Subject: Fwd: draft-ietf-httpbis-p1-messaging-25

From: Julian Reschke <[email-masked]>
Date: Mon, 16 Dec 2013 17:10:21 +0100

-------- Original Message --------
Subject: draft-ietf-httpbis-p1-messaging-25
Resent-To: [email-masked], [email-masked], 
[email-masked], [email-masked],
Date: Mon, 16 Dec 2013 10:49:42 -0500
From: Thomas Nadeau <[email-masked]>
To: [email-masked], [email-masked]

	Hello,

	I am the assigned OPS-DIR reviewer for 
draft-ietf-httpbis-p1-messaging-25. The OPS-DIR review have as principal 
goal helping the OPS ADs in their evaluation and balloting of documents 
at IESG reviews. Please consider these comments together with the other 
IETF Last Call comments.

This document is of high quality and ready generally speaking, but can 
be improved with a few editorial clarifications as mentioned below. My 
comments begin with "TOM:" so you can easily search for them below.

	--Tom

HTTPbis Working Group                                   R. Fielding, Ed.
Internet-Draft                                                     Adobe
Obsoletes: 2145,2616 (if approved)                       J. Reschke, Ed.
Updates: 2817,2818 (if approved)                              greenbytes
Intended status: Standards Track                       November 17, 2013
Expires: May 21, 2014

    Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing
                    draft-ietf-httpbis-p1-messaging-25

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 ([email-masked]), 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 C.2.

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 May 21, 2014.

Copyright Notice

    Copyright (c) 2013 IETF Trust and the persons identified as the
    document authors.  All rights reserved.

    This document is subject to BCP 78 and the IETF Trust's Legal
    Provisions Relating to IETF Documents
    (http://trustee.ietf.org/license-info) in effect on the date of
    publication of this document.  Please review these documents
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    include Simplified BSD License text as described in Section 4.e of
    the Trust Legal Provisions and are provided without warranty as
    described in the Simplified BSD License.

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

Table of Contents

    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  . . . . . . . . . . . . . . . . . . . 14
      2.7.  Uniform Resource Identifiers . . . . . . . . . . . . . . . 16
        2.7.1.  http URI scheme  . . . . . . . . . . . . . . . . . . . 17
        2.7.2.  https URI scheme . . . . . . . . . . . . . . . . . . . 18
        2.7.3.  http and https URI Normalization and Comparison  . . . 19
    3.  Message Format . . . . . . . . . . . . . . . . . . . . . . . . 19

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      3.1.  Start Line . . . . . . . . . . . . . . . . . . . . . . . . 20
        3.1.1.  Request Line . . . . . . . . . . . . . . . . . . . . . 21
        3.1.2.  Status Line  . . . . . . . . . . . . . . . . . . . . . 22
      3.2.  Header Fields  . . . . . . . . . . . . . . . . . . . . . . 22
        3.2.1.  Field Extensibility  . . . . . . . . . . . . . . . . . 23
        3.2.2.  Field Order  . . . . . . . . . . . . . . . . . . . . . 23
        3.2.3.  Whitespace . . . . . . . . . . . . . . . . . . . . . . 24
        3.2.4.  Field Parsing  . . . . . . . . . . . . . . . . . . . . 24
        3.2.5.  Field Limits . . . . . . . . . . . . . . . . . . . . . 26
        3.2.6.  Field value components . . . . . . . . . . . . . . . . 26
      3.3.  Message Body . . . . . . . . . . . . . . . . . . . . . . . 27
        3.3.1.  Transfer-Encoding  . . . . . . . . . . . . . . . . . . 28
        3.3.2.  Content-Length . . . . . . . . . . . . . . . . . . . . 30
        3.3.3.  Message Body Length  . . . . . . . . . . . . . . . . . 31
      3.4.  Handling Incomplete Messages . . . . . . . . . . . . . . . 33
      3.5.  Message Parsing Robustness . . . . . . . . . . . . . . . . 34
    4.  Transfer Codings . . . . . . . . . . . . . . . . . . . . . . . 35
      4.1.  Chunked Transfer Coding  . . . . . . . . . . . . . . . . . 35
        4.1.1.  Chunk Extensions . . . . . . . . . . . . . . . . . . . 36
        4.1.2.  Chunked Trailer Part . . . . . . . . . . . . . . . . . 36
        4.1.3.  Decoding Chunked . . . . . . . . . . . . . . . . . . . 37
      4.2.  Compression Codings  . . . . . . . . . . . . . . . . . . . 38
        4.2.1.  Compress Coding  . . . . . . . . . . . . . . . . . . . 38
        4.2.2.  Deflate Coding . . . . . . . . . . . . . . . . . . . . 38
        4.2.3.  Gzip Coding  . . . . . . . . . . . . . . . . . . . . . 38
      4.3.  TE . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
      4.4.  Trailer  . . . . . . . . . . . . . . . . . . . . . . . . . 40
    5.  Message Routing  . . . . . . . . . . . . . . . . . . . . . . . 40
      5.1.  Identifying a Target Resource  . . . . . . . . . . . . . . 40
      5.2.  Connecting Inbound . . . . . . . . . . . . . . . . . . . . 40
      5.3.  Request Target . . . . . . . . . . . . . . . . . . . . . . 41
      5.4.  Host . . . . . . . . . . . . . . . . . . . . . . . . . . . 43
      5.5.  Effective Request URI  . . . . . . . . . . . . . . . . . . 44
      5.6.  Associating a Response to a Request  . . . . . . . . . . . 46
      5.7.  Message Forwarding . . . . . . . . . . . . . . . . . . . . 46
        5.7.1.  Via  . . . . . . . . . . . . . . . . . . . . . . . . . 46
        5.7.2.  Transformations  . . . . . . . . . . . . . . . . . . . 48
    6.  Connection Management  . . . . . . . . . . . . . . . . . . . . 49
      6.1.  Connection . . . . . . . . . . . . . . . . . . . . . . . . 49
      6.2.  Establishment  . . . . . . . . . . . . . . . . . . . . . . 51
      6.3.  Persistence  . . . . . . . . . . . . . . . . . . . . . . . 51
        6.3.1.  Retrying Requests  . . . . . . . . . . . . . . . . . . 52
        6.3.2.  Pipelining . . . . . . . . . . . . . . . . . . . . . . 53
      6.4.  Concurrency  . . . . . . . . . . . . . . . . . . . . . . . 53
      6.5.  Failures and Time-outs . . . . . . . . . . . . . . . . . . 54
      6.6.  Tear-down  . . . . . . . . . . . . . . . . . . . . . . . . 55
      6.7.  Upgrade  . . . . . . . . . . . . . . . . . . . . . . . . . 56
    7.  ABNF list extension: #rule . . . . . . . . . . . . . . . . . . 58

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    8.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 59
      8.1.  Header Field Registration  . . . . . . . . . . . . . . . . 59
      8.2.  URI Scheme Registration  . . . . . . . . . . . . . . . . . 60
      8.3.  Internet Media Type Registration . . . . . . . . . . . . . 60
        8.3.1.  Internet Media Type message/http . . . . . . . . . . . 60
        8.3.2.  Internet Media Type application/http . . . . . . . . . 61
      8.4.  Transfer Coding Registry . . . . . . . . . . . . . . . . . 63
        8.4.1.  Procedure  . . . . . . . . . . . . . . . . . . . . . . 63
        8.4.2.  Registration . . . . . . . . . . . . . . . . . . . . . 63
      8.5.  Content Coding Registration  . . . . . . . . . . . . . . . 64
      8.6.  Upgrade Token Registry . . . . . . . . . . . . . . . . . . 64
        8.6.1.  Procedure  . . . . . . . . . . . . . . . . . . . . . . 64
        8.6.2.  Upgrade Token Registration . . . . . . . . . . . . . . 65
    9.  Security Considerations  . . . . . . . . . . . . . . . . . . . 65
      9.1.  DNS-related Attacks  . . . . . . . . . . . . . . . . . . . 65
      9.2.  Intermediaries and Caching . . . . . . . . . . . . . . . . 65
      9.3.  Buffer Overflows . . . . . . . . . . . . . . . . . . . . . 66
      9.4.  Message Integrity  . . . . . . . . . . . . . . . . . . . . 66
      9.5.  Server Log Information . . . . . . . . . . . . . . . . . . 67
    10. Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 68
    11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 69
      11.1. Normative References . . . . . . . . . . . . . . . . . . . 69
      11.2. Informative References . . . . . . . . . . . . . . . . . . 71
    Appendix A.  HTTP Version History  . . . . . . . . . . . . . . . . 72
      A.1.  Changes from HTTP/1.0  . . . . . . . . . . . . . . . . . . 73
        A.1.1.  Multi-homed Web Servers  . . . . . . . . . . . . . . . 73
        A.1.2.  Keep-Alive Connections . . . . . . . . . . . . . . . . 74
        A.1.3.  Introduction of Transfer-Encoding  . . . . . . . . . . 74
      A.2.  Changes from RFC 2616  . . . . . . . . . . . . . . . . . . 74
    Appendix B.  Collected ABNF  . . . . . . . . . . . . . . . . . . . 77
    Appendix C.  Change Log (to be removed by RFC Editor before
                 publication)  . . . . . . . . . . . . . . . . . . . . 79
      C.1.  Since RFC 2616 . . . . . . . . . . . . . . . . . . . . . . 79
      C.2.  Since draft-ietf-httpbis-p1-messaging-24 . . . . . . . . . 79
    Index  . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80

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1.  Introduction

    The Hypertext Transfer Protocol (HTTP) is an application-level
    request/response protocol that uses extensible semantics and self-
    descriptive 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, and RFC 2145 (on HTTP
    versioning).  This specification also updates the use of CONNECT to
    establish a tunnel, previously defined in RFC 2817, and defines the
    "https" URI scheme that was described informally in RFC 2818.

    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 this 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

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    the communication is considered in isolation, then successful actions
    ought to be reflected in corresponding changes to the observable
    interface provided by servers.  However, since multiple clients might
    act in parallel and perhaps at cross-purposes, we cannot require that
    such changes be observable beyond the scope of a single response.

    This document 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
    Section 7.  Appendix B shows the collected ABNF with the list rule
    expanded.

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

TOM: Change "HTTP responses" to "HTTP response messages in order to 
reply to HTTP requests."

    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 any of the various client programs
    that initiate a request, including (but not limited to) browsers,
    spiders (web-based robots), command-line tools, native applications,
    and mobile apps.  The term "origin server" is used to refer to the
    program that can originate authoritative responses to a request.  For
    general requirements, we use the terms "sender" and "recipient" to
    refer to any component that sends or receives, respectively, a given
    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

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    response messages, each beginning with a status line that includes
    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 6.3.

    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: 51
      Vary: Accept-Encoding
      Content-Type: text/plain

      Hello World! My payload includes a trailing CRLF.

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,

TOM: scales?  Do you mean like a "weight scale"?

    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 might be met via advance
    configuration choices, run-time options, or simple avoidance of the
    unsafe action; confirmation does not imply any specific user
    interface or interruption of normal processing if the user has
    already made that choice.

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.  Likewise, later requests might be sent through a different
    path of connections, often based on dynamic configuration for load
    balancing.

    We use the terms "upstream" and "downstream" to describe various

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

    A "proxy" is a message forwarding agent that is selected by the
    client, usually via local configuration rules, to receive requests
    for some type(s) of absolute URI and attempt to satisfy those
    requests via translation through the HTTP interface.  Some
    translations are minimal, such as for proxy requests for "http" URIs,
    whereas other requests might require translation to and from entirely
    different application-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 6.3.4 of [Part2] and Section 5.5 of [Part6]
    for status and warning codes related to transformations.

    A "gateway" (a.k.a., "reverse proxy") is an intermediary that acts as
    an origin server for the outbound connection, but translates received
    requests and forwards them inbound to another server or servers.
    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.

    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 ought to conform to user agent requirements
    on the gateway's inbound connection.

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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, a
    server 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.,

TOM: Do you mean "non-standards track"?  Many people regard anything 
with an RFC number to be some form of "standard".

    [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 that 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

TOM: You should say, "store a copy of the response message for some 
usually non-permanent period of time..."

    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 deployed across the World Wide Web and inside large
    organizations.  These include national hierarchies of proxy caches to
    save transoceanic bandwidth, collaborative systems that broadcast or
    multicast cache entries, archives of pre-fetched cache entries for
    use in off-line or high-latency environments, 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.

    Conformance includes both the syntax and semantics of 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 corresponding ABNF rules.  Within a given message, a sender MUST
    NOT generate protocol elements or syntax alternatives that are only
    allowed to be generated by participants in other roles (i.e., a role
    that the sender does not have for that message).

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    When a received protocol element is parsed, the recipient MUST be
    able to parse any value of reasonable length that is applicable to
    the recipient's role and matches the grammar defined by the
    corresponding ABNF rules.  Note, however, that some received protocol
    elements might not be parsed.  For example, an intermediary
    forwarding a message might parse a header-field into generic field-
    name and field-value components, but then forward the header field
    without further parsing inside the field-value.

    HTTP does not have specific length limitations for many of its
    protocol elements because the lengths that might be appropriate will
    vary widely, depending on the deployment context and purpose of the
    implementation.  Hence, interoperability between senders and
    recipients depends on shared expectations regarding what is a
    reasonable length for each protocol element.  Furthermore, what is
    commonly understood to be a reasonable length for some protocol
    elements has changed over the course of the past two decades of HTTP
    use, and is expected to continue changing in the future.

    At a minimum, a recipient MUST be able to parse and process protocol
    element lengths that are at least as long as the values that it
    generates for those same protocol elements in other messages.  For
    example, an origin server that publishes very long URI references to
    its own resources needs to be able to parse and process those same
    references when received as a request target.

    A recipient MUST interpret a received protocol element according to
    the semantics defined for it by this specification, including
    extensions to this specification, unless the recipient has determined
    (through experience or configuration) that the sender incorrectly
    implements what is implied by those semantics.  For example, an
    origin server might disregard the contents of a received Accept-
    Encoding header field if inspection of the User-Agent header field
    indicates a specific implementation version that is known to fail on
    receipt of certain content codings.

    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.

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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 within that major
    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.

    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 introduced without changing the protocol
    version if their defined semantics allow them to be safely ignored by
    recipients that do not recognize them.  Header field extensibility is
    discussed in Section 3.2.1.

    Intermediaries that process HTTP messages (i.e., all intermediaries

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    other than those acting as tunnels) MUST send their own HTTP-version
    in forwarded messages.  In other words, they are not allowed to
    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.

    A 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.  A client MUST NOT send a version to which it is not
    conformant.

    A 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 code or header fields (e.g., Server) that
    the server improperly handles higher request versions.

    A server SHOULD send a response version equal to the highest version
    to which the server is conformant that has a major version less than
    or equal to the one received in the request.  A server MUST NOT send
    a version to which it is not conformant.  A server can send a 505
    (HTTP Version Not Supported) response if it wishes, for any reason,
    to refuse service of the client's major protocol version.

    A server MAY send an HTTP/1.0 response to a 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 has
    specifically avoided any such changes to the protocol.

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    When an HTTP message is received with a major version number that the
    recipient implements, but a higher minor version number than what the
    recipient implements, the recipient SHOULD process the message as if
    it were in the highest minor version within that major version to
    which the recipient is conformant.  A recipient can assume that a
    message with a higher minor version, when sent to a recipient that
    has not yet indicated support for that higher version, is
    sufficiently backwards-compatible to be safely processed by any
    implementation of the same major version.

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", "authority", "port", "host", "path-
    abempty", "segment", "query", and "fragment" from the URI generic
    syntax.  In addition, we define an "absolute-path" rule (that differs
    from RFC 3986's "path-absolute" in that it allows a leading "//") and
    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>
      uri-host      = <host, defined in [RFC3986], Section 3.2.2>
      port          = <port, defined in [RFC3986], Section 3.2.3>
      path-abempty  = <path-abempty, defined in [RFC3986], Section 3.3>
      segment       = <segment, defined in [RFC3986], Section 3.3>
      query         = <query, defined in [RFC3986], Section 3.4>
      fragment      = <fragment, defined in [RFC3986], Section 3.5>

      absolute-path = 1*( "/" segment )
      partial-URI   = relative-part [ "?" query ]

    Each protocol element in HTTP that allows a URI reference will
    indicate in its ABNF production whether the element allows any form
    of reference (URI-reference), only a URI in absolute form (absolute-
    URI), only the path and optional query components, or some
    combination of the above.  Unless otherwise indicated, URI references
    are parsed relative to the effective request URI (Section 5.5).

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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 ([RFC0793]) connections on a given port.

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

    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.

    A sender MUST NOT generate an "http" URI with an empty host
    identifier.  A recipient that processes such a URI reference MUST
    reject it as invalid.

    If the host identifier is provided as an IP 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.  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
    HTTP response message, as described in Section 6 of [Part2], then
    that response is considered an authoritative answer to the client's
    request.

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    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 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.  A sender MUST NOT
    generate the userinfo subcomponent (and its "@" delimiter) when an
    "http" URI reference is generated within a message as a request
    target or header field value.  Before making use of an "http" URI
    reference received from an untrusted source, a recipient ought to
    parse for userinfo and treat its presence as an error; it is likely
    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 ([RFC0793], [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 user agent MUST ensure that its connection to the origin server
    is secured through the use of strong encryption, end-to-end, prior to

TOM: Rather than "end-to-end" I would rather use "..., on the connection 
between the client and server, ..."

    sending the first HTTP request.

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

    Note that the "https" URI scheme depends on both TLS and TCP for
    establishing authority.  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

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    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 omit the port subcomponent.  When not being used in
    absolute form as the request target of an OPTIONS request, 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 ]

    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

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    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.

    A recipient 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.

    A sender MUST NOT send whitespace between the start-line and the
    first header field.  A recipient that receives whitespace between the
    start-line and the first header field MUST either reject the message
    as invalid or consume each whitespace-preceded line without further
    processing of it (i.e., ignore the entire line, along with any
    subsequent lines preceded by whitespace, until a properly formed
    header field is received or the header section is terminated).

    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.

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

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    clients are implemented to only expect a response.

      start-line     = request-line / status-line

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

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

      method         = token

    The request methods defined by this specification can be found in
    Section 4 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.

    Recipients typically parse the request-line into its component parts
    by splitting on whitespace (see Section 3.5), since no whitespace is
    allowed in the three components.  Unfortunately, some user agents
    fail to properly encode or exclude whitespace found in hypertext
    references, resulting in those disallowed characters being sent in a
    request-target.

    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.  A recipient 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 a 501 (Not Implemented) status code.
    A server ought to be prepared to receive URIs of unbounded length, as
    described in Section 2.5, and MUST respond with a 414 (URI Too Long)
    status code if the received request-target is longer than the server
    wishes to parse (see Section 6.5.12 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 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

    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 6 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 leading whitespace, the field
    value, and optional trailing whitespace.

      header-field   = field-name ":" OWS field-value OWS
      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 3.2.4

    The field-name token labels the corresponding field-value as having
    the semantics defined by that header field.  For example, the Date
    header field is defined in Section 7.1.1.2 of [Part2] as containing

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    the origination timestamp for the message in which it appears.

3.2.1.  Field Extensibility

    Header fields are fully extensible: there is no limit on the
    introduction of new field names, each presumably defining new
    semantics, nor 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 core
    standard.

    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, define preconditions on request
    evaluation, or refine the meaning of responses.

    A proxy MUST forward unrecognized header fields 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.  Other recipients SHOULD ignore unrecognized header fields.
    These requirements allow HTTP's functionality to be enhanced without
    requiring prior update of deployed intermediaries.

    All defined header fields ought to be registered with IANA in the
    Message Header Field Registry, as described in Section 8.3 of
    [Part2].

3.2.2.  Field Order

    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.

    A sender MUST NOT generate multiple header fields with the same field
    name in a message unless either the entire field value for that
    header field is defined as a comma-separated list [i.e., #(values)]
    or the header field is a well-known exception (as noted below).

    A recipient MAY combine multiple header fields with the same field
    name 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

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    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: In practice, the "Set-Cookie" header field ([RFC6265]) often
       appears multiple times in a response message and does not use the
       list syntax, violating the above requirements on multiple header
       fields with the same name.  Since it cannot be combined into a
       single field-value, recipients ought to handle "Set-Cookie" as a
       special case while processing header fields.  (See Appendix A.2.3
       of [Kri2001] for details.)

3.2.3.  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.  For protocol elements where optional whitespace is
    preferred to improve readability, a sender SHOULD generate the
    optional whitespace as a single SP; otherwise, a sender SHOULD NOT
    generate optional whitespace except as needed to white-out invalid or
    unwanted protocol elements during in-place message filtering.

    The RWS rule is used when at least one linear whitespace octet is
    required to separate field tokens.  A sender SHOULD generate RWS as a
    single SP.

    The BWS rule is used where the grammar allows optional whitespace
    only for historical reasons.  A sender MUST NOT generate BWS in
    messages.  A recipient MUST parse for such bad whitespace and remove
    it before interpreting the protocol element.

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

3.2.4.  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.  A

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    server MUST reject any received request message that contains
    whitespace between a header field-name and colon 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 is 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 ought to be excluded by parsers when extracting the
    field value from a header field.

    A recipient of field-content containing multiple sequential octets of
    optional (OWS) or required (RWS) whitespace SHOULD either replace the
    sequence with a single SP or transform any non-SP octets in the
    sequence to SP octets before interpreting the field value or
    forwarding the message downstream.

    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 8.3.1).  A sender MUST NOT generate a message that includes
    line folding (i.e., that has any field-value that contains a match to
    the obs-fold rule) unless the message is intended for packaging
    within the message/http media type.

    A server that receives an obs-fold in a request message that is not
    within a message/http container MUST either reject the message by
    sending a 400 (Bad Request), preferably with a representation
    explaining that obsolete line folding is unacceptable, or replace
    each received obs-fold with one or more SP octets prior to
    interpreting the field value or forwarding the message downstream.

    A proxy or gateway that receives an obs-fold in a response message
    that is not within a message/http container MUST either discard the
    message and replace it with a 502 (Bad Gateway) response, preferably
    with a representation explaining that unacceptable line folding was
    received, or replace each received obs-fold with one or more SP
    octets prior to interpreting the field value or forwarding the
    message downstream.

    A user agent that receives an obs-fold in a response message that is
    not within a message/http container MUST replace each received obs-
    fold with one or more SP octets prior to interpreting the field
    value.

    Historically, HTTP has allowed field content with text in the ISO-

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    8859-1 [ISO-8859-1] charset, supporting other charsets only through
    use of [RFC2047] encoding.  In practice, most HTTP header field
    values use only a subset of the US-ASCII charset [USASCII].  Newly
    defined header fields SHOULD limit their field values to US-ASCII
    octets.  A recipient SHOULD treat other octets in field content (obs-
    text) as opaque data.

3.2.5.  Field Limits

    HTTP does not place a pre-defined limit on the length of each header
    field or on the length of the header section as a whole, as described
    in Section 2.5.  Various ad-hoc limitations on individual header
    field length are found in practice, often depending on the specific
    field semantics.

    A server ought to be prepared to receive request header fields of
    unbounded length and MUST respond with an appropriate 4xx (Client
    Error) status code if the received header field(s) are larger than
    the server wishes to process.

    A client ought to be prepared to receive response header fields of
    unbounded length.  A client MAY discard or truncate received header
    fields that are larger than the client wishes to process if the field
    semantics are such that the dropped value(s) can be safely ignored
    without changing the message framing or response semantics.

3.2.6.  Field value components

    Many HTTP header field values consist of words (token or quoted-
    string) separated by whitespace or special characters.

      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.

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      quoted-string  = DQUOTE *( qdtext / quoted-pair ) DQUOTE
      qdtext         = HTAB / SP /%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 a quoted-string MUST handle a
    quoted-pair as if it were replaced by the octet following the
    backslash.

    A sender SHOULD NOT generate a quoted-pair in a quoted-string except
    where necessary to quote DQUOTE and backslash octets occurring within
    that string.

    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          = HTAB / SP / %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 )

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

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

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    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 4.3.2 of [Part2]). 2xx (Successful) responses to CONNECT
    switch to tunnel mode instead of having a message body (Section 4.3.6
    of [Part2]).  All 1xx (Informational), 204 (No Content), and 304 (Not
    Modified) responses do not include a message body.  All other
    responses do include a message body, although the body might be of
    zero length.

3.3.1.  Transfer-Encoding

    The Transfer-Encoding header field lists the transfer coding names
    corresponding to the sequence of transfer codings that have been (or
    will be) applied to the payload body in order to form the message
    body.  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 selected resource.

    A recipient MUST be able to parse the chunked transfer coding

TOM: Do you mean "encoding" rather than "coding"?

    (Section 4.1) because it plays a crucial role in framing messages
    when the payload body size is not known in advance.  A sender MUST
    NOT apply chunked more than once to a message body (i.e., chunking an

TOM: "NOT apply chunked" does not parse well. Please consider rewriting. 
Please look over the remainder of this section to maintain consistent 
use of "chunked" as a verb, adjective or both.

    already chunked message is not allowed).  If any transfer coding
    other than chunked is applied to a request payload body, the sender
    MUST apply chunked as the final transfer coding to ensure that the
    message is properly framed.  If any transfer coding other than

TOM: coding -> encoding. Please look over the rest of this section as 
there are other instances of this.

    chunked is applied to a response payload body, the sender MUST either
    apply chunked as the final transfer coding or terminate the message
    by closing the connection.

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    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.

    Unlike Content-Encoding (Section 3.1.2.1 of [Part2]), Transfer-
    Encoding is a property of the message, not of the representation, and
    any recipient along the request/response chain MAY decode the
    received transfer coding(s) or apply additional transfer coding(s) to
    the message body, assuming that corresponding changes are made to the
    Transfer-Encoding field-value.  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.

    A server MUST NOT send a Transfer-Encoding header field in any
    response with a status code of 1xx (Informational) or 204 (No
    Content).  A server MUST NOT send a Transfer-Encoding header field in
    any 2xx (Successful) response to a CONNECT request (Section 4.3.6 of
    [Part2]).

    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
    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).

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3.3.2.  Content-Length

    When a message does not have a Transfer-Encoding header field, a
    Content-Length header field can provide the anticipated size, as a
    decimal number of octets, for a potential payload body.  For messages
    that do include a payload body, the Content-Length field-value
    provides the framing information necessary for determining where the
    body (and message) ends.  For messages that do not include a payload
    body, the Content-Length indicates the size of the selected
    representation (Section 3 of [Part2]).

      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 user agent SHOULD send a Content-Length in a request message when
    no Transfer-Encoding is sent and the request method defines a meaning
    for an enclosed payload body.  For example, a Content-Length header
    field is normally sent in a POST request even when the value is 0
    (indicating an empty payload body).  A user agent SHOULD NOT send a
    Content-Length header field when the request message does not contain
    a payload body and the method semantics do not anticipate such a
    body.

    A server MAY send a Content-Length header field in a response to a
    HEAD request (Section 4.3.2 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 MUST NOT send a Content-Length header field in any 2xx
    (Successful) response to a CONNECT request (Section 4.3.6 of
    [Part2]).

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    Aside from the cases defined above, in the absence of Transfer-
    Encoding, an origin server SHOULD send a Content-Length header field
    when the payload body size is known prior to sending the complete
    header section.  This will allow downstream recipients to measure
    transfer progress, know when a received message is complete, and
    potentially reuse the connection for additional requests.

    Any Content-Length field value greater than or equal to zero is
    valid.  Since there is no predefined limit to the length of a
    payload, a recipient MUST anticipate potentially large decimal
    numerals and prevent parsing errors due to integer conversion
    overflows (Section 9.3).

    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 or forwarding the message.

       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

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        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
        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.  A sender MUST remove the received Content-
        Length field prior to forwarding such a message downstream.

    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 the
        recipient MUST treat it as an unrecoverable 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 close the connection to the server,
        discard the received response, and send a 502 (Bad Gateway)
        response to the client.  If this is a response message received
        by a user agent, the user agent MUST close the connection to the
        server and discard the received response.

    5.  If a valid Content-Length header field is present without
        Transfer-Encoding, its decimal value defines the expected message
        body length in octets.  If the sender closes the connection or
        the recipient times out before the indicated number of octets are
        received, the recipient MUST consider the message to be
        incomplete and close the connection.

    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.

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    Since there is no way to distinguish a successfully completed, close-
    delimited message from a partially-received message interrupted by
    network failure, a server SHOULD generate 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 transfer coding, since some
    existing services respond to chunked with a 411 (Length Required)
    status code even though they understand the chunked transfer coding.
    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 user agent that sends a request containing a message body MUST send
    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.

    If the final response to the last request on a connection has been
    completely received and there remains additional data to read, a user
    agent MAY discard the remaining data or attempt to determine if that
    data belongs as part of the prior response body, which might be the
    case if the prior message's Content-Length value is incorrect.  A
    client MUST NOT process, cache, or forward such extra data as a
    separate response, since such behavior would be vulnerable to cache
    poisoning.

3.4.  Handling Incomplete Messages

    A server that receives an incomplete request message, usually due to
    a canceled request or a triggered time-out exception, MAY send an
    error response prior to closing the connection.

TOM: Isn't this a case where the server SHOULD send an error rather than 
MAY?

    A client that receives an incomplete response message, which can
    occur when a connection is closed prematurely or when decoding a
    supposedly chunked transfer coding fails, MUST record the message as
    incomplete.  Cache requirements for incomplete responses are defined
    in Section 3 of [Part6].

    If a response terminates in the middle of the header section (before

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    the empty line is received) and the status code might rely on header
    fields to convey the full meaning of the response, then the client
    cannot assume that meaning has been conveyed; the client might need
    to repeat the request in order to determine what action to take next.

    A message body that uses the chunked transfer coding 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 coding 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 section was
    received intact.

3.5.  Message Parsing Robustness

    Older HTTP/1.0 user agent implementations might send an extra CRLF
    after a POST request as a 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 user agent 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 user agent MUST
    count the terminating CRLF octets as part of the message body length.

    In the interest of robustness, a server that is expecting to receive
    and parse a request-line SHOULD ignore at least one empty line (CRLF)
    received prior to the request-line.

    Although the line terminator for the start-line and header fields is
    the sequence CRLF, a recipient MAY recognize a single LF as a line
    terminator and ignore any preceding CR.

    Although the request-line and status-line grammar rules require that
    each of the component elements be separated by a single SP octet,
    recipients MAY instead parse on whitespace-delimited word boundaries
    and, aside from the CRLF terminator, treat any form of whitespace as
    the SP separator while ignoring preceding or trailing whitespace;
    such whitespace includes one or more of the following octets: SP,
    HTAB, VT (%x0B), FF (%x0C), or bare 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
    SHOULD respond with a 400 (Bad Request) response.

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4.  Transfer Codings

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

      transfer-coding    = "chunked" ; Section 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 names are case-insensitive and ought to be
    registered within the HTTP Transfer Coding registry, as defined in
    Section 8.4.  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 transfer coding wraps the payload body in order to
    transfer it as a series of chunks, each with its own size indicator,
    followed by an OPTIONAL trailer containing header fields.  Chunked
    enables content streams of unknown size to be transferred as a
    sequence of length-delimited buffers, which enables the sender to
    retain connection persistence and the recipient to know when it has
    received the entire 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-data     = 1*OCTET ; a sequence of chunk-size octets

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    The chunk-size field is a string of hex digits indicating the size of
    the chunk-data in octets.  The chunked transfer coding is complete
    when a chunk with a chunk-size of zero is received, possibly followed
    by a trailer, and finally terminated by an empty line.

    A recipient MUST be able to parse and decode the chunked transfer
    coding.

4.1.1.  Chunk Extensions

    The chunked encoding allows each chunk to include zero or more chunk
    extensions, immediately following the chunk-size, for the sake of
    supplying per-chunk metadata (such as a signature or hash), mid-
    message control information, or randomization of message body size.

      chunk-ext      = *( ";" chunk-ext-name [ "=" chunk-ext-val ] )

      chunk-ext-name = token
      chunk-ext-val  = token / quoted-str-nf

      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

    The chunked encoding is specific to each connection and is likely to
    be removed or recoded by each recipient (including intermediaries)
    before any higher-level application would have a chance to inspect
    the extensions.  Hence, use of chunk extensions is generally limited
    to specialized HTTP services such as "long polling" (where client and
    server can have shared expectations regarding the use of chunk
    extensions) or for padding within an end-to-end secured connection.

    A recipient MUST ignore unrecognized chunk extensions.  A server
    ought to limit the total length of chunk extensions received in a
    request to an amount reasonable for the services provided, in the
    same way that it applies length limitations and timeouts for other
    parts of a message, and generate an appropriate 4xx (Client Error)
    response if that amount is exceeded.

4.1.2.  Chunked Trailer Part

    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 fields are identical to header fields, except
    they are sent in a chunked trailer instead of the message's header
    section.

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      trailer-part   = *( header-field CRLF )

    A sender MUST NOT generate a trailer that contains a field which
    needs to be known by the recipient before it can begin processing the
    message body.  For example, most recipients need to know the values
    of Content-Encoding and Content-Type in order to select a content
    handler, so placing those fields in a trailer would force the
    recipient to buffer the entire body before it could begin, greatly
    increasing user-perceived latency and defeating one of the main
    advantages of using chunked to send data streams of unknown length.
    A sender MUST NOT generate a trailer containing a Transfer-Encoding,
    Content-Length, or Trailer field.

    A server MUST generate 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
        generating server) without receiving that metadata.  In other
        words, the generating server 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
    forwarded to an HTTP/1.0 recipient.

4.1.3.  Decoding Chunked

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

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      length := 0
      read chunk-size, chunk-ext (if any), and CRLF
      while (chunk-size > 0) {
         read chunk-data and CRLF
         append chunk-data to decoded-body
         length := length + chunk-size
         read chunk-size, chunk-ext (if any), 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

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" coding is an adaptive Lempel-Ziv-Welch (LZW) coding
    [Welch] that is commonly produced by the UNIX file compression
    program "compress".  A recipient SHOULD consider "x-compress" to be
    equivalent to "compress".

4.2.2.  Deflate Coding

    The "deflate" coding is a "zlib" data format [RFC1950] containing a
    "deflate" compressed data stream [RFC1951] that uses a combination of
    the Lempel-Ziv (LZ77) compression algorithm and Huffman coding.

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

4.2.3.  Gzip Coding

    The "gzip" coding is an LZ77 coding with a 32 bit CRC that is
    commonly produced by the gzip file compression program [RFC1952].  A
    recipient 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

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    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".  A client 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 4.1.2, on behalf of itself and any downstream
    clients.  For requests from an intermediary, this implies that
    either: (a) all downstream clients are willing to accept trailer
    fields in the forwarded response; or, (b) the intermediary 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 an intermediary 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
    5.3.1 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.

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4.4.  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
    generate 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

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 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 cache [Part6] and the request can be satisfied by
    it, then the request is usually directed there first.

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

    origin-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.

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    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.

    absolute-form

    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 5.7.

    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, a server MUST accept the absolute-form in
    requests, even though HTTP/1.1 clients will only send them in
    requests to proxies.

    authority-form

    The authority-form of request-target is only used for CONNECT
    requests (Section 4.3.6 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 and
    its "@" delimiter) as the request-target.  For example,

      CONNECT www.example.com:80 HTTP/1.1

    asterisk-form

    The asterisk-form of request-target is only used for a server-wide
    OPTIONS request (Section 4.3.7 of [Part2]).  When a client wishes to

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    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,

      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.

      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 a
    client MUST send a field-value for Host that is identical to that
    authority component, excluding any userinfo subcomponent and its "@"
    delimiter (Section 2.7.1).  If the authority component is missing or
    undefined for the target URI, then a client MUST send a Host header
    field with an empty field-value.

    Since the Host field-value is critical information for handling a
    request, a user agent SHOULD generate Host as the first header field
    following the request-line.

    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

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      Host: www.example.org

    A client MUST send a Host header field 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.

    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.  A proxy that forwards such a request 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

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    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.

    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.

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5.6.  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 6.2 of
    [Part2]) precede a final response to the same request.

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

5.7.  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.

    An intermediary not acting as a tunnel MUST implement the Connection
    header field, as specified in Section 6.1, and exclude fields from
    being forwarded that are only intended for the incoming connection.

    An intermediary MUST NOT forward a message to itself unless it is
    protected from an infinite request loop.  In general, an intermediary
    ought to recognize its own server names, including any aliases, local
    variations, or literal IP addresses, and respond to such requests
    directly.

5.7.1.  Via

    The "Via" header field indicates the presence of intermediate
    protocols and recipients between the user agent and the server (on
    requests) or between the origin server and the client (on responses),
    similar to the "Received" header field in email (Section 3.6.7 of
    [RFC5322]).  Via can be used for tracking message forwards, avoiding
    request loops, and identifying the protocol capabilities of senders
    along the request/response chain.

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      Via = 1#( received-protocol RWS received-by [ RWS comment ] )

      received-protocol = [ protocol-name "/" ] protocol-version
                          ; see Section 6.7
      received-by       = ( uri-host [ ":" port ] ) / pseudonym
      pseudonym         = token

    Multiple Via field values represent each proxy or gateway that has
    forwarded the message.  Each intermediary appends its own information
    about how the message was received, such that the end result is
    ordered according to the sequence of forwarding recipients.

    A proxy MUST send an appropriate Via header field, as described
    below, in each message that it forwards.  An HTTP-to-HTTP gateway
    MUST send an appropriate Via header field in each inbound request
    message and MAY send a Via header field in forwarded response
    messages.

    For each intermediary, the received-protocol indicates the protocol
    and protocol version used by the upstream sender of the message.
    Hence, the Via field value records the advertised protocol
    capabilities of the request/response chain such that they remain
    visible to downstream recipients; this can be useful for determining
    what backwards-incompatible features might be safe to use in
    response, or within a later request, as described in Section 2.6.
    For brevity, the protocol-name is omitted when the received protocol
    is 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, a sender MAY replace it with a pseudonym.  If a port is
    not provided, a recipient MAY interpret that as meaning it was
    received on the default TCP port, if any, for the received-protocol.

    A sender MAY generate comments 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 a recipient MAY remove them 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:

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      Via: 1.0 fred, 1.1 p.example.net

    An intermediary 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, such an
    intermediary SHOULD replace each received-by host of any host behind
    the firewall by an appropriate pseudonym for that host.

    An intermediary 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

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

5.7.2.  Transformations

    Some intermediaries include features for transforming messages and
    their payloads.  A transforming proxy might, for example, convert
    between image formats in order to save cache space or to reduce the
    amount of traffic on a slow link.  However, operational problems
    might occur when these transformations are applied to payloads
    intended for critical applications, such as medical imaging or
    scientific data analysis, particularly when integrity checks or
    digital signatures are used to ensure that the payload received is
    identical to the original.

    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 proxy MUST NOT modify the "absolute-path" 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 proxy MUST NOT modify header fields that provide information about
    the end points of the communication chain, the resource state, or the
    selected representation.  A proxy MAY change the message body through

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    application or removal of a transfer coding (Section 4).

    A non-transforming proxy MUST NOT modify the message payload (Section
    3.3 of [Part2]).  A transforming proxy MUST NOT modify the payload of
    a message that contains the no-transform cache-control directive.

    A transforming proxy MAY transform the payload of a message that does
    not contain the no-transform cache-control directive; if the payload
    is transformed, the transforming proxy MUST add a Warning header
    field with the warn-code of 214 ("Transformation Applied") if one
    does not already appear in the message (see Section 5.5 of [Part6]).
    If the payload of a 200 (OK) response is transformed, the
    transforming proxy can also inform downstream recipients that a
    transformation has been applied by changing the response status code
    to 203 (Non-Authoritative Information) (Section 6.3.4 of [Part2]).

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.

TOM: Do you want to refer to the security section for the DoS Attack?

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 aside from Connection is used to supply control
    information for or about the current connection, the sender MUST 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 send a connection option corresponding to a header
    field that is intended for all recipients of the payload.  For
    example, Cache-Control is never appropriate as a connection option
    (Section 5.2 of [Part6]).

    The connection options do not always 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 a
    connection option.  In contrast, a connection-specific header field
    that is received without a corresponding connection option usually
    indicates that the field has been improperly forwarded by an
    intermediary and ought to be ignored by the recipient.

    When defining new connection options, specification authors ought to
    survey existing header field names and ensure that the new connection
    option does not share the same name as an already deployed header
    field.  Defining a 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.

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    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 sender is going to close the connection after the current
    request/response is complete (Section 6.6).

    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.  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.

6.3.  Persistence

    HTTP/1.1 defaults to the use of "persistent connections", allowing
    multiple requests and responses to be carried over a single
    connection.  The "close" connection-option is used to signal that a
    connection will not persist after the current request/response.  HTTP
    implementations SHOULD support persistent connections.

    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.

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    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.

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

    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).

    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.

6.3.1.  Retrying Requests

    Connections can be closed at any time, with or without intention.
    Implementations ought to anticipate the need to recover from
    asynchronous close events.

    When an inbound connection is closed prematurely, a client MAY open a
    new connection and automatically retransmit an aborted sequence of
    requests if all of those requests have idempotent methods (Section
    4.2.2 of [Part2]).  A proxy MUST NOT automatically retry non-
    idempotent requests.

    A user agent MUST NOT automatically retry a request with a non-
    idempotent method unless it has some means to know that the request
    semantics are actually idempotent, regardless of the method, or some
    means to detect that the original request was never applied.  For
    example, a user agent that knows (through design or configuration)
    that a POST request to a given resource is safe can repeat that
    request automatically.  Likewise, a user agent designed specifically
    to operate on a version control repository might be able to recover
    from partial failure conditions by checking the target resource

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    revision(s) after a failed connection, reverting or fixing any
    changes that were partially applied, and then automatically retrying
    the requests that failed.

    A client SHOULD NOT automatically retry a failed automatic retry.

6.3.2.  Pipelining

    A client that supports persistent connections MAY "pipeline" its
    requests (i.e., send multiple requests without waiting for each
    response).  A server MAY process a sequence of pipelined requests in
    parallel if they all have safe methods (Section 4.2.1 of [Part2]),
    but MUST send the corresponding responses in the same order that the
    requests were received.

TOM: Have you considered that packets may be delivered out-of-order? 
While not ideal, this is a very real possibility especially with 
multi-path TCP and other load-balancing techniques. In these cases, I 
think the statement of requiring the server to return responses  in the 
same order of receiving requests is fine, but it might be useful to 
mention this point so that clients are not confused if respones come 
back in a different order than they were sent (in reality).

    A client that pipelines requests SHOULD retry unanswered requests if
    the connection closes before it receives all of the corresponding
    responses.  When retrying pipelined requests after a failed
    connection (a connection not explicitly closed by the server in its
    last complete response), a client MUST NOT pipeline immediately after
    connection establishment, since the first remaining request in the
    prior pipeline might have caused an error response that can be lost
    again if multiple requests are sent on a prematurely closed
    connection (see the TCP reset problem described in Section 6.6).

    Idempotent methods (Section 4.2.2 of [Part2]) are significant to
    pipelining because they can be automatically retried after a
    connection failure.  A user agent SHOULD NOT pipeline requests after
    a non-idempotent method, until the final response status code for
    that method has been received, unless the user agent has a means to
    detect and recover from partial failure conditions involving the
    pipelined sequence.

    An intermediary that receives pipelined requests MAY pipeline those
    requests when forwarding them inbound, since it can rely on the
    outbound user agent(s) to determine what requests can be safely
    pipelined.  If the inbound connection fails before receiving a
    response, the pipelining intermediary MAY attempt to retry a sequence
    of requests that have yet to receive a response if the requests all
    have idempotent methods; otherwise, the pipelining intermediary
    SHOULD forward any received responses and then close the
    corresponding outbound connection(s) so that the outbound user
    agent(s) can recover accordingly.

6.4.  Concurrency

    A client SHOULD limit the number of simultaneous open connections
    that it maintains to a given server.

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    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.

6.5.  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.

    A client or server that wishes to time-out SHOULD issue a graceful
    close on the connection.  Implementations SHOULD constantly monitor
    open connections for a received closure signal and respond to it as
    appropriate, since prompt closure of both sides of a connection
    enables allocated system resources to be reclaimed.

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

    A server SHOULD sustain persistent connections, when possible, 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 response while it is transmitting the request.  If the
    client sees a response that indicates the server does not wish to
    receive the message body and is closing the connection, the client

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    SHOULD immediately cease transmitting the body and close its side of
    the connection.

6.6.  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
    close of the connection (see below) after it sends the final response
    to the request that contained close.  The server SHOULD send 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 close
    of the connection (see below) 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 NOT
    assume that they will 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
    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, servers typically close a connection
    in stages.  First, the server performs a half-close by closing only
    the write side of the read/write connection.  The server then
    continues to read from the connection until it receives a
    corresponding close by the client, or until the server is reasonably
    certain that its own TCP stack has received the client's

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    acknowledgement of the packet(s) containing the server's last
    response.  Finally, the server fully closes the connection.

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

6.7.  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, in order of descending preference, before
    sending the final response.  A server MAY ignore a received Upgrade
    header field if it wishes to continue using the current protocol on
    that connection.

      Upgrade          = 1#protocol

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

    A server that sends a 101 (Switching Protocols) response MUST send an
    Upgrade header field to indicate the new protocol(s) to which the
    connection is being switched; if multiple protocol layers are being
    switched, the sender MUST list the protocols in layer-ascending
    order.  A server MUST NOT switch to a protocol that was not indicated
    by the client in the corresponding request's Upgrade header field.  A
    server MAY choose to ignore the order of preference indicated by the
    client and select the new protocol(s) based on other factors, such as
    the nature of the request or the current load on the server.

    A server that sends a 426 (Upgrade Required) response MUST send an
    Upgrade header field to indicate the acceptable protocols, in order
    of descending preference.

    A server MAY send an Upgrade header field in any other response to
    advertise that it implements support for upgrading to the listed
    protocols, in order of descending preference, when appropriate for a
    future request.

    The following is a hypothetical example sent by a client:

      GET /hello.txt HTTP/1.1
      Host: www.example.com
      Connection: upgrade
      Upgrade: HTTP/2.0, SHTTP/1.3, IRC/6.9, RTA/x11

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    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(s) chosen.  However,
    immediately after sending the 101 response, the server is expected to
    continue responding to the original request as if it had received its
    equivalent within the new protocol (i.e., the server still has an
    outstanding request to satisfy after the protocol has been changed,
    and is expected to do so without requiring the request to be
    repeated).

    For example, if the Upgrade header field is received in a GET request
    and the server decides to switch protocols, it first responds with a
    101 (Switching Protocols) message in HTTP/1.1 and then immediately
    follows 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.

    The following is an example response to the above hypothetical
    request:

      HTTP/1.1 101 Switching Protocols
      Connection: upgrade
      Upgrade: HTTP/2.0

      [... data stream switches to HTTP/2.0 with an appropriate response
      (as defined by new protocol) to the "GET /hello.txt" request ...]

    When Upgrade is sent, the sender MUST also send a Connection header
    field (Section 6.1) that contains an "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.

    A client cannot begin using an upgraded protocol on the connection
    until it has completely sent the request message (i.e., the client
    can't change the protocol it is sending in the middle of a message).
    If a server receives both Upgrade and an Expect header field with the
    "100-continue" expectation (Section 5.1.1 of [Part2]), the server
    MUST send a 100 (Continue) response before sending a 101 (Switching
    Protocols) response.

    The Upgrade header field only applies to switching protocols on top
    of the existing connection; it cannot be used to switch the

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    underlying connection (transport) protocol, nor to switch the
    existing communication to a different connection.  For those
    purposes, it is more appropriate to use a 3xx (Redirection) response
    (Section 6.4 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 ought to be registered with IANA
    using the registration procedure defined in Section 8.6.

7.  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, a sender MUST expand the list construct as follows:

      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, a recipient MUST parse and
    ignore a reasonable number of empty list elements: enough to handle
    common mistakes by senders that merge values, but not so much that
    they could be used as a denial of service mechanism.  In other words,
    a recipient MUST expand the list construct as follows:

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

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

    Empty elements do not contribute to the count of elements present.
    For example, given these ABNF productions:

      example-list      = 1#example-list-elmt
      example-list-elmt = token ; see Section 3.2.6

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

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

    In contrast, the following values would be invalid, since at least
    one non-empty element is required by the example-list production:

      ""
      ","
      ",   ,"

    Appendix B shows the collected ABNF after the list constructs have
    been expanded, as described above, for recipients.

8.  IANA Considerations

8.1.  Header Field Registration

    HTTP header fields are registered within the Message Header Field
    Registry maintained at
    <http://www.iana.org/assignments/message-headers/>.

    This document defines the following HTTP header fields, so their
    associated registry entries shall be updated according to the
    permanent registrations below (see [BCP90]):

    +-------------------+----------+----------+---------------+
    | 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 4.4   |
    | Transfer-Encoding | http     | standard | Section 3.3.1 |
    | Upgrade           | http     | standard | Section 6.7   |
    | Via               | http     | standard | Section 5.7.1 |
    +-------------------+----------+----------+---------------+

    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).

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    +-------------------+----------+----------+-------------+
    | Header Field Name | Protocol | Status   | Reference   |
    +-------------------+----------+----------+-------------+
    | Close             | http     | reserved | Section 8.1 |
    +-------------------+----------+----------+-------------+

    The change controller is: "IETF ([email-masked]) - Internet
    Engineering Task Force".

8.2.  URI Scheme Registration

    IANA maintains the registry of URI Schemes [BCP115] at
    <http://www.iana.org/assignments/uri-schemes/>.

    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 |
    +------------+------------------------------------+---------------+

8.3.  Internet Media Type Registration

    IANA maintains the registry of Internet media types [BCP13] at
    <http://www.iana.org/assignments/media-types>.

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

8.3.1.  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

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    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 8.3.1).

    Applications that use this media type:

    Additional information:

       Magic number(s):  none

       File extension(s):  none

       Macintosh file type code(s):  none

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

    Intended usage:  COMMON

    Restrictions on usage:  none

    Author:  See Authors Section.

    Change controller:  IESG

8.3.2.  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).

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    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 8.3.2).

    Applications that use this media type:

    Additional information:

       Magic number(s):  none

       File extension(s):  none

       Macintosh file type code(s):  none

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

    Intended usage:  COMMON

    Restrictions on usage:  none

    Author:  See Authors Section.

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    Change controller:  IESG

8.4.  Transfer Coding Registry

    The HTTP Transfer Coding Registry defines the name space for transfer
    coding names.  It is maintained at
    <http://www.iana.org/assignments/http-parameters>.

8.4.1.  Procedure

    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 specification.

    Use of program names for the identification of encoding formats is
    not desirable and is discouraged for future encodings.

8.4.2.  Registration

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

    +------------+--------------------------------------+---------------+
    | Name       | Description                          | Reference     |
    +------------+--------------------------------------+---------------+
    | chunked    | Transfer in a series of chunks       | Section 4.1   |
    | compress   | UNIX "compress" data format [Welch]  | Section 4.2.1 |
    | deflate    | "deflate" compressed data            | Section 4.2.2 |
    |            | ([RFC1951]) inside the "zlib" data   |               |
    |            | format ([RFC1950])                   |               |
    | gzip       | GZIP file format [RFC1952]           | Section 4.2.3 |
    | x-compress | Deprecated (alias for compress)      | Section 4.2.1 |
    | x-gzip     | Deprecated (alias for gzip)          | Section 4.2.3 |
    +------------+--------------------------------------+---------------+

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8.5.  Content Coding Registration

    IANA maintains the registry of HTTP Content Codings at
    <http://www.iana.org/assignments/http-parameters>.

    The HTTP Content Codings Registry shall be updated with the
    registrations below:

    +------------+--------------------------------------+---------------+
    | Name       | Description                          | Reference     |
    +------------+--------------------------------------+---------------+
    | compress   | UNIX "compress" data format [Welch]  | Section 4.2.1 |
    | deflate    | "deflate" compressed data            | Section 4.2.2 |
    |            | ([RFC1951]) inside the "zlib" data   |               |
    |            | format ([RFC1950])                   |               |
    | gzip       | GZIP file format [RFC1952]           | Section 4.2.3 |
    | x-compress | Deprecated (alias for compress)      | Section 4.2.1 |
    | x-gzip     | Deprecated (alias for gzip)          | Section 4.2.3 |
    +------------+--------------------------------------+---------------+

8.6.  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.
    The registry is maintained at
    <http://www.iana.org/assignments/http-upgrade-tokens>.

8.6.1.  Procedure

    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.

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    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].

8.6.2.  Upgrade Token Registration

    The "HTTP" entry in 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 ([email-masked]) - Internet
    Engineering Task Force".

9.  Security Considerations

    This section is meant to inform developers, information providers,
    and users of known security concerns relevant to HTTP/1.1 message
    syntax, parsing, and routing.

9.1.  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]).

9.2.  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

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    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.

    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.

9.3.  Buffer 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 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 6.5.12 of
    [Part2]) or request entities that are too large (Section 6.5 of
    [Part2]).  Additional status codes related to capacity limits have
    been defined by extensions to HTTP [RFC6585].

    Recipients ought to 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.

9.4.  Message Integrity

    HTTP does not define a specific mechanism for ensuring message
    integrity, instead relying on the error-detection ability of
    underlying transport protocols and the use of length or chunk-

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    delimited framing to detect completeness.  Additional integrity
    mechanisms, such as hash functions or digital signatures applied to
    the content, can be selectively added to messages via extensible
    metadata header fields.  Historically, the lack of a single integrity
    mechanism has been justified by the informal nature of most HTTP
    communication.  However, the prevalence of HTTP as an information
    access mechanism has resulted in its increasing use within
    environments where verification of message integrity is crucial.

    User agents are encouraged to implement configurable means for
    detecting and reporting failures of message integrity such that those
    means can be enabled within environments for which integrity is
    necessary.  For example, a browser being used to view medical history
    or drug interaction information needs to indicate to the user when
    such information is detected by the protocol to be incomplete,
    expired, or corrupted during transfer.  Such mechanisms might be
    selectively enabled via user agent extensions or the presence of
    message integrity metadata in a response.  At a minimum, user agents
    ought to provide some indication that allows a user to distinguish
    between a complete and incomplete response message (Section 3.4) when
    such verification is desired.

9.5.  Server Log Information

    A server is in the position to save personal data about a user's
    requests over time, 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
    characteristics.  As such, access traces that are keyed to a specific
    client are unsafe to publish even if the key is pseudonymous.

    To minimize the risk of theft or accidental publication, log
    information ought to 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.

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10.  Acknowledgments

    This edition of HTTP/1.1 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,
    and Paul J. Leach.  Mark Nottingham oversaw this effort as working
    group chair.

    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:

    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,
    Andrei Popov, Anil Sharma, Anne van Kesteren, Anthony Bryan, Asbjorn
    Ulsberg, Ashok Kumar, Balachander Krishnamurthy, Barry Leiba, Ben
    Laurie, Benjamin Carlyle, Benjamin Niven-Jenkins, Bil Corry, Bill
    Burke, Bjoern Hoehrmann, Bob Scheifler, Boris Zbarsky, Brett Slatkin,
    Brian Kell, Brian McBarron, Brian Pane, Brian Raymor, 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, Darrel Miller, Dave Cridland,
    Dave Crocker, Dave Kristol, Dave Thaler, David Booth, David Singer,
    David W. Morris, Diwakar Shetty, Dmitry Kurochkin, Drummond Reed,
    Duane Wessels, Edward Lee, Eitan Adler, Eliot Lear, Emile Stephan,
    Eran Hammer-Lahav, Eric D. Williams, Eric J. Bowman, Eric Lawrence,
    Eric Rescorla, Erik Aronesty, EungJun Yi, Evan Prodromou, Felix
    Geisendoerfer, Florian Weimer, Frank Ellermann, Fred Akalin, Fred
    Bohle, Frederic Kayser, Gabor Molnar, Gabriel Montenegro, Geoffrey
    Sneddon, Gervase Markham, Gili Tzabari, Grahame Grieve, Greg Wilkins,
    Grzegorz Calkowski, Harald Tveit Alvestrand, Harry Halpin, Helge
    Hess, Henrik Nordstrom, Henry S. Thompson, Henry Story, Herbert van
    de Sompel, Herve Ruellan, Howard Melman, Hugo Haas, Ian Fette, Ian
    Hickson, Ido Safruti, Ilari Liusvaara, Ilya Grigorik, Ingo Struck, J.
    Ross Nicoll, James Cloos, James H. Manger, James Lacey, James M.
    Snell, Jamie Lokier, Jan Algermissen, Jeff Hodges (who came up with
    the term 'effective Request-URI'), Jeff Pinner, Jeff Walden, Jim
    Luther, Jitu Padhye, 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 A.
    Rees, Jonathan Billington, Jonathan Moore, 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, Ken Murchison, Koen Holtman,

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    Konstantin Voronkov, Kris Zyp, Leif Hedstrom, Lisa Dusseault, Maciej
    Stachowiak, Manu Sporny, 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, Michael
    Scharf, Michael Sweet, Michael Tuexen, Michael Welzl, Mike Amundsen,
    Mike Belshe, Mike Bishop, Mike Kelly, Mike Schinkel, Miles Sabin,
    Murray S. Kucherawy, Mykyta Yevstifeyev, Nathan Rixham, Nicholas
    Shanks, Nico Williams, Nicolas Alvarez, Nicolas Mailhot, Noah Slater,
    Osama Mazahir, Pablo Castro, Pat Hayes, Patrick R. McManus, Paul E.
    Jones, Paul Hoffman, Paul Marquess, Peter Lepeska, Peter Occil, Peter
    Saint-Andre, Peter Watkins, Phil Archer, Philippe Mougin, Phillip
    Hallam-Baker, Piotr Dobrogost, Poul-Henning Kamp, Preethi Natarajan,
    Rajeev Bector, Ray Polk, Reto Bachmann-Gmuer, Richard Cyganiak, Robby
    Simpson, Robert Brewer, Robert Collins, Robert Mattson, Robert
    O'Callahan, Robert Olofsson, Robert Sayre, Robert Siemer, Robert de
    Wilde, Roberto Javier Godoy, Roberto Peon, Roland Zink, Ronny
    Widjaja, Ryan Hamilton, S. Mike Dierken, Salvatore Loreto, Sam
    Johnston, Sam Pullara, Sam Ruby, Saurabh Kulkarni, Scott Lawrence
    (who maintained the original issues list), Sean B. Palmer, Sebastien
    Barnoud, Shane McCarron, Shigeki Ohtsu, Stefan Eissing, Stefan
    Tilkov, Stefanos Harhalakis, Stephane Bortzmeyer, Stephen Farrell,
    Stephen Ludin, Stuart Williams, Subbu Allamaraju, Subramanian
    Moonesamy, Sylvain Hellegouarch, Tapan Divekar, Tatsuhiro Tsujikawa,
    Tatsuya Hayashi, Ted Hardie, Thomas Broyer, Thomas Fossati, Thomas
    Maslen, 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, Yuchung Cheng,
    Yutaka Oiwa, Yves Lafon (long-time member of the editor team), Zed A.
    Shaw, and Zhong Yu.

    See Section 16 of [RFC2616] for additional acknowledgements from
    prior revisions.

11.  References

11.1.  Normative References

    [Part2]       Fielding, R., Ed. and J. Reschke, Ed., "Hypertext
                  Transfer Protocol (HTTP/1.1): Semantics and Content",
                  draft-ietf-httpbis-p2-semantics-25 (work in progress),
                  November 2013.

    [Part4]       Fielding, R., Ed. and J. Reschke, Ed., "Hypertext
                  Transfer Protocol (HTTP/1.1): Conditional Requests",
                  draft-ietf-httpbis-p4-conditional-25 (work in

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                  progress), November 2013.

    [Part5]       Fielding, R., Ed., Lafon, Y., Ed., and J. Reschke, Ed.,
                  "Hypertext Transfer Protocol (HTTP/1.1): Range
                  Requests", draft-ietf-httpbis-p5-range-25 (work in
                  progress), November 2013.

    [Part6]       Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
                  Ed., "Hypertext Transfer Protocol (HTTP/1.1): Caching",
                  draft-ietf-httpbis-p6-cache-25 (work in progress),
                  November 2013.

    [Part7]       Fielding, R., Ed. and J. Reschke, Ed., "Hypertext
                  Transfer Protocol (HTTP/1.1): Authentication",
                  draft-ietf-httpbis-p7-auth-25 (work in progress),
                  November 2013.

    [RFC0793]     Postel, J., "Transmission Control Protocol", STD 7,
                  RFC 793, September 1981.

    [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.

    [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.

    [Welch]       Welch, T., "A Technique for High Performance Data
                  Compression", IEEE Computer 17(6), June 1984.

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11.2.  Informative References

    [BCP115]      Hansen, T., Hardie, T., and L. Masinter, "Guidelines
                  and Registration Procedures for New URI Schemes",
                  BCP 115, RFC 4395, February 2006.

    [BCP13]       Freed, N., Klensin, J., and T. Hansen, "Media Type
                  Specifications and Registration Procedures", BCP 13,
                  RFC 6838, January 2013.

    [BCP90]       Klyne, G., Nottingham, M., and J. Mogul, "Registration
                  Procedures for Message Header Fields", BCP 90,
                  RFC 3864, September 2004.

    [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 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",
                  RFC 2145, May 1997.

    [RFC2616]     Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,

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                  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.

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

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

    [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.

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

    [RFC6585]     Nottingham, M. and R. Fielding, "Additional HTTP Status
                  Codes", RFC 6585, April 2012.

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

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    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.

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

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    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 an 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 will need to
    monitor the connection for "hung" requests (which indicate that the
    client ought stop sending the header field), and this mechanism 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).  Transfer codings need to be decoded prior to
    forwarding an HTTP message over a MIME-compliant protocol.

A.2.  Changes from RFC 2616

    HTTP's approach to error handling has been explained.  (Section 2.5)

    The HTTP-version ABNF production has been clarified to be case-

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    sensitive.  Additionally, version numbers has been restricted to
    single digits, due to the fact that implementations are known to
    handle multi-digit version numbers incorrectly.  (Section 2.6)

    Userinfo (i.e., username and password) are now disallowed in HTTP and
    HTTPS URIs, because of security issues related to their transmission
    on the wire.  (Section 2.7.1)

    The HTTPS URI scheme is now defined by this specification;
    previously, it was done in Section 2.4 of [RFC2818].  Furthermore, it
    implies end-to-end security.  (Section 2.7.2)

    HTTP messages can be (and often are) buffered by implementations;
    despite it sometimes being available as a stream, HTTP is
    fundamentally a message-oriented protocol.  Minimum supported sizes
    for various protocol elements have been suggested, to improve
    interoperability.  (Section 3)

    Invalid whitespace around field-names is now required to be rejected,
    because accepting it represents a security vulnerability.  The ABNF
    productions defining header fields now only list 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 3.2.3)

    Header fields that span multiple lines ("line folding") are
    deprecated.  (Section 3.2.4)

    The NUL octet is no longer allowed in comment and quoted-string text,
    and handling of backslash-escaping in them has been clarified.  The
    quoted-pair rule no longer allows escaping control characters other
    than HTAB.  Non-ASCII content in header fields and the reason phrase
    has been obsoleted and made opaque (the TEXT rule was removed).
    (Section 3.2.6)

    Bogus "Content-Length" header fields are now required to be handled
    as errors by recipients.  (Section 3.3.2)

    The algorithm for determining the message body length has been
    clarified to indicate all of the special cases (e.g., driven by
    methods or status codes) that affect it, and that new protocol
    elements cannot define such special cases.  CONNECT is a new, special
    case in determining message body length. "multipart/byteranges" is no
    longer a way of determining message body length detection.
    (Section 3.3.3)

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    The "identity" transfer coding token has been removed.  (Sections 3.3
    and 4)

    Chunk length does not include the count of the octets in the chunk
    header and trailer.  Line folding in chunk extensions is disallowed.
    (Section 4.1)

    The meaning of the "deflate" content coding has been clarified.
    (Section 4.2.2)

    The segment + query components of RFC 3986 have been used to define
    the request-target, instead of abs_path from RFC 1808.  The asterisk-
    form of the request-target is only allowed with the OPTIONS method.
    (Section 5.3)

    The term "Effective Request URI" has been introduced.  (Section 5.5)

    Gateways do not need to generate Via header fields anymore.
    (Section 5.7.1)

    Exactly when "close" connection options have to be sent has been
    clarified.  Also, "hop-by-hop" header fields are required to appear
    in the Connection header field; just because they're defined as hop-
    by-hop in this specification doesn't exempt them.  (Section 6.1)

    The limit of two connections per server has been removed.  An
    idempotent sequence of requests is no longer required to be retried.
    The requirement to retry requests under certain circumstances when
    the server prematurely closes the connection has been removed.  Also,
    some extraneous requirements about when servers are allowed to close
    connections prematurely have been removed.  (Section 6.3)

    The semantics of the Upgrade header field is now defined in responses
    other than 101 (this was incorporated from [RFC2817]).  Furthermore,
    the ordering in the field value is now significant.  (Section 6.7)

    Empty list elements in list productions (e.g., a list header field
    containing ", ,") have been deprecated.  (Section 7)

    Registration of Transfer Codings now requires IETF Review
    (Section 8.4)

    This specification now defines the Upgrade Token Registry, previously
    defined in Section 7.2 of [RFC2817].  (Section 8.6)

    The expectation to support HTTP/0.9 requests has been removed.
    (Appendix A)

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    Issues with the Keep-Alive and Proxy-Connection header fields in
    requests are pointed out, with use of the latter being discouraged
    altogether.  (Appendix A.1.2)

Appendix B.  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 ] )

    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
    absolute-path = 1*( "/" segment )
    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

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    chunked-body = *chunk last-chunk trailer-part CRLF
    comment = "(" *( ctext / quoted-cpair / comment ) ")"
    connection-option = token
    ctext = HTAB / SP / %x21-27 ; '!'-'''
     / %x2A-5B ; '*'-'['
     / %x5D-7E ; ']'-'~'
     / obs-text

    field-content = *( HTAB / SP / VCHAR / obs-text )
    field-name = token
    field-value = *( field-content / obs-fold )
    fragment = <fragment, defined in [RFC3986], Section 3.5>

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

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

    message-body = *OCTET
    method = token

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

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

    qdtext = HTAB / SP / "!" / %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

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    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

    segment = <segment, defined in [RFC3986], Section 3.3>
    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

    word = token / quoted-string

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

C.1.  Since RFC 2616

    Changes up to the IETF Last Call draft are summarized in <http://
    trac.tools.ietf.org/html/
    draft-ietf-httpbis-p1-messaging-24#appendix-C>.

C.2.  Since draft-ietf-httpbis-p1-messaging-24

    Closed issues:

    o  <http://tools.ietf.org/wg/httpbis/trac/ticket/502>: "APPSDIR
       review of draft-ietf-httpbis-p1-messaging-24"

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    o  <http://tools.ietf.org/wg/httpbis/trac/ticket/507>: "integer value
       parsing"

    o  <http://tools.ietf.org/wg/httpbis/trac/ticket/517>: "move IANA
       registrations to correct draft"

Index

    A
       absolute-form (of request-target)  42
       accelerator  10
       application/http Media Type  61
       asterisk-form (of request-target)  42
       authority-form (of request-target)  42

    B
       browser  7

    C
       cache  11
       cacheable  12
       captive portal  11
       chunked (Coding Format)  28, 31, 35
       client  7
       close  49, 55
       compress (Coding Format)  38
       connection  7
       Connection header field  49, 55
       Content-Length header field  30

    D
       deflate (Coding Format)  38
       downstream  9

    E
       effective request URI  44

    G
       gateway  10
       Grammar
          absolute-form  41
          absolute-path  16
          absolute-URI  16
          ALPHA  6
          asterisk-form  41
          attribute  35
          authority  16
          authority-form  41

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          BWS  24
          chunk  35-36
          chunk-data  35-36
          chunk-ext  35-36
          chunk-ext-name  35-36
          chunk-ext-val  35-36
          chunk-size  35-36
          chunked-body  35-36
          comment  27
          Connection  50
          connection-option  50
          Content-Length  30
          CR  6
          CRLF  6
          ctext  27
          CTL  6
          date2  35
          date3  35
          DIGIT  6
          DQUOTE  6
          field-content  22
          field-name  22
          field-value  22
          fragment  16
          header-field  22
          HEXDIG  6
          Host  43
          HTAB  6
          HTTP-message  19
          HTTP-name  14
          http-URI  17
          HTTP-version  14
          https-URI  18
          last-chunk  35-36
          LF  6
          message-body  27
          method  21
          obs-fold  22
          obs-text  27
          OCTET  6
          origin-form  41
          OWS  24
          partial-URI  16
          port  16
          protocol-name  47
          protocol-version  47
          pseudonym  47
          qdtext  27

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          qdtext-nf  35-36
          query  16
          quoted-cpair  27
          quoted-pair  27
          quoted-str-nf  35-36
          quoted-string  27
          rank  39
          reason-phrase  22
          received-by  47
          received-protocol  47
          request-line  21
          request-target  41
          RWS  24
          segment  16
          SP  6
          special  26
          start-line  21
          status-code  22
          status-line  22
          t-codings  39
          t-ranking  39
          tchar  26
          TE  39
          token  26
          Trailer  40
          trailer-part  35-37
          transfer-coding  35
          Transfer-Encoding  28
          transfer-extension  35
          transfer-parameter  35
          Upgrade  56
          uri-host  16
          URI-reference  16
          value  35
          VCHAR  6
          Via  47
          word  26
       gzip (Coding Format)  38

    H
       header field  19
       header section  19
       headers  19
       Host header field  43
       http URI scheme  17
       https URI scheme  18

    I

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       inbound  9
       interception proxy  11
       intermediary  9

    M
       Media Type
          application/http  61
          message/http  60
       message  7
       message/http Media Type  60
       method  21

    N
       non-transforming proxy  10

    O
       origin server  7
       origin-form (of request-target)  41
       outbound  9

    P
       proxy  10

    R
       recipient  7
       request  7
       request-target  21
       resource  16
       response  7
       reverse proxy  10

    S
       sender  7
       server  7
       spider  7

    T
       target resource  40
       target URI  40
       TE header field  38
       Trailer header field  40
       Transfer-Encoding header field  28
       transforming proxy  10
       transparent proxy  11
       tunnel  11

    U
       Upgrade header field  56

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       upstream  9
       URI scheme
          http  17
          https  18
       user agent  7

    V
       Via header field  46

Authors' Addresses

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

    EMail: [email-masked]
    URI:   http://roy.gbiv.com/

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

    EMail: [email-masked]
    URI:   http://greenbytes.de/tech/webdav/

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