rfc7230 Varnish source references

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.

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Copyright Notice

   Copyright (c) 2014 IETF Trust and the persons identified as the

   document authors.  All rights reserved.

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   Contributions published or made publicly available before November

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   Without obtaining an adequate license from the person(s) controlling

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   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

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           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

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   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

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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

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   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.

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2.1.  Client/Server Messaging

   HTTP is a stateless request/response protocol that operates by

   exchanging messages (Section 3) across a reliable transport- or

   session-layer "connection" (Section 6).  An HTTP "client" is a

   program that establishes a connection to a server for the purpose of

   sending one or more HTTP requests.  An HTTP "server" is a program

   that accepts connections in order to service HTTP requests by sending

   HTTP responses.

   The terms "client" and "server" refer only to the roles that these

   programs perform for a particular connection.  The same program might

   act as a client on some connections and a server on others.  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).

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   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

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   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

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   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.

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   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

                     <             <

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   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

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   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.

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   The version of an HTTP message is indicated by an HTTP-version field

   in the first line of the message.  HTTP-version is case-sensitive.

     HTTP-version  = HTTP-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

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   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

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   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).

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2.7.1.  http URI Scheme

   The "http" URI scheme is hereby defined for the purpose of minting

   identifiers according to their association with the hierarchical

   namespace governed by a potential HTTP origin server listening for

   TCP ([RFC0793]) connections on a given port.

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

                [ "#" fragment ]

   The 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

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   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

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   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

   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).

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   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

     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

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   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

   by a colon (":"), optional leading whitespace, the field value, and

   optional trailing whitespace.

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

     field-name     = token

     field-value    = *( field-content / obs-fold )

     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

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   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.

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     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.

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   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.

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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.

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3.3.  Message Body

   The message body (if any) of an HTTP message is used to carry the

   payload body of that request or response.  The message body is

   identical to the payload body unless a transfer coding has been

   applied, as described in Section 3.3.1.

     message-body = *OCTET

   The rules for when a message body is allowed in a message differ for

   requests and responses.

   The presence of a message body in a request is signaled by a

   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.

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   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