wdiff rfc7230.original rfc7230.txt

HTTPbis Working Group
Internet Engineering Task Force (IETF) R. Fielding, Ed.
Internet-Draft
Request for Comments: 7230 Adobe
Obsoletes: 2145,2616 (if approved) 2145, 2616 J. Reschke, Ed.
Updates: 2817,2818 (if approved) 2817, 2818 greenbytes
Intended status:
Category: Standards Track February 6, 2014
Expires: August 10, June 2014
ISSN: 2070-1721

Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing
draft-ietf-httpbis-p1-messaging-26

Abstract

The Hypertext Transfer Protocol (HTTP) is a stateless application-
level protocol for distributed, collaborative, hypertext information
systems. 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 related security
concerns for implementations.

Editorial Note (To be removed by RFC Editor)

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

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

The changes in this draft are summarized in Appendix C.3.

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 an Internet Standards Track document.

This document is a product of the Internet Engineering Task Force
(IETF). Note that other groups may also distribute
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Internet-Drafts are draft documents valid the IETF community. It has
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Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of six months RFC 5741.

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This Internet-Draft will expire on August 10, 2014.
http://www.rfc-editor.org/info/rfc7230.

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Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 5 ....................................................5
1.1. Requirement Requirements Notation . . . . . . . . . . . . . . . . . . . 6 ......................................6
1.2. Syntax Notation . . . . . . . . . . . . . . . . . . . . . 6 ............................................6
2. Architecture . . . . . . . . . . . . . . . . . . . . . . . . . 6 ....................................................6
2.1. Client/Server Messaging . . . . . . . . . . . . . . . . . 7 ....................................7
2.2. Implementation Diversity . . . . . . . . . . . . . . . . . 8 ...................................8
2.3. Intermediaries . . . . . . . . . . . . . . . . . . . . . . 9 .............................................9
2.4. Caches . . . . . . . . . . . . . . . . . . . . . . . . . . 11 ....................................................11
2.5. Conformance and Error Handling . . . . . . . . . . . . . . 12 ............................12
2.6. Protocol Versioning . . . . . . . . . . . . . . . . . . . 13 .......................................13
2.7. Uniform Resource Identifiers . . . . . . . . . . . . . . . 16 ..............................16
2.7.1. http URI scheme . . . . . . . . . . . . . . . . . . . 16 Scheme ....................................17
2.7.2. https URI scheme . . . . . . . . . . . . . . . . . . . 18 Scheme ...................................18
2.7.3. http and https URI Normalization and Comparison . . . 19 ....19
3. Message Format . . . . . . . . . . . . . . . . . . . . . . . . 19 .................................................19
3.1. Start Line . . . . . . . . . . . . . . . . . . . . . . . . 20 ................................................20
3.1.1. Request Line . . . . . . . . . . . . . . . . . . . . . 21 .......................................21
3.1.2. Status Line . . . . . . . . . . . . . . . . . . . . . 22 ........................................22
3.2. Header Fields . . . . . . . . . . . . . . . . . . . . . . 22 .............................................22
3.2.1. Field Extensibility . . . . . . . . . . . . . . . . . 23 ................................23
3.2.2. Field Order . . . . . . . . . . . . . . . . . . . . . 23 ........................................23
3.2.3. Whitespace . . . . . . . . . . . . . . . . . . . . . . 24 .........................................24
3.2.4. Field Parsing . . . . . . . . . . . . . . . . . . . . 25 ......................................25
3.2.5. Field Limits . . . . . . . . . . . . . . . . . . . . . 26 .......................................26
3.2.6. Field value components . . . . . . . . . . . . . . . . 26 Value Components .............................27
3.3. Message Body . . . . . . . . . . . . . . . . . . . . . . . 27 ..............................................28
3.3.1. Transfer-Encoding . . . . . . . . . . . . . . . . . . 28 ..................................28
3.3.2. Content-Length . . . . . . . . . . . . . . . . . . . . 30 .....................................30
3.3.3. Message Body Length . . . . . . . . . . . . . . . . . 31 ................................32
3.4. Handling Incomplete Messages . . . . . . . . . . . . . . . 33 ..............................34
3.5. Message Parsing Robustness . . . . . . . . . . . . . . . . 34 ................................34
4. Transfer Codings . . . . . . . . . . . . . . . . . . . . . . . 35 ...............................................35
4.1. Chunked Transfer Coding . . . . . . . . . . . . . . . . . 35 ...................................36
4.1.1. Chunk Extensions . . . . . . . . . . . . . . . . . . . 36 ...................................36
4.1.2. Chunked Trailer Part . . . . . . . . . . . . . . . . . 36 ...............................37
4.1.3. Decoding Chunked . . . . . . . . . . . . . . . . . . . 37 ...................................38
4.2. Compression Codings . . . . . . . . . . . . . . . . . . . 38 .......................................38
4.2.1. Compress Coding . . . . . . . . . . . . . . . . . . . 38 ....................................38
4.2.2. Deflate Coding . . . . . . . . . . . . . . . . . . . . 38 .....................................38
4.2.3. Gzip Coding . . . . . . . . . . . . . . . . . . . . . 38 ........................................39
4.3. TE . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38 ........................................................39
4.4. Trailer . . . . . . . . . . . . . . . . . . . . . . . . . 39 ...................................................40
5. Message Routing . . . . . . . . . . . . . . . . . . . . . . . 40 ................................................40
5.1. Identifying a Target Resource . . . . . . . . . . . . . . 40 .............................40
5.2. Connecting Inbound . . . . . . . . . . . . . . . . . . . . 40 ........................................41
5.3. Request Target . . . . . . . . . . . . . . . . . . . . . . 41 ............................................41
5.3.1. origin-form . . . . . . . . . . . . . . . . . . . . . 41 ........................................42
5.3.2. absolute-form . . . . . . . . . . . . . . . . . . . . 42 ......................................42
5.3.3. authority-form . . . . . . . . . . . . . . . . . . . . 42 .....................................43
5.3.4. asterisk-form . . . . . . . . . . . . . . . . . . . . 42 ......................................43
5.4. Host . . . . . . . . . . . . . . . . . . . . . . . . . . . 43 ......................................................44
5.5. Effective Request URI . . . . . . . . . . . . . . . . . . 44 .....................................45
5.6. Associating a Response to a Request . . . . . . . . . . . 46 .......................46
5.7. Message Forwarding . . . . . . . . . . . . . . . . . . . . 46 ........................................47
5.7.1. Via . . . . . . . . . . . . . . . . . . . . . . . . . 47 ................................................47
5.7.2. Transformations . . . . . . . . . . . . . . . . . . . 48 ....................................49
6. Connection Management . . . . . . . . . . . . . . . . . . . . 49 ..........................................50
6.1. Connection . . . . . . . . . . . . . . . . . . . . . . . . 50 ................................................51
6.2. Establishment . . . . . . . . . . . . . . . . . . . . . . 51 .............................................52
6.3. Persistence . . . . . . . . . . . . . . . . . . . . . . . 52 ...............................................52
6.3.1. Retrying Requests . . . . . . . . . . . . . . . . . . 53 ..................................53
6.3.2. Pipelining . . . . . . . . . . . . . . . . . . . . . . 53 .........................................54
6.4. Concurrency . . . . . . . . . . . . . . . . . . . . . . . 54 ...............................................55
6.5. Failures and Time-outs . . . . . . . . . . . . . . . . . . 54 Timeouts .....................................55
6.6. Tear-down . . . . . . . . . . . . . . . . . . . . . . . . 55 .................................................56
6.7. Upgrade . . . . . . . . . . . . . . . . . . . . . . . . . 56 ...................................................57
7. ABNF list extension: List Extension: #rule . . . . . . . . . . . . . . . . . . 58 .....................................59
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 60 ............................................61
8.1. Header Field Registration . . . . . . . . . . . . . . . . 60 .................................61
8.2. URI Scheme Registration . . . . . . . . . . . . . . . . . 60 ...................................62
8.3. Internet Media Type Registration . . . . . . . . . . . . . 61 ..........................62
8.3.1. Internet Media Type message/http . . . . . . . . . . . 61 ...................62
8.3.2. Internet Media Type application/http . . . . . . . . . 62 ...............63
8.4. Transfer Coding Registry . . . . . . . . . . . . . . . . . 63 ..................................64
8.4.1. Procedure . . . . . . . . . . . . . . . . . . . . . . 63 ..........................................65
8.4.2. Registration . . . . . . . . . . . . . . . . . . . . . 64 .......................................65
8.5. Content Coding Registration . . . . . . . . . . . . . . . 64 ...............................66
8.6. Upgrade Token Registry . . . . . . . . . . . . . . . . . . 65 ....................................66
8.6.1. Procedure . . . . . . . . . . . . . . . . . . . . . . 65 ..........................................66
8.6.2. Upgrade Token Registration . . . . . . . . . . . . . . 66 .........................67
9. Security Considerations . . . . . . . . . . . . . . . . . . . 66 ........................................67
9.1. Establishing Authority . . . . . . . . . . . . . . . . . . 66 ....................................67
9.2. Risks of Intermediaries . . . . . . . . . . . . . . . . . 67 ...................................68
9.3. Attacks via Protocol Element Length . . . . . . . . . . . 68 .......................69
9.4. Response Splitting . . . . . . . . . . . . . . . . . . . . 68 ........................................69
9.5. Request Smuggling . . . . . . . . . . . . . . . . . . . . 69 .........................................70
9.6. Message Integrity . . . . . . . . . . . . . . . . . . . . 69 .........................................70
9.7. Message Confidentiality . . . . . . . . . . . . . . . . . 70 ...................................71
9.8. Privacy of Server Log Information . . . . . . . . . . . . 70 .........................71
10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 71 ...............................................72
11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 72 ....................................................74
11.1. Normative References . . . . . . . . . . . . . . . . . . . 72 .....................................74
11.2. Informative References . . . . . . . . . . . . . . . . . . 74 ...................................75
Appendix A. HTTP Version History . . . . . . . . . . . . . . . . 76 ..................................78
A.1. Changes from HTTP/1.0 . . . . . . . . . . . . . . . . . . 77 ....................................78
A.1.1. Multi-homed Multihomed Web Servers . . . . . . . . . . . . . . . 77 ............................78
A.1.2. Keep-Alive Connections . . . . . . . . . . . . . . . . 77 ............................79
A.1.3. Introduction of Transfer-Encoding . . . . . . . . . . 78 .................79
A.2. Changes from RFC 2616 . . . . . . . . . . . . . . . . . . 78 ....................................80
Appendix B. Collected ABNF . . . . . . . . . . . . . . . . . . . 80
Appendix C. Change Log (to be removed by RFC Editor before
publication) . . . . . . . . . . . . . . . . . . . . 82
C.1. Since RFC 2616 . . . . . . . . . . . . . . . . . . . . . . 82
C.2. Since draft-ietf-httpbis-p1-messaging-24 . . . . . . . . . 83
C.3. Since draft-ietf-httpbis-p1-messaging-25 . . . . . . . . . 83 ........................................82
Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84 .............................................................85

1. Introduction

The Hypertext Transfer Protocol (HTTP) is a stateless 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

1. "Message Syntax and Routing

RFC xxx2: Semantics Routing" (this document)

2. "Semantics and Content

RFC xxx3: Conditional Requests

RFC xxx4: Range Requests

RFC xxx5: Caching

RFC xxx6: Authentication Content" [RFC7231]

3. "Conditional Requests" [RFC7232]

4. "Range Requests" [RFC7233]

5. "Caching" [RFC7234]

6. "Authentication" [RFC7235]

This HTTP/1.1 specification obsoletes RFC 2616 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
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 Requirements 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 a list extension, defined in Section 7,
that allows for compact definition of comma-separated lists using a
'#' operator (similar to how the '*' operator indicates repetition).
Appendix B shows the collected grammar with all list operators
expanded to standard ABNF notation.

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 (WWW) architecture and has
evolved over time to support the scalability needs of a worldwide
hypertext system. Much of that architecture is reflected in the
terminology and syntax productions used to define HTTP.

2.1. Client/Server Messaging

HTTP is a stateless request/response protocol that operates by
exchanging messages (Section 3) across a reliable transport transport- or
session-layer "connection" (Section 6). An HTTP "client" is a
program that establishes a connection to a server for the purpose of
sending one or more HTTP requests. An HTTP "server" is a program
that accepts connections in order to service HTTP requests by sending
HTTP responses.

The terms client "client" and server "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. The term
"user agent" refers to any of the various client programs that
initiate a request, including (but not limited to) browsers, spiders
(web-based robots), command-line tools, custom applications, and
mobile apps. The term "origin server" refers to the program that can
originate authoritative responses for a given target resource. The
terms "sender" and "recipient" refer to any implementation that sends
or receives a given message, respectively.

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]
[RFC7231] for the differences between HTTP and MIME messages).

Most HTTP communication consists of a retrieval request (GET) for a
representation of some resource identified by a URI. In the simplest
case, this might be accomplished via a single bidirectional
connection (===) between the user agent (UA) and the origin
server (O).

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

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

A server responds to a client's request by sending one or more HTTP
response messages, each beginning with a status line that includes
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 (Section 4.3.1 of [Part2]) [RFC7231]) 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,
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
video-delivery platforms.

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 not all user agents
can make interactive suggestions to their user or provide adequate
warning for security or privacy concerns. 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 endpoints 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.

The terms "upstream" and "downstream" are used to describe
directional requirements in relation to the message flow: all
messages flow from upstream to downstream. The terms inbound "inbound" and
outbound
"outbound" are used to describe directional requirements in relation
to the request route: "inbound" means toward the origin server and
"outbound" means toward the user agent.

A "proxy" is a message forwarding 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.
Some proxies are designed to apply transformations to selected
messages or payloads while they are being forwarded, as described in
Section 5.7.2.

A "gateway" (a.k.a., (a.k.a. "reverse proxy") is an intermediary that acts as
an origin server for the outbound connection, 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.

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 are
indistinguishable (at a protocol level) from a man-in-the-middle
attack, often introducing security flaws or interoperability problems
due to mistakenly 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.

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 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.,
[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
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
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].
[RFC7234].

There are is 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).

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 that 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
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,
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
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.

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
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
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.

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]). [RFC7231]).
URI references are used to target requests, indicate redirects, and
define relationships.

The definitions of "URI-reference", "absolute-URI", "relative-part",
"scheme", "authority", "port", "host", "path-abempty", "segment",
"query", and "fragment" are adopted from the URI generic syntax. An
"absolute-path" rule is defined for protocol elements that can
contain a non-empty path component. (This rule differs slightly from
RFC 3986's
the path-abempty rule, rule of RFC 3986, which allows for an empty path to
be used in references, and path-absolute rule, which does not allow
paths that begin with "//".) A "partial-URI" rule is defined for
protocol elements that can contain a relative URI but not a fragment
component.

URI-reference = <URI-reference, defined in see [RFC3986], Section 4.1>
absolute-URI = <absolute-URI, defined in see [RFC3986], Section 4.3>
relative-part = <relative-part, defined in see [RFC3986], Section 4.2>
scheme = <scheme, defined in see [RFC3986], Section 3.1>
authority = <authority, defined in see [RFC3986], Section 3.2>
uri-host = <host, defined in see [RFC3986], Section 3.2.2>
port = <port, defined in see [RFC3986], Section 3.2.3>
path-abempty = <path-abempty, defined in see [RFC3986], Section 3.3>
segment = <segment, defined in see [RFC3986], Section 3.3>
query = <query, defined in see [RFC3986], Section 3.4>
fragment = <fragment, defined in see [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),
(absolute-URI), only the path and optional query components, or some
combination of the above. Unless otherwise indicated, URI references
are parsed relative to the effective request URI (Section 5.5).

2.7.1. http URI scheme 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 origin server for an "http" URI is identified by the authority
component, which includes a host identifier and optional TCP port
([RFC3986], Section 3.2.2). The hierarchical path component and
optional query component serve as an identifier for a potential
target resource within that origin server's name space. The optional
fragment component allows for indirect identification of a secondary
resource, independent of the URI scheme, as defined in Section 3.5 of
[RFC3986].

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, the origin
server is the listener (if any) on the indicated TCP port at that IP
address. If host is a registered name, the registered name is an
indirect identifier for use with a name resolution service, such as
DNS, to find an address for that origin server. If the port
subcomponent is empty or not given, TCP port 80 (the reserved port
for WWW services) is the default.

Note that the presence of a URI with a given authority component does
not imply that there is always an HTTP server listening for
connections on that host and port. Anyone can mint a URI. What the
authority component determines is who has the right to respond
authoritatively to requests that target the identified resource. The
delegated nature of registered names and IP addresses creates a
federated namespace, based on control over the indicated host and
port, whether or not an HTTP server is present. See Section 9.1 for
security considerations related to establishing authority.

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], [RFC7231], then
that response is considered an authoritative answer to the client's
request.

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

The "https" URI scheme is hereby defined for the purpose of minting
identifiers according to their association with the hierarchical
namespace governed by a potential HTTP origin server listening to a
given TCP port for TLS-secured connections ([RFC5246]).

All of the requirements listed above for the "http" scheme are also
requirements for the "https" scheme, except that TCP port 443 is the
default 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 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 namespaces and
are considered to be distinct origin servers. However, an extension
to HTTP that is defined to apply to entire host domains, such as the
Cookie protocol [RFC6265], can allow information set by one service
to impact communication with other services within a matching group
of host domains.

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

2.7.3. http and https URI Normalization and Comparison

Since the "http" and "https" schemes conform to the URI generic
syntax, such URIs are normalized and compared according to the
algorithm defined in Section 6 of [RFC3986], 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
percent-encoded octets: the normal form is to not encode them (see
Sections 2.1 and 2.2 of [RFC3986]).

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
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 either 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
but, in practice practice, servers are implemented to only expect a request (a
response is interpreted as an unknown or invalid request method) and
clients are implemented to only expect a response.

start-line = request-line / status-line

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 ends 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], [RFC7231], 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 predefined limit on the length of a request-
line,
request-line, as described in Section 2.5. A server that receives a
method longer than any that it implements SHOULD respond with a 501
(Not Implemented) status code. A server that receives a
request-target longer than any URI it wishes to parse MUST respond
with a 414 (URI Too Long) status code (see Section 6.5.12 of [Part2]).
[RFC7231]).

Various ad-hoc ad hoc limitations on request-line length are found in
practice. It is RECOMMENDED that all HTTP senders and recipients
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 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] [RFC7231] 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 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 = field-vchar [ 1*( SP / HTAB ) field-vchar ]
field-vchar = VCHAR / obs-text

obs-fold = CRLF 1*( 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] [RFC7231] as containing
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 this document
set.

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,
"Message Headers" registry, as described in Section 8.3 of
[Part2]. [RFC7231].

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 NOT
apply a request to the target resource until the entire request
header section is received, 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
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 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

Messages are parsed using a generic algorithm, independent of the
individual header field names. The contents within a given field
value are not parsed until a later stage of message interpretation
(usually after the message's entire header section has been
processed). Consequently, this specification does not use ABNF rules
to define each "Field-Name: Field Value" pair, as was done in
previous editions. Instead, this specification uses ABNF rules which that
are named according to each registered field name, wherein the rule
defines the valid grammar for that field's corresponding field values
(i.e., after the field-value has been extracted from the header
section by a generic field parser).

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
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 might be preceded and/or followed by optional
whitespace (OWS); a single SP preceding the field-value is preferred
for consistent readability by humans. The field value does not
include any leading or trailing white space: whitespace: OWS occurring before the
first non-whitespace octet of the field value or after the last non-
whitespace
non-whitespace octet of the field value ought to be excluded by
parsers when extracting the field value from a header field.

Historically, HTTP header field values could be extended over
multiple lines by preceding each extra line with at least one space
or horizontal tab (obs-fold). This specification deprecates such
line folding except within the message/http media type
(Section 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
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-
8859-1 [ISO-8859-1] charset,
ISO-8859-1 charset [ISO-8859-1], 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) (obs-text) as opaque data.

3.2.5. Field Limits

HTTP does not place a pre-defined predefined 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 ad hoc limitations on individual header
field length are found in practice, often depending on the specific
field semantics.

A server that receives a request header field, or set of fields,
larger than it wishes to process MUST respond with an appropriate 4xx
(Client Error) status code. Ignoring such header fields would
increase the server's vulnerability to request smuggling attacks
(Section 9.5).

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 Value Components

Most HTTP header field values are defined using common syntax
components (token, quoted-string, and comment) separated by
whitespace or specific delimiting characters. Delimiters are chosen
from the set of US-ASCII visual characters not allowed in a token
(DQUOTE and "(),/:;<=>?@[\]{}").

token = 1*tchar

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

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

quoted-string = DQUOTE *( qdtext / quoted-pair ) DQUOTE
qdtext = HTAB / SP /%x21 / %x23-5B / %x5D-7E / obs-text
obs-text = %x80-FF

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-pair / 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 quoted-string and comment constructs. 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.

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

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. A sender SHOULD NOT generate a quoted-pair in a comment
except where necessary to quote parentheses ["(" and ")"] and
backslash octets occurring within that comment.

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

The presence of a message body in a response depends on both the
request method to which it is responding and the response status code
(Section 3.1.2). Responses to the HEAD request method (Section 4.3.2
of [Part2]) [RFC7231]) 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 4.3.1 of [Part2]). [RFC7231]). 2xx
(Successful) responses to a CONNECT request method (Section 4.3.6 of
[Part2])
[RFC7231]) switch to tunnel mode instead of having a message body.
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
(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
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
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.

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 [RFC7231]),
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 can 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]) [RFC7232]) 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]).
[RFC7231]).

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).

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]). [RFC7231]).

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]); [RFC7231]); 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]);
[RFC7232]); 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]).
[RFC7231]).

Aside from the cases defined above, in the absence of Transfer-
Encoding,
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.
media-type.

3.3.3. Message Body Length

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

1. Any response to a HEAD request and any response with a 1xx
(Informational), 204 (No Content), or 304 (Not Modified) status
code is always terminated by the first empty line after the
header fields, regardless of the header fields present in the
message, and thus cannot contain a message body.

2. Any 2xx (Successful) response to a CONNECT request implies that
the connection will become a tunnel immediately after the empty
line that concludes the header fields. A client MUST ignore any
Content-Length or Transfer-Encoding header fields received in
such a message.

3. If a Transfer-Encoding header field is present and the chunked
transfer coding (Section 4.1) is the final encoding, the message
body length is determined by reading and decoding the chunked
data until the transfer coding indicates the data is complete.

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

If a message is received with both a Transfer-Encoding and a
Content-Length header field, the Transfer-Encoding overrides the
Content-Length. Such a message might indicate an attempt to
perform request smuggling (Section 9.5) or response splitting
(Section 9.4) and 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. 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.

Since there is no way to distinguish a successfully completed, close-
delimited
close-delimited message from a partially-received partially received message interrupted
by network failure, a server SHOULD generate encoding or length-
delimited
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 timeout exception, MAY send an
error response prior to closing the connection.

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]. [RFC7234].

If a response terminates in the middle of the header section (before
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
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. However, lenient parsing can
result in security vulnerabilities if there are multiple recipients
of the message and each has its own unique interpretation of
robustness (see Section 9.5).

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.

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 a name or name=value pair.

transfer-parameter = token BWS "=" BWS ( token / quoted-string )

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
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

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
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-string

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.

trailer-part = *( header-field CRLF )

A sender MUST NOT generate a trailer that contains a field necessary
for message framing (e.g., Transfer-Encoding and Content-Length),
routing (e.g., Host), request modifiers (e.g., controls and
conditionals in Section 5 of [Part2]), [RFC7231]), authentication (e.g., see
[Part7]
[RFC7235] and [RFC6265]), response control data (e.g., see Section
7.1 of [Part2]), [RFC7231]), or determining how to process the payload (e.g.,
Content-Encoding, Content-Type, Content-Range, and Trailer).

When a chunked message containing a non-empty trailer is received,
the recipient MAY process the fields (aside from those forbidden
above) as if they were appended to the message's header section. A
recipient MUST ignore (or consider as an error) any fields that are
forbidden to be sent in a trailer, since processing them as if they
were present in the header section might bypass external security
filters.

Unless the request includes a TE header field indicating "trailers"
is acceptable, as described in Section 4.3, a server SHOULD NOT
generate trailer fields that it believes are necessary for the user
agent to receive. Without a TE containing "trailers", the server
ought to assume that the trailer fields might be silently discarded
along the path to the user agent. This requirement allows
intermediaries to forward a de-chunked message to an HTTP/1.0
recipient without buffering the entire response.

4.1.3. Decoding Chunked

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

length := 0
read chunk-size, chunk-ext (if any), and CRLF
while (chunk-size > 0) {
read chunk-data and CRLF
append chunk-data to decoded-body
length := length + chunk-size
read chunk-size, chunk-ext (if any), and CRLF
}
read trailer field
while (trailer field is not empty) {
if (trailer field is allowed to be sent in a trailer) {
append trailer field to existing header fields
}
read trailer-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 non-conformant 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 32-bit Cyclic Redundancy
Check (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
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]). [RFC7231]). 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.

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
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], [RFC7231], 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] [RFC7234] and the request can be satisfied
by it, then the request is usually directed there first.

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

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

HTTP requirements regarding connection management are defined in
Section 6.

5.3. Request Target

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

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

5.3.1. origin-form

The most common form of request-target is the origin-form.

origin-form = absolute-path [ "?" query ]

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, 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.

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.

5.3.2. 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.

absolute-form = absolute-URI

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.

5.3.3. authority-form

The authority-form of request-target is only used for CONNECT
requests (Section 4.3.6 of [Part2]). [RFC7231]).

authority-form = authority

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

5.3.4. asterisk-form

The asterisk-form of request-target is only used for a server-wide
OPTIONS request (Section 4.3.7 of [Part2]). [RFC7231]).

asterisk-form = "*"

When a client wishes to request OPTIONS for the server as a whole, as
opposed to a specific named resource of that server, the client MUST
send only "*" (%x2A) as the request-target. For example,

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
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,
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.
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

Since the request-target often contains only part of the user agent's
target URI, a server reconstructs the intended target as an
"effective request URI" to properly service the request. This
reconstruction involves both the server's local configuration and
information communicated in the request-target, Host header field,
and connection context.

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

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

If the server's configuration (or outbound gateway) provides a
fixed URI scheme, that scheme is used for the effective request
URI. Otherwise, if the request is received over a TLS-secured TCP
connection, the effective request URI's scheme is "https"; if not,
the scheme is "http".

If the server's configuration (or outbound gateway) provides a
fixed URI authority component, that authority is used for the
effective request URI. If not, then if the request-target is in
authority-form, the effective request URI's authority component is
the same as the request-target. If not, then if a Host header
field is supplied with a non-empty field-value, the authority
component is the same as the Host field-value. Otherwise, the
authority component is assigned the default name configured for
the server and, if the connection's incoming TCP port number
differs from the default port for the effective request URI's
scheme, then a colon (":") and the incoming port number (in
decimal form) are appended to the authority component.

If the request-target is in authority-form or asterisk-form, 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

Recipients of an HTTP/1.0 request that lacks a Host header field
might need 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.

Once the effective request URI has been constructed, an origin server
needs to decide whether or not to provide service for that URI via
the connection in which the request was received. For example, the
request might have been misdirected, deliberately or accidentally,
such that the information within a received request-target or Host
header field differs from the host or port upon which the connection
has been made. If the connection is from a trusted gateway, that
inconsistency might be expected; otherwise, it might indicate an
attempt to bypass security filters, trick the server into delivering
non-public content, or poison a cache. See Section 9 for security
considerations regarding message routing.

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])
[RFC7231]) 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)
(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.

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 portion of the field value 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
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:

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 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.

An HTTP-to-HTTP proxy is called a "transforming proxy" if it is
designed or configured to modify 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 a privacy filter. Such transformations are
presumed to be desired by whichever client (or client organization)
selected the proxy.

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 the request-target contains 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 MAY modify the message body through application or removal of
a transfer coding (Section 4).

A proxy MUST NOT transform the payload (Section 3.3 of [Part2]) [RFC7231]) of
a message that contains a no-transform cache-control directive
(Section 5.2 of [Part6]). [RFC7234]).

A proxy MAY transform the payload of a message that does not contain
a no-transform cache-control directive. A proxy that transforms a
payload MUST add a Warning header field with the warn-code of 214
("Transformation Applied") if one is not already in the message (see
Section 5.5 of [Part6]). [RFC7234]). A proxy that transforms the payload of a
200 (OK) response can further 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]). [RFC7231]).

A proxy SHOULD NOT modify header fields that provide information
about the end points endpoints of the communication chain, the resource state,
or the selected representation (other than the payload) unless the
field's definition specifically allows such modification or the
modification is deemed necessary for privacy or security.

6. Connection Management

HTTP messaging is independent of the underlying transport transport- or session-
layer
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 denial-of-service attacks.

6.1. Connection

The "Connection" header field allows the sender to indicate desired
control options for the current connection. In order to avoid
confusing downstream recipients, a proxy or gateway MUST remove or
replace any received connection options before forwarding the
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" 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]). [RFC7234]).

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.

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

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

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

o The connection will close after the current response.

A client MAY send additional requests on a persistent connection
until it sends or receives a close "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).

See Appendix A.1.2 for more information on backward backwards 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]). [RFC7231]). A proxy MUST NOT automatically retry non-
idempotent
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
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]), [RFC7231]),
but it MUST send the corresponding responses in the same order that
the requests were received.

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]) [RFC7231]) 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 ought to limit the number of simultaneous open connections
that it maintains to a given server.

Previous revisions of HTTP gave a specific number of connections as a
ceiling, but this was found to be impractical for many applications.
As a result, this specification does not mandate a particular maximum
number of connections, but instead 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
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 a server might reject traffic that it deems abusive or
characteristic of a denial of service denial-of-service attack, such as an excessive
number of open connections from a single client.

6.5. Failures and Time-outs Timeouts

Servers will usually have some time-out timeout 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 proxy server. The use of
persistent connections places no requirements on the length (or
existence) of this time-out timeout for either the client or the server.

A client or server that wishes to time-out 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 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
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 "close" connection option MUST NOT send further
requests on that connection (after the one containing close) "close") and
MUST close the connection after reading the final response message
corresponding to this request.

A server that receives a close "close" connection option MUST initiate a
close of the connection (see below) after it sends the final response
to the request that contained close. "close". The server SHOULD send a close
"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 "close" connection option MUST initiate a close
of the connection (see below) after it sends the response containing
close.
"close". The server MUST NOT process any further requests received
on that connection.

A client that receives a close "close" connection option MUST cease sending
requests on that connection and close the connection after reading
the response message containing the close; "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 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
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 cannot be used to insist on a protocol
change.

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

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
(Switching Protocols) 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. 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 an Upgrade and an Expect header field with
the "100-continue" expectation (Section 5.1.1 of [Part2]), [RFC7231]), 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
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]). [RFC7231]).

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: 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
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).

In any production that uses the list construct, a sender MUST NOT
generate empty list elements. In other words, a sender MUST generate
lists that satisfy the following syntax:

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 denial-of-service mechanism. In other words,
a recipient MUST accept lists that satisfy the following syntax:

#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

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 for recipients after the list
constructs have been expanded.

8. IANA Considerations

8.1. Header Field Registration

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

This document defines the following HTTP header fields, so their
associated the
"Permanent Message Header Field Names" registry entries shall be has been updated according to the
permanent registrations below
accordingly (see [BCP90]): [BCP90]).

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

The change controller is: "IETF (iesg@ietf.org) - 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: "Permanent
URI Schemes" registry has been updated accordingly.

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 has been
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: N/A

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: see Section 9

Interoperability considerations: N/A

Published specification: This specification (see Section 8.3.1).

Applications that use this media type: N/A

Fragment identifier considerations: N/A

Additional information:

Magic number(s): N/A

Deprecated alias names for this type: N/A

File extension(s): N/A

Macintosh file type code(s): N/A

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

Intended usage: COMMON

Restrictions on usage: N/A

Author: See Authors Section. Authors' Addresses 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).

Type name: application

Subtype name: http

Required parameters: N/A

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. email.

Security considerations: see Section 9

Interoperability considerations: N/A

Published specification: This specification (see Section 8.3.2).

Applications that use this media type: N/A

Fragment identifier considerations: N/A

Additional information:

Deprecated alias names for this type: N/A

Magic number(s): N/A

File extension(s): N/A

Macintosh file type code(s): N/A

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

Intended usage: COMMON

Restrictions on usage: N/A

Author: See Authors Section. Authors' Addresses section.

Change controller: IESG

8.4. Transfer Coding Registry

The HTTP "HTTP Transfer Coding Registry Registry" defines the name space namespace 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]) [RFC7231]) 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 namespace 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 "HTTP Transfer Coding Registry shall be Registry" has been updated with the
registrations below:

8.5. Content Coding Registration

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

The HTTP "HTTP Content Codings Registry shall be Coding Registry" has been updated with the
registrations below:

8.6. Upgrade Token Registry

The HTTP "Hypertext Transfer Protocol (HTTP) Upgrade Token Registry Registry"
defines the name space namespace for protocol-
name 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.

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 upgrade token registry has been updated with
the registration below:

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

9. Security Considerations

This section is meant to inform developers, information providers,
and users of known security considerations relevant to HTTP message
syntax, parsing, and routing. Security considerations about HTTP
semantics and payloads are addressed in [Part2]. [RFC7231].

9.1. Establishing Authority

HTTP relies on the notion of an authoritative response: a response
that has been determined by (or at the direction of) the authority
identified within the target URI to be the most appropriate response
for that request given the state of the target resource at the time
of response message origination. Providing a response from a non-
authoritative
non-authoritative source, such as a shared cache, is often useful to
improve performance and availability, but only to the extent that the
source can be trusted or the distrusted response can be safely used.

Unfortunately, establishing authority can be difficult. For example,
phishing is an attack on the user's perception of authority, where
that perception can be misled by presenting similar branding in
hypertext, possibly aided by userinfo obfuscating the authority
component (see Section 2.7.1). User agents can reduce the impact of
phishing attacks by enabling users to easily inspect a target URI
prior to making an action, by prominently distinguishing (or
rejecting) userinfo when present, and by not sending stored
credentials and cookies when the referring document is from an
unknown or untrusted source.

When a registered name is used in the authority component, the "http"
URI scheme (Section 2.7.1) relies on the user's local name resolution
service to determine where it can find authoritative responses. This
means that any attack on a user's network host table, cached names,
or name resolution libraries becomes an avenue for attack on
establishing authority. Likewise, the user's choice of server for
Domain Name Service (DNS), and the hierarchy of servers from which it
obtains resolution results, could impact the authenticity of address
mappings; DNSSEC ([RFC4033]) is DNS Security Extensions (DNSSEC, [RFC4033]) are one way to
improve authenticity.

Furthermore, after an IP address is obtained, establishing authority
for an "http" URI is vulnerable to attacks on Internet Protocol
routing.

The "https" scheme (Section 2.7.2) is intended to prevent (or at
least reveal) many of these potential attacks on establishing
authority, provided that the negotiated TLS connection is secured and
the client properly verifies that the communicating server's identity
matches the target URI's authority component (see [RFC2818]).
Correctly implementing such verification can be difficult (see
[Georgiev]).

9.2. Risks of Intermediaries

By their very nature, HTTP intermediaries are men-in-the-middle, and
thus men-in-the-middle and,
thus, 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 might have
access to security-related information, personal information about
individual users and organizations, and proprietary information
belonging to users and content providers. A compromised
intermediary, or an intermediary implemented or configured without
regard to security and privacy considerations, might be used in the
commission of a wide range of potential attacks.

Intermediaries that contain a shared cache are especially vulnerable
to cache poisoning attacks, as described in Section 8 of [Part6]. [RFC7234].

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. Attacks via Protocol Element Length

Because HTTP uses mostly textual, character-delimited fields, parsers
are often vulnerable to attacks based on sending very long (or very
slow) streams of data, particularly where an implementation is
expecting a protocol element with no predefined length.

To promote interoperability, specific recommendations are made 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.

A server can reject a message that has a request-target that is too
long (Section 6.5.12 of [Part2]) [RFC7231]) or a request payload that is too
large (Section 6.5.11 of [Part2]). [RFC7231]). 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 process
other protocol elements, including (but not limited to) request
methods, response status phrases, header field-names, numeric values,
and body chunks. Failure to limit such processing can result in
buffer overflows, arithmetic overflows, or increased vulnerability to
denial of service
denial-of-service attacks.

9.4. Response Splitting

Response splitting (a.k.a, CRLF injection) is a common technique,
used in various attacks on Web usage, that exploits the line-based
nature of HTTP message framing and the ordered association of
requests to responses on persistent connections [Klein]. This
technique can be particularly damaging when the requests pass through
a shared cache.

Response splitting exploits a vulnerability in servers (usually
within an application server) where an attacker can send encoded data
within some parameter of the request that is later decoded and echoed
within any of the response header fields of the response. If the
decoded data is crafted to look like the response has ended and a
subsequent response has begun, the response has been split and the
content within the apparent second response is controlled by the
attacker. The attacker can then make any other request on the same
persistent connection and trick the recipients (including
intermediaries) into believing that the second half of the split is
an authoritative answer to the second request.

For example, a parameter within the request-target might be read by
an application server and reused within a redirect, resulting in the
same parameter being echoed in the Location header field of the
response. If the parameter is decoded by the application and not
properly encoded when placed in the response field, the attacker can
send encoded CRLF octets and other content that will make the
application's single response look like two or more responses.

A common defense against response splitting is to filter requests for
data that looks like encoded CR and LF (e.g., "%0D" and "%0A").
However, that assumes the application server is only performing URI
decoding, rather than more obscure data transformations like charset
transcoding, XML entity translation, base64 decoding, sprintf
reformatting, etc. A more effective mitigation is to prevent
anything other than the server's core protocol libraries from sending
a CR or LF within the header section, which means restricting the
output of header fields to APIs that filter for bad octets and not
allowing application servers to write directly to the protocol
stream.

9.5. Request Smuggling

Request smuggling ([Linhart]) is a technique that exploits
differences in protocol parsing among various recipients to hide
additional requests (which might otherwise be blocked or disabled by
policy) within an apparently harmless request. Like response
splitting, request smuggling can lead to a variety of attacks on HTTP
usage.

This specification has introduced new requirements on request
parsing, particularly with regard to message framing in
Section 3.3.3, to reduce the effectiveness of request smuggling.

9.6. 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-
delimited
chunk-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.

9.7. Message Confidentiality

HTTP relies on underlying transport protocols to provide message
confidentiality when that is desired. HTTP has been specifically
designed to be independent of the transport protocol, such that it
can be used over many different forms of encrypted connection, with
the selection of such transports being identified by the choice of
URI scheme or within user agent configuration.

The "https" scheme can be used to identify resources that require a
confidential connection, as described in Section 2.7.2.

9.8. Privacy of 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 it 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
user-provided query parameters, as soon as that information is no
longer necessary to support operational needs for security, auditing,
or fraud control.

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:
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. 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, Adrian Cole,
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, Benoit Claise, Bil Corry, Bill Burke, Bjoern
Hoehrmann, Bob Scheifler, Boris Zbarsky, Brett Slatkin, Brian Kell,
Brian McBarron, Brian Pane, Brian Raymor, Brian Smith, Bruce Perens,
Bryce Nesbitt, Cameron Heavon-Jones, Carl Kugler, Carsten Bormann,
Charles Fry, Chris Burdess, Chris Newman, Christian Huitema, 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 Slepak, 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, Jari Arkko, 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, Joel
Jaeggli, 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, Kathleen Moriarty, Keith
Hoffman, Keith Moore, Ken Murchison, Koen Holtman, Konstantin
Voronkov, Kris Zyp, Leif Hedstrom, Lionel Morand, 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, Matthew Kerwin, Max Clark, Menachem Dodge,
Meral Shirazipour, 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, Pete Resnick, Peter Lepeska,
Peter Occil, Peter Saint-Andre, Peter Watkins, Phil Archer, Phil
Hunt, Philippe Mougin, Phillip Hallam-Baker, Piotr Dobrogost, Poul-
Henning Kamp, Preethi Natarajan, Rajeev Bector, Ray Polk, Reto
Bachmann-Gmuer, Richard Barnes, Richard Cyganiak, Rob Trace, 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, Sean
Turner, Sebastien Barnoud, Shane McCarron, Shigeki Ohtsu, Simon
Yarde, Stefan Eissing, Stefan Tilkov, Stefanos Harhalakis, Stephane
Bortzmeyer, Stephen Farrell, Stephen Kent, Stephen Ludin, Stuart
Williams, Subbu Allamaraju, Subramanian Moonesamy, Susan Hares,
Sylvain Hellegouarch, Tapan Divekar, Tatsuhiro Tsujikawa, Tatsuya
Hayashi, Ted Hardie, Ted Lemon, Thomas Broyer, Thomas Fossati, Thomas
Maslen, Thomas Nadeau, 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-26 (work in progress),
February 2014.

[Part4] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext
Transfer Protocol (HTTP/1.1): Conditional Requests",
draft-ietf-httpbis-p4-conditional-26 (work in
progress), February 2014.

[Part5] Fielding, R., Ed., Lafon, Y., Ed., and J. Reschke, Ed.,
"Hypertext Transfer Protocol (HTTP/1.1): Range
Requests", draft-ietf-httpbis-p5-range-26 (work in
progress), February 2014.

[Part6] Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
Ed., "Hypertext Transfer Protocol (HTTP/1.1): Caching",
draft-ietf-httpbis-p6-cache-26 (work in progress),
February 2014.

[Part7] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext
Transfer Protocol (HTTP/1.1): Authentication",
draft-ietf-httpbis-p7-auth-26 (work in progress),
February 2014.

[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., 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.

[RFC7231] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext
Transfer Protocol (HTTP/1.1): Semantics and Content",
RFC 7231, June 2014.

[RFC7232] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext
Transfer Protocol (HTTP/1.1): Conditional Requests",
RFC 7232, June 2014.

[RFC7233] Fielding, R., Ed., Lafon, Y., Ed., and J. Reschke, Ed.,
"Hypertext Transfer Protocol (HTTP/1.1): Range
Requests", RFC 7233, June 2014.

[RFC7234] Fielding, R., Ed., Nottingham, M., Ed., and L. Masinter,
"Uniform Resource Identifier (URI): Generic Syntax",
STD 66, J. Reschke,
Ed., "Hypertext Transfer Protocol (HTTP/1.1): Caching",
RFC 3986, January 2005.

[RFC5234] Crocker, D., 7234, June 2014.

[RFC7235] Fielding, R., Ed. and P. Overell, "Augmented BNF for
Syntax Specifications: ABNF", STD 68, J. Reschke, Ed., "Hypertext
Transfer Protocol (HTTP/1.1): Authentication",
RFC 5234,
January 2008. 7235, June 2014.

[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 High-Performance Data
Compression", IEEE Computer 17(6), June 1984.

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.

[Georgiev] Georgiev, M., Iyengar, S., Jana, S., Anubhai, R.,
Boneh, D., and V. Shmatikov, "The Most Dangerous Code
in the World: Validating SSL Certificates in Non-
browser Software", In Proceedings of the 2012 ACM
Conference on Computer and Communications Security (CCS
'12), pp. 38-49, October 2012,
<http://doi.acm.org/10.1145/2382196.2382204>.

[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.

[Klein] Klein, A., "Divide and Conquer - HTTP Response
Splitting, Web Cache Poisoning Attacks, and Related
Topics", March 2004, <http://packetstormsecurity.com/
papers/general/whitepaper_httpresponse.pdf>.

[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>.

[Linhart] Linhart, C., Klein, A., Heled, R., and S. Orrin, "HTTP
Request Smuggling", June 2005,
<http://www.watchfire.com/news/whitepapers.aspx>.

[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.,
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 since 1990. The first version, later referred
to as HTTP/0.9, was a simple protocol for hypertext data transfer
across the Internet, using 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, allowing for metadata to be
transferred and modifiers placed on the request/response semantics.
However, HTTP/1.0 did not sufficiently take into consideration the
effects of hierarchical proxies, caching, the need for persistent
connections, or name-based virtual hosts. The proliferation of
incompletely-implemented
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
features that can either be safely ignored by an HTTP/1.0 recipient
or only be sent when communicating with a party advertising
conformance with HTTP/1.1.

HTTP/1.1 has been designed to make supporting previous versions easy.
A general-purpose HTTP/1.1 server ought to be able to understand any
valid request in the format of HTTP/1.0, responding appropriately
with an HTTP/1.1 message that only uses features understood (or
safely ignored) by HTTP/1.0 clients. Likewise, an HTTP/1.1 client
can be expected 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 caused by a client failing to
properly encode 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 Multihomed Web Servers

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

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

A.1.2. Keep-Alive Connections

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

Some clients and servers might wish to be compatible with these
previous approaches to persistent connections, by explicitly
negotiating for them with a "Connection: keep-alive" request header
field. However, some experimental implementations of HTTP/1.0
persistent connections are faulty; for example, if 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
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-
sensitive. Additionally, version numbers has have 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 Non-US-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" 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)

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
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)

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 see [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 see [RFC3986], Section 4.3>
absolute-form = absolute-URI
absolute-path = 1*( "/" segment )
asterisk-form = "*"
authority = <authority, defined in see [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-string
chunk-size = 1*HEXDIG
chunked-body = *chunk last-chunk trailer-part CRLF
comment = "(" *( ctext / quoted-pair / comment ) ")"
connection-option = token
ctext = HTAB / SP / %x21-27 ; '!'-'''
/ %x2A-5B ; '*'-'['
/ %x5D-7E ; ']'-'~'
/ obs-text

field-content = field-vchar [ 1*( SP / HTAB ) field-vchar ]
field-name = token
field-value = *( field-content / obs-fold )
field-vchar = VCHAR / obs-text
fragment = <fragment, defined in see [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 1*( SP / HTAB )
obs-text = %x80-FF
origin-form = absolute-path [ "?" query ]

partial-URI = relative-part [ "?" query ]
path-abempty = <path-abempty, defined in see [RFC3986], Section 3.3>
port = <port, defined in see [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
query = <query, defined in see [RFC3986], Section 3.4>
quoted-pair = "\" ( HTAB / SP / VCHAR / obs-text )
quoted-string = DQUOTE *( qdtext / quoted-pair ) DQUOTE

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

scheme = <scheme, defined in see [RFC3986], Section 3.1>
segment = <segment, defined in see [RFC3986], Section 3.3>
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 = token BWS "=" BWS ( token / quoted-string )

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

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"

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"

C.3. Since draft-ietf-httpbis-p1-messaging-25

Closed issues:

o <http://tools.ietf.org/wg/httpbis/trac/ticket/526>: "check media
type registration templates"

o <http://tools.ietf.org/wg/httpbis/trac/ticket/528>: "Redundant
rule quoted-str-nf"

o <http://tools.ietf.org/wg/httpbis/trac/ticket/531>: "IESG ballot
on draft-ietf-httpbis-p1-messaging-25"

o <http://tools.ietf.org/wg/httpbis/trac/ticket/538>: "add
'stateless' to Abstract"

o <http://tools.ietf.org/wg/httpbis/trac/ticket/540>: "clarify ABNF
layering"

o <http://tools.ietf.org/wg/httpbis/trac/ticket/541>: "use of 'word'
ABNF production"

o <http://tools.ietf.org/wg/httpbis/trac/ticket/542>: "improve
introduction of list rule"

o <http://tools.ietf.org/wg/httpbis/trac/ticket/544>: "moving 2616/
2068/2145 to historic"

o <http://tools.ietf.org/wg/httpbis/trac/ticket/549>: "augment
security considerations with pointers to current research"

o <http://tools.ietf.org/wg/httpbis/trac/ticket/551>:
"intermediaries handling trailers"
o <http://tools.ietf.org/wg/httpbis/trac/ticket/552>: "allow privacy
proxies to be conformant"

Index

A
absolute-form (of request-target) 42
accelerator 10
application/http Media Type 62 63
asterisk-form (of request-target) 42 43
authoritative response 66 67
authority-form (of request-target) 42 42-43

B
browser 7

C
cache 11
cacheable 11 12
captive portal 11
chunked (Coding Format) 28, 31, 35 32, 36
client 7
close 50, 55 51, 56
compress (Coding Format) 38
connection 7
Connection header field 50, 55 51, 56
Content-Length header field 30

D
deflate (Coding Format) 38
Delimiters 26 27
downstream 9 10

E
effective request URI 44 45

G
gateway 10
Grammar
absolute-form 41-42 42
absolute-path 16
absolute-URI 16
ALPHA 6
asterisk-form 41-42 41, 43
authority 16
authority-form 41-42 42-43
BWS 24 25
chunk 35 36
chunk-data 35 36
chunk-ext 35-36 36
chunk-ext-name 36
chunk-ext-val 36
chunk-size 35 36
chunked-body 35-36 36
comment 27
Connection 51
connection-option 51
Content-Length 30
CR 6
CRLF 6
ctext 27
CTL 6
DIGIT 6
DQUOTE 6
field-content 22 23
field-name 22, 39 23, 40
field-value 22 23
field-vchar 22 23
fragment 16
header-field 22, 36 23, 37
HEXDIG 6
Host 43 44
HTAB 6
HTTP-message 19
HTTP-name 13 14
http-URI 16 17
HTTP-version 13 14
https-URI 18
last-chunk 35 36
LF 6
message-body 27 28
method 21
obs-fold 22 23
obs-text 27
OCTET 6
origin-form 41 42
OWS 24 25
partial-URI 16
port 16
protocol-name 47
protocol-version 47
pseudonym 47
qdtext 27
query 16
quoted-pair 27
quoted-string 27
rank 38 39
reason-phrase 22
received-by 47
received-protocol 47
request-line 21
request-target 41
RWS 24 25
scheme 16
segment 16
SP 6
start-line 20 21
status-code 22
status-line 22
t-codings 38 39
t-ranking 38 39
tchar 27
TE 38 39
token 27
Trailer 39 40
trailer-part 35-36 37
transfer-coding 35
Transfer-Encoding 28
transfer-extension 35
transfer-parameter 35
Upgrade 56 57
uri-host 16
URI-reference 16
VCHAR 6
Via 47
gzip (Coding Format) 38 39

H
header field 19
header section 19
headers 19
Host header field 43 44
http URI scheme 16 17
https URI scheme 18 17
I
inbound 9
interception proxy 11
intermediary 9

M
Media Type
application/http 62 63
message/http 61 62
message 7
message/http Media Type 61 62
method 21
N
non-transforming proxy 48 49

O
origin server 7
origin-form (of request-target) 41 42
outbound 9 10

P
phishing 66 67
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 39
Trailer header field 39 40
Transfer-Encoding header field 28
transforming proxy 48 49
transparent proxy 11
tunnel 10

U
Upgrade header field 56 57
upstream 9
URI scheme
http 16 17
https 18 17
user agent 7

V
Via header field 47

Authors' Addresses

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

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

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

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