source: draft-ietf-httpbis/21/draft-ietf-httpbis-p1-messaging-21.txt @ 2132

HTTPbis Working Group                                   R. Fielding, Ed.
Internet-Draft                                                     Adobe
Obsoletes: 2145,2616 (if approved)                       J. Reschke, Ed.
Updates: 2817 (if approved)                                   greenbytes
Intended status: Standards Track                         October 4, 2012
Expires: April 7, 2013


   Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing
                   draft-ietf-httpbis-p1-messaging-21

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 provides an
   overview of HTTP architecture and its associated terminology, defines
   the "http" and "https" Uniform Resource Identifier (URI) schemes,
   defines the HTTP/1.1 message syntax and parsing requirements, and
   describes general security concerns for implementations.

Editorial Note (To be removed by RFC Editor)

   Discussion of this draft takes 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/>.

   The changes in this draft are summarized in Appendix D.22.

Status of This Memo

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

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

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference



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   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on April 7, 2013.

Copyright Notice

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

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
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   publication of this document.  Please review these documents
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   described in the Simplified BSD License.

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

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  5
     1.1.  Requirement Notation . . . . . . . . . . . . . . . . . . .  6
     1.2.  Syntax Notation  . . . . . . . . . . . . . . . . . . . . .  6
   2.  Architecture . . . . . . . . . . . . . . . . . . . . . . . . .  6
     2.1.  Client/Server Messaging  . . . . . . . . . . . . . . . . .  7
     2.2.  Implementation Diversity . . . . . . . . . . . . . . . . .  8
     2.3.  Intermediaries . . . . . . . . . . . . . . . . . . . . . .  9
     2.4.  Caches . . . . . . . . . . . . . . . . . . . . . . . . . . 11
     2.5.  Conformance and Error Handling . . . . . . . . . . . . . . 12
     2.6.  Protocol Versioning  . . . . . . . . . . . . . . . . . . . 13
     2.7.  Uniform Resource Identifiers . . . . . . . . . . . . . . . 15
       2.7.1.  http URI scheme  . . . . . . . . . . . . . . . . . . . 16
       2.7.2.  https URI scheme . . . . . . . . . . . . . . . . . . . 17
       2.7.3.  http and https URI Normalization and Comparison  . . . 18
   3.  Message Format . . . . . . . . . . . . . . . . . . . . . . . . 18



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     3.1.  Start Line . . . . . . . . . . . . . . . . . . . . . . . . 19
       3.1.1.  Request Line . . . . . . . . . . . . . . . . . . . . . 20
       3.1.2.  Status Line  . . . . . . . . . . . . . . . . . . . . . 21
     3.2.  Header Fields  . . . . . . . . . . . . . . . . . . . . . . 21
       3.2.1.  Whitespace . . . . . . . . . . . . . . . . . . . . . . 23
       3.2.2.  Field Parsing  . . . . . . . . . . . . . . . . . . . . 23
       3.2.3.  Field Length . . . . . . . . . . . . . . . . . . . . . 24
       3.2.4.  Field value components . . . . . . . . . . . . . . . . 24
     3.3.  Message Body . . . . . . . . . . . . . . . . . . . . . . . 26
       3.3.1.  Transfer-Encoding  . . . . . . . . . . . . . . . . . . 26
       3.3.2.  Content-Length . . . . . . . . . . . . . . . . . . . . 28
       3.3.3.  Message Body Length  . . . . . . . . . . . . . . . . . 29
     3.4.  Handling Incomplete Messages . . . . . . . . . . . . . . . 31
     3.5.  Message Parsing Robustness . . . . . . . . . . . . . . . . 32
   4.  Transfer Codings . . . . . . . . . . . . . . . . . . . . . . . 32
     4.1.  Chunked Transfer Coding  . . . . . . . . . . . . . . . . . 33
       4.1.1.  Trailer  . . . . . . . . . . . . . . . . . . . . . . . 34
       4.1.2.  Decoding chunked . . . . . . . . . . . . . . . . . . . 35
     4.2.  Compression Codings  . . . . . . . . . . . . . . . . . . . 35
       4.2.1.  Compress Coding  . . . . . . . . . . . . . . . . . . . 35
       4.2.2.  Deflate Coding . . . . . . . . . . . . . . . . . . . . 35
       4.2.3.  Gzip Coding  . . . . . . . . . . . . . . . . . . . . . 36
     4.3.  TE . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
   5.  Message Routing  . . . . . . . . . . . . . . . . . . . . . . . 37
     5.1.  Identifying a Target Resource  . . . . . . . . . . . . . . 37
     5.2.  Connecting Inbound . . . . . . . . . . . . . . . . . . . . 37
     5.3.  Request Target . . . . . . . . . . . . . . . . . . . . . . 38
     5.4.  Host . . . . . . . . . . . . . . . . . . . . . . . . . . . 40
     5.5.  Effective Request URI  . . . . . . . . . . . . . . . . . . 41
     5.6.  Message Forwarding . . . . . . . . . . . . . . . . . . . . 42
     5.7.  Via  . . . . . . . . . . . . . . . . . . . . . . . . . . . 43
     5.8.  Message Transforming . . . . . . . . . . . . . . . . . . . 44
     5.9.  Associating a Response to a Request  . . . . . . . . . . . 46
   6.  Connection Management  . . . . . . . . . . . . . . . . . . . . 46
     6.1.  Connection . . . . . . . . . . . . . . . . . . . . . . . . 46
     6.2.  Persistent Connections . . . . . . . . . . . . . . . . . . 48
       6.2.1.  Establishment  . . . . . . . . . . . . . . . . . . . . 49
       6.2.2.  Reuse  . . . . . . . . . . . . . . . . . . . . . . . . 50
       6.2.3.  Concurrency  . . . . . . . . . . . . . . . . . . . . . 51
       6.2.4.  Failures and Time-outs . . . . . . . . . . . . . . . . 51
       6.2.5.  Tear-down  . . . . . . . . . . . . . . . . . . . . . . 52
     6.3.  Upgrade  . . . . . . . . . . . . . . . . . . . . . . . . . 53
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 54
     7.1.  Header Field Registration  . . . . . . . . . . . . . . . . 54
     7.2.  URI Scheme Registration  . . . . . . . . . . . . . . . . . 55
     7.3.  Internet Media Type Registrations  . . . . . . . . . . . . 56
       7.3.1.  Internet Media Type message/http . . . . . . . . . . . 56
       7.3.2.  Internet Media Type application/http . . . . . . . . . 57



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     7.4.  Transfer Coding Registry . . . . . . . . . . . . . . . . . 58
     7.5.  Transfer Coding Registrations  . . . . . . . . . . . . . . 58
     7.6.  Upgrade Token Registry . . . . . . . . . . . . . . . . . . 59
     7.7.  Upgrade Token Registration . . . . . . . . . . . . . . . . 60
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . . 60
     8.1.  Personal Information . . . . . . . . . . . . . . . . . . . 60
     8.2.  Abuse of Server Log Information  . . . . . . . . . . . . . 60
     8.3.  Attacks Based On File and Path Names . . . . . . . . . . . 61
     8.4.  DNS-related Attacks  . . . . . . . . . . . . . . . . . . . 61
     8.5.  Intermediaries and Caching . . . . . . . . . . . . . . . . 61
     8.6.  Protocol Element Size Overflows  . . . . . . . . . . . . . 62
   9.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 62
   10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 64
     10.1. Normative References . . . . . . . . . . . . . . . . . . . 64
     10.2. Informative References . . . . . . . . . . . . . . . . . . 65
   Appendix A.  HTTP Version History  . . . . . . . . . . . . . . . . 67
     A.1.  Changes from HTTP/1.0  . . . . . . . . . . . . . . . . . . 67
       A.1.1.  Multi-homed Web Servers  . . . . . . . . . . . . . . . 68
       A.1.2.  Keep-Alive Connections . . . . . . . . . . . . . . . . 68
       A.1.3.  Introduction of Transfer-Encoding  . . . . . . . . . . 69
     A.2.  Changes from RFC 2616  . . . . . . . . . . . . . . . . . . 69
   Appendix B.  ABNF list extension: #rule  . . . . . . . . . . . . . 70
   Appendix C.  Collected ABNF  . . . . . . . . . . . . . . . . . . . 71
   Appendix D.  Change Log (to be removed by RFC Editor before
                publication)  . . . . . . . . . . . . . . . . . . . . 74
     D.1.  Since RFC 2616 . . . . . . . . . . . . . . . . . . . . . . 74
     D.2.  Since draft-ietf-httpbis-p1-messaging-00 . . . . . . . . . 74
     D.3.  Since draft-ietf-httpbis-p1-messaging-01 . . . . . . . . . 75
     D.4.  Since draft-ietf-httpbis-p1-messaging-02 . . . . . . . . . 76
     D.5.  Since draft-ietf-httpbis-p1-messaging-03 . . . . . . . . . 77
     D.6.  Since draft-ietf-httpbis-p1-messaging-04 . . . . . . . . . 77
     D.7.  Since draft-ietf-httpbis-p1-messaging-05 . . . . . . . . . 78
     D.8.  Since draft-ietf-httpbis-p1-messaging-06 . . . . . . . . . 79
     D.9.  Since draft-ietf-httpbis-p1-messaging-07 . . . . . . . . . 79
     D.10. Since draft-ietf-httpbis-p1-messaging-08 . . . . . . . . . 80
     D.11. Since draft-ietf-httpbis-p1-messaging-09 . . . . . . . . . 80
     D.12. Since draft-ietf-httpbis-p1-messaging-10 . . . . . . . . . 81
     D.13. Since draft-ietf-httpbis-p1-messaging-11 . . . . . . . . . 81
     D.14. Since draft-ietf-httpbis-p1-messaging-12 . . . . . . . . . 82
     D.15. Since draft-ietf-httpbis-p1-messaging-13 . . . . . . . . . 82
     D.16. Since draft-ietf-httpbis-p1-messaging-14 . . . . . . . . . 83
     D.17. Since draft-ietf-httpbis-p1-messaging-15 . . . . . . . . . 83
     D.18. Since draft-ietf-httpbis-p1-messaging-16 . . . . . . . . . 83
     D.19. Since draft-ietf-httpbis-p1-messaging-17 . . . . . . . . . 84
     D.20. Since draft-ietf-httpbis-p1-messaging-18 . . . . . . . . . 84
     D.21. Since draft-ietf-httpbis-p1-messaging-19 . . . . . . . . . 84
     D.22. Since draft-ietf-httpbis-p1-messaging-20 . . . . . . . . . 85
   Index  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85



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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.  This document is the first in a
   series of documents that collectively form the HTTP/1.1
   specification:

      RFC xxx1: Message Syntax and Routing

      RFC xxx2: Semantics and Content

      RFC xxx3: Conditional Requests

      RFC xxx4: Range Requests

      RFC xxx5: Caching

      RFC xxx6: Authentication

   This HTTP/1.1 specification obsoletes and moves to historic status
   RFC 2616, its predecessor RFC 2068, RFC 2145 (on HTTP versioning),
   and RFC 2817 (on using CONNECT for TLS upgrades).

   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



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   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 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 and message-
   forwarding intermediaries.

1.1.  Requirement Notation

   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].

   Conformance criteria and considerations regarding error handling are
   defined in Section 2.5.

1.2.  Syntax Notation

   This specification uses the Augmented Backus-Naur Form (ABNF)
   notation of [RFC5234] with the list rule extension defined in
   Appendix B.  Appendix C shows the collected ABNF with the list rule
   expanded.

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

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

2.  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.






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2.1.  Client/Server Messaging

   HTTP is a stateless request/response protocol that operates by
   exchanging messages (Section 3) across a reliable transport or
   session-layer "connection" (Section 6).  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.

   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.

   HTTP relies upon the Uniform Resource Identifier (URI) standard
   [RFC3986] to indicate the target resource (Section 5.1) 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 [Part2]
   for the differences between HTTP and MIME messages).

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

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

   A client sends an HTTP request to a server in the form of a request
   message, beginning with a request-line that includes a method, URI,
   and protocol version (Section 3.1.1), followed by header fields
   containing request modifiers, client information, and representation
   metadata (Section 3.2), an empty line to indicate the end of the
   header section, and finally a message body containing the payload
   body (if any, Section 3.3).

   A server responds to a client's request by sending one or more HTTP
   response messages, each beginning with a status line that includes



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   the protocol version, a success or error code, and textual reason
   phrase (Section 3.1.2), possibly followed by header fields containing
   server information, resource metadata, and representation metadata
   (Section 3.2), an empty line to indicate the end of the header
   section, and finally a message body containing the payload body (if
   any, Section 3.3).

   A connection might be used for multiple request/response exchanges,
   as defined in Section 6.2.

   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-Language: en, mi


   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!

2.2.  Implementation Diversity

   When considering the design of HTTP, it is easy to fall into a trap
   of thinking that all user agents are general-purpose browsers and all
   origin servers are large public websites.  That is not the case in
   practice.  Common HTTP user agents include household appliances,
   stereos, scales, firmware update scripts, command-line programs,
   mobile apps, and communication devices in a multitude of shapes and
   sizes.  Likewise, common HTTP origin servers include home automation
   units, configurable networking components, office machines,
   autonomous robots, news feeds, traffic cameras, ad selectors, and
   video delivery platforms.




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   The term "user agent" does not imply that there is a human user
   directly interacting with the software agent at the time of a
   request.  In many cases, a user agent is installed or configured to
   run in the background and save its results for later inspection (or
   save only a subset of those results that might be interesting or
   erroneous).  Spiders, for example, are typically given a start URI
   and configured to follow certain behavior while crawling the Web as a
   hypertext graph.

   The implementation diversity of HTTP means that we cannot assume the
   user agent can make interactive suggestions to a user or provide
   adequate warning for security or privacy options.  In the few cases
   where this specification requires reporting of errors to the user, it
   is acceptable for such reporting to only be observable in an error
   console or log file.  Likewise, requirements that an automated action
   be confirmed by the user before proceeding can me met via advance
   configuration choices, run-time options, or simply not proceeding
   with the unsafe action.

2.3.  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"



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   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-level 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
   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 7.3.4 of [Part2] and Section 7.5 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 conform to HTTP user agent requirements
   on the gateway's inbound connection and MUST implement the Connection
   (Section 6.1) and Via (Section 5.7) header fields for both
   connections.



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   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 (TLS, [RFC5246]) is used to establish
   confidential communication through a shared firewall proxy.

   The above categories for intermediary only consider those acting as
   participants in the HTTP communication.  There are also
   intermediaries that can act on lower layers of the network protocol
   stack, filtering or redirecting HTTP traffic without the knowledge or
   permission of message senders.  Network intermediaries often
   introduce security flaws or interoperability problems by violating
   HTTP semantics.  For example, an "interception proxy" [RFC3040] (also
   commonly known as a "transparent proxy" [RFC1919] or "captive
   portal") differs from an HTTP proxy because it is not selected by the
   client.  Instead, an interception proxy filters or redirects outgoing
   TCP port 80 packets (and occasionally other common port traffic).
   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.

   HTTP is defined as a stateless protocol, meaning that each request
   message can be understood in isolation.  Many implementations depend
   on HTTP's stateless design in order to reuse proxied connections or
   dynamically load balance requests across multiple servers.  Hence,
   servers MUST NOT assume that two requests on the same connection are
   from the same user agent unless the connection is secured and
   specific to that agent.  Some non-standard HTTP extensions (e.g.,
   [RFC4559]) have been known to violate this requirement, resulting in
   security and interoperability problems.

2.4.  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



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   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.

2.5.  Conformance and Error Handling

   This specification targets conformance criteria according to the role
   of a participant in HTTP communication.  Hence, HTTP requirements are
   placed on senders, recipients, clients, servers, user agents,
   intermediaries, origin servers, proxies, gateways, or caches,
   depending on what behavior is being constrained by the requirement.
   Additional (social) requirements are placed on implementations,
   resource owners, and protocol element registrations when they apply
   beyond the scope of a single communication.

   The verb "generate" is used instead of "send" where a requirement
   differentiates between creating a protocol element and merely
   forwarding a received element downstream.

   An implementation is considered conformant if it complies with all of
   the requirements associated with the roles it partakes in HTTP.  Note
   that SHOULD-level requirements are relevant here, unless one of the
   documented exceptions is applicable.

   Conformance applies to both the syntax and semantics of HTTP protocol
   elements.  A sender MUST NOT generate protocol elements that convey a
   meaning that is known by that sender to be false.  A sender MUST NOT
   generate protocol elements that do not match the grammar defined by
   the ABNF rules for those protocol elements that are applicable to the
   sender's role.  If a received protocol element is processed, the



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   recipient MUST be able to parse any value that would match the ABNF
   rules for that protocol element, excluding only those rules not
   applicable to the recipient's role.

   Unless noted otherwise, a recipient MAY attempt to recover a usable
   protocol element from an invalid construct.  HTTP does not define
   specific error handling mechanisms except when they have a direct
   impact on security, since different applications of the protocol
   require different error handling strategies.  For example, a Web
   browser might wish to transparently recover from a response where the
   Location header field doesn't parse according to the ABNF, whereas a
   systems control client might consider any form of error recovery to
   be dangerous.

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 conformance with
   the set of requirements laid out in that version's corresponding
   specification of HTTP.

   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-name "/" DIGIT "." DIGIT
     HTTP-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
   conformant 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
   conformant 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.




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   The interpretation of a header field does not change between minor
   versions of the same major HTTP 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 conformance 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 6.1).  These
   requirements allow HTTP's functionality to be enhanced without
   requiring prior update of deployed intermediaries.

   Intermediaries that process HTTP messages (i.e., all intermediaries
   other than those acting as tunnels) MUST send their own HTTP-version
   in forwarded messages.  In other words, they MUST NOT blindly forward
   the first line of an HTTP message without ensuring that the protocol
   version in that message matches a version to which that intermediary
   is conformant 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 to which the client is conformant 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 to which it is not
   conformant.

   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 to which the server is conformant and whose major version is
   less than or equal to the one received in the request.  An HTTP
   server MUST NOT send a version to which it is not conformant.  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



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   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 conform to 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.

2.7.  Uniform Resource Identifiers

   Uniform Resource Identifiers (URIs) [RFC3986] are used throughout
   HTTP as the means for identifying resources (Section 2 of [Part2]).
   URI references are used to target requests, indicate redirects, and
   define relationships.

   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.  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



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   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 (Section 5.5).

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
   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
   (Section 3) containing the URI's identifying data (Section 5) to the
   server.  If the server responds to that request with a non-interim



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   HTTP response message, as described in Section 7 of [Part2], 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 resources that require an end-to-end
   secured 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
   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 to a
   given TCP port for TLS-secured connections [RFC5246].

   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, end-to-end, 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 7.2 of [Part6]).



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   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
   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.

     HTTP-message   = start-line
                      *( header-field CRLF )
                      CRLF
                      [ message-body ]



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   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.

   Recipients MUST parse an HTTP message as a sequence of octets in an
   encoding that is a superset of US-ASCII [USASCII].  Parsing an HTTP
   message as a stream of Unicode characters, without regard for the
   specific encoding, creates security vulnerabilities due to the
   varying ways that string processing libraries handle invalid
   multibyte character sequences that contain the octet LF (%x0A).
   String-based parsers can only be safely used within protocol elements
   after the element has been extracted from the message, such as within
   a header field-value after message parsing has delineated the
   individual fields.

   An HTTP message can be parsed as a stream for incremental processing
   or forwarding downstream.  However, recipients cannot rely on
   incremental delivery of partial messages, since some implementations
   will buffer or delay message forwarding for the sake of network
   efficiency, security checks, or payload transformations.

3.1.  Start Line

   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.

     start-line     = request-line / status-line

   A sender 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
   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.




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3.1.1.  Request Line

   A 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

   A server MUST be able to parse any received message that begins with
   a request-line and matches the ABNF rule for HTTP-message.

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

     method         = token

   The methods defined by this specification can be found in Section 5
   of [Part2], along with information regarding the HTTP method registry
   and considerations for defining new methods.

   The request-target identifies the target resource upon which to apply
   the request, as defined in Section 5.3.

   No whitespace is allowed inside the method, request-target, and
   protocol version.  Hence, recipients typically parse the request-line
   into its component parts by splitting on the SP characters.

   Unfortunately, some user agents fail to properly encode hypertext
   references that have embedded whitespace, sending the characters
   directly instead of properly percent-encoding the disallowed
   characters.  Recipients of an invalid request-line SHOULD respond
   with either a 400 (Bad Request) error or a 301 (Moved Permanently)
   redirect with the request-target properly encoded.  Recipients SHOULD
   NOT attempt to autocorrect and then process the request without a
   redirect, since the invalid request-line might be deliberately
   crafted to bypass security filters along the request chain.

   HTTP does not place a pre-defined limit on the length of a request-
   line.  A server that receives a method longer than any that it
   implements SHOULD respond with either a 405 (Method Not Allowed), if
   it is an origin server, or a 501 (Not Implemented) status code.  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 7.5.12 of [Part2]).

   Various ad-hoc limitations on request-line length are found in
   practice.  It is RECOMMENDED that all HTTP senders and recipients



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   support, at a minimum, request-line lengths of up to 8000 octets.

3.1.2.  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,
   and ending with CRLF.

     status-line = HTTP-version SP status-code SP reason-phrase CRLF

   A client MUST be able to parse any received message that begins with
   a status-line and matches the ABNF rule for HTTP-message.

   The status-code element is a 3-digit integer code describing the
   result of the server's attempt to understand and satisfy the client's
   corresponding request.  The rest of the response message is to be
   interpreted in light of the semantics defined for that status code.
   See Section 7 of [Part2] for information about the semantics of
   status codes, including the classes of status code (indicated by the
   first digit), the status codes defined by this specification,
   considerations for the definition of new status codes, and the IANA
   registry.

     status-code    = 3DIGIT

   The reason-phrase element exists for the sole purpose of providing a
   textual description associated with the numeric status code, mostly
   out of deference to earlier Internet application protocols that were
   more frequently used with interactive text clients.  A client SHOULD
   ignore the reason-phrase content.

     reason-phrase  = *( HTAB / SP / VCHAR / obs-text )

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.

     header-field   = field-name ":" OWS field-value BWS
     field-name     = token
     field-value    = *( field-content / obs-fold )
     field-content  = *( HTAB / SP / VCHAR / obs-text )
     obs-fold       = CRLF ( SP / HTAB )
                    ; obsolete line folding
                    ; see Section 3.2.2

   The field-name token labels the corresponding field-value as having



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   the semantics defined by that header field.  For example, the Date
   header field is defined in Section 8.1.1.2 of [Part2] as containing
   the origination timestamp for the message in which it appears.

   HTTP header fields are fully extensible: there is no limit on the
   introduction of new field names, each presumably defining new
   semantics, or on the number of header fields used in a given message.
   Existing fields are defined in each part of this specification and in
   many other specifications outside the standards process.  New header
   fields can be introduced without changing the protocol version if
   their defined semantics allow them to be safely ignored by recipients
   that do not recognize them.

   New HTTP header fields SHOULD be registered with IANA in the Message
   Header Field Registry, as described in Section 9.3 of [Part2].
   Unrecognized header fields MUST be forwarded by a proxy unless the
   field-name is listed in the Connection header field (Section 6.1) or
   the proxy is specifically configured to block or otherwise transform
   such fields.  Unrecognized header fields SHOULD be ignored by other
   recipients.

   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.




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3.2.1.  Whitespace

   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 either be replaced with a single SP or transformed to all SP
   octets (each octet other than SP replaced with 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 either be
   replaced with a single SP or transformed to all SP octets 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;
   recipients MUST accept such bad optional whitespace and remove it
   before interpreting the field value or forwarding the message
   downstream.


     OWS            = *( SP / HTAB )
                    ; "optional" whitespace
     RWS            = 1*( SP / HTAB )
                    ; "required" whitespace
     BWS            = OWS
                    ; "bad" whitespace

3.2.2.  Field Parsing

   No whitespace is allowed between the header field-name and colon.  In
   the past, differences in the handling of such whitespace have led to
   security vulnerabilities in request routing and response handling.
   Any received request message that contains whitespace between a
   header field-name and colon 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



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   processing (as this does not change the meaning of the header field).

   Historically, HTTP header field values could be extended over
   multiple lines by preceding each extra line with at least one space
   or horizontal tab (obs-fold).  This specification deprecates such
   line folding except within the message/http media type
   (Section 7.3.1).  HTTP senders MUST NOT produce messages that include
   line folding (i.e., that contain any field-value that matches the
   obs-fold rule) unless the message is intended for packaging within
   the message/http media type.  HTTP recipients SHOULD accept line
   folding and replace any embedded obs-fold whitespace with either a
   single SP or a matching number of SP octets (to avoid buffer copying)
   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.

3.2.3.  Field Length

   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 (Client Error) status code if the received header field(s)
   would be longer than the server wishes to handle.

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

   Various ad-hoc limitations on header field 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.

3.2.4.  Field value components

   Many HTTP 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 4).






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     word           = token / quoted-string

     token          = 1*tchar

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

     special        = "(" / ")" / "<" / ">" / "@" / ","
                    / ";" / ":" / "\" / DQUOTE / "/" / "["
                    / "]" / "?" / "=" / "{" / "}"

   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
     obs-text       = %x80-FF

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

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

   Recipients that process the value of the quoted-string MUST handle a
   quoted-pair as if it were replaced by the octet following the
   backslash.

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

   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

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

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

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




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3.3.  Message Body

   The message body (if any) of an HTTP message is used to carry the
   payload body of that request or response.  The message body is
   identical to the payload body unless a transfer coding has been
   applied, as described in Section 3.3.1.

     message-body = *OCTET

   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 a a
   Content-Length or Transfer-Encoding header field.  Request message
   framing is independent of method semantics, even if the method does
   not define any use for a message body.

   The presence of a message body in a response depends on both the
   request method to which it is responding and the response status code
   (Section 3.1.2).  Responses to the HEAD request method never include
   a message body because the associated response header fields (e.g.,
   Transfer-Encoding, Content-Length, etc.), if present, indicate only
   what their values would have been if the request method had been GET
   (Section 5.3.2 of [Part2]). 2xx (Successful) responses to CONNECT
   switch to tunnel mode instead of having a message body (Section 5.3.6
   of [Part2]).  All 1xx (Informational), 204 (No Content), and 304 (Not
   Modified) responses MUST NOT include a message body.  All other
   responses do include a message body, although the body MAY be of zero
   length.

3.3.1.  Transfer-Encoding

   When one or more transfer codings are applied to a payload body in
   order to form the message body, a Transfer-Encoding header field MUST
   be sent in the message and MUST contain the list of corresponding
   transfer-coding names in the same order that they were applied.
   Transfer codings are defined in Section 4.

     Transfer-Encoding = 1#transfer-coding

   Transfer-Encoding is analogous to the Content-Transfer-Encoding field
   of MIME, which was 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's case, Transfer-Encoding is primarily intended to accurately
   delimit a dynamically generated payload and to distinguish payload
   encodings that are only applied for transport efficiency or security
   from those that are characteristics of the target resource.



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   The "chunked" transfer-coding (Section 4.1) MUST be implemented by
   all HTTP/1.1 recipients because it plays a crucial role in delimiting
   messages when the payload body size is not known in advance.  When
   the "chunked" transfer-coding is used, it MUST be the last transfer-
   coding applied to form the message body and MUST NOT be applied more
   than once in a message body.  If any transfer-coding is applied to a
   request payload body, the final transfer-coding applied MUST be
   "chunked".  If any 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.

   For example,

     Transfer-Encoding: gzip, chunked

   indicates that the payload body has been compressed using the gzip
   coding and then chunked using the chunked coding while forming the
   message body.

   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.

   Unlike Content-Encoding (Section 3.1.2.1 of [Part2]), 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.  Additional information about the encoding parameters
   MAY be provided by other header fields not defined by this
   specification.

   Transfer-Encoding MAY be sent in a response to a HEAD request or in a
   304 (Not Modified) response (Section 4.1 of [Part4]) to a GET
   request, neither of which includes a message body, to indicate that
   the origin server would have applied a transfer coding to the message
   body if the request had been an unconditional GET.  This indication
   is not required, however, because any recipient on the response chain
   (including the origin server) can remove transfer codings when they
   are not needed.

   Transfer-Encoding was added in HTTP/1.1.  It is generally assumed
   that implementations advertising only HTTP/1.0 support will not
   understand how to process a transfer-encoded payload.  A client MUST
   NOT send a request containing Transfer-Encoding unless it knows the
   server will handle HTTP/1.1 (or later) requests; such knowledge might
   be in the form of specific user configuration or by remembering the
   version of a prior received response.  A server MUST NOT send a
   response containing Transfer-Encoding unless the corresponding



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   request indicates HTTP/1.1 (or later).

   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.

3.3.2.  Content-Length

   When a message is allowed to contain a message body, does not have a
   Transfer-Encoding header field, and has a payload body length that is
   known to the sender before the message header section has been sent,
   the sender SHOULD send a Content-Length header field to indicate the
   length of the payload body as a decimal number of octets.

     Content-Length = 1*DIGIT

   An example is

     Content-Length: 3495

   A sender MUST NOT send a Content-Length header field in any message
   that contains a Transfer-Encoding header field.

   A server MAY send a Content-Length header field in a response to a
   HEAD request (Section 5.3.2 of [Part2]); a server MUST NOT send
   Content-Length in such a response unless its field-value equals the
   decimal number of octets that would have been sent in the payload
   body of a response if the same request had used the GET method.

   A server MAY send a Content-Length header field in a 304 (Not
   Modified) response to a conditional GET request (Section 4.1 of
   [Part4]); a server MUST NOT send Content-Length in such a response
   unless its field-value equals the decimal number of octets that would
   have been sent in the payload body of a 200 (OK) response to the same
   request.

   A server MUST NOT send a Content-Length header field in any response
   with a status code of 1xx (Informational) or 204 (No Content).  A
   server SHOULD NOT send a Content-Length header field in any 2xx
   (Successful) response to a CONNECT request (Section 5.3.6 of
   [Part2]).

   Any Content-Length field value greater than or equal to zero is
   valid.  Since there is no predefined limit to the length of an HTTP
   payload, recipients SHOULD anticipate potentially large decimal
   numerals and prevent parsing errors due to integer conversion
   overflows (Section 8.6).




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   If a message is received that has multiple Content-Length header
   fields 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.

      Note: HTTP's use of Content-Length for message framing differs
      significantly from the same field's use in MIME, where it is an
      optional field used only within the "message/external-body" media-
      type.

3.3.3.  Message Body Length

   The length of a 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 1xx
       (Informational), 204 (No Content), or 304 (Not Modified) status
       code 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.  Any 2xx (Successful) response to a CONNECT request implies that
       the connection will become a tunnel immediately after the empty
       line that concludes the header fields.  A client MUST ignore any
       Content-Length or Transfer-Encoding header fields received in
       such a message.

   3.  If a Transfer-Encoding header field is present and the "chunked"
       transfer-coding (Section 4.1) 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 and a



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       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.

   4.  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
       connection.

   5.  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 a connection and the connection
       has been closed by the server.

   6.  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).

   7.  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



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   network failure, a server 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
   the form of specific user configuration or by remembering the version
   of a prior received response.

3.4.  Handling Incomplete Messages

   Request messages that are prematurely terminated, possibly due to a
   canceled connection or a server-imposed time-out exception, MUST
   result in closure of the connection; sending an 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 response that terminates in the middle of
   the header block (before the empty line is received) cannot be
   assumed to convey the full semantics of the response and MUST be
   treated as an error.

   A message body that uses the chunked transfer encoding is incomplete
   if the zero-sized chunk that terminates the encoding has not been
   received.  A message that uses a valid Content-Length is incomplete
   if the size of the message body received (in octets) is less than the
   value given by Content-Length.  A response that has neither chunked
   transfer encoding nor Content-Length is terminated by closure of the
   connection, and thus is considered complete regardless of the number
   of message body octets received, provided that the header block was
   received intact.



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   A user agent MUST NOT render an incomplete response message body as
   if it were complete (i.e., some indication needs to be given to the
   user that an error occurred).  Cache requirements for incomplete
   responses are defined in Section 3 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 6.2.2.1.

3.5.  Message Parsing Robustness

   Older HTTP/1.0 client implementations might 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.

   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.
   Likewise, although the line terminator for the start-line and header
   fields is the sequence CRLF, we recommend that recipients recognize a
   single LF as a line terminator and ignore any CR.

   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.

4.  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.





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     transfer-coding    = "chunked" ; Section 4.1
                        / "compress" ; Section 4.2.1
                        / "deflate" ; Section 4.2.2
                        / "gzip" ; Section 4.2.3
                        / 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

   All transfer-coding values are case-insensitive and SHOULD be
   registered within the HTTP Transfer Coding registry, as defined in
   Section 7.4.  They are used in the TE (Section 4.3) and Transfer-
   Encoding (Section 3.3.1) header fields.

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

     chunk-ext      = *( ";" chunk-ext-name [ "=" chunk-ext-val ] )
     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      = HTAB / SP / %x21 / %x23-5B / %x5D-7E / obs-text

   Chunk extensions within the chucked encoding are deprecated.  Senders



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   SHOULD NOT send chunk-ext.  Definition of new chunk extensions is
   discouraged.

   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.

4.1.1.  Trailer

   A trailer allows the sender to include additional fields at the end
   of a chunked message in order to supply metadata that might be
   dynamically generated while the message body is sent, such as a
   message integrity check, digital signature, or post-processing
   status.  The trailer MUST NOT contain fields that need to be known
   before a recipient processes the body, such as Transfer-Encoding,
   Content-Length, and Trailer.

   When a message includes a message body encoded with the chunked
   transfer-coding and the sender desires to send metadata in the form
   of trailer fields at the end of the message, the sender SHOULD send a
   Trailer header field before the message body to indicate which fields
   will be present in the trailers.  This allows the recipient to
   prepare for receipt of that metadata before it starts processing the
   body, which is useful if the message is being streamed and the
   recipient wishes to confirm an integrity check on the fly.

     Trailer = 1#field-name

   If no Trailer header field is present, the sender of a chunked
   message body SHOULD send an empty trailer.

   A server MUST send an empty trailer with the chunked transfer-coding
   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 4.3; 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 that
       metadata.  In other words, the server that generated the header
       field is willing to accept the possibility that the trailer
       fields might be silently discarded along the path to the client.

   The above requirement prevents the need for an infinite buffer when a
   message is being received by an HTTP/1.1 (or later) proxy and



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   forwarded to an HTTP/1.0 recipient.

4.1.2.  Decoding chunked

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

     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
     Remove Trailer from existing header fields

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

4.2.  Compression Codings

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

4.2.1.  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).  Recipients SHOULD consider "x-compress" to be
   equivalent to "compress".

4.2.2.  Deflate Coding

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






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

4.2.3.  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.  Recipients SHOULD consider "x-gzip"
   to be equivalent to "gzip".

4.3.  TE

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

   The TE field-value consists of a comma-separated list of transfer-
   coding names, each allowing for optional parameters (as described in
   Section 4), and/or the keyword "trailers".  Clients MUST NOT send the
   chunked transfer-coding name in TE; chunked is always acceptable for
   HTTP/1.1 recipients.

     TE        = #t-codings
     t-codings = "trailers" / ( transfer-coding [ t-ranking ] )
     t-ranking = OWS ";" OWS "q=" rank
     rank      = ( "0" [ "." 0*3DIGIT ] )
                / ( "1" [ "." 0*3("0") ] )

   Three examples of TE use are below.

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

   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 4.1, on behalf of itself and any downstream
   clients.  For chained requests, this implies that either: (a) all
   downstream clients are willing to accept trailer fields in the
   forwarded response; or, (b) the client will attempt to buffer the
   response on behalf of downstream recipients.  Note that 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.

   When multiple transfer-codings are acceptable, the client MAY rank
   the codings by preference using a case-insensitive "q" parameter
   (similar to the qvalues used in content negotiation fields, Section



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   6.3.1 of [Part2]).  The rank value is a real number in the range 0
   through 1, where 0.001 is the least preferred and 1 is the most
   preferred; a value of 0 means "not acceptable".

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

   Since the TE header field only applies to the immediate connection, a
   sender of TE MUST also send a "TE" connection option within the
   Connection header field (Section 6.1) in order to prevent the TE
   field from being forwarded by intermediaries that do not support its
   semantics.

5.  Message Routing

   HTTP request message routing is determined by each client based on
   the target resource, the client's proxy configuration, and
   establishment or reuse of an inbound connection.  The corresponding
   response routing follows the same connection chain back to the
   client.

5.1.  Identifying a Target Resource

   HTTP is used in a wide variety of applications, ranging from general-
   purpose computers to home appliances.  In some cases, communication
   options are hard-coded in a client's configuration.  However, most
   HTTP clients rely on the same resource identification mechanism and
   configuration techniques as general-purpose Web browsers.

   HTTP communication is initiated by a user agent for some purpose.
   The purpose is a combination of request semantics, which are defined
   in [Part2], and a target resource upon which to apply those
   semantics.  A URI reference (Section 2.7) is typically used as an
   identifier for the "target resource", which a user agent would
   resolve to its absolute form in order to obtain the "target URI".
   The target URI excludes the reference's fragment identifier
   component, if any, since fragment identifiers are reserved for
   client-side processing ([RFC3986], Section 3.5).

5.2.  Connecting Inbound

   Once the target URI is determined, a client needs to decide whether a
   network request is necessary to accomplish the desired semantics and,
   if so, where that request is to be directed.

   If the client has a response cache and the request semantics can be
   satisfied by a cache ([Part6]), then the request is usually directed



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   to the cache first.

   If the request is not satisfied by a cache, then a typical client
   will check its configuration to determine whether a proxy is to be
   used to satisfy the request.  Proxy configuration is implementation-
   dependent, but is often based on URI prefix matching, selective
   authority matching, or both, and the proxy itself is usually
   identified by an "http" or "https" URI.  If a proxy is applicable,
   the client connects inbound by establishing (or reusing) a connection
   to that proxy.

   If no proxy is applicable, a typical client will invoke a handler
   routine, usually specific to the target URI's scheme, to connect
   directly to an authority for the target resource.  How that is
   accomplished is dependent on the target URI scheme and defined by its
   associated specification, similar to how this specification defines
   origin server access for resolution of the "http" (Section 2.7.1) and
   "https" (Section 2.7.2) schemes.

   HTTP requirements regarding connection management are defined in
   Section 6.

5.3.  Request Target

   Once an inbound connection is obtained, the client sends an HTTP
   request message (Section 3) with a request-target derived from the
   target URI.  There are four distinct formats for the request-target,
   depending on both the method being requested and whether the request
   is to a proxy.

     request-target = origin-form
                    / absolute-form
                    / authority-form
                    / asterisk-form

     origin-form    = path-absolute [ "?" query ]
     absolute-form  = absolute-URI
     authority-form = authority
     asterisk-form  = "*"

   The most common form of request-target is the origin-form.  When
   making a request directly to an origin server, other than a CONNECT
   or server-wide OPTIONS request (as detailed below), a client MUST
   send only the absolute path and query components of the target URI as
   the request-target.  If the target URI's path component is empty,
   then the client MUST send "/" as the path within the origin-form of
   request-target.  A Host header field is also sent, as defined in
   Section 5.4, containing the target URI's authority component



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   (excluding any userinfo).

   For example, a client wishing to retrieve a representation of the
   resource identified as

     http://www.example.org/where?q=now

   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 /where?q=now HTTP/1.1
     Host: www.example.org

   followed by the remainder of the request message.

   When making a request to a proxy, other than a CONNECT or server-wide
   OPTIONS request (as detailed below), a client MUST send the target
   URI in absolute-form as the request-target.  The proxy is requested
   to either service that request from a valid cache, if possible, or
   make the same request on the client's behalf to either the next
   inbound proxy server or directly to the origin server indicated by
   the request-target.  Requirements on such "forwarding" of messages
   are defined in Section 5.6.

   An example absolute-form of request-line would be:

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

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

   The authority-form of request-target is only used for CONNECT
   requests (Section 5.3.6 of [Part2]).  When making a CONNECT request
   to establish a tunnel through one or more proxies, a client MUST send
   only the target URI's authority component (excluding any userinfo) as
   the request-target.  For example,

     CONNECT www.example.com:80 HTTP/1.1

   The asterisk-form of request-target is only used for a server-wide
   OPTIONS request (Section 5.3.7 of [Part2]).  When a client wishes to
   request OPTIONS for the server as a whole, as opposed to a specific
   named resource of that server, the client MUST send only "*" (%x2A)
   as the request-target.  For example,




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     OPTIONS * HTTP/1.1

   If a proxy receives an OPTIONS request with an absolute-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 send 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

   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".

5.4.  Host

   The "Host" header field in a request provides the host and port
   information from the target URI, enabling the origin server to
   distinguish among 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 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 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 absolute-form, since this allows the Host
   information to be forwarded through ancient HTTP/1.0 proxies that
   might not have implemented Host.




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

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

5.5.  Effective Request URI

   A server that receives an HTTP request message MUST reconstruct the
   user agent's original target URI, based on the pieces of information
   learned from the request-target, Host header field, and connection
   context, in order to identify the intended target resource and
   properly service the request.  The URI derived from this
   reconstruction process is referred to as the "effective request URI".

   For a user agent, the effective request URI is the target URI.

   If the request-target is in absolute-form, then the effective request
   URI is the same as the request-target.  Otherwise, the effective
   request URI is constructed as follows.

   If the request is received over a TLS-secured TCP connection, then
   the effective request URI's scheme is "https"; otherwise, the scheme
   is "http".

   If the request-target is in authority-form, then the effective
   request URI's authority component is the same as the request-target.
   Otherwise, if a Host header field is supplied with a non-empty field-
   value, then the authority component is the same as the Host field-
   value.  Otherwise, the authority component is the concatenation of
   the default host name configured for the server, a colon (":"), and
   the connection's incoming TCP port number in decimal form.




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   If the request-target is in authority-form or asterisk-form, then the
   effective request URI's combined path and query component is empty.
   Otherwise, the combined path and query component is the same as the
   request-target.

   The components of the effective request URI, once determined as
   above, can be combined into absolute-URI form by concatenating the
   scheme, "://", authority, and combined path and query component.

   Example 1: the following message received over an insecure TCP
   connection

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

   has an effective request URI of

     http://www.example.org:8080/pub/WWW/TheProject.html

   Example 2: the following message received over a TLS-secured TCP
   connection

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

   has an effective request URI of

     https://www.example.org

   An origin server that does not allow resources to differ by requested
   host MAY ignore the Host field-value and instead replace it with a
   configured server name when constructing the effective request URI.

   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 guess the
   effective request URI's authority component.

5.6.  Message Forwarding

   As described in Section 2.3, intermediaries can serve a variety of
   roles in the processing of HTTP requests and responses.  Some
   intermediaries are used to improve performance or availability.
   Others are used for access control or to filter content.  Since an
   HTTP stream has characteristics similar to a pipe-and-filter
   architecture, there are no inherent limits to the extent an
   intermediary can enhance (or interfere) with either direction of the
   stream.



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   Intermediaries that forward a message MUST implement the Connection
   header field, as specified in Section 6.1, to exclude fields that are
   only intended for the incoming connection.

   In order to 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, or literal IP
   addresses.

5.7.  Via

   The "Via" header field MUST be sent by a proxy or gateway in
   forwarded messages 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]).  Via is used in HTTP 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
     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
   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



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   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.

   A proxy or gateway MAY combine an ordered subsequence of Via header
   field entries into a single such entry if the entries have identical
   received-protocol values.  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.

5.8.  Message Transforming

   If a proxy receives a request-target with a host name that is not a
   fully qualified domain name, it MAY add its own domain to the host
   name it received when forwarding the request.  A proxy MUST NOT
   change the host name if it is a fully qualified domain name.

   A non-transforming proxy MUST NOT modify the "path-absolute" and
   "query" parts of the received request-target when forwarding it to
   the next inbound server, except as noted above to replace an empty
   path with "/" or "*".

   A non-transforming proxy MUST preserve the message payload (Section
   3.3 of [Part2]), though it MAY change the message body through
   application or removal of a transfer-coding (Section 4).




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   A non-transforming proxy SHOULD NOT modify header fields that provide
   information about the end points of the communication chain, the
   resource state, or the selected representation.

   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 (Section 8.4.1 of [Part2])

   o  Content-Location (Section 3.1.4.2 of [Part2])

   o  Content-MD5 (Section 14.15 of [RFC2616])

   o  ETag (Section 2.3 of [Part4])

   o  Last-Modified (Section 2.2 of [Part4])

   o  Server (Section 8.4.2 of [Part2])

   A non-transforming proxy MUST NOT modify an Expires header field
   (Section 7.3 of [Part6]) if already present in a response, but it MAY
   add an Expires header field with a field-value identical to that of
   the Date header field.

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

   o  Content-Encoding (Section 3.1.2.2 of [Part2])

   o  Content-Range (Section 5.2 of [Part5])

   o  Content-Type (Section 3.1.1.5 of [Part2])

   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 7.5 of [Part6]).

      Warning: Unnecessary modification of 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.







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5.9.  Associating a Response to a Request

   HTTP does not include a request identifier for associating a given
   request message with its corresponding one or more response messages.
   Hence, it relies on the order of response arrival to correspond
   exactly to the order in which requests are made on the same
   connection.  More than one response message per request only occurs
   when one or more informational responses (1xx, see Section 7.2 of
   [Part2]) precede a final response to the same request.

   A client that uses persistent connections and sends more than one
   request per connection MUST maintain a list of outstanding requests
   in the order sent on that connection and MUST associate each received
   response message to the highest ordered request that has not yet
   received a final (non-1xx) response.

6.  Connection Management

   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 an underlying transport
   protocol is outside the scope of this specification.

   As described in Section 5.2, the specific connection protocols to be
   used for an HTTP interaction are determined by client configuration
   and the target 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.

   HTTP implementations are expected to engage in connection management,
   which includes maintaining the state of current connections,
   establishing a new connection or reusing an existing connection,
   processing messages received on a connection, detecting connection
   failures, and closing each connection.  Most clients maintain
   multiple connections in parallel, including more than one connection
   per server endpoint.  Most servers are designed to maintain thousands
   of concurrent connections, while controlling request queues to enable
   fair use and detect denial of service attacks.

6.1.  Connection

   The "Connection" header field allows the sender to indicate desired
   control options for the current connection.  In order to avoid
   confusing downstream recipients, a proxy or gateway MUST remove or
   replace any received connection options before forwarding the



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   message.

   When a header field is used to supply control information for or
   about the current connection, the sender SHOULD list the
   corresponding field-name within the "Connection" header field.  A
   proxy or gateway MUST parse a received Connection header field before
   a message is forwarded and, for each connection-option in this field,
   remove any header field(s) from the message with the same name as the
   connection-option, and then remove the Connection header field itself
   (or replace it with the intermediary's own connection options for the
   forwarded message).

   Hence, the Connection header field provides a declarative way of
   distinguishing header fields that are only intended for the immediate
   recipient ("hop-by-hop") from those fields that are intended for all
   recipients on the chain ("end-to-end"), enabling the message to be
   self-descriptive and allowing future connection-specific extensions
   to be deployed without fear that they will be blindly forwarded by
   older intermediaries.

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

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

   Connection options are case-insensitive.

   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 7.2 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-option 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 conformant.

   When defining new connection options, specifications ought to
   carefully consider existing deployed header fields and ensure that
   the new connection option does not share the same name as an
   unrelated header field that might already be deployed.  Defining a



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   new connection option 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.

   The "close" connection option is defined for a sender to signal that
   this 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 be closed after the current request/response is
   complete (Section 6.2.5).

   A client that does not support persistent connections MUST send the
   "close" connection option in every request message.

   A server that does not support persistent connections MUST send the
   "close" connection option in every response message that does not
   have a 1xx (Informational) status code.

6.2.  Persistent Connections

   HTTP was originally designed to use a separate connection for each
   request/response pair.  As the Web evolved and embedded requests
   became common for inline images, the connection establishment
   overhead was a significant drain on performance and a concern for
   Internet congestion.  Message framing (via Content-Length) and
   optional long-lived connections (via Keep-Alive) were added to
   HTTP/1.0 in order to improve performance for some requests.  However,
   these extensions were insufficient for dynamically generated
   responses and difficult to use with intermediaries.

   HTTP/1.1 defaults to the use of "persistent connections", which allow
   multiple requests and responses to be carried over a single
   connection.  The "close" connection-option is used to signal that a
   connection will close after the current request/response.  Persistent
   connections have a number of advantages:

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

   o  Most 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 connection to be used



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      much more efficiently and with less overall latency.

   o  For TCP connections, network congestion is reduced by eliminating
      the packets associated with the three way handshake and graceful
      close procedures, and by allowing sufficient time to determine the
      congestion state of the network.

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

   o  HTTP can evolve more gracefully, since most errors can be reported
      without the penalty of closing the 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.

6.2.1.  Establishment

   It is beyond the scope of this specification to describe how
   connections are established via various transport or session-layer
   protocols.  Each connection applies to only one transport link.

   A recipient determines whether a connection is persistent or not
   based on the most recently received message's protocol version and
   Connection header field (if any):

   o  If the close connection option is present, the connection will not
      persist after the current response; else,

   o  If the received protocol is HTTP/1.1 (or later), the connection
      will persist after the current response; else,

   o  If the received protocol is HTTP/1.0, the "keep-alive" connection
      option is present, the recipient is not a proxy, and the recipient
      wishes to honor the HTTP/1.0 "keep-alive" mechanism, the
      connection will persist after the current response; otherwise,

   o  The connection will close after the current response.

   A proxy server MUST NOT maintain a persistent connection with an
   HTTP/1.0 client (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).






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6.2.2.  Reuse

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

   A server MAY assume that an HTTP/1.1 client intends to maintain a
   persistent connection until a close connection option is received in
   a request.

   A client MAY reuse a persistent connection until it sends or receives
   a close connection option or receives an HTTP/1.0 response without a
   "keep-alive" connection option.

   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 A.1.2 for more information on backward
   compatibility with HTTP/1.0 clients.

6.2.2.1.  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
   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
   5.2.2 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.

6.2.2.2.  Retrying Requests

   Senders can close the transport connection at any time.  Therefore,
   clients, servers, and proxies MUST be able to recover from
   asynchronous close events.  Client software MAY reopen the transport
   connection and retransmit the aborted sequence of requests without



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   user interaction so long as the request sequence is idempotent (see
   Section 5.2.2 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.

6.2.3.  Concurrency

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

   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.

   Multiple connections are typically used to avoid the "head-of-line
   blocking" problem, wherein a request that takes significant server-
   side processing and/or has a large payload blocks subsequent requests
   on the same connection.  However, each connection consumes server
   resources.  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.

6.2.4.  Failures and Time-outs

   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



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   at the same time that the server has decided to close the "idle"
   connection.  From the server's point of view, the connection is being
   closed while it was idle, but from the client's point of view, a
   request is in progress.

   Servers SHOULD maintain persistent connections and allow the
   underlying transport's flow control mechanisms to resolve temporary
   overloads, rather than terminate connections with the expectation
   that clients will retry.  The latter technique can exacerbate network
   congestion.

   A 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 and close the connection.

6.2.5.  Tear-down

   The Connection header field (Section 6.1) provides a "close"
   connection option that a sender SHOULD send when it wishes to close
   the connection after the current request/response pair.

   A client that sends a close connection option MUST NOT send further
   requests on that connection (after the one containing close) and MUST
   close the connection after reading the final response message
   corresponding to this request.

   A server that receives a close connection option MUST initiate a
   lingering close of the connection after it sends the final response
   to the request that contained close.  The server SHOULD include a
   close connection option in its final response on that connection.
   The server MUST NOT process any further requests received on that
   connection.

   A server that sends a close connection option MUST initiate a
   lingering close of the connection after it sends the response
   containing close.  The server MUST NOT process any further requests
   received on that connection.

   A client that receives a close connection option MUST cease sending
   requests on that connection and close the connection after reading
   the response message containing the close; if additional pipelined
   requests had been sent on the connection, the client SHOULD assume
   that they will not be processed by the server.

   If a server performs an immediate close of a TCP connection, there is
   a significant risk that the client will not be able to read the last
   HTTP response.  If the server receives additional data from the



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   client on a fully-closed connection, such as another request that was
   sent by the client before receiving the server's response, the
   server's TCP stack will send a reset packet to the client;
   unfortunately, the reset packet might erase the client's
   unacknowledged input buffers before they can be read and interpreted
   by the client's HTTP parser.

   To avoid the TCP reset problem, a server can perform a lingering
   close on a connection by closing only the write side of the read/
   write connection (a half-close) and continuing to read from the
   connection until the connection is closed by the client or the server
   is reasonably certain that its own TCP stack has received the
   client's acknowledgement of the packet(s) containing the server's
   last response.  It is then safe for the server to fully close the
   connection.

   It is unknown whether the reset problem is exclusive to TCP or might
   also be found in other transport connection protocols.

6.3.  Upgrade

   The "Upgrade" header field is intended to provide a simple mechanism
   for transitioning from HTTP/1.1 to some other protocol on the same
   connection.  A client MAY send a list of protocols in the Upgrade
   header field of a request to invite the server to switch to one or
   more of those protocols before sending the final response.  A server
   MUST send an Upgrade header field in 101 (Switching Protocols)
   responses to indicate which protocol(s) are being switched to, and
   MUST send it in 426 (Upgrade Required) responses to indicate
   acceptable protocols.  A server MAY send an Upgrade header field in
   any other response to indicate that they might be willing to upgrade
   to one of the specified protocols for a future request.

     Upgrade          = 1#protocol

     protocol         = protocol-name ["/" protocol-version]
     protocol-name    = token
     protocol-version = token

   For example,

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

   Upgrade 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



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   or target resource).

   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-level communication after the protocol change is
   entirely dependent upon the new protocol chosen, although the first
   action after changing the protocol MUST be a response to the initial
   HTTP request that contained the Upgrade header field.

   For example, if the Upgrade header field is received in a GET request
   and the server decides to switch protocols, then it MUST first
   respond with a 101 (Switching Protocols) message in HTTP/1.1 and then
   immediately follow that with the new protocol's equivalent of a
   response to a GET on the target resource.  This allows a connection
   to be upgraded to protocols with the same semantics as HTTP without
   the latency cost of an additional round-trip.  A server MUST NOT
   switch protocols unless the received message semantics can be honored
   by the new protocol; an OPTIONS request can be honored by any
   protocol.

   When Upgrade is sent, a sender MUST also send a Connection header
   field (Section 6.1) that contains the "upgrade" connection option, in
   order to prevent Upgrade from being accidentally forwarded by
   intermediaries that might not implement the listed protocols.  A
   server MUST ignore an Upgrade header field that is received in an
   HTTP/1.0 request.

   The Upgrade header field only applies to switching application-level
   protocols on the existing connection; it cannot be used to switch to
   a protocol on a different connection.  For that purpose, it is more
   appropriate to use a 3xx (Redirection) response (Section 7.4 of
   [Part2]).

   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 in Section 7.6.

7.  IANA Considerations

7.1.  Header Field Registration

   HTTP header fields are registered within the Message Header Field
   Registry [RFC3864] maintained by IANA at <http://www.iana.org/
   assignments/message-headers/message-header-index.html>.

   This document defines the following HTTP header fields, so their



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   associated registry entries shall be updated according to the
   permanent registrations below:

   +-------------------+----------+----------+---------------+
   | Header Field Name | Protocol | Status   | Reference     |
   +-------------------+----------+----------+---------------+
   | Connection        | http     | standard | Section 6.1   |
   | Content-Length    | http     | standard | Section 3.3.2 |
   | Host              | http     | standard | Section 5.4   |
   | TE                | http     | standard | Section 4.3   |
   | Trailer           | http     | standard | Section 4.1.1 |
   | Transfer-Encoding | http     | standard | Section 3.3.1 |
   | Upgrade           | http     | standard | Section 6.3   |
   | Via               | http     | standard | Section 5.7   |
   +-------------------+----------+----------+---------------+

   Furthermore, the header field-name "Close" shall be registered as
   "reserved", since using that name as an HTTP header field might
   conflict with the "close" connection option of the "Connection"
   header field (Section 6.1).

   +-------------------+----------+----------+-------------+
   | Header Field Name | Protocol | Status   | Reference   |
   +-------------------+----------+----------+-------------+
   | Close             | http     | reserved | Section 7.1 |
   +-------------------+----------+----------+-------------+

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

7.2.  URI Scheme Registration

   IANA maintains the registry of URI Schemes [RFC4395] at
   <http://www.iana.org/assignments/uri-schemes.html>.

   This document defines the following URI schemes, so their associated
   registry entries shall be updated according to the permanent
   registrations below:

   +------------+------------------------------------+---------------+
   | URI Scheme | Description                        | Reference     |
   +------------+------------------------------------+---------------+
   | http       | Hypertext Transfer Protocol        | Section 2.7.1 |
   | https      | Hypertext Transfer Protocol Secure | Section 2.7.2 |
   +------------+------------------------------------+---------------+






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7.3.  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]).

7.3.1.  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.

      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 7.3.1).

   Applications that use this media type:

   Additional information:

      Magic number(s):  none

      File extension(s):  none







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      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

7.3.2.  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

      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 7.3.2).

   Applications that use this media type:

   Additional information:






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      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

7.4.  Transfer Coding Registry

   The HTTP Transfer Coding Registry defines the name space for 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 3.1.2.1 of [Part2]) unless the encoding
   transformation is identical, as is the case for the compression
   codings defined in Section 4.2.

   Values to be added to this name space require IETF Review (see
   Section 4.1 of [RFC5226]), and MUST conform to the purpose of
   transfer coding defined in this section.  Use of program names for
   the identification of encoding formats is not desirable and is
   discouraged for future encodings.

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

7.5.  Transfer Coding Registrations

   The HTTP Transfer Coding Registry shall be updated with the
   registrations below:





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   +----------+----------------------------------------+---------------+
   | Name     | Description                            | Reference     |
   +----------+----------------------------------------+---------------+
   | chunked  | Transfer in a series of chunks         | Section 4.1   |
   | compress | UNIX "compress" program method         | Section 4.2.1 |
   | deflate  | "deflate" compression mechanism        | Section 4.2.2 |
   |          | ([RFC1951]) used inside the "zlib"     |               |
   |          | data format ([RFC1950])                |               |
   | gzip     | Same as GNU zip [RFC1952]              | Section 4.2.3 |
   +----------+----------------------------------------+---------------+

7.6.  Upgrade Token Registry

   The HTTP Upgrade Token Registry defines the name space for protocol-
   name tokens used to identify protocols in the Upgrade header field.
   Each registered protocol name 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 happen on a "First Come First Served" basis (see
   Section 4.1 of [RFC5226]) and are subject to the following rules:

   1.  A protocol-name 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.

   5.  The registration SHOULD name a set of expected "protocol-version"
       tokens associated with that token at the time of registration.

   6.  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.

   7.  The IESG MAY reassign responsibility for a protocol token.  This
       will normally only be used in the case when a responsible party
       cannot be contacted.

   This registration procedure for HTTP Upgrade Tokens replaces that
   previously defined in Section 7.2 of [RFC2817].






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7.7.  Upgrade Token Registration

   The HTTP Upgrade Token Registry shall be updated with the
   registration below:

   +-------+----------------------+----------------------+-------------+
   | Value | Description          | Expected Version     | Reference   |
   |       |                      | Tokens               |             |
   +-------+----------------------+----------------------+-------------+
   | HTTP  | Hypertext Transfer   | any DIGIT.DIGIT      | Section 2.6 |
   |       | Protocol             | (e.g, "2.0")         |             |
   +-------+----------------------+----------------------+-------------+

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

8.  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.

8.1.  Personal Information

   HTTP clients are often privy to large amounts of personal
   information, including both information provided by the user to
   interact with resources (e.g., the user's name, location, mail
   address, passwords, encryption keys, etc.) and information about the
   user's browsing activity over time (e.g., history, bookmarks, etc.).
   HTTP implementations need to prevent unintentional leakage of this
   information.

8.2.  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.  In particular, log information gathered at an intermediary
   often contains a history of user agent interaction, across a
   multitude of sites, that can be traced to individual users.

   HTTP log information is confidential in nature; its handling is often
   constrained by laws and regulations.  Log information needs to be
   securely stored and appropriate guidelines followed for its analysis.
   Anonymization of personal information within individual entries
   helps, but is generally not sufficient to prevent real log traces
   from being re-identified based on correlation with other access



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   characteristics.  As such, access traces that are keyed to a specific
   client should not be published even if the key is pseudonymous.

   To minimize the risk of theft or accidental publication, log
   information should be purged of personally identifiable information,
   including user identifiers, IP addresses, and user-provided query
   parameters, as soon as that information is no longer necessary to
   support operational needs for security, auditing, or fraud control.

8.3.  Attacks Based On File and Path Names

   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.

8.4.  DNS-related Attacks

   HTTP clients rely heavily on the Domain Name Service (DNS), and are
   thus generally prone to security attacks based on the deliberate
   misassociation of IP addresses and DNS names not protected by DNSSec.
   Clients need to be cautious in assuming the validity of an IP number/
   DNS name association unless the response is protected by DNSSec
   ([RFC4033]).

8.5.  Intermediaries and Caching

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




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   Intermediaries that contain a shared cache are especially vulnerable
   to cache poisoning attacks.

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

   Users need to be aware that intermediaries are no more trustworthy
   than the people who run them; HTTP itself cannot solve this problem.

8.6.  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 minimum size limits on request-line
   (Section 3.1.1) 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 7.5.12 of
   [Part2]) or request entities that are too large (Section 7.5 of
   [Part2]).

   Recipients SHOULD carefully limit the extent to which they read other
   fields, including (but not limited to) request methods, response
   status phrases, header field-names, and body chunks, so as to avoid
   denial of service attacks without impeding interoperability.

9.  Acknowledgments

   This edition of HTTP builds on the many contributions that went into
   RFC 1945, RFC 2068, RFC 2145, and RFC 2616, including substantial
   contributions made by the previous authors, editors, and working
   group chairs: Tim Berners-Lee, Ari Luotonen, Roy T. Fielding, Henrik
   Frystyk Nielsen, Jim Gettys, Jeffrey C. Mogul, Larry Masinter, Paul
   J. Leach, and Mark Nottingham.  See Section 16 of [RFC2616] for
   additional acknowledgements from prior revisions.

   Since 1999, the following contributors have helped improve the HTTP
   specification by reporting bugs, asking smart questions, drafting or
   reviewing text, and evaluating open issues:



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   Adam Barth, Adam Roach, Addison Phillips, Adrian Chadd, Adrien W. de
   Croy, Alan Ford, Alan Ruttenberg, Albert Lunde, Alek Storm, Alex
   Rousskov, Alexandre Morgaut, Alexey Melnikov, Alisha Smith, Amichai
   Rothman, Amit Klein, Amos Jeffries, Andreas Maier, Andreas Petersson,
   Anil Sharma, Anne van Kesteren, Anthony Bryan, Asbjorn Ulsberg,
   Balachander Krishnamurthy, Barry Leiba, Ben Laurie, Benjamin Niven-
   Jenkins, Bil Corry, Bill Burke, Bjoern Hoehrmann, Bob Scheifler,
   Boris Zbarsky, Brett Slatkin, Brian Kell, Brian McBarron, Brian Pane,
   Brian Smith, Bryce Nesbitt, Cameron Heavon-Jones, Carl Kugler,
   Carsten Bormann, Charles Fry, Chris Newman, Cyrus Daboo, Dale Robert
   Anderson, Dan Wing, Dan Winship, Daniel Stenberg, Dave Cridland, Dave
   Crocker, Dave Kristol, David Booth, David Singer, David W. Morris,
   Diwakar Shetty, Dmitry Kurochkin, Drummond Reed, Duane Wessels,
   Edward Lee, Eliot Lear, Eran Hammer-Lahav, Eric D. Williams, Eric J.
   Bowman, Eric Lawrence, Eric Rescorla, Erik Aronesty, Evan Prodromou,
   Florian Weimer, Frank Ellermann, Fred Bohle, Gabriel Montenegro,
   Geoffrey Sneddon, Gervase Markham, Grahame Grieve, Greg Wilkins,
   Harald Tveit Alvestrand, Harry Halpin, Helge Hess, Henrik Nordstrom,
   Henry S. Thompson, Henry Story, Herbert van de Sompel, Howard Melman,
   Hugo Haas, Ian Fette, Ian Hickson, Ido Safruti, Ingo Struck, J. Ross
   Nicoll, James H. Manger, James Lacey, James M. Snell, Jamie Lokier,
   Jan Algermissen, Jeff Hodges (who came up with the term 'effective
   Request-URI'), Jeff Walden, Jim Luther, Joe D. Williams, Joe
   Gregorio, Joe Orton, John C. Klensin, John C. Mallery, John Cowan,
   John Kemp, John Panzer, John Schneider, John Stracke, John Sullivan,
   Jonas Sicking, Jonathan Billington, Jonathan Moore, Jonathan Rees,
   Jonathan Silvera, Jordi Ros, Joris Dobbelsteen, Josh Cohen, Julien
   Pierre, Jungshik Shin, Justin Chapweske, Justin Erenkrantz, Justin
   James, Kalvinder Singh, Karl Dubost, Keith Hoffman, Keith Moore, Koen
   Holtman, Konstantin Voronkov, Kris Zyp, Lisa Dusseault, Maciej
   Stachowiak, Marc Schneider, Marc Slemko, Mark Baker, Mark Pauley,
   Mark Watson, Markus Isomaki, Markus Lanthaler, Martin J. Duerst,
   Martin Musatov, Martin Nilsson, Martin Thomson, Matt Lynch, Matthew
   Cox, Max Clark, Michael Burrows, Michael Hausenblas, Mike Amundsen,
   Mike Belshe, Mike Kelly, Mike Schinkel, Miles Sabin, Murray S.
   Kucherawy, Mykyta Yevstifeyev, Nathan Rixham, Nicholas Shanks, Nico
   Williams, Nicolas Alvarez, Nicolas Mailhot, Noah Slater, Pablo
   Castro, Pat Hayes, Patrick R. McManus, Paul E. Jones, Paul Hoffman,
   Paul Marquess, Peter Lepeska, Peter Saint-Andre, Peter Watkins, Phil
   Archer, Philippe Mougin, Phillip Hallam-Baker, Poul-Henning Kamp,
   Preethi Natarajan, Rajeev Bector, Ray Polk, Reto Bachmann-Gmuer,
   Richard Cyganiak, Robert Brewer, Robert Collins, Robert O'Callahan,
   Robert Olofsson, Robert Sayre, Robert Siemer, Robert de Wilde,
   Roberto Javier Godoy, Roberto Peon, Ronny Widjaja, S. Mike Dierken,
   Salvatore Loreto, Sam Johnston, Sam Ruby, Scott Lawrence (who
   maintained the original issues list), Sean B. Palmer, Shane McCarron,
   Stefan Eissing, Stefan Tilkov, Stefanos Harhalakis, Stephane
   Bortzmeyer, Stephen Farrell, Stephen Ludin, Stuart Williams, Subbu



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   Allamaraju, Sylvain Hellegouarch, Tapan Divekar, Tatsuya Hayashi, Ted
   Hardie, Thomas Broyer, Thomas Nordin, Thomas Roessler, Tim Bray, Tim
   Morgan, Tim Olsen, Tom Zhou, Travis Snoozy, Tyler Close, Vincent
   Murphy, Wenbo Zhu, Werner Baumann, Wilbur Streett, Wilfredo Sanchez
   Vega, William A. Rowe Jr., William Chan, Willy Tarreau, Xiaoshu Wang,
   Yaron Goland, Yngve Nysaeter Pettersen, Yoav Nir, Yogesh Bang, Yutaka
   Oiwa, Yves Lafon (long-time member of the editor team), Zed A. Shaw,
   and Zhong Yu.

10.  References

10.1.  Normative References

   [Part2]       Fielding, R., Ed. and J. Reschke, Ed., "Hypertext
                 Transfer Protocol (HTTP/1.1): Semantics and Content",
                 draft-ietf-httpbis-p2-semantics-21 (work in progress),
                 October 2012.

   [Part4]       Fielding, R., Ed. and J. Reschke, Ed., "Hypertext
                 Transfer Protocol (HTTP/1.1): Conditional Requests",
                 draft-ietf-httpbis-p4-conditional-21 (work in
                 progress), October 2012.

   [Part5]       Fielding, R., Ed., Lafon, Y., Ed., and J. Reschke, Ed.,
                 "Hypertext Transfer Protocol (HTTP/1.1): Range
                 Requests", draft-ietf-httpbis-p5-range-21 (work in
                 progress), October 2012.

   [Part6]       Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
                 Ed., "Hypertext Transfer Protocol (HTTP/1.1): Caching",
                 draft-ietf-httpbis-p6-cache-21 (work in progress),
                 October 2012.

   [Part7]       Fielding, R., Ed. and J. Reschke, Ed., "Hypertext
                 Transfer Protocol (HTTP/1.1): Authentication",
                 draft-ietf-httpbis-p7-auth-21 (work in progress),
                 October 2012.

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

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

   [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.




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

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

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

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

10.2.  Informative 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.

   [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>.

   [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,
                 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",



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                 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.

   [RFC4033]     Arends, R., Austein, R., Larson, M., Massey, D., and S.
                 Rose, "DNS Security Introduction and Requirements",
                 RFC 4033, March 2005.

   [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
                 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.

   [RFC5246]     Dierks, T. and E. Rescorla, "The Transport Layer
                 Security (TLS) Protocol Version 1.2", RFC 5246,
                 August 2008.

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



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

Appendix A.  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
   conformance with HTTP/1.1.

   It is beyond the scope of a protocol specification to mandate
   conformance 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, we 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
   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.

A.1.  Changes from HTTP/1.0

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



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A.1.1.  Multi-homed Web Servers

   The requirements that clients and servers support the Host header
   field (Section 5.4), report an error if it is missing from an
   HTTP/1.1 request, and accept absolute URIs (Section 5.3) 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.

A.1.2.  Keep-Alive Connections

   In 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 explicitly negotiated
   ("Keep-Alive") version of persistent connections described in Section
   19.7.1 of [RFC2068].

   Some clients and servers might wish to be compatible with these
   previous approaches to persistent connections, by explicitly
   negotiating for them with a "Connection: keep-alive" request header
   field.  However, some experimental implementations of HTTP/1.0
   persistent connections are faulty; for example, if a HTTP/1.0 proxy
   server doesn't understand Connection, it will erroneously forward
   that header field to the next inbound server, which would result in a
   hung connection.

   One attempted solution was the introduction of a Proxy-Connection
   header field, targeted specifically at proxies.  In practice, this
   was also unworkable, because proxies are often deployed in multiple
   layers, bringing about the same problem discussed above.

   As a result, clients are encouraged not to send the Proxy-Connection
   header field in any requests.

   Clients are also encouraged to consider the use of Connection: keep-
   alive in requests carefully; while they can enable persistent
   connections with HTTP/1.0 servers, clients using them need will need
   to monitor the connection for "hung" requests (which indicate that
   the client ought stop sending the header field), and this mechanism



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   ought not be used by clients at all when a proxy is being used.

A.1.3.  Introduction of Transfer-Encoding

   HTTP/1.1 introduces the Transfer-Encoding header field
   (Section 3.3.1).  Proxies/gateways MUST remove any transfer-coding
   prior to forwarding a message via a MIME-compliant protocol.

A.2.  Changes from RFC 2616

   Clarify that the string "HTTP" in the HTTP-version ABNF 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.
   Change ABNF productions for header fields to only define the field
   value.  (Section 3.2)

   Rules about implicit linear whitespace between certain grammar
   productions have been removed; now whitespace is only allowed where
   specifically defined in the ABNF.  (Section 3.2.1)

   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 3.2.4)

   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 4)

   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, and deprecate their use.  (Section 4.1)

   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 5.3)

   Clarify exactly when "close" connection options have to be sent; drop
   notion of header fields being "hop-by-hop" without being listed in
   the Connection header field.  (Section 6.1)

   Remove hard limit of two connections per server.  Remove requirement



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   to retry a sequence of requests as long it was idempotent.  Remove
   requirements about when servers are allowed to close connections
   prematurely.  (Section 6.2)

   Remove requirement to retry requests under certain circumstances when
   the server prematurely closes the connection.  (Section 6.2.2)

   Define the semantics of the Upgrade header field in responses other
   than 101 (this was incorporated from [RFC2817]).  (Section 6.3)

   Registration of Transfer Codings now requires IETF Review
   (Section 7.4)

   Take over the Upgrade Token Registry, previously defined in Section
   7.2 of [RFC2817].  (Section 7.6)

   Empty list elements in list productions have been deprecated.
   (Appendix B)

Appendix B.  ABNF list extension: #rule

   A #rule extension to the ABNF rules of [RFC5234] is used to improve
   readability in the definitions of some header field values.

   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).

   Thus,

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

   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 ] )



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   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 3.2.4

   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   "

   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.

Appendix C.  Collected ABNF

   BWS = OWS

   Connection = *( "," OWS ) connection-option *( OWS "," [ OWS
    connection-option ] )
   Content-Length = 1*DIGIT

   HTTP-message = start-line *( header-field CRLF ) CRLF [ message-body
    ]
   HTTP-name = %x48.54.54.50 ; HTTP
   HTTP-version = HTTP-name "/" DIGIT "." DIGIT
   Host = uri-host [ ":" port ]

   OWS = *( SP / HTAB )

   RWS = 1*( SP / HTAB )

   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 ) protocol *( OWS "," [ OWS protocol ] )

   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>
   absolute-form = absolute-URI
   asterisk-form = "*"
   attribute = token
   authority = <authority, defined in [RFC3986], Section 3.2>
   authority-form = authority

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

   field-content = *( HTAB / SP / VCHAR / obs-text )
   field-name = token
   field-value = *( field-content / obs-fold )

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

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

   message-body = *OCTET
   method = token

   obs-fold = CRLF ( SP / HTAB )
   obs-text = %x80-FF
   origin-form = path-absolute [ "?" query ]

   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>



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   port = <port, defined in [RFC3986], Section 3.2.3>
   protocol = protocol-name [ "/" protocol-version ]
   protocol-name = token
   protocol-version = token
   pseudonym = token

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

   rank = ( "0" [ "." *3DIGIT ] ) / ( "1" [ "." *3"0" ] )
   reason-phrase = *( HTAB / SP / VCHAR / obs-text )
   received-by = ( uri-host [ ":" port ] ) / pseudonym
   received-protocol = [ protocol-name "/" ] protocol-version
   relative-part = <relative-part, defined in [RFC3986], Section 4.2>
   request-line = method SP request-target SP HTTP-version CRLF
   request-target = origin-form / absolute-form / authority-form /
    asterisk-form

   special = "(" / ")" / "<" / ">" / "@" / "," / ";" / ":" / "\" /
    DQUOTE / "/" / "[" / "]" / "?" / "=" / "{" / "}"
   start-line = request-line / status-line
   status-code = 3DIGIT
   status-line = HTTP-version SP status-code SP reason-phrase CRLF

   t-codings = "trailers" / ( transfer-coding [ t-ranking ] )
   t-ranking = OWS ";" OWS "q=" rank
   tchar = "!" / "#" / "$" / "%" / "&" / "'" / "*" / "+" / "-" / "." /
    "^" / "_" / "`" / "|" / "~" / DIGIT / ALPHA
   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




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   word = token / quoted-string

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>)

   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"



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   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 HTAB, fix broken ABNF
      for chunk-data (work in progress on
      <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>):




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   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".

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
      header fields 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).




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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
      header fields vs Set-Cookie"

   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-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.




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   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"

   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:




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   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 payload body MUST NOT be included)

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

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

   Closed issues:

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

   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"



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   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"

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



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   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 header fields"

   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"

   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:





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

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 Field
      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"

   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/95>: "Handling
      multiple Content-Length header fields"

   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"





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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"

D.17.  Since draft-ietf-httpbis-p1-messaging-15

   Closed issues:

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/100>: "DNS Spoofing
      / DNS Binding advice"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/254>: "move RFCs
      2145, 2616, 2817 to Historic status"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/270>: "\-escaping in
      quoted strings"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/305>: "'Close'
      should be reserved in the HTTP header field registry"

D.18.  Since draft-ietf-httpbis-p1-messaging-16

   Closed issues:

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

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/215>: "Explain
      header field registration"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/219>: "Revise
      Acknowledgements Sections"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/297>: "Retrying
      Requests"




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   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/318>: "Closing the
      connection on server error"

D.19.  Since draft-ietf-httpbis-p1-messaging-17

   Closed issues:

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/158>: "Proxy-
      Connection and Keep-Alive"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/166>: "Clarify 'User
      Agent'"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/300>: "Define non-
      final responses"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/323>: "intended
      maturity level vs normative references"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/324>: "Intermediary
      rewriting of queries"

D.20.  Since draft-ietf-httpbis-p1-messaging-18

   Closed issues:

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/250>: "message-body
      in CONNECT response"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/302>: "Misplaced
      text on connection handling in p2"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/335>: "wording of
      line folding rule"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/343>: "chunk-
      extensions"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/346>: "make IANA
      policy definitions consistent"

D.21.  Since draft-ietf-httpbis-p1-messaging-19

   Closed issues:

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/346>: "make IANA
      policy definitions consistent"




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   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/359>: "clarify
      connection header field values are case-insensitive"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/361>: "ABNF
      requirements for recipients"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/368>: "note
      introduction of new IANA registries as normative changes"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/374>: "Reference to
      ISO-8859-1 is informative"

D.22.  Since draft-ietf-httpbis-p1-messaging-20

   Closed issues:

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/378>: "is 'q=' case-
      sensitive?"

   o  <http://tools.ietf.org/wg/httpbis/trac/ticket/383>: "Semantics of
      HTTPS"

   Other changes:

   o  Drop notion of header fields being "hop-by-hop" without being
      listed in the Connection header field.

   o  Section about connection management rewritten; dropping some
      historic information.

   o  Move description of "100-continue" into Part 2.

   o  Rewrite the persistent connection and Upgrade requirements to be
      actionable by role and consistent with the rest of HTTP.

Index

   A
      absolute-form (of request-target)  39
      accelerator  10
      application/http Media Type  57
      asterisk-form (of request-target)  39
      authority-form (of request-target)  39

   B
      browser  7

   C



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      cache  11
      cacheable  12
      captive portal  11
      chunked (Coding Format)  33
      client  7
      close  46, 52
      compress (Coding Format)  35
      connection  7
      Connection header field  46, 52
      Content-Length header field  28

   D
      deflate (Coding Format)  35
      downstream  9

   E
      effective request URI  41

   G
      gateway  10
      Grammar
         absolute-form  38
         absolute-URI  15
         ALPHA  6
         asterisk-form  38
         attribute  33
         authority  15
         authority-form  38
         BWS  23
         chunk  33
         chunk-data  33
         chunk-ext  33
         chunk-ext-name  33
         chunk-ext-val  33
         chunk-size  33
         chunked-body  33
         comment  25
         Connection  47
         connection-option  47
         Content-Length  28
         CR  6
         CRLF  6
         ctext  25
         CTL  6
         date2  33
         date3  33
         DIGIT  6
         DQUOTE  6



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         field-content  21
         field-name  21
         field-value  21
         header-field  21
         HEXDIG  6
         Host  40
         HTAB  6
         HTTP-message  18
         HTTP-name  13
         http-URI  16
         HTTP-version  13
         https-URI  17
         last-chunk  33
         LF  6
         message-body  26
         method  20
         obs-fold  21
         obs-text  25
         OCTET  6
         origin-form  38
         OWS  23
         partial-URI  15
         path-absolute  15
         port  15
         protocol-name  43
         protocol-version  43
         pseudonym  43
         qdtext  25
         qdtext-nf  33
         query  15
         quoted-cpair  25
         quoted-pair  25
         quoted-str-nf  33
         quoted-string  25
         rank  36
         reason-phrase  21
         received-by  43
         received-protocol  43
         request-line  20
         request-target  38
         RWS  23
         SP  6
         special  25
         start-line  19
         status-code  21
         status-line  21
         t-codings  36
         t-ranking  36



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         tchar  25
         TE  36
         token  25
         Trailer  34
         trailer-part  33
         transfer-coding  33
         Transfer-Encoding  26
         transfer-extension  33
         transfer-parameter  33
         Upgrade  53
         uri-host  15
         URI-reference  15
         value  33
         VCHAR  6
         Via  43
         word  25
      gzip (Coding Format)  36

   H
      header field  18
      header section  18
      headers  18
      Host header field  40
      http URI scheme  16
      https URI scheme  17

   I
      inbound  9
      interception proxy  11
      intermediary  9

   M
      Media Type
         application/http  57
         message/http  56
      message  7
      message/http Media Type  56
      method  20

   N
      non-transforming proxy  10

   O
      origin server  7
      origin-form (of request-target)  38
      outbound  9

   P



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      proxy  10

   R
      recipient  7
      request  7
      request-target  20
      resource  15
      response  7
      reverse proxy  10

   S
      sender  7
      server  7
      spider  7

   T
      target resource  37
      target URI  37
      TE header field  36
      Trailer header field  34
      Transfer-Encoding header field  26
      transforming proxy  10
      transparent proxy  11
      tunnel  11

   U
      Upgrade header field  53
      upstream  9
      URI scheme
         http  16
         https  17
      user agent  7

   V
      Via header field  43

Authors' Addresses

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

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





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

   EMail: julian.reschke@greenbytes.de
   URI:   http://greenbytes.de/tech/webdav/











































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