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Internet Engineering Task Force (IETF) R. Fielding, Ed. Request for Comments: 7230 Adobe Obsoletes: 2145, 2616 J. Reschke, Ed. Updates: 2817, 2818 greenbytes Category: Standards Track June 2014 ISSN: 2070-1721

 Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing

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.

Status of This Memo

 This is an Internet Standards Track document.
 This document is a product of the Internet Engineering Task Force
 (IETF).  It represents the consensus of the IETF community.  It has
 received public review and has been approved for publication by the
 Internet Engineering Steering Group (IESG).  Further information on
 Internet Standards is available in Section 2 of RFC 5741.
 Information about the current status of this document, any errata,
 and how to provide feedback on it may be obtained at
 http://www.rfc-editor.org/info/rfc7230.

Fielding & Reschke Standards Track [Page 1] RFC 7230 HTTP/1.1 Message Syntax and Routing June 2014

Copyright Notice

 Copyright (c) 2014 IETF Trust and the persons identified as the
 document authors.  All rights reserved.
 This document is subject to BCP 78 and the IETF Trust's Legal
 Provisions Relating to IETF Documents
 (http://trustee.ietf.org/license-info) in effect on the date of
 publication of this document.  Please review these documents
 carefully, as they describe your rights and restrictions with respect
 to this document.  Code Components extracted from this document must
 include Simplified BSD License text as described in Section 4.e of
 the Trust Legal Provisions and are provided without warranty as
 described in the Simplified BSD License.
 This document may contain material from IETF Documents or IETF
 Contributions published or made publicly available before November
 10, 2008.  The person(s) controlling the copyright in some of this
 material may not have granted the IETF Trust the right to allow
 modifications of such material outside the IETF Standards Process.
 Without obtaining an adequate license from the person(s) controlling
 the copyright in such materials, this document may not be modified
 outside the IETF Standards Process, and derivative works of it may
 not be created outside the IETF Standards Process, except to format
 it for publication as an RFC or to translate it into languages other
 than English.

Table of Contents

 1. Introduction ....................................................5
    1.1. Requirements Notation ......................................6
    1.2. Syntax Notation ............................................6
 2. Architecture ....................................................6
    2.1. Client/Server Messaging ....................................7
    2.2. Implementation Diversity ...................................8
    2.3. Intermediaries .............................................9
    2.4. Caches ....................................................11
    2.5. Conformance and Error Handling ............................12
    2.6. Protocol Versioning .......................................13
    2.7. Uniform Resource Identifiers ..............................16
         2.7.1. http URI Scheme ....................................17
         2.7.2. https URI Scheme ...................................18
         2.7.3. http and https URI Normalization and Comparison ....19
 3. Message Format .................................................19
    3.1. Start Line ................................................20
         3.1.1. Request Line .......................................21
         3.1.2. Status Line ........................................22
    3.2. Header Fields .............................................22

Fielding & Reschke Standards Track [Page 2] RFC 7230 HTTP/1.1 Message Syntax and Routing June 2014

         3.2.1. Field Extensibility ................................23
         3.2.2. Field Order ........................................23
         3.2.3. Whitespace .........................................24
         3.2.4. Field Parsing ......................................25
         3.2.5. Field Limits .......................................26
         3.2.6. Field Value Components .............................27
    3.3. Message Body ..............................................28
         3.3.1. Transfer-Encoding ..................................28
         3.3.2. Content-Length .....................................30
         3.3.3. Message Body Length ................................32
    3.4. Handling Incomplete Messages ..............................34
    3.5. Message Parsing Robustness ................................34
 4. Transfer Codings ...............................................35
    4.1. Chunked Transfer Coding ...................................36
         4.1.1. Chunk Extensions ...................................36
         4.1.2. Chunked Trailer Part ...............................37
         4.1.3. Decoding Chunked ...................................38
    4.2. Compression Codings .......................................38
         4.2.1. Compress Coding ....................................38
         4.2.2. Deflate Coding .....................................38
         4.2.3. Gzip Coding ........................................39
    4.3. TE ........................................................39
    4.4. Trailer ...................................................40
 5. Message Routing ................................................40
    5.1. Identifying a Target Resource .............................40
    5.2. Connecting Inbound ........................................41
    5.3. Request Target ............................................41
         5.3.1. origin-form ........................................42
         5.3.2. absolute-form ......................................42
         5.3.3. authority-form .....................................43
         5.3.4. asterisk-form ......................................43
    5.4. Host ......................................................44
    5.5. Effective Request URI .....................................45
    5.6. Associating a Response to a Request .......................46
    5.7. Message Forwarding ........................................47
         5.7.1. Via ................................................47
         5.7.2. Transformations ....................................49
 6. Connection Management ..........................................50
    6.1. Connection ................................................51
    6.2. Establishment .............................................52
    6.3. Persistence ...............................................52
         6.3.1. Retrying Requests ..................................53
         6.3.2. Pipelining .........................................54
    6.4. Concurrency ...............................................55
    6.5. Failures and Timeouts .....................................55
    6.6. Tear-down .................................................56
    6.7. Upgrade ...................................................57
 7. ABNF List Extension: #rule .....................................59

Fielding & Reschke Standards Track [Page 3] RFC 7230 HTTP/1.1 Message Syntax and Routing June 2014

 8. IANA Considerations ............................................61
    8.1. Header Field Registration .................................61
    8.2. URI Scheme Registration ...................................62
    8.3. Internet Media Type Registration ..........................62
         8.3.1. Internet Media Type message/http ...................62
         8.3.2. Internet Media Type application/http ...............63
    8.4. Transfer Coding Registry ..................................64
         8.4.1. Procedure ..........................................65
         8.4.2. Registration .......................................65
    8.5. Content Coding Registration ...............................66
    8.6. Upgrade Token Registry ....................................66
         8.6.1. Procedure ..........................................66
         8.6.2. Upgrade Token Registration .........................67
 9. Security Considerations ........................................67
    9.1. Establishing Authority ....................................67
    9.2. Risks of Intermediaries ...................................68
    9.3. Attacks via Protocol Element Length .......................69
    9.4. Response Splitting ........................................69
    9.5. Request Smuggling .........................................70
    9.6. Message Integrity .........................................70
    9.7. Message Confidentiality ...................................71
    9.8. Privacy of Server Log Information .........................71
 10. Acknowledgments ...............................................72
 11. References ....................................................74
    11.1. Normative References .....................................74
    11.2. Informative References ...................................75
 Appendix A. HTTP Version History ..................................78
    A.1. Changes from HTTP/1.0  ....................................78
         A.1.1.  Multihomed Web Servers ............................78
         A.1.2.  Keep-Alive Connections ............................79
         A.1.3.  Introduction of Transfer-Encoding .................79
    A.2.  Changes from RFC 2616 ....................................80
 Appendix B. Collected ABNF ........................................82
 Index .............................................................85

Fielding & Reschke Standards Track [Page 4] RFC 7230 HTTP/1.1 Message Syntax and Routing June 2014

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:
 1.  "Message Syntax and Routing" (this document)
 2.  "Semantics and 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

Fielding & Reschke Standards Track [Page 5] RFC 7230 HTTP/1.1 Message Syntax and Routing June 2014

 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. 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.

Fielding & Reschke Standards Track [Page 6] RFC 7230 HTTP/1.1 Message Syntax and Routing June 2014

2.1. Client/Server Messaging

 HTTP is a stateless request/response protocol that operates by
 exchanging messages (Section 3) across a reliable transport- or
 session-layer "connection" (Section 6).  An HTTP "client" is a
 program that establishes a connection to a server for the purpose of
 sending one or more HTTP requests.  An HTTP "server" is a program
 that accepts connections in order to service HTTP requests by sending
 HTTP responses.
 The terms "client" and "server" refer only to the roles that these
 programs perform for a particular connection.  The same program might
 act as a client on some connections and a server on others.  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
 [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).

Fielding & Reschke Standards Track [Page 7] RFC 7230 HTTP/1.1 Message Syntax and Routing June 2014

 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 [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

Fielding & Reschke Standards Track [Page 8] RFC 7230 HTTP/1.1 Message Syntax and Routing June 2014

 units, configurable networking components, office machines,
 autonomous robots, news feeds, traffic cameras, ad selectors, and
 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 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

Fielding & Reschke Standards Track [Page 9] RFC 7230 HTTP/1.1 Message Syntax and Routing June 2014

 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" and
 "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 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. "reverse proxy") is an intermediary that acts as
 an origin server for the outbound connection but translates received
 requests and forwards them inbound to another server or servers.
 Gateways are often used to encapsulate legacy or untrusted
 information services, to improve server performance through
 "accelerator" caching, and to enable partitioning or load balancing
 of HTTP services across multiple machines.
 All HTTP requirements applicable to an origin server also apply to
 the outbound communication of a gateway.  A gateway communicates with
 inbound servers using any protocol that it desires, including private
 extensions to HTTP that are outside the scope of this specification.
 However, an HTTP-to-HTTP gateway that wishes to interoperate with
 third-party HTTP servers ought to conform to user agent requirements
 on the gateway's inbound connection.
 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.

Fielding & Reschke Standards Track [Page 10] RFC 7230 HTTP/1.1 Message Syntax and Routing June 2014

 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 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
                   <             <

Fielding & Reschke Standards Track [Page 11] RFC 7230 HTTP/1.1 Message Syntax and Routing June 2014

 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
 [RFC7234].
 There 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

Fielding & Reschke Standards Track [Page 12] RFC 7230 HTTP/1.1 Message Syntax and Routing June 2014

 forwarding a message might parse a header-field into generic
 field-name and field-value components, but then forward the header
 field without further parsing inside the field-value.
 HTTP does not have specific length limitations for many of its
 protocol elements because the lengths that might be appropriate will
 vary widely, depending on the deployment context and purpose of the
 implementation.  Hence, interoperability between senders and
 recipients depends on shared expectations regarding what is a
 reasonable length for each protocol element.  Furthermore, what is
 commonly understood to be a reasonable length for some protocol
 elements has changed over the course of the past two decades of HTTP
 use and is expected to continue changing in the future.
 At a minimum, a recipient MUST be able to parse and process protocol
 element lengths that are at least as long as the values that it
 generates for those same protocol elements in other messages.  For
 example, an origin server that publishes very long URI references to
 its own resources needs to be able to parse and process those same
 references when received as a request target.
 A recipient MUST interpret a received protocol element according to
 the semantics defined for it by this specification, including
 extensions to this specification, unless the recipient has determined
 (through experience or configuration) that the sender incorrectly
 implements what is implied by those semantics.  For example, an
 origin server might disregard the contents of a received
 Accept-Encoding header field if inspection of the User-Agent header
 field indicates a specific implementation version that is known to
 fail on receipt of certain content codings.
 Unless noted otherwise, a recipient MAY attempt to recover a usable
 protocol element from an invalid construct.  HTTP does not define
 specific error handling mechanisms except when they have a direct
 impact on security, since different applications of the protocol
 require different error handling strategies.  For example, a Web
 browser might wish to transparently recover from a response where the
 Location header field doesn't parse according to the ABNF, whereas a
 systems control client might consider any form of error recovery to
 be dangerous.

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.

Fielding & Reschke Standards Track [Page 13] RFC 7230 HTTP/1.1 Message Syntax and Routing June 2014

 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

Fielding & Reschke Standards Track [Page 14] RFC 7230 HTTP/1.1 Message Syntax and Routing June 2014

 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

Fielding & Reschke Standards Track [Page 15] RFC 7230 HTTP/1.1 Message Syntax and Routing June 2014

 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 [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
 the path-abempty 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, see [RFC3986], Section 4.1>
   absolute-URI  = <absolute-URI, see [RFC3986], Section 4.3>
   relative-part = <relative-part, see [RFC3986], Section 4.2>
   scheme        = <scheme, see [RFC3986], Section 3.1>
   authority     = <authority, see [RFC3986], Section 3.2>
   uri-host      = <host, see [RFC3986], Section 3.2.2>
   port          = <port, see [RFC3986], Section 3.2.3>
   path-abempty  = <path-abempty, see [RFC3986], Section 3.3>
   segment       = <segment, see [RFC3986], Section 3.3>
   query         = <query, see [RFC3986], Section 3.4>
   fragment      = <fragment, 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), 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).

Fielding & Reschke Standards Track [Page 16] RFC 7230 HTTP/1.1 Message Syntax and Routing June 2014

2.7.1. http URI Scheme

 The "http" URI scheme is hereby defined for the purpose of minting
 identifiers according to their association with the hierarchical
 namespace governed by a potential HTTP origin server listening for
 TCP ([RFC0793]) connections on a given port.
   http-URI = "http:" "//" authority path-abempty [ "?" query ]
              [ "#" fragment ]
 The 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

Fielding & Reschke Standards Track [Page 17] RFC 7230 HTTP/1.1 Message Syntax and Routing June 2014

 HTTP response message, as described in Section 6 of [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

 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

Fielding & Reschke Standards Track [Page 18] RFC 7230 HTTP/1.1 Message Syntax and Routing June 2014

 resource identifiers indicate the same authority (the same host
 listening to the same TCP port).  They are distinct 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 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 ]

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 The normal procedure for parsing an HTTP message is to read the
 start-line into a structure, read each header field into a hash table
 by field name until the empty line, and then use the parsed data to
 determine if a message body is expected.  If a message body has been
 indicated, then it is read as a stream until an amount of octets
 equal to the message body length is read or the connection is closed.
 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 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).

Fielding & Reschke Standards Track [Page 20] RFC 7230 HTTP/1.1 Message Syntax and Routing June 2014

 In theory, a client could receive requests and a server could receive
 responses, distinguishing them by their different start-line formats,
 but, in practice, servers are implemented to only expect a request (a
 response is interpreted as an unknown or invalid request method) and
 clients are implemented to only expect a response.
   start-line     = request-line / status-line

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 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 [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 predefined limit on the length of a
 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

Fielding & Reschke Standards Track [Page 21] RFC 7230 HTTP/1.1 Message Syntax and Routing June 2014

 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
 [RFC7231]).
 Various 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 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 [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.

Fielding & Reschke Standards Track [Page 22] RFC 7230 HTTP/1.1 Message Syntax and Routing June 2014

   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 [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 Headers" registry, as described in Section 8.3 of [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

Fielding & Reschke Standards Track [Page 23] RFC 7230 HTTP/1.1 Message Syntax and Routing June 2014

 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 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.

Fielding & Reschke Standards Track [Page 24] RFC 7230 HTTP/1.1 Message Syntax and Routing June 2014

   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 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 whitespace: OWS occurring before the
 first non-whitespace octet of the field value or after the last
 non-whitespace octet of the field value ought to be excluded by
 parsers when extracting the field value from a header field.
 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.

Fielding & Reschke Standards Track [Page 25] RFC 7230 HTTP/1.1 Message Syntax and Routing June 2014

 A server that receives an obs-fold in a request message that is not
 within a message/http container MUST either reject the message by
 sending a 400 (Bad Request), preferably with a representation
 explaining that obsolete line folding is unacceptable, or replace
 each received obs-fold with one or more SP octets prior to
 interpreting the field value or forwarding the message downstream.
 A proxy or gateway that receives an obs-fold in a response message
 that is not within a message/http container MUST either discard the
 message and replace it with a 502 (Bad Gateway) response, preferably
 with a representation explaining that unacceptable line folding was
 received, or replace each received obs-fold with one or more SP
 octets prior to interpreting the field value or forwarding the
 message downstream.
 A user agent that receives an obs-fold in a response message that is
 not within a message/http container MUST replace each received
 obs-fold with one or more SP octets prior to interpreting the field
 value.
 Historically, HTTP has allowed field content with text in the
 ISO-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) as opaque data.

3.2.5. Field Limits

 HTTP does not place a 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 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.

Fielding & Reschke Standards Track [Page 26] RFC 7230 HTTP/1.1 Message Syntax and Routing June 2014

3.2.6. Field 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.

Fielding & Reschke Standards Track [Page 27] RFC 7230 HTTP/1.1 Message Syntax and Routing June 2014

3.3. Message Body

 The message body (if any) of an HTTP message is used to carry the
 payload body of that request or response.  The message body is
 identical to the payload body unless a transfer coding has been
 applied, as described in Section 3.3.1.
   message-body = *OCTET
 The rules for when a message body is allowed in a message differ for
 requests and responses.
 The presence of a message body in a request is signaled by a
 Content-Length or Transfer-Encoding header field.  Request message
 framing is independent of method semantics, even if the method does
 not define 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 [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 [RFC7231]). 2xx
 (Successful) responses to a CONNECT request method (Section 4.3.6 of
 [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.

Fielding & Reschke Standards Track [Page 28] RFC 7230 HTTP/1.1 Message Syntax and Routing June 2014

 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 [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 [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
 [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

Fielding & Reschke Standards Track [Page 29] RFC 7230 HTTP/1.1 Message Syntax and Routing June 2014

 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 [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 [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
 [RFC7232]); a server MUST NOT send Content-Length in such a response

Fielding & Reschke Standards Track [Page 30] RFC 7230 HTTP/1.1 Message Syntax and Routing June 2014

 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
 [RFC7231]).
 Aside from the cases defined above, in the absence of
 Transfer-Encoding, an origin server SHOULD send a Content-Length
 header field when the payload body size is known prior to sending the
 complete header section.  This will allow downstream recipients to
 measure transfer progress, know when a received message is complete,
 and potentially reuse the connection for additional requests.
 Any Content-Length field value greater than or equal to zero is
 valid.  Since there is no predefined limit to the length of a
 payload, a recipient MUST anticipate potentially large decimal
 numerals and prevent parsing errors due to integer conversion
 overflows (Section 9.3).
 If a message is received that has multiple Content-Length header
 fields with field-values consisting of the same decimal value, or a
 single Content-Length header field with a field value containing a
 list of identical decimal values (e.g., "Content-Length: 42, 42"),
 indicating that duplicate Content-Length header fields have been
 generated or combined by an upstream message processor, then the
 recipient MUST either reject the message as invalid or replace the
 duplicated field-values with a single valid Content-Length field
 containing that decimal value prior to determining the message body
 length or forwarding the message.
    Note: HTTP's use of Content-Length for message framing differs
    significantly from the same field's use in MIME, where it is an
    optional field used only within the "message/external-body"
    media-type.

Fielding & Reschke Standards Track [Page 31] RFC 7230 HTTP/1.1 Message Syntax and Routing June 2014

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

Fielding & Reschke Standards Track [Page 32] RFC 7230 HTTP/1.1 Message Syntax and Routing June 2014

     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 message from a partially received message interrupted
 by network failure, a server SHOULD generate encoding or
 length-delimited messages whenever possible.  The close-delimiting
 feature exists primarily for backwards compatibility with HTTP/1.0.
 A server MAY reject a request that contains a message body but not a
 Content-Length by responding with 411 (Length Required).
 Unless a transfer coding other than chunked has been applied, a
 client that sends a request containing a message body SHOULD use a
 valid Content-Length header field if the message body length is known
 in advance, rather than the chunked transfer coding, since some
 existing services respond to chunked with a 411 (Length Required)
 status code even though they understand the chunked transfer coding.
 This is typically because such services are implemented via a gateway
 that requires a content-length in advance of being called and the
 server is unable or unwilling to buffer the entire request before
 processing.
 A user agent that sends a request containing a message body MUST send
 a valid Content-Length header field if it does not know the server
 will handle HTTP/1.1 (or later) requests; such knowledge can be in
 the form of specific user configuration or by remembering the version
 of a prior received response.
 If the final response to the last request on a connection has been
 completely received and there remains additional data to read, a user
 agent MAY discard the remaining data or attempt to determine if that

Fielding & Reschke Standards Track [Page 33] RFC 7230 HTTP/1.1 Message Syntax and Routing June 2014

 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 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 [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, 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.

Fielding & Reschke Standards Track [Page 34] RFC 7230 HTTP/1.1 Message Syntax and Routing June 2014

 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 (Section 3.3.1) header fields.

Fielding & Reschke Standards Track [Page 35] RFC 7230 HTTP/1.1 Message Syntax and Routing June 2014

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 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

Fielding & Reschke Standards Track [Page 36] RFC 7230 HTTP/1.1 Message Syntax and Routing June 2014

 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 [RFC7231]), authentication (e.g., see
 [RFC7235] and [RFC6265]), response control data (e.g., see Section
 7.1 of [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.

Fielding & Reschke Standards Track [Page 37] RFC 7230 HTTP/1.1 Message Syntax and Routing June 2014

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.

Fielding & Reschke Standards Track [Page 38] RFC 7230 HTTP/1.1 Message Syntax and Routing June 2014

4.2.3. Gzip Coding

 The "gzip" coding is an LZ77 coding with a 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

Fielding & Reschke Standards Track [Page 39] RFC 7230 HTTP/1.1 Message Syntax and Routing June 2014

 5.3.1 of [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 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 [RFC7231], and a target resource upon which to apply those
 semantics.  A URI reference (Section 2.7) is typically used as an

Fielding & Reschke Standards Track [Page 40] RFC 7230 HTTP/1.1 Message Syntax and Routing June 2014

 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 [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

Fielding & Reschke Standards Track [Page 41] RFC 7230 HTTP/1.1 Message Syntax and Routing June 2014

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

Fielding & Reschke Standards Track [Page 42] RFC 7230 HTTP/1.1 Message Syntax and Routing June 2014

 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 [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 [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".

Fielding & Reschke Standards Track [Page 43] RFC 7230 HTTP/1.1 Message Syntax and Routing June 2014

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, the proxy MUST ignore the received Host header field
 (if any) and instead replace it with the host information of the
 request-target.  A proxy that forwards such a request MUST generate a
 new Host field-value based on the received request-target rather than
 forward the received Host field-value.
 Since the Host header field acts as an application-level routing
 mechanism, it is a frequent target for malware seeking to poison a
 shared cache or redirect a request to an unintended server.  An
 interception proxy is particularly vulnerable if it relies on the
 Host field-value for redirecting requests to internal servers, or for
 use as a cache key in a shared cache, without first verifying that
 the intercepted connection is targeting a valid IP address for that
 host.

Fielding & Reschke Standards Track [Page 44] RFC 7230 HTTP/1.1 Message Syntax and Routing June 2014

 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.

Fielding & Reschke Standards Track [Page 45] RFC 7230 HTTP/1.1 Message Syntax and Routing June 2014

 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
 [RFC7231]) precede a final response to the same request.

Fielding & Reschke Standards Track [Page 46] RFC 7230 HTTP/1.1 Message Syntax and Routing June 2014

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

5.7. Message Forwarding

 As described in Section 2.3, intermediaries can serve a variety of
 roles in the processing of HTTP requests and responses.  Some
 intermediaries are used to improve performance or availability.
 Others are used for access control or to filter content.  Since an
 HTTP stream has characteristics similar to a pipe-and-filter
 architecture, there are no inherent limits to the extent an
 intermediary can enhance (or interfere) with either direction of the
 stream.
 An intermediary not acting as a tunnel MUST implement the Connection
 header field, as specified in Section 6.1, and exclude fields from
 being forwarded that are only intended for the incoming connection.
 An intermediary MUST NOT forward a message to itself unless it is
 protected from an infinite request loop.  In general, an intermediary
 ought to recognize its own server names, including any aliases, local
 variations, or literal IP addresses, and respond to such requests
 directly.

5.7.1. Via

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

Fielding & Reschke Standards Track [Page 47] RFC 7230 HTTP/1.1 Message Syntax and Routing June 2014

 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, 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.

Fielding & Reschke Standards Track [Page 48] RFC 7230 HTTP/1.1 Message Syntax and Routing June 2014

 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.

Fielding & Reschke Standards Track [Page 49] RFC 7230 HTTP/1.1 Message Syntax and Routing June 2014

 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 [RFC7231]) of
 a message that contains a no-transform cache-control directive
 (Section 5.2 of [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 [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 [RFC7231]).
 A proxy SHOULD NOT modify header fields that provide information
 about the 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- or
 session-layer connection protocol(s).  HTTP only presumes a reliable
 transport with in-order delivery of requests and the corresponding
 in-order delivery of responses.  The mapping of HTTP request and
 response structures onto the data units of an underlying transport
 protocol is outside the scope of this specification.
 As described in Section 5.2, the specific connection protocols to be
 used for an HTTP interaction are determined by client configuration
 and the target URI.  For example, the "http" URI scheme
 (Section 2.7.1) indicates a default connection of TCP over IP, with a
 default TCP port of 80, but the client might be configured to use a
 proxy via some other connection, port, or protocol.

Fielding & Reschke Standards Track [Page 50] RFC 7230 HTTP/1.1 Message Syntax and Routing June 2014

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

6.1. Connection

 The "Connection" header field allows the sender to indicate desired
 control options for the current connection.  In order to avoid
 confusing downstream recipients, a proxy or gateway MUST remove or
 replace any received connection options before forwarding the
 message.
 When a header field aside from Connection is used to supply control
 information for or about the current connection, the sender MUST list
 the corresponding field-name within the Connection header field.  A
 proxy or gateway MUST parse a received Connection header field before
 a message is forwarded and, for each connection-option in this field,
 remove any header field(s) from the message with the same name as the
 connection-option, and then remove the Connection header field itself
 (or replace it with the intermediary's own connection options for the
 forwarded message).
 Hence, the Connection header field provides a declarative way of
 distinguishing header fields that are only intended for the immediate
 recipient ("hop-by-hop") from those fields that are intended for all
 recipients on the chain ("end-to-end"), enabling the message to be
 self-descriptive and allowing future connection-specific extensions
 to be deployed without fear that they will be blindly forwarded by
 older intermediaries.
 The Connection header field's value has the following grammar:
   Connection        = 1#connection-option
   connection-option = token
 Connection options are case-insensitive.
 A sender MUST NOT send a connection option corresponding to a header
 field that is intended for all recipients of the payload.  For
 example, Cache-Control is never appropriate as a connection option
 (Section 5.2 of [RFC7234]).

Fielding & Reschke Standards Track [Page 51] RFC 7230 HTTP/1.1 Message Syntax and Routing June 2014

 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- or session-layer
 protocols.  Each connection applies to only one transport link.

6.3. Persistence

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

Fielding & Reschke Standards Track [Page 52] RFC 7230 HTTP/1.1 Message Syntax and Routing June 2014

 A recipient determines whether a connection is persistent or not
 based on the most recently received message's protocol version and
 Connection header field (if any):
 o  If the "close" connection option is present, the connection will
    not persist after the current response; else,
 o  If the received protocol is HTTP/1.1 (or later), the connection
    will persist after the current response; else,
 o  If the received protocol is HTTP/1.0, the "keep-alive" connection
    option is present, the recipient is not a proxy, and the recipient
    wishes to honor the HTTP/1.0 "keep-alive" mechanism, the
    connection will persist after the current response; otherwise,
 o  The connection will close after the current response.
 A client MAY send additional requests on a persistent connection
 until it sends or receives a "close" connection option or receives an
 HTTP/1.0 response without a "keep-alive" connection option.
 In order to remain persistent, all messages on a connection need to
 have a self-defined message length (i.e., one not defined by closure
 of the connection), as described in Section 3.3.  A server MUST read
 the entire request message body or close the connection after sending
 its response, since otherwise the remaining data on a persistent
 connection would be misinterpreted as the next request.  Likewise, a
 client MUST read the entire response message body if it intends to
 reuse the same connection for a subsequent request.
 A proxy server MUST NOT maintain a persistent connection with an
 HTTP/1.0 client (see Section 19.7.1 of [RFC2068] for information and
 discussion of the problems with the Keep-Alive header field
 implemented by many HTTP/1.0 clients).
 See Appendix A.1.2 for more information on 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.

Fielding & Reschke Standards Track [Page 53] RFC 7230 HTTP/1.1 Message Syntax and Routing June 2014

 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 [RFC7231]).  A proxy MUST NOT automatically retry
 non-idempotent requests.
 A user agent MUST NOT automatically retry a request with a non-
 idempotent method unless it has some means to know that the request
 semantics are actually idempotent, regardless of the method, or some
 means to detect that the original request was never applied.  For
 example, a user agent that knows (through design or configuration)
 that a POST request to a given resource is safe can repeat that
 request automatically.  Likewise, a user agent designed specifically
 to operate on a version control repository might be able to recover
 from partial failure conditions by checking the target resource
 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 [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 [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.

Fielding & Reschke Standards Track [Page 54] RFC 7230 HTTP/1.1 Message Syntax and Routing June 2014

 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, encourages clients to be
 conservative when opening multiple connections.
 Multiple connections are typically used to avoid the "head-of-line
 blocking" problem, wherein a request that takes significant
 server-side processing and/or has a large payload blocks subsequent
 requests on the same connection.  However, each connection consumes
 server resources.  Furthermore, using multiple connections can cause
 undesirable side effects in congested networks.
 Note that a server might reject traffic that it deems abusive or
 characteristic of a denial-of-service attack, such as an excessive
 number of open connections from a single client.

6.5. Failures and Timeouts

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

Fielding & Reschke Standards Track [Page 55] RFC 7230 HTTP/1.1 Message Syntax and Routing June 2014

 A client, server, or proxy MAY close the transport connection at any
 time.  For example, a client might have started to send a new request
 at the same time that the server has decided to close the "idle"
 connection.  From the server's point of view, the connection is being
 closed while it was idle, but from the client's point of view, a
 request is in progress.
 A server SHOULD sustain persistent connections, when possible, and
 allow the underlying transport's flow-control mechanisms to resolve
 temporary overloads, rather than terminate connections with the
 expectation that clients will retry.  The latter technique can
 exacerbate network congestion.
 A client sending a message body SHOULD monitor the network connection
 for an error response while it is transmitting the request.  If the
 client sees a response that indicates the server does not wish to
 receive the message body and is closing the connection, the client
 SHOULD immediately cease transmitting the body and close its side of
 the connection.

6.6. Tear-down

 The Connection header field (Section 6.1) provides a "close"
 connection option that a sender SHOULD send when it wishes to close
 the connection after the current request/response pair.
 A client that sends a "close" connection option MUST NOT send further
 requests on that connection (after the one containing "close") and
 MUST close the connection after reading the final response message
 corresponding to this request.
 A server that receives a "close" connection option MUST initiate a
 close of the connection (see below) after it sends the final response
 to the request that contained "close".  The server SHOULD send a
 "close" connection option in its final response on that connection.
 The server MUST NOT process any further requests received on that
 connection.
 A server that sends a "close" connection option MUST initiate a close
 of the connection (see below) after it sends the response containing
 "close".  The server MUST NOT process any further requests received
 on that connection.
 A client that receives a "close" connection option MUST cease sending
 requests on that connection and close the connection after reading
 the response message containing the "close"; if additional pipelined
 requests had been sent on the connection, the client SHOULD NOT
 assume that they will be processed by the server.

Fielding & Reschke Standards Track [Page 56] RFC 7230 HTTP/1.1 Message Syntax and Routing June 2014

 If a server performs an immediate close of a TCP connection, there is
 a significant risk that the client will not be able to read the last
 HTTP response.  If the server receives additional data from the
 client on a fully closed connection, such as another request that was
 sent by the client before receiving the server's response, the
 server's TCP stack will send a reset packet to the client;
 unfortunately, the reset packet might erase the client's
 unacknowledged input buffers before they can be read and interpreted
 by the client's HTTP parser.
 To avoid the TCP reset problem, servers typically close a connection
 in stages.  First, the server performs a half-close by closing only
 the write side of the read/write connection.  The server then
 continues to read from the connection until it receives a
 corresponding close by the client, or until the server is reasonably
 certain that its own TCP stack has received the client's
 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

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 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.  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.

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 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 [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 [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: #rule

 A #rule extension to the ABNF rules of [RFC5234] is used to improve
 readability in the definitions of some header field values.
 A construct "#" is defined, similar to "*", for defining
 comma-delimited lists of elements.  The full form is "<n>#<m>element"
 indicating at least <n> and at most <m> elements, each separated by a
 single comma (",") and optional whitespace (OWS).

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 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 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.

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8. IANA Considerations

8.1. Header Field Registration

 HTTP header fields are registered within the "Message Headers"
 registry maintained at
 <http://www.iana.org/assignments/message-headers/>.
 This document defines the following HTTP header fields, so the
 "Permanent Message Header Field Names" registry has been updated
 accordingly (see [BCP90]).
 +-------------------+----------+----------+---------------+
 | Header Field Name | Protocol | Status   | Reference     |
 +-------------------+----------+----------+---------------+
 | Connection        | http     | standard | Section 6.1   |
 | Content-Length    | http     | standard | Section 3.3.2 |
 | Host              | http     | standard | Section 5.4   |
 | TE                | http     | standard | Section 4.3   |
 | Trailer           | http     | standard | Section 4.4   |
 | Transfer-Encoding | http     | standard | Section 3.3.1 |
 | Upgrade           | http     | standard | Section 6.7   |
 | Via               | http     | standard | Section 5.7.1 |
 +-------------------+----------+----------+---------------+
 Furthermore, the header field-name "Close" has been registered as
 "reserved", since using that name as an HTTP header field might
 conflict with the "close" connection option of the Connection header
 field (Section 6.1).
 +-------------------+----------+----------+-------------+
 | Header Field Name | Protocol | Status   | Reference   |
 +-------------------+----------+----------+-------------+
 | Close             | http     | reserved | Section 8.1 |
 +-------------------+----------+----------+-------------+
 The change controller is: "IETF (iesg@ietf.org) - Internet
 Engineering Task Force".

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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 the "Permanent
 URI Schemes" registry has been updated accordingly.
 +------------+------------------------------------+---------------+
 | URI Scheme | Description                        | Reference     |
 +------------+------------------------------------+---------------+
 | http       | Hypertext Transfer Protocol        | Section 2.7.1 |
 | https      | Hypertext Transfer Protocol Secure | Section 2.7.2 |
 +------------+------------------------------------+---------------+

8.3. Internet Media Type Registration

 IANA maintains the registry of Internet media types [BCP13] at
 <http://www.iana.org/assignments/media-types>.
 This document serves as the specification for the Internet media
 types "message/http" and "application/http".  The following 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

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 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' Addresses section.
 Intended usage:  COMMON
 Restrictions on usage:  N/A
 Author:  See 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.

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    msgtype:  The message type -- "request" or "response".  If not
       present, the type can be determined from the first line of the
       body.
 Encoding considerations:  HTTP messages enclosed by this type are in
    "binary" format; use of an appropriate Content-Transfer-Encoding
    is required when transmitted via 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' Addresses section.
 Intended usage:  COMMON
 Restrictions on usage:  N/A
 Author:  See Authors' Addresses section.
 Change controller:  IESG

8.4. Transfer Coding Registry

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

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

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8.5. Content Coding Registration

 IANA maintains the "HTTP Content Coding Registry" at
 <http://www.iana.org/assignments/http-parameters>.
 The "HTTP Content Coding Registry" has been updated with the
 registrations below:
 +------------+--------------------------------------+---------------+
 | Name       | Description                          | Reference     |
 +------------+--------------------------------------+---------------+
 | compress   | UNIX "compress" data format [Welch]  | Section 4.2.1 |
 | deflate    | "deflate" compressed data            | Section 4.2.2 |
 |            | ([RFC1951]) inside the "zlib" data   |               |
 |            | format ([RFC1950])                   |               |
 | gzip       | GZIP file format [RFC1952]           | Section 4.2.3 |
 | x-compress | Deprecated (alias for compress)      | Section 4.2.1 |
 | x-gzip     | Deprecated (alias for gzip)          | Section 4.2.3 |
 +------------+--------------------------------------+---------------+

8.6. Upgrade Token Registry

 The "Hypertext Transfer Protocol (HTTP) Upgrade Token Registry"
 defines the namespace for protocol-name tokens used to identify
 protocols in the Upgrade header field.  The registry is maintained at
 <http://www.iana.org/assignments/http-upgrade-tokens>.

8.6.1. Procedure

 Each registered protocol name is associated with contact information
 and an optional set of specifications that details how the connection
 will be processed after it has been upgraded.
 Registrations happen on a "First Come First Served" basis (see
 Section 4.1 of [RFC5226]) and are subject to the following rules:
 1.  A protocol-name token, once registered, stays registered forever.
 2.  The registration MUST name a responsible party for the
     registration.
 3.  The registration MUST name a point of contact.
 4.  The registration MAY name a set of specifications associated with
     that token.  Such specifications need not be publicly available.
 5.  The registration SHOULD name a set of expected "protocol-version"
     tokens associated with that token at the time of registration.

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 6.  The responsible party MAY change the registration at any time.
     The IANA will keep a record of all such changes, and make them
     available upon request.
 7.  The IESG MAY reassign responsibility for a protocol token.  This
     will normally only be used in the case when a responsible party
     cannot be contacted.
 This registration procedure for HTTP Upgrade Tokens replaces that
 previously defined in Section 7.2 of [RFC2817].

8.6.2. Upgrade Token Registration

 The "HTTP" entry in the upgrade token registry has been updated with
 the registration below:
 +-------+----------------------+----------------------+-------------+
 | Value | Description          | Expected Version     | Reference   |
 |       |                      | Tokens               |             |
 +-------+----------------------+----------------------+-------------+
 | HTTP  | Hypertext Transfer   | any DIGIT.DIGIT      | Section 2.6 |
 |       | Protocol             | (e.g, "2.0")         |             |
 +-------+----------------------+----------------------+-------------+
 The responsible party is: "IETF (iesg@ietf.org) - Internet
 Engineering Task Force".

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 [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 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

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 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; 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, 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 [RFC7234].

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 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 [RFC7231]) or a request payload that is too
 large (Section 6.5.11 of [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 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

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 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 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

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 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.

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

10. Acknowledgments

 This edition of HTTP/1.1 builds on the many contributions that went
 into RFC 1945, RFC 2068, RFC 2145, and RFC 2616, including
 substantial contributions made by the previous authors, editors, and
 Working Group Chairs: Tim Berners-Lee, Ari Luotonen, Roy T. Fielding,
 Henrik Frystyk Nielsen, Jim Gettys, Jeffrey C. Mogul, Larry Masinter,
 and Paul J. Leach.  Mark Nottingham oversaw this effort as Working
 Group Chair.
 Since 1999, the following contributors have helped improve the HTTP
 specification by reporting bugs, asking smart questions, drafting or
 reviewing text, and evaluating open issues:
 Adam Barth, Adam Roach, Addison Phillips, Adrian Chadd, 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

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 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.

Fielding & Reschke Standards Track [Page 73] RFC 7230 HTTP/1.1 Message Syntax and Routing June 2014

11. References

11.1. Normative References

 [RFC0793]     Postel, J., "Transmission Control Protocol", STD 7,
               RFC 793, September 1981.
 [RFC1950]     Deutsch, L. and J-L. Gailly, "ZLIB Compressed Data
               Format Specification version 3.3", RFC 1950, May 1996.
 [RFC1951]     Deutsch, P., "DEFLATE Compressed Data Format
               Specification version 1.3", RFC 1951, May 1996.
 [RFC1952]     Deutsch, P., Gailly, J-L., Adler, M., Deutsch, L., and
               G. Randers-Pehrson, "GZIP file format specification
               version 4.3", RFC 1952, May 1996.
 [RFC2119]     Bradner, S., "Key words for use in RFCs to Indicate
               Requirement Levels", BCP 14, RFC 2119, March 1997.
 [RFC3986]     Berners-Lee, T., Fielding, R., and L. Masinter,
               "Uniform Resource Identifier (URI): Generic Syntax",
               STD 66, RFC 3986, January 2005.
 [RFC5234]     Crocker, D., Ed. and P. Overell, "Augmented BNF for
               Syntax Specifications: ABNF", STD 68, RFC 5234,
               January 2008.
 [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 J. Reschke,
               Ed., "Hypertext Transfer Protocol (HTTP/1.1): Caching",
               RFC 7234, June 2014.
 [RFC7235]     Fielding, R., Ed. and J. Reschke, Ed., "Hypertext
               Transfer Protocol (HTTP/1.1): Authentication",
               RFC 7235, June 2014.

Fielding & Reschke Standards Track [Page 74] RFC 7230 HTTP/1.1 Message Syntax and Routing 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 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>.

Fielding & Reschke Standards Track [Page 75] RFC 7230 HTTP/1.1 Message Syntax and Routing June 2014

 [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.

Fielding & Reschke Standards Track [Page 76] RFC 7230 HTTP/1.1 Message Syntax and Routing June 2014

 [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.

Fielding & Reschke Standards Track [Page 77] RFC 7230 HTTP/1.1 Message Syntax and Routing June 2014

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 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. 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.

Fielding & Reschke Standards Track [Page 78] RFC 7230 HTTP/1.1 Message Syntax and Routing June 2014

 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 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.

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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 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-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 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

Fielding & Reschke Standards Track [Page 80] RFC 7230 HTTP/1.1 Message Syntax and Routing June 2014

 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 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)

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 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, 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, see [RFC3986], Section 4.3>
 absolute-form = absolute-URI
 absolute-path = 1*( "/" segment )
 asterisk-form = "*"
 authority = <authority, see [RFC3986], Section 3.2>
 authority-form = authority

Fielding & Reschke Standards Track [Page 82] RFC 7230 HTTP/1.1 Message Syntax and Routing June 2014

 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, 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, see [RFC3986], Section 3.3>
 port = <port, 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, see [RFC3986], Section 3.4>
 quoted-pair = "\" ( HTAB / SP / VCHAR / obs-text )

Fielding & Reschke Standards Track [Page 83] RFC 7230 HTTP/1.1 Message Syntax and Routing June 2014

 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, 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, see [RFC3986], Section 3.1>
 segment = <segment, 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, see [RFC3986], Section 3.2.2>

Fielding & Reschke Standards Track [Page 84] RFC 7230 HTTP/1.1 Message Syntax and Routing June 2014

Index

 A
    absolute-form (of request-target)  42
    accelerator  10
    application/http Media Type  63
    asterisk-form (of request-target)  43
    authoritative response  67
    authority-form (of request-target)  42-43
 B
    browser  7
 C
    cache  11
    cacheable  12
    captive portal  11
    chunked (Coding Format)  28, 32, 36
    client  7
    close  51, 56
    compress (Coding Format)  38
    connection  7
    Connection header field  51, 56
    Content-Length header field  30
 D
    deflate (Coding Format)  38
    Delimiters  27
    downstream  10
 E
    effective request URI  45
 G
    gateway  10
    Grammar
       absolute-form  42
       absolute-path  16
       absolute-URI  16
       ALPHA  6
       asterisk-form  41, 43
       authority  16
       authority-form  42-43
       BWS  25
       chunk  36
       chunk-data  36
       chunk-ext  36
       chunk-ext-name  36

Fielding & Reschke Standards Track [Page 85] RFC 7230 HTTP/1.1 Message Syntax and Routing June 2014

       chunk-ext-val  36
       chunk-size  36
       chunked-body  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  23
       field-name  23, 40
       field-value  23
       field-vchar  23
       fragment  16
       header-field  23, 37
       HEXDIG  6
       Host  44
       HTAB  6
       HTTP-message  19
       HTTP-name  14
       http-URI  17
       HTTP-version  14
       https-URI  18
       last-chunk  36
       LF  6
       message-body  28
       method  21
       obs-fold  23
       obs-text  27
       OCTET  6
       origin-form  42
       OWS  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  39
       reason-phrase  22
       received-by  47

Fielding & Reschke Standards Track [Page 86] RFC 7230 HTTP/1.1 Message Syntax and Routing June 2014

       received-protocol  47
       request-line  21
       request-target  41
       RWS  25
       scheme  16
       segment  16
       SP  6
       start-line  21
       status-code  22
       status-line  22
       t-codings  39
       t-ranking  39
       tchar  27
       TE  39
       token  27
       Trailer  40
       trailer-part  37
       transfer-coding  35
       Transfer-Encoding  28
       transfer-extension  35
       transfer-parameter  35
       Upgrade  57
       uri-host  16
       URI-reference  16
       VCHAR  6
       Via  47
    gzip (Coding Format)  39
 H
    header field  19
    header section  19
    headers  19
    Host header field  44
    http URI scheme  17
    https URI scheme  17
 I
    inbound  9
    interception proxy  11
    intermediary  9
 M
    Media Type
       application/http  63
       message/http  62
    message  7
    message/http Media Type  62
    method  21

Fielding & Reschke Standards Track [Page 87] RFC 7230 HTTP/1.1 Message Syntax and Routing June 2014

 N
    non-transforming proxy  49
 O
    origin server  7
    origin-form (of request-target)  42
    outbound  10
 P
    phishing  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  39
    Trailer header field  40
    Transfer-Encoding header field  28
    transforming proxy  49
    transparent proxy  11
    tunnel  10
 U
    Upgrade header field  57
    upstream  9
    URI scheme
       http  17
       https  17
    user agent  7
 V
    Via header field  47

Fielding & Reschke Standards Track [Page 88] RFC 7230 HTTP/1.1 Message Syntax and Routing June 2014

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/

Fielding & Reschke Standards Track [Page 89]

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