source: draft-ietf-httpbis/15/draft-ietf-httpbis-p1-messaging-15.txt @ 1630

HTTPbis Working Group                                   R. Fielding, Ed.
Internet-Draft                                                     Adobe
Obsoletes: 2145,2616 (if approved)                             J. Gettys
Updates: 2817 (if approved)                               Alcatel-Lucent
Intended status: Standards Track                                J. Mogul
Expires: January 12, 2012                                             HP
                                                              H. Frystyk
                                                               Microsoft
                                                             L. Masinter
                                                                   Adobe
                                                                P. Leach
                                                               Microsoft
                                                          T. Berners-Lee
                                                                 W3C/MIT
                                                           Y. Lafon, Ed.
                                                                     W3C
                                                         J. Reschke, Ed.
                                                              greenbytes
                                                           July 11, 2011


        HTTP/1.1, part 1: URIs, Connections, and Message Parsing
                   draft-ietf-httpbis-p1-messaging-15

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 1 of the
   seven-part specification that defines the protocol referred to as
   "HTTP/1.1" and, taken together, obsoletes RFC 2616.  Part 1 provides
   an overview of HTTP and its associated terminology, defines the
   "http" and "https" Uniform Resource Identifier (URI) schemes, defines
   the generic message syntax and parsing requirements for HTTP message
   frames, and describes general security concerns for implementations.

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 D.16.

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
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   Internet-Drafts are draft documents valid for a maximum of six months
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   This Internet-Draft will expire on January 12, 2012.

Copyright Notice

   Copyright (c) 2011 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   10, 2008.  The person(s) controlling the copyright in some of this
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   than English.

Table of Contents




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   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  6
     1.1.  Requirements . . . . . . . . . . . . . . . . . . . . . . .  7
     1.2.  Syntax Notation  . . . . . . . . . . . . . . . . . . . . .  7
       1.2.1.  ABNF Extension: #rule  . . . . . . . . . . . . . . . .  7
       1.2.2.  Basic Rules  . . . . . . . . . . . . . . . . . . . . .  8
   2.  HTTP-related architecture  . . . . . . . . . . . . . . . . . . 10
     2.1.  Client/Server Messaging  . . . . . . . . . . . . . . . . . 10
     2.2.  Message Orientation and Buffering  . . . . . . . . . . . . 12
     2.3.  Connections and Transport Independence . . . . . . . . . . 12
     2.4.  Intermediaries . . . . . . . . . . . . . . . . . . . . . . 13
     2.5.  Caches . . . . . . . . . . . . . . . . . . . . . . . . . . 15
     2.6.  Protocol Versioning  . . . . . . . . . . . . . . . . . . . 15
     2.7.  Uniform Resource Identifiers . . . . . . . . . . . . . . . 18
       2.7.1.  http URI scheme  . . . . . . . . . . . . . . . . . . . 18
       2.7.2.  https URI scheme . . . . . . . . . . . . . . . . . . . 20
       2.7.3.  http and https URI Normalization and Comparison  . . . 20
   3.  Message Format . . . . . . . . . . . . . . . . . . . . . . . . 21
     3.1.  Message Parsing Robustness . . . . . . . . . . . . . . . . 22
     3.2.  Header Fields  . . . . . . . . . . . . . . . . . . . . . . 22
     3.3.  Message Body . . . . . . . . . . . . . . . . . . . . . . . 25
     3.4.  General Header Fields  . . . . . . . . . . . . . . . . . . 28
   4.  Request  . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
     4.1.  Request-Line . . . . . . . . . . . . . . . . . . . . . . . 29
       4.1.1.  Method . . . . . . . . . . . . . . . . . . . . . . . . 29
       4.1.2.  request-target . . . . . . . . . . . . . . . . . . . . 29
     4.2.  The Resource Identified by a Request . . . . . . . . . . . 31
     4.3.  Effective Request URI  . . . . . . . . . . . . . . . . . . 32
   5.  Response . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
     5.1.  Status-Line  . . . . . . . . . . . . . . . . . . . . . . . 33
       5.1.1.  Status Code and Reason Phrase  . . . . . . . . . . . . 34
   6.  Protocol Parameters  . . . . . . . . . . . . . . . . . . . . . 34
     6.1.  Date/Time Formats: Full Date . . . . . . . . . . . . . . . 34
     6.2.  Transfer Codings . . . . . . . . . . . . . . . . . . . . . 37
       6.2.1.  Chunked Transfer Coding  . . . . . . . . . . . . . . . 38
       6.2.2.  Compression Codings  . . . . . . . . . . . . . . . . . 40
       6.2.3.  Transfer Coding Registry . . . . . . . . . . . . . . . 41
     6.3.  Product Tokens . . . . . . . . . . . . . . . . . . . . . . 42
     6.4.  Quality Values . . . . . . . . . . . . . . . . . . . . . . 42
   7.  Connections  . . . . . . . . . . . . . . . . . . . . . . . . . 42
     7.1.  Persistent Connections . . . . . . . . . . . . . . . . . . 43
       7.1.1.  Purpose  . . . . . . . . . . . . . . . . . . . . . . . 43
       7.1.2.  Overall Operation  . . . . . . . . . . . . . . . . . . 43
       7.1.3.  Proxy Servers  . . . . . . . . . . . . . . . . . . . . 45
       7.1.4.  Practical Considerations . . . . . . . . . . . . . . . 47
     7.2.  Message Transmission Requirements  . . . . . . . . . . . . 48
       7.2.1.  Persistent Connections and Flow Control  . . . . . . . 48
       7.2.2.  Monitoring Connections for Error Status Messages . . . 49
       7.2.3.  Use of the 100 (Continue) Status . . . . . . . . . . . 49



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       7.2.4.  Client Behavior if Server Prematurely Closes
               Connection . . . . . . . . . . . . . . . . . . . . . . 51
   8.  Miscellaneous notes that might disappear . . . . . . . . . . . 52
     8.1.  Scheme aliases considered harmful  . . . . . . . . . . . . 52
     8.2.  Use of HTTP for proxy communication  . . . . . . . . . . . 52
     8.3.  Interception of HTTP for access control  . . . . . . . . . 52
     8.4.  Use of HTTP by other protocols . . . . . . . . . . . . . . 52
     8.5.  Use of HTTP by media type specification  . . . . . . . . . 52
   9.  Header Field Definitions . . . . . . . . . . . . . . . . . . . 52
     9.1.  Connection . . . . . . . . . . . . . . . . . . . . . . . . 52
     9.2.  Content-Length . . . . . . . . . . . . . . . . . . . . . . 54
     9.3.  Date . . . . . . . . . . . . . . . . . . . . . . . . . . . 54
       9.3.1.  Clockless Origin Server Operation  . . . . . . . . . . 55
     9.4.  Host . . . . . . . . . . . . . . . . . . . . . . . . . . . 56
     9.5.  TE . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57
     9.6.  Trailer  . . . . . . . . . . . . . . . . . . . . . . . . . 58
     9.7.  Transfer-Encoding  . . . . . . . . . . . . . . . . . . . . 58
     9.8.  Upgrade  . . . . . . . . . . . . . . . . . . . . . . . . . 59
       9.8.1.  Upgrade Token Registry . . . . . . . . . . . . . . . . 60
     9.9.  Via  . . . . . . . . . . . . . . . . . . . . . . . . . . . 61
   10. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 62
     10.1. Header Field Registration  . . . . . . . . . . . . . . . . 62
     10.2. URI Scheme Registration  . . . . . . . . . . . . . . . . . 63
     10.3. Internet Media Type Registrations  . . . . . . . . . . . . 63
       10.3.1. Internet Media Type message/http . . . . . . . . . . . 63
       10.3.2. Internet Media Type application/http . . . . . . . . . 64
     10.4. Transfer Coding Registry . . . . . . . . . . . . . . . . . 65
     10.5. Upgrade Token Registration . . . . . . . . . . . . . . . . 66
   11. Security Considerations  . . . . . . . . . . . . . . . . . . . 66
     11.1. Personal Information . . . . . . . . . . . . . . . . . . . 66
     11.2. Abuse of Server Log Information  . . . . . . . . . . . . . 67
     11.3. Attacks Based On File and Path Names . . . . . . . . . . . 67
     11.4. DNS Spoofing . . . . . . . . . . . . . . . . . . . . . . . 67
     11.5. Proxies and Caching  . . . . . . . . . . . . . . . . . . . 68
     11.6. Protocol Element Size Overflows  . . . . . . . . . . . . . 69
     11.7. Denial of Service Attacks on Proxies . . . . . . . . . . . 69
   12. Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 69
   13. References . . . . . . . . . . . . . . . . . . . . . . . . . . 70
     13.1. Normative References . . . . . . . . . . . . . . . . . . . 70
     13.2. Informative References . . . . . . . . . . . . . . . . . . 72
   Appendix A.  Tolerant Applications . . . . . . . . . . . . . . . . 74
   Appendix B.  HTTP Version History  . . . . . . . . . . . . . . . . 75
     B.1.  Changes from HTTP/1.0  . . . . . . . . . . . . . . . . . . 76
       B.1.1.  Multi-homed Web Servers  . . . . . . . . . . . . . . . 76
       B.1.2.  Keep-Alive Connections . . . . . . . . . . . . . . . . 76
     B.2.  Changes from RFC 2616  . . . . . . . . . . . . . . . . . . 77
   Appendix C.  Collected ABNF  . . . . . . . . . . . . . . . . . . . 78
   Appendix D.  Change Log (to be removed by RFC Editor before



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                publication)  . . . . . . . . . . . . . . . . . . . . 82
     D.1.  Since RFC 2616 . . . . . . . . . . . . . . . . . . . . . . 82
     D.2.  Since draft-ietf-httpbis-p1-messaging-00 . . . . . . . . . 82
     D.3.  Since draft-ietf-httpbis-p1-messaging-01 . . . . . . . . . 84
     D.4.  Since draft-ietf-httpbis-p1-messaging-02 . . . . . . . . . 85
     D.5.  Since draft-ietf-httpbis-p1-messaging-03 . . . . . . . . . 85
     D.6.  Since draft-ietf-httpbis-p1-messaging-04 . . . . . . . . . 86
     D.7.  Since draft-ietf-httpbis-p1-messaging-05 . . . . . . . . . 86
     D.8.  Since draft-ietf-httpbis-p1-messaging-06 . . . . . . . . . 87
     D.9.  Since draft-ietf-httpbis-p1-messaging-07 . . . . . . . . . 88
     D.10. Since draft-ietf-httpbis-p1-messaging-08 . . . . . . . . . 88
     D.11. Since draft-ietf-httpbis-p1-messaging-09 . . . . . . . . . 89
     D.12. Since draft-ietf-httpbis-p1-messaging-10 . . . . . . . . . 89
     D.13. Since draft-ietf-httpbis-p1-messaging-11 . . . . . . . . . 90
     D.14. Since draft-ietf-httpbis-p1-messaging-12 . . . . . . . . . 90
     D.15. Since draft-ietf-httpbis-p1-messaging-13 . . . . . . . . . 91
     D.16. Since draft-ietf-httpbis-p1-messaging-14 . . . . . . . . . 91
   Index  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91

































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1.  Introduction

   The Hypertext Transfer Protocol (HTTP) is an application-level
   request/response protocol that uses extensible semantics and MIME-
   like message payloads for flexible interaction with network-based
   hypertext information systems.  HTTP relies upon the Uniform Resource
   Identifier (URI) standard [RFC3986] to indicate the target resource
   and relationships between resources.  Messages are passed in a format
   similar to that used by Internet mail [RFC5322] and the Multipurpose
   Internet Mail Extensions (MIME) [RFC2045] (see Appendix A of [Part3]
   for the differences between HTTP and MIME messages).

   HTTP is a generic interface protocol for information systems.  It is
   designed to hide the details of how a service is implemented by
   presenting a uniform interface to clients that is independent of the
   types of resources provided.  Likewise, servers do not need to be
   aware of each client's purpose: an HTTP request can be considered in
   isolation rather than being associated with a specific type of client
   or a predetermined sequence of application steps.  The result is a
   protocol that can be used effectively in many different contexts and
   for which implementations can evolve independently over time.

   HTTP is also designed for use as an intermediation protocol for
   translating communication to and from non-HTTP information systems.
   HTTP proxies and gateways can provide access to alternative
   information services by translating their diverse protocols into a
   hypertext format that can be viewed and manipulated by clients in the
   same way as HTTP services.

   One consequence of HTTP flexibility is that the protocol cannot be
   defined in terms of what occurs behind the interface.  Instead, we
   are limited to defining the syntax of communication, the intent of
   received communication, and the expected behavior of recipients.  If
   the communication is considered in isolation, then successful actions
   ought to be reflected in corresponding changes to the observable
   interface provided by servers.  However, since multiple clients might
   act in parallel and perhaps at cross-purposes, we cannot require that
   such changes be observable beyond the scope of a single response.

   This document is Part 1 of the seven-part specification of HTTP,
   defining the protocol referred to as "HTTP/1.1", obsoleting [RFC2616]
   and [RFC2145].  Part 1 describes the architectural elements that are
   used or referred to in HTTP, defines the "http" and "https" URI
   schemes, describes overall network operation and connection
   management, and defines HTTP message framing and forwarding
   requirements.  Our goal is to define all of the mechanisms necessary
   for HTTP message handling that are independent of message semantics,
   thereby defining the complete set of requirements for message parsers



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   and message-forwarding intermediaries.

1.1.  Requirements

   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].

   An implementation is not compliant if it fails to satisfy one or more
   of the "MUST" or "REQUIRED" level requirements for the protocols it
   implements.  An implementation that satisfies all the "MUST" or
   "REQUIRED" level and all the "SHOULD" level requirements for its
   protocols is said to be "unconditionally compliant"; one that
   satisfies all the "MUST" level requirements but not all the "SHOULD"
   level requirements for its protocols is said to be "conditionally
   compliant".

1.2.  Syntax Notation

   This specification uses the Augmented Backus-Naur Form (ABNF)
   notation of [RFC5234].

   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), LF (line feed), OCTET (any 8-bit
   sequence of data), SP (space), VCHAR (any visible [USASCII]
   character), and WSP (whitespace).

   As a syntactic convention, ABNF rule names prefixed with "obs-"
   denote "obsolete" grammar rules that appear for historical reasons.

1.2.1.  ABNF Extension: #rule

   The #rule extension to the ABNF rules of [RFC5234] is used to improve
   readability.

   A construct "#" is defined, similar to "*", for defining comma-
   delimited lists of elements.  The full form is "<n>#<m>element"
   indicating at least <n> and at most <m> elements, each separated by a
   single comma (",") and optional whitespace (OWS, Section 1.2.2).

   Thus,

     1#element => element *( OWS "," OWS element )






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   and:

     #element => [ 1#element ]

   and for n >= 1 and m > 1:

     <n>#<m>element => element <n-1>*<m-1>( OWS "," OWS element )

   For compatibility with legacy list rules, recipients SHOULD accept
   empty list elements.  In other words, consumers would follow the list
   productions:

     #element => [ ( "," / element ) *( OWS "," [ OWS element ] ) ]

     1#element => *( "," OWS ) element *( OWS "," [ OWS element ] )

   Note that empty elements do not contribute to the count of elements
   present, though.

   For example, given these ABNF productions:

     example-list      = 1#example-list-elmt
     example-list-elmt = token ; see Section 1.2.2

   Then these are valid values for example-list (not including the
   double quotes, which are present for delimitation only):

     "foo,bar"
     " foo ,bar,"
     "  foo , ,bar,charlie   "
     "foo ,bar,   charlie "

   But these values would be invalid, as at least one non-empty element
   is required:

     ""
     ","
     ",   ,"

   Appendix C shows the collected ABNF, with the list rules expanded as
   explained above.

1.2.2.  Basic Rules

   HTTP/1.1 defines the sequence CR LF as the end-of-line marker for all
   protocol elements other than the message-body (see Appendix A for
   tolerant applications).




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   This specification uses three rules to denote the use of linear
   whitespace: OWS (optional whitespace), RWS (required whitespace), and
   BWS ("bad" whitespace).

   The OWS rule is used where zero or more linear whitespace octets
   might appear.  OWS SHOULD either not be produced or be produced as a
   single SP.  Multiple OWS octets that occur within field-content
   SHOULD be replaced with a single SP before interpreting the field
   value or forwarding the message downstream.

   RWS is used when at least one linear whitespace octet is required to
   separate field tokens.  RWS SHOULD be produced as a single SP.
   Multiple RWS octets that occur within field-content SHOULD be
   replaced with a single SP before interpreting the field value or
   forwarding the message downstream.

   BWS is used where the grammar allows optional whitespace for
   historical reasons but senders SHOULD NOT produce it in messages.
   HTTP/1.1 recipients MUST accept such bad optional whitespace and
   remove it before interpreting the field value or forwarding the
   message downstream.


     OWS            = *( [ obs-fold ] WSP )
                    ; "optional" whitespace
     RWS            = 1*( [ obs-fold ] WSP )
                    ; "required" whitespace
     BWS            = OWS
                    ; "bad" whitespace
     obs-fold       = CRLF
                    ; see Section 3.2

   Many HTTP/1.1 header field values consist of words (token or quoted-
   string) separated by whitespace or special characters.  These special
   characters MUST be in a quoted string to be used within a parameter
   value (as defined in Section 6.2).

     word           = token / quoted-string

     token          = 1*tchar

     tchar          = "!" / "#" / "$" / "%" / "&" / "'" / "*"
                    / "+" / "-" / "." / "^" / "_" / "`" / "|" / "~"
                    / DIGIT / ALPHA
                    ; any VCHAR, except special

     special        = "(" / ")" / "<" / ">" / "@" / ","
                    / ";" / ":" / "\" / DQUOTE / "/" / "["



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                    / "]" / "?" / "=" / "{" / "}"

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

     quoted-string  = DQUOTE *( qdtext / quoted-pair ) DQUOTE
     qdtext         = OWS / %x21 / %x23-5B / %x5D-7E / obs-text
                    ; OWS / <VCHAR except DQUOTE and "\"> / obs-text
     obs-text       = %x80-FF

   The backslash octet ("\") can be used as a single-octet quoting
   mechanism within quoted-string constructs:

     quoted-pair    = "\" ( WSP / VCHAR / obs-text )

   Senders SHOULD NOT escape octets that do not require escaping (i.e.,
   other than DQUOTE and the backslash octet).

2.  HTTP-related architecture

   HTTP was created for the World Wide Web architecture and has evolved
   over time to support the scalability needs of a worldwide hypertext
   system.  Much of that architecture is reflected in the terminology
   and syntax productions used to define HTTP.

2.1.  Client/Server Messaging

   HTTP is a stateless request/response protocol that operates by
   exchanging messages across a reliable transport or session-layer
   "connection".  An HTTP "client" is a program that establishes a
   connection to a server for the purpose of sending one or more HTTP
   requests.  An HTTP "server" is a program that accepts connections in
   order to service HTTP requests by sending HTTP responses.

   Note that the terms client and server refer only to the roles that
   these programs perform for a particular connection.  The same program
   might act as a client on some connections and a server on others.  We
   use the term "user agent" to refer to the program that initiates a
   request, such as a WWW browser, editor, or spider (web-traversing
   robot), and the term "origin server" to refer to the program that can
   originate authoritative responses to a request.  For general
   requirements, we use the term "sender" to refer to whichever
   component sent a given message and the term "recipient" to refer to
   any component that receives the message.

   Most HTTP communication consists of a retrieval request (GET) for a
   representation of some resource identified by a URI.  In the simplest
   case, this might be accomplished via a single bidirectional



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   connection (===) between the user agent (UA) and the origin server
   (O).

            request   >
       UA ======================================= O
                                   <   response

   A client sends an HTTP request to the server in the form of a request
   message (Section 4), beginning with a method, URI, and protocol
   version, followed by MIME-like header fields containing request
   modifiers, client information, and payload metadata, an empty line to
   indicate the end of the header section, and finally the payload body
   (if any).

   A server responds to the client's request by sending an HTTP response
   message (Section 5), beginning with a status line that includes the
   protocol version, a success or error code, and textual reason phrase,
   followed by MIME-like header fields containing server information,
   resource metadata, and payload metadata, an empty line to indicate
   the end of the header section, and finally the payload body (if any).

   The following example illustrates a typical message exchange for a
   GET request on the URI "http://www.example.com/hello.txt":

   client request:

     GET /hello.txt HTTP/1.1
     User-Agent: curl/7.16.3 libcurl/7.16.3 OpenSSL/0.9.7l zlib/1.2.3
     Host: www.example.com
     Accept: */*


   server response:

     HTTP/1.1 200 OK
     Date: Mon, 27 Jul 2009 12:28:53 GMT
     Server: Apache
     Last-Modified: Wed, 22 Jul 2009 19:15:56 GMT
     ETag: "34aa387-d-1568eb00"
     Accept-Ranges: bytes
     Content-Length: 14
     Vary: Accept-Encoding
     Content-Type: text/plain

     Hello World!






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2.2.  Message Orientation and Buffering

   Fundamentally, HTTP is a message-based protocol.  Although message
   bodies can be chunked (Section 6.2.1) and implementations often make
   parts of a message available progressively, this is not required, and
   some widely-used implementations only make a message available when
   it is complete.  Furthermore, while most proxies will progressively
   stream messages, some amount of buffering will take place, and some
   proxies might buffer messages to perform transformations, check
   content or provide other services.

   Therefore, extensions to and uses of HTTP cannot rely on the
   availability of a partial message, or assume that messages will not
   be buffered.  There are strategies that can be used to test for
   buffering in a given connection, but it should be understood that
   behaviors can differ across connections, and between requests and
   responses.

   Recipients MUST consider every message in a connection in isolation;
   because HTTP is a stateless protocol, it cannot be assumed that two
   requests on the same connection are from the same client or share any
   other common attributes.  In particular, intermediaries might mix
   requests from different clients into a single server connection.
   Note that some existing HTTP extensions (e.g., [RFC4559]) violate
   this requirement, thereby potentially causing interoperability and
   security problems.

2.3.  Connections and Transport Independence

   HTTP messaging is independent of the underlying transport or session-
   layer connection protocol(s).  HTTP only presumes a reliable
   transport with in-order delivery of requests and the corresponding
   in-order delivery of responses.  The mapping of HTTP request and
   response structures onto the data units of the underlying transport
   protocol is outside the scope of this specification.

   The specific connection protocols to be used for an interaction are
   determined by client configuration and the target resource's URI.
   For example, the "http" URI scheme (Section 2.7.1) indicates a
   default connection of TCP over IP, with a default TCP port of 80, but
   the client might be configured to use a proxy via some other
   connection port or protocol instead of using the defaults.

   A connection might be used for multiple HTTP request/response
   exchanges, as defined in Section 7.1.






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2.4.  Intermediaries

   HTTP enables the use of intermediaries to satisfy requests through a
   chain of connections.  There are three common forms of HTTP
   intermediary: proxy, gateway, and tunnel.  In some cases, a single
   intermediary might act as an origin server, proxy, gateway, or
   tunnel, switching behavior based on the nature of each request.

            >             >             >             >
       UA =========== A =========== B =========== C =========== O
                  <             <             <             <

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

   We use the terms "upstream" and "downstream" to describe various
   requirements in relation to the directional flow of a message: all
   messages flow from upstream to downstream.  Likewise, we use the
   terms inbound and outbound to refer to directions in relation to the
   request path: "inbound" means toward the origin server and "outbound"
   means toward the user agent.

   A "proxy" is a message forwarding agent that is selected by the
   client, usually via local configuration rules, to receive requests
   for some type(s) of absolute URI and attempt to satisfy those
   requests via translation through the HTTP interface.  Some
   translations are minimal, such as for proxy requests for "http" URIs,
   whereas other requests might require translation to and from entirely
   different application-layer protocols.  Proxies are often used to
   group an organization's HTTP requests through a common intermediary
   for the sake of security, annotation services, or shared caching.

   An HTTP-to-HTTP proxy is called a "transforming proxy" if it is
   designed or configured to modify request or response messages in a
   semantically meaningful way (i.e., modifications, beyond those
   required by normal HTTP processing, that change the message in a way
   that would be significant to the original sender or potentially
   significant to downstream recipients).  For example, a transforming
   proxy might be acting as a shared annotation server (modifying



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   responses to include references to a local annotation database), a
   malware filter, a format transcoder, or an intranet-to-Internet
   privacy filter.  Such transformations are presumed to be desired by
   the client (or client organization) that selected the proxy and are
   beyond the scope of this specification.  However, when a proxy is not
   intended to transform a given message, we use the term "non-
   transforming proxy" to target requirements that preserve HTTP message
   semantics.  See Section 8.2.4 of [Part2] and Section 3.6 of [Part6]
   for status and warning codes related to transformations.

   A "gateway" (a.k.a., "reverse proxy") is a receiving agent that acts
   as a layer above some other server(s) and translates the received
   requests to the underlying server's protocol.  Gateways are often
   used to encapsulate legacy or untrusted information services, to
   improve server performance through "accelerator" caching, and to
   enable partitioning or load-balancing of HTTP services across
   multiple machines.

   A gateway behaves as an origin server on its outbound connection and
   as a user agent on its inbound connection.  All HTTP requirements
   applicable to an origin server also apply to the outbound
   communication of a gateway.  A gateway communicates with inbound
   servers using any protocol that it desires, including private
   extensions to HTTP that are outside the scope of this specification.
   However, an HTTP-to-HTTP gateway that wishes to interoperate with
   third-party HTTP servers MUST comply with HTTP user agent
   requirements on the gateway's inbound connection and MUST implement
   the Connection (Section 9.1) and Via (Section 9.9) header fields for
   both connections.

   A "tunnel" acts as a blind relay between two connections without
   changing the messages.  Once active, a tunnel is not considered a
   party to the HTTP communication, though the tunnel might have been
   initiated by an HTTP request.  A tunnel ceases to exist when both
   ends of the relayed connection are closed.  Tunnels are used to
   extend a virtual connection through an intermediary, such as when
   transport-layer security is used to establish private communication
   through a shared firewall proxy.

   In addition, there may exist network intermediaries that are not
   considered part of the HTTP communication but nevertheless act as
   filters or redirecting agents (usually violating HTTP semantics,
   causing security problems, and otherwise making a mess of things).
   Such a network intermediary, often referred to as an "interception
   proxy" [RFC3040], "transparent proxy" [RFC1919], or "captive portal",
   differs from an HTTP proxy because it has not been selected by the
   client.  Instead, the network intermediary redirects outgoing TCP
   port 80 packets (and occasionally other common port traffic) to an



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   internal HTTP server.  Interception proxies are commonly found on
   public network access points, as a means of enforcing account
   subscription prior to allowing use of non-local Internet services,
   and within corporate firewalls to enforce network usage policies.
   They are indistinguishable from a man-in-the-middle attack.

2.5.  Caches

   A "cache" is a local store of previous response messages and the
   subsystem that controls its message storage, retrieval, and deletion.
   A cache stores cacheable responses in order to reduce the response
   time and network bandwidth consumption on future, equivalent
   requests.  Any client or server MAY employ a cache, though a cache
   cannot be used by a server while it is acting as a tunnel.

   The effect of a cache is that the request/response chain is shortened
   if one of the participants along the chain has a cached response
   applicable to that request.  The following illustrates the resulting
   chain if B has a cached copy of an earlier response from O (via C)
   for a request which has not been cached by UA or A.

               >             >
          UA =========== A =========== B - - - - - - C - - - - - - O
                     <             <

   A response is "cacheable" if a cache is allowed to store a copy of
   the response message for use in answering subsequent requests.  Even
   when a response is cacheable, there might be additional constraints
   placed by the client or by the origin server on when that cached
   response can be used for a particular request.  HTTP requirements for
   cache behavior and cacheable responses are defined in Section 2 of
   [Part6].

   There are a wide variety of architectures and configurations of
   caches and proxies deployed across the World Wide Web and inside
   large organizations.  These systems include national hierarchies of
   proxy caches to save transoceanic bandwidth, systems that broadcast
   or multicast cache entries, organizations that distribute subsets of
   cached data via optical media, and so on.

2.6.  Protocol Versioning

   HTTP uses a "<major>.<minor>" numbering scheme to indicate versions
   of the protocol.  This specification defines version "1.1".  The
   protocol version as a whole indicates the sender's compliance with
   the set of requirements laid out in that version's corresponding
   specification of HTTP.




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   The version of an HTTP message is indicated by an HTTP-Version field
   in the first line of the message.  HTTP-Version is case-sensitive.

     HTTP-Version   = HTTP-Prot-Name "/" DIGIT "." DIGIT
     HTTP-Prot-Name = %x48.54.54.50 ; "HTTP", case-sensitive

   The HTTP version number consists of two decimal digits separated by a
   "." (period or decimal point).  The first digit ("major version")
   indicates the HTTP messaging syntax, whereas the second digit ("minor
   version") indicates the highest minor version to which the sender is
   at least conditionally compliant and able to understand for future
   communication.  The minor version advertises the sender's
   communication capabilities even when the sender is only using a
   backwards-compatible subset of the protocol, thereby letting the
   recipient know that more advanced features can be used in response
   (by servers) or in future requests (by clients).

   When an HTTP/1.1 message is sent to an HTTP/1.0 recipient [RFC1945]
   or a recipient whose version is unknown, the HTTP/1.1 message is
   constructed such that it can be interpreted as a valid HTTP/1.0
   message if all of the newer features are ignored.  This specification
   places recipient-version requirements on some new features so that a
   compliant sender will only use compatible features until it has
   determined, through configuration or the receipt of a message, that
   the recipient supports HTTP/1.1.

   The interpretation of an HTTP header field does not change between
   minor versions of the same major version, though the default behavior
   of a recipient in the absence of such a field can change.  Unless
   specified otherwise, header fields defined in HTTP/1.1 are defined
   for all versions of HTTP/1.x.  In particular, the Host and Connection
   header fields ought to be implemented by all HTTP/1.x implementations
   whether or not they advertise compliance with HTTP/1.1.

   New header fields can be defined such that, when they are understood
   by a recipient, they might override or enhance the interpretation of
   previously defined header fields.  When an implementation receives an
   unrecognized header field, the recipient MUST ignore that header
   field for local processing regardless of the message's HTTP version.
   An unrecognized header field received by a proxy MUST be forwarded
   downstream unless the header field's field-name is listed in the
   message's Connection header-field (see Section 9.1).  These
   requirements allow HTTP's functionality to be enhanced without
   requiring prior update of all compliant intermediaries.

   Intermediaries that process HTTP messages (i.e., all intermediaries
   other than those acting as a tunnel) MUST send their own HTTP-Version
   in forwarded messages.  In other words, they MUST NOT blindly forward



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   the first line of an HTTP message without ensuring that the protocol
   version matches what the intermediary understands, and is at least
   conditionally compliant to, for both the receiving and sending of
   messages.  Forwarding an HTTP message without rewriting the HTTP-
   Version might result in communication errors when downstream
   recipients use the message sender's version to determine what
   features are safe to use for later communication with that sender.

   An HTTP client SHOULD send a request version equal to the highest
   version for which the client is at least conditionally compliant and
   whose major version is no higher than the highest version supported
   by the server, if this is known.  An HTTP client MUST NOT send a
   version for which it is not at least conditionally compliant.

   An HTTP client MAY send a lower request version if it is known that
   the server incorrectly implements the HTTP specification, but only
   after the client has attempted at least one normal request and
   determined from the response status or header fields (e.g., Server)
   that the server improperly handles higher request versions.

   An HTTP server SHOULD send a response version equal to the highest
   version for which the server is at least conditionally compliant and
   whose major version is less than or equal to the one received in the
   request.  An HTTP server MUST NOT send a version for which it is not
   at least conditionally compliant.  A server MAY send a 505 (HTTP
   Version Not Supported) response if it cannot send a response using
   the major version used in the client's request.

   An HTTP server MAY send an HTTP/1.0 response to an HTTP/1.0 request
   if it is known or suspected that the client incorrectly implements
   the HTTP specification and is incapable of correctly processing later
   version responses, such as when a client fails to parse the version
   number correctly or when an intermediary is known to blindly forward
   the HTTP-Version even when it doesn't comply with the given minor
   version of the protocol.  Such protocol downgrades SHOULD NOT be
   performed unless triggered by specific client attributes, such as
   when one or more of the request header fields (e.g., User-Agent)
   uniquely match the values sent by a client known to be in error.

   The intention of HTTP's versioning design is that the major number
   will only be incremented if an incompatible message syntax is
   introduced, and that the minor number will only be incremented when
   changes made to the protocol have the effect of adding to the message
   semantics or implying additional capabilities of the sender.
   However, the minor version was not incremented for the changes
   introduced between [RFC2068] and [RFC2616], and this revision is
   specifically avoiding any such changes to the protocol.




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2.7.  Uniform Resource Identifiers

   Uniform Resource Identifiers (URIs) [RFC3986] are used throughout
   HTTP as the means for identifying resources.  URI references are used
   to target requests, indicate redirects, and define relationships.
   HTTP does not limit what a resource might be; it merely defines an
   interface that can be used to interact with a resource via HTTP.
   More information on the scope of URIs and resources can be found in
   [RFC3986].

   This specification adopts the definitions of "URI-reference",
   "absolute-URI", "relative-part", "port", "host", "path-abempty",
   "path-absolute", "query", and "authority" from the URI generic syntax
   [RFC3986].  In addition, we define a partial-URI rule for protocol
   elements that allow a relative URI but not a fragment.

     URI-reference = <URI-reference, defined in [RFC3986], Section 4.1>
     absolute-URI  = <absolute-URI, defined in [RFC3986], Section 4.3>
     relative-part = <relative-part, defined in [RFC3986], Section 4.2>
     authority     = <authority, defined in [RFC3986], Section 3.2>
     path-abempty  = <path-abempty, defined in [RFC3986], Section 3.3>
     path-absolute = <path-absolute, defined in [RFC3986], Section 3.3>
     port          = <port, defined in [RFC3986], Section 3.2.3>
     query         = <query, defined in [RFC3986], Section 3.4>
     uri-host      = <host, defined in [RFC3986], Section 3.2.2>

     partial-URI   = relative-part [ "?" query ]

   Each protocol element in HTTP that allows a URI reference will
   indicate in its ABNF production whether the element allows any form
   of reference (URI-reference), only a URI in absolute form (absolute-
   URI), only the path and optional query components, or some
   combination of the above.  Unless otherwise indicated, URI references
   are parsed relative to the effective request URI, which defines the
   default base URI for references in both the request and its
   corresponding response.

2.7.1.  http URI scheme

   The "http" URI scheme is hereby defined for the purpose of minting
   identifiers according to their association with the hierarchical
   namespace governed by a potential HTTP origin server listening for
   TCP connections on a given port.

     http-URI = "http:" "//" authority path-abempty [ "?" query ]

   The HTTP origin server is identified by the generic syntax's
   authority component, which includes a host identifier and optional



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   TCP port ([RFC3986], Section 3.2.2).  The remainder of the URI,
   consisting of both the hierarchical path component and optional query
   component, serves as an identifier for a potential resource within
   that origin server's name space.

   If the host identifier is provided as an IP literal or IPv4 address,
   then the origin server is any listener on the indicated TCP port at
   that IP address.  If host is a registered name, then that name is
   considered an indirect identifier and the recipient might use a name
   resolution service, such as DNS, to find the address of a listener
   for that host.  The host MUST NOT be empty; if an "http" URI is
   received with an empty host, then it MUST be rejected as invalid.  If
   the port subcomponent is empty or not given, then TCP port 80 is
   assumed (the default reserved port for WWW services).

   Regardless of the form of host identifier, access to that host is not
   implied by the mere presence of its name or address.  The host might
   or might not exist and, even when it does exist, might or might not
   be running an HTTP server or listening to the indicated port.  The
   "http" URI scheme makes use of the delegated nature of Internet names
   and addresses to establish a naming authority (whatever entity has
   the ability to place an HTTP server at that Internet name or address)
   and allows that authority to determine which names are valid and how
   they might be used.

   When an "http" URI is used within a context that calls for access to
   the indicated resource, a client MAY attempt access by resolving the
   host to an IP address, establishing a TCP connection to that address
   on the indicated port, and sending an HTTP request message to the
   server containing the URI's identifying data as described in
   Section 4.  If the server responds to that request with a non-interim
   HTTP response message, as described in Section 5, then that response
   is considered an authoritative answer to the client's request.

   Although HTTP is independent of the transport protocol, the "http"
   scheme is specific to TCP-based services because the name delegation
   process depends on TCP for establishing authority.  An HTTP service
   based on some other underlying connection protocol would presumably
   be identified using a different URI scheme, just as the "https"
   scheme (below) is used for servers that require an SSL/TLS transport
   layer on a connection.  Other protocols might also be used to provide
   access to "http" identified resources -- it is only the authoritative
   interface used for mapping the namespace that is specific to TCP.

   The URI generic syntax for authority also includes a deprecated
   userinfo subcomponent ([RFC3986], Section 3.2.1) for including user
   authentication information in the URI.  Some implementations make use
   of the userinfo component for internal configuration of



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   authentication information, such as within command invocation
   options, configuration files, or bookmark lists, even though such
   usage might expose a user identifier or password.  Senders MUST NOT
   include a userinfo subcomponent (and its "@" delimiter) when
   transmitting an "http" URI in a message.  Recipients of HTTP messages
   that contain a URI reference SHOULD parse for the existence of
   userinfo and treat its presence as an error, likely indicating that
   the deprecated subcomponent is being used to obscure the authority
   for the sake of phishing attacks.

2.7.2.  https URI scheme

   The "https" URI scheme is hereby defined for the purpose of minting
   identifiers according to their association with the hierarchical
   namespace governed by a potential HTTP origin server listening for
   SSL/TLS-secured connections on a given TCP port.

   All of the requirements listed above for the "http" scheme are also
   requirements for the "https" scheme, except that a default TCP port
   of 443 is assumed if the port subcomponent is empty or not given, and
   the TCP connection MUST be secured for privacy through the use of
   strong encryption prior to sending the first HTTP request.

     https-URI = "https:" "//" authority path-abempty [ "?" query ]

   Unlike the "http" scheme, responses to "https" identified requests
   are never "public" and thus MUST NOT be reused for shared caching.
   They can, however, be reused in a private cache if the message is
   cacheable by default in HTTP or specifically indicated as such by the
   Cache-Control header field (Section 3.2 of [Part6]).

   Resources made available via the "https" scheme have no shared
   identity with the "http" scheme even if their resource identifiers
   indicate the same authority (the same host listening to the same TCP
   port).  They are distinct name spaces and are considered to be
   distinct origin servers.  However, an extension to HTTP that is
   defined to apply to entire host domains, such as the Cookie protocol
   [RFC6265], can allow information set by one service to impact
   communication with other services within a matching group of host
   domains.

   The process for authoritative access to an "https" identified
   resource is defined in [RFC2818].

2.7.3.  http and https URI Normalization and Comparison

   Since the "http" and "https" schemes conform to the URI generic
   syntax, such URIs are normalized and compared according to the



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   algorithm defined in [RFC3986], Section 6, using the defaults
   described above for each scheme.

   If the port is equal to the default port for a scheme, the normal
   form is to elide the port subcomponent.  Likewise, an empty path
   component is equivalent to an absolute path of "/", so the normal
   form is to provide a path of "/" instead.  The scheme and host are
   case-insensitive and normally provided in lowercase; all other
   components are compared in a case-sensitive manner.  Characters other
   than those in the "reserved" set are equivalent to their percent-
   encoded octets (see [RFC3986], Section 2.1): the normal form is to
   not encode them.

   For example, the following three URIs are equivalent:

      http://example.com:80/~smith/home.html
      http://EXAMPLE.com/%7Esmith/home.html
      http://EXAMPLE.com:/%7esmith/home.html

3.  Message Format

   All HTTP/1.1 messages consist of a start-line followed by a sequence
   of octets in a format similar to the Internet Message Format
   [RFC5322]: zero or more header fields (collectively referred to as
   the "headers" or the "header section"), an empty line indicating the
   end of the header section, and an optional message-body.

   An HTTP message can either be a request from client to server or a
   response from server to client.  Syntactically, the two types of
   message differ only in the start-line, which is either a Request-Line
   (for requests) or a Status-Line (for responses), and in the algorithm
   for determining the length of the message-body (Section 3.3).  In
   theory, a client could receive requests and a server could receive
   responses, distinguishing them by their different start-line formats,
   but in practice servers are implemented to only expect a request (a
   response is interpreted as an unknown or invalid request method) and
   clients are implemented to only expect a response.

     HTTP-message    = start-line
                       *( header-field CRLF )
                       CRLF
                       [ message-body ]
     start-line      = Request-Line / Status-Line

   Implementations MUST NOT send whitespace between the start-line and
   the first header field.  The presence of such whitespace in a request
   might be an attempt to trick a server into ignoring that field or
   processing the line after it as a new request, either of which might



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   result in a security vulnerability if other implementations within
   the request chain interpret the same message differently.  Likewise,
   the presence of such whitespace in a response might be ignored by
   some clients or cause others to cease parsing.

3.1.  Message Parsing Robustness

   In the interest of robustness, servers SHOULD ignore at least one
   empty line received where a Request-Line is expected.  In other
   words, if the server is reading the protocol stream at the beginning
   of a message and receives a CRLF first, it SHOULD ignore the CRLF.

   Some old HTTP/1.0 client implementations send an extra CRLF after a
   POST request as a lame workaround for some early server applications
   that failed to read message-body content that was not terminated by a
   line-ending.  An HTTP/1.1 client MUST NOT preface or follow a request
   with an extra CRLF.  If terminating the request message-body with a
   line-ending is desired, then the client MUST include the terminating
   CRLF octets as part of the message-body length.

   When a server listening only for HTTP request messages, or processing
   what appears from the start-line to be an HTTP request message,
   receives a sequence of octets that does not match the HTTP-message
   grammar aside from the robustness exceptions listed above, the server
   MUST respond with an HTTP/1.1 400 (Bad Request) response.

   The normal procedure for parsing an HTTP message is to read the
   start-line into a structure, read each header field into a hash table
   by field name until the empty line, and then use the parsed data to
   determine if a message-body is expected.  If a message-body has been
   indicated, then it is read as a stream until an amount of octets
   equal to the message-body length is read or the connection is closed.
   Care must be taken to parse an HTTP message as a sequence of octets
   in an encoding that is a superset of US-ASCII.  Attempting to parse
   HTTP as a stream of Unicode characters in a character encoding like
   UTF-16 might introduce security flaws due to the differing ways that
   such parsers interpret invalid characters.

   HTTP allows the set of defined header fields to be extended without
   changing the protocol version (see Section 10.1).  Unrecognized
   header fields MUST be forwarded by a proxy unless the proxy is
   specifically configured to block or otherwise transform such fields.
   Unrecognized header fields SHOULD be ignored by other recipients.

3.2.  Header Fields

   Each HTTP header field consists of a case-insensitive field name
   followed by a colon (":"), optional whitespace, and the field value.



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     header-field   = field-name ":" OWS [ field-value ] OWS
     field-name     = token
     field-value    = *( field-content / OWS )
     field-content  = *( WSP / VCHAR / obs-text )

   No whitespace is allowed between the header field name and colon.
   For security reasons, any request message received containing such
   whitespace MUST be rejected with a response code of 400 (Bad
   Request).  A proxy MUST remove any such whitespace from a response
   message before forwarding the message downstream.

   A field value MAY be preceded by optional whitespace (OWS); a single
   SP is preferred.  The field value does not include any leading or
   trailing white space: OWS occurring before the first non-whitespace
   octet of the field value or after the last non-whitespace octet of
   the field value is ignored and SHOULD be removed before further
   processing (as this does not change the meaning of the header field).

   The order in which header fields with differing field names are
   received is not significant.  However, it is "good practice" to send
   header fields that contain control data first, such as Host on
   requests and Date on responses, so that implementations can decide
   when not to handle a message as early as possible.  A server MUST
   wait until the entire header section is received before interpreting
   a request message, since later header fields might include
   conditionals, authentication credentials, or deliberately misleading
   duplicate header fields that would impact request processing.

   Multiple header fields with the same field name MUST NOT be sent in a
   message unless the entire field value for that header field is
   defined as a comma-separated list [i.e., #(values)].  Multiple header
   fields with the same field name can be combined into one "field-name:
   field-value" pair, without changing the semantics of the message, by
   appending each subsequent field value to the combined field value in
   order, separated by a comma.  The order in which header fields with
   the same field name are received is therefore significant to the
   interpretation of the combined field value; a proxy MUST NOT change
   the order of these field values when forwarding a message.

      Note: The "Set-Cookie" header field as implemented in practice can
      occur multiple times, but does not use the list syntax, and thus
      cannot be combined into a single line ([RFC6265]).  (See Appendix
      A.2.3 of [Kri2001] for details.)  Also note that the Set-Cookie2
      header field specified in [RFC2965] does not share this problem.

   Historically, HTTP header field values could be extended over
   multiple lines by preceding each extra line with at least one space
   or horizontal tab octet (line folding).  This specification



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   deprecates such line folding except within the message/http media
   type (Section 10.3.1).  HTTP/1.1 senders MUST NOT produce messages
   that include line folding (i.e., that contain any field-content that
   matches the obs-fold rule) unless the message is intended for
   packaging within the message/http media type.  HTTP/1.1 recipients
   SHOULD accept line folding and replace any embedded obs-fold
   whitespace with a single SP prior to interpreting the field value or
   forwarding the message downstream.

   Historically, HTTP has allowed field content with text in the ISO-
   8859-1 [ISO-8859-1] character encoding and supported other character
   sets only through use of [RFC2047] encoding.  In practice, most HTTP
   header field values use only a subset of the US-ASCII character
   encoding [USASCII].  Newly defined header fields SHOULD limit their
   field values to US-ASCII octets.  Recipients SHOULD treat other (obs-
   text) octets in field content as opaque data.

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

     comment        = "(" *( ctext / quoted-cpair / comment ) ")"
     ctext          = OWS / %x21-27 / %x2A-5B / %x5D-7E / obs-text
                    ; OWS / <VCHAR except "(", ")", and "\"> / obs-text

   The backslash octet ("\") can be used as a single-octet quoting
   mechanism within comment constructs:

     quoted-cpair    = "\" ( WSP / VCHAR / obs-text )

   Senders SHOULD NOT escape octets that do not require escaping (i.e.,
   other than the backslash octet "\" and the parentheses "(" and ")").

   HTTP does not place a pre-defined limit on the length of header
   fields, either in isolation or as a set.  A server MUST be prepared
   to receive request header fields of unbounded length and respond with
   a 4xx status code if the received header field(s) would be longer
   than the server wishes to handle.

   A client that receives response headers that are longer than it
   wishes to handle can only treat it as a server error.

   Various ad-hoc limitations on header length are found in practice.
   It is RECOMMENDED that all HTTP senders and recipients support
   messages whose combined header fields have 4000 or more octets.






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3.3.  Message Body

   The message-body (if any) of an HTTP message is used to carry the
   payload body associated with the request or response.

     message-body = *OCTET

   The message-body differs from the payload body only when a transfer-
   coding has been applied, as indicated by the Transfer-Encoding header
   field (Section 9.7).  If more than one Transfer-Encoding header field
   is present in a message, the multiple field-values MUST be combined
   into one field-value, according to the algorithm defined in
   Section 3.2, before determining the message-body length.

   When one or more transfer-codings are applied to a payload in order
   to form the message-body, the Transfer-Encoding header field MUST
   contain the list of transfer-codings applied.  Transfer-Encoding is a
   property of the message, not of the payload, and thus MAY be added or
   removed by any implementation along the request/response chain under
   the constraints found in Section 6.2.

   If a message is received that has multiple Content-Length header
   fields (Section 9.2) with field-values consisting of the same decimal
   value, or a single Content-Length header field with a field value
   containing a list of identical decimal values (e.g., "Content-Length:
   42, 42"), indicating that duplicate Content-Length header fields have
   been generated or combined by an upstream message processor, then the
   recipient MUST either reject the message as invalid or replace the
   duplicated field-values with a single valid Content-Length field
   containing that decimal value prior to determining the message-body
   length.

   The rules for when a message-body is allowed in a message differ for
   requests and responses.

   The presence of a message-body in a request is signaled by the
   inclusion of a Content-Length or Transfer-Encoding header field in
   the request's header fields, even if the request method does not
   define any use for a message-body.  This allows the request message
   framing algorithm to be independent of method semantics.

   For response messages, whether or not a message-body is included with
   a message is dependent on both the request method and the response
   status code (Section 5.1.1).  Responses to the HEAD request method
   never include a message-body because the associated response header
   fields (e.g., Transfer-Encoding, Content-Length, etc.) only indicate
   what their values would have been if the request method had been GET.
   All 1xx (Informational), 204 (No Content), and 304 (Not Modified)



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   responses MUST NOT include a message-body.  All other responses do
   include a message-body, although the body MAY be of zero length.

   The length of the message-body is determined by one of the following
   (in order of precedence):

   1.  Any response to a HEAD request and any response with a status
       code of 100-199, 204, or 304 is always terminated by the first
       empty line after the header fields, regardless of the header
       fields present in the message, and thus cannot contain a message-
       body.

   2.  If a Transfer-Encoding header field is present and the "chunked"
       transfer-coding (Section 6.2) is the final encoding, the message-
       body length is determined by reading and decoding the chunked
       data until the transfer-coding indicates the data is complete.

       If a Transfer-Encoding header field is present in a response and
       the "chunked" transfer-coding is not the final encoding, the
       message-body length is determined by reading the connection until
       it is closed by the server.  If a Transfer-Encoding header field
       is present in a request and the "chunked" transfer-coding is not
       the final encoding, the message-body length cannot be determined
       reliably; the server MUST respond with the 400 (Bad Request)
       status code and then close the connection.

       If a message is received with both a Transfer-Encoding header
       field and a Content-Length header field, the Transfer-Encoding
       overrides the Content-Length.  Such a message might indicate an
       attempt to perform request or response smuggling (bypass of
       security-related checks on message routing or content) and thus
       ought to be handled as an error.  The provided Content-Length
       MUST be removed, prior to forwarding the message downstream, or
       replaced with the real message-body length after the transfer-
       coding is decoded.

   3.  If a message is received without Transfer-Encoding and with
       either multiple Content-Length header fields having differing
       field-values or a single Content-Length header field having an
       invalid value, then the message framing is invalid and MUST be
       treated as an error to prevent request or response smuggling.  If
       this is a request message, the server MUST respond with a 400
       (Bad Request) status code and then close the connection.  If this
       is a response message received by a proxy, the proxy MUST discard
       the received response, send a 502 (Bad Gateway) status code as
       its downstream response, and then close the connection.  If this
       is a response message received by a user-agent, it MUST be
       treated as an error by discarding the message and closing the



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       connection.

   4.  If a valid Content-Length header field is present without
       Transfer-Encoding, its decimal value defines the message-body
       length in octets.  If the actual number of octets sent in the
       message is less than the indicated Content-Length, the recipient
       MUST consider the message to be incomplete and treat the
       connection as no longer usable.  If the actual number of octets
       sent in the message is more than the indicated Content-Length,
       the recipient MUST only process the message-body up to the field
       value's number of octets; the remainder of the message MUST
       either be discarded or treated as the next message in a pipeline.
       For the sake of robustness, a user-agent MAY attempt to detect
       and correct such an error in message framing if it is parsing the
       response to the last request on on a connection and the
       connection has been closed by the server.

   5.  If this is a request message and none of the above are true, then
       the message-body length is zero (no message-body is present).

   6.  Otherwise, this is a response message without a declared message-
       body length, so the message-body length is determined by the
       number of octets received prior to the server closing the
       connection.

   Since there is no way to distinguish a successfully completed, close-
   delimited message from a partially-received message interrupted by
   network failure, implementations SHOULD use encoding or length-
   delimited messages whenever possible.  The close-delimiting feature
   exists primarily for backwards compatibility with HTTP/1.0.

   A server MAY reject a request that contains a message-body but not a
   Content-Length by responding with 411 (Length Required).

   Unless a transfer-coding other than "chunked" has been applied, a
   client that sends a request containing a message-body SHOULD use a
   valid Content-Length header field if the message-body length is known
   in advance, rather than the "chunked" encoding, since some existing
   services respond to "chunked" with a 411 (Length Required) status
   code even though they understand the chunked encoding.  This is
   typically because such services are implemented via a gateway that
   requires a content-length in advance of being called and the server
   is unable or unwilling to buffer the entire request before
   processing.

   A client that sends a request containing a message-body MUST include
   a valid Content-Length header field if it does not know the server
   will handle HTTP/1.1 (or later) requests; such knowledge can be in



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   the form of specific user configuration or by remembering the version
   of a prior received response.

   Request messages that are prematurely terminated, possibly due to a
   cancelled connection or a server-imposed time-out exception, MUST
   result in closure of the connection; sending an HTTP/1.1 error
   response prior to closing the connection is OPTIONAL.  Response
   messages that are prematurely terminated, usually by closure of the
   connection prior to receiving the expected number of octets or by
   failure to decode a transfer-encoded message-body, MUST be recorded
   as incomplete.  A user agent MUST NOT render an incomplete response
   message-body as if it were complete (i.e., some indication must be
   given to the user that an error occurred).  Cache requirements for
   incomplete responses are defined in Section 2.1.1 of [Part6].

   A server MUST read the entire request message-body or close the
   connection after sending its response, since otherwise the remaining
   data on a persistent connection would be misinterpreted as the next
   request.  Likewise, a client MUST read the entire response message-
   body if it intends to reuse the same connection for a subsequent
   request.  Pipelining multiple requests on a connection is described
   in Section 7.1.2.2.

3.4.  General Header Fields

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

   +-------------------+---------------+
   | Header Field Name | Defined in... |
   +-------------------+---------------+
   | Connection        | Section 9.1   |
   | Date              | Section 9.3   |
   | Trailer           | Section 9.6   |
   | Transfer-Encoding | Section 9.7   |
   | Upgrade           | Section 9.8   |
   | Via               | Section 9.9   |
   +-------------------+---------------+

4.  Request

   A request message from a client to a server begins with a Request-
   Line, followed by zero or more header fields, an empty line
   signifying the end of the header block, and an optional message body.





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     Request       = Request-Line              ; Section 4.1
                     *( header-field CRLF )    ; Section 3.2
                     CRLF
                     [ message-body ]          ; Section 3.3

4.1.  Request-Line

   The Request-Line begins with a method token, followed by a single
   space (SP), the request-target, another single space (SP), the
   protocol version, and ending with CRLF.

     Request-Line   = Method SP request-target SP HTTP-Version CRLF

4.1.1.  Method

   The Method token indicates the request method to be performed on the
   target resource.  The request method is case-sensitive.

     Method         = token

4.1.2.  request-target

   The request-target identifies the target resource upon which to apply
   the request.  In most cases, the user agent is provided a URI
   reference from which it determines an absolute URI for identifying
   the target resource.  When a request to the resource is initiated,
   all or part of that URI is used to construct the HTTP request-target.

     request-target = "*"
                    / absolute-URI
                    / ( path-absolute [ "?" query ] )
                    / authority

   The four options for request-target are dependent on the nature of
   the request.

   The asterisk "*" form of request-target, which MUST NOT be used with
   any request method other than OPTIONS, means that the request applies
   to the server as a whole (the listening process) rather than to a
   specific named resource at that server.  For example,

     OPTIONS * HTTP/1.1

   The "absolute-URI" form is REQUIRED when the request is being made to
   a proxy.  The proxy is requested to either forward the request or
   service it from a valid cache, and then return the response.  Note
   that the proxy MAY forward the request on to another proxy or
   directly to the server specified by the absolute-URI.  In order to



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   avoid request loops, a proxy that forwards requests to other proxies
   MUST be able to recognize and exclude all of its own server names,
   including any aliases, local variations, and the numeric IP address.
   An example Request-Line would be:

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

   To allow for transition to absolute-URIs in all requests in future
   versions of HTTP, all HTTP/1.1 servers MUST accept the absolute-URI
   form in requests, even though HTTP/1.1 clients will only generate
   them in requests to proxies.

   If a proxy receives a host name that is not a fully qualified domain
   name, it MAY add its domain to the host name it received.  If a proxy
   receives a fully qualified domain name, the proxy MUST NOT change the
   host name.

   The "authority form" is only used by the CONNECT request method
   (Section 7.9 of [Part2]).

   The most common form of request-target is that used when making a
   request to an origin server ("origin form").  In this case, the
   absolute path and query components of the URI MUST be transmitted as
   the request-target, and the authority component MUST be transmitted
   in a Host header field.  For example, a client wishing to retrieve a
   representation of the resource, as identified above, directly from
   the origin server would open (or reuse) a TCP connection to port 80
   of the host "www.example.org" and send the lines:

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

   followed by the remainder of the Request.  Note that the origin form
   of request-target always starts with an absolute path; if the target
   resource's URI path is empty, then an absolute path of "/" MUST be
   provided in the request-target.

   If a proxy receives an OPTIONS request with an absolute-URI form of
   request-target in which the URI has an empty path and no query
   component, then the last proxy on the request chain MUST use a
   request-target of "*" when it forwards the request to the indicated
   origin server.

   For example, the request

     OPTIONS http://www.example.org:8001 HTTP/1.1





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   would be forwarded by the final proxy as

     OPTIONS * HTTP/1.1
     Host: www.example.org:8001

   after connecting to port 8001 of host "www.example.org".

   The request-target is transmitted in the format specified in
   Section 2.7.1.  If the request-target is percent-encoded ([RFC3986],
   Section 2.1), the origin server MUST decode the request-target in
   order to properly interpret the request.  Servers SHOULD respond to
   invalid request-targets with an appropriate status code.

   A non-transforming proxy MUST NOT rewrite the "path-absolute" part of
   the received request-target when forwarding it to the next inbound
   server, except as noted above to replace a null path-absolute with
   "/" or "*".

      Note: The "no rewrite" rule prevents the proxy from changing the
      meaning of the request when the origin server is improperly using
      a non-reserved URI character for a reserved purpose.  Implementors
      need to be aware that some pre-HTTP/1.1 proxies have been known to
      rewrite the request-target.

   HTTP does not place a pre-defined limit on the length of a request-
   target.  A server MUST be prepared to receive URIs of unbounded
   length and respond with the 414 (URI Too Long) status code if the
   received request-target would be longer than the server wishes to
   handle (see Section 8.4.15 of [Part2]).

   Various ad-hoc limitations on request-target length are found in
   practice.  It is RECOMMENDED that all HTTP senders and recipients
   support request-target lengths of 8000 or more octets.

      Note: Fragments ([RFC3986], Section 3.5) are not part of the
      request-target and thus will not be transmitted in an HTTP
      request.

4.2.  The Resource Identified by a Request

   The exact resource identified by an Internet request is determined by
   examining both the request-target and the Host header field.

   An origin server that does not allow resources to differ by the
   requested host MAY ignore the Host header field value when
   determining the resource identified by an HTTP/1.1 request.  (But see
   Appendix B.1.1 for other requirements on Host support in HTTP/1.1.)




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   An origin server that does differentiate resources based on the host
   requested (sometimes referred to as virtual hosts or vanity host
   names) MUST use the following rules for determining the requested
   resource on an HTTP/1.1 request:

   1.  If request-target is an absolute-URI, the host is part of the
       request-target.  Any Host header field value in the request MUST
       be ignored.

   2.  If the request-target is not an absolute-URI, and the request
       includes a Host header field, the host is determined by the Host
       header field value.

   3.  If the host as determined by rule 1 or 2 is not a valid host on
       the server, the response MUST be a 400 (Bad Request) error
       message.

   Recipients of an HTTP/1.0 request that lacks a Host header field MAY
   attempt to use heuristics (e.g., examination of the URI path for
   something unique to a particular host) in order to determine what
   exact resource is being requested.

4.3.  Effective Request URI

   HTTP requests often do not carry the absolute URI ([RFC3986], Section
   4.3) for the target resource; instead, the URI needs to be inferred
   from the request-target, Host header field, and connection context.
   The result of this process is called the "effective request URI".
   The "target resource" is the resource identified by the effective
   request URI.

   If the request-target is an absolute-URI, then the effective request
   URI is the request-target.

   If the request-target uses the path-absolute form or the asterisk
   form, and the Host header field is present, then the effective
   request URI is constructed by concatenating

   o  the scheme name: "http" if the request was received over an
      insecure TCP connection, or "https" when received over a SSL/
      TLS-secured TCP connection,

   o  the octet sequence "://",

   o  the authority component, as specified in the Host header field
      (Section 9.4), and





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   o  the request-target obtained from the Request-Line, unless the
      request-target is just the asterisk "*".

   If the request-target uses the path-absolute form or the asterisk
   form, and the Host header field is not present, then the effective
   request URI is undefined.

   Otherwise, when request-target uses the authority form, the effective
   request URI is undefined.

   Example 1: the effective request URI for the message

     GET /pub/WWW/TheProject.html HTTP/1.1
     Host: www.example.org:8080

   (received over an insecure TCP connection) is "http", plus "://",
   plus the authority component "www.example.org:8080", plus the
   request-target "/pub/WWW/TheProject.html", thus
   "http://www.example.org:8080/pub/WWW/TheProject.html".

   Example 2: the effective request URI for the message

     GET * HTTP/1.1
     Host: www.example.org

   (received over an SSL/TLS secured TCP connection) is "https", plus
   "://", plus the authority component "www.example.org", thus
   "https://www.example.org".

   Effective request URIs are compared using the rules described in
   Section 2.7.3, except that empty path components MUST NOT be treated
   as equivalent to an absolute path of "/".

5.  Response

   After receiving and interpreting a request message, a server responds
   with an HTTP response message.

     Response      = Status-Line               ; Section 5.1
                     *( header-field CRLF )    ; Section 3.2
                     CRLF
                     [ message-body ]          ; Section 3.3

5.1.  Status-Line

   The first line of a Response message is the Status-Line, consisting
   of the protocol version, a space (SP), the status code, another
   space, a possibly-empty textual phrase describing the status code,



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   and ending with CRLF.

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

5.1.1.  Status Code and Reason Phrase

   The Status-Code element is a 3-digit integer result code of the
   attempt to understand and satisfy the request.  These codes are fully
   defined in Section 8 of [Part2].  The Reason Phrase exists for the
   sole purpose of providing a textual description associated with the
   numeric status code, out of deference to earlier Internet application
   protocols that were more frequently used with interactive text
   clients.  A client SHOULD ignore the content of the Reason Phrase.

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

   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

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


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

6.  Protocol Parameters

6.1.  Date/Time Formats: Full Date

   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.



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     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.  See Appendix A for further information.

   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.

6.2.  Transfer Codings

   Transfer-coding values are used to indicate an encoding
   transformation that has been, can be, or might need to be applied to
   a payload body in order to ensure "safe transport" through the
   network.  This differs from a content coding in that the transfer-
   coding is a property of the message rather than a property of the
   representation that is being transferred.

     transfer-coding         = "chunked" ; Section 6.2.1
                             / "compress" ; Section 6.2.2.1
                             / "deflate" ; Section 6.2.2.2
                             / "gzip" ; Section 6.2.2.3
                             / transfer-extension
     transfer-extension      = token *( OWS ";" OWS transfer-parameter )

   Parameters are in the form of attribute/value pairs.

     transfer-parameter      = attribute BWS "=" BWS value
     attribute               = token
     value                   = word



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   All transfer-coding values are case-insensitive.  HTTP/1.1 uses
   transfer-coding values in the TE header field (Section 9.5) and in
   the Transfer-Encoding header field (Section 9.7).

   Transfer-codings are analogous to the Content-Transfer-Encoding
   values of MIME, which were designed to enable safe transport of
   binary data over a 7-bit transport service ([RFC2045], Section 6).
   However, safe transport has a different focus for an 8bit-clean
   transfer protocol.  In HTTP, the only unsafe characteristic of
   message-bodies is the difficulty in determining the exact message
   body length (Section 3.3), or the desire to encrypt data over a
   shared transport.

   A server that receives a request message with a transfer-coding it
   does not understand SHOULD respond with 501 (Not Implemented) and
   then close the connection.  A server MUST NOT send transfer-codings
   to an HTTP/1.0 client.

6.2.1.  Chunked Transfer Coding

   The chunked encoding modifies the body of a message in order to
   transfer it as a series of chunks, each with its own size indicator,
   followed by an OPTIONAL trailer containing header fields.  This
   allows dynamically produced content to be transferred along with the
   information necessary for the recipient to verify that it has
   received the full message.

     Chunked-Body   = *chunk
                      last-chunk
                      trailer-part
                      CRLF

     chunk          = chunk-size *WSP [ chunk-ext ] CRLF
                      chunk-data CRLF
     chunk-size     = 1*HEXDIG
     last-chunk     = 1*("0") *WSP [ chunk-ext ] CRLF

     chunk-ext      = *( ";" *WSP chunk-ext-name
                         [ "=" chunk-ext-val ] *WSP )
     chunk-ext-name = token
     chunk-ext-val  = token / quoted-str-nf
     chunk-data     = 1*OCTET ; a sequence of chunk-size octets
     trailer-part   = *( header-field CRLF )

     quoted-str-nf  = DQUOTE *( qdtext-nf / quoted-pair ) DQUOTE
                    ; like quoted-string, but disallowing line folding
     qdtext-nf      = WSP / %x21 / %x23-5B / %x5D-7E / obs-text
                    ; WSP / <VCHAR except DQUOTE and "\"> / obs-text



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   The chunk-size field is a string of hex digits indicating the size of
   the chunk-data in octets.  The chunked encoding is ended by any chunk
   whose size is zero, followed by the trailer, which is terminated by
   an empty line.

   The trailer allows the sender to include additional HTTP header
   fields at the end of the message.  The Trailer header field can be
   used to indicate which header fields are included in a trailer (see
   Section 9.6).

   A server using chunked transfer-coding in a response MUST NOT use the
   trailer for any header fields unless at least one of the following is
   true:

   1.  the request included a TE header field that indicates "trailers"
       is acceptable in the transfer-coding of the response, as
       described in Section 9.5; or,

   2.  the trailer fields consist entirely of optional metadata, and the
       recipient could use the message (in a manner acceptable to the
       server where the field originated) without receiving it.  In
       other words, the server that generated the header (often but not
       always the origin server) is willing to accept the possibility
       that the trailer fields might be silently discarded along the
       path to the client.

   This requirement prevents an interoperability failure when the
   message is being received by an HTTP/1.1 (or later) proxy and
   forwarded to an HTTP/1.0 recipient.  It avoids a situation where
   compliance with the protocol would have necessitated a possibly
   infinite buffer on the proxy.

   A process for decoding the "chunked" transfer-coding can be
   represented in pseudo-code as:

















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     length := 0
     read chunk-size, chunk-ext (if any) and CRLF
     while (chunk-size > 0) {
        read chunk-data and CRLF
        append chunk-data to decoded-body
        length := length + chunk-size
        read chunk-size and CRLF
     }
     read header-field
     while (header-field not empty) {
        append header-field to existing header fields
        read header-field
     }
     Content-Length := length
     Remove "chunked" from Transfer-Encoding

   All HTTP/1.1 applications MUST be able to receive and decode the
   "chunked" transfer-coding and MUST ignore chunk-ext extensions they
   do not understand.

   Since "chunked" is the only transfer-coding required to be understood
   by HTTP/1.1 recipients, it plays a crucial role in delimiting
   messages on a persistent connection.  Whenever a transfer-coding is
   applied to a payload body in a request, the final transfer-coding
   applied MUST be "chunked".  If a transfer-coding is applied to a
   response payload body, then either the final transfer-coding applied
   MUST be "chunked" or the message MUST be terminated by closing the
   connection.  When the "chunked" transfer-coding is used, it MUST be
   the last transfer-coding applied to form the message-body.  The
   "chunked" transfer-coding MUST NOT be applied more than once in a
   message-body.

6.2.2.  Compression Codings

   The codings defined below can be used to compress the payload of a
   message.

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

      Note: For compatibility with previous implementations of HTTP,
      applications SHOULD consider "x-gzip" and "x-compress" to be
      equivalent to "gzip" and "compress" respectively.






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6.2.2.1.  Compress Coding

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

6.2.2.2.  Deflate Coding

   The "deflate" format is defined as the "deflate" compression
   mechanism (described in [RFC1951]) used inside the "zlib" data format
   ([RFC1950]).

      Note: Some incorrect implementations send the "deflate" compressed
      data without the zlib wrapper.

6.2.2.3.  Gzip Coding

   The "gzip" format is produced by the file compression program "gzip"
   (GNU zip), as described in [RFC1952].  This format is a Lempel-Ziv
   coding (LZ77) with a 32 bit CRC.

6.2.3.  Transfer Coding Registry

   The HTTP Transfer Coding Registry defines the name space for the
   transfer coding names.

   Registrations MUST include the following fields:

   o  Name

   o  Description

   o  Pointer to specification text

   Names of transfer codings MUST NOT overlap with names of content
   codings (Section 2.2 of [Part3]), unless the encoding transformation
   is identical (as it is the case for the compression codings defined
   in Section 6.2.2).

   Values to be added to this name space require a specification (see
   "Specification Required" in Section 4.1 of [RFC5226]), and MUST
   conform to the purpose of transfer coding defined in this section.

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






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6.3.  Product Tokens

   Product tokens are used to allow communicating applications to
   identify themselves by software name and version.  Most fields using
   product tokens also allow sub-products which form a significant part
   of the application to be listed, separated by whitespace.  By
   convention, the products are listed in order of their significance
   for identifying the application.

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

   Examples:

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

   Product tokens SHOULD be short and to the point.  They MUST NOT be
   used for advertising or other non-essential information.  Although
   any token octet MAY appear in a product-version, this token SHOULD
   only be used for a version identifier (i.e., successive versions of
   the same product SHOULD only differ in the product-version portion of
   the product value).

6.4.  Quality Values

   Both transfer codings (TE request header field, Section 9.5) and
   content negotiation (Section 5 of [Part3]) use short "floating point"
   numbers to indicate the relative importance ("weight") of various
   negotiable parameters.  A weight is normalized to a real number in
   the range 0 through 1, where 0 is the minimum and 1 the maximum
   value.  If a parameter has a quality value of 0, then content with
   this parameter is "not acceptable" for the client.  HTTP/1.1
   applications MUST NOT generate more than three digits after the
   decimal point.  User configuration of these values SHOULD also be
   limited in this fashion.

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

      Note: "Quality values" is a misnomer, since these values merely
      represent relative degradation in desired quality.

7.  Connections







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7.1.  Persistent Connections

7.1.1.  Purpose

   Prior to persistent connections, a separate TCP connection was
   established for each request, increasing the load on HTTP servers and
   causing congestion on the Internet.  The use of inline images and
   other associated data often requires a client to make multiple
   requests of the same server in a short amount of time.  Analysis of
   these performance problems and results from a prototype
   implementation are available [Pad1995] [Spe].  Implementation
   experience and measurements of actual HTTP/1.1 implementations show
   good results [Nie1997].  Alternatives have also been explored, for
   example, T/TCP [Tou1998].

   Persistent HTTP connections have a number of advantages:

   o  By opening and closing fewer TCP connections, CPU time is saved in
      routers and hosts (clients, servers, proxies, gateways, tunnels,
      or caches), and memory used for TCP protocol control blocks can be
      saved in hosts.

   o  HTTP requests and responses can be pipelined on a connection.
      Pipelining allows a client to make multiple requests without
      waiting for each response, allowing a single TCP connection to be
      used much more efficiently, with much lower elapsed time.

   o  Network congestion is reduced by reducing the number of packets
      caused by TCP opens, and by allowing TCP sufficient time to
      determine the congestion state of the network.

   o  Latency on subsequent requests is reduced since there is no time
      spent in TCP's connection opening handshake.

   o  HTTP can evolve more gracefully, since errors can be reported
      without the penalty of closing the TCP connection.  Clients using
      future versions of HTTP might optimistically try a new feature,
      but if communicating with an older server, retry with old
      semantics after an error is reported.

   HTTP implementations SHOULD implement persistent connections.

7.1.2.  Overall Operation

   A significant difference between HTTP/1.1 and earlier versions of
   HTTP is that persistent connections are the default behavior of any
   HTTP connection.  That is, unless otherwise indicated, the client
   SHOULD assume that the server will maintain a persistent connection,



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   even after error responses from the server.

   Persistent connections provide a mechanism by which a client and a
   server can signal the close of a TCP connection.  This signaling
   takes place using the Connection header field (Section 9.1).  Once a
   close has been signaled, the client MUST NOT send any more requests
   on that connection.

7.1.2.1.  Negotiation

   An HTTP/1.1 server MAY assume that a HTTP/1.1 client intends to
   maintain a persistent connection unless a Connection header field
   including the connection-token "close" was sent in the request.  If
   the server chooses to close the connection immediately after sending
   the response, it SHOULD send a Connection header field including the
   connection-token "close".

   An HTTP/1.1 client MAY expect a connection to remain open, but would
   decide to keep it open based on whether the response from a server
   contains a Connection header field with the connection-token close.
   In case the client does not want to maintain a connection for more
   than that request, it SHOULD send a Connection header field including
   the connection-token close.

   If either the client or the server sends the close token in the
   Connection header field, that request becomes the last one for the
   connection.

   Clients and servers SHOULD NOT assume that a persistent connection is
   maintained for HTTP versions less than 1.1 unless it is explicitly
   signaled.  See Appendix B.1.2 for more information on backward
   compatibility with HTTP/1.0 clients.

   In order to remain persistent, all messages on the connection MUST
   have a self-defined message length (i.e., one not defined by closure
   of the connection), as described in Section 3.3.

7.1.2.2.  Pipelining

   A client that supports persistent connections MAY "pipeline" its
   requests (i.e., send multiple requests without waiting for each
   response).  A server MUST send its responses to those requests in the
   same order that the requests were received.

   Clients which assume persistent connections and pipeline immediately
   after connection establishment SHOULD be prepared to retry their
   connection if the first pipelined attempt fails.  If a client does
   such a retry, it MUST NOT pipeline before it knows the connection is



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   persistent.  Clients MUST also be prepared to resend their requests
   if the server closes the connection before sending all of the
   corresponding responses.

   Clients SHOULD NOT pipeline requests using non-idempotent request
   methods or non-idempotent sequences of request methods (see Section
   7.1.2 of [Part2]).  Otherwise, a premature termination of the
   transport connection could lead to indeterminate results.  A client
   wishing to send a non-idempotent request SHOULD wait to send that
   request until it has received the response status line for the
   previous request.

7.1.3.  Proxy Servers

   It is especially important that proxies correctly implement the
   properties of the Connection header field as specified in
   Section 9.1.

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

   A proxy server MUST NOT establish a HTTP/1.1 persistent connection
   with an HTTP/1.0 client (but see Section 19.7.1 of [RFC2068] for
   information and discussion of the problems with the Keep-Alive header
   field implemented by many HTTP/1.0 clients).

7.1.3.1.  End-to-end and Hop-by-hop Header Fields

   For the purpose of defining the behavior of caches and non-caching
   proxies, we divide HTTP header fields into two categories:

   o  End-to-end header fields, which are transmitted to the ultimate
      recipient of a request or response.  End-to-end header fields in
      responses MUST be stored as part of a cache entry and MUST be
      transmitted in any response formed from a cache entry.

   o  Hop-by-hop header fields, which are meaningful only for a single
      transport-level connection, and are not stored by caches or
      forwarded by proxies.

   The following HTTP/1.1 header fields are hop-by-hop header fields:

   o  Connection

   o  Keep-Alive




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   o  Proxy-Authenticate

   o  Proxy-Authorization

   o  TE

   o  Trailer

   o  Transfer-Encoding

   o  Upgrade

   All other header fields defined by HTTP/1.1 are end-to-end header
   fields.

   Other hop-by-hop header fields MUST be listed in a Connection header
   field (Section 9.1).

7.1.3.2.  Non-modifiable Header Fields

   Some features of HTTP/1.1, such as Digest Authentication, depend on
   the value of certain end-to-end header fields.  A non-transforming
   proxy SHOULD NOT modify an end-to-end header field unless the
   definition of that header field requires or specifically allows that.

   A non-transforming proxy MUST NOT modify any of the following fields
   in a request or response, and it MUST NOT add any of these fields if
   not already present:

   o  Allow

   o  Content-Location

   o  Content-MD5

   o  ETag

   o  Last-Modified

   o  Server

   A non-transforming proxy MUST NOT modify any of the following fields
   in a response:

   o  Expires

   but it MAY add any of these fields if not already present.  If an
   Expires header field is added, it MUST be given a field-value



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   identical to that of the Date header field in that response.

   A proxy MUST NOT modify or add any of the following fields in a
   message that contains the no-transform cache-control directive, or in
   any request:

   o  Content-Encoding

   o  Content-Range

   o  Content-Type

   A transforming proxy MAY modify or add these fields to a message that
   does not include no-transform, but if it does so, it MUST add a
   Warning 214 (Transformation applied) if one does not already appear
   in the message (see Section 3.6 of [Part6]).

      Warning: Unnecessary modification of end-to-end header fields
      might cause authentication failures if stronger authentication
      mechanisms are introduced in later versions of HTTP.  Such
      authentication mechanisms MAY rely on the values of header fields
      not listed here.

   A non-transforming proxy MUST preserve the message payload ([Part3]),
   though it MAY change the message-body through application or removal
   of a transfer-coding (Section 6.2).

7.1.4.  Practical Considerations

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

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

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



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   request is in progress.

   This means that clients, servers, and proxies MUST be able to recover
   from asynchronous close events.  Client software SHOULD reopen the
   transport connection and retransmit the aborted sequence of requests
   without user interaction so long as the request sequence is
   idempotent (see Section 7.1.2 of [Part2]).  Non-idempotent request
   methods or sequences MUST NOT be automatically retried, although user
   agents MAY offer a human operator the choice of retrying the
   request(s).  Confirmation by user-agent software with semantic
   understanding of the application MAY substitute for user
   confirmation.  The automatic retry SHOULD NOT be repeated if the
   second sequence of requests fails.

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

   Clients (including proxies) SHOULD limit the number of simultaneous
   connections that they maintain to a given server (including proxies).

   Previous revisions of HTTP gave a specific number of connections as a
   ceiling, but this was found to be impractical for many applications.
   As a result, this specification does not mandate a particular maximum
   number of connections, but instead encourages clients to be
   conservative when opening multiple connections.

   In particular, while using multiple connections avoids the "head-of-
   line blocking" problem (whereby a request that takes significant
   server-side processing and/or has a large payload can block
   subsequent requests on the same connection), each connection used
   consumes server resources (sometimes significantly), and furthermore
   using multiple connections can cause undesirable side effects in
   congested networks.

   Note that servers might reject traffic that they deem abusive,
   including an excessive number of connections from a client.

7.2.  Message Transmission Requirements

7.2.1.  Persistent Connections and Flow Control

   HTTP/1.1 servers SHOULD maintain persistent connections and use TCP's
   flow control mechanisms to resolve temporary overloads, rather than
   terminating connections with the expectation that clients will retry.
   The latter technique can exacerbate network congestion.




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7.2.2.  Monitoring Connections for Error Status Messages

   An HTTP/1.1 (or later) client sending a message-body SHOULD monitor
   the network connection for an error status code while it is
   transmitting the request.  If the client sees an error status code,
   it SHOULD immediately cease transmitting the body.  If the body is
   being sent using a "chunked" encoding (Section 6.2), a zero length
   chunk and empty trailer MAY be used to prematurely mark the end of
   the message.  If the body was preceded by a Content-Length header
   field, the client MUST close the connection.

7.2.3.  Use of the 100 (Continue) Status

   The purpose of the 100 (Continue) status code (see Section 8.1.1 of
   [Part2]) is to allow a client that is sending a request message with
   a request body to determine if the origin server is willing to accept
   the request (based on the request header fields) before the client
   sends the request body.  In some cases, it might either be
   inappropriate or highly inefficient for the client to send the body
   if the server will reject the message without looking at the body.

   Requirements for HTTP/1.1 clients:

   o  If a client will wait for a 100 (Continue) response before sending
      the request body, it MUST send an Expect header field (Section 9.2
      of [Part2]) with the "100-continue" expectation.

   o  A client MUST NOT send an Expect header field (Section 9.2 of
      [Part2]) with the "100-continue" expectation if it does not intend
      to send a request body.

   Because of the presence of older implementations, the protocol allows
   ambiguous situations in which a client might send "Expect: 100-
   continue" without receiving either a 417 (Expectation Failed) or a
   100 (Continue) status code.  Therefore, when a client sends this
   header field to an origin server (possibly via a proxy) from which it
   has never seen a 100 (Continue) status code, the client SHOULD NOT
   wait for an indefinite period before sending the request body.

   Requirements for HTTP/1.1 origin servers:

   o  Upon receiving a request which includes an Expect header field
      with the "100-continue" expectation, an origin server MUST either
      respond with 100 (Continue) status code and continue to read from
      the input stream, or respond with a final status code.  The origin
      server MUST NOT wait for the request body before sending the 100
      (Continue) response.  If it responds with a final status code, it
      MAY close the transport connection or it MAY continue to read and



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      discard the rest of the request.  It MUST NOT perform the request
      method if it returns a final status code.

   o  An origin server SHOULD NOT send a 100 (Continue) response if the
      request message does not include an Expect header field with the
      "100-continue" expectation, and MUST NOT send a 100 (Continue)
      response if such a request comes from an HTTP/1.0 (or earlier)
      client.  There is an exception to this rule: for compatibility
      with [RFC2068], a server MAY send a 100 (Continue) status code in
      response to an HTTP/1.1 PUT or POST request that does not include
      an Expect header field with the "100-continue" expectation.  This
      exception, the purpose of which is to minimize any client
      processing delays associated with an undeclared wait for 100
      (Continue) status code, applies only to HTTP/1.1 requests, and not
      to requests with any other HTTP-version value.

   o  An origin server MAY omit a 100 (Continue) response if it has
      already received some or all of the request body for the
      corresponding request.

   o  An origin server that sends a 100 (Continue) response MUST
      ultimately send a final status code, once the request body is
      received and processed, unless it terminates the transport
      connection prematurely.

   o  If an origin server receives a request that does not include an
      Expect header field with the "100-continue" expectation, the
      request includes a request body, and the server responds with a
      final status code before reading the entire request body from the
      transport connection, then the server SHOULD NOT close the
      transport connection until it has read the entire request, or
      until the client closes the connection.  Otherwise, the client
      might not reliably receive the response message.  However, this
      requirement is not be construed as preventing a server from
      defending itself against denial-of-service attacks, or from badly
      broken client implementations.

   Requirements for HTTP/1.1 proxies:

   o  If a proxy receives a request that includes an Expect header field
      with the "100-continue" expectation, and the proxy either knows
      that the next-hop server complies with HTTP/1.1 or higher, or does
      not know the HTTP version of the next-hop server, it MUST forward
      the request, including the Expect header field.

   o  If the proxy knows that the version of the next-hop server is
      HTTP/1.0 or lower, it MUST NOT forward the request, and it MUST
      respond with a 417 (Expectation Failed) status code.



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   o  Proxies SHOULD maintain a cache recording the HTTP version numbers
      received from recently-referenced next-hop servers.

   o  A proxy MUST NOT forward a 100 (Continue) response if the request
      message was received from an HTTP/1.0 (or earlier) client and did
      not include an Expect header field with the "100-continue"
      expectation.  This requirement overrides the general rule for
      forwarding of 1xx responses (see Section 8.1 of [Part2]).

7.2.4.  Client Behavior if Server Prematurely Closes Connection

   If an HTTP/1.1 client sends a request which includes a request body,
   but which does not include an Expect header field with the "100-
   continue" expectation, and if the client is not directly connected to
   an HTTP/1.1 origin server, and if the client sees the connection
   close before receiving a status line from the server, the client
   SHOULD retry the request.  If the client does retry this request, it
   MAY use the following "binary exponential backoff" algorithm to be
   assured of obtaining a reliable response:

   1.  Initiate a new connection to the server

   2.  Transmit the request-line, header fields, and the CRLF that
       indicates the end of header fields.

   3.  Initialize a variable R to the estimated round-trip time to the
       server (e.g., based on the time it took to establish the
       connection), or to a constant value of 5 seconds if the round-
       trip time is not available.

   4.  Compute T = R * (2**N), where N is the number of previous retries
       of this request.

   5.  Wait either for an error response from the server, or for T
       seconds (whichever comes first)

   6.  If no error response is received, after T seconds transmit the
       body of the request.

   7.  If client sees that the connection is closed prematurely, repeat
       from step 1 until the request is accepted, an error response is
       received, or the user becomes impatient and terminates the retry
       process.

   If at any point an error status code is received, the client

   o  SHOULD NOT continue and




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   o  SHOULD close the connection if it has not completed sending the
      request message.

8.  Miscellaneous notes that might disappear

8.1.  Scheme aliases considered harmful

   [[TBD-aliases-harmful: describe why aliases like webcal are
   harmful.]]

8.2.  Use of HTTP for proxy communication

   [[TBD-proxy-other: Configured to use HTTP to proxy HTTP or other
   protocols.]]

8.3.  Interception of HTTP for access control

   [[TBD-intercept: Interception of HTTP traffic for initiating access
   control.]]

8.4.  Use of HTTP by other protocols

   [[TBD-profiles: Profiles of HTTP defined by other protocol.
   Extensions of HTTP like WebDAV.]]

8.5.  Use of HTTP by media type specification

   [[TBD-hypertext: Instructions on composing HTTP requests via
   hypertext formats.]]

9.  Header Field Definitions

   This section defines the syntax and semantics of HTTP header fields
   related to message framing and transport protocols.

9.1.  Connection

   The "Connection" header field allows the sender to specify options
   that are desired only for that particular connection.  Such
   connection options MUST be removed or replaced before the message can
   be forwarded downstream by a proxy or gateway.  This mechanism also
   allows the sender to indicate which HTTP header fields used in the
   message are only intended for the immediate recipient ("hop-by-hop"),
   as opposed to all recipients on the chain ("end-to-end"), enabling
   the message to be self-descriptive and allowing future connection-
   specific extensions to be deployed in HTTP without fear that they
   will be blindly forwarded by previously deployed intermediaries.




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   The Connection header field's value has the following grammar:

     Connection       = 1#connection-token
     connection-token = token

   A proxy or gateway MUST parse a received Connection header field
   before a message is forwarded and, for each connection-token in this
   field, remove any header field(s) from the message with the same name
   as the connection-token, and then remove the Connection header field
   itself or replace it with the sender's own connection options for the
   forwarded message.

   A sender MUST NOT include field-names in the Connection header field-
   value for fields that are defined as expressing constraints for all
   recipients in the request or response chain, such as the Cache-
   Control header field (Section 3.2 of [Part6]).

   The connection options do not have to correspond to a header field
   present in the message, since a connection-specific header field
   might not be needed if there are no parameters associated with that
   connection option.  Recipients that trigger certain connection
   behavior based on the presence of connection options MUST do so based
   on the presence of the connection-token rather than only the presence
   of the optional header field.  In other words, if the connection
   option is received as a header field but not indicated within the
   Connection field-value, then the recipient MUST ignore the
   connection-specific header field because it has likely been forwarded
   by an intermediary that is only partially compliant.

   When defining new connection options, specifications ought to
   carefully consider existing deployed header fields and ensure that
   the new connection-token does not share the same name as an unrelated
   header field that might already be deployed.  Defining a new
   connection-token essentially reserves that potential field-name for
   carrying additional information related to the connection option,
   since it would be unwise for senders to use that field-name for
   anything else.

   HTTP/1.1 defines the "close" connection option for the sender to
   signal that the connection will be closed after completion of the
   response.  For example,

     Connection: close

   in either the request or the response header fields indicates that
   the connection SHOULD NOT be considered "persistent" (Section 7.1)
   after the current request/response is complete.




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   An HTTP/1.1 client that does not support persistent connections MUST
   include the "close" connection option in every request message.

   An HTTP/1.1 server that does not support persistent connections MUST
   include the "close" connection option in every response message that
   does not have a 1xx (Informational) status code.

9.2.  Content-Length

   The "Content-Length" header field indicates the size of the message-
   body, in decimal number of octets, for any message other than a
   response to a HEAD request or a response with a status code of 304.
   In the case of a response to a HEAD request, Content-Length indicates
   the size of the payload body (not including any potential transfer-
   coding) that would have been sent had the request been a GET.  In the
   case of a 304 (Not Modified) response to a GET request, Content-
   Length indicates the size of the payload body (not including any
   potential transfer-coding) that would have been sent in a 200 (OK)
   response.

     Content-Length = 1*DIGIT

   An example is

     Content-Length: 3495

   Implementations SHOULD use this field to indicate the message-body
   length when no transfer-coding is being applied and the payload's
   body length can be determined prior to being transferred.
   Section 3.3 describes how recipients determine the length of a
   message-body.

   Any Content-Length greater than or equal to zero is a valid value.

   Note that the use of this field in HTTP is significantly different
   from the corresponding definition in MIME, where it is an optional
   field used within the "message/external-body" content-type.

9.3.  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 described in Section 6.1; it MUST be
   sent in rfc1123-date format.

     Date = HTTP-date




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   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.  In this case, the rules in Section 9.3.1
       MUST be followed.

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

9.3.1.  Clockless Origin Server Operation

   Some origin server implementations might not have a clock available.
   An origin server without a clock MUST NOT assign Expires or Last-
   Modified values to a response, unless these values were associated
   with the resource by a system or user with a reliable clock.  It MAY
   assign an Expires value that is known, at or before server
   configuration time, to be in the past (this allows "pre-expiration"
   of responses without storing separate Expires values for each
   resource).



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9.4.  Host

   The "Host" header field in a request provides the host and port
   information from the target resource's URI, enabling the origin
   server to distinguish between resources while servicing requests for
   multiple host names on a single IP address.  Since the Host field-
   value is critical information for handling a request, it SHOULD be
   sent as the first header field following the Request-Line.

     Host = uri-host [ ":" port ] ; Section 2.7.1

   A client MUST send a Host header field in all HTTP/1.1 request
   messages.  If the target resource's URI includes an authority
   component, then the Host field-value MUST be identical to that
   authority component after excluding any userinfo (Section 2.7.1).  If
   the authority component is missing or undefined for the target
   resource's URI, then the Host header field MUST be sent with an empty
   field-value.

   For example, a GET request to the origin server for
   <http://www.example.org/pub/WWW/> would begin with:

     GET /pub/WWW/ HTTP/1.1
     Host: www.example.org

   The Host header field MUST be sent in an HTTP/1.1 request even if the
   request-target is in the form of an absolute-URI, since this allows
   the Host information to be forwarded through ancient HTTP/1.0 proxies
   that might not have implemented Host.

   When an HTTP/1.1 proxy receives a request with a request-target in
   the form of an absolute-URI, the proxy MUST ignore the received Host
   header field (if any) and instead replace it with the host
   information of the request-target.  When a proxy forwards a request,
   it MUST generate the Host header field based on the received
   absolute-URI rather than the received Host.

   Since the Host header field acts as an application-level routing
   mechanism, it is a frequent target for malware seeking to poison a
   shared cache or redirect a request to an unintended server.  An
   interception proxy is particularly vulnerable if it relies on the
   Host header field value for redirecting requests to internal servers,
   or for use as a cache key in a shared cache, without first verifying
   that the intercepted connection is targeting a valid IP address for
   that host.

   A server MUST respond with a 400 (Bad Request) status code to any
   HTTP/1.1 request message that lacks a Host header field and to any



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   request message that contains more than one Host header field or a
   Host header field with an invalid field-value.

   See Sections 4.2 and B.1.1 for other requirements relating to Host.

9.5.  TE

   The "TE" header field indicates what extension transfer-codings it is
   willing to accept in the response, and whether or not it is willing
   to accept trailer fields in a chunked transfer-coding.

   Its value consists of the keyword "trailers" and/or a comma-separated
   list of extension transfer-coding names with optional accept
   parameters (as described in Section 6.2).

     TE        = #t-codings
     t-codings = "trailers" / ( transfer-extension [ te-params ] )
     te-params = OWS ";" OWS "q=" qvalue *( te-ext )
     te-ext    = OWS ";" OWS token [ "=" word ]

   The presence of the keyword "trailers" indicates that the client is
   willing to accept trailer fields in a chunked transfer-coding, as
   defined in Section 6.2.1.  This keyword is reserved for use with
   transfer-coding values even though it does not itself represent a
   transfer-coding.

   Examples of its use are:

     TE: deflate
     TE:
     TE: trailers, deflate;q=0.5

   The TE header field only applies to the immediate connection.
   Therefore, the keyword MUST be supplied within a Connection header
   field (Section 9.1) whenever TE is present in an HTTP/1.1 message.

   A server tests whether a transfer-coding is acceptable, according to
   a TE field, using these rules:

   1.  The "chunked" transfer-coding is always acceptable.  If the
       keyword "trailers" is listed, the client indicates that it is
       willing to accept trailer fields in the chunked response on
       behalf of itself and any downstream clients.  The implication is
       that, if given, the client is stating that either all downstream
       clients are willing to accept trailer fields in the forwarded
       response, or that it will attempt to buffer the response on
       behalf of downstream recipients.




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       Note: HTTP/1.1 does not define any means to limit the size of a
       chunked response such that a client can be assured of buffering
       the entire response.

   2.  If the transfer-coding being tested is one of the transfer-
       codings listed in the TE field, then it is acceptable unless it
       is accompanied by a qvalue of 0.  (As defined in Section 6.4, a
       qvalue of 0 means "not acceptable".)

   3.  If multiple transfer-codings are acceptable, then the acceptable
       transfer-coding with the highest non-zero qvalue is preferred.
       The "chunked" transfer-coding always has a qvalue of 1.

   If the TE field-value is empty or if no TE field is present, the only
   transfer-coding is "chunked".  A message with no transfer-coding is
   always acceptable.

9.6.  Trailer

   The "Trailer" header field indicates that the given set of header
   fields is present in the trailer of a message encoded with chunked
   transfer-coding.

     Trailer = 1#field-name

   An HTTP/1.1 message SHOULD include a Trailer header field in a
   message using chunked transfer-coding with a non-empty trailer.
   Doing so allows the recipient to know which header fields to expect
   in the trailer.

   If no Trailer header field is present, the trailer SHOULD NOT include
   any header fields.  See Section 6.2.1 for restrictions on the use of
   trailer fields in a "chunked" transfer-coding.

   Message header fields listed in the Trailer header field MUST NOT
   include the following header fields:

   o  Transfer-Encoding

   o  Content-Length

   o  Trailer

9.7.  Transfer-Encoding

   The "Transfer-Encoding" header field indicates what transfer-codings
   (if any) have been applied to the message body.  It differs from
   Content-Encoding (Section 2.2 of [Part3]) in that transfer-codings



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   are a property of the message (and therefore are removed by
   intermediaries), whereas content-codings are not.

     Transfer-Encoding = 1#transfer-coding

   Transfer-codings are defined in Section 6.2.  An example is:

     Transfer-Encoding: chunked

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

   Many older HTTP/1.0 applications do not understand the Transfer-
   Encoding header field.

9.8.  Upgrade

   The "Upgrade" header field allows the client to specify what
   additional communication protocols it would like to use, if the
   server chooses to switch protocols.  Servers can use it to indicate
   what protocols they are willing to switch to.

     Upgrade = 1#product

   For example,

     Upgrade: HTTP/2.0, SHTTP/1.3, IRC/6.9, RTA/x11

   The Upgrade header field is intended to provide a simple mechanism
   for transition from HTTP/1.1 to some other, incompatible protocol.
   It does so by allowing the client to advertise its desire to use
   another protocol, such as a later version of HTTP with a higher major
   version number, even though the current request has been made using
   HTTP/1.1.  This eases the difficult transition between incompatible
   protocols by allowing the client to initiate a request in the more
   commonly supported protocol while indicating to the server that it
   would like to use a "better" protocol if available (where "better" is
   determined by the server, possibly according to the nature of the
   request method or target resource).

   The Upgrade header field only applies to switching application-layer
   protocols upon the existing transport-layer connection.  Upgrade
   cannot be used to insist on a protocol change; its acceptance and use
   by the server is optional.  The capabilities and nature of the
   application-layer communication after the protocol change is entirely
   dependent upon the new protocol chosen, although the first action



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   after changing the protocol MUST be a response to the initial HTTP
   request containing the Upgrade header field.

   The Upgrade header field only applies to the immediate connection.
   Therefore, the upgrade keyword MUST be supplied within a Connection
   header field (Section 9.1) whenever Upgrade is present in an HTTP/1.1
   message.

   The Upgrade header field cannot be used to indicate a switch to a
   protocol on a different connection.  For that purpose, it is more
   appropriate to use a 3xx redirection response (Section 8.3 of
   [Part2]).

   Servers MUST include the "Upgrade" header field in 101 (Switching
   Protocols) responses to indicate which protocol(s) are being switched
   to, and MUST include it in 426 (Upgrade Required) responses to
   indicate acceptable protocols to upgrade to.  Servers MAY include it
   in any other response to indicate that they are willing to upgrade to
   one of the specified protocols.

   This specification only defines the protocol name "HTTP" for use by
   the family of Hypertext Transfer Protocols, as defined by the HTTP
   version rules of Section 2.6 and future updates to this
   specification.  Additional tokens can be registered with IANA using
   the registration procedure defined below.

9.8.1.  Upgrade Token Registry

   The HTTP Upgrade Token Registry defines the name space for product
   tokens used to identify protocols in the Upgrade header field.  Each
   registered token is associated with contact information and an
   optional set of specifications that details how the connection will
   be processed after it has been upgraded.

   Registrations are allowed on a First Come First Served basis as
   described in Section 4.1 of [RFC5226].  The specifications need not
   be IETF documents or be subject to IESG review.  Registrations are
   subject to the following rules:

   1.  A token, once registered, stays registered forever.

   2.  The registration MUST name a responsible party for the
       registration.

   3.  The registration MUST name a point of contact.

   4.  The registration MAY name a set of specifications associated with
       that token.  Such specifications need not be publicly available.



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   5.  The responsible party MAY change the registration at any time.
       The IANA will keep a record of all such changes, and make them
       available upon request.

   6.  The responsible party for the first registration of a "product"
       token MUST approve later registrations of a "version" token
       together with that "product" token before they can be registered.

   7.  If absolutely required, the IESG MAY reassign the responsibility
       for a token.  This will normally only be used in the case when a
       responsible party cannot be contacted.

9.9.  Via

   The "Via" header field MUST be sent by a proxy or gateway to indicate
   the intermediate protocols and recipients between the user agent and
   the server on requests, and between the origin server and the client
   on responses.  It is analogous to the "Received" field used by email
   systems (Section 3.6.7 of [RFC5322]) and is intended to be used for
   tracking message forwards, avoiding request loops, and identifying
   the protocol capabilities of all senders along the request/response
   chain.

     Via               = 1#( received-protocol RWS received-by
                             [ RWS comment ] )
     received-protocol = [ protocol-name "/" ] protocol-version
     protocol-name     = token
     protocol-version  = token
     received-by       = ( uri-host [ ":" port ] ) / pseudonym
     pseudonym         = token

   The received-protocol indicates the protocol version of the message
   received by the server or client along each segment of the request/
   response chain.  The received-protocol version is appended to the Via
   field value when the message is forwarded so that information about
   the protocol capabilities of upstream applications remains visible to
   all recipients.

   The protocol-name is excluded if and only if it would be "HTTP".  The
   received-by field is normally the host and optional port number of a
   recipient server or client that subsequently forwarded the message.
   However, if the real host is considered to be sensitive information,
   it MAY be replaced by a pseudonym.  If the port is not given, it MAY
   be assumed to be the default port of the received-protocol.

   Multiple Via field values represent each proxy or gateway that has
   forwarded the message.  Each recipient MUST append its information
   such that the end result is ordered according to the sequence of



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   forwarding applications.

   Comments MAY be used in the Via header field to identify the software
   of each recipient, analogous to the User-Agent and Server header
   fields.  However, all comments in the Via field are optional and MAY
   be removed by any recipient prior to forwarding the message.

   For example, a request message could be sent from an HTTP/1.0 user
   agent to an internal proxy code-named "fred", which uses HTTP/1.1 to
   forward the request to a public proxy at p.example.net, which
   completes the request by forwarding it to the origin server at
   www.example.com.  The request received by www.example.com would then
   have the following Via header field:

     Via: 1.0 fred, 1.1 p.example.net (Apache/1.1)

   A proxy or gateway used as a portal through a network firewall SHOULD
   NOT forward the names and ports of hosts within the firewall region
   unless it is explicitly enabled to do so.  If not enabled, the
   received-by host of any host behind the firewall SHOULD be replaced
   by an appropriate pseudonym for that host.

   For organizations that have strong privacy requirements for hiding
   internal structures, a proxy or gateway MAY combine an ordered
   subsequence of Via header field entries with identical received-
   protocol values into a single such entry.  For example,

     Via: 1.0 ricky, 1.1 ethel, 1.1 fred, 1.0 lucy

   could be collapsed to

     Via: 1.0 ricky, 1.1 mertz, 1.0 lucy

   Senders SHOULD NOT combine multiple entries unless they are all under
   the same organizational control and the hosts have already been
   replaced by pseudonyms.  Senders MUST NOT combine entries which have
   different received-protocol values.

10.  IANA Considerations

10.1.  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   |
   +-------------------+----------+----------+-------------+
   | Connection        | http     | standard | Section 9.1 |
   | Content-Length    | http     | standard | Section 9.2 |
   | Date              | http     | standard | Section 9.3 |
   | Host              | http     | standard | Section 9.4 |
   | TE                | http     | standard | Section 9.5 |
   | Trailer           | http     | standard | Section 9.6 |
   | Transfer-Encoding | http     | standard | Section 9.7 |
   | Upgrade           | http     | standard | Section 9.8 |
   | Via               | http     | standard | Section 9.9 |
   +-------------------+----------+----------+-------------+

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

10.2.  URI Scheme Registration

   The entries for the "http" and "https" URI Schemes in the registry
   located at <http://www.iana.org/assignments/uri-schemes.html> shall
   be updated to point to Sections 2.7.1 and 2.7.2 of this document (see
   [RFC4395]).

10.3.  Internet Media Type Registrations

   This document serves as the specification for the Internet media
   types "message/http" and "application/http".  The following is to be
   registered with IANA (see [RFC4288]).

10.3.1.  Internet Media Type message/http

   The message/http type can be used to enclose a single HTTP request or
   response message, provided that it obeys the MIME restrictions for
   all "message" types regarding line length and encodings.

   Type name:  message

   Subtype name:  http

   Required parameters:  none

   Optional parameters:  version, msgtype

      version:  The HTTP-Version number of the enclosed message (e.g.,
         "1.1").  If not present, the version can be determined from the
         first line of the body.




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      msgtype:  The message type -- "request" or "response".  If not
         present, the type can be determined from the first line of the
         body.

   Encoding considerations:  only "7bit", "8bit", or "binary" are
      permitted

   Security considerations:  none

   Interoperability considerations:  none

   Published specification:  This specification (see Section 10.3.1).

   Applications that use this media type:

   Additional information:

      Magic number(s):  none

      File extension(s):  none

      Macintosh file type code(s):  none

   Person and email address to contact for further information:  See
      Authors Section.

   Intended usage:  COMMON

   Restrictions on usage:  none

   Author/Change controller:  IESG

10.3.2.  Internet Media Type application/http

   The application/http type can be used to enclose a pipeline of one or
   more HTTP request or response messages (not intermixed).

   Type name:  application

   Subtype name:  http

   Required parameters:  none

   Optional parameters:  version, msgtype







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      version:  The HTTP-Version number of the enclosed messages (e.g.,
         "1.1").  If not present, the version can be determined from the
         first line of the body.

      msgtype:  The message type -- "request" or "response".  If not
         present, the type can be determined from the first line of the
         body.

   Encoding considerations:  HTTP messages enclosed by this type are in
      "binary" format; use of an appropriate Content-Transfer-Encoding
      is required when transmitted via E-mail.

   Security considerations:  none

   Interoperability considerations:  none

   Published specification:  This specification (see Section 10.3.2).

   Applications that use this media type:

   Additional information:

      Magic number(s):  none

      File extension(s):  none

      Macintosh file type code(s):  none

   Person and email address to contact for further information:  See
      Authors Section.

   Intended usage:  COMMON

   Restrictions on usage:  none

   Author/Change controller:  IESG

10.4.  Transfer Coding Registry

   The registration procedure for HTTP Transfer Codings is now defined
   by Section 6.2.3 of this document.

   The HTTP Transfer Codings Registry located at
   <http://www.iana.org/assignments/http-parameters> shall be updated
   with the registrations below:






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   +----------+--------------------------------------+-----------------+
   | Name     | Description                          | Reference       |
   +----------+--------------------------------------+-----------------+
   | chunked  | Transfer in a series of chunks       | Section 6.2.1   |
   | compress | UNIX "compress" program method       | Section 6.2.2.1 |
   | deflate  | "deflate" compression mechanism      | Section 6.2.2.2 |
   |          | ([RFC1951]) used inside the "zlib"   |                 |
   |          | data format ([RFC1950])              |                 |
   | gzip     | Same as GNU zip [RFC1952]            | Section 6.2.2.3 |
   +----------+--------------------------------------+-----------------+

10.5.  Upgrade Token Registration

   The registration procedure for HTTP Upgrade Tokens -- previously
   defined in Section 7.2 of [RFC2817] -- is now defined by
   Section 9.8.1 of this document.

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

   +-------+---------------------------+-------------------------------+
   | Value | Description               | Reference                     |
   +-------+---------------------------+-------------------------------+
   | HTTP  | Hypertext Transfer        | Section 2.6 of this           |
   |       | Protocol                  | specification                 |
   +-------+---------------------------+-------------------------------+

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.  Personal Information

   HTTP clients are often privy to large amounts of personal information
   (e.g., the user's name, location, mail address, passwords, encryption
   keys, etc.), and SHOULD be very careful to prevent unintentional
   leakage of this information.  We very strongly recommend that a
   convenient interface be provided for the user to control
   dissemination of such information, and that designers and
   implementors be particularly careful in this area.  History shows
   that errors in this area often create serious security and/or privacy
   problems and generate highly adverse publicity for the implementor's
   company.



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11.2.  Abuse of Server Log Information

   A server is in the position to save personal data about a user's
   requests which might identify their reading patterns or subjects of
   interest.  This information is clearly confidential in nature and its
   handling can be constrained by law in certain countries.  People
   using HTTP to provide data are responsible for ensuring that such
   material is not distributed without the permission of any individuals
   that are identifiable by the published results.

11.3.  Attacks Based On File and Path Names

   Implementations of HTTP origin servers SHOULD be careful to restrict
   the documents returned by HTTP requests to be only those that were
   intended by the server administrators.  If an HTTP server translates
   HTTP URIs directly into file system calls, the server MUST take
   special care not to serve files that were not intended to be
   delivered to HTTP clients.  For example, UNIX, Microsoft Windows, and
   other operating systems use ".." as a path component to indicate a
   directory level above the current one.  On such a system, an HTTP
   server MUST disallow any such construct in the request-target if it
   would otherwise allow access to a resource outside those intended to
   be accessible via the HTTP server.  Similarly, files intended for
   reference only internally to the server (such as access control
   files, configuration files, and script code) MUST be protected from
   inappropriate retrieval, since they might contain sensitive
   information.  Experience has shown that minor bugs in such HTTP
   server implementations have turned into security risks.

11.4.  DNS Spoofing

   Clients using HTTP rely heavily on the Domain Name Service, and are
   thus generally prone to security attacks based on the deliberate mis-
   association of IP addresses and DNS names.  Clients need to be
   cautious in assuming the continuing validity of an IP number/DNS name
   association.

   In particular, HTTP clients SHOULD rely on their name resolver for
   confirmation of an IP number/DNS name association, rather than
   caching the result of previous host name lookups.  Many platforms
   already can cache host name lookups locally when appropriate, and
   they SHOULD be configured to do so.  It is proper for these lookups
   to be cached, however, only when the TTL (Time To Live) information
   reported by the name server makes it likely that the cached
   information will remain useful.

   If HTTP clients cache the results of host name lookups in order to
   achieve a performance improvement, they MUST observe the TTL



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   information reported by DNS.

   If HTTP clients do not observe this rule, they could be spoofed when
   a previously-accessed server's IP address changes.  As network
   renumbering is expected to become increasingly common [RFC1900], the
   possibility of this form of attack will grow.  Observing this
   requirement thus reduces this potential security vulnerability.

   This requirement also improves the load-balancing behavior of clients
   for replicated servers using the same DNS name and reduces the
   likelihood of a user's experiencing failure in accessing sites which
   use that strategy.

11.5.  Proxies and Caching

   By their very nature, HTTP proxies are men-in-the-middle, and
   represent an opportunity for man-in-the-middle attacks.  Compromise
   of the systems on which the proxies run can result in serious
   security and privacy problems.  Proxies have access to security-
   related information, personal information about individual users and
   organizations, and proprietary information belonging to users and
   content providers.  A compromised proxy, or a proxy implemented or
   configured without regard to security and privacy considerations,
   might be used in the commission of a wide range of potential attacks.

   Proxy operators need to protect the systems on which proxies run as
   they would protect any system that contains or transports sensitive
   information.  In particular, log information gathered at proxies
   often contains highly sensitive personal information, and/or
   information about organizations.  Log information needs to be
   carefully guarded, and appropriate guidelines for use need to be
   developed and followed.  (Section 11.2).

   Proxy implementors need to consider the privacy and security
   implications of their design and coding decisions, and of the
   configuration options they provide to proxy operators (especially the
   default configuration).

   Users of a proxy need to be aware that proxies are no trustworthier
   than the people who run them; HTTP itself cannot solve this problem.

   The judicious use of cryptography, when appropriate, might suffice to
   protect against a broad range of security and privacy attacks.  Such
   cryptography is beyond the scope of the HTTP/1.1 specification.







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11.6.  Protocol Element Size Overflows

   Because HTTP uses mostly textual, character-delimited fields,
   attackers can overflow buffers in implementations, and/or perform a
   Denial of Service against implementations that accept fields with
   unlimited lengths.

   To promote interoperability, this specification makes specific
   recommendations for size limits on request-targets (Section 4.1.2)
   and blocks of header fields (Section 3.2).  These are minimum
   recommendations, chosen to be supportable even by implementations
   with limited resources; it is expected that most implementations will
   choose substantially higher limits.

   This specification also provides a way for servers to reject messages
   that have request-targets that are too long (Section 8.4.15 of
   [Part2]) or request entities that are too large (Section 8.4 of
   [Part2]).

   Other fields (including but not limited to request methods, response
   status phrases, header field-names, and body chunks) SHOULD be
   limited by implementations carefully, so as to not impede
   interoperability.

11.7.  Denial of Service Attacks on Proxies

   They exist.  They are hard to defend against.  Research continues.
   Beware.

12.  Acknowledgments

   HTTP has evolved considerably over the years.  It has benefited from
   a large and active developer community -- the many people who have
   participated on the www-talk mailing list -- and it is that community
   which has been most responsible for the success of HTTP and of the
   World-Wide Web in general.  Marc Andreessen, Robert Cailliau, Daniel
   W. Connolly, Bob Denny, John Franks, Jean-Francois Groff, Phillip M.
   Hallam-Baker, Hakon W. Lie, Ari Luotonen, Rob McCool, Lou Montulli,
   Dave Raggett, Tony Sanders, and Marc VanHeyningen deserve special
   recognition for their efforts in defining early aspects of the
   protocol.

   This document has benefited greatly from the comments of all those
   participating in the HTTP-WG.  In addition to those already
   mentioned, the following individuals have contributed to this
   specification:

   Gary Adams, Harald Tveit Alvestrand, Keith Ball, Brian Behlendorf,



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   Paul Burchard, Maurizio Codogno, Josh Cohen, Mike Cowlishaw, Roman
   Czyborra, Michael A. Dolan, Daniel DuBois, David J. Fiander, Alan
   Freier, Marc Hedlund, Greg Herlihy, Koen Holtman, Alex Hopmann, Bob
   Jernigan, Shel Kaphan, Rohit Khare, John Klensin, Martijn Koster,
   Alexei Kosut, David M. Kristol, Daniel LaLiberte, Ben Laurie, Paul J.
   Leach, Albert Lunde, John C. Mallery, Jean-Philippe Martin-Flatin,
   Mitra, David Morris, Gavin Nicol, Ross Patterson, Bill Perry, Jeffrey
   Perry, Scott Powers, Owen Rees, Luigi Rizzo, David Robinson, Marc
   Salomon, Rich Salz, Allan M. Schiffman, Jim Seidman, Chuck Shotton,
   Eric W. Sink, Simon E. Spero, Richard N. Taylor, Robert S. Thau, Bill
   (BearHeart) Weinman, Francois Yergeau, Mary Ellen Zurko.

   Thanks to the "cave men" of Palo Alto.  You know who you are.

   Jim Gettys (the editor of [RFC2616]) wishes particularly to thank Roy
   Fielding, the editor of [RFC2068], along with John Klensin, Jeff
   Mogul, Paul Leach, Dave Kristol, Koen Holtman, John Franks, Josh
   Cohen, Alex Hopmann, Scott Lawrence, and Larry Masinter for their
   help.  And thanks go particularly to Jeff Mogul and Scott Lawrence
   for performing the "MUST/MAY/SHOULD" audit.

   The Apache Group, Anselm Baird-Smith, author of Jigsaw, and Henrik
   Frystyk implemented RFC 2068 early, and we wish to thank them for the
   discovery of many of the problems that this document attempts to
   rectify.

   This specification makes heavy use of the augmented BNF and generic
   constructs defined by David H. Crocker for [RFC5234].  Similarly, it
   reuses many of the definitions provided by Nathaniel Borenstein and
   Ned Freed for MIME [RFC2045].  We hope that their inclusion in this
   specification will help reduce past confusion over the relationship
   between HTTP and Internet mail message formats.

13.  References

13.1.  Normative References

   [ISO-8859-1]  International Organization for Standardization,
                 "Information technology -- 8-bit single-byte coded
                 graphic character sets -- Part 1: Latin alphabet No.
                 1", ISO/IEC 8859-1:1998, 1998.

   [Part2]       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 2: Message
                 Semantics", draft-ietf-httpbis-p2-semantics-15 (work in
                 progress), July 2011.




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   [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-15 (work in progress),
                 July 2011.

   [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-15 (work in progress),
                 July 2011.

   [RFC1950]     Deutsch, L. and J-L. Gailly, "ZLIB Compressed Data
                 Format Specification version 3.3", RFC 1950, May 1996.

                 RFC 1950 is an Informational RFC, thus it might be less
                 stable than this specification.  On the other hand,
                 this downward reference was present since the
                 publication of RFC 2068 in 1997 ([RFC2068]), therefore
                 it is unlikely to cause problems in practice.  See also
                 [BCP97].

   [RFC1951]     Deutsch, P., "DEFLATE Compressed Data Format
                 Specification version 1.3", RFC 1951, May 1996.

                 RFC 1951 is an Informational RFC, thus it might be less
                 stable than this specification.  On the other hand,
                 this downward reference was present since the
                 publication of RFC 2068 in 1997 ([RFC2068]), therefore
                 it is unlikely to cause problems in practice.  See also
                 [BCP97].

   [RFC1952]     Deutsch, P., Gailly, J-L., Adler, M., Deutsch, L., and
                 G. Randers-Pehrson, "GZIP file format specification
                 version 4.3", RFC 1952, May 1996.

                 RFC 1952 is an Informational RFC, thus it might be less
                 stable than this specification.  On the other hand,
                 this downward reference was present since the
                 publication of RFC 2068 in 1997 ([RFC2068]), therefore
                 it is unlikely to cause problems in practice.  See also
                 [BCP97].

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




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   [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.

   [USASCII]     American National Standards Institute, "Coded Character
                 Set -- 7-bit American Standard Code for Information
                 Interchange", ANSI X3.4, 1986.

13.2.  Informative References

   [BCP97]       Klensin, J. and S. Hartman, "Handling Normative
                 References to Standards-Track Documents", BCP 97,
                 RFC 4897, June 2007.

   [Kri2001]     Kristol, D., "HTTP Cookies: Standards, Privacy, and
                 Politics", ACM Transactions on Internet Technology Vol.
                 1, #2, November 2001,
                 <http://arxiv.org/abs/cs.SE/0105018>.

   [Nie1997]     Frystyk, H., Gettys, J., Prud'hommeaux, E., Lie, H.,
                 and C. Lilley, "Network Performance Effects of
                 HTTP/1.1, CSS1, and PNG", ACM Proceedings of the ACM
                 SIGCOMM '97 conference on Applications, technologies,
                 architectures, and protocols for computer communication
                 SIGCOMM '97, September 1997,
                 <http://doi.acm.org/10.1145/263105.263157>.

   [Pad1995]     Padmanabhan, V. and J. Mogul, "Improving HTTP Latency",
                 Computer Networks and ISDN Systems v. 28, pp. 25-35,
                 December 1995,
                 <http://portal.acm.org/citation.cfm?id=219094>.

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

   [RFC1900]     Carpenter, B. and Y. Rekhter, "Renumbering Needs Work",
                 RFC 1900, February 1996.

   [RFC1919]     Chatel, M., "Classical versus Transparent IP Proxies",
                 RFC 1919, March 1996.

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



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                 May 1996.

   [RFC2045]     Freed, N. and N. Borenstein, "Multipurpose Internet
                 Mail Extensions (MIME) Part One: Format of Internet
                 Message Bodies", RFC 2045, November 1996.

   [RFC2047]     Moore, K., "MIME (Multipurpose Internet Mail
                 Extensions) Part Three: Message Header Extensions for
                 Non-ASCII Text", RFC 2047, November 1996.

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

   [RFC2145]     Mogul, J., Fielding, R., Gettys, J., and H. Nielsen,
                 "Use and Interpretation of HTTP Version Numbers",
                 RFC 2145, May 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.

   [RFC2818]     Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000.

   [RFC2965]     Kristol, D. and L. Montulli, "HTTP State Management
                 Mechanism", RFC 2965, October 2000.

   [RFC3040]     Cooper, I., Melve, I., and G. Tomlinson, "Internet Web
                 Replication and Caching Taxonomy", RFC 3040,
                 January 2001.

   [RFC3864]     Klyne, G., Nottingham, M., and J. Mogul, "Registration
                 Procedures for Message Header Fields", BCP 90,
                 RFC 3864, September 2004.

   [RFC4288]     Freed, N. and J. Klensin, "Media Type Specifications
                 and Registration Procedures", BCP 13, RFC 4288,
                 December 2005.

   [RFC4395]     Hansen, T., Hardie, T., and L. Masinter, "Guidelines
                 and Registration Procedures for New URI Schemes",
                 BCP 115, RFC 4395, February 2006.

   [RFC4559]     Jaganathan, K., Zhu, L., and J. Brezak, "SPNEGO-based
                 Kerberos and NTLM HTTP Authentication in Microsoft



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                 Windows", RFC 4559, June 2006.

   [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.

   [RFC6265]     Barth, A., "HTTP State Management Mechanism", RFC 6265,
                 April 2011.

   [Spe]         Spero, S., "Analysis of HTTP Performance Problems",
                 <http://sunsite.unc.edu/mdma-release/http-prob.html>.

   [Tou1998]     Touch, J., Heidemann, J., and K. Obraczka, "Analysis of
                 HTTP Performance", ISI Research Report ISI/RR-98-463,
                 Aug 1998, <http://www.isi.edu/touch/pubs/http-perf96/>.

                 (original report dated Aug. 1996)

Appendix A.  Tolerant Applications

   Although this document specifies the requirements for the generation
   of HTTP/1.1 messages, not all applications will be correct in their
   implementation.  We therefore recommend that operational applications
   be tolerant of deviations whenever those deviations can be
   interpreted unambiguously.

   The line terminator for header fields is the sequence CRLF.  However,
   we recommend that applications, when parsing such headers fields,
   recognize a single LF as a line terminator and ignore the leading CR.

   The character encoding of a representation SHOULD be labeled as the
   lowest common denominator of the character codes used within that
   representation, with the exception that not labeling the
   representation is preferred over labeling the representation with the
   labels US-ASCII or ISO-8859-1.  See [Part3].

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

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

   o  Although all date formats are specified to be case-sensitive,
      recipients SHOULD match day, week and timezone names case-



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      insensitively.

   o  An HTTP/1.1 implementation MAY internally represent a parsed
      Expires date as earlier than the proper value, but MUST NOT
      internally represent a parsed Expires date as later than the
      proper value.

   o  All expiration-related calculations MUST be done in GMT.  The
      local time zone MUST NOT influence the calculation or comparison
      of an age or expiration time.

   o  If an HTTP header field incorrectly carries a date value with a
      time zone other than GMT, it MUST be converted into GMT using the
      most conservative possible conversion.

Appendix B.  HTTP Version History

   HTTP has been in use by the World-Wide Web global information
   initiative since 1990.  The first version of HTTP, later referred to
   as HTTP/0.9, was a simple protocol for hypertext data transfer across
   the Internet with only a single request method (GET) and no metadata.
   HTTP/1.0, as defined by [RFC1945], added a range of request methods
   and MIME-like messaging that could include metadata about the data
   transferred and modifiers on the request/response semantics.
   However, HTTP/1.0 did not sufficiently take into consideration the
   effects of hierarchical proxies, caching, the need for persistent
   connections, or name-based virtual hosts.  The proliferation of
   incompletely-implemented applications calling themselves "HTTP/1.0"
   further necessitated a protocol version change in order for two
   communicating applications to determine each other's true
   capabilities.

   HTTP/1.1 remains compatible with HTTP/1.0 by including more stringent
   requirements that enable reliable implementations, adding only those
   new features that will either be safely ignored by an HTTP/1.0
   recipient or only sent when communicating with a party advertising
   compliance with HTTP/1.1.

   It is beyond the scope of a protocol specification to mandate
   compliance with previous versions.  HTTP/1.1 was deliberately
   designed, however, to make supporting previous versions easy.  We
   would expect a general-purpose HTTP/1.1 server to understand any
   valid request in the format of HTTP/1.0 and respond appropriately
   with an HTTP/1.1 message that only uses features understood (or
   safely ignored) by HTTP/1.0 clients.  Likewise, would expect an
   HTTP/1.1 client to understand any valid HTTP/1.0 response.

   Since HTTP/0.9 did not support header fields in a request, there is



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   no mechanism for it to support name-based virtual hosts (selection of
   resource by inspection of the Host header field).  Any server that
   implements name-based virtual hosts ought to disable support for
   HTTP/0.9.  Most requests that appear to be HTTP/0.9 are, in fact,
   badly constructed HTTP/1.x requests wherein a buggy client failed to
   properly encode linear whitespace found in a URI reference and placed
   in the request-target.

B.1.  Changes from HTTP/1.0

   This section summarizes major differences between versions HTTP/1.0
   and HTTP/1.1.

B.1.1.  Multi-homed Web Servers

   The requirements that clients and servers support the Host header
   field (Section 9.4), report an error if it is missing from an
   HTTP/1.1 request, and accept absolute URIs (Section 4.1.2) are among
   the most important changes defined by HTTP/1.1.

   Older HTTP/1.0 clients assumed a one-to-one relationship of IP
   addresses and servers; there was no other established mechanism for
   distinguishing the intended server of a request than the IP address
   to which that request was directed.  The Host header field was
   introduced during the development of HTTP/1.1 and, though it was
   quickly implemented by most HTTP/1.0 browsers, additional
   requirements were placed on all HTTP/1.1 requests in order to ensure
   complete adoption.  At the time of this writing, most HTTP-based
   services are dependent upon the Host header field for targeting
   requests.

B.1.2.  Keep-Alive Connections

   For most implementations of HTTP/1.0, each connection is established
   by the client prior to the request and closed by the server after
   sending the response.  However, some implementations implement the
   Keep-Alive version of persistent connections described in Section
   19.7.1 of [RFC2068].

   Some clients and servers might wish to be compatible with some
   previous implementations of persistent connections in HTTP/1.0
   clients and servers.  Persistent connections in HTTP/1.0 are
   explicitly negotiated as they are not the default behavior.  HTTP/1.0
   experimental implementations of persistent connections are faulty,
   and the new facilities in HTTP/1.1 are designed to rectify these
   problems.  The problem was that some existing HTTP/1.0 clients might
   send Keep-Alive to a proxy server that doesn't understand Connection,
   which would then erroneously forward it to the next inbound server,



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   which would establish the Keep-Alive connection and result in a hung
   HTTP/1.0 proxy waiting for the close on the response.  The result is
   that HTTP/1.0 clients must be prevented from using Keep-Alive when
   talking to proxies.

   However, talking to proxies is the most important use of persistent
   connections, so that prohibition is clearly unacceptable.  Therefore,
   we need some other mechanism for indicating a persistent connection
   is desired, which is safe to use even when talking to an old proxy
   that ignores Connection.  Persistent connections are the default for
   HTTP/1.1 messages; we introduce a new keyword (Connection: close) for
   declaring non-persistence.  See Section 9.1.

B.2.  Changes from RFC 2616

   Empty list elements in list productions have been deprecated.
   (Section 1.2.1)

   Rules about implicit linear whitespace between certain grammar
   productions have been removed; now it's only allowed when
   specifically pointed out in the ABNF.  The NUL octet is no longer
   allowed in comment and quoted-string text.  The quoted-pair rule no
   longer allows escaping control characters other than HTAB.  Non-ASCII
   content in header fields and reason phrase has been obsoleted and
   made opaque (the TEXT rule was removed) (Section 1.2.2)

   Clarify that the string "HTTP" in the HTTP-Version ABFN production is
   case sensitive.  Restrict the version numbers to be single digits due
   to the fact that implementations are known to handle multi-digit
   version numbers incorrectly.  (Section 2.6)

   Require that invalid whitespace around field-names be rejected.
   (Section 3.2)

   Require recipients to handle bogus Content-Length header fields as
   errors.  (Section 3.3)

   Remove reference to non-existent identity transfer-coding value
   tokens.  (Sections 3.3 and 6.2)

   Update use of abs_path production from RFC 1808 to the path-absolute
   + query components of RFC 3986.  State that the asterisk form is
   allowed for the OPTIONS request method only.  (Section 4.1.2)

   Clarification that the chunk length does not include the count of the
   octets in the chunk header and trailer.  Furthermore disallowed line
   folding in chunk extensions.  (Section 6.2.1)




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   Remove hard limit of two connections per server.  (Section 7.1.4)

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

   Clarify exactly when close connection options must be sent.
   (Section 9.1)

   Define the semantics of the "Upgrade" header field in responses other
   than 101 (this was incorporated from [RFC2817]).  (Section 9.8)

Appendix C.  Collected ABNF

   BWS = OWS

   Chunked-Body = *chunk last-chunk trailer-part CRLF
   Connection = *( "," OWS ) connection-token *( OWS "," [ OWS
    connection-token ] )
   Content-Length = 1*DIGIT

   Date = HTTP-date

   GMT = %x47.4D.54 ; GMT

   HTTP-Prot-Name = %x48.54.54.50 ; HTTP
   HTTP-Version = HTTP-Prot-Name "/" DIGIT "." DIGIT
   HTTP-date = rfc1123-date / obs-date
   HTTP-message = start-line *( header-field CRLF ) CRLF [ message-body
    ]
   Host = uri-host [ ":" port ]

   Method = token

   OWS = *( [ obs-fold ] WSP )

   RWS = 1*( [ obs-fold ] WSP )
   Reason-Phrase = *( WSP / VCHAR / obs-text )
   Request = Request-Line *( header-field CRLF ) CRLF [ message-body ]
   Request-Line = Method SP request-target SP HTTP-Version CRLF
   Response = Status-Line *( header-field CRLF ) CRLF [ message-body ]

   Status-Code = 3DIGIT
   Status-Line = HTTP-Version SP Status-Code SP Reason-Phrase CRLF

   TE = [ ( "," / t-codings ) *( OWS "," [ OWS t-codings ] ) ]
   Trailer = *( "," OWS ) field-name *( OWS "," [ OWS field-name ] )
   Transfer-Encoding = *( "," OWS ) transfer-coding *( OWS "," [ OWS
    transfer-coding ] )



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   URI-reference = <URI-reference, defined in [RFC3986], Section 4.1>
   Upgrade = *( "," OWS ) product *( OWS "," [ OWS product ] )

   Via = *( "," OWS ) received-protocol RWS received-by [ RWS comment ]
    *( OWS "," [ OWS received-protocol RWS received-by [ RWS comment ] ]
    )

   absolute-URI = <absolute-URI, defined in [RFC3986], Section 4.3>
   asctime-date = day-name SP date3 SP time-of-day SP year
   attribute = token
   authority = <authority, defined in [RFC3986], Section 3.2>

   chunk = chunk-size *WSP [ chunk-ext ] CRLF chunk-data CRLF
   chunk-data = 1*OCTET
   chunk-ext = *( ";" *WSP chunk-ext-name [ "=" chunk-ext-val ] *WSP )
   chunk-ext-name = token
   chunk-ext-val = token / quoted-str-nf
   chunk-size = 1*HEXDIG
   comment = "(" *( ctext / quoted-cpair / comment ) ")"
   connection-token = token
   ctext = OWS / %x21-27 ; '!'-'''
    / %x2A-5B ; '*'-'['
    / %x5D-7E ; ']'-'~'
    / obs-text

   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

   field-content = *( WSP / VCHAR / obs-text )
   field-name = token
   field-value = *( field-content / OWS )




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   header-field = field-name ":" OWS [ field-value ] OWS
   hour = 2DIGIT
   http-URI = "http://" authority path-abempty [ "?" query ]
   https-URI = "https://" authority path-abempty [ "?" query ]

   last-chunk = 1*"0" *WSP [ chunk-ext ] CRLF

   message-body = *OCTET
   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-fold = CRLF
   obs-text = %x80-FF

   partial-URI = relative-part [ "?" query ]
   path-abempty = <path-abempty, defined in [RFC3986], Section 3.3>
   path-absolute = <path-absolute, defined in [RFC3986], Section 3.3>
   port = <port, defined in [RFC3986], Section 3.2.3>
   product = token [ "/" product-version ]
   product-version = token
   protocol-name = token
   protocol-version = token
   pseudonym = token

   qdtext = OWS / "!" / %x23-5B ; '#'-'['
    / %x5D-7E ; ']'-'~'
    / obs-text
   qdtext-nf = WSP / "!" / %x23-5B ; '#'-'['
    / %x5D-7E ; ']'-'~'
    / obs-text
   query = <query, defined in [RFC3986], Section 3.4>
   quoted-cpair = "\" ( WSP / VCHAR / obs-text )
   quoted-pair = "\" ( WSP / VCHAR / obs-text )
   quoted-str-nf = DQUOTE *( qdtext-nf / quoted-pair ) DQUOTE
   quoted-string = DQUOTE *( qdtext / quoted-pair ) DQUOTE
   qvalue = ( "0" [ "." *3DIGIT ] ) / ( "1" [ "." *3"0" ] )



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   received-by = ( uri-host [ ":" port ] ) / pseudonym
   received-protocol = [ protocol-name "/" ] protocol-version
   relative-part = <relative-part, defined in [RFC3986], Section 4.2>
   request-target = "*" / absolute-URI / ( path-absolute [ "?" query ] )
    / authority
   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
   special = "(" / ")" / "<" / ">" / "@" / "," / ";" / ":" / "\" /
    DQUOTE / "/" / "[" / "]" / "?" / "=" / "{" / "}"
   start-line = Request-Line / Status-Line

   t-codings = "trailers" / ( transfer-extension [ te-params ] )
   tchar = "!" / "#" / "$" / "%" / "&" / "'" / "*" / "+" / "-" / "." /
    "^" / "_" / "`" / "|" / "~" / DIGIT / ALPHA
   te-ext = OWS ";" OWS token [ "=" word ]
   te-params = OWS ";" OWS "q=" qvalue *te-ext
   time-of-day = hour ":" minute ":" second
   token = 1*tchar
   trailer-part = *( header-field CRLF )
   transfer-coding = "chunked" / "compress" / "deflate" / "gzip" /
    transfer-extension
   transfer-extension = token *( OWS ";" OWS transfer-parameter )
   transfer-parameter = attribute BWS "=" BWS value

   uri-host = <host, defined in [RFC3986], Section 3.2.2>

   value = word

   word = token / quoted-string

   year = 4DIGIT


















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   ABNF diagnostics:

   ; Chunked-Body defined but not used
   ; Connection defined but not used
   ; Content-Length defined but not used
   ; Date defined but not used
   ; HTTP-message defined but not used
   ; Host defined but not used
   ; Request defined but not used
   ; Response defined but not used
   ; TE defined but not used
   ; Trailer defined but not used
   ; Transfer-Encoding defined but not used
   ; URI-reference defined but not used
   ; Upgrade defined but not used
   ; Via defined but not used
   ; http-URI defined but not used
   ; https-URI defined but not used
   ; partial-URI defined but not used
   ; special defined but not used

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

D.1.  Since RFC 2616

   Extracted relevant partitions from [RFC2616].

D.2.  Since draft-ietf-httpbis-p1-messaging-00

   Closed issues:

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/1>: "HTTP Version
      should be case sensitive"
      (<http://purl.org/NET/http-errata#verscase>)

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/2>: "'unsafe'
      characters" (<http://purl.org/NET/http-errata#unsafe-uri>)

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/3>: "Chunk Size
      Definition" (<http://purl.org/NET/http-errata#chunk-size>)

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/4>: "Message Length"
      (<http://purl.org/NET/http-errata#msg-len-chars>)

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/8>: "Media Type
      Registrations" (<http://purl.org/NET/http-errata#media-reg>)





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   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/11>: "URI includes
      query" (<http://purl.org/NET/http-errata#uriquery>)

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/15>: "No close on
      1xx responses" (<http://purl.org/NET/http-errata#noclose1xx>)

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/16>: "Remove
      'identity' token references"
      (<http://purl.org/NET/http-errata#identity>)

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/26>: "Import query
      BNF"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/31>: "qdtext BNF"

   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/45>: "RFC977
      reference"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/46>: "RFC1700
      references"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/47>: "inconsistency
      in date format explanation"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/48>: "Date reference
      typo"

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

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/66>: "ISO-8859-1
      Reference"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/86>: "Normative up-
      to-date references"

   Other changes:

   o  Update media type registrations to use RFC4288 template.

   o  Use names of RFC4234 core rules DQUOTE and WSP, fix broken ABNF
      for chunk-data (work in progress on



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      <http://tools.ietf.org/wg/httpbis/trac/ticket/36>)

D.3.  Since draft-ietf-httpbis-p1-messaging-01

   Closed issues:

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/19>: "Bodies on GET
      (and other) requests"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/55>: "Updating to
      RFC4288"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/57>: "Status Code
      and Reason Phrase"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/82>: "rel_path not
      used"

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

   o  Get rid of duplicate BNF rule names ("host" -> "uri-host",
      "trailer" -> "trailer-part").

   o  Avoid underscore character in rule names ("http_URL" -> "http-
      URL", "abs_path" -> "path-absolute").

   o  Add rules for terms imported from URI spec ("absoluteURI",
      "authority", "path-absolute", "port", "query", "relativeURI",
      "host) -- these will have to be updated when switching over to
      RFC3986.

   o  Synchronize core rules with RFC5234.

   o  Get rid of prose rules that span multiple lines.

   o  Get rid of unused rules LOALPHA and UPALPHA.

   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  Rewrite prose rule "token" in terms of "tchar", rewrite prose rule
      "TEXT".





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D.4.  Since draft-ietf-httpbis-p1-messaging-02

   Closed issues:

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/51>: "HTTP-date vs.
      rfc1123-date"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/64>: "WS in quoted-
      pair"

   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>):

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

D.5.  Since draft-ietf-httpbis-p1-messaging-03

   Closed issues:

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/28>: "Connection
      closing"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/97>: "Move
      registrations and registry information to IANA Considerations"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/120>: "need new URL
      for PAD1995 reference"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/127>: "IANA
      Considerations: update HTTP URI scheme registration"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/128>: "Cite HTTPS
      URI scheme definition"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/129>: "List-type
      headers vs Set-Cookie"

   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
      (HTTP-Date).

   o  Replace HEX by HEXDIG for future consistence with RFC 5234's core
      rules.

D.6.  Since draft-ietf-httpbis-p1-messaging-04

   Closed issues:

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/34>: "Out-of-date
      reference for URIs"

   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  Get rid of RFC822 dependency; use RFC5234 plus extensions instead.

   o  Only reference RFC 5234's core rules.

   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.

D.7.  Since draft-ietf-httpbis-p1-messaging-05

   Closed issues:

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/30>: "Header LWS"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/52>: "Sort 1.3
      Terminology"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/63>: "RFC2047
      encoded words"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/74>: "Character
      Encodings in TEXT"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/77>: "Line Folding"




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   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/83>: "OPTIONS * and
      proxies"

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

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/111>: "Use of TEXT"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/118>: "Join
      "Differences Between HTTP Entities and RFC 2045 Entities"?"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/134>: "RFC822
      reference left in discussion of date formats"

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

   o  Rewrite definition of list rules, deprecate empty list elements.

   o  Add appendix containing collected and expanded ABNF.

   Other changes:

   o  Rewrite introduction; add mostly new Architecture Section.

   o  Move definition of quality values from Part 3 into Part 1; make TE
      request header field grammar independent of accept-params (defined
      in Part 3).

D.8.  Since draft-ietf-httpbis-p1-messaging-06

   Closed issues:

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/161>: "base for
      numeric protocol elements"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/162>: "comment ABNF"

   Partly resolved issues:

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/88>: "205 Bodies"
      (took out language that implied that there might be methods for
      which a request body MUST NOT be included)

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/163>: "editorial
      improvements around HTTP-date"





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D.9.  Since draft-ietf-httpbis-p1-messaging-07

   Closed issues:

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/93>: "Repeating
      single-value headers"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/131>: "increase
      connection limit"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/157>: "IP addresses
      in URLs"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/172>: "take over
      HTTP Upgrade Token Registry"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/173>: "CR and LF in
      chunk extension values"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/184>: "HTTP/0.9
      support"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/188>: "pick IANA
      policy (RFC5226) for Transfer Coding / Content Coding"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/189>: "move
      definitions of gzip/deflate/compress to part 1"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/194>: "disallow
      control characters in quoted-pair"

   Partly resolved issues:

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/148>: "update IANA
      requirements wrt Transfer-Coding values" (add the IANA
      Considerations subsection)

D.10.  Since draft-ietf-httpbis-p1-messaging-08

   Closed issues:

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/201>: "header
      parsing, treatment of leading and trailing OWS"

   Partly resolved issues:

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/60>: "Placement of
      13.5.1 and 13.5.2"



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   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/200>: "use of term
      "word" when talking about header structure"

D.11.  Since draft-ietf-httpbis-p1-messaging-09

   Closed issues:

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/73>: "Clarification
      of the term 'deflate'"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/83>: "OPTIONS * and
      proxies"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/122>: "MIME-Version
      not listed in P1, general header fields"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/143>: "IANA registry
      for content/transfer encodings"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/165>: "Case-
      sensitivity of HTTP-date"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/200>: "use of term
      "word" when talking about header structure"

   Partly resolved issues:

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

D.12.  Since draft-ietf-httpbis-p1-messaging-10

   Closed issues:

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/28>: "Connection
      Closing"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/90>: "Delimiting
      messages with multipart/byteranges"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/95>: "Handling
      multiple Content-Length headers"

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

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



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   Partly resolved issues:

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/159>: "HTTP(s) URI
      scheme definitions"

D.13.  Since draft-ietf-httpbis-p1-messaging-11

   Closed issues:

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/193>: "Trailer
      requirements"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/204>: "Text about
      clock requirement for caches belongs in p6"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/221>: "effective
      request URI: handling of missing host in HTTP/1.0"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/248>: "confusing
      Date requirements for clients"

   Partly resolved issues:

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/95>: "Handling
      multiple Content-Length headers"

D.14.  Since draft-ietf-httpbis-p1-messaging-12

   Closed issues:

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/75>: "RFC2145
      Normative"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/159>: "HTTP(s) URI
      scheme definitions" (tune the requirements on userinfo)

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/210>: "define
      'transparent' proxy"

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

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/233>: "Is * usable
      as a request-uri for new methods?"

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




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

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/279>: "update RFC
      2109 reference"

D.15.  Since draft-ietf-httpbis-p1-messaging-13

   Closed issues:

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/53>: "Allow is not
      in 13.5.2"

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

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/286>: "Content-
      Length ABNF broken"

D.16.  Since draft-ietf-httpbis-p1-messaging-14

   Closed issues:

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/273>: "HTTP-Version
      should be redefined as fixed length pair of DIGIT .  DIGIT"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/282>: "Recommend
      minimum sizes for protocol elements"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/283>: "Set
      expectations around buffering"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/288>: "Considering
      messages in isolation"

Index

   A
      absolute-URI form (of request-target)  29
      accelerator  14
      application/http Media Type  64
      asterisk form (of request-target)  29
      authority form (of request-target)  30

   B
      browser  10

   C



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      cache  15
      cacheable  15
      captive portal  14
      chunked (Coding Format)  38
      client  10
      Coding Format
         chunked  38
         compress  41
         deflate  41
         gzip  41
      compress (Coding Format)  41
      connection  10
      Connection header field  52
      Content-Length header field  54

   D
      Date header field  54
      deflate (Coding Format)  41
      downstream  13

   E
      effective request URI  32

   G
      gateway  14
      Grammar
         absolute-URI  18
         ALPHA  7
         asctime-date  37
         attribute  37
         authority  18
         BWS  9
         chunk  38
         chunk-data  38
         chunk-ext  38
         chunk-ext-name  38
         chunk-ext-val  38
         chunk-size  38
         Chunked-Body  38
         comment  24
         Connection  53
         connection-token  53
         Content-Length  54
         CR  7
         CRLF  7
         ctext  24
         CTL  7
         Date  54



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         date1  36
         date2  37
         date3  37
         day  36
         day-name  36
         day-name-l  36
         DIGIT  7
         DQUOTE  7
         field-content  23
         field-name  23
         field-value  23
         GMT  36
         header-field  23
         HEXDIG  7
         Host  56
         hour  36
         HTTP-date  35
         HTTP-message  21
         HTTP-Prot-Name  16
         http-URI  18
         HTTP-Version  16
         https-URI  20
         last-chunk  38
         LF  7
         message-body  25
         Method  29
         minute  36
         month  36
         obs-date  36
         obs-text  10
         OCTET  7
         OWS  9
         path-absolute  18
         port  18
         product  42
         product-version  42
         protocol-name  61
         protocol-version  61
         pseudonym  61
         qdtext  10
         qdtext-nf  38
         query  18
         quoted-cpair  24
         quoted-pair  10
         quoted-str-nf  38
         quoted-string  10
         qvalue  42
         Reason-Phrase  34



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         received-by  61
         received-protocol  61
         Request  29
         Request-Line  29
         request-target  29
         Response  33
         rfc850-date  37
         rfc1123-date  36
         RWS  9
         second  36
         SP  7
         special  9
         Status-Code  34
         Status-Line  34
         t-codings  57
         tchar  9
         TE  57
         te-ext  57
         te-params  57
         time-of-day  36
         token  9
         Trailer  58
         trailer-part  38
         transfer-coding  37
         Transfer-Encoding  59
         transfer-extension  37
         transfer-parameter  37
         Upgrade  59
         uri-host  18
         URI-reference  18
         value  37
         VCHAR  7
         Via  61
         word  9
         WSP  7
         year  36
      gzip (Coding Format)  41

   H
      header field  21
      Header Fields
         Connection  52
         Content-Length  54
         Date  54
         Host  56
         TE  57
         Trailer  58
         Transfer-Encoding  58



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         Upgrade  59
         Via  61
      header section  21
      headers  21
      Host header field  56
      http URI scheme  18
      https URI scheme  20

   I
      inbound  13
      interception proxy  14
      intermediary  13

   M
      Media Type
         application/http  64
         message/http  63
      message  11
      message/http Media Type  63

   N
      non-transforming proxy  13

   O
      origin form (of request-target)  30
      origin server  10
      outbound  13

   P
      proxy  13

   R
      recipient  10
      request  11
      resource  18
      response  11
      reverse proxy  14

   S
      sender  10
      server  10
      spider  10

   T
      target resource  32
      TE header field  57
      Trailer header field  58
      Transfer-Encoding header field  58



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      transforming proxy  13
      transparent proxy  14
      tunnel  14

   U
      Upgrade header field  59
      upstream  13
      URI scheme
         http  18
         https  20
      user agent  10

   V
      Via header field  61

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/


   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






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   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


   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/














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   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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