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HTTP/1.1, part 2: Semantics and Payloads

HTTP/1.1, part 2: Semantics and Payloads Abstract

The Hypertext Transfer Protocol (HTTP) is an application-level protocol for distributed, collaborative, hypertext information systems. This document defines the semantics of HTTP/1.1 messages, as expressed by request methods, request header fields, response status codes, and response header fields, along with the payload of messages (metadata and body content) and mechanisms for content negotiation.

Status of This Memo

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

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

Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as “work in progress”.

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

Copyright Notice

Copyright (c) 2012 IETF Trust and the persons identified as the document authors. All rights reserved.

This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.

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

Editorial Note (To be removed by RFC Editor)

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

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

The changes in this draft are summarized in Appendix F.40.

Table of Contents 1. Introduction

Each HTTP message is either a request or a response. A server listens on a connection for a request, parses each message received, interprets the message semantics in relation to the identified request target, and responds to that request with one or more response messages. This document defines HTTP/1.1 request and response semantics in terms of the architecture, syntax notation, and conformance criteria defined in [Part1].

HTTP provides a uniform interface for interacting with resources regardless of their type, nature, or implementation. HTTP semantics includes the intentions defined by each request method, extensions to those semantics that might be described in request header fields, the meaning of status codes to indicate a machine-readable response, and additional control data and resource metadata that might be given in response header fields.

In addition, this document defines the payload of messages (a.k.a., content), the associated metadata header fields that define how the payload is intended to be interpreted by a recipient, the request header fields that might influence content selection, and the various selection algorithms that are collectively referred to as "content negotiation".

Note: This document is currently disorganized in order to minimize changes between drafts and enable reviewers to see the smaller errata changes. A future draft will reorganize the sections to better reflect the content. In particular, the sections will be ordered according to the typical processing of an HTTP request message (after message parsing): resource mapping, methods, request modifying header fields, response status, status modifying header fields, and resource metadata. The current mess reflects how widely dispersed these topics and associated requirements had become in [RFC2616].

1.1. Conformance and Error Handling

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].

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. See Section 2 of [Part1] for definitions of these terms.

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.

This document also uses ABNF to define valid protocol elements (Section 1.2). 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.

1.2. Syntax Notation

This specification uses the Augmented Backus-Naur Form (ABNF) notation of [RFC5234] with the list rule extension defined in Section 1.2 of [Part1]. Appendix D describes rules imported from other documents. Appendix E shows the collected ABNF with the list rule expanded.

2. Methods

The method token indicates the request method to be performed on the target resource (Section 5.5 of [Part1]). The method is case-sensitive.

The list of methods allowed by a resource can be specified in an Allow header field (Section 9.5). The status code of the response always notifies the client whether a method is currently allowed on a resource, since the set of allowed methods can change dynamically. An origin server SHOULD respond with the status code 405 (Method Not Allowed) if the method is known by the origin server but not allowed for the resource, and 501 (Not Implemented) if the method is unrecognized or not implemented by the origin server. The methods GET and HEAD MUST be supported by all general-purpose servers. All other methods are OPTIONAL; however, if the above methods are implemented, they MUST be implemented with the same semantics as those specified in Section 2.3.

2.1. Safe and Idempotent Methods 2.1.1. Safe Methods

Implementers need to be aware that the software represents the user in their interactions over the Internet, and need to allow the user to be aware of any actions they take which might have an unexpected significance to themselves or others.

In particular, the convention has been established that the GET, HEAD, OPTIONS, and TRACE request methods SHOULD NOT have the significance of taking an action other than retrieval. These request methods ought to be considered "safe". This allows user agents to represent other methods, such as POST, PUT and DELETE, in a special way, so that the user is made aware of the fact that a possibly unsafe action is being requested.

Naturally, it is not possible to ensure that the server does not generate side-effects as a result of performing a GET request; in fact, some dynamic resources consider that a feature. The important distinction here is that the user did not request the side-effects, so therefore cannot be held accountable for them.

2.1.2. Idempotent Methods

Request methods can also have the property of "idempotence" in that, aside from error or expiration issues, the intended effect of multiple identical requests is the same as for a single request. PUT, DELETE, and all safe request methods are idempotent. It is important to note that idempotence refers only to changes requested by the client: a server is free to change its state due to multiple requests for the purpose of tracking those requests, versioning of results, etc.

2.2. Method Registry

The HTTP Method Registry defines the name space for the method token in the Request line of an HTTP request.

Registrations MUST include the following fields:

Values to be added to this name space require IETF Review (see [RFC5226], Section 4.1).

The registry itself is maintained at <http://www.iana.org/assignments/http-methods>.

2.2.1. Considerations for New Methods

When it is necessary to express new semantics for a HTTP request that aren't specific to a single application or media type, and currently defined methods are inadequate, it might be appropriate to register a new method.

HTTP methods are generic; that is, they are potentially applicable to any resource, not just one particular media type, "type" of resource, or application. As such, it is preferred that new HTTP methods be registered in a document that isn't specific to a single application, so that this is clear.

Due to the parsing rules defined in Section 3.3 of [Part1], definitions of HTTP methods cannot prohibit the presence of a message body on either the request or the response message (with responses to HEAD requests being the single exception). Definitions of new methods cannot change this rule, but they can specify that only zero-length bodies (as opposed to absent bodies) are allowed.

New method definitions need to indicate whether they are safe (Section 2.1.1), what semantics (if any) the request body has, and whether they are idempotent (Section 2.1.2). They also need to state whether they can be cached ([Part6]); in particular under what conditions a cache can store the response, and under what conditions such a stored response can be used to satisfy a subsequent request.

2.3. Method Definitions 2.3.1. OPTIONS

The OPTIONS method requests information about the communication options available on the request/response chain identified by the effective request URI. This method allows a client to determine the options and/or requirements associated with a resource, or the capabilities of a server, without implying a resource action or initiating a resource retrieval.

Responses to the OPTIONS method are not cacheable.

If the OPTIONS request includes a message body (as indicated by the presence of Content-Length or Transfer-Encoding), then the media type MUST be indicated by a Content-Type field. Although this specification does not define any use for such a body, future extensions to HTTP might use the OPTIONS body to make more detailed queries on the server.

If the request-target (Section 5.3 of [Part1]) is an asterisk ("*"), the OPTIONS request is intended to apply to the server in general rather than to a specific resource. Since a server's communication options typically depend on the resource, the "*" request is only useful as a "ping" or "no-op" type of method; it does nothing beyond allowing the client to test the capabilities of the server. For example, this can be used to test a proxy for HTTP/1.1 conformance (or lack thereof).

If the request-target is not an asterisk, the OPTIONS request applies only to the options that are available when communicating with that resource.

A 200 (OK) response SHOULD include any header fields that indicate optional features implemented by the server and applicable to that resource (e.g., Allow), possibly including extensions not defined by this specification. The response body, if any, SHOULD also include information about the communication options. The format for such a body is not defined by this specification, but might be defined by future extensions to HTTP. Content negotiation MAY be used to select the appropriate response format. If no response body is included, the response MUST include a Content-Length field with a field-value of "0".

The Max-Forwards header field MAY be used to target a specific proxy in the request chain (see Section 9.14). If no Max-Forwards field is present in the request, then the forwarded request MUST NOT include a Max-Forwards field.

2.3.2. GET

The GET method requests transfer of a current representation of the target resource.

If the target resource is a data-producing process, it is the produced data which shall be returned as the representation in the response and not the source text of the process, unless that text happens to be the output of the process.

The semantics of the GET method change to a "conditional GET" if the request message includes an If-Modified-Since, If-Unmodified-Since, If-Match, If-None-Match, or If-Range header field ([Part4]). A conditional GET requests that the representation be transferred only under the circumstances described by the conditional header field(s). The conditional GET request is intended to reduce unnecessary network usage by allowing cached representations to be refreshed without requiring multiple requests or transferring data already held by the client.

The semantics of the GET method change to a "partial GET" if the request message includes a Range header field ([Part5]). A partial GET requests that only part of the representation be transferred, as described in Section 5.4 of [Part5]. The partial GET request is intended to reduce unnecessary network usage by allowing partially-retrieved representations to be completed without transferring data already held by the client.

Bodies on GET requests have no defined semantics. Note that sending a body on a GET request might cause some existing implementations to reject the request.

The response to a GET request is cacheable and MAY be used to satisfy subsequent GET and HEAD requests (see [Part6]).

See Section 11.2 for security considerations when used for forms.

2.3.3. HEAD

The HEAD method is identical to GET except that the server MUST NOT return a message body in the response. The metadata contained in the HTTP header fields in response to a HEAD request SHOULD be identical to the information sent in response to a GET request. This method can be used for obtaining metadata about the representation implied by the request without transferring the representation body. This method is often used for testing hypertext links for validity, accessibility, and recent modification.

The response to a HEAD request is cacheable and MAY be used to satisfy a subsequent HEAD request. It also has potential side effects on previously stored responses to GET; see Section 5 of [Part6].

Bodies on HEAD requests have no defined semantics. Note that sending a body on a HEAD request might cause some existing implementations to reject the request.

2.3.4. POST

The POST method requests that the origin server accept the representation enclosed in the request as data to be processed by the target resource. POST is designed to allow a uniform method to cover the following functions:

The actual function performed by the POST method is determined by the server and is usually dependent on the effective request URI.

The action performed by the POST method might not result in a resource that can be identified by a URI. In this case, either 200 (OK) or 204 (No Content) is the appropriate response status code, depending on whether or not the response includes a representation that describes the result.

If a resource has been created on the origin server, the response SHOULD be 201 (Created) and contain a representation which describes the status of the request and refers to the new resource, and a Location header field (see Section 9.13).

Responses to POST requests are only cacheable when they include explicit freshness information (see Section 4.1.1 of [Part6]). A cached POST response with a Content-Location header field (see Section 9.8) whose value is the effective Request URI MAY be used to satisfy subsequent GET and HEAD requests.

Note that POST caching is not widely implemented. However, the 303 (See Other) response can be used to direct the user agent to retrieve a cacheable representation of the resource.

2.3.5. PUT

The PUT method requests that the state of the target resource be created or replaced with the state defined by the representation enclosed in the request message payload. A successful PUT of a given representation would suggest that a subsequent GET on that same target resource will result in an equivalent representation being returned in a 200 (OK) response. However, there is no guarantee that such a state change will be observable, since the target resource might be acted upon by other user agents in parallel, or might be subject to dynamic processing by the origin server, before any subsequent GET is received. A successful response only implies that the user agent's intent was achieved at the time of its processing by the origin server.

If the target resource does not have a current representation and the PUT successfully creates one, then the origin server MUST inform the user agent by sending a 201 (Created) response. If the target resource does have a current representation and that representation is successfully modified in accordance with the state of the enclosed representation, then either a 200 (OK) or 204 (No Content) response SHOULD be sent to indicate successful completion of the request.

Unrecognized header fields SHOULD be ignored (i.e., not saved as part of the resource state).

An origin server SHOULD verify that the PUT representation is consistent with any constraints which the server has for the target resource that cannot or will not be changed by the PUT. This is particularly important when the origin server uses internal configuration information related to the URI in order to set the values for representation metadata on GET responses. When a PUT representation is inconsistent with the target resource, the origin server SHOULD either make them consistent, by transforming the representation or changing the resource configuration, or respond with an appropriate error message containing sufficient information to explain why the representation is unsuitable. The 409 (Conflict) or 415 (Unsupported Media Type) status codes are suggested, with the latter being specific to constraints on Content-Type values.

For example, if the target resource is configured to always have a Content-Type of "text/html" and the representation being PUT has a Content-Type of "image/jpeg", then the origin server SHOULD do one of:

  1. reconfigure the target resource to reflect the new media type;
  2. transform the PUT representation to a format consistent with that of the resource before saving it as the new resource state; or,
  3. reject the request with a 415 (Unsupported Media Type) response indicating that the target resource is limited to "text/html", perhaps including a link to a different resource that would be a suitable target for the new representation.

HTTP does not define exactly how a PUT method affects the state of an origin server beyond what can be expressed by the intent of the user agent request and the semantics of the origin server response. It does not define what a resource might be, in any sense of that word, beyond the interface provided via HTTP. It does not define how resource state is "stored", nor how such storage might change as a result of a change in resource state, nor how the origin server translates resource state into representations. Generally speaking, all implementation details behind the resource interface are intentionally hidden by the server.

The fundamental difference between the POST and PUT methods is highlighted by the different intent for the target resource. The target resource in a POST request is intended to handle the enclosed representation as a data-accepting process, such as for a gateway to some other protocol or a document that accepts annotations. In contrast, the target resource in a PUT request is intended to take the enclosed representation as a new or replacement value. Hence, the intent of PUT is idempotent and visible to intermediaries, even though the exact effect is only known by the origin server.

Proper interpretation of a PUT request presumes that the user agent knows what target resource is desired. A service that is intended to select a proper URI on behalf of the client, after receiving a state-changing request, SHOULD be implemented using the POST method rather than PUT. If the origin server will not make the requested PUT state change to the target resource and instead wishes to have it applied to a different resource, such as when the resource has been moved to a different URI, then the origin server MUST send a 301 (Moved Permanently) response; the user agent MAY then make its own decision regarding whether or not to redirect the request.

A PUT request applied to the target resource MAY have side-effects on other resources. For example, an article might have a URI for identifying "the current version" (a resource) which is separate from the URIs identifying each particular version (different resources that at one point shared the same state as the current version resource). A successful PUT request on "the current version" URI might therefore create a new version resource in addition to changing the state of the target resource, and might also cause links to be added between the related resources.

An origin server SHOULD reject any PUT request that contains a Content-Range header field (Section 5.2 of [Part5]), since it might be misinterpreted as partial content (or might be partial content that is being mistakenly PUT as a full representation). Partial content updates are possible by targeting a separately identified resource with state that overlaps a portion of the larger resource, or by using a different method that has been specifically defined for partial updates (for example, the PATCH method defined in [RFC5789]).

Responses to the PUT method are not cacheable. If a PUT request passes through a cache that has one or more stored responses for the effective request URI, those stored responses will be invalidated (see Section 6 of [Part6]).

2.3.6. DELETE

The DELETE method requests that the origin server delete the target resource. This method MAY be overridden by human intervention (or other means) on the origin server. The client cannot be guaranteed that the operation has been carried out, even if the status code returned from the origin server indicates that the action has been completed successfully. However, the server SHOULD NOT indicate success unless, at the time the response is given, it intends to delete the resource or move it to an inaccessible location.

A successful response SHOULD be 200 (OK) if the response includes a representation describing the status, 202 (Accepted) if the action has not yet been enacted, or 204 (No Content) if the action has been enacted but the response does not include a representation.

Bodies on DELETE requests have no defined semantics. Note that sending a body on a DELETE request might cause some existing implementations to reject the request.

Responses to the DELETE method are not cacheable. If a DELETE request passes through a cache that has one or more stored responses for the effective request URI, those stored responses will be invalidated (see Section 6 of [Part6]).

2.3.7. TRACE

The TRACE method requests a remote, application-layer loop-back of the request message. The final recipient of the request SHOULD reflect the message received back to the client as the message body of a 200 (OK) response. The final recipient is either the origin server or the first proxy to receive a Max-Forwards value of zero (0) in the request (see Section 9.14). A TRACE request MUST NOT include a message body.

TRACE allows the client to see what is being received at the other end of the request chain and use that data for testing or diagnostic information. The value of the Via header field (Section 6.2 of [Part1]) is of particular interest, since it acts as a trace of the request chain. Use of the Max-Forwards header field allows the client to limit the length of the request chain, which is useful for testing a chain of proxies forwarding messages in an infinite loop.

If the request is valid, the response SHOULD have a Content-Type of "message/http" (see Section 7.3.1 of [Part1]) and contain a message body that encloses a copy of the entire request message. Responses to the TRACE method are not cacheable.

2.3.8. CONNECT

The CONNECT method requests that the proxy establish a tunnel to the request-target and, if successful, thereafter restrict its behavior to blind forwarding of packets until the connection is closed.

When using CONNECT, the request-target MUST use the authority form (Section 5.3 of [Part1]); i.e., the request-target consists of only the host name and port number of the tunnel destination, separated by a colon. For example,

CONNECT server.example.com:80 HTTP/1.1
Host: server.example.com:80

Any 2xx (Successful) response to a CONNECT request indicates that the proxy has established a connection to the requested host and port, and has switched to tunneling the current connection to that server connection. The tunneled data from the server begins immediately after the blank line that concludes the successful response's header block.

A server SHOULD NOT send any Transfer-Encoding or Content-Length header fields in a successful response. A client MUST ignore any Content-Length or Transfer-Encoding header fields received in a successful response.

Any response other than a successful response indicates that the tunnel has not yet been formed and that the connection remains governed by HTTP.

Proxy authentication might be used to establish the authority to create a tunnel:

CONNECT server.example.com:80 HTTP/1.1
Host: server.example.com:80
Proxy-Authorization: basic aGVsbG86d29ybGQ=

A message body on a CONNECT request has no defined semantics. Sending a body on a CONNECT request might cause existing implementations to reject the request.

Similar to a pipelined HTTP/1.1 request, data to be tunneled from client to server MAY be sent immediately after the request (before a response is received). The usual caveats also apply: data can be discarded if the eventual response is negative, and the connection can be reset with no response if more than one TCP segment is outstanding.

It might be the case that the proxy itself can only reach the requested origin server through another proxy. In this case, the first proxy SHOULD make a CONNECT request of that next proxy, requesting a tunnel to the authority. A proxy MUST NOT respond with any 2xx status code unless it has either a direct or tunnel connection established to the authority.

If at any point either one of the peers gets disconnected, any outstanding data that came from that peer will be passed to the other one, and after that also the other connection will be terminated by the proxy. If there is outstanding data to that peer undelivered, that data will be discarded.

An origin server which receives a CONNECT request for itself MAY respond with a 2xx status code to indicate that a connection is established. However, most origin servers do not implement CONNECT.

4. Status Codes

The status-code element is a 3-digit integer result code of the attempt to understand and satisfy the request.

HTTP status codes are extensible. HTTP applications are not required to understand the meaning of all registered status codes, though such understanding is obviously desirable. However, applications MUST understand the class of any status code, as indicated by the first digit, and treat any unrecognized response as being equivalent to the x00 status code of that class, with the exception that an unrecognized response MUST NOT be cached. For example, if an unrecognized status code of 431 is received by the client, it can safely assume that there was something wrong with its request and treat the response as if it had received a 400 status code. In such cases, user agents SHOULD present to the user the representation enclosed with the response, since that representation is likely to include human-readable information which will explain the unusual status.

The first digit of the status-code defines the class of response. The last two digits do not have any categorization role. There are 5 values for the first digit:

For most status codes the response can carry a payload, in which case a Content-Type header field indicates the payload's media type (Section 9.9).

4.1. Overview of Status Codes

The status codes listed below are defined in this specification, Section 4 of [Part4], Section 3 of [Part5], and Section 3 of [Part7]. The reason phrases listed here are only recommendations — they can be replaced by local equivalents without affecting the protocol.

Note that this list is not exhaustive — it does not include extension status codes defined in other specifications.

4.2. Status Code Registry

The HTTP Status Code Registry defines the name space for the status-code token in the status-line of an HTTP response.

Values to be added to this name space require IETF Review (see [RFC5226], Section 4.1).

The registry itself is maintained at <http://www.iana.org/assignments/http-status-codes>.

4.2.1. Considerations for New Status Codes

When it is necessary to express new semantics for a HTTP response that aren't specific to a single application or media type, and currently defined status codes are inadequate, a new status code can be registered.

HTTP status codes are generic; that is, they are potentially applicable to any resource, not just one particular media type, "type" of resource, or application. As such, it is preferred that new HTTP status codes be registered in a document that isn't specific to a single application, so that this is clear.

Definitions of new HTTP status codes typically explain the request conditions that produce a response containing the status code (e.g., combinations of request header fields and/or method(s)), along with any interactions with response header fields (e.g., those that are required, those that modify the semantics of the response).

New HTTP status codes are required to fall under one of the categories defined in Section 4. To allow existing parsers to properly handle them, new status codes cannot disallow a response body, although they can mandate a zero-length response body. They can require the presence of one or more particular HTTP response header field(s).

Likewise, their definitions can specify that caches are allowed to use heuristics to determine their freshness (see [Part6]; by default, it is not allowed), and can define how to determine the resource which they carry a representation for (see Section 7.1; by default, it is anonymous).

4.3. Informational 1xx

This class of status code indicates a provisional response, consisting only of the status-line and optional header fields, and is terminated by an empty line. There are no required header fields for this class of status code. Since HTTP/1.0 did not define any 1xx status codes, servers MUST NOT send a 1xx response to an HTTP/1.0 client except under experimental conditions.

A client MUST be prepared to accept one or more 1xx status responses prior to a regular response, even if the client does not expect a 100 (Continue) status message. Unexpected 1xx status responses MAY be ignored by a user agent.

Proxies MUST forward 1xx responses, unless the connection between the proxy and its client has been closed, or unless the proxy itself requested the generation of the 1xx response. (For example, if a proxy adds an "Expect: 100-continue" field when it forwards a request, then it need not forward the corresponding 100 (Continue) response(s).)

4.3.1. 100 Continue

The client SHOULD continue with its request. This interim response is used to inform the client that the initial part of the request has been received and has not yet been rejected by the server. The client SHOULD continue by sending the remainder of the request or, if the request has already been completed, ignore this response. The server MUST send a final response after the request has been completed. See Section 6.4.3 of [Part1] for detailed discussion of the use and handling of this status code.

4.3.2. 101 Switching Protocols

The server understands and is willing to comply with the client's request, via the Upgrade message header field (Section 6.5 of [Part1]), for a change in the application protocol being used on this connection. The server will switch protocols to those defined by the response's Upgrade header field immediately after the empty line which terminates the 101 response.

The protocol SHOULD be switched only when it is advantageous to do so. For example, switching to a newer version of HTTP is advantageous over older versions, and switching to a real-time, synchronous protocol might be advantageous when delivering resources that use such features.

4.4. Successful 2xx

This class of status code indicates that the client's request was successfully received, understood, and accepted.

4.4.1. 200 OK

The request has succeeded. The payload returned with the response is dependent on the method used in the request, for example:

GET
a representation of the target resource is sent in the response;
HEAD
the same representation as GET, except without the message body;
POST
a representation describing or containing the result of the action;
TRACE
a representation containing the request message as received by the end server.

Caches MAY use a heuristic (see Section 4.1.2 of [Part6]) to determine freshness for 200 responses.

4.4.2. 201 Created

The request has been fulfilled and has resulted in one or more new resources being created.

Newly created resources are typically linked to from the response payload, with the most relevant URI also being carried in the Location header field. If the newly created resource's URI is the same as the Effective Request URI, this information can be omitted (e.g., in the case of a response to a PUT request).

The origin server MUST create the resource(s) before returning the 201 status code. If the action cannot be carried out immediately, the server SHOULD respond with 202 (Accepted) response instead.

A 201 response MAY contain an ETag response header field indicating the current value of the entity-tag for the representation of the resource identified by the Location header field or, in case the Location header field was omitted, by the Effective Request URI (see Section 2.3 of [Part4]).

4.4.3. 202 Accepted

The request has been accepted for processing, but the processing has not been completed. The request might or might not eventually be acted upon, as it might be disallowed when processing actually takes place. There is no facility for re-sending a status code from an asynchronous operation such as this.

The 202 response is intentionally non-committal. Its purpose is to allow a server to accept a request for some other process (perhaps a batch-oriented process that is only run once per day) without requiring that the user agent's connection to the server persist until the process is completed. The representation returned with this response SHOULD include an indication of the request's current status and either a pointer to a status monitor or some estimate of when the user can expect the request to be fulfilled.

4.4.4. 203 Non-Authoritative Information

The representation in the response has been transformed or otherwise modified by a transforming proxy (Section 2.4 of [Part1]). Note that the behavior of transforming intermediaries is controlled by the no-transform Cache-Control directive (Section 7.2 of [Part6]).

This status code is only appropriate when the response status code would have been 200 (OK) otherwise. When the status code before transformation would have been different, the 214 Transformation Applied warn-code (Section 7.6 of [Part6]) is appropriate.

Caches MAY use a heuristic (see Section 4.1.2 of [Part6]) to determine freshness for 203 responses.

4.4.5. 204 No Content

The 204 (No Content) status code indicates that the server has successfully fulfilled the request and that there is no additional content to return in the response payload body. Metadata in the response header fields refer to the target resource and its current representation after the requested action.

For example, if a 204 status code is received in response to a PUT request and the response contains an ETag header field, then the PUT was successful and the ETag field-value contains the entity-tag for the new representation of that target resource.

The 204 response allows a server to indicate that the action has been successfully applied to the target resource while implying that the user agent SHOULD NOT traverse away from its current "document view" (if any). The server assumes that the user agent will provide some indication of the success to its user, in accord with its own interface, and apply any new or updated metadata in the response to the active representation.

For example, a 204 status code is commonly used with document editing interfaces corresponding to a "save" action, such that the document being saved remains available to the user for editing. It is also frequently used with interfaces that expect automated data transfers to be prevalent, such as within distributed version control systems.

The 204 response MUST NOT include a message body, and thus is always terminated by the first empty line after the header fields.

4.4.6. 205 Reset Content

The server has fulfilled the request and the user agent SHOULD reset the document view which caused the request to be sent. This response is primarily intended to allow input for actions to take place via user input, followed by a clearing of the form in which the input is given so that the user can easily initiate another input action.

The message body included with the response MUST be empty. Note that receivers still need to parse the response according to the algorithm defined in Section 3.3 of [Part1].

4.5. Redirection 3xx

This class of status code indicates that further action needs to be taken by the user agent in order to fulfill the request. If the required action involves a subsequent HTTP request, it MAY be carried out by the user agent without interaction with the user if and only if the method used in the second request is known to be "safe", as defined in Section 2.1.1.

There are several types of redirects:

  1. Redirects of the request to another URI, either temporarily or permanently. The new URI is specified in the Location header field. In this specification, the status codes 301 (Moved Permanently), 302 (Found), and 307 (Temporary Redirect) fall under this category.

  2. Redirection to a new location that represents an indirect response to the request, such as the result of a POST operation to be retrieved with a subsequent GET request. This is status code 303 (See Other).

  3. Redirection offering a choice of matching resources for use by agent-driven content negotiation (Section 8.2). This is status code 300 (Multiple Choices).

  4. Other kinds of redirection, such as to a cached result (status code 304 (Not Modified), see Section 4.1 of [Part4]).

Note: In HTTP/1.0, only the status codes 301 (Moved Permanently) and 302 (Found) were defined for the first type of redirect, and the second type did not exist at all ([RFC1945], Section 9.3). However it turned out that web forms using POST expected redirects to change the operation for the subsequent request to retrieval (GET). To address this use case, HTTP/1.1 introduced the second type of redirect with the status code 303 (See Other) ([RFC2068], Section 10.3.4). As user agents did not change their behavior to maintain backwards compatibility, the first revision of HTTP/1.1 added yet another status code, 307 (Temporary Redirect), for which the backwards compatibility problems did not apply ([RFC2616], Section 10.3.8). Over 10 years later, most user agents still do method rewriting for status codes 301 and 302, therefore this specification makes that behavior conformant in case the original request was POST.

A Location header field on a 3xx response indicates that a client MAY automatically redirect to the URI provided; see Section 9.13.

Note that for methods not known to be "safe", as defined in Section 2.1.1, automatic redirection needs to done with care, since the redirect might change the conditions under which the request was issued.

Clients SHOULD detect and intervene in cyclical redirections (i.e., "infinite" redirection loops).

Note: An earlier version of this specification recommended a maximum of five redirections ([RFC2068], Section 10.3). Content developers need to be aware that some clients might implement such a fixed limitation.

4.5.1. 300 Multiple Choices

The target resource has more than one representation, each with its own specific location, and agent-driven negotiation information (Section 8) is being provided so that the user (or user agent) can select a preferred representation by redirecting its request to that location.

Unless it was a HEAD request, the response SHOULD include a representation containing a list of representation metadata and location(s) from which the user or user agent can choose the one most appropriate. Depending upon the format and the capabilities of the user agent, selection of the most appropriate choice MAY be performed automatically. However, this specification does not define any standard for such automatic selection.

If the server has a preferred choice of representation, it SHOULD include the specific URI for that representation in the Location field; user agents MAY use the Location field value for automatic redirection.

Caches MAY use a heuristic (see Section 4.1.2 of [Part6]) to determine freshness for 300 responses.

4.5.2. 301 Moved Permanently

The target resource has been assigned a new permanent URI and any future references to this resource SHOULD use one of the returned URIs. Clients with link editing capabilities ought to automatically re-link references to the effective request URI to one or more of the new references returned by the server, where possible.

Caches MAY use a heuristic (see Section 4.1.2 of [Part6]) to determine freshness for 301 responses.

The new permanent URI SHOULD be given by the Location field in the response. A response payload can contain a short hypertext note with a hyperlink to the new URI(s).

Note: For historic reasons, user agents MAY change the request method from POST to GET for the subsequent request. If this behavior is undesired, status code 307 (Temporary Redirect) can be used instead.

4.5.3. 302 Found

The target resource resides temporarily under a different URI. Since the redirection might be altered on occasion, the client SHOULD continue to use the effective request URI for future requests.

The temporary URI SHOULD be given by the Location field in the response. A response payload can contain a short hypertext note with a hyperlink to the new URI(s).

Note: For historic reasons, user agents MAY change the request method from POST to GET for the subsequent request. If this behavior is undesired, status code 307 (Temporary Redirect) can be used instead.

4.5.4. 303 See Other

The 303 status code indicates that the server is redirecting the user agent to a different resource, as indicated by a URI in the Location header field, that is intended to provide an indirect response to the original request. In order to satisfy the original request, a user agent SHOULD perform a retrieval request using the Location URI (a GET or HEAD request if using HTTP), which can itself be redirected further, and present the eventual result as an answer to the original request. Note that the new URI in the Location header field is not considered equivalent to the effective request URI.

This status code is generally applicable to any HTTP method. It is primarily used to allow the output of a POST action to redirect the user agent to a selected resource, since doing so provides the information corresponding to the POST response in a form that can be separately identified, bookmarked, and cached independent of the original request.

A 303 response to a GET request indicates that the requested resource does not have a representation of its own that can be transferred by the server over HTTP. The Location URI indicates a resource that is descriptive of the target resource, such that the follow-on representation might be useful to recipients without implying that it adequately represents the target resource. Note that answers to the questions of what can be represented, what representations are adequate, and what might be a useful description are outside the scope of HTTP and thus entirely determined by the URI owner(s).

Except for responses to a HEAD request, the representation of a 303 response SHOULD contain a short hypertext note with a hyperlink to the Location URI.

4.5.5. 305 Use Proxy

The 305 status code was defined in a previous version of this specification (see Appendix C), and is now deprecated.

4.5.6. 306 (Unused)

The 306 status code was used in a previous version of the specification, is no longer used, and the code is reserved.

4.5.7. 307 Temporary Redirect

The target resource resides temporarily under a different URI. Since the redirection can change over time, the client SHOULD continue to use the effective request URI for future requests.

The temporary URI SHOULD be given by the Location field in the response. A response payload can contain a short hypertext note with a hyperlink to the new URI(s).

4.6. Client Error 4xx

The 4xx class of status code is intended for cases in which the client seems to have erred. Except when responding to a HEAD request, the server SHOULD include a representation containing an explanation of the error situation, and whether it is a temporary or permanent condition. These status codes are applicable to any request method. User agents SHOULD display any included representation to the user.

4.6.1. 400 Bad Request

The server cannot or will not process the request, due to a client error (e.g., malformed syntax).

4.6.2. 402 Payment Required

This code is reserved for future use.

4.6.3. 403 Forbidden

The server understood the request, but refuses to authorize it. Providing different user authentication credentials might be successful, but any credentials that were provided in the request are insufficient. The request SHOULD NOT be repeated with the same credentials.

If the request method was not HEAD and the server wishes to make public why the request has not been fulfilled, it SHOULD describe the reason for the refusal in the representation. If the server does not wish to make this information available to the client, the status code 404 (Not Found) MAY be used instead.

4.6.4. 404 Not Found

The server has not found anything matching the effective request URI. No indication is given of whether the condition is temporary or permanent. The 410 (Gone) status code SHOULD be used if the server knows, through some internally configurable mechanism, that an old resource is permanently unavailable and has no forwarding address. This status code is commonly used when the server does not wish to reveal exactly why the request has been refused, or when no other response is applicable.

4.6.5. 405 Method Not Allowed

The method specified in the request-line is not allowed for the target resource. The response MUST include an Allow header field containing a list of valid methods for the requested resource.

4.6.6. 406 Not Acceptable

The resource identified by the request is only capable of generating response representations which have content characteristics not acceptable according to the Accept and Accept-* header fields sent in the request.

Unless it was a HEAD request, the response SHOULD include a representation containing a list of available representation characteristics and location(s) from which the user or user agent can choose the one most appropriate. Depending upon the format and the capabilities of the user agent, selection of the most appropriate choice MAY be performed automatically. However, this specification does not define any standard for such automatic selection.

Note: HTTP/1.1 servers are allowed to return responses which are not acceptable according to the accept header fields sent in the request. In some cases, this might even be preferable to sending a 406 response. User agents are encouraged to inspect the header fields of an incoming response to determine if it is acceptable.

If the response could be unacceptable, a user agent SHOULD temporarily stop receipt of more data and query the user for a decision on further actions.

4.6.7. 408 Request Timeout

The client did not produce a request within the time that the server was prepared to wait. The client MAY repeat the request without modifications at any later time.

4.6.8. 409 Conflict

The request could not be completed due to a conflict with the current state of the resource. This code is only allowed in situations where it is expected that the user might be able to resolve the conflict and resubmit the request. The response body SHOULD include enough information for the user to recognize the source of the conflict. Ideally, the response representation would include enough information for the user or user agent to fix the problem; however, that might not be possible and is not required.

Conflicts are most likely to occur in response to a PUT request. For example, if versioning were being used and the representation being PUT included changes to a resource which conflict with those made by an earlier (third-party) request, the server might use the 409 response to indicate that it can't complete the request. In this case, the response representation would likely contain a list of the differences between the two versions.

4.6.9. 410 Gone

The target resource is no longer available at the server and no forwarding address is known. This condition is expected to be considered permanent. Clients with link editing capabilities SHOULD delete references to the effective request URI after user approval. If the server does not know, or has no facility to determine, whether or not the condition is permanent, the status code 404 (Not Found) SHOULD be used instead.

The 410 response is primarily intended to assist the task of web maintenance by notifying the recipient that the resource is intentionally unavailable and that the server owners desire that remote links to that resource be removed. Such an event is common for limited-time, promotional services and for resources belonging to individuals no longer working at the server's site. It is not necessary to mark all permanently unavailable resources as "gone" or to keep the mark for any length of time — that is left to the discretion of the server owner.

Caches MAY use a heuristic (see Section 4.1.2 of [Part6]) to determine freshness for 410 responses.

4.6.10. 411 Length Required

The server refuses to accept the request without a defined Content-Length. The client MAY repeat the request if it adds a valid Content-Length header field containing the length of the message body in the request message.

4.6.11. 413 Request Representation Too Large

The server is refusing to process a request because the request representation is larger than the server is willing or able to process. The server MAY close the connection to prevent the client from continuing the request.

If the condition is temporary, the server SHOULD include a Retry-After header field to indicate that it is temporary and after what time the client MAY try again.

4.6.12. 414 URI Too Long

The server is refusing to service the request because the effective request URI is longer than the server is willing to interpret. This rare condition is only likely to occur when a client has improperly converted a POST request to a GET request with long query information, when the client has descended into a URI "black hole" of redirection (e.g., a redirected URI prefix that points to a suffix of itself), or when the server is under attack by a client attempting to exploit security holes present in some servers using fixed-length buffers for reading or manipulating the request-target.

4.6.13. 415 Unsupported Media Type

The server is refusing to service the request because the request payload is in a format not supported by this request method on the target resource.

4.6.14. 417 Expectation Failed

The expectation given in an Expect header field (see Section 9.11) could not be met by this server, or, if the server is a proxy, the server has unambiguous evidence that the request could not be met by the next-hop server.

4.6.15. 426 Upgrade Required

The request can not be completed without a prior protocol upgrade. This response MUST include an Upgrade header field (Section 6.5 of [Part1]) specifying the required protocols.

Example:

HTTP/1.1 426 Upgrade Required
Upgrade: HTTP/3.0
Connection: Upgrade
Content-Length: 53
Content-Type: text/plain

This service requires use of the HTTP/3.0 protocol.

The server SHOULD include a message body in the 426 response which indicates in human readable form the reason for the error and describes any alternative courses which might be available to the user.

4.7. Server Error 5xx

Response status codes beginning with the digit "5" indicate cases in which the server is aware that it has erred or is incapable of performing the request. Except when responding to a HEAD request, the server SHOULD include a representation containing an explanation of the error situation, and whether it is a temporary or permanent condition. User agents SHOULD display any included representation to the user. These response codes are applicable to any request method.

4.7.1. 500 Internal Server Error

The server encountered an unexpected condition which prevented it from fulfilling the request.

4.7.2. 501 Not Implemented

The server does not support the functionality required to fulfill the request. This is the appropriate response when the server does not recognize the request method and is not capable of supporting it for any resource.

4.7.3. 502 Bad Gateway

The server, while acting as a gateway or proxy, received an invalid response from the upstream server it accessed in attempting to fulfill the request.

4.7.4. 503 Service Unavailable

The server is currently unable to handle the request due to a temporary overloading or maintenance of the server.

The implication is that this is a temporary condition which will be alleviated after some delay. If known, the length of the delay MAY be indicated in a Retry-After header field (Section 9.16). If no Retry-After is given, the client SHOULD handle the response as it would for a 500 (Internal Server Error) response.

Note: The existence of the 503 status code does not imply that a server has to use it when becoming overloaded. Some servers might wish to simply refuse the connection.

4.7.5. 504 Gateway Timeout

The server, while acting as a gateway or proxy, did not receive a timely response from the upstream server specified by the URI (e.g., HTTP, FTP, LDAP) or some other auxiliary server (e.g., DNS) it needed to access in attempting to complete the request.

4.7.6. 505 HTTP Version Not Supported

The server does not support, or refuses to support, the protocol version that was used in the request message. The server is indicating that it is unable or unwilling to complete the request using the same major version as the client, as described in Section 2.7 of [Part1], other than with this error message. The response SHOULD contain a representation describing why that version is not supported and what other protocols are supported by that server.

5. Protocol Parameters 5.1. Date/Time Formats

HTTP applications have historically allowed three different formats for date/time stamps. However, the preferred format is a fixed-length subset of that defined by [RFC1123]:

Sun, 06 Nov 1994 08:49:37 GMT  ; RFC 1123

The other formats are described here only for compatibility with obsolete implementations.

Sunday, 06-Nov-94 08:49:37 GMT ; obsolete RFC 850 format
Sun Nov  6 08:49:37 1994       ; ANSI C's asctime() format

HTTP/1.1 clients and servers that parse a date value MUST accept all three formats (for compatibility with HTTP/1.0), though they MUST only generate the RFC 1123 format for representing HTTP-date values in header fields.

All HTTP date/time stamps MUST be represented in Greenwich Mean Time (GMT), without exception. For the purposes of HTTP, GMT is exactly equal to UTC (Coordinated Universal Time). This is indicated in the first two formats by the inclusion of "GMT" as the three-letter abbreviation for time zone, and MUST be assumed when reading the asctime format. HTTP-date is case sensitive and MUST NOT include additional whitespace beyond that specifically included as SP in the grammar.

Preferred format:

  rfc1123-date = day-name "," SP date1 SP time-of-day SP GMT
  ; fixed length subset of the format defined in
  ; Section 5.2.14 of [RFC1123]
  
  day-name     = %x4D.6F.6E ; "Mon", case-sensitive
               / %x54.75.65 ; "Tue", case-sensitive
               / %x57.65.64 ; "Wed", case-sensitive
               / %x54.68.75 ; "Thu", case-sensitive
               / %x46.72.69 ; "Fri", case-sensitive
               / %x53.61.74 ; "Sat", case-sensitive
               / %x53.75.6E ; "Sun", case-sensitive
               
  date1        = day SP month SP year
               ; e.g., 02 Jun 1982

  day          = 2DIGIT
  month        = %x4A.61.6E ; "Jan", case-sensitive
               / %x46.65.62 ; "Feb", case-sensitive
               / %x4D.61.72 ; "Mar", case-sensitive
               / %x41.70.72 ; "Apr", case-sensitive
               / %x4D.61.79 ; "May", case-sensitive
               / %x4A.75.6E ; "Jun", case-sensitive
               / %x4A.75.6C ; "Jul", case-sensitive
               / %x41.75.67 ; "Aug", case-sensitive
               / %x53.65.70 ; "Sep", case-sensitive
               / %x4F.63.74 ; "Oct", case-sensitive
               / %x4E.6F.76 ; "Nov", case-sensitive
               / %x44.65.63 ; "Dec", case-sensitive
  year         = 4DIGIT

  GMT   = %x47.4D.54 ; "GMT", case-sensitive

  time-of-day  = hour ":" minute ":" second
                 ; 00:00:00 - 23:59:59
                 
  hour         = 2DIGIT               
  minute       = 2DIGIT               
  second       = 2DIGIT

The semantics of day-name, day, month, year, and time-of-day are the same as those defined for the RFC 5322 constructs with the corresponding name ([RFC5322], Section 3.3).

Obsolete formats:

  rfc850-date  = day-name-l "," SP date2 SP time-of-day SP GMT
  date2        = day "-" month "-" 2DIGIT
                 ; day-month-year (e.g., 02-Jun-82)

  day-name-l   = %x4D.6F.6E.64.61.79 ; "Monday", case-sensitive
         / %x54.75.65.73.64.61.79 ; "Tuesday", case-sensitive
         / %x57.65.64.6E.65.73.64.61.79 ; "Wednesday", case-sensitive
         / %x54.68.75.72.73.64.61.79 ; "Thursday", case-sensitive
         / %x46.72.69.64.61.79 ; "Friday", case-sensitive
         / %x53.61.74.75.72.64.61.79 ; "Saturday", case-sensitive
         / %x53.75.6E.64.61.79 ; "Sunday", case-sensitive

Note: Recipients of date values are encouraged to be robust in accepting date values that might have been sent by non-HTTP applications, as is sometimes the case when retrieving or posting messages via proxies/gateways to SMTP or NNTP.

Note: HTTP requirements for the date/time stamp format apply only to their usage within the protocol stream. Clients and servers are not required to use these formats for user presentation, request logging, etc.

5.2. Product Tokens

Product tokens are used to allow communicating applications to identify themselves by software name and version. Most fields using product tokens also allow sub-products which form a significant part of the application to be listed, separated by whitespace. By convention, the products are listed in order of their significance for identifying the application.

Examples:

  User-Agent: CERN-LineMode/2.15 libwww/2.17b3
  Server: Apache/0.8.4

Product tokens SHOULD be short and to the point. They MUST NOT be used for advertising or other non-essential information. Although any token octet MAY appear in a product-version, this token SHOULD only be used for a version identifier (i.e., successive versions of the same product SHOULD only differ in the product-version portion of the product value).

5.3. Character Encodings (charset)

HTTP uses charset names to indicate the character encoding of a textual representation.

A character encoding is identified by a case-insensitive token. The complete set of tokens is defined by the IANA Character Set registry (<http://www.iana.org/assignments/character-sets>).

Although HTTP allows an arbitrary token to be used as a charset value, any token that has a predefined value within the IANA Character Set registry MUST represent the character encoding defined by that registry. Applications SHOULD limit their use of character encodings to those defined within the IANA registry.

HTTP uses charset in two contexts: within an Accept-Charset request header field (in which the charset value is an unquoted token) and as the value of a parameter in a Content-Type header field (within a request or response), in which case the parameter value of the charset parameter can be quoted.

Implementers need to be aware of IETF character set requirements [RFC3629] [RFC2277].

5.4. Content Codings

Content coding values indicate an encoding transformation that has been or can be applied to a representation. Content codings are primarily used to allow a representation to be compressed or otherwise usefully transformed without losing the identity of its underlying media type and without loss of information. Frequently, the representation is stored in coded form, transmitted directly, and only decoded by the recipient.

All content-coding values are case-insensitive. HTTP/1.1 uses content-coding values in the Accept-Encoding (Section 9.3) and Content-Encoding (Section 9.6) header fields. Although the value describes the content-coding, what is more important is that it indicates what decoding mechanism will be required to remove the encoding.

5.4.1. Content Coding Registry

The HTTP Content Coding Registry defines the name space for the content coding names.

Registrations MUST include the following fields:

Names of content codings MUST NOT overlap with names of transfer codings (Section 4 of [Part1]), unless the encoding transformation is identical (as is the case for the compression codings defined in Section 4.2 of [Part1]).

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

The registry itself is maintained at <http://www.iana.org/assignments/http-parameters>.

5.5. Media Types

HTTP uses Internet Media Types [RFC2046] in the Content-Type (Section 9.9) and Accept (Section 9.1) header fields in order to provide open and extensible data typing and type negotiation.

The type/subtype MAY be followed by parameters in the form of attribute/value pairs.

The type, subtype, and parameter attribute names are case-insensitive. Parameter values might or might not be case-sensitive, depending on the semantics of the parameter name. The presence or absence of a parameter might be significant to the processing of a media-type, depending on its definition within the media type registry.

A parameter value that matches the token production can be transmitted as either a token or within a quoted-string. The quoted and unquoted values are equivalent.

Note that some older HTTP applications do not recognize media type parameters. When sending data to older HTTP applications, implementations SHOULD only use media type parameters when they are required by that type/subtype definition.

Media-type values are registered with the Internet Assigned Number Authority (IANA). The media type registration process is outlined in [RFC4288]. Use of non-registered media types is discouraged.

5.5.1. Canonicalization and Text Defaults

Internet media types are registered with a canonical form. A representation transferred via HTTP messages MUST be in the appropriate canonical form prior to its transmission except for "text" types, as defined in the next paragraph.

When in canonical form, media subtypes of the "text" type use CRLF as the text line break. HTTP relaxes this requirement and allows the transport of text media with plain CR or LF alone representing a line break when it is done consistently for an entire representation. HTTP applications MUST accept CRLF, bare CR, and bare LF as indicating a line break in text media received via HTTP. In addition, if the text is in a character encoding that does not use octets 13 and 10 for CR and LF respectively, as is the case for some multi-byte character encodings, HTTP allows the use of whatever octet sequences are defined by that character encoding to represent the equivalent of CR and LF for line breaks. This flexibility regarding line breaks applies only to text media in the payload body; a bare CR or LF MUST NOT be substituted for CRLF within any of the HTTP control structures (such as header fields and multipart boundaries).

If a representation is encoded with a content-coding, the underlying data MUST be in a form defined above prior to being encoded.

5.5.2. Multipart Types

MIME provides for a number of "multipart" types — encapsulations of one or more representations within a single message body. All multipart types share a common syntax, as defined in Section 5.1.1 of [RFC2046], and MUST include a boundary parameter as part of the media type value. The message body is itself a protocol element and MUST therefore use only CRLF to represent line breaks between body-parts.

In general, HTTP treats a multipart message body no differently than any other media type: strictly as payload. HTTP does not use the multipart boundary as an indicator of message body length. In all other respects, an HTTP user agent SHOULD follow the same or similar behavior as a MIME user agent would upon receipt of a multipart type. The MIME header fields within each body-part of a multipart message body do not have any significance to HTTP beyond that defined by their MIME semantics.

If an application receives an unrecognized multipart subtype, the application MUST treat it as being equivalent to "multipart/mixed".

Note: The "multipart/form-data" type has been specifically defined for carrying form data suitable for processing via the POST request method, as described in [RFC2388].

5.6. Language Tags

A language tag, as defined in [RFC5646], identifies a natural language spoken, written, or otherwise conveyed by human beings for communication of information to other human beings. Computer languages are explicitly excluded. HTTP uses language tags within the Accept-Language and Content-Language fields.

In summary, a language tag is composed of one or more parts: A primary language subtag followed by a possibly empty series of subtags:

White space is not allowed within the tag and all tags are case-insensitive. The name space of language subtags is administered by the IANA (see <http://www.iana.org/assignments/language-subtag-registry>).

Example tags include:

  en, en-US, es-419, az-Arab, x-pig-latin, man-Nkoo-GN

See [RFC5646] for further information.

6. Payload

HTTP messages MAY transfer a payload if not otherwise restricted by the request method or response status code. The payload consists of metadata, in the form of header fields, and data, in the form of the sequence of octets in the message body after any transfer-coding has been decoded.

A "payload" in HTTP is always a partial or complete representation of some resource. We use separate terms for payload and representation because some messages contain only the associated representation's header fields (e.g., responses to HEAD) or only some part(s) of the representation (e.g., the 206 (Partial Content) status code).

6.2. Payload Body

A payload body is only present in a message when a message body is present, as described in Section 3.3 of [Part1]. The payload body is obtained from the message body by decoding any Transfer-Encoding that might have been applied to ensure safe and proper transfer of the message.

7. Representation

A "representation" is information in a format that can be readily communicated from one party to another. A resource representation is information that reflects the state of that resource, as observed at some point in the past (e.g., in a response to GET) or to be desired at some point in the future (e.g., in a PUT request).

Most, but not all, representations transferred via HTTP are intended to be a representation of the target resource (the resource identified by the effective request URI). The precise semantics of a representation are determined by the type of message (request or response), the request method, the response status code, and the representation metadata. For example, the above semantic is true for the representation in any 200 (OK) response to GET and for the representation in any PUT request. A 200 response to PUT, in contrast, contains either a representation that describes the successful action or a representation of the target resource, with the latter indicated by a Content-Location header field with the same value as the effective request URI. Likewise, response messages with an error status code usually contain a representation that describes the error and what next steps are suggested for resolving it.

Request and Response messages MAY transfer a representation if not otherwise restricted by the request method or response status code. A representation consists of metadata (representation header fields) and data (representation body). When a complete or partial representation is enclosed in an HTTP message, it is referred to as the payload of the message.

A representation body is only present in a message when a message body is present, as described in Section 3.3 of [Part1]. The representation body is obtained from the message body by decoding any Transfer-Encoding that might have been applied to ensure safe and proper transfer of the message.

7.1. Identifying the Resource Associated with a Representation

It is sometimes necessary to determine an identifier for the resource associated with a representation.

An HTTP request representation, when present, is always associated with an anonymous (i.e., unidentified) resource.

In the common case, an HTTP response is a representation of the target resource (see Section 5.5 of [Part1]). However, this is not always the case. To determine the URI of the resource a response is associated with, the following rules are used (with the first applicable one being selected):

  1. If the response status code is 200 (OK) or 203 (Non-Authoritative Information) and the request method was GET, the response payload is a representation of the target resource.
  2. If the response status code is 204 (No Content), 206 (Partial Content), or 304 (Not Modified) and the request method was GET or HEAD, the response payload is a partial representation of the target resource.
  3. If the response has a Content-Location header field, and that URI is the same as the effective request URI, the response payload is a representation of the target resource.
  4. If the response has a Content-Location header field, and that URI is not the same as the effective request URI, then the response asserts that its payload is a representation of the resource identified by the Content-Location URI. However, such an assertion cannot be trusted unless it can be verified by other means (not defined by HTTP).
  5. Otherwise, the response is a representation of an anonymous (i.e., unidentified) resource.

7.3. Representation Data

The representation body associated with an HTTP message is either provided as the payload body of the message or referred to by the message semantics and the effective request URI. The representation data is in a format and encoding defined by the representation metadata header fields.

The data type of the representation data is determined via the header fields Content-Type and Content-Encoding. These define a two-layer, ordered encoding model:

  representation-data := Content-Encoding( Content-Type( bits ) )

Content-Type specifies the media type of the underlying data, which defines both the data format and how that data SHOULD be processed by the recipient (within the scope of the request method semantics). Any HTTP/1.1 message containing a payload body SHOULD include a Content-Type header field defining the media type of the associated representation unless that metadata is unknown to the sender. If the Content-Type header field is not present, it indicates that the sender does not know the media type of the representation; recipients MAY either assume that the media type is "application/octet-stream" ([RFC2046], Section 4.5.1) or examine the content to determine its type.

In practice, resource owners do not always properly configure their origin server to provide the correct Content-Type for a given representation, with the result that some clients will examine a response body's content and override the specified type. Clients that do so risk drawing incorrect conclusions, which might expose additional security risks (e.g., "privilege escalation"). Furthermore, it is impossible to determine the sender's intent by examining the data format: many data formats match multiple media types that differ only in processing semantics. Implementers are encouraged to provide a means of disabling such "content sniffing" when it is used.

Content-Encoding is used to indicate any additional content codings applied to the data, usually for the purpose of data compression, that are a property of the representation. If Content-Encoding is not present, then there is no additional encoding beyond that defined by the Content-Type header field.

8. Content Negotiation

HTTP responses include a representation which contains information for interpretation, whether by a human user or for further processing. Often, the server has different ways of representing the same information; for example, in different formats, languages, or using different character encodings.

HTTP clients and their users might have different or variable capabilities, characteristics or preferences which would influence which representation, among those available from the server, would be best for the server to deliver. For this reason, HTTP provides mechanisms for "content negotiation" — a process of allowing selection of a representation of a given resource, when more than one is available.

This specification defines two patterns of content negotiation; "server-driven", where the server selects the representation based upon the client's stated preferences, and "agent-driven" negotiation, where the server provides a list of representations for the client to choose from, based upon their metadata. In addition, there are other patterns: some applications use an "active content" pattern, where the server returns active content which runs on the client and, based on client available parameters, selects additional resources to invoke. "Transparent Content Negotiation" ([RFC2295]) has also been proposed.

These patterns are all widely used, and have trade-offs in applicability and practicality. In particular, when the number of preferences or capabilities to be expressed by a client are large (such as when many different formats are supported by a user-agent), server-driven negotiation becomes unwieldy, and might not be appropriate. Conversely, when the number of representations to choose from is very large, agent-driven negotiation might not be appropriate.

Note that in all cases, the supplier of representations has the responsibility for determining which representations might be considered to be the "same information".

8.1. Server-driven Negotiation

If the selection of the best representation for a response is made by an algorithm located at the server, it is called server-driven negotiation. Selection is based on the available representations of the response (the dimensions over which it can vary; e.g., language, content-coding, etc.) and the contents of particular header fields in the request message or on other information pertaining to the request (such as the network address of the client).

Server-driven negotiation is advantageous when the algorithm for selecting from among the available representations is difficult to describe to the user agent, or when the server desires to send its "best guess" to the client along with the first response (hoping to avoid the round-trip delay of a subsequent request if the "best guess" is good enough for the user). In order to improve the server's guess, the user agent MAY include request header fields (Accept, Accept-Language, Accept-Encoding, etc.) which describe its preferences for such a response.

Server-driven negotiation has disadvantages:

  1. It is impossible for the server to accurately determine what might be "best" for any given user, since that would require complete knowledge of both the capabilities of the user agent and the intended use for the response (e.g., does the user want to view it on screen or print it on paper?).
  2. Having the user agent describe its capabilities in every request can be both very inefficient (given that only a small percentage of responses have multiple representations) and a potential violation of the user's privacy.
  3. It complicates the implementation of an origin server and the algorithms for generating responses to a request.
  4. It might limit a public cache's ability to use the same response for multiple user's requests.

Server-driven negotiation allows the user agent to specify its preferences, but it cannot expect responses to always honor them. For example, the origin server might not implement server-driven negotiation, or it might decide that sending a response that doesn't conform to them is better than sending a 406 (Not Acceptable) response.

Many of the mechanisms for expressing preferences use quality values to declare relative preference. See Section 4.3.1 of [Part1] for more information.

HTTP/1.1 includes the following header fields for enabling server-driven negotiation through description of user agent capabilities and user preferences: Accept (Section 9.1), Accept-Charset (Section 9.2), Accept-Encoding (Section 9.3), Accept-Language (Section 9.4), and User-Agent (Section 9.18). However, an origin server is not limited to these dimensions and MAY vary the response based on any aspect of the request, including aspects of the connection (e.g., IP address) or information within extension header fields not defined by this specification.

Note: In practice, User-Agent based negotiation is fragile, because new clients might not be recognized.

The Vary header field (Section 7.5 of [Part6]) can be used to express the parameters the server uses to select a representation that is subject to server-driven negotiation.

8.2. Agent-driven Negotiation

With agent-driven negotiation, selection of the best representation for a response is performed by the user agent after receiving an initial response from the origin server. Selection is based on a list of the available representations of the response included within the header fields or body of the initial response, with each representation identified by its own URI. Selection from among the representations can be performed automatically (if the user agent is capable of doing so) or manually by the user selecting from a generated (possibly hypertext) menu.

Agent-driven negotiation is advantageous when the response would vary over commonly-used dimensions (such as type, language, or encoding), when the origin server is unable to determine a user agent's capabilities from examining the request, and generally when public caches are used to distribute server load and reduce network usage.

Agent-driven negotiation suffers from the disadvantage of needing a second request to obtain the best alternate representation. This second request is only efficient when caching is used. In addition, this specification does not define any mechanism for supporting automatic selection, though it also does not prevent any such mechanism from being developed as an extension and used within HTTP/1.1.

This specification defines the 300 (Multiple Choices) and 406 (Not Acceptable) status codes for enabling agent-driven negotiation when the server is unwilling or unable to provide a varying response using server-driven negotiation.

10. IANA Considerations 10.1. Method Registry

The registration procedure for HTTP request methods is defined by Section 2.2 of this document.

The HTTP Method Registry shall be created at <http://www.iana.org/assignments/http-methods> and be populated with the registrations below:

10.2. Status Code Registry

The registration procedure for HTTP Status Codes — previously defined in Section 7.1 of [RFC2817] — is now defined by Section 4.2 of this document.

The HTTP Status Code Registry located at <http://www.iana.org/assignments/http-status-codes> shall be updated with the registrations below:

10.4. Content Coding Registry

The registration procedure for HTTP Content Codings is now defined by Section 5.4.1 of this document.

The HTTP Content Codings Registry located at <http://www.iana.org/assignments/http-parameters> shall be updated with the registration below:

11. 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.

11.1. Transfer of Sensitive Information

Like any generic data transfer protocol, HTTP cannot regulate the content of the data that is transferred, nor is there any a priori method of determining the sensitivity of any particular piece of information within the context of any given request. Therefore, applications SHOULD supply as much control over this information as possible to the provider of that information. Four header fields are worth special mention in this context: Server, Via, Referer and From.

Revealing the specific software version of the server might allow the server machine to become more vulnerable to attacks against software that is known to contain security holes. Implementers SHOULD make the Server header field a configurable option.

Proxies which serve as a portal through a network firewall SHOULD take special precautions regarding the transfer of header information that identifies the hosts behind the firewall. In particular, they SHOULD remove, or replace with sanitized versions, any Via fields generated behind the firewall.

The Referer header field allows reading patterns to be studied and reverse links drawn. Although it can be very useful, its power can be abused if user details are not separated from the information contained in the Referer. Even when the personal information has been removed, the Referer header field might indicate a private document's URI whose publication would be inappropriate.

The information sent in the From field might conflict with the user's privacy interests or their site's security policy, and hence it SHOULD NOT be transmitted without the user being able to disable, enable, and modify the contents of the field. The user MUST be able to set the contents of this field within a user preference or application defaults configuration.

We suggest, though do not require, that a convenient toggle interface be provided for the user to enable or disable the sending of From and Referer information.

The User-Agent (Section 9.18) or Server (Section 9.17) header fields can sometimes be used to determine that a specific client or server has a particular security hole which might be exploited. Unfortunately, this same information is often used for other valuable purposes for which HTTP currently has no better mechanism.

Furthermore, the User-Agent header field might contain enough entropy to be used, possibly in conjunction with other material, to uniquely identify the user.

Some request methods, like TRACE (Section 2.3.7), expose information that was sent in request header fields within the body of their response. Clients SHOULD be careful with sensitive information, like Cookies, Authorization credentials, and other header fields that might be used to collect data from the client.

11.2. Encoding Sensitive Information in URIs

Because the source of a link might be private information or might reveal an otherwise private information source, it is strongly recommended that the user be able to select whether or not the Referer field is sent. For example, a browser client could have a toggle switch for browsing openly/anonymously, which would respectively enable/disable the sending of Referer and From information.

Clients SHOULD NOT include a Referer header field in a (non-secure) HTTP request if the referring page was transferred with a secure protocol.

Authors of services SHOULD NOT use GET-based forms for the submission of sensitive data because that data will be placed in the request-target. Many existing servers, proxies, and user agents log or display the request-target in places where it might be visible to third parties. Such services can use POST-based form submission instead.

11.3. Location Header Fields: Spoofing and Information Leakage

If a single server supports multiple organizations that do not trust one another, then it MUST check the values of Location and Content-Location header fields in responses that are generated under control of said organizations to make sure that they do not attempt to invalidate resources over which they have no authority.

Furthermore, appending the fragment identifier from one URI to another one obtained from a Location header field might leak confidential information to the target server — although the fragment identifier is not transmitted in the final request, it might be visible to the user agent through other means, such as scripting.

11.4. Security Considerations for CONNECT

Since tunneled data is opaque to the proxy, there are additional risks to tunneling to other well-known or reserved ports. A HTTP client CONNECTing to port 25 could relay spam via SMTP, for example. As such, proxies SHOULD restrict CONNECT access to a small number of known ports.

12. Acknowledgments

See Section 9 of [Part1].

13. References 13.1. Normative References
[Part1]
Fielding, R., Ed., Lafon, Y., Ed., and J. Reschke, Ed., “HTTP/1.1, part 1: Message Routing and Syntax"”, Internet-Draft draft-ietf-httpbis-p1-messaging-20 (work in progress), July 2012.
[Part4]
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.
[Part5]
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.
[Part6]
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.
[Part7]
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.
[RFC1950]
Deutsch, L. and J-L. Gailly, “ZLIB Compressed Data Format Specification version 3.3”, RFC 1950, May 1996.
[RFC1951]
Deutsch, P., “DEFLATE Compressed Data Format Specification version 1.3”, RFC 1951, May 1996.
[RFC1952]
Deutsch, P., Gailly, J-L., Adler, M., Deutsch, L., and G. Randers-Pehrson, “GZIP file format specification version 4.3”, RFC 1952, May 1996.
[RFC2045]
Freed, N. and N. Borenstein, “Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies”, RFC 2045, November 1996.
[RFC2046]
Freed, N. and N. Borenstein, “Multipurpose Internet Mail Extensions (MIME) Part Two: Media Types”, RFC 2046, November 1996.
[RFC2119]
Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels”, BCP 14, RFC 2119, March 1997.
[RFC3986]
Berners-Lee, T., Fielding, R., and L. Masinter, “Uniform Resource Identifier (URI): Generic Syntax”, STD 66, RFC 3986, January 2005.
[RFC4647]
Phillips, A., Ed. and M. Davis, Ed., “Matching of Language Tags”, BCP 47, RFC 4647, September 2006.
[RFC5234]
Crocker, D., Ed. and P. Overell, “Augmented BNF for Syntax Specifications: ABNF”, STD 68, RFC 5234, January 2008.
[RFC5646]
Phillips, A., Ed. and M. Davis, Ed., “Tags for Identifying Languages”, BCP 47, RFC 5646, September 2009.
13.2. Informative References
[draft-reschke-http-status-308]
Reschke, J., “The Hypertext Transfer Protocol (HTTP) Status Code 308 (Permanent Redirect)”, Internet-Draft draft-reschke-http-status-308-07 (work in progress), March 2012.
[RFC1123]
Braden, R., “Requirements for Internet Hosts - Application and Support”, STD 3, RFC 1123, October 1989.
[RFC1945]
Berners-Lee, T., Fielding, R., and H. Nielsen, “Hypertext Transfer Protocol -- HTTP/1.0”, RFC 1945, May 1996.
[RFC2049]
Freed, N. and N. Borenstein, “Multipurpose Internet Mail Extensions (MIME) Part Five: Conformance Criteria and Examples”, RFC 2049, November 1996.
[RFC2068]
Fielding, R., Gettys, J., Mogul, J., Nielsen, H., and T. Berners-Lee, “Hypertext Transfer Protocol -- HTTP/1.1”, RFC 2068, January 1997.
[RFC2076]
Palme, J., “Common Internet Message Headers”, RFC 2076, February 1997.
[RFC2277]
Alvestrand, H., “IETF Policy on Character Sets and Languages”, BCP 18, RFC 2277, January 1998.
[RFC2295]
Holtman, K. and A. Mutz, “Transparent Content Negotiation in HTTP”, RFC 2295, March 1998.
[RFC2388]
Masinter, L., “Returning Values from Forms: multipart/form-data”, RFC 2388, August 1998.
[RFC2557]
Palme, F., Hopmann, A., Shelness, N., and E. Stefferud, “MIME Encapsulation of Aggregate Documents, such as HTML (MHTML)”, RFC 2557, March 1999.
[RFC2616]
Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Masinter, L., Leach, P., and T. Berners-Lee, “Hypertext Transfer Protocol -- HTTP/1.1”, RFC 2616, June 1999.
[RFC2817]
Khare, R. and S. Lawrence, “Upgrading to TLS Within HTTP/1.1”, RFC 2817, May 2000.
[RFC3629]
Yergeau, F., “UTF-8, a transformation format of ISO 10646”, STD 63, RFC 3629, November 2003.
[RFC3864]
Klyne, G., Nottingham, M., and J. Mogul, “Registration Procedures for Message Header Fields”, BCP 90, RFC 3864, September 2004.
[RFC4288]
Freed, N. and J. Klensin, “Media Type Specifications and Registration Procedures”, BCP 13, RFC 4288, December 2005.
[RFC5226]
Narten, T. and H. Alvestrand, “Guidelines for Writing an IANA Considerations Section in RFCs”, BCP 26, RFC 5226, May 2008.
[RFC5322]
Resnick, P., “Internet Message Format”, RFC 5322, October 2008.
[RFC5789]
Dusseault, L. and J. Snell, “PATCH Method for HTTP”, RFC 5789, March 2010.
[RFC5987]
Reschke, J., “Character Set and Language Encoding for Hypertext Transfer Protocol (HTTP) Header Field Parameters”, RFC 5987, August 2010.
[RFC6151]
Turner, S. and L. Chen, “Updated Security Considerations for the MD5 Message-Digest and the HMAC-MD5 Algorithms”, RFC 6151, March 2011.
[RFC6266]
Reschke, J., “Use of the Content-Disposition Header Field in the Hypertext Transfer Protocol (HTTP)”, RFC 6266, June 2011.
Appendix A. Differences between HTTP and MIME

HTTP/1.1 uses many of the constructs defined for Internet Mail ([RFC5322]) and the Multipurpose Internet Mail Extensions (MIME [RFC2045]) to allow a message body to be transmitted in an open variety of representations and with extensible mechanisms. However, RFC 2045 discusses mail, and HTTP has a few features that are different from those described in MIME. These differences were carefully chosen to optimize performance over binary connections, to allow greater freedom in the use of new media types, to make date comparisons easier, and to acknowledge the practice of some early HTTP servers and clients.

This appendix describes specific areas where HTTP differs from MIME. Proxies and gateways to strict MIME environments SHOULD be aware of these differences and provide the appropriate conversions where necessary. Proxies and gateways from MIME environments to HTTP also need to be aware of the differences because some conversions might be required.

A.1. MIME-Version

HTTP is not a MIME-compliant protocol. However, HTTP/1.1 messages MAY include a single MIME-Version header field to indicate what version of the MIME protocol was used to construct the message. Use of the MIME-Version header field indicates that the message is in full conformance with the MIME protocol (as defined in [RFC2045]). Proxies/gateways are responsible for ensuring full conformance (where possible) when exporting HTTP messages to strict MIME environments.

MIME version "1.0" is the default for use in HTTP/1.1. However, HTTP/1.1 message parsing and semantics are defined by this document and not the MIME specification.

A.2. Conversion to Canonical Form

MIME requires that an Internet mail body-part be converted to canonical form prior to being transferred, as described in Section 4 of [RFC2049]. Section 5.5.1 of this document describes the forms allowed for subtypes of the "text" media type when transmitted over HTTP. [RFC2046] requires that content with a type of "text" represent line breaks as CRLF and forbids the use of CR or LF outside of line break sequences. HTTP allows CRLF, bare CR, and bare LF to indicate a line break within text content when a message is transmitted over HTTP.

Where it is possible, a proxy or gateway from HTTP to a strict MIME environment SHOULD translate all line breaks within the text media types described in Section 5.5.1 of this document to the RFC 2049 canonical form of CRLF. Note, however, that this might be complicated by the presence of a Content-Encoding and by the fact that HTTP allows the use of some character encodings which do not use octets 13 and 10 to represent CR and LF, respectively, as is the case for some multi-byte character encodings.

Conversion will break any cryptographic checksums applied to the original content unless the original content is already in canonical form. Therefore, the canonical form is recommended for any content that uses such checksums in HTTP.

A.3. Conversion of Date Formats

HTTP/1.1 uses a restricted set of date formats (Section 5.1) to simplify the process of date comparison. Proxies and gateways from other protocols SHOULD ensure that any Date header field present in a message conforms to one of the HTTP/1.1 formats and rewrite the date if necessary.

A.4. Introduction of Content-Encoding

MIME does not include any concept equivalent to HTTP/1.1's Content-Encoding header field. Since this acts as a modifier on the media type, proxies and gateways from HTTP to MIME-compliant protocols MUST either change the value of the Content-Type header field or decode the representation before forwarding the message. (Some experimental applications of Content-Type for Internet mail have used a media-type parameter of ";conversions=<content-coding>" to perform a function equivalent to Content-Encoding. However, this parameter is not part of the MIME standards).

A.5. No Content-Transfer-Encoding

HTTP does not use the Content-Transfer-Encoding field of MIME. Proxies and gateways from MIME-compliant protocols to HTTP MUST remove any Content-Transfer-Encoding prior to delivering the response message to an HTTP client.

Proxies and gateways from HTTP to MIME-compliant protocols are responsible for ensuring that the message is in the correct format and encoding for safe transport on that protocol, where "safe transport" is defined by the limitations of the protocol being used. Such a proxy or gateway SHOULD label the data with an appropriate Content-Transfer-Encoding if doing so will improve the likelihood of safe transport over the destination protocol.

A.6. MHTML and Line Length Limitations

HTTP implementations which share code with MHTML [RFC2557] implementations need to be aware of MIME line length limitations. Since HTTP does not have this limitation, HTTP does not fold long lines. MHTML messages being transported by HTTP follow all conventions of MHTML, including line length limitations and folding, canonicalization, etc., since HTTP transports all message-bodies as payload (see Section 5.5.2) and does not interpret the content or any MIME header lines that might be contained therein.

Appendix B. Additional Features

[RFC1945] and [RFC2068] document protocol elements used by some existing HTTP implementations, but not consistently and correctly across most HTTP/1.1 applications. Implementers are advised to be aware of these features, but cannot rely upon their presence in, or interoperability with, other HTTP/1.1 applications. Some of these describe proposed experimental features, and some describe features that experimental deployment found lacking that are now addressed in the base HTTP/1.1 specification.

A number of other header fields, such as Content-Disposition and Title, from SMTP and MIME are also often implemented (see [RFC6266] and [RFC2076]).

Appendix C. Changes from RFC 2616

Introduce Method Registry. (Section 2.2)

Clarify definition of POST. (Section 2.3.4)

Remove requirement to handle all Content-* header fields; ban use of Content-Range with PUT. (Section 2.3.5)

Take over definition of CONNECT method from [RFC2817]. (Section 2.3.8)

Take over the Status Code Registry, previously defined in Section 7.1 of [RFC2817]. (Section 4.2)

Broadened the definition of 203 (Non-Authoritative Information) to include cases of payload transformations as well. (Section 4.4.4)

Status codes 301, 302, and 307: removed the normative requirements on both response payloads and user interaction. (Section 4.5)

Failed to consider that there are many other request methods that are safe to automatically redirect, and further that the user agent is able to make that determination based on the request method semantics. Furthermore, allow user agents to rewrite the method from POST to GET for status codes 301 and 302. (Sections 4.5.2, 4.5.3 and 4.5.7)

Deprecate 305 (Use Proxy) status code, because user agents did not implement it. It used to indicate that the target resource needs to be accessed through the proxy given by the Location field. The Location field gave the URI of the proxy. The recipient was expected to repeat this single request via the proxy. (Section 4.5.5)

Define status 426 (Upgrade Required) (this was incorporated from [RFC2817]). (Section 4.6.15)

Change ABNF productions for header fields to only define the field value. (Section 9)

Reclassify "Allow" as response header field, removing the option to specify it in a PUT request. Relax the server requirement on the contents of the Allow header field and remove requirement on clients to always trust the header field value. (Section 9.5)

The ABNF for the Expect header field has been both fixed (allowing parameters for value-less expectations as well) and simplified (allowing trailing semicolons after "100-continue" when they were invalid before). (Section 9.11)

Correct syntax of Location header field to allow URI references (including relative references and fragments), as referred symbol "absoluteURI" wasn't what was expected, and add some clarifications as to when use of fragments would not be appropriate. (Section 9.13)

Restrict Max-Forwards header field to OPTIONS and TRACE (previously, extension methods could have used it as well). (Section 9.14)

Allow Referer field value of "about:blank" as alternative to not specifying it. (Section 9.15)

In the description of the Server header field, the Via field was described as a SHOULD. The requirement was and is stated correctly in the description of the Via header field in Section 6.2 of [Part1]. (Section 9.17)

Clarify contexts that charset is used in. (Section 5.3)

Registration of Content Codings now requires IETF Review (Section 5.4.1)

Remove the default character encoding of "ISO-8859-1" for text media types; the default now is whatever the media type definition says. (Section 5.5.1)

Change ABNF productions for header fields to only define the field value. (Section 9)

Remove definition of Content-MD5 header field because it was inconsistently implemented with respect to partial responses, and also because of known deficiencies in the hash algorithm itself (see [RFC6151] for details). (Section 9)

Remove ISO-8859-1 special-casing in Accept-Charset. (Section 9.2)

Remove base URI setting semantics for Content-Location due to poor implementation support, which was caused by too many broken servers emitting bogus Content-Location header fields, and also the potentially undesirable effect of potentially breaking relative links in content-negotiated resources. (Section 9.8)

Remove reference to non-existant identity transfer-coding value tokens. (Appendix A.5)

Remove discussion of Content-Disposition header field, it is now defined by [RFC6266]. (Appendix B)

Appendix D. Imported ABNF

The following core rules are included by reference, as defined in Appendix B.1 of [RFC5234]: 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 US-ASCII character).

The rules below are defined in [Part1]:

  BWS           = <BWS, defined in [Part1], Section 3.2.1>
  OWS           = <OWS, defined in [Part1], Section 3.2.1>
  RWS           = <RWS, defined in [Part1], Section 3.2.1>
  quoted-string = <quoted-string, defined in [Part1], Section 3.2.4>
  token         = <token, defined in [Part1], Section 3.2.4>
  word          = <word, defined in [Part1], Section 3.2.4>

  absolute-URI  = <absolute-URI, defined in [Part1], Section 2.8>
  comment       = <comment, defined in [Part1], Section 3.2.4>
  partial-URI   = <partial-URI, defined in [Part1], Section 2.8>
  qvalue        = <qvalue, defined in [Part1], Section 4.3.1>
  URI-reference = <URI-reference, defined in [Part1], Section 2.8>
Appendix E. Collected ABNF 
Accept = [ ( "," / ( media-range [ accept-params ] ) ) *( OWS "," [
 OWS ( media-range [ accept-params ] ) ] ) ]
Accept-Charset = *( "," OWS ) ( ( charset / "*" ) [ OWS ";" OWS "q="
 qvalue ] ) *( OWS "," [ OWS ( ( charset / "*" ) [ OWS ";" OWS "q="
 qvalue ] ) ] )
Accept-Encoding = [ ( "," / ( codings [ OWS ";" OWS "q=" qvalue ] ) )
 *( OWS "," [ OWS ( codings [ OWS ";" OWS "q=" qvalue ] ) ] ) ]
Accept-Language = *( "," OWS ) ( language-range [ OWS ";" OWS "q="
 qvalue ] ) *( OWS "," [ OWS ( language-range [ OWS ";" OWS "q="
 qvalue ] ) ] )
Allow = [ ( "," / method ) *( OWS "," [ OWS method ] ) ]

BWS = <BWS, defined in [Part1], Section 3.2.1>

Content-Encoding = *( "," OWS ) content-coding *( OWS "," [ OWS
 content-coding ] )
Content-Language = *( "," OWS ) language-tag *( OWS "," [ OWS
 language-tag ] )
Content-Location = absolute-URI / partial-URI
Content-Type = media-type

Date = HTTP-date

Expect = *( "," OWS ) expectation *( OWS "," [ OWS expectation ] )

From = mailbox

GMT = %x47.4D.54 ; GMT

HTTP-date = rfc1123-date / obs-date

Location = URI-reference

MIME-Version = 1*DIGIT "." 1*DIGIT
Max-Forwards = 1*DIGIT

OWS = <OWS, defined in [Part1], Section 3.2.1>

RWS = <RWS, defined in [Part1], Section 3.2.1>
Referer = absolute-URI / partial-URI
Retry-After = HTTP-date / delta-seconds

Server = product *( RWS ( product / comment ) )

URI-reference = <URI-reference, defined in [Part1], Section 2.8>
User-Agent = product *( RWS ( product / comment ) )

absolute-URI = <absolute-URI, defined in [Part1], Section 2.8>
accept-ext = OWS ";" OWS token [ "=" word ]
accept-params = OWS ";" OWS "q=" qvalue *accept-ext
asctime-date = day-name SP date3 SP time-of-day SP year
attribute = token

charset = token
codings = content-coding / "identity" / "*"
comment = <comment, defined in [Part1], Section 3.2.4>
content-coding = token

date1 = day SP month SP year
date2 = day "-" month "-" 2DIGIT
date3 = month SP ( 2DIGIT / ( SP DIGIT ) )
day = 2DIGIT
day-name = %x4D.6F.6E ; Mon
 / %x54.75.65 ; Tue
 / %x57.65.64 ; Wed
 / %x54.68.75 ; Thu
 / %x46.72.69 ; Fri
 / %x53.61.74 ; Sat
 / %x53.75.6E ; Sun
day-name-l = %x4D.6F.6E.64.61.79 ; Monday
 / %x54.75.65.73.64.61.79 ; Tuesday
 / %x57.65.64.6E.65.73.64.61.79 ; Wednesday
 / %x54.68.75.72.73.64.61.79 ; Thursday
 / %x46.72.69.64.61.79 ; Friday
 / %x53.61.74.75.72.64.61.79 ; Saturday
 / %x53.75.6E.64.61.79 ; Sunday
delta-seconds = 1*DIGIT

expect-name = token
expect-param = expect-name [ BWS "=" BWS expect-value ]
expect-value = token / quoted-string
expectation = expect-name [ BWS "=" BWS expect-value ] *( OWS ";" [
 OWS expect-param ] )

hour = 2DIGIT

language-range = <language-range, defined in [RFC4647], Section 2.1>
language-tag = <Language-Tag, defined in [RFC5646], Section 2.1>

mailbox = <mailbox, defined in [RFC5322], Section 3.4>
media-range = ( "*/*" / ( type "/*" ) / ( type "/" subtype ) ) *( OWS
 ";" OWS parameter )
media-type = type "/" subtype *( OWS ";" OWS parameter )
method = token
minute = 2DIGIT
month = %x4A.61.6E ; Jan
 / %x46.65.62 ; Feb
 / %x4D.61.72 ; Mar
 / %x41.70.72 ; Apr
 / %x4D.61.79 ; May
 / %x4A.75.6E ; Jun
 / %x4A.75.6C ; Jul
 / %x41.75.67 ; Aug
 / %x53.65.70 ; Sep
 / %x4F.63.74 ; Oct
 / %x4E.6F.76 ; Nov
 / %x44.65.63 ; Dec

obs-date = rfc850-date / asctime-date

parameter = attribute "=" value
partial-URI = <partial-URI, defined in [Part1], Section 2.8>
product = token [ "/" product-version ]
product-version = token

quoted-string = <quoted-string, defined in [Part1], Section 3.2.4>
qvalue = <qvalue, defined in [Part1], Section 4.3.1>

rfc1123-date = day-name "," SP date1 SP time-of-day SP GMT
rfc850-date = day-name-l "," SP date2 SP time-of-day SP GMT

second = 2DIGIT
subtype = token

time-of-day = hour ":" minute ":" second
token = <token, defined in [Part1], Section 3.2.4>
type = token

value = word

word = <word, defined in [Part1], Section 3.2.4>

year = 4DIGIT
Appendix F. Change Log (to be removed by RFC Editor before publication) F.1. Since RFC 2616

Extracted relevant partitions from [RFC2616].

F.2. Since draft-ietf-httpbis-p2-semantics-00

Other changes:

F.3. Since draft-ietf-httpbis-p3-payload-00 F.4. Since draft-ietf-httpbis-p2-semantics-01

Ongoing work on ABNF conversion (<http://tools.ietf.org/wg/httpbis/trac/ticket/36>):

F.5. Since draft-ietf-httpbis-p3-payload-01 F.6. Since draft-ietf-httpbis-p2-semantics-02 F.7. Since draft-ietf-httpbis-p3-payload-02 F.8. Since draft-ietf-httpbis-p2-semantics-03 F.9. Since draft-ietf-httpbis-p3-payload-03 F.10. Since draft-ietf-httpbis-p2-semantics-04

Ongoing work on ABNF conversion (<http://tools.ietf.org/wg/httpbis/trac/ticket/36>):

F.11. Since draft-ietf-httpbis-p3-payload-04

Ongoing work on ABNF conversion (<http://tools.ietf.org/wg/httpbis/trac/ticket/36>):

F.12. Since draft-ietf-httpbis-p2-semantics-05 F.13. Since draft-ietf-httpbis-p3-payload-05

Other changes:

F.14. Since draft-ietf-httpbis-p2-semantics-06 F.15. Since draft-ietf-httpbis-p3-payload-06 F.16. Since draft-ietf-httpbis-p2-semantics-07 F.17. Since draft-ietf-httpbis-p3-payload-07 F.18. Since draft-ietf-httpbis-p2-semantics-08 F.19. Since draft-ietf-httpbis-p3-payload-08 F.20. Since draft-ietf-httpbis-p2-semantics-09 F.21. Since draft-ietf-httpbis-p3-payload-09 F.22. Since draft-ietf-httpbis-p2-semantics-10 F.23. Since draft-ietf-httpbis-p3-payload-10 F.24. Since draft-ietf-httpbis-p2-semantics-11 F.25. Since draft-ietf-httpbis-p3-payload-11 F.26. Since draft-ietf-httpbis-p2-semantics-12 F.27. Since draft-ietf-httpbis-p3-payload-12 F.28. Since draft-ietf-httpbis-p2-semantics-13 F.29. Since draft-ietf-httpbis-p3-payload-13 F.30. Since draft-ietf-httpbis-p2-semantics-14 F.31. Since draft-ietf-httpbis-p3-payload-14

None.

F.32. Since draft-ietf-httpbis-p2-semantics-15 F.33. Since draft-ietf-httpbis-p3-payload-15 F.34. Since draft-ietf-httpbis-p2-semantics-16 F.35. Since draft-ietf-httpbis-p3-payload-16 F.36. Since draft-ietf-httpbis-p2-semantics-17 F.37. Since draft-ietf-httpbis-p3-payload-17 F.38. Since draft-ietf-httpbis-p2-semantics-18 F.39. Since draft-ietf-httpbis-p3-payload-18 F.40. Since draft-ietf-httpbis-p2-semantics-19 and draft-ietf-httpbis-p3-payload-19 Index

1 2 3 4 5 A C D E F G H I L M O P R S T U

Authors' Addresses Roy T. Fielding (editor)
Adobe Systems Incorporated
345 Park Ave
San Jose, CA 95110
USA
Email: fielding@gbiv.com
URI: http://roy.gbiv.com/ Yves Lafon (editor)
World Wide Web Consortium
W3C / ERCIM
2004, rte des Lucioles
Sophia-Antipolis, AM 06902
France
Email: ylafon@w3.org
URI: http://www.raubacapeu.net/people/yves/ Julian F. Reschke (editor)
greenbytes GmbH
Hafenweg 16
Muenster, NW 48155
Germany
Email: julian.reschke@greenbytes.de
URI: http://greenbytes.de/tech/webdav/

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