Dec", case-sensitive
     year         = 4DIGIT

     GMT   = %x47.4D.54 ; "GMT", case-sensitive

     time-of-day  = hour ":" minute ":" second
                    ; 00:00:00 - 23:59:59

     hour         = 2DIGIT
     minute       = 2DIGIT
     second       = 2DIGIT

   The semantics of day-name, day, month, year, and time-of-day are the
   same as those defined for the RFC 5322 constructs with the
   corresponding name ([RFC5322], Section 3.3).

   Obsolete formats:

     obs-date     = rfc850-date / asctime-date





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     rfc850-date  = day-name-l "," SP date2 SP time-of-day SP GMT
     date2        = day "-" month "-" 2DIGIT
                    ; day-month-year (e.g., 02-Jun-82)

     day-name-l   = %x4D.6F.6E.64.61.79 ; "Monday", case-sensitive
            / %x54.75.65.73.64.61.79 ; "Tuesday", case-sensitive
            / %x57.65.64.6E.65.73.64.61.79 ; "Wednesday", case-sensitive
            / %x54.68.75.72.73.64.61.79 ; "Thursday", case-sensitive
            / %x46.72.69.64.61.79 ; "Friday", case-sensitive
            / %x53.61.74.75.72.64.61.79 ; "Saturday", case-sensitive
            / %x53.75.6E.64.61.79 ; "Sunday", case-sensitive


     asctime-date = day-name SP date3 SP time-of-day SP year
     date3        = month SP ( 2DIGIT / ( SP 1DIGIT ))
                    ; month day (e.g., Jun  2)

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

      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.

9.  Header Field Definitions

   This section defines the syntax and semantics of HTTP/1.1 header
   fields related to request and response semantics.

9.1.  Allow

   The "Allow" header field lists the set of methods advertised as
   supported by the target resource.  The purpose of this field is
   strictly to inform the recipient of valid request methods associated
   with the resource.

     Allow = #Method

   Example of use:

     Allow: GET, HEAD, PUT

   The actual set of allowed methods is defined by the origin server at
   the time of each request.




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   A proxy MUST NOT modify the Allow header field -- it does not need to
   understand all the methods specified in order to handle them
   according to the generic message handling rules.

9.2.  Date

   The "Date" header field represents the date and time at which the
   message was originated, having the same semantics as the Origination
   Date Field (orig-date) defined in Section 3.6.1 of [RFC5322].  The
   field value is an HTTP-date, as defined in Section 8; it MUST be sent
   in rfc1123-date format.

     Date = HTTP-date

   An example is

     Date: Tue, 15 Nov 1994 08:12:31 GMT

   Origin servers MUST include a Date header field in all responses,
   except in these cases:

   1.  If the response status code is 100 (Continue) or 101 (Switching
       Protocols), the response MAY include a Date header field, at the
       server's option.

   2.  If the response status code conveys a server error, e.g., 500
       (Internal Server Error) or 503 (Service Unavailable), and it is
       inconvenient or impossible to generate a valid Date.

   3.  If the server does not have a clock that can provide a reasonable
       approximation of the current time, its responses MUST NOT include
       a Date header field.

   A received message that does not have a Date header field MUST be
   assigned one by the recipient if the message will be cached by that
   recipient.

   Clients can use the Date header field as well; in order to keep
   request messages small, they are advised not to include it when it
   doesn't convey any useful information (as it is usually the case for
   requests that do not contain a payload).

   The HTTP-date sent in a Date header field SHOULD NOT represent a date
   and time subsequent to the generation of the message.  It SHOULD
   represent the best available approximation of the date and time of
   message generation, unless the implementation has no means of
   generating a reasonably accurate date and time.  In theory, the date
   ought to represent the moment just before the payload is generated.



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   In practice, the date can be generated at any time during the message
   origination without affecting its semantic value.

9.3.  Expect

   The "Expect" header field is used to indicate that particular server
   behaviors are required by the client.

     Expect       = 1#expectation

     expectation  = expect-name [ BWS "=" BWS expect-value ]
                                *( OWS ";" [ OWS expect-param ] )
     expect-param = expect-name [ BWS "=" BWS expect-value ]

     expect-name  = token
     expect-value = token / quoted-string

   If all received Expect header field(s) are syntactically valid but
   contain an expectation that the recipient does not understand or
   cannot comply with, the recipient MUST respond with a 417
   (Expectation Failed) status code.  A recipient of a syntactically
   invalid Expectation header field MUST respond with a 4xx status code
   other than 417.

   The only expectation defined by this specification is:

   100-continue

      The "100-continue" expectation is defined Section 6.2.3 of
      [Part1].  It does not support any expect-params.

   Comparison is case-insensitive for names (expect-name), and case-
   sensitive for values (expect-value).

   The Expect mechanism is hop-by-hop: the above requirements apply to
   any server, including proxies.  However, the Expect header field
   itself is end-to-end; it MUST be forwarded if the request is
   forwarded.

   Many older HTTP/1.0 and HTTP/1.1 applications do not understand the
   Expect header field.

9.4.  From

   The "From" 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
   Section 3.4 of [RFC5322]:



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     From    = mailbox

     mailbox = <mailbox, defined in [RFC5322], Section 3.4>

   An example is:

     From: webmaster@example.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 field 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.

   The client SHOULD NOT send the From header field without the user's
   approval, as it might 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.

9.5.  Location

   The "Location" header field is used to identify a newly created
   resource, or to redirect the recipient to a different location for
   completion of the request.

   For 201 (Created) responses, the Location is the URI of the new
   resource which was created by the request.  For 3xx responses, the
   location SHOULD indicate the server's preferred URI for automatic
   redirection to the resource.

   The field value consists of a single URI-reference.  When it has the
   form of a relative reference ([RFC3986], Section 4.2), the final
   value is computed by resolving it against the effective request URI
   ([RFC3986], Section 5).

     Location = URI-reference






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   Examples are:

     Location: http://www.example.org/pub/WWW/People.html#tim

     Location: /index.html

      Note: Some recipients attempt to recover from Location fields that
      are not valid URI references.  This specification does not mandate
      or define such processing, but does allow it (see Section 1.1).

   There are circumstances in which a fragment identifier in a Location
   URI would not be appropriate.  For instance, when it appears in a 201
   Created response, where the Location header field specifies the URI
   for the entire created resource.

      Note: This specification does not define precedence rules for the
      case where the original URI, as navigated to by the user agent,
      and the Location header field value both contain fragment
      identifiers.  Thus be aware that including fragment identifiers
      might inconvenience anyone relying on the semantics of the
      original URI's fragment identifier.

      Note: The Content-Location header field (Section 6.7 of [Part3])
      differs from Location in that the Content-Location identifies the
      most specific resource corresponding to the enclosed
      representation.  It is therefore possible for a response to
      contain header fields for both Location and Content-Location.

9.6.  Max-Forwards

   The "Max-Forwards" header field provides a mechanism with the TRACE
   (Section 6.8) and OPTIONS (Section 6.2) methods to limit the number
   of times that the request is forwarded by proxies.  This can be
   useful when the client is attempting to trace a request which appears
   to be failing or looping in mid-chain.

     Max-Forwards = 1*DIGIT

   The Max-Forwards value is a decimal integer indicating the remaining
   number of times this request message can be forwarded.

   Each recipient of a TRACE or OPTIONS request containing a Max-
   Forwards header field MUST check and update its value prior to
   forwarding the request.  If the received value is zero (0), the
   recipient MUST NOT forward the request; instead, it MUST respond as
   the final recipient.  If the received Max-Forwards value is greater
   than zero, then the forwarded message MUST contain an updated Max-
   Forwards field with a value decremented by one (1).



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   The Max-Forwards header field MAY be ignored for all other request
   methods.

9.7.  Referer

   The "Referer" [sic] header field allows the client to specify the URI
   of the resource from which the effective request URI was obtained
   (the "referrer", although the header field is misspelled.).

   The Referer header field allows servers 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.
   Some servers use Referer as a means of controlling where they allow
   links from (so-called "deep linking"), but legitimate requests do not
   always contain a Referer header field.

   If the effective request URI was obtained from a source that does not
   have its own URI (e.g., input from the user keyboard), the Referer
   field MUST either be sent with the value "about:blank", or not be
   sent at all.  Note that this requirement does not apply to sources
   with non-HTTP URIs (e.g., FTP).

     Referer = absolute-URI / partial-URI

   Example:

     Referer: http://www.example.org/hypertext/Overview.html

   If the field value is a relative URI, it SHOULD be interpreted
   relative to the effective request URI.  The URI MUST NOT include a
   fragment.  See Section 11.2 for security considerations.

9.8.  Retry-After

   The header "Retry-After" 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.  This field MAY also be used
   with any 3xx (Redirection) response to indicate the minimum time the
   user-agent is asked wait before issuing the redirected request.

   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 = HTTP-date / delta-seconds

   Time spans are non-negative decimal integers, representing time in
   seconds.




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     delta-seconds  = 1*DIGIT

   Two examples of its use are

     Retry-After: Fri, 31 Dec 1999 23:59:59 GMT
     Retry-After: 120

   In the latter example, the delay is 2 minutes.

9.9.  Server

   The "Server" header field contains information about the software
   used by the origin server to handle the request.

   The field can contain multiple product tokens (Section 5.2 of
   [Part1]) and comments (Section 3.2 of [Part1]) identifying the server
   and any significant subproducts.  The product tokens are listed in
   order of their significance for identifying the application.

     Server = product *( RWS ( product / comment ) )

   Example:

     Server: CERN/3.0 libwww/2.17

   If the response is being forwarded through a proxy, the proxy
   application MUST NOT modify the Server header field.  Instead, it
   MUST include a Via field (as described in Section 8.8 of [Part1]).

      Note: Revealing the specific software version of the server might
      allow the server machine to become more vulnerable to attacks
      against software that is known to contain security holes.  Server
      implementors are encouraged to make this field a configurable
      option.

9.10.  User-Agent

   The "User-Agent" header field contains information about the user
   agent originating the request.  User agents SHOULD include this field
   with requests.

   Typically, it is used for statistical purposes, the tracing of
   protocol violations, and tailoring responses to avoid particular user
   agent limitations.

   The field can contain multiple product tokens (Section 5.2 of
   [Part1]) and comments (Section 3.2 of [Part1]) identifying the agent
   and its significant subproducts.  By convention, the product tokens



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   are listed in order of their significance for identifying the
   application.

   Because this field is usually sent on every request a user agent
   makes, implementations are encouraged not to include needlessly fine-
   grained detail, and to limit (or even prohibit) the addition of
   subproducts by third parties.  Overly long and detailed User-Agent
   field values make requests larger and can also be used to identify
   ("fingerprint") the user against their wishes.

   Likewise, implementations are encouraged not to use the product
   tokens of other implementations in order to declare compatibility
   with them, as this circumvents the purpose of the field.  Finally,
   they are encouraged not to use comments to identify products; doing
   so makes the field value more difficult to parse.

     User-Agent = product *( RWS ( product / comment ) )

   Example:

     User-Agent: CERN-LineMode/2.15 libwww/2.17b3

10.  IANA Considerations

10.1.  Method Registry

   The registration procedure for HTTP request methods is defined by
   Section 2.2 of this document.

   The HTTP Method Registry shall be created at
   <http://www.iana.org/assignments/http-methods> and be populated with
   the registrations below:

   +---------+------+-------------+
   | Method  | Safe | Reference   |
   +---------+------+-------------+
   | CONNECT | no   | Section 6.9 |
   | DELETE  | no   | Section 6.7 |
   | GET     | yes  | Section 6.3 |
   | HEAD    | yes  | Section 6.4 |
   | OPTIONS | yes  | Section 6.2 |
   | POST    | no   | Section 6.5 |
   | PUT     | no   | Section 6.6 |
   | TRACE   | yes  | Section 6.8 |
   +---------+------+-------------+






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10.2.  Status Code Registry

   The registration procedure for HTTP Status Codes -- previously
   defined in Section 7.1 of [RFC2817] -- is now defined by Section 4.2
   of this document.

   The HTTP Status Code Registry located at
   <http://www.iana.org/assignments/http-status-codes> shall be updated
   with the registrations below:










































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   +-------+----------------------------------+----------------+
   | Value | Description                      | Reference      |
   +-------+----------------------------------+----------------+
   | 100   | Continue                         | Section 7.1.1  |
   | 101   | Switching Protocols              | Section 7.1.2  |
   | 200   | OK                               | Section 7.2.1  |
   | 201   | Created                          | Section 7.2.2  |
   | 202   | Accepted                         | Section 7.2.3  |
   | 203   | Non-Authoritative Information    | Section 7.2.4  |
   | 204   | No Content                       | Section 7.2.5  |
   | 205   | Reset Content                    | Section 7.2.6  |
   | 300   | Multiple Choices                 | Section 7.3.1  |
   | 301   | Moved Permanently                | Section 7.3.2  |
   | 302   | Found                            | Section 7.3.3  |
   | 303   | See Other                        | Section 7.3.4  |
   | 305   | Use Proxy                        | Section 7.3.6  |
   | 306   | (Unused)                         | Section 7.3.7  |
   | 307   | Temporary Redirect               | Section 7.3.8  |
   | 400   | Bad Request                      | Section 7.4.1  |
   | 402   | Payment Required                 | Section 7.4.3  |
   | 403   | Forbidden                        | Section 7.4.4  |
   | 404   | Not Found                        | Section 7.4.5  |
   | 405   | Method Not Allowed               | Section 7.4.6  |
   | 406   | Not Acceptable                   | Section 7.4.7  |
   | 407   | Proxy Authentication Required    | Section 7.4.8  |
   | 408   | Request Timeout                  | Section 7.4.9  |
   | 409   | Conflict                         | Section 7.4.10 |
   | 410   | Gone                             | Section 7.4.11 |
   | 411   | Length Required                  | Section 7.4.12 |
   | 413   | Request Representation Too Large | Section 7.4.14 |
   | 414   | URI Too Long                     | Section 7.4.15 |
   | 415   | Unsupported Media Type           | Section 7.4.16 |
   | 417   | Expectation Failed               | Section 7.4.18 |
   | 426   | Upgrade Required                 | Section 7.4.19 |
   | 500   | Internal Server Error            | Section 7.5.1  |
   | 501   | Not Implemented                  | Section 7.5.2  |
   | 502   | Bad Gateway                      | Section 7.5.3  |
   | 503   | Service Unavailable              | Section 7.5.4  |
   | 504   | Gateway Timeout                  | Section 7.5.5  |
   | 505   | HTTP Version Not Supported       | Section 7.5.6  |
   +-------+----------------------------------+----------------+

10.3.  Header Field Registration

   The Message Header Field Registry located at <http://www.iana.org/
   assignments/message-headers/message-header-index.html> shall be
   updated with the permanent registrations below (see [RFC3864]):




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   +-------------------+----------+----------+--------------+
   | Header Field Name | Protocol | Status   | Reference    |
   +-------------------+----------+----------+--------------+
   | Allow             | http     | standard | Section 9.1  |
   | Date              | http     | standard | Section 9.2  |
   | Expect            | http     | standard | Section 9.3  |
   | From              | http     | standard | Section 9.4  |
   | Location          | http     | standard | Section 9.5  |
   | Max-Forwards      | http     | standard | Section 9.6  |
   | Referer           | http     | standard | Section 9.7  |
   | Retry-After       | http     | standard | Section 9.8  |
   | Server            | http     | standard | Section 9.9  |
   | User-Agent        | http     | standard | Section 9.10 |
   +-------------------+----------+----------+--------------+

   The change controller is: "IETF (iesg@ietf.org) - Internet
   Engineering Task Force".

11.  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 solutions to the problems revealed, though it does make
   some suggestions for reducing security risks.

11.1.  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 might allow the
   server machine to become more vulnerable to attacks against software
   that is known to contain security holes.  Implementors 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.

   The Referer header field allows reading patterns to be studied and



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   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 header field might 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.

   The User-Agent (Section 9.10) or Server (Section 9.9) header fields
   can sometimes be used to determine that a specific client or server
   have a particular security hole which might be exploited.
   Unfortunately, this same information is often used for other valuable
   purposes for which HTTP currently has no better mechanism.

   Furthermore, the User-Agent header field may contain enough entropy
   to be used, possibly in conjunction with other material, to uniquely
   identify the user.

   Some request methods, like TRACE (Section 6.8), expose information
   that was sent in request header fields within the body of their
   response.  Clients SHOULD be careful with sensitive information, like
   Cookies, Authorization credentials, and other header fields that
   might be used to collect data from the client.

11.2.  Encoding Sensitive Information in URIs

   Because the source of a link might be private information or might
   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.

   Clients SHOULD NOT include a Referer header field in a (non-secure)
   HTTP request if the referring page was transferred with a secure
   protocol.

   Authors of services SHOULD NOT use GET-based forms for the submission



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   of sensitive data because that data will be placed in the request-
   target.  Many existing servers, proxies, and user agents log or
   display the request-target in places where it might be visible to
   third parties.  Such services can use POST-based form submission
   instead.

11.3.  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 header fields 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.

11.4.  Security Considerations for CONNECT

   Since tunneled data is opaque to the proxy, there are additional
   risks to tunneling to other well-known or reserved ports.  A HTTP
   client CONNECTing to port 25 could relay spam via SMTP, for example.
   As such, proxies SHOULD restrict CONNECT access to a small number of
   known ports.

12.  Acknowledgments

   See Section 11 of [Part1].

13.  References

13.1.  Normative References

   [Part1]    Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H.,
              Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., Ed.,
              and J. Reschke, Ed., "HTTP/1.1, part 1: URIs, Connections,
              and Message Parsing", draft-ietf-httpbis-p1-messaging-18
              (work in progress), January 2012.

   [Part3]    Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H.,
              Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., Ed.,
              and J. Reschke, Ed., "HTTP/1.1, part 3: Message Payload
              and Content Negotiation", draft-ietf-httpbis-p3-payload-18
              (work in progress), January 2012.

   [Part4]    Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H.,
              Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., Ed.,
              and J. Reschke, Ed., "HTTP/1.1, part 4: Conditional
              Requests", draft-ietf-httpbis-p4-conditional-18 (work in
              progress), January 2012.




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   [Part5]    Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H.,
              Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., Ed.,
              and J. Reschke, Ed., "HTTP/1.1, part 5: Range Requests and
              Partial Responses", draft-ietf-httpbis-p5-range-18 (work
              in progress), January 2012.

   [Part6]    Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H.,
              Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., Ed.,
              Nottingham, M., Ed., and J. Reschke, Ed., "HTTP/1.1, part
              6: Caching", draft-ietf-httpbis-p6-cache-18 (work in
              progress), January 2012.

   [Part7]    Fielding, R., Ed., Gettys, J., Mogul, J., Frystyk, H.,
              Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., Ed.,
              and J. Reschke, Ed., "HTTP/1.1, part 7: Authentication",
              draft-ietf-httpbis-p7-auth-18 (work in progress),
              January 2012.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66,
              RFC 3986, January 2005.

   [RFC5234]  Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234, January 2008.

13.2.  Informative References

   [RFC1123]  Braden, R., "Requirements for Internet Hosts - Application
              and Support", STD 3, RFC 1123, October 1989.

   [RFC1945]  Berners-Lee, T., Fielding, R., and H. Nielsen, "Hypertext
              Transfer Protocol -- HTTP/1.0", RFC 1945, May 1996.

   [RFC2068]  Fielding, R., Gettys, J., Mogul, J., Nielsen, H., and T.
              Berners-Lee, "Hypertext Transfer Protocol -- HTTP/1.1",
              RFC 2068, January 1997.

   [RFC2616]  Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
              Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
              Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.

   [RFC2817]  Khare, R. and S. Lawrence, "Upgrading to TLS Within
              HTTP/1.1", RFC 2817, May 2000.

   [RFC3864]  Klyne, G., Nottingham, M., and J. Mogul, "Registration



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              Procedures for Message Header Fields", BCP 90, RFC 3864,
              September 2004.

   [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", BCP 26, RFC 5226,
              May 2008.

   [RFC5322]  Resnick, P., "Internet Message Format", RFC 5322,
              October 2008.

   [RFC5789]  Dusseault, L. and J. Snell, "PATCH Method for HTTP",
              RFC 5789, March 2010.

   [RFC5987]  Reschke, J., "Character Set and Language Encoding for
              Hypertext Transfer Protocol (HTTP) Header Field
              Parameters", RFC 5987, August 2010.

Appendix A.  Changes from RFC 2616

   This document takes over the Status Code Registry, previously defined
   in Section 7.1 of [RFC2817].  (Section 4.2)

   Clarify definition of POST.  (Section 6.5)

   Remove requirement to handle all Content-* header fields; ban use of
   Content-Range with PUT.  (Section 6.6)

   Take over definition of CONNECT method from [RFC2817].  (Section 6.9)

   Broadened the definition of 203 (Non-Authoritative Information) to
   include cases of payload transformations as well.  (Section 7.2.4)

   Failed to consider that there are many other request methods that are
   safe to automatically redirect, and further that the user agent is
   able to make that determination based on the request method
   semantics.  Furthermore, allow user agents to rewrite the method from
   POST to GET for status codes 301 and 302.  (Sections 7.3.2, 7.3.3 and
   7.3.8)

   Deprecate 305 Use Proxy status code, because user agents did not
   implement it.  It used to indicate that the target resource must be
   accessed through the proxy given by the Location field.  The Location
   field gave the URI of the proxy.  The recipient was expected to
   repeat this single request via the proxy.  (Section 7.3.6)

   Define status 426 (Upgrade Required) (this was incorporated from
   [RFC2817]).  (Section 7.4.19)




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   Change ABNF productions for header fields to only define the field
   value.  (Section 9)

   Reclassify "Allow" as response header field, removing the option to
   specify it in a PUT request.  Relax the server requirement on the
   contents of the Allow header field and remove requirement on clients
   to always trust the header field value.  (Section 9.1)

   The ABNF for the Expect header field has been both fixed (allowing
   parameters for value-less expectations as well) and simplified
   (allowing trailing semicolons after "100-continue" when they were
   invalid before).  (Section 9.3)

   Correct syntax of Location header field to allow URI references
   (including relative references and fragments), as referred symbol
   "absoluteURI" wasn't what was expected, and add some clarifications
   as to when use of fragments would not be appropriate.  (Section 9.5)

   Restrict Max-Forwards header field to OPTIONS and TRACE (previously,
   extension methods could have used it as well).  (Section 9.6)

   Allow Referer field value of "about:blank" as alternative to not
   specifying it.  (Section 9.7)

   In the description of the Server header field, the Via field was
   described as a SHOULD.  The requirement was and is stated correctly
   in the description of the Via header field in Section 8.8 of [Part1].
   (Section 9.9)

Appendix B.  Collected ABNF

   Allow = [ ( "," / Method ) *( OWS "," [ OWS Method ] ) ]

   BWS = <BWS, defined in [Part1], Section 1.2.2>

   Date = HTTP-date

   Expect = *( "," OWS ) expectation *( OWS "," [ OWS expectation ] )

   From = mailbox

   GMT = %x47.4D.54 ; GMT

   HTTP-date = rfc1123-date / obs-date

   Location = URI-reference

   Max-Forwards = 1*DIGIT



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   Method = token

   OWS = <OWS, defined in [Part1], Section 1.2.2>

   RWS = <RWS, defined in [Part1], Section 1.2.2>
   Reason-Phrase = *( HTAB / SP / VCHAR / obs-text )
   Referer = absolute-URI / partial-URI
   Retry-After = HTTP-date / delta-seconds

   Server = product *( RWS ( product / comment ) )
   Status-Code = 3DIGIT

   URI-reference = <URI-reference, defined in [Part1], Section 2.7>
   User-Agent = product *( RWS ( product / comment ) )

   absolute-URI = <absolute-URI, defined in [Part1], Section 2.7>
   asctime-date = day-name SP date3 SP time-of-day SP year

   comment = <comment, defined in [Part1], Section 3.2>

   date1 = day SP month SP year
   date2 = day "-" month "-" 2DIGIT
   date3 = month SP ( 2DIGIT / ( SP DIGIT ) )
   day = 2DIGIT
   day-name = %x4D.6F.6E ; Mon
    / %x54.75.65 ; Tue
    / %x57.65.64 ; Wed
    / %x54.68.75 ; Thu
    / %x46.72.69 ; Fri
    / %x53.61.74 ; Sat
    / %x53.75.6E ; Sun
   day-name-l = %x4D.6F.6E.64.61.79 ; Monday
    / %x54.75.65.73.64.61.79 ; Tuesday
    / %x57.65.64.6E.65.73.64.61.79 ; Wednesday
    / %x54.68.75.72.73.64.61.79 ; Thursday
    / %x46.72.69.64.61.79 ; Friday
    / %x53.61.74.75.72.64.61.79 ; Saturday
    / %x53.75.6E.64.61.79 ; Sunday
   delta-seconds = 1*DIGIT

   expect-name = token
   expect-param = expect-name [ BWS "=" BWS expect-value ]
   expect-value = token / quoted-string
   expectation = expect-name [ BWS "=" BWS expect-value ] *( OWS ";" [
    OWS expect-param ] )

   hour = 2DIGIT




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   mailbox = <mailbox, defined in [RFC5322], Section 3.4>
   minute = 2DIGIT
   month = %x4A.61.6E ; Jan
    / %x46.65.62 ; Feb
    / %x4D.61.72 ; Mar
    / %x41.70.72 ; Apr
    / %x4D.61.79 ; May
    / %x4A.75.6E ; Jun
    / %x4A.75.6C ; Jul
    / %x41.75.67 ; Aug
    / %x53.65.70 ; Sep
    / %x4F.63.74 ; Oct
    / %x4E.6F.76 ; Nov
    / %x44.65.63 ; Dec

   obs-date = rfc850-date / asctime-date
   obs-text = <obs-text, defined in [Part1], Section 1.2.2>

   partial-URI = <partial-URI, defined in [Part1], Section 2.7>
   product = <product, defined in [Part1], Section 5.2>

   quoted-string = <quoted-string, defined in [Part1], Section 3.2.3>

   rfc1123-date = day-name "," SP date1 SP time-of-day SP GMT
   rfc850-date = day-name-l "," SP date2 SP time-of-day SP GMT

   second = 2DIGIT

   time-of-day = hour ":" minute ":" second
   token = <token, defined in [Part1], Section 3.2.3>

   year = 4DIGIT

   ABNF diagnostics:

   ; Allow defined but not used
   ; Date defined but not used
   ; Expect defined but not used
   ; From defined but not used
   ; Location defined but not used
   ; Max-Forwards defined but not used
   ; Reason-Phrase defined but not used
   ; Referer defined but not used
   ; Retry-After defined but not used
   ; Server defined but not used
   ; Status-Code defined but not used
   ; User-Agent defined but not used




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Appendix C.  Change Log (to be removed by RFC Editor before publication)

C.1.  Since RFC 2616

   Extracted relevant partitions from [RFC2616].

C.2.  Since draft-ietf-httpbis-p2-semantics-00

   Closed issues:

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/5>: "Via is a MUST"
      (<http://purl.org/NET/http-errata#via-must>)

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/6>: "Fragments
      allowed in Location"
      (<http://purl.org/NET/http-errata#location-fragments>)

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/10>: "Safe Methods
      vs Redirection" (<http://purl.org/NET/http-errata#saferedirect>)

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/17>: "Revise
      description of the POST method"
      (<http://purl.org/NET/http-errata#post>)

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/35>: "Normative and
      Informative references"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/42>: "RFC2606
      Compliance"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/65>: "Informative
      references"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/84>: "Redundant
      cross-references"

   Other changes:

   o  Move definitions of 304 and 412 condition codes to [Part4]

C.3.  Since draft-ietf-httpbis-p2-semantics-01

   Closed issues:

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/21>: "PUT side
      effects"





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   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/91>: "Duplicate Host
      header requirements"

   Ongoing work on ABNF conversion
   (<http://tools.ietf.org/wg/httpbis/trac/ticket/36>):

   o  Move "Product Tokens" section (back) into Part 1, as "token" is
      used in the definition of the Upgrade header field.

   o  Add explicit references to BNF syntax and rules imported from
      other parts of the specification.

   o  Copy definition of delta-seconds from Part6 instead of referencing
      it.

C.4.  Since draft-ietf-httpbis-p2-semantics-02

   Closed issues:

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/24>: "Requiring
      Allow in 405 responses"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/59>: "Status Code
      Registry"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/61>: "Redirection
      vs. Location"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/70>: "Cacheability
      of 303 response"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/76>: "305 Use Proxy"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/105>:
      "Classification for Allow header"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/112>: "PUT - 'store
      under' vs 'store at'"

   Ongoing work on IANA Message Header Field Registration
   (<http://tools.ietf.org/wg/httpbis/trac/ticket/40>):

   o  Reference RFC 3984, and update header field registrations for
      headers defined in this document.

   Ongoing work on ABNF conversion
   (<http://tools.ietf.org/wg/httpbis/trac/ticket/36>):




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   o  Replace string literals when the string really is case-sensitive
      (method).

C.5.  Since draft-ietf-httpbis-p2-semantics-03

   Closed issues:

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/98>: "OPTIONS
      request bodies"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/119>: "Description
      of CONNECT should refer to RFC2817"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/125>: "Location
      Content-Location reference request/response mixup"

   Ongoing work on Method Registry
   (<http://tools.ietf.org/wg/httpbis/trac/ticket/72>):

   o  Added initial proposal for registration process, plus initial
      content (non-HTTP/1.1 methods to be added by a separate
      specification).

C.6.  Since draft-ietf-httpbis-p2-semantics-04

   Closed issues:

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/103>: "Content-*"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/132>: "RFC 2822 is
      updated by RFC 5322"

   Ongoing work on ABNF conversion
   (<http://tools.ietf.org/wg/httpbis/trac/ticket/36>):

   o  Use "/" instead of "|" for alternatives.

   o  Introduce new ABNF rules for "bad" whitespace ("BWS"), optional
      whitespace ("OWS") and required whitespace ("RWS").

   o  Rewrite ABNFs to spell out whitespace rules, factor out header
      field value format definitions.

C.7.  Since draft-ietf-httpbis-p2-semantics-05

   Closed issues:





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   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/94>: "Reason-Phrase
      BNF"

   Final work on ABNF conversion
   (<http://tools.ietf.org/wg/httpbis/trac/ticket/36>):

   o  Add appendix containing collected and expanded ABNF, reorganize
      ABNF introduction.

C.8.  Since draft-ietf-httpbis-p2-semantics-06

   Closed issues:

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/144>: "Clarify when
      Referer is sent"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/164>: "status codes
      vs methods"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/170>: "Do not
      require "updates" relation for specs that register status codes or
      method names"

C.9.  Since draft-ietf-httpbis-p2-semantics-07

   Closed issues:

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/27>: "Idempotency"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/33>: "TRACE security
      considerations"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/110>: "Clarify rules
      for determining what entities a response carries"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/140>: "update note
      citing RFC 1945 and 2068"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/182>: "update note
      about redirect limit"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/191>: "Location
      header ABNF should use 'URI'"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/192>: "fragments in
      Location vs status 303"





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   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/198>: "move IANA
      registrations for optional status codes"

   Partly resolved issues:

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/171>: "Are OPTIONS
      and TRACE safe?"

C.10.  Since draft-ietf-httpbis-p2-semantics-08

   Closed issues:

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/10>: "Safe Methods
      vs Redirection" (we missed the introduction to the 3xx status
      codes when fixing this previously)

C.11.  Since draft-ietf-httpbis-p2-semantics-09

   Closed issues:

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/43>: "Fragment
      combination / precedence during redirects"

   Partly resolved issues:

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/185>: "Location
      header payload handling"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/196>: "Term for the
      requested resource's URI"

C.12.  Since draft-ietf-httpbis-p2-semantics-10

   Closed issues:

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/69>: "Clarify
      'Requested Variant'"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/109>: "Clarify
      entity / representation / variant terminology"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/139>: "Methods and
      Caching"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/190>: "OPTIONS vs
      Max-Forwards"





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   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/199>: "Status codes
      and caching"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/220>: "consider
      removing the 'changes from 2068' sections"

C.13.  Since draft-ietf-httpbis-p2-semantics-11

   Closed issues:

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/229>:
      "Considerations for new status codes"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/230>:
      "Considerations for new methods"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/232>: "User-Agent
      guidelines" (relating to the 'User-Agent' header field)

C.14.  Since draft-ietf-httpbis-p2-semantics-12

   Closed issues:

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/43>: "Fragment
      combination / precedence during redirects" (added warning about
      having a fragid on the redirect may cause inconvenience in some
      cases)

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/79>: "Content-* vs.
      PUT"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/88>: "205 Bodies"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/102>: "Understanding
      Content-* on non-PUT requests"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/103>: "Content-*"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/104>: "Header type
      defaulting"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/112>: "PUT - 'store
      under' vs 'store at'"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/137>: "duplicate
      ABNF for Reason-Phrase"





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   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/180>: "Note special
      status of Content-* prefix in header registration procedures"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/203>: "Max-Forwards
      vs extension methods"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/213>: "What is the
      value space of HTTP status codes?" (actually fixed in
      draft-ietf-httpbis-p2-semantics-11)

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/224>: "Header
      Classification"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/225>: "PUT side
      effect: invalidation or just stale?"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/226>: "proxies not
      supporting certain methods"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/239>: "Migrate
      CONNECT from RFC2817 to p2"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/240>: "Migrate
      Upgrade details from RFC2817"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/267>: "clarify PUT
      semantics'"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/275>: "duplicate
      ABNF for 'Method'"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/276>: "untangle
      ABNFs for header fields"

C.15.  Since draft-ietf-httpbis-p2-semantics-13

   Closed issues:

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/276>: "untangle
      ABNFs for header fields"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/251>: "message-body
      in CONNECT request"








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C.16.  Since draft-ietf-httpbis-p2-semantics-14

   Closed issues:

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/255>: "Clarify
      status code for rate limiting"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/294>: "clarify 403
      forbidden"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/296>: "Clarify 203
      Non-Authoritative Information"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/298>: "update
      default reason phrase for 413"

C.17.  Since draft-ietf-httpbis-p2-semantics-15

   Closed issues:

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/285>: "Strength of
      requirements on Accept re: 406"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/303>: "400 response
      isn't generic"

C.18.  Since draft-ietf-httpbis-p2-semantics-16

   Closed issues:

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/160>: "Redirects and
      non-GET methods"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/186>: "Document
      HTTP's error-handling philosophy"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/231>:
      "Considerations for new headers"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/310>: "clarify 303
      redirect on HEAD"

C.19.  Since draft-ietf-httpbis-p2-semantics-17

   Closed issues:

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/185>: "Location
      header payload handling"



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   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/255>: "Clarify
      status code for rate limiting" (change backed out because a new
      status code is being defined for this purpose)

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/312>: "should there
      be a permanent variant of 307"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/325>: "When are
      Location's semantics triggered?"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/327>: "'expect'
      grammar missing OWS"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/329>: "header field
      considerations: quoted-string vs use of double quotes"

Index

   1
      100 Continue (status code)  26
      100-continue (expect value)  44
      101 Switching Protocols (status code)  26

   2
      200 OK (status code)  27
      201 Created (status code)  27
      202 Accepted (status code)  28
      203 Non-Authoritative Information (status code)  28
      204 No Content (status code)  28
      205 Reset Content (status code)  29
      206 Partial Content (status code)  29

   3
      300 Multiple Choices (status code)  30
      301 Moved Permanently (status code)  31
      302 Found (status code)  32
      303 See Other (status code)  32
      304 Not Modified (status code)  33
      305 Use Proxy (status code)  33
      306 (Unused) (status code)  33
      307 Temporary Redirect (status code)  33

   4
      400 Bad Request (status code)  34
      401 Unauthorized (status code)  34
      402 Payment Required (status code)  34
      403 Forbidden (status code)  34
      404 Not Found (status code)  35



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      405 Method Not Allowed (status code)  35
      406 Not Acceptable (status code)  35
      407 Proxy Authentication Required (status code)  36
      408 Request Timeout (status code)  36
      409 Conflict (status code)  36
      410 Gone (status code)  36
      411 Length Required (status code)  37
      412 Precondition Failed (status code)  37
      413 Request Representation Too Large (status code)  37
      414 URI Too Long (status code)  37
      415 Unsupported Media Type (status code)  37
      416 Requested Range Not Satisfiable (status code)  38
      417 Expectation Failed (status code)  38
      426 Upgrade Required (status code)  38

   5
      500 Internal Server Error (status code)  38
      501 Not Implemented (status code)  39
      502 Bad Gateway (status code)  39
      503 Service Unavailable (status code)  39
      504 Gateway Timeout (status code)  39
      505 HTTP Version Not Supported (status code)  39

   A
      Allow header field  42

   C
      CONNECT method  24

   D
      Date header field  43
      DELETE method  23

   E
      Expect header field  44
      Expect Values
         100-continue  44

   F
      From header field  44

   G
      GET method  19
      Grammar
         Allow  42
         asctime-date  42
         Date  43
         date1  41



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         day  41
         day-name  41
         day-name-l  41
         delta-seconds  47
         Expect  44
         expect-name  44
         expect-param  44
         expect-value  44
         expectation  44
         extension-code  13
         From  45
         GMT  41
         hour  41
         HTTP-date  40
         Location  45
         Max-Forwards  46
         Method  7
         minute  41
         month  41
         obs-date  41
         Reason-Phrase  13
         Referer  47
         Retry-After  47
         rfc850-date  42
         rfc1123-date  41
         second  41
         Server  48
         Status-Code  13
         time-of-day  41
         User-Agent  49
         year  41

   H
      HEAD method  19
      Header Fields
         Allow  42
         Date  43
         Expect  44
         From  44
         Location  45
         Max-Forwards  46
         Referer  47
         Retry-After  47
         Server  48
         User-Agent  48

   I
      Idempotent Methods  17



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   L
      Location header field  45

   M
      Max-Forwards header field  46
      Methods
         CONNECT  24
         DELETE  23
         GET  19
         HEAD  19
         OPTIONS  18
         POST  20
         PUT  21
         TRACE  23

   O
      OPTIONS method  18

   P
      POST method  20
      PUT method  21

   R
      Referer header field  47
      Retry-After header field  47

   S
      Safe Methods  17
      Server header field  48
      Status Codes
         100 Continue  26
         101 Switching Protocols  26
         200 OK  27
         201 Created  27
         202 Accepted  28
         203 Non-Authoritative Information  28
         204 No Content  28
         205 Reset Content  29
         206 Partial Content  29
         300 Multiple Choices  30
         301 Moved Permanently  31
         302 Found  32
         303 See Other  32
         304 Not Modified  33
         305 Use Proxy  33
         306 (Unused)  33
         307 Temporary Redirect  33
         400 Bad Request  34



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         401 Unauthorized  34
         402 Payment Required  34
         403 Forbidden  34
         404 Not Found  35
         405 Method Not Allowed  35
         406 Not Acceptable  35
         407 Proxy Authentication Required  36
         408 Request Timeout  36
         409 Conflict  36
         410 Gone  36
         411 Length Required  37
         412 Precondition Failed  37
         413 Request Representation Too Large  37
         414 URI Too Long  37
         415 Unsupported Media Type  37
         416 Requested Range Not Satisfiable  38
         417 Expectation Failed  38
         426 Upgrade Required  38
         500 Internal Server Error  38
         501 Not Implemented  39
         502 Bad Gateway  39
         503 Service Unavailable  39
         504 Gateway Timeout  39
         505 HTTP Version Not Supported  39

   T
      TRACE method  23

   U
      User-Agent header field  48

Authors' Addresses

   Roy T. Fielding (editor)
   Adobe Systems Incorporated
   345 Park Ave
   San Jose, CA  95110
   USA

   EMail: fielding@gbiv.com
   URI:   http://roy.gbiv.com/










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   Jim Gettys
   Alcatel-Lucent Bell Labs
   21 Oak Knoll Road
   Carlisle, MA  01741
   USA

   EMail: jg@freedesktop.org
   URI:   http://gettys.wordpress.com/


   Jeffrey C. Mogul
   Hewlett-Packard Company
   HP Labs, Large Scale Systems Group
   1501 Page Mill Road, MS 1177
   Palo Alto, CA  94304
   USA

   EMail: JeffMogul@acm.org


   Henrik Frystyk Nielsen
   Microsoft Corporation
   1 Microsoft Way
   Redmond, WA  98052
   USA

   EMail: henrikn@microsoft.com


   Larry Masinter
   Adobe Systems Incorporated
   345 Park Ave
   San Jose, CA  95110
   USA

   EMail: LMM@acm.org
   URI:   http://larry.masinter.net/


   Paul J. Leach
   Microsoft Corporation
   1 Microsoft Way
   Redmond, WA  98052

   EMail: paulle@microsoft.com






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   Tim Berners-Lee
   World Wide Web Consortium
   MIT Computer Science and Artificial Intelligence Laboratory
   The Stata Center, Building 32
   32 Vassar Street
   Cambridge, MA  02139
   USA

   EMail: timbl@w3.org
   URI:   http://www.w3.org/People/Berners-Lee/


   Yves Lafon (editor)
   World Wide Web Consortium
   W3C / ERCIM
   2004, rte des Lucioles
   Sophia-Antipolis, AM  06902
   France

   EMail: ylafon@w3.org
   URI:   http://www.raubacapeu.net/people/yves/


   Julian F. Reschke (editor)
   greenbytes GmbH
   Hafenweg 16
   Muenster, NW  48155
   Germany

   Phone: +49 251 2807760
   Fax:   +49 251 2807761
   EMail: julian.reschke@greenbytes.de
   URI:   http://greenbytes.de/tech/webdav/


















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