HTTPbis Working GroupR. Fielding, Editor
Obsoletes: 2145, 2616 (if approved)Y. Lafon, Editor
Updates: 2817 (if approved)W3C
Intended status: Standards TrackJ. Reschke, Editor
Expires: January 17, 2013greenbytes
July 16, 2012

HTTP/1.1, part 1: Message Routing and Syntax"


The Hypertext Transfer Protocol (HTTP) is an application-level protocol for distributed, collaborative, hypertext information systems. HTTP has been in use by the World Wide Web global information initiative since 1990. This document provides an overview of HTTP architecture and its associated terminology, defines the "http" and "https" Uniform Resource Identifier (URI) schemes, defines the HTTP/1.1 message syntax and parsing requirements, and describes general security concerns for implementations.

Status of This Memo

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

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

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This Internet-Draft will expire on January 17, 2013.

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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.

Editorial Note (To be removed by RFC Editor)

Discussion of this draft takes place on the HTTPBIS working group mailing list (, which is archived at <>.

The current issues list is at <> and related documents (including fancy diffs) can be found at <>.

The changes in this draft are summarized in Appendix D.21.

1. Introduction

The Hypertext Transfer Protocol (HTTP) is an application-level request/response protocol that uses extensible semantics and MIME-like message payloads for flexible interaction with network-based hypertext information systems. This document is the first in a series of documents that collectively form the HTTP/1.1 specification:

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

HTTP is a generic interface protocol for information systems. It is designed to hide the details of how a service is implemented by presenting a uniform interface to clients that is independent of the types of resources provided. Likewise, servers do not need to be aware of each client's purpose: an HTTP request can be considered in isolation rather than being associated with a specific type of client or a predetermined sequence of application steps. The result is a protocol that can be used effectively in many different contexts and for which implementations can evolve independently over time.

HTTP is also designed for use as an intermediation protocol for translating communication to and from non-HTTP information systems. HTTP proxies and gateways can provide access to alternative information services by translating their diverse protocols into a hypertext format that can be viewed and manipulated by clients in the same way as HTTP services.

One consequence of HTTP flexibility is that the protocol cannot be defined in terms of what occurs behind the interface. Instead, we are limited to defining the syntax of communication, the intent of received communication, and the expected behavior of recipients. If the communication is considered in isolation, then successful actions ought to be reflected in corresponding changes to the observable interface provided by servers. However, since multiple clients might act in parallel and perhaps at cross-purposes, we cannot require that such changes be observable beyond the scope of a single response.

This document describes the architectural elements that are used or referred to in HTTP, defines the "http" and "https" URI schemes, describes overall network operation and connection management, and defines HTTP message framing and forwarding requirements. Our goal is to define all of the mechanisms necessary for HTTP message handling that are independent of message semantics, thereby defining the complete set of requirements for message parsers and message-forwarding intermediaries.

1.1. Requirement Notation

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119].

1.2. Syntax Notation

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

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

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

2. Architecture

HTTP was created for the World Wide Web architecture and has evolved over time to support the scalability needs of a worldwide hypertext system. Much of that architecture is reflected in the terminology and syntax productions used to define HTTP.

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". An HTTP "client" is a program that establishes a connection to a server for the purpose of sending one or more HTTP requests. An HTTP "server" is a program that accepts connections in order to service HTTP requests by sending HTTP responses.

The terms client and server refer only to the roles that these programs perform for a particular connection. The same program might act as a client on some connections and a server on others. We use the term "user agent" to refer to the program that initiates a request, such as a WWW browser, editor, or spider (web-traversing robot), and the term "origin server" to refer to the program that can originate authoritative responses to a request. For general requirements, we use the term "sender" to refer to whichever component sent a given message and the term "recipient" to refer to any component that receives the message.

HTTP relies upon the Uniform Resource Identifier (URI) standard [RFC3986] to indicate the target resource (Section 5.1) and relationships between resources. Messages are passed in a format similar to that used by Internet mail [RFC5322] and the Multipurpose Internet Mail Extensions (MIME) [RFC2045] (see Appendix A of [Part2] for the differences between HTTP and MIME messages).

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

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

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

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

The following example illustrates a typical message exchange for a GET request on the URI "":

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
Accept-Language: en, mi

server response:

HTTP/1.1 200 OK
Date: Mon, 27 Jul 2009 12:28:53 GMT
Server: Apache
Last-Modified: Wed, 22 Jul 2009 19:15:56 GMT
ETag: "34aa387-d-1568eb00"
Accept-Ranges: bytes
Content-Length: 14
Vary: Accept-Encoding
Content-Type: text/plain

Hello World!

2.2. Implementation Diversity

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

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

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

2.3. Connections and Transport Independence

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

The specific connection protocols to be used for an interaction are determined by client configuration and the target URI (Section 5.1). For example, the "http" URI scheme (Section 2.8.1) 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 instead of using the defaults.

A connection might be used for multiple HTTP request/response exchanges, as defined in Section 6.3.

2.4. Intermediaries

HTTP enables the use of intermediaries to satisfy requests through a chain of connections. There are three common forms of HTTP intermediary: proxy, gateway, and tunnel. In some cases, a single intermediary might act as an origin server, proxy, gateway, or tunnel, switching behavior based on the nature of each request.

         >             >             >             >
    UA =========== A =========== B =========== C =========== O
               <             <             <             <

The figure above shows three intermediaries (A, B, and C) between the user agent and origin server. A request or response message that travels the whole chain will pass through four separate connections. Some HTTP communication options might apply only to the connection with the nearest, non-tunnel neighbor, only to the end-points of the chain, or to all connections along the chain. Although the diagram is linear, each participant might be engaged in multiple, simultaneous communications. For example, B might be receiving requests from many clients other than A, and/or forwarding requests to servers other than C, at the same time that it is handling A's request.

We use the terms "upstream" and "downstream" to describe various requirements in relation to the directional flow of a message: all messages flow from upstream to downstream. Likewise, we use the terms inbound and outbound to refer to directions in relation to the request path: "inbound" means toward the origin server and "outbound" 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-layer protocols. Proxies are often used to group an organization's HTTP requests through a common intermediary for the sake of security, annotation services, or shared caching.

An HTTP-to-HTTP proxy is called a "transforming proxy" if it is designed or configured to modify request or response messages in a semantically meaningful way (i.e., modifications, beyond those required by normal HTTP processing, that change the message in a way that would be significant to the original sender or potentially significant to downstream recipients). For example, a transforming proxy might be acting as a shared annotation server (modifying responses to include references to a local annotation database), a malware filter, a format transcoder, or an intranet-to-Internet privacy filter. Such transformations are presumed to be desired by the client (or client organization) that selected the proxy and are beyond the scope of this specification. However, when a proxy is not intended to transform a given message, we use the term "non-transforming proxy" to target requirements that preserve HTTP message semantics. See Section 4.4.4 of [Part2] and Section 7.6 of [Part6] for status and warning codes related to transformations.

A "gateway" (a.k.a., "reverse proxy") is a receiving agent that acts as a layer above some other server(s) and translates the received requests to the underlying server's protocol. Gateways are often used to encapsulate legacy or untrusted information services, to improve server performance through "accelerator" caching, and to enable partitioning or load-balancing of HTTP services across multiple machines.

A gateway behaves as an origin server on its outbound connection and as a user agent on its inbound connection. All HTTP requirements applicable to an origin server also apply to the outbound communication of a gateway. A gateway communicates with inbound servers using any protocol that it desires, including private extensions to HTTP that are outside the scope of this specification. However, an HTTP-to-HTTP gateway that wishes to interoperate with third-party HTTP servers MUST conform to HTTP user agent requirements on the gateway's inbound connection and MUST implement the Connection (Section 6.1) and Via (Section 6.2) header fields for both connections.

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 is used to establish private communication through a shared firewall proxy.

The above categories for intermediary only consider those acting as participants in the HTTP communication. There are also intermediaries that can act on lower layers of the network protocol stack, filtering or redirecting HTTP traffic without the knowledge or permission of message senders. Network intermediaries often introduce security flaws or interoperability problems by violating HTTP semantics. For example, an "interception proxy" [RFC3040] (also commonly known as a "transparent proxy" [RFC1919] or "captive portal") differs from an HTTP proxy because it is not selected by the client. Instead, an interception proxy filters or redirects outgoing TCP port 80 packets (and occasionally other common port traffic). Interception proxies are commonly found on public network access points, as a means of enforcing account subscription prior to allowing use of non-local Internet services, and within corporate firewalls to enforce network usage policies. They are indistinguishable from a man-in-the-middle attack.

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

2.5. 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 which has not been cached by UA or A.

            >             >
       UA =========== A =========== B - - - - - - C - - - - - - O
                  <             <

A response is "cacheable" if a cache is allowed to store a copy of the response message for use in answering subsequent requests. Even when a response is cacheable, there might be additional constraints placed by the client or by the origin server on when that cached response can be used for a particular request. HTTP requirements for cache behavior and cacheable responses are defined in Section 2 of [Part6].

There are a wide variety of architectures and configurations of caches and proxies deployed across the World Wide Web and inside large organizations. These systems include national hierarchies of proxy caches to save transoceanic bandwidth, systems that broadcast or multicast cache entries, organizations that distribute subsets of cached data via optical media, and so on.

2.6. 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.

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

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

In addition to the prose requirements placed upon them, senders MUST NOT generate protocol elements that do not match the grammar defined by the ABNF rules for those protocol elements that are applicable to the sender's role. If a received protocol element is processed, the recipient MUST be able to parse any value that would match the ABNF rules for that protocol element, excluding only those rules not applicable to the recipient's role.

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

2.7. Protocol Versioning

HTTP uses a "<major>.<minor>" numbering scheme to indicate versions of the protocol. This specification defines version "1.1". The protocol version as a whole indicates the sender's conformance with the set of requirements laid out in that version's corresponding specification of HTTP.

The version of an HTTP message is indicated by an HTTP-version field in the first line of the message. HTTP-version is case-sensitive.

  HTTP-version  = HTTP-name "/" DIGIT "." DIGIT
  HTTP-name     = %x48.54.54.50 ; "HTTP", case-sensitive 

The HTTP version number consists of two decimal digits separated by a "." (period or decimal point). The first digit ("major version") indicates the HTTP messaging syntax, whereas the second digit ("minor version") indicates the highest minor version to which the sender is conformant and able to understand for future communication. The minor version advertises the sender's communication capabilities even when the sender is only using a backwards-compatible subset of the protocol, thereby letting the recipient know that more advanced features can be used in response (by servers) or in future requests (by clients).

When an HTTP/1.1 message is sent to an HTTP/1.0 recipient [RFC1945] or a recipient whose version is unknown, the HTTP/1.1 message is constructed such that it can be interpreted as a valid HTTP/1.0 message if all of the newer features are ignored. This specification places recipient-version requirements on some new features so that a conformant sender will only use compatible features until it has determined, through configuration or the receipt of a message, that the recipient supports HTTP/1.1.

The interpretation of a header field does not change between minor versions of the same major HTTP version, though the default behavior of a recipient in the absence of such a field can change. Unless specified otherwise, header fields defined in HTTP/1.1 are defined for all versions of HTTP/1.x. In particular, the Host and Connection header fields ought to be implemented by all HTTP/1.x implementations whether or not they advertise conformance with HTTP/1.1.

New header fields can be defined such that, when they are understood by a recipient, they might override or enhance the interpretation of previously defined header fields. When an implementation receives an unrecognized header field, the recipient MUST ignore that header field for local processing regardless of the message's HTTP version. An unrecognized header field received by a proxy MUST be forwarded downstream unless the header field's field-name is listed in the message's Connection header field (see Section 6.1). These requirements allow HTTP's functionality to be enhanced without requiring prior update of deployed intermediaries.

Intermediaries that process HTTP messages (i.e., all intermediaries other than those acting as tunnels) MUST send their own HTTP-version in forwarded messages. In other words, they MUST NOT blindly forward the first line of an HTTP message without ensuring that the protocol version in that message matches a version to which that intermediary is conformant for both the receiving and sending of messages. Forwarding an HTTP message without rewriting the HTTP-version might result in communication errors when downstream recipients use the message sender's version to determine what features are safe to use for later communication with that sender.

An HTTP client SHOULD send a request version equal to the highest version to which the client is conformant and whose major version is no higher than the highest version supported by the server, if this is known. An HTTP client MUST NOT send a version to which it is not conformant.

An HTTP client MAY send a lower request version if it is known that the server incorrectly implements the HTTP specification, but only after the client has attempted at least one normal request and determined from the response status or header fields (e.g., Server) that the server improperly handles higher request versions.

An HTTP server SHOULD send a response version equal to the highest version to which the server is conformant and whose major version is less than or equal to the one received in the request. An HTTP server MUST NOT send a version to which it is not conformant. A server MAY send a 505 (HTTP Version Not Supported) response if it cannot send a response using the major version used in the client's request.

An HTTP server MAY send an HTTP/1.0 response to an HTTP/1.0 request if it is known or suspected that the client incorrectly implements the HTTP specification and is incapable of correctly processing later version responses, such as when a client fails to parse the version number correctly or when an intermediary is known to blindly forward the HTTP-version even when it doesn't conform to the given minor version of the protocol. Such protocol downgrades SHOULD NOT be performed unless triggered by specific client attributes, such as when one or more of the request header fields (e.g., User-Agent) uniquely match the values sent by a client known to be in error.

The intention of HTTP's versioning design is that the major number will only be incremented if an incompatible message syntax is introduced, and that the minor number will only be incremented when changes made to the protocol have the effect of adding to the message semantics or implying additional capabilities of the sender. However, the minor version was not incremented for the changes introduced between [RFC2068] and [RFC2616], and this revision is specifically avoiding any such changes to the protocol.

2.8. Uniform Resource Identifiers

Uniform Resource Identifiers (URIs) [RFC3986] are used throughout HTTP as the means for identifying resources. URI references are used to target requests, indicate redirects, and define relationships. HTTP does not limit what a resource might be; it merely defines an interface that can be used to interact with a resource via HTTP. More information on the scope of URIs and resources can be found in [RFC3986].

This specification adopts the definitions of "URI-reference", "absolute-URI", "relative-part", "port", "host", "path-abempty", "path-absolute", "query", and "authority" from the URI generic syntax [RFC3986]. In addition, we define a partial-URI rule for protocol elements that allow a relative URI but not a fragment.

  URI-reference = <URI-reference, defined in [RFC3986], Section 4.1>
  absolute-URI  = <absolute-URI, defined in [RFC3986], Section 4.3>
  relative-part = <relative-part, defined in [RFC3986], Section 4.2>
  authority     = <authority, defined in [RFC3986], Section 3.2>
  path-abempty  = <path-abempty, defined in [RFC3986], Section 3.3>
  path-absolute = <path-absolute, defined in [RFC3986], Section 3.3>
  port          = <port, defined in [RFC3986], Section 3.2.3>
  query         = <query, defined in [RFC3986], Section 3.4>
  uri-host      = <host, defined in [RFC3986], Section 3.2.2>
  partial-URI   = relative-part [ "?" query ]

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

2.8.1. http URI scheme

The "http" URI scheme is hereby defined for the purpose of minting identifiers according to their association with the hierarchical namespace governed by a potential HTTP origin server listening for TCP connections on a given port.

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

The HTTP origin server is identified by the generic syntax's authority component, which includes a host identifier and optional TCP port ([RFC3986], Section 3.2.2). The remainder of the URI, consisting of both the hierarchical path component and optional query component, serves as an identifier for a potential resource within that origin server's name space.

If the host identifier is provided as an IP literal or IPv4 address, then the origin server is any listener on the indicated TCP port at that IP address. If host is a registered name, then that name is considered an indirect identifier and the recipient might use a name resolution service, such as DNS, to find the address of a listener for that host. The host MUST NOT be empty; if an "http" URI is received with an empty host, then it MUST be rejected as invalid. If the port subcomponent is empty or not given, then TCP port 80 is assumed (the default reserved port for WWW services).

Regardless of the form of host identifier, access to that host is not implied by the mere presence of its name or address. The host might or might not exist and, even when it does exist, might or might not be running an HTTP server or listening to the indicated port. The "http" URI scheme makes use of the delegated nature of Internet names and addresses to establish a naming authority (whatever entity has the ability to place an HTTP server at that Internet name or address) and allows that authority to determine which names are valid and how they might be used.

When an "http" URI is used within a context that calls for access to the indicated resource, a client MAY attempt access by resolving the host to an IP address, establishing a TCP connection to that address on the indicated port, and sending an HTTP request message (Section 3) containing the URI's identifying data (Section 5) to the server. If the server responds to that request with a non-interim HTTP response message, as described in Section 4 of [Part2], then that response is considered an authoritative answer to the client's request.

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

The URI generic syntax for authority also includes a deprecated userinfo subcomponent ([RFC3986], Section 3.2.1) for including user authentication information in the URI. Some implementations make use of the userinfo component for internal configuration of authentication information, such as within command invocation options, configuration files, or bookmark lists, even though such usage might expose a user identifier or password. Senders MUST NOT include a userinfo subcomponent (and its "@" delimiter) when transmitting an "http" URI in a message. Recipients of HTTP messages that contain a URI reference SHOULD parse for the existence of userinfo and treat its presence as an error, likely indicating that the deprecated subcomponent is being used to obscure the authority for the sake of phishing attacks.

2.8.2. 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 for SSL/TLS-secured connections on a given TCP port.

All of the requirements listed above for the "http" scheme are also requirements for the "https" scheme, except that a default TCP port of 443 is assumed if the port subcomponent is empty or not given, and the TCP connection MUST be secured for privacy through the use of strong encryption prior to sending the first HTTP request.

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

Unlike the "http" scheme, responses to "https" identified requests are never "public" and thus MUST NOT be reused for shared caching. They can, however, be reused in a private cache if the message is cacheable by default in HTTP or specifically indicated as such by the Cache-Control header field (Section 7.2 of [Part6]).

Resources made available via the "https" scheme have no shared identity with the "http" scheme even if their resource identifiers indicate the same authority (the same host listening to the same TCP port). They are distinct name spaces and are considered to be distinct origin servers. However, an extension to HTTP that is defined to apply to entire host domains, such as the Cookie protocol [RFC6265], can allow information set by one service to impact communication with other services within a matching group of host domains.

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

2.8.3. http and https URI Normalization and Comparison

Since the "http" and "https" schemes conform to the URI generic syntax, such URIs are normalized and compared according to the algorithm defined in [RFC3986], Section 6, using the defaults described above for each scheme.

If the port is equal to the default port for a scheme, the normal form is to elide the port subcomponent. Likewise, an empty path component is equivalent to an absolute path of "/", so the normal form is to provide a path of "/" instead. The scheme and host are case-insensitive and normally provided in lowercase; all other components are compared in a case-sensitive manner. Characters other than those in the "reserved" set are equivalent to their percent-encoded octets (see [RFC3986], Section 2.1): the normal form is to not encode them.

For example, the following three URIs are equivalent:

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 )
                   [ message-body ]

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

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

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

3.1. Start Line

An HTTP message can either be a request from client to server or a response from server to client. Syntactically, the two types of message differ only in the start-line, which is either a request-line (for requests) or a status-line (for responses), and in the algorithm for determining the length of the message body (Section 3.3). In theory, a client could receive requests and a server could receive responses, distinguishing them by their different start-line formats, but in practice servers are implemented to only expect a request (a response is interpreted as an unknown or invalid request method) and clients are implemented to only expect a response.

Implementations MUST NOT send whitespace between the start-line and the first header field. The presence of such whitespace in a request might be an attempt to trick a server into ignoring that field or processing the line after it as a new request, either of which might result in a security vulnerability if other implementations within the request chain interpret the same message differently. Likewise, the presence of such whitespace in a response might be ignored by some clients or cause others to cease parsing.

3.1.1. Request Line

A request-line begins with a method token, followed by a single space (SP), the request-target, another single space (SP), the protocol version, and ending with CRLF.

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

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

The methods defined by this specification can be found in Section 2 of [Part2], along with information regarding the HTTP method registry and considerations for defining new methods.

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

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

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

HTTP does not place a pre-defined limit on the length of a request-line. A server that receives a method longer than any that it implements SHOULD respond with either a 405 (Method Not Allowed), if it is an origin server, or a 501 (Not Implemented) status code. A server MUST be prepared to receive URIs of unbounded length and respond with the 414 (URI Too Long) status code if the received request-target would be longer than the server wishes to handle (see Section 4.6.12 of [Part2]).

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 up to 8000 octets.

3.1.2. Status Line

The first line of a response message is the status-line, consisting of the protocol version, a space (SP), the status code, another space, a possibly-empty textual phrase describing the status code, and ending with CRLF.

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

The status-code element is a 3-digit integer code describing the result of the server's attempt to understand and satisfy the client's corresponding request. The rest of the response message is to be interpreted in light of the semantics defined for that status code. See Section 4 of [Part2] for information about the semantics of status codes, including the classes of status code (indicated by the first digit), the status codes defined by this specification, considerations for the definition of new status codes, and the IANA registry.

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.

3.2. Header Fields

Each HTTP header field consists of a case-insensitive field name followed by a colon (":"), optional whitespace, and the field value.

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 9.10 of [Part2] as containing the origination timestamp for the message in which it appears.

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

New HTTP header fields SHOULD be registered with IANA according to the procedures in Section 3.1 of [Part2]. Unrecognized header fields MUST be forwarded by a proxy unless the field-name is listed in the Connection header field (Section 6.1) or the proxy is specifically configured to block or otherwise transform such fields. Unrecognized header fields SHOULD be ignored by other recipients.

The order in which header fields with differing field names are received is not significant. However, it is "good practice" to send header fields that contain control data first, such as Host on requests and Date on responses, so that implementations can decide when not to handle a message as early as possible. A server MUST wait until the entire header section is received before interpreting a request message, since later header fields might include conditionals, authentication credentials, or deliberately misleading duplicate header fields that would impact request processing.

Multiple header fields with the same field name MUST NOT be sent in a message unless the entire field value for that header field is defined as a comma-separated list [i.e., #(values)]. Multiple header fields with the same field name can be combined into one "field-name: field-value" pair, without changing the semantics of the message, by appending each subsequent field value to the combined field value in order, separated by a comma. The order in which header fields with the same field name are received is therefore significant to the interpretation of the combined field value; a proxy MUST NOT change the order of these field values when forwarding a message.

3.2.1. Whitespace

This specification uses three rules to denote the use of linear whitespace: OWS (optional whitespace), RWS (required whitespace), and BWS ("bad" whitespace).

The OWS rule is used where zero or more linear whitespace octets might appear. OWS SHOULD either not be produced or be produced as a single SP. Multiple OWS octets that occur within field-content SHOULD either be replaced with a single SP or transformed to all SP octets (each octet other than SP replaced with SP) before interpreting the field value or forwarding the message downstream.

RWS is used when at least one linear whitespace octet is required to separate field tokens. RWS SHOULD be produced as a single SP. Multiple RWS octets that occur within field-content SHOULD either be replaced with a single SP or transformed to all SP octets before interpreting the field value or forwarding the message downstream.

BWS is used where the grammar allows optional whitespace for historical reasons but senders SHOULD NOT produce it in messages. HTTP/1.1 recipients MUST accept such bad optional whitespace and remove it before interpreting the field value or forwarding the message downstream.

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

3.2.2. Field Parsing

No whitespace is allowed between the header field-name and colon. In the past, differences in the handling of such whitespace have led to security vulnerabilities in request routing and response handling. Any received request message that contains whitespace between a header field-name and colon MUST be rejected with a response code of 400 (Bad Request). A proxy MUST remove any such whitespace from a response message before forwarding the message downstream.

A field value MAY be preceded by optional whitespace (OWS); a single SP is preferred. The field value does not include any leading or trailing white space: OWS occurring before the first non-whitespace octet of the field value or after the last non-whitespace octet of the field value is ignored and SHOULD be removed before further processing (as this does not change the meaning of the header field).

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

Historically, HTTP has allowed field content with text in the ISO-8859-1 [ISO-8859-1] character encoding and supported other character sets only through use of [RFC2047] encoding. In practice, most HTTP header field values use only a subset of the US-ASCII character encoding [USASCII]. Newly defined header fields SHOULD limit their field values to US-ASCII octets. Recipients SHOULD treat other (obs-text) octets in field content as opaque data.

3.2.3. Field Length

HTTP does not place a pre-defined limit on the length of header fields, either in isolation or as a set. A server MUST be prepared to receive request header fields of unbounded length and respond with a 4xx (Client Error) status code if the received header field(s) would be longer than the server wishes to handle.

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

Various ad-hoc limitations on header field length are found in practice. It is RECOMMENDED that all HTTP senders and recipients support messages whose combined header fields have 4000 or more octets.

3.2.4. Field value components

Many HTTP/1.1 header field values consist of words (token or quoted-string) separated by whitespace or special characters. These special characters MUST be in a quoted string to be used within a parameter value (as defined in Section 4).

  word           = token / quoted-string

  token          = 1*tchar

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

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

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

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

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

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

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

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

Comments can be included in some HTTP header fields by surrounding the comment text with parentheses. Comments are only allowed in fields containing "comment" as part of their field value definition.

  comment        = "(" *( ctext / quoted-cpair / comment ) ")"
  ctext          = OWS / %x21-27 / %x2A-5B / %x5D-7E / obs-text

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

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

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

3.3. Message Body

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

  message-body = *OCTET

The rules for when a message body is allowed in a message differ for requests and responses.

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

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

3.3.1. Transfer-Encoding

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

Transfer-Encoding is analogous to the Content-Transfer-Encoding field of MIME, which was designed to enable safe transport of binary data over a 7-bit transport service ([RFC2045], Section 6). However, safe transport has a different focus for an 8bit-clean transfer protocol. In HTTP's case, Transfer-Encoding is primarily intended to accurately delimit a dynamically generated payload and to distinguish payload encodings that are only applied for transport efficiency or security from those that are characteristics of the target resource.

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

For example,

  Transfer-Encoding: gzip, chunked

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

If more than one Transfer-Encoding header field is present in a message, the multiple field-values MUST be combined into one field-value, according to the algorithm defined in Section 3.2, before determining the message body length.

Unlike Content-Encoding (Section 5.4 of [Part2]), Transfer-Encoding is a property of the message, not of the payload, and thus MAY be added or removed by any implementation along the request/response chain. Additional information about the encoding parameters MAY be provided by other header fields not defined by this specification.

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

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

A server that receives a request message with a transfer-coding it does not understand SHOULD respond with 501 (Not Implemented) and then close the connection.

3.3.2. Content-Length

When a message does not have a Transfer-Encoding header field and the payload body length can be determined prior to being transferred, a Content-Length header field SHOULD be sent to indicate the length of the payload body that is either present as the message body, for requests and non-HEAD responses other than 304 (Not Modified), or would have been present had the request been an unconditional GET. The length is expressed as a decimal number of octets.

An example is

  Content-Length: 3495

In the case of a response to a HEAD request, Content-Length indicates the size of the payload body (without any potential transfer-coding) that would have been sent had the request been a GET. In the case of a 304 (Not Modified) response (Section 4.1 of [Part4]) to a GET request, Content-Length indicates the size of the payload body (without any potential transfer-coding) that would have been sent in a 200 (OK) response.

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

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.

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 or response smuggling (bypass of security-related checks on message routing or content) and thus ought to be handled as an error. The provided Content-Length MUST be removed, prior to forwarding the message downstream, or replaced with the real message body length after the transfer-coding is decoded.

  4. If a message is received without Transfer-Encoding and with either multiple Content-Length header fields having differing field-values or a single Content-Length header field having an invalid value, then the message framing is invalid and MUST be treated as an error to prevent request or response smuggling. If this is a request message, the server MUST respond with a 400 (Bad Request) status code and then close the connection. If this is a response message received by a proxy, the proxy MUST discard the received response, send a 502 (Bad Gateway) status code as its downstream response, and then close the connection. If this is a response message received by a user-agent, it MUST be treated as an error by discarding the message and closing the connection.

  5. If a valid Content-Length header field is present without Transfer-Encoding, its decimal value defines the message body length in octets. If the actual number of octets sent in the message is less than the indicated Content-Length, the recipient MUST consider the message to be incomplete and treat the connection as no longer usable. If the actual number of octets sent in the message is more than the indicated Content-Length, the recipient MUST only process the message body up to the field value's number of octets; the remainder of the message MUST either be discarded or treated as the next message in a pipeline. For the sake of robustness, a user-agent MAY attempt to detect and correct such an error in message framing if it is parsing the response to the last request on a connection and the connection has been closed by the server.

  6. If this is a request message and none of the above are true, then the message body length is zero (no message body is present).

  7. Otherwise, this is a response message without a declared message body length, so the message body length is determined by the number of octets received prior to the server closing the connection.

Since there is no way to distinguish a successfully completed, close-delimited message from a partially-received message interrupted by network failure, implementations SHOULD use encoding or length-delimited messages whenever possible. The close-delimiting feature exists primarily for backwards compatibility with HTTP/1.0.

A server MAY reject a request that contains a message body but not a Content-Length by responding with 411 (Length Required).

Unless a transfer-coding other than "chunked" has been applied, a client that sends a request containing a message body SHOULD use a valid Content-Length header field if the message body length is known in advance, rather than the "chunked" encoding, since some existing services respond to "chunked" with a 411 (Length Required) status code even though they understand the chunked encoding. This is typically because such services are implemented via a gateway that requires a content-length in advance of being called and the server is unable or unwilling to buffer the entire request before processing.

A client that sends a request containing a message body MUST include a valid Content-Length header field if it does not know the server will handle HTTP/1.1 (or later) requests; such knowledge can be in the form of specific user configuration or by remembering the version of a prior received response.

3.4. Handling Incomplete Messages

Request messages that are prematurely terminated, possibly due to a canceled connection or a server-imposed time-out exception, MUST result in closure of the connection; sending an HTTP/1.1 error response prior to closing the connection is OPTIONAL.

Response messages that are prematurely terminated, usually by closure of the connection prior to receiving the expected number of octets or by failure to decode a transfer-encoded message body, MUST be recorded as incomplete. A response that terminates in the middle of the header block (before the empty line is received) cannot be assumed to convey the full semantics of the response and MUST be treated as an error.

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

A user agent MUST NOT render an incomplete response message body as if it were complete (i.e., some indication needs to be given to the user that an error occurred). Cache requirements for incomplete responses are defined in Section 3 of [Part6].

A server MUST read the entire request message body or close the connection after sending its response, since otherwise the remaining data on a persistent connection would be misinterpreted as the next request. Likewise, a client MUST read the entire response message body if it intends to reuse the same connection for a subsequent request. Pipelining multiple requests on a connection is described in Section

3.5. Message Parsing Robustness

Older HTTP/1.0 client implementations might send an extra CRLF after a POST request as a lame workaround for some early server applications that failed to read message body content that was not terminated by a line-ending. An HTTP/1.1 client MUST NOT preface or follow a request with an extra CRLF. If terminating the request message body with a line-ending is desired, then the client MUST include the terminating CRLF octets as part of the message body length.

In the interest of robustness, servers SHOULD ignore at least one empty line received where a request-line is expected. In other words, if the server is reading the protocol stream at the beginning of a message and receives a CRLF first, it SHOULD ignore the CRLF. Likewise, although the line terminator for the start-line and header fields is the sequence CRLF, we recommend that recipients recognize a single LF as a line terminator and ignore any CR.

When a server listening only for HTTP request messages, or processing what appears from the start-line to be an HTTP request message, receives a sequence of octets that does not match the HTTP-message grammar aside from the robustness exceptions listed above, the server MUST respond with an HTTP/1.1 400 (Bad Request) response.

4. Transfer Codings

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

  transfer-coding    = "chunked" ; Section 4.1
                     / "compress" ; Section 4.2.1
                     / "deflate" ; Section 4.2.2
                     / "gzip" ; Section 4.2.3
                     / transfer-extension
  transfer-extension = token *( OWS ";" OWS transfer-parameter )

Parameters are in the form of attribute/value pairs.

  transfer-parameter = attribute BWS "=" BWS value
  attribute          = token
  value              = word

All transfer-coding values are case-insensitive. The HTTP Transfer Coding registry is defined in Section 7.4. HTTP/1.1 uses transfer-coding values in the TE header field (Section 4.3) and in the Transfer-Encoding header field (Section 3.3.1).

4.1. Chunked Transfer Coding

The chunked encoding modifies the body of a message in order to transfer it as a series of chunks, each with its own size indicator, followed by an OPTIONAL trailer containing header fields. This allows dynamically produced content to be transferred along with the information necessary for the recipient to verify that it has received the full message.

The chunk-size field is a string of hex digits indicating the size of the chunk-data in octets. The chunked encoding is ended by any chunk whose size is zero, followed by the trailer, which is terminated by an empty line.

The trailer allows the sender to include additional HTTP header fields at the end of the message. The Trailer header field can be used to indicate which header fields are included in a trailer (see Section 4.4).

A server using chunked transfer-coding in a response MUST NOT use the trailer for any header fields unless at least one of the following is true:

  1. the request included a TE header field that indicates "trailers" is acceptable in the transfer-coding of the response, as described in Section 4.3; or,
  2. the trailer fields consist entirely of optional metadata, and the recipient could use the message (in a manner acceptable to the server where the field originated) without receiving it. In other words, the server that generated the header field (often but not always the origin server) is willing to accept the possibility that the trailer fields might be silently discarded along the path to the client.

This requirement prevents an interoperability failure when the message is being received by an HTTP/1.1 (or later) proxy and forwarded to an HTTP/1.0 recipient. It avoids a situation where conformance with the protocol would have necessitated a possibly infinite buffer on the proxy.

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

  length := 0
  read chunk-size, chunk-ext (if any) and CRLF
  while (chunk-size > 0) {
     read chunk-data and CRLF
     append chunk-data to decoded-body
     length := length + chunk-size
     read chunk-size and CRLF
  read header-field
  while (header-field not empty) {
     append header-field to existing header fields
     read header-field
  Content-Length := length
  Remove "chunked" from Transfer-Encoding

All HTTP/1.1 applications MUST be able to receive and decode the "chunked" transfer-coding and MUST ignore chunk-ext extensions they do not understand.

Use of chunk-ext extensions by senders is deprecated; they SHOULD NOT be sent and definition of new chunk-extensions is discouraged.

4.2. Compression Codings

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

4.2.1. Compress Coding

The "compress" format is produced by the common UNIX file compression program "compress". This format is an adaptive Lempel-Ziv-Welch coding (LZW).

4.2.2. Deflate Coding

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

4.2.3. Gzip Coding

The "gzip" format is produced by the file compression program "gzip" (GNU zip), as described in [RFC1952]. This format is a Lempel-Ziv coding (LZ77) with a 32 bit CRC.

4.3. TE

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

Its value consists of the keyword "trailers" and/or a comma-separated list of extension transfer-coding names with optional accept parameters (as described in Section 4).

  TE        = #t-codings
  t-codings = "trailers" / ( transfer-extension [ te-params ] )
  te-params = OWS ";" OWS "q=" qvalue *( te-ext )
  te-ext    = OWS ";" OWS token [ "=" word ]

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. This keyword is reserved for use with transfer-coding values even though it does not itself represent a transfer-coding.

Examples of its use are:

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

The TE header field only applies to the immediate connection. Therefore, the keyword MUST be supplied within a Connection header field (Section 6.1) whenever TE is present in an HTTP/1.1 message.

A server tests whether a transfer-coding is acceptable, according to a TE field, using these rules:

  1. The "chunked" transfer-coding is always acceptable. If the keyword "trailers" is listed, the client indicates that it is willing to accept trailer fields in the chunked response on behalf of itself and any downstream clients. The implication is that, if given, the client is stating that either all downstream clients are willing to accept trailer fields in the forwarded response, or that it will attempt to buffer the response on behalf of downstream recipients.

    Note: HTTP/1.1 does not define any means to limit the size of a chunked response such that a client can be assured of buffering the entire response.

  2. If the transfer-coding being tested is one of the transfer-codings listed in the TE field, then it is acceptable unless it is accompanied by a qvalue of 0. (As defined in Section 4.3.1, a qvalue of 0 means "not acceptable".)

  3. If multiple transfer-codings are acceptable, then the acceptable transfer-coding with the highest non-zero qvalue is preferred. The "chunked" transfer-coding always has a qvalue of 1.

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.

4.3.1. Quality Values

Both transfer codings (TE request header field, Section 4.3) and content negotiation (Section 8 of [Part2]) use short "floating point" numbers to indicate the relative importance ("weight") of various negotiable parameters. A weight is normalized to a real number in the range 0 through 1, where 0 is the minimum and 1 the maximum value. If a parameter has a quality value of 0, then content with this parameter is "not acceptable" for the client. HTTP/1.1 applications MUST NOT generate more than three digits after the decimal point. User configuration of these values SHOULD also be limited in this fashion.

  qvalue         = ( "0" [ "." 0*3DIGIT ] )
                 / ( "1" [ "." 0*3("0") ] )

4.4. Trailer

The "Trailer" header field indicates that the given set of header fields is present in the trailer of a message encoded with chunked transfer-coding.

An HTTP/1.1 message SHOULD include a Trailer header field in a message using chunked transfer-coding with a non-empty trailer. Doing so allows the recipient to know which header fields to expect in the trailer.

If no Trailer header field is present, the trailer SHOULD NOT include any header fields. See Section 4.1 for restrictions on the use of trailer fields in a "chunked" transfer-coding.

Message header fields listed in the Trailer header field MUST NOT include the following header fields:

5. Message Routing

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

5.1. Identifying a Target Resource

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

HTTP communication is initiated by a user agent for some purpose. The purpose is a combination of request semantics, which are defined in [Part2], and a target resource upon which to apply those semantics. A URI reference (Section 2.8) is typically used as an identifier for the "target resource", which a user agent would resolve to its absolute form in order to obtain the "target URI". The target URI excludes the reference's fragment identifier component, if any, since fragment identifiers are reserved for client-side processing ([RFC3986], Section 3.5).

HTTP intermediaries obtain the request semantics and target URI from the request-line of an incoming request message.

5.2. Connecting Inbound

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

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

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

If no proxy is applicable, a typical client will invoke a handler routine, usually specific to the target URI's scheme, to connect directly to an authority for the target resource. How that is accomplished is dependent on the target URI scheme and defined by its associated specification, similar to how this specification defines origin server access for resolution of the "http" (Section 2.8.1) and "https" (Section 2.8.2) schemes.

5.3. Request Target

Once an inbound connection is obtained (Section 6), 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.

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

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

directly from the origin server would open (or reuse) a TCP connection to port 80 of the host "" and send the lines:

GET /where?q=now HTTP/1.1

followed by the remainder of the request message.

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

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


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

The authority-form of request-target is only used for CONNECT requests (Section 2.3.8 of [Part2]). When making a CONNECT request to establish a tunnel through one or more proxies, a client MUST send only the target URI's authority component (excluding any userinfo) as the request-target. For example,


The asterisk-form of request-target is only used for a server-wide OPTIONS request (Section 2.3.1 of [Part2]). When a client wishes to request OPTIONS for the server as a whole, as opposed to a specific named resource of that server, the client MUST send only "*" (%x2A) as the request-target. For example,


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


would be forwarded by the final proxy as


after connecting to port 8001 of host "".

5.4. Host

The "Host" header field in a request provides the host and port information from the target URI, enabling the origin server to distinguish among resources while servicing requests for multiple host names on a single IP address. Since the Host field-value is critical information for handling a request, it SHOULD be sent as the first header field following the request-line.

A client MUST send a Host header field in all HTTP/1.1 request messages. If the target URI includes an authority component, then the Host field-value MUST be identical to that authority component after excluding any userinfo (Section 2.8.1). If the authority component is missing or undefined for the target URI, then the Host header field MUST be sent with an empty field-value.

For example, a GET request to the origin server for <> would begin with:

GET /pub/WWW/ HTTP/1.1

The Host header field MUST be sent in an HTTP/1.1 request even if the request-target is in the absolute-form, since this allows the Host information to be forwarded through ancient HTTP/1.0 proxies that might not have implemented Host.

When an HTTP/1.1 proxy receives a request with an absolute-form of request-target, the proxy MUST ignore the received Host header field (if any) and instead replace it with the host information of the request-target. If the proxy forwards the request, it MUST generate a new Host field-value based on the received request-target rather than forward the received Host field-value.

Since the Host header field acts as an application-level routing mechanism, it is a frequent target for malware seeking to poison a shared cache or redirect a request to an unintended server. An interception proxy is particularly vulnerable if it relies on the Host field-value for redirecting requests to internal servers, or for use as a cache key in a shared cache, without first verifying that the intercepted connection is targeting a valid IP address for that host.

A server MUST respond with a 400 (Bad Request) status code to any HTTP/1.1 request message that lacks a Host header field and to any request message that contains more than one Host header field or a Host header field with an invalid field-value.

5.5. Effective Request URI

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

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

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

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

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

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

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

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

GET /pub/WWW/TheProject.html HTTP/1.1

has an effective request URI of

Example 2: the following message received over an SSL/TLS-secured TCP connection


has an effective request URI of

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

Recipients of an HTTP/1.0 request that lacks a Host header field MAY attempt to use heuristics (e.g., examination of the URI path for something unique to a particular host) in order to guess the effective request URI's authority component.

5.6. Intermediary Forwarding

As described in Section 2.4, 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.

In order to avoid request loops, a proxy that forwards requests to other proxies MUST be able to recognize and exclude all of its own server names, including any aliases, local variations, or literal IP addresses.

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

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

Intermediaries that forward a message MUST implement the Connection header field as specified in Section 6.1.

5.6.1. End-to-end and Hop-by-hop Header Fields

For the purpose of defining the behavior of caches and non-caching proxies, we divide HTTP header fields into two categories:

  • End-to-end header fields, which are transmitted to the ultimate recipient of a request or response. End-to-end header fields in responses MUST be stored as part of a cache entry and MUST be transmitted in any response formed from a cache entry.
  • Hop-by-hop header fields, which are meaningful only for a single transport-level connection, and are not stored by caches or forwarded by proxies.

The following HTTP/1.1 header fields are hop-by-hop header fields:

All other header fields defined by HTTP/1.1 are end-to-end header fields.

Other hop-by-hop header fields MUST be listed in a Connection header field (Section 6.1).

5.6.2. Non-modifiable Header Fields

Some features of HTTP/1.1, such as Digest Authentication, depend on the value of certain end-to-end header fields. A non-transforming proxy SHOULD NOT modify an end-to-end header field unless the definition of that header field requires or specifically allows that.

A non-transforming proxy MUST NOT modify any of the following fields in a request or response, and it MUST NOT add any of these fields if not already present:

A non-transforming proxy MUST NOT modify any of the following fields in a response:

but it MAY add any of these fields if not already present. If an Expires header field is added, it MUST be given a field value identical to that of the Date header field in that response.

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

A transforming proxy MAY modify or add these fields to a message that does not include no-transform, but if it does so, it MUST add a Warning 214 (Transformation applied) if one does not already appear in the message (see Section 7.6 of [Part6]).

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

5.7. 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 4.3 of [Part2]) precede a final response to the same request.

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

6. Connection Management

6.1. Connection

The "Connection" header field allows the sender to specify options that are desired only for that particular connection. Such connection options MUST be removed or replaced before the message can be forwarded downstream by a proxy or gateway. This mechanism also allows the sender to indicate which HTTP header fields used in the message are only intended for the immediate recipient ("hop-by-hop"), as opposed to all recipients on the chain ("end-to-end"), enabling the message to be self-descriptive and allowing future connection-specific extensions to be deployed in HTTP without fear that they will be blindly forwarded by previously deployed intermediaries.

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

Connection options are compared case-insensitively.

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 sender's own connection options for the forwarded message.

A sender MUST NOT include field-names in the Connection header field-value for fields that are defined as expressing constraints for all recipients in the request or response chain, such as the Cache-Control header field (Section 7.2 of [Part6]).

The connection options do not have to correspond to a header field present in the message, since a connection-specific header field might not be needed if there are no parameters associated with that connection option. Recipients that trigger certain connection behavior based on the presence of connection options MUST do so based on the presence of the connection-option rather than only the presence of the optional header field. In other words, if the connection option is received as a header field but not indicated within the Connection field-value, then the recipient MUST ignore the connection-specific header field because it has likely been forwarded by an intermediary that is only partially conformant.

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

HTTP/1.1 defines the "close" connection option for the sender to signal that the connection will be closed after completion of the response. For example,

  Connection: close

in either the request or the response header fields indicates that the connection SHOULD NOT be considered "persistent" (Section 6.3) after the current request/response is complete.

An HTTP/1.1 client that does not support persistent connections MUST include the "close" connection option in every request message.

An HTTP/1.1 server that does not support persistent connections MUST include the "close" connection option in every response message that does not have a 1xx (Informational) status code.

6.2. Via

The "Via" header field MUST be sent by a proxy or gateway to indicate the intermediate protocols and recipients between the user agent and the server on requests, and between the origin server and the client on responses. It is analogous to the "Received" field used by email systems (Section 3.6.7 of [RFC5322]) and is intended to be used for tracking message forwards, avoiding request loops, and identifying the protocol capabilities of all senders along the request/response chain.

The received-protocol indicates the protocol version of the message received by the server or client along each segment of the request/response chain. The received-protocol version is appended to the Via field value when the message is forwarded so that information about the protocol capabilities of upstream applications remains visible to all recipients.

The protocol-name is excluded if and only if it would be "HTTP". The received-by field is normally the host and optional port number of a recipient server or client that subsequently forwarded the message. However, if the real host is considered to be sensitive information, it MAY be replaced by a pseudonym. If the port is not given, it MAY be assumed to be the default port of the received-protocol.

Multiple Via field values represent each proxy or gateway that has forwarded the message. Each recipient MUST append its information such that the end result is ordered according to the sequence of forwarding applications.

Comments MAY be used in the Via header field to identify the software of each recipient, analogous to the User-Agent and Server header fields. However, all comments in the Via field are optional and MAY be removed by any recipient prior to forwarding the message.

For example, a request message could be sent from an HTTP/1.0 user agent to an internal proxy code-named "fred", which uses HTTP/1.1 to forward the request to a public proxy at, which completes the request by forwarding it to the origin server at The request received by would then have the following Via header field:

  Via: 1.0 fred, 1.1 (Apache/1.1)

A proxy or gateway used as a portal through a network firewall SHOULD NOT forward the names and ports of hosts within the firewall region unless it is explicitly enabled to do so. If not enabled, the received-by host of any host behind the firewall SHOULD be replaced by an appropriate pseudonym for that host.

For organizations that have strong privacy requirements for hiding internal structures, a proxy or gateway MAY combine an ordered subsequence of Via header field entries with identical received-protocol values into a single such entry. For example,

  Via: 1.0 ricky, 1.1 ethel, 1.1 fred, 1.0 lucy

could be collapsed to

  Via: 1.0 ricky, 1.1 mertz, 1.0 lucy

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

6.3. Persistent Connections

6.3.1. Purpose

Prior to persistent connections, a separate TCP connection was established for each request, increasing the load on HTTP servers and causing congestion on the Internet. The use of inline images and other associated data often requires a client to make multiple requests of the same server in a short amount of time. Analysis of these performance problems and results from a prototype implementation are available [Pad1995] [Spe]. Implementation experience and measurements of actual HTTP/1.1 implementations show good results [Nie1997]. Alternatives have also been explored, for example, T/TCP [Tou1998].

Persistent HTTP connections have a number of advantages:

  • By opening and closing fewer TCP connections, CPU time is saved in routers and hosts (clients, servers, proxies, gateways, tunnels, or caches), and memory used for TCP protocol control blocks can be saved in hosts.
  • HTTP requests and responses can be pipelined on a connection. Pipelining allows a client to make multiple requests without waiting for each response, allowing a single TCP connection to be used much more efficiently, with much lower elapsed time.
  • Network congestion is reduced by reducing the number of packets caused by TCP opens, and by allowing TCP sufficient time to determine the congestion state of the network.
  • Latency on subsequent requests is reduced since there is no time spent in TCP's connection opening handshake.
  • HTTP can evolve more gracefully, since errors can be reported without the penalty of closing the TCP connection. Clients using future versions of HTTP might optimistically try a new feature, but if communicating with an older server, retry with old semantics after an error is reported.

HTTP implementations SHOULD implement persistent connections.

6.3.2. Overall Operation

A significant difference between HTTP/1.1 and earlier versions of HTTP is that persistent connections are the default behavior of any HTTP connection. That is, unless otherwise indicated, the client SHOULD assume that the server will maintain a persistent connection, even after error responses from the server.

Persistent connections provide a mechanism by which a client and a server can signal the close of a TCP connection. This signaling takes place using the Connection header field (Section 6.1). Once a close has been signaled, the client MUST NOT send any more requests on that connection. Negotiation

An HTTP/1.1 server MAY assume that a HTTP/1.1 client intends to maintain a persistent connection unless a Connection header field including the connection option "close" was sent in the request. If the server chooses to close the connection immediately after sending the response, it SHOULD send a Connection header field including the connection option "close".

An HTTP/1.1 client MAY expect a connection to remain open, but would decide to keep it open based on whether the response from a server contains a Connection header field with the connection option "close". In case the client does not want to maintain a connection for more than that request, it SHOULD send a Connection header field including the connection option "close".

If either the client or the server sends the "close" option in the Connection header field, that request becomes the last one for the connection.

Clients and servers SHOULD NOT assume that a persistent connection is maintained for HTTP versions less than 1.1 unless it is explicitly signaled. See Appendix A.1.2 for more information on backward compatibility with HTTP/1.0 clients.

Each persistent connection applies to only one transport link.

A proxy server MUST NOT establish a HTTP/1.1 persistent connection with an HTTP/1.0 client (but 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).

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

A client that supports persistent connections MAY "pipeline" its requests (i.e., send multiple requests without waiting for each response). A server MUST send its responses to those requests in the same order that the requests were received.

Clients which assume persistent connections and pipeline immediately after connection establishment SHOULD be prepared to retry their connection if the first pipelined attempt fails. If a client does such a retry, it MUST NOT pipeline before it knows the connection is persistent. Clients MUST also be prepared to resend their requests if the server closes the connection before sending all of the corresponding responses.

Clients SHOULD NOT pipeline requests using non-idempotent request methods or non-idempotent sequences of request methods (see Section 2.1.2 of [Part2]). Otherwise, a premature termination of the transport connection could lead to indeterminate results. A client wishing to send a non-idempotent request SHOULD wait to send that request until it has received the response status line for the previous request.

6.3.3. Practical Considerations

Servers will usually have some time-out value beyond which they will no longer maintain an inactive connection. Proxy servers might make this a higher value since it is likely that the client will be making more connections through the same server. The use of persistent connections places no requirements on the length (or existence) of this time-out for either the client or the server.

When a client or server wishes to time-out it SHOULD issue a graceful close on the transport connection. Clients and servers SHOULD both constantly watch for the other side of the transport close, and respond to it as appropriate. If a client or server does not detect the other side's close promptly it could cause unnecessary resource drain on the network.

A client, server, or proxy MAY close the transport connection at any time. For example, a client might have started to send a new request 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.

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

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.

In particular, while using multiple connections avoids the "head-of-line blocking" problem (whereby a request that takes significant server-side processing and/or has a large payload can block subsequent requests on the same connection), each connection used consumes server resources (sometimes significantly), and furthermore using multiple connections can cause undesirable side effects in congested networks.

Note that servers might reject traffic that they deem abusive, including an excessive number of connections from a client.

6.3.4. Retrying Requests

Senders can close the transport connection at any time. Therefore, clients, servers, and proxies MUST be able to recover from asynchronous close events. Client software MAY reopen the transport connection and retransmit the aborted sequence of requests without user interaction so long as the request sequence is idempotent (see Section 2.1.2 of [Part2]). Non-idempotent request methods or sequences MUST NOT be automatically retried, although user agents MAY offer a human operator the choice of retrying the request(s). Confirmation by user-agent software with semantic understanding of the application MAY substitute for user confirmation. The automatic retry SHOULD NOT be repeated if the second sequence of requests fails.

6.4. Message Transmission Requirements

6.4.1. Persistent Connections and Flow Control

HTTP/1.1 servers SHOULD maintain persistent connections and use TCP's flow control mechanisms to resolve temporary overloads, rather than terminating connections with the expectation that clients will retry. The latter technique can exacerbate network congestion.

6.4.2. Monitoring Connections for Error Status Messages

An HTTP/1.1 (or later) client sending a message body SHOULD monitor the network connection for an error status code while it is transmitting the request. If the client sees an error status code, it SHOULD immediately cease transmitting the body. If the body is being sent using a "chunked" encoding (Section 4), a zero length chunk and empty trailer MAY be used to prematurely mark the end of the message. If the body was preceded by a Content-Length header field, the client MUST close the connection.

6.4.3. Use of the 100 (Continue) Status

The purpose of the 100 (Continue) status code (see Section 4.3.1 of [Part2]) is to allow a client that is sending a request message with a request body to determine if the origin server is willing to accept the request (based on the request header fields) before the client sends the request body. In some cases, it might either be inappropriate or highly inefficient for the client to send the body if the server will reject the message without looking at the body.

Requirements for HTTP/1.1 clients:

  • If a client will wait for a 100 (Continue) response before sending the request body, it MUST send an Expect header field (Section 9.11 of [Part2]) with the "100-continue" expectation.
  • A client MUST NOT send an Expect header field with the "100-continue" expectation if it does not intend to send a request body.

Because of the presence of older implementations, the protocol allows ambiguous situations in which a client might send "Expect: 100-continue" without receiving either a 417 (Expectation Failed) or a 100 (Continue) status code. Therefore, when a client sends this header field to an origin server (possibly via a proxy) from which it has never seen a 100 (Continue) status code, the client SHOULD NOT wait for an indefinite period before sending the request body.

Requirements for HTTP/1.1 origin servers:

  • Upon receiving a request which includes an Expect header field with the "100-continue" expectation, an origin server MUST either respond with 100 (Continue) status code and continue to read from the input stream, or respond with a final status code. The origin server MUST NOT wait for the request body before sending the 100 (Continue) response. If it responds with a final status code, it MAY close the transport connection or it MAY continue to read and discard the rest of the request. It MUST NOT perform the request method if it returns a final status code.
  • An origin server SHOULD NOT send a 100 (Continue) response if the request message does not include an Expect header field with the "100-continue" expectation, and MUST NOT send a 100 (Continue) response if such a request comes from an HTTP/1.0 (or earlier) client. There is an exception to this rule: for compatibility with [RFC2068], a server MAY send a 100 (Continue) status code in response to an HTTP/1.1 PUT or POST request that does not include an Expect header field with the "100-continue" expectation. This exception, the purpose of which is to minimize any client processing delays associated with an undeclared wait for 100 (Continue) status code, applies only to HTTP/1.1 requests, and not to requests with any other HTTP-version value.
  • An origin server MAY omit a 100 (Continue) response if it has already received some or all of the request body for the corresponding request.
  • An origin server that sends a 100 (Continue) response MUST ultimately send a final status code, once the request body is received and processed, unless it terminates the transport connection prematurely.
  • If an origin server receives a request that does not include an Expect header field with the "100-continue" expectation, the request includes a request body, and the server responds with a final status code before reading the entire request body from the transport connection, then the server SHOULD NOT close the transport connection until it has read the entire request, or until the client closes the connection. Otherwise, the client might not reliably receive the response message. However, this requirement ought not be construed as preventing a server from defending itself against denial-of-service attacks, or from badly broken client implementations.

Requirements for HTTP/1.1 proxies:

  • If a proxy receives a request that includes an Expect header field with the "100-continue" expectation, and the proxy either knows that the next-hop server complies with HTTP/1.1 or higher, or does not know the HTTP version of the next-hop server, it MUST forward the request, including the Expect header field.
  • If the proxy knows that the version of the next-hop server is HTTP/1.0 or lower, it MUST NOT forward the request, and it MUST respond with a 417 (Expectation Failed) status code.
  • Proxies SHOULD maintain a record of the HTTP version numbers received from recently-referenced next-hop servers.
  • A proxy MUST NOT forward a 100 (Continue) response if the request message was received from an HTTP/1.0 (or earlier) client and did not include an Expect header field with the "100-continue" expectation. This requirement overrides the general rule for forwarding of 1xx responses (see Section 4.3 of [Part2]).

6.4.4. Closing Connections on Error

If the client is sending data, a server implementation using TCP SHOULD be careful to ensure that the client acknowledges receipt of the packet(s) containing the response, before the server closes the input connection. If the client continues sending data to the server after the close, the server's TCP stack will send a reset packet to the client, which might erase the client's unacknowledged input buffers before they can be read and interpreted by the HTTP application.

6.5. Upgrade

The "Upgrade" header field allows the client to specify what additional communication protocols it would like to use, if the server chooses to switch protocols. Servers can use it to indicate what protocols they are willing to switch to.

For example,

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

The Upgrade header field is intended to provide a simple mechanism for transitioning from HTTP/1.1 to some other, incompatible protocol. It does so by allowing the client to advertise its desire to use another protocol, such as a later version of HTTP with a higher major version number, even though the current request has been made using HTTP/1.1. This eases the difficult transition between incompatible protocols by allowing the client to initiate a request in the more commonly supported protocol while indicating to the server that it would like to use a "better" protocol if available (where "better" is determined by the server, possibly according to the nature of the request method or target resource).

The Upgrade header field only applies to switching application-layer protocols upon the existing transport-layer connection. Upgrade cannot be used to insist on a protocol change; its acceptance and use by the server is optional. The capabilities and nature of the application-layer communication after the protocol change is entirely dependent upon the new protocol chosen, although the first action after changing the protocol MUST be a response to the initial HTTP request containing the Upgrade header field.

The Upgrade header field only applies to the immediate connection. Therefore, the upgrade keyword MUST be supplied within a Connection header field (Section 6.1) whenever Upgrade is present in an HTTP/1.1 message.

The Upgrade header field cannot be used to indicate a switch to a protocol on a different connection. For that purpose, it is more appropriate to use a 3xx (Redirection) response (Section 4.5 of [Part2]).

Servers MUST include the "Upgrade" header field in 101 (Switching Protocols) responses to indicate which protocol(s) are being switched to, and MUST include it in 426 (Upgrade Required) responses to indicate acceptable protocols to upgrade to. Servers MAY include it in any other response to indicate that they are willing to upgrade to one of the specified protocols.

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 2.7 and future updates to this specification. Additional tokens can be registered with IANA using the registration procedure defined in Section 7.6.

7. IANA Considerations

7.1. Header Field Registration

HTTP header fields are registered within the Message Header Field Registry [RFC3864] maintained by IANA at <>.

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

Header Field NameProtocolStatusReference
ConnectionhttpstandardSection 6.1
Content-LengthhttpstandardSection 3.3.2
HosthttpstandardSection 5.4
TEhttpstandardSection 4.3
TrailerhttpstandardSection 4.4
Transfer-EncodinghttpstandardSection 3.3.1
UpgradehttpstandardSection 6.5
ViahttpstandardSection 6.2

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

Header Field NameProtocolStatusReference
ClosehttpreservedSection 7.1

The change controller is: "IETF ( - Internet Engineering Task Force".

7.2. URI Scheme Registration

IANA maintains the registry of URI Schemes [RFC4395] at <>.

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

URI SchemeDescriptionReference
httpHypertext Transfer ProtocolSection 2.8.1
httpsHypertext Transfer Protocol SecureSection 2.8.2

7.3. Internet Media Type Registrations

This document serves as the specification for the Internet media types "message/http" and "application/http". The following is to be registered with IANA (see [RFC4288]).

7.3.1. Internet Media Type message/http

The message/http type can be used to enclose a single HTTP request or response message, provided that it obeys the MIME restrictions for all "message" types regarding line length and encodings.

Type name:
Subtype name:
Required parameters:
Optional parameters:
version, msgtype
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.
The message type — "request" or "response". If not present, the type can be determined from the first line of the body.
Encoding considerations:
only "7bit", "8bit", or "binary" are permitted
Security considerations:
Interoperability considerations:
Published specification:
This specification (see Section 7.3.1).
Applications that use this media type:
Additional information:
Magic number(s):
File extension(s):
Macintosh file type code(s):
Person and email address to contact for further information:
See Authors Section.
Intended usage:
Restrictions on usage:
Author/Change controller:

7.3.2. Internet Media Type application/http

The application/http type can be used to enclose a pipeline of one or more HTTP request or response messages (not intermixed).

Type name:
Subtype name:
Required parameters:
Optional parameters:
version, msgtype
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.
The message type — "request" or "response". If not present, the type can be determined from the first line of the body.
Encoding considerations:
HTTP messages enclosed by this type are in "binary" format; use of an appropriate Content-Transfer-Encoding is required when transmitted via E-mail.
Security considerations:
Interoperability considerations:
Published specification:
This specification (see Section 7.3.2).
Applications that use this media type:
Additional information:
Magic number(s):
File extension(s):
Macintosh file type code(s):
Person and email address to contact for further information:
See Authors Section.
Intended usage:
Restrictions on usage:
Author/Change controller:

7.4. Transfer Coding Registry

The HTTP Transfer Coding Registry defines the name space for transfer coding names.

Registrations MUST include the following fields:

  • Name
  • Description
  • Pointer to specification text

Names of transfer codings MUST NOT overlap with names of content codings (Section 5.4 of [Part2]) unless the encoding transformation is identical, as it is the case for the compression codings defined in Section 4.2.

Values to be added to this name space require IETF Review (see Section 4.1 of [RFC5226]), and MUST conform to the purpose of transfer coding defined in this section.

The registry itself is maintained at <>.

7.5. Transfer Coding Registrations

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

chunkedTransfer in a series of chunksSection 4.1
compressUNIX "compress" program methodSection 4.2.1
deflate"deflate" compression mechanism ([RFC1951]) used inside the "zlib" data format ([RFC1950])Section 4.2.2
gzipSame as GNU zip [RFC1952]Section 4.2.3

7.6. Upgrade Token Registry

The HTTP Upgrade Token Registry defines the name space for protocol-name tokens used to identify protocols in the Upgrade header field. Each registered protocol name is associated with contact information and an optional set of specifications that details how the connection will be processed after it has been upgraded.

Registrations happen on a "First Come First Served" basis (see Section 4.1 of [RFC5226]) and are subject to the following rules:

  1. A protocol-name token, once registered, stays registered forever.
  2. The registration MUST name a responsible party for the registration.
  3. The registration MUST name a point of contact.
  4. The registration MAY name a set of specifications associated with that token. Such specifications need not be publicly available.
  5. The registration SHOULD name a set of expected "protocol-version" tokens associated with that token at the time of registration.
  6. The responsible party MAY change the registration at any time. The IANA will keep a record of all such changes, and make them available upon request.
  7. The IESG MAY reassign responsibility for a protocol token. This will normally only be used in the case when a responsible party cannot be contacted.

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

7.7. Upgrade Token Registration

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

ValueDescriptionExpected Version TokensReference
HTTPHypertext Transfer Protocolany DIGIT.DIGIT (e.g, "2.0")Section 2.7

The responsible party is: "IETF ( - Internet Engineering Task Force".

8. Security Considerations

This section is meant to inform application developers, information providers, and users of the security limitations in HTTP/1.1 as described by this document. The discussion does not include definitive solutions to the problems revealed, though it does make some suggestions for reducing security risks.

8.1. Personal Information

HTTP clients are often privy to large amounts of personal information (e.g., the user's name, location, mail address, passwords, encryption keys, etc.), and SHOULD be very careful to prevent unintentional leakage of this information. We very strongly recommend that a convenient interface be provided for the user to control dissemination of such information, and that designers and implementers be particularly careful in this area. History shows that errors in this area often create serious security and/or privacy problems and generate highly adverse publicity for the implementer's company.

8.2. Abuse of Server Log Information

A server is in the position to save personal data about a user's requests which might identify their reading patterns or subjects of interest. In particular, log information gathered at an intermediary often contains a history of user agent interaction, across a multitude of sites, that can be traced to individual users.

HTTP log information is confidential in nature; its handling is often constrained by laws and regulations. Log information needs to be securely stored and appropriate guidelines followed for its analysis. Anonymization of personal information within individual entries helps, but is generally not sufficient to prevent real log traces from being re-identified based on correlation with other access characteristics. As such, access traces that are keyed to a specific client should not be published even if the key is pseudonymous.

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

8.3. Attacks Based On File and Path Names

Implementations of HTTP origin servers SHOULD be careful to restrict the documents returned by HTTP requests to be only those that were intended by the server administrators. If an HTTP server translates HTTP URIs directly into file system calls, the server MUST take special care not to serve files that were not intended to be delivered to HTTP clients. For example, UNIX, Microsoft Windows, and other operating systems use ".." as a path component to indicate a directory level above the current one. On such a system, an HTTP server MUST disallow any such construct in the request-target if it would otherwise allow access to a resource outside those intended to be accessible via the HTTP server. Similarly, files intended for reference only internally to the server (such as access control files, configuration files, and script code) MUST be protected from inappropriate retrieval, since they might contain sensitive information. Experience has shown that minor bugs in such HTTP server implementations have turned into security risks.

8.4. DNS-related Attacks

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

8.5. Intermediaries and Caching

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

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

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

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

The judicious use of cryptography, when appropriate, might suffice to protect against a broad range of security and privacy attacks. Such cryptography is beyond the scope of the HTTP/1.1 specification.

8.6. Protocol Element Size Overflows

Because HTTP uses mostly textual, character-delimited fields, attackers can overflow buffers in implementations, and/or perform a Denial of Service against implementations that accept fields with unlimited lengths.

To promote interoperability, this specification makes specific recommendations for minimum size limits on request-line (Section 3.1.1) and blocks of header fields (Section 3.2). These are minimum recommendations, chosen to be supportable even by implementations with limited resources; it is expected that most implementations will choose substantially higher limits.

This specification also provides a way for servers to reject messages that have request-targets that are too long (Section 4.6.12 of [Part2]) or request entities that are too large (Section 4.6 of [Part2]).

Other fields (including but not limited to request methods, response status phrases, header field-names, and body chunks) SHOULD be limited by implementations carefully, so as to not impede interoperability.

9. Acknowledgments

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

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

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

10. References

10.1. Normative References

Fielding, R., Ed., Lafon, Y., Ed., and J. Reschke, Ed., “HTTP/1.1, part 2: Semantics and Payloads”, Internet-Draft draft-ietf-httpbis-p2-semantics-20 (work in progress), July 2012.
Fielding, R., Ed., Lafon, Y., Ed., and J. Reschke, Ed., “HTTP/1.1, part 4: Conditional Requests”, Internet-Draft draft-ietf-httpbis-p4-conditional-20 (work in progress), July 2012.
Fielding, R., Ed., Lafon, Y., Ed., and J. Reschke, Ed., “HTTP/1.1, part 5: Range Requests”, Internet-Draft draft-ietf-httpbis-p5-range-20 (work in progress), July 2012.
Fielding, R., Ed., Lafon, Y., Ed., Nottingham, M., Ed., and J. Reschke, Ed., “HTTP/1.1, part 6: Caching”, Internet-Draft draft-ietf-httpbis-p6-cache-20 (work in progress), July 2012.
Fielding, R., Ed., Lafon, Y., Ed., and J. Reschke, Ed., “HTTP/1.1, part 7: Authentication”, Internet-Draft draft-ietf-httpbis-p7-auth-20 (work in progress), July 2012.
Deutsch, L. and J-L. Gailly, “ZLIB Compressed Data Format Specification version 3.3”, RFC 1950, May 1996.
Deutsch, P., “DEFLATE Compressed Data Format Specification version 1.3”, RFC 1951, May 1996.
Deutsch, P., Gailly, J-L., Adler, M., Deutsch, L., and G. Randers-Pehrson, “GZIP file format specification version 4.3”, RFC 1952, May 1996.
Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels”, BCP 14, RFC 2119, March 1997.
Berners-Lee, T., Fielding, R., and L. Masinter, “Uniform Resource Identifier (URI): Generic Syntax”, STD 66, RFC 3986, January 2005.
Crocker, D., Ed. and P. Overell, “Augmented BNF for Syntax Specifications: ABNF”, STD 68, RFC 5234, January 2008.
American National Standards Institute, “Coded Character Set -- 7-bit American Standard Code for Information Interchange”, ANSI X3.4, 1986.

10.2. Informative References

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.
Kristol, D., “HTTP Cookies: Standards, Privacy, and Politics”, ACM Transactions on Internet Technology Vol. 1, #2, November 2001, <>.
Frystyk, H., Gettys, J., Prud'hommeaux, E., Lie, H., and C. Lilley, “Network Performance Effects of HTTP/1.1, CSS1, and PNG”, ACM Proceedings of the ACM SIGCOMM '97 conference on Applications, technologies, architectures, and protocols for computer communication SIGCOMM '97, September 1997, <>.
Padmanabhan, V. and J. Mogul, “Improving HTTP Latency”, Computer Networks and ISDN Systems v. 28, pp. 25-35, December 1995, <>.
Chatel, M., “Classical versus Transparent IP Proxies”, RFC 1919, March 1996.
Berners-Lee, T., Fielding, R., and H. Nielsen, “Hypertext Transfer Protocol -- HTTP/1.0”, RFC 1945, May 1996.
Freed, N. and N. Borenstein, “Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies”, RFC 2045, November 1996.
Moore, K., “MIME (Multipurpose Internet Mail Extensions) Part Three: Message Header Extensions for Non-ASCII Text”, RFC 2047, November 1996.
Fielding, R., Gettys, J., Mogul, J., Nielsen, H., and T. Berners-Lee, “Hypertext Transfer Protocol -- HTTP/1.1”, RFC 2068, January 1997.
Mogul, J., Fielding, R., Gettys, J., and H. Nielsen, “Use and Interpretation of HTTP Version Numbers”, RFC 2145, May 1997.
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.
Khare, R. and S. Lawrence, “Upgrading to TLS Within HTTP/1.1”, RFC 2817, May 2000.
Rescorla, E., “HTTP Over TLS”, RFC 2818, May 2000.
Kristol, D. and L. Montulli, “HTTP State Management Mechanism”, RFC 2965, October 2000.
Cooper, I., Melve, I., and G. Tomlinson, “Internet Web Replication and Caching Taxonomy”, RFC 3040, January 2001.
Klyne, G., Nottingham, M., and J. Mogul, “Registration Procedures for Message Header Fields”, BCP 90, RFC 3864, September 2004.
Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose, “DNS Security Introduction and Requirements”, RFC 4033, March 2005.
Freed, N. and J. Klensin, “Media Type Specifications and Registration Procedures”, BCP 13, RFC 4288, December 2005.
Hansen, T., Hardie, T., and L. Masinter, “Guidelines and Registration Procedures for New URI Schemes”, BCP 115, RFC 4395, February 2006.
Jaganathan, K., Zhu, L., and J. Brezak, “SPNEGO-based Kerberos and NTLM HTTP Authentication in Microsoft Windows”, RFC 4559, June 2006.
Narten, T. and H. Alvestrand, “Guidelines for Writing an IANA Considerations Section in RFCs”, BCP 26, RFC 5226, May 2008.
Resnick, P., “Internet Message Format”, RFC 5322, October 2008.
Barth, A., “HTTP State Management Mechanism”, RFC 6265, April 2011.
Spero, S., “Analysis of HTTP Performance Problems”, <>.
Touch, J., Heidemann, J., and K. Obraczka, “Analysis of HTTP Performance”, ISI Research Report ISI/RR-98-463, Aug 1998, <>.
(original report dated Aug. 1996)

Appendix A. HTTP Version History

HTTP has been in use by the World-Wide Web global information initiative since 1990. The first version of HTTP, later referred to as HTTP/0.9, was a simple protocol for hypertext data transfer across the Internet with only a single request method (GET) and no metadata. HTTP/1.0, as defined by [RFC1945], added a range of request methods and MIME-like messaging that could include metadata about the data transferred and modifiers on the request/response semantics. However, HTTP/1.0 did not sufficiently take into consideration the effects of hierarchical proxies, caching, the need for persistent connections, or name-based virtual hosts. The proliferation of incompletely-implemented applications calling themselves "HTTP/1.0" further necessitated a protocol version change in order for two communicating applications to determine each other's true capabilities.

HTTP/1.1 remains compatible with HTTP/1.0 by including more stringent requirements that enable reliable implementations, adding only those new features that will either be safely ignored by an HTTP/1.0 recipient or only sent when communicating with a party advertising conformance with HTTP/1.1.

It is beyond the scope of a protocol specification to mandate conformance with previous versions. HTTP/1.1 was deliberately designed, however, to make supporting previous versions easy. We would expect a general-purpose HTTP/1.1 server to understand any valid request in the format of HTTP/1.0 and respond appropriately with an HTTP/1.1 message that only uses features understood (or safely ignored) by HTTP/1.0 clients. Likewise, we would expect an HTTP/1.1 client to understand any valid HTTP/1.0 response.

Since HTTP/0.9 did not support header fields in a request, there is no mechanism for it to support name-based virtual hosts (selection of resource by inspection of the Host header field). Any server that implements name-based virtual hosts ought to disable support for HTTP/0.9. Most requests that appear to be HTTP/0.9 are, in fact, badly constructed HTTP/1.x requests wherein a buggy client failed to properly encode linear whitespace found in a URI reference and placed in the request-target.

A.1. Changes from HTTP/1.0

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

A.1.1. Multi-homed Web Servers

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

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

A.1.2. Keep-Alive Connections

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

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

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

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

Clients are also encouraged to consider the use of Connection: keep-alive in requests carefully; while they can enable persistent connections with HTTP/1.0 servers, clients using them need will need to monitor the connection for "hung" requests (which indicate that the client ought stop sending the header field), and this mechanism ought not be used by clients at all when a proxy is being used.

A.1.3. Introduction of Transfer-Encoding

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

A.2. Changes from RFC 2616

Clarify that the string "HTTP" in the HTTP-version ABNF production is case sensitive. Restrict the version numbers to be single digits due to the fact that implementations are known to handle multi-digit version numbers incorrectly. (Section 2.7)

Update use of abs_path production from RFC 1808 to the path-absolute + query components of RFC 3986. State that the asterisk form is allowed for the OPTIONS request method only. (Section 5.3)

Require that invalid whitespace around field-names be rejected. (Section 3.2)

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

The NUL octet is no longer allowed in comment and quoted-string text. The quoted-pair rule no longer allows escaping control characters other than HTAB. Non-ASCII content in header fields and reason phrase has been obsoleted and made opaque (the TEXT rule was removed). (Section 3.2.4)

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

Require recipients to handle bogus Content-Length header fields as errors. (Section 3.3)

Remove reference to non-existent identity transfer-coding value tokens. (Sections 3.3 and 4)

Clarification that the chunk length does not include the count of the octets in the chunk header and trailer. Furthermore disallowed line folding in chunk extensions, and deprecate their use. (Section 4.1)

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

Remove hard limit of two connections per server. Remove requirement to retry a sequence of requests as long it was idempotent. Remove requirements about when servers are allowed to close connections prematurely. (Section 6.3.3)

Remove requirement to retry requests under certain circumstances when the server prematurely closes the connection. (Section 6.4)

Change ABNF productions for header fields to only define the field value.

Clarify exactly when "close" connection options have to be sent. (Section 6.1)

Define the semantics of the Upgrade header field in responses other than 101 (this was incorporated from [RFC2817]). (Section 6.5)

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

Appendix B. ABNF list extension: #rule

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

A construct "#" is defined, similar to "*", for defining comma-delimited lists of elements. The full form is "<n>#<m>element" indicating at least <n> and at most <m> elements, each separated by a single comma (",") and optional whitespace (OWS).


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


  #element => [ 1#element ]

and for n >= 1 and m > 1:

  <n>#<m>element => element <n-1>*<m-1>( OWS "," OWS element )

For compatibility with legacy list rules, recipients SHOULD accept empty list elements. In other words, consumers would follow the list productions:

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

Note that empty elements do not contribute to the count of elements present, though.

For example, given these ABNF productions:

  example-list      = 1#example-list-elmt
  example-list-elmt = token ; see Section 3.2.4 

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

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

But these values would be invalid, as at least one non-empty element is required:

  ",   ,"

Appendix C shows the collected ABNF, with the list rules expanded as explained above.

Appendix C. Collected ABNF


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

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

OWS = *( SP / HTAB )

RWS = 1*( SP / HTAB )

TE = [ ( "," / t-codings ) *( OWS "," [ OWS t-codings ] ) ]
Trailer = *( "," OWS ) field-name *( OWS "," [ OWS field-name ] )
Transfer-Encoding = *( "," OWS ) transfer-coding *( OWS "," [ OWS
 transfer-coding ] )

URI-reference = <URI-reference, defined in [RFC3986], Section 4.1>
Upgrade = *( "," OWS ) protocol *( OWS "," [ OWS protocol ] )

Via = *( "," OWS ) ( received-protocol RWS received-by [ RWS comment
 ] ) *( OWS "," [ OWS ( received-protocol RWS received-by [ RWS
 comment ] ) ] )

absolute-URI = <absolute-URI, defined in [RFC3986], Section 4.3>
absolute-form = absolute-URI
asterisk-form = "*"
attribute = token
authority = <authority, defined in [RFC3986], Section 3.2>
authority-form = authority

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

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

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

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

message-body = *OCTET
method = token

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

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

qdtext = OWS / "!" / %x23-5B ; '#'-'['
 / %x5D-7E ; ']'-'~'
 / obs-text
qdtext-nf = HTAB / SP / "!" / %x23-5B ; '#'-'['
 / %x5D-7E ; ']'-'~'
 / obs-text
query = <query, defined in [RFC3986], Section 3.4>
quoted-cpair = "\" ( HTAB / SP / VCHAR / obs-text )
quoted-pair = "\" ( HTAB / SP / VCHAR / obs-text )
quoted-str-nf = DQUOTE *( qdtext-nf / quoted-pair ) DQUOTE
quoted-string = DQUOTE *( qdtext / quoted-pair ) DQUOTE
qvalue = ( "0" [ "." *3DIGIT ] ) / ( "1" [ "." *3"0" ] )

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

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

t-codings = "trailers" / ( transfer-extension [ te-params ] )
tchar = "!" / "#" / "$" / "%" / "&" / "'" / "*" / "+" / "-" / "." /
 "^" / "_" / "`" / "|" / "~" / DIGIT / ALPHA
te-ext = OWS ";" OWS token [ "=" word ]
te-params = OWS ";" OWS "q=" qvalue *te-ext
token = 1*tchar
trailer-part = *( header-field CRLF )
transfer-coding = "chunked" / "compress" / "deflate" / "gzip" /
transfer-extension = token *( OWS ";" OWS transfer-parameter )
transfer-parameter = attribute BWS "=" BWS value

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

value = word

word = token / quoted-string

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

D.1. Since RFC 2616

Extracted relevant partitions from [RFC2616].

D.2. Since draft-ietf-httpbis-p1-messaging-00

Closed issues:

Other changes:

D.3. Since draft-ietf-httpbis-p1-messaging-01

Closed issues:

Ongoing work on ABNF conversion (<>):

  • Get rid of duplicate BNF rule names ("host" -> "uri-host", "trailer" -> "trailer-part").
  • Avoid underscore character in rule names ("http_URL" -> "http-URL", "abs_path" -> "path-absolute").
  • Add rules for terms imported from URI spec ("absoluteURI", "authority", "path-absolute", "port", "query", "relativeURI", "host) — these will have to be updated when switching over to RFC3986.
  • Synchronize core rules with RFC5234.
  • Get rid of prose rules that span multiple lines.
  • Get rid of unused rules LOALPHA and UPALPHA.
  • Move "Product Tokens" section (back) into Part 1, as "token" is used in the definition of the Upgrade header field.
  • Add explicit references to BNF syntax and rules imported from other parts of the specification.
  • Rewrite prose rule "token" in terms of "tchar", rewrite prose rule "TEXT".

D.4. Since draft-ietf-httpbis-p1-messaging-02

Closed issues:

Ongoing work on IANA Message Header Field Registration (<>):

  • Reference RFC 3984, and update header field registrations for header fields defined in this document.

Ongoing work on ABNF conversion (<>):

  • Replace string literals when the string really is case-sensitive (HTTP-version).

D.5. Since draft-ietf-httpbis-p1-messaging-03

Closed issues:

Ongoing work on ABNF conversion (<>):

  • Replace string literals when the string really is case-sensitive (HTTP-Date).
  • Replace HEX by HEXDIG for future consistence with RFC 5234's core rules.

D.6. Since draft-ietf-httpbis-p1-messaging-04

Closed issues:

Ongoing work on ABNF conversion (<>):

  • Use "/" instead of "|" for alternatives.
  • Get rid of RFC822 dependency; use RFC5234 plus extensions instead.
  • Only reference RFC 5234's core rules.
  • Introduce new ABNF rules for "bad" whitespace ("BWS"), optional whitespace ("OWS") and required whitespace ("RWS").
  • Rewrite ABNFs to spell out whitespace rules, factor out header field value format definitions.

D.7. Since draft-ietf-httpbis-p1-messaging-05

Closed issues:

Final work on ABNF conversion (<>):

  • Rewrite definition of list rules, deprecate empty list elements.
  • Add appendix containing collected and expanded ABNF.

Other changes:

  • Rewrite introduction; add mostly new Architecture Section.
  • Move definition of quality values from Part 3 into Part 1; make TE request header field grammar independent of accept-params (defined in Part 3).

D.8. Since draft-ietf-httpbis-p1-messaging-06

Closed issues:

Partly resolved issues:

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

Closed issues:

Partly resolved issues:

D.10. Since draft-ietf-httpbis-p1-messaging-08

Closed issues:

Partly resolved issues:

D.11. Since draft-ietf-httpbis-p1-messaging-09

Closed issues:

Partly resolved issues:

D.12. Since draft-ietf-httpbis-p1-messaging-10

Closed issues:

Partly resolved issues:

D.13. Since draft-ietf-httpbis-p1-messaging-11

Closed issues:

Partly resolved issues:

D.14. Since draft-ietf-httpbis-p1-messaging-12

Closed issues:

D.15. Since draft-ietf-httpbis-p1-messaging-13

Closed issues:

D.16. Since draft-ietf-httpbis-p1-messaging-14

Closed issues:

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

Closed issues:

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

Closed issues:

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

Closed issues:

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

Closed issues:

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

Closed issues:



Authors' Addresses

Roy T. Fielding (editor)
Adobe Systems Incorporated
345 Park Ave
San Jose, CA 95110
Yves Lafon (editor)
World Wide Web Consortium
2004, rte des Lucioles
Sophia-Antipolis, AM 06902
Julian F. Reschke (editor)
greenbytes GmbH
Hafenweg 16
Muenster, NW 48155