HTTP/1.1, part 2: Semantics and Payloads

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.¶

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.¶

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: ¶

• Method Name (see Section 2)
• Safe ("yes" or "no", see Section 2.1.1)
• Idempotent ("yes" or "no", see Section 2.1.1)
• Pointer to specification text

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>.¶

New header fields are registered using the procedures described in [RFC3864].¶

The requirements for header field names are defined in Section 4.1 of [RFC3864]. Authors of specifications defining new fields are advised to keep the name as short as practical, and not to prefix them with "X-" if they are to be registered (either immediately or in the future).¶

New header field values typically have their syntax defined using ABNF ([RFC5234]), using the extension defined in Appendix B of [Part1] as necessary, and are usually constrained to the range of ASCII characters. Header fields needing a greater range of characters can use an encoding such as the one defined in [RFC5987].¶

Because commas (",") are used as a generic delimiter between field-values, they need to be treated with care if they are allowed in the field-value's payload. Typically, components that might contain a comma are protected with double-quotes using the quoted-string ABNF production (Section 3.2.4 of [Part1]).¶

Note that double quote delimiters almost always are used with the quoted-string production; using a different syntax inside double quotes will likely cause unnecessary confusion.¶

Many header fields use a format including (case-insensitively) named parameters (for instance, Content-Type, defined in Section 9.9). Allowing both unquoted (token) and quoted (quoted-string) syntax for the parameter value enables recipients to use existing parser components. When allowing both forms, the meaning of a parameter value ought to be independent of the syntax used for it (for an example, see the notes on parameter handling for media types in Section 5.5).¶

Authors of specifications defining new header fields are advised to consider documenting: ¶

• Whether the field is a single value, or whether it can be a list (delimited by commas; see Section 3.2 of [Part1]).

  If it does not use the list syntax, document how to treat messages where the header field occurs multiple times (a sensible default would be to ignore the header field, but this might not always be the right choice).

  Note that intermediaries and software libraries might combine multiple header field instances into a single one, despite the header field not allowing this. A robust format enables recipients to discover these situations (good example: "Content-Type", as the comma can only appear inside quoted strings; bad example: "Location", as a comma can occur inside a URI).

• Under what conditions the header field can be used; e.g., only in responses or requests, in all messages, only on responses to a particular request method.

• Whether it is appropriate to list the field-name in the Connection header field (i.e., if the header field is to be hop-by-hop, see Section 6.1 of [Part1]).

• Under what conditions intermediaries are allowed to modify the header field's value, insert or delete it.

• How the header field might interact with caching (see [Part6]).

• Whether the header field is useful or allowable in trailers (see Section 4.1 of [Part1]).

• Whether the header field ought to be preserved across redirects.

The request header fields allow the client to pass additional information about the request, and about the client itself, to the server. These fields act as request modifiers, with semantics equivalent to the parameters on a programming language method invocation.¶

The response header fields allow the server to pass additional information about the response which cannot be placed in the status-line. These header fields give information about the server and about further access to the target resource (Section 5.5 of [Part1]).¶

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.¶

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>.¶

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).)¶

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

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]).

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).¶

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.¶

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.¶

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

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.¶

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).¶

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.¶

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).¶

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].¶

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.¶

compress ¶

• See Section 4.2.1 of [Part1].

deflate ¶

• See Section 4.2.2 of [Part1].

gzip ¶

• See Section 4.2.3 of [Part1].

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.¶

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.¶

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.¶

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.¶

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.¶

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.

[TODO-req-uri: The comparison function is going to have to be defined somewhere, because we already need to compare URIs for things like cache invalidation.] ¶

Representation header fields define metadata about the representation data enclosed in the message body or, if no message body is present, about the representation that would have been transferred in a 200 (OK) response to a simultaneous GET request with the same effective request URI.¶

We use the term "selected representation" to refer to the the current representation of a target resource that would have been selected in a successful response if the same request had used the method GET and excluded any conditional request header fields.¶

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.¶

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.¶

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.¶

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.¶

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.¶

The "Accept" header field can be used by user agents to specify response media types that are acceptable. Accept header fields can be used to indicate that the request is specifically limited to a small set of desired types, as in the case of a request for an in-line image.¶

The asterisk "*" character is used to group media types into ranges, with "*/*" indicating all media types and "type/*" indicating all subtypes of that type. The media-range MAY include media type parameters that are applicable to that range.¶

Each media-range MAY be followed by one or more accept-params, beginning with the "q" parameter for indicating a relative quality factor. The first "q" parameter (if any) separates the media-range parameter(s) from the accept-params. Quality factors allow the user or user agent to indicate the relative degree of preference for that media-range, using the qvalue scale from 0 to 1 (Section 4.3.1 of [Part1]). The default value is q=1.¶

The example¶

SHOULD be interpreted as "I prefer audio/basic, but send me any audio type if it is the best available after an 80% mark-down in quality".¶

A request without any Accept header field implies that the user agent will accept any media type in response. If an Accept header field is present in a request and none of the available representations for the response have a media type that is listed as acceptable, the origin server MAY either honor the Accept header field by sending a 406 (Not Acceptable) response or disregard the Accept header field by treating the response as if it is not subject to content negotiation.¶

A more elaborate example is¶

Verbally, this would be interpreted as "text/html and text/x-c are the preferred media types, but if they do not exist, then send the text/x-dvi representation, and if that does not exist, send the text/plain representation".¶

Media ranges can be overridden by more specific media ranges or specific media types. If more than one media range applies to a given type, the most specific reference has precedence. For example,¶

have the following precedence: ¶

1. text/plain;format=flowed
2. text/plain
3. text/*
4. */*

The media type quality factor associated with a given type is determined by finding the media range with the highest precedence which matches that type. For example,¶

Note: A user agent might be provided with a default set of quality values for certain media ranges. However, unless the user agent is a closed system which cannot interact with other rendering agents, this default set ought to be configurable by the user.¶

The "Accept-Charset" header field can be used by user agents to indicate what character encodings are acceptable in a response payload. This field allows clients capable of understanding more comprehensive or special-purpose character encodings to signal that capability to a server which is capable of representing documents in those character encodings.¶

Character encoding values (a.k.a., charsets) are described in Section 5.3. Each charset MAY be given an associated quality value which represents the user's preference for that charset. The default value is q=1. An example is¶

The special value "*", if present in the Accept-Charset field, matches every character encoding which is not mentioned elsewhere in the Accept-Charset field. If no "*" is present in an Accept-Charset field, then all character encodings not explicitly mentioned get a quality value of 0.¶

A request without any Accept-Charset header field implies that the user agent will accept any character encoding in response. If an Accept-Charset header field is present in a request and none of the available representations for the response have a character encoding that is listed as acceptable, the origin server MAY either honor the Accept-Charset header field by sending a 406 (Not Acceptable) response or disregard the Accept-Charset header field by treating the response as if it is not subject to content negotiation.¶

The "Accept-Encoding" header field can be used by user agents to indicate what response content-codings (Section 5.4) are acceptable in the response. An "identity" token is used as a synonym for "no encoding" in order to communicate when no encoding is preferred.¶

Each codings value MAY be given an associated quality value which represents the preference for that encoding. The default value is q=1.¶

For example,¶

A server tests whether a content-coding for a given representation is acceptable, according to an Accept-Encoding field, using these rules: ¶

1. The special "*" symbol in an Accept-Encoding field matches any available content-coding not explicitly listed in the header field.
2. If the representation has no content-coding, then it is acceptable by default unless specifically excluded by the Accept-Encoding field stating either "identity;q=0" or "*;q=0" without a more specific entry for "identity".
3. If the representation's content-coding is one of the content-codings listed in the Accept-Encoding field, then it is acceptable unless it is accompanied by a qvalue of 0. (As defined in Section 4.3.1 of [Part1], a qvalue of 0 means "not acceptable".)
4. If multiple content-codings are acceptable, then the acceptable content-coding with the highest non-zero qvalue is preferred.

An Accept-Encoding header field with a combined field-value that is empty implies that the user agent does not want any content-coding in response. If an Accept-Encoding header field is present in a request and none of the available representations for the response have a content-coding that is listed as acceptable, the origin server SHOULD send a response without any content-coding.¶

A request without an Accept-Encoding header field implies that the user agent will accept any content-coding in response, but a representation without content-coding is preferred for compatibility with the widest variety of user agents.¶

The "Accept-Language" header field can be used by user agents to indicate the set of natural languages that are preferred in the response. Language tags are defined in Section 5.6.¶

Each language-range can be given an associated quality value which represents an estimate of the user's preference for the languages specified by that range. The quality value defaults to "q=1". For example,¶

would mean: "I prefer Danish, but will accept British English and other types of English". (see also Section 2.3 of [RFC4647])¶

For matching, Section 3 of [RFC4647] defines several matching schemes. Implementations can offer the most appropriate matching scheme for their requirements.¶

It might be contrary to the privacy expectations of the user to send an Accept-Language header field with the complete linguistic preferences of the user in every request. For a discussion of this issue, see Section 11.5.¶

As intelligibility is highly dependent on the individual user, it is recommended that client applications make the choice of linguistic preference available to the user. If the choice is not made available, then the Accept-Language header field MUST NOT be given in the request.¶

The "Allow" header field lists the set of methods advertised as supported by the target resource. The purpose of this field is strictly to inform the recipient of valid request methods associated with the resource.¶

The actual set of allowed methods is defined by the origin server at the time of each request.¶

A proxy MUST NOT modify the Allow header field — it does not need to understand all the methods specified in order to handle them according to the generic message handling rules.¶

The "Content-Encoding" header field indicates what content-codings have been applied to the representation beyond those inherent in the media type, and thus what decoding mechanisms have to be applied in order to obtain the media-type referenced by the Content-Type header field. Content-Encoding is primarily used to allow a representation to be compressed without losing the identity of its underlying media type.¶

Content codings are defined in Section 5.4. An example of its use is¶

The content-coding is a characteristic of the representation. Typically, the representation body is stored with this encoding and is only decoded before rendering or analogous usage. However, a transforming proxy MAY modify the content-coding if the new coding is known to be acceptable to the recipient, unless the "no-transform" cache-control directive is present in the message.¶

If the media type includes an inherent encoding, such as a data format that is always compressed, then that encoding would not be restated as a Content-Encoding even if it happens to be the same algorithm as one of the content-codings. Such a content-coding would only be listed if, for some bizarre reason, it is applied a second time to form the representation. Likewise, an origin server might choose to publish the same payload data as multiple representations that differ only in whether the coding is defined as part of Content-Type or Content-Encoding, since some user agents will behave differently in their handling of each response (e.g., open a "Save as ..." dialog instead of automatic decompression and rendering of content).¶

A representation that has a content-coding applied to it MUST include a Content-Encoding header field that lists the content-coding(s) applied.¶

If multiple encodings have been applied to a representation, the content codings MUST be listed in the order in which they were applied. Additional information about the encoding parameters MAY be provided by other header fields not defined by this specification.¶

If the content-coding of a representation in a request message is not acceptable to the origin server, the server SHOULD respond with a status code of 415 (Unsupported Media Type).¶

The "Content-Language" header field describes the natural language(s) of the intended audience for the representation. Note that this might not be equivalent to all the languages used within the representation.¶

Language tags are defined in Section 5.6. The primary purpose of Content-Language is to allow a user to identify and differentiate representations according to the user's own preferred language. Thus, if the body content is intended only for a Danish-literate audience, the appropriate field is¶

If no Content-Language is specified, the default is that the content is intended for all language audiences. This might mean that the sender does not consider it to be specific to any natural language, or that the sender does not know for which language it is intended.¶

Multiple languages MAY be listed for content that is intended for multiple audiences. For example, a rendition of the "Treaty of Waitangi", presented simultaneously in the original Maori and English versions, would call for¶

However, just because multiple languages are present within a representation does not mean that it is intended for multiple linguistic audiences. An example would be a beginner's language primer, such as "A First Lesson in Latin", which is clearly intended to be used by an English-literate audience. In this case, the Content-Language would properly only include "en".¶

Content-Language MAY be applied to any media type — it is not limited to textual documents.¶

The "Content-Location" header field supplies a URI that can be used as a specific identifier for the representation in this message. In other words, if one were to perform a GET on this URI at the time of this message's generation, then a 200 (OK) response would contain the same representation that is enclosed as payload in this message.¶

The Content-Location value is not a replacement for the effective Request URI (Section 5.5 of [Part1]). It is representation metadata. It has the same syntax and semantics as the header field of the same name defined for MIME body parts in Section 4 of [RFC2557]. However, its appearance in an HTTP message has some special implications for HTTP recipients.¶

If Content-Location is included in a response message and its value is the same as the effective request URI, then the response payload SHOULD be considered a current representation of that resource. For a GET or HEAD request, this is the same as the default semantics when no Content-Location is provided by the server. For a state-changing request like PUT or POST, it implies that the server's response contains the new representation of that resource, thereby distinguishing it from representations that might only report about the action (e.g., "It worked!"). This allows authoring applications to update their local copies without the need for a subsequent GET request.¶

If Content-Location is included in a response message and its value differs from the effective request URI, then the origin server is informing recipients that this representation has its own, presumably more specific, identifier. For a GET or HEAD request, this is an indication that the effective request URI identifies a resource that is subject to content negotiation and the selected representation for this response can also be found at the identified URI. For other methods, such a Content-Location indicates that this representation contains a report on the action's status and the same report is available (for future access with GET) at the given URI. For example, a purchase transaction made via a POST request might include a receipt document as the payload of the 200 (OK) response; the Content-Location value provides an identifier for retrieving a copy of that same receipt in the future.¶

If Content-Location is included in a request message, then it MAY be interpreted by the origin server as an indication of where the user agent originally obtained the content of the enclosed representation (prior to any subsequent modification of the content by that user agent). In other words, the user agent is providing the same representation metadata that it received with the original representation. However, such interpretation MUST NOT be used to alter the semantics of the method requested by the client. For example, if a client makes a PUT request on a negotiated resource and the origin server accepts that PUT (without redirection), then the new set of values for that resource is expected to be consistent with the one representation supplied in that PUT; the Content-Location cannot be used as a form of reverse content selection that identifies only one of the negotiated representations to be updated. If the user agent had wanted the latter semantics, it would have applied the PUT directly to the Content-Location URI.¶

A Content-Location field received in a request message is transitory information that SHOULD NOT be saved with other representation metadata for use in later responses. The Content-Location's value might be saved for use in other contexts, such as within source links or other metadata.¶

A cache cannot assume that a representation with a Content-Location different from the URI used to retrieve it can be used to respond to later requests on that Content-Location URI.¶

If the Content-Location value is a partial URI, the partial URI is interpreted relative to the effective request URI.¶

The "Content-Type" header field indicates the media type of the representation. In the case of responses to the HEAD method, the media type is that which would have been sent had the request been a GET.¶

Media types are defined in Section 5.5. An example of the field is¶

Further discussion of Content-Type is provided in Section 7.3.¶

The "Date" header field represents the date and time at which the message was originated, having the same semantics as the Origination Date Field (orig-date) defined in Section 3.6.1 of [RFC5322]. The field value is an HTTP-date, as defined in Section 5.1; it MUST be sent in rfc1123-date format.¶

An example is¶

Origin servers MUST include a Date header field in all responses, except in these cases: ¶

1. If the response status code is 100 (Continue) or 101 (Switching Protocols), the response MAY include a Date header field, at the server's option.
2. If the response status code conveys a server error, e.g., 500 (Internal Server Error) or 503 (Service Unavailable), and it is inconvenient or impossible to generate a valid Date.
3. If the server does not have a clock that can provide a reasonable approximation of the current time, its responses MUST NOT include a Date header field.

A received message that does not have a Date header field MUST be assigned one by the recipient if the message will be cached by that recipient.¶

Clients can use the Date header field as well; in order to keep request messages small, they are advised not to include it when it doesn't convey any useful information (as is usually the case for requests that do not contain a payload).¶

The HTTP-date sent in a Date header field SHOULD NOT represent a date and time subsequent to the generation of the message. It SHOULD represent the best available approximation of the date and time of message generation, unless the implementation has no means of generating a reasonably accurate date and time. In theory, the date ought to represent the moment just before the payload is generated. In practice, the date can be generated at any time during the message origination without affecting its semantic value.¶

The "Expect" header field is used to indicate that particular server behaviors are required by the client.¶

If all received Expect header field(s) are syntactically valid but contain an expectation that the recipient does not understand or cannot comply with, the recipient MUST respond with a 417 (Expectation Failed) status code. A recipient of a syntactically invalid Expectation header field MUST respond with a 4xx status code other than 417.¶

100-continue ¶

• The "100-continue" expectation is defined Section 6.4.3 of [Part1]. It does not support any expect-params.

Comparison is case-insensitive for names (expect-name), and case-sensitive for values (expect-value).¶

The Expect mechanism is hop-by-hop: the above requirements apply to any server, including proxies. However, the Expect header field itself is end-to-end; it MUST be forwarded if the request is forwarded.¶

Many older HTTP/1.0 and HTTP/1.1 applications do not understand the Expect header field.¶

This header field MAY be used for logging purposes and as a means for identifying the source of invalid or unwanted requests. It SHOULD NOT be used as an insecure form of access protection. The interpretation of this field is that the request is being performed on behalf of the person given, who accepts responsibility for the method performed. In particular, robot agents SHOULD include this header field so that the person responsible for running the robot can be contacted if problems occur on the receiving end.¶

The Internet e-mail address in this field MAY be separate from the Internet host which issued the request. For example, when a request is passed through a proxy the original issuer's address SHOULD be used.¶

The client SHOULD NOT send the From header field without the user's approval, as it might conflict with the user's privacy interests or their site's security policy. It is strongly recommended that the user be able to disable, enable, and modify the value of this field at any time prior to a request.¶

The "Location" header field MAY be sent in responses to refer to a specific resource in accordance with the semantics of the status code.¶

For 201 (Created) responses, the Location is the URI of the new resource which was created by the request. For 3xx (Redirection) responses, the location SHOULD indicate the server's preferred URI for automatic redirection to the resource.¶

The field value consists of a single URI-reference. When it has the form of a relative reference ([RFC3986], Section 4.2), the final value is computed by resolving it against the effective request URI ([RFC3986], Section 5). If the original URI, as navigated to by the user agent, did contain a fragment identifier, and the final value does not, then the original URI's fragment identifier is added to the final value.¶

For example, the original URI "http://www.example.org/~tim", combined with a field value given as:

  Location: /pub/WWW/People.html#tim

would result in a final value of "http://www.example.org/pub/WWW/People.html#tim"

An original URI "http://www.example.org/index.html#larry", combined with a field value given as:

  Location: http://www.example.net/index.html

would result in a final value of "http://www.example.net/index.html#larry", preserving the original fragment identifier.

There are circumstances in which a fragment identifier in a Location URI would not be appropriate. For instance, when it appears in a 201 (Created) response, where the Location header field specifies the URI for the entire created resource.¶

The "Max-Forwards" header field provides a mechanism with the TRACE (Section 2.3.7) and OPTIONS (Section 2.3.1) methods to limit the number of times that the request is forwarded by proxies. This can be useful when the client is attempting to trace a request which appears to be failing or looping mid-chain.¶

The Max-Forwards value is a decimal integer indicating the remaining number of times this request message can be forwarded.¶

Each recipient of a TRACE or OPTIONS request containing a Max-Forwards header field MUST check and update its value prior to forwarding the request. If the received value is zero (0), the recipient MUST NOT forward the request; instead, it MUST respond as the final recipient. If the received Max-Forwards value is greater than zero, then the forwarded message MUST contain an updated Max-Forwards field with a value decremented by one (1).¶

The Max-Forwards header field MAY be ignored for all other request methods.¶

The "Referer" [sic] header field allows the client to specify the URI of the resource from which the target URI was obtained (the "referrer", although the header field is misspelled.).¶

The Referer header field allows servers to generate lists of back-links to resources for interest, logging, optimized caching, etc. It also allows obsolete or mistyped links to be traced for maintenance. Some servers use Referer as a means of controlling where they allow links from (so-called "deep linking"), but legitimate requests do not always contain a Referer header field.¶

If the target URI was obtained from a source that does not have its own URI (e.g., input from the user keyboard), the Referer field MUST either be sent with the value "about:blank", or not be sent at all. Note that this requirement does not apply to sources with non-HTTP URIs (e.g., FTP).¶

If the field value is a relative URI, it SHOULD be interpreted relative to the effective request URI. The URI MUST NOT include a fragment. See Section 11.2 for security considerations.¶

The header "Retry-After" field can be used with a 503 (Service Unavailable) response to indicate how long the service is expected to be unavailable to the requesting client. This field MAY also be used with any 3xx (Redirection) response to indicate the minimum time the user-agent is asked to wait before issuing the redirected request.¶

The value of this field can be either an HTTP-date or an integer number of seconds (in decimal) after the time of the response.¶

Time spans are non-negative decimal integers, representing time in seconds.¶

Two examples of its use are¶

In the latter example, the delay is 2 minutes.¶

The "Server" header field contains information about the software used by the origin server to handle the request.¶

The field can contain multiple product tokens (Section 5.2) and comments (Section 3.2 of [Part1]) identifying the server and any significant subproducts. The product tokens are listed in order of their significance for identifying the application.¶

If the response is being forwarded through a proxy, the proxy application MUST NOT modify the Server header field. Instead, it MUST include a Via field (as described in Section 6.2 of [Part1]).¶

The "User-Agent" header field contains information about the user agent originating the request. User agents SHOULD include this field with requests.¶

Typically, it is used for statistical purposes, the tracing of protocol violations, and tailoring responses to avoid particular user agent limitations.¶

The field can contain multiple product tokens (Section 5.2) and comments (Section 3.2 of [Part1]) identifying the agent and its significant subproducts. By convention, the product tokens are listed in order of their significance for identifying the application.¶

Because this field is usually sent on every request a user agent makes, implementations are encouraged not to include needlessly fine-grained detail, and to limit (or even prohibit) the addition of subproducts by third parties. Overly long and detailed User-Agent field values make requests larger and can also be used to identify ("fingerprint") the user against their wishes.¶

Likewise, implementations are encouraged not to use the product tokens of other implementations in order to declare compatibility with them, as this circumvents the purpose of the field. Finally, they are encouraged not to use comments to identify products; doing so makes the field value more difficult to parse.¶

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

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 change controller is: "IETF (iesg@ietf.org) - Internet Engineering Task Force".¶

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

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.¶

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.¶

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.¶

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.¶

Accept header fields can reveal information about the user to all servers which are accessed. The Accept-Language header field in particular can reveal information the user would consider to be of a private nature, because the understanding of particular languages is often strongly correlated to the membership of a particular ethnic group. User agents which offer the option to configure the contents of an Accept-Language header field to be sent in every request are strongly encouraged to let the configuration process include a message which makes the user aware of the loss of privacy involved.¶

An approach that limits the loss of privacy would be for a user agent to omit the sending of Accept-Language header fields by default, and to ask the user whether or not to start sending Accept-Language header fields to a server if it detects, by looking for any Vary header fields generated by the server, that such sending could improve the quality of service.¶

Elaborate user-customized accept header fields sent in every request, in particular if these include quality values, can be used by servers as relatively reliable and long-lived user identifiers. Such user identifiers would allow content providers to do click-trail tracking, and would allow collaborating content providers to match cross-server click-trails or form submissions of individual users. Note that for many users not behind a proxy, the network address of the host running the user agent will also serve as a long-lived user identifier. In environments where proxies are used to enhance privacy, user agents ought to be conservative in offering accept header field configuration options to end users. As an extreme privacy measure, proxies could filter the accept header fields in relayed requests. General purpose user agents which provide a high degree of header field configurability SHOULD warn users about the loss of privacy which can be involved.¶

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.

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.¶

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.¶

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.¶

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).¶

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.¶

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.¶

Extracted relevant partitions from [RFC2616].¶

Closed issues: ¶

• <http://tools.ietf.org/wg/httpbis/trac/ticket/5>: "Via is a MUST" (<http://purl.org/NET/http-errata#via-must>)
• <http://tools.ietf.org/wg/httpbis/trac/ticket/6>: "Fragments allowed in Location" (<http://purl.org/NET/http-errata#location-fragments>)
• <http://tools.ietf.org/wg/httpbis/trac/ticket/10>: "Safe Methods vs Redirection" (<http://purl.org/NET/http-errata#saferedirect>)
• <http://tools.ietf.org/wg/httpbis/trac/ticket/17>: "Revise description of the POST method" (<http://purl.org/NET/http-errata#post>)
• <http://tools.ietf.org/wg/httpbis/trac/ticket/35>: "Normative and Informative references"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/42>: "RFC2606 Compliance"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/65>: "Informative references"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/84>: "Redundant cross-references"

Other changes: ¶

• Move definitions of 304 and 412 condition codes to [Part4]

Closed issues: ¶

• <http://tools.ietf.org/wg/httpbis/trac/ticket/8>: "Media Type Registrations" (<http://purl.org/NET/http-errata#media-reg>)
• <http://tools.ietf.org/wg/httpbis/trac/ticket/14>: "Clarification regarding quoting of charset values" (<http://purl.org/NET/http-errata#charactersets>)
• <http://tools.ietf.org/wg/httpbis/trac/ticket/16>: "Remove 'identity' token references" (<http://purl.org/NET/http-errata#identity>)
• <http://tools.ietf.org/wg/httpbis/trac/ticket/25>: "Accept-Encoding BNF"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/35>: "Normative and Informative references"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/46>: "RFC1700 references"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/55>: "Updating to RFC4288"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/65>: "Informative references"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/66>: "ISO-8859-1 Reference"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/68>: "Encoding References Normative"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/86>: "Normative up-to-date references"

Closed issues: ¶

• <http://tools.ietf.org/wg/httpbis/trac/ticket/21>: "PUT side effects"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/91>: "Duplicate Host header requirements"

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

• Move "Product Tokens" section (back) into Part 1, as "token" is used in the definition of the Upgrade header field.
• Add explicit references to BNF syntax and rules imported from other parts of the specification.
• Copy definition of delta-seconds from Part6 instead of referencing it.

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

• Add explicit references to BNF syntax and rules imported from other parts of the specification.

Closed issues: ¶

• <http://tools.ietf.org/wg/httpbis/trac/ticket/24>: "Requiring Allow in 405 responses"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/59>: "Status Code Registry"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/61>: "Redirection vs. Location"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/70>: "Cacheability of 303 response"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/76>: "305 Use Proxy"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/105>: "Classification for Allow header field"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/112>: "PUT - 'store under' vs 'store at'"

Ongoing work on IANA Message Header Field Registration (<http://tools.ietf.org/wg/httpbis/trac/ticket/40>): ¶

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

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

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

Closed issues: ¶

• <http://tools.ietf.org/wg/httpbis/trac/ticket/67>: "Quoting Charsets"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/105>: "Classification for Allow header field"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/115>: "missing default for qvalue in description of Accept-Encoding"

Ongoing work on IANA Message Header Field Registration (<http://tools.ietf.org/wg/httpbis/trac/ticket/40>): ¶

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

Closed issues: ¶

• <http://tools.ietf.org/wg/httpbis/trac/ticket/98>: "OPTIONS request bodies"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/119>: "Description of CONNECT should refer to RFC2817"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/125>: "Location Content-Location reference request/response mixup"

Ongoing work on Method Registry (<http://tools.ietf.org/wg/httpbis/trac/ticket/72>): ¶

• Added initial proposal for registration process, plus initial content (non-HTTP/1.1 methods to be added by a separate specification).

Closed issues: ¶

• <http://tools.ietf.org/wg/httpbis/trac/ticket/67>: "Quoting Charsets"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/113>: "language tag matching (Accept-Language) vs RFC4647"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/121>: "RFC 1806 has been replaced by RFC2183"

Other changes: ¶

• <http://tools.ietf.org/wg/httpbis/trac/ticket/68>: "Encoding References Normative" — rephrase the annotation and reference BCP97.

Closed issues: ¶

• <http://tools.ietf.org/wg/httpbis/trac/ticket/103>: "Content-*"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/132>: "RFC 2822 is updated by RFC 5322"

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

• Use "/" instead of "|" for alternatives.
• Introduce new ABNF rules for "bad" whitespace ("BWS"), optional whitespace ("OWS") and required whitespace ("RWS").
• Rewrite ABNFs to spell out whitespace rules, factor out header field value format definitions.

Closed issues: ¶

• <http://tools.ietf.org/wg/httpbis/trac/ticket/132>: "RFC 2822 is updated by RFC 5322"

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

• Use "/" instead of "|" for alternatives.
• Introduce new ABNF rules for "bad" whitespace ("BWS"), optional whitespace ("OWS") and required whitespace ("RWS").
• Rewrite ABNFs to spell out whitespace rules, factor out header field value format definitions.

Closed issues: ¶

• <http://tools.ietf.org/wg/httpbis/trac/ticket/94>: "reason-phrase BNF"

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

• Add appendix containing collected and expanded ABNF, reorganize ABNF introduction.

Closed issues: ¶

• <http://tools.ietf.org/wg/httpbis/trac/ticket/118>: "Join "Differences Between HTTP Entities and RFC 2045 Entities"?"

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

• Add appendix containing collected and expanded ABNF, reorganize ABNF introduction.

Other changes: ¶

• Move definition of quality values into Part 1.

Closed issues: ¶

• <http://tools.ietf.org/wg/httpbis/trac/ticket/144>: "Clarify when Referer is sent"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/164>: "status codes vs methods"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/170>: "Do not require "updates" relation for specs that register status codes or method names"

Closed issues: ¶

• <http://tools.ietf.org/wg/httpbis/trac/ticket/80>: "Content-Location isn't special"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/155>: "Content Sniffing"

Closed issues: ¶

• <http://tools.ietf.org/wg/httpbis/trac/ticket/27>: "Idempotency"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/33>: "TRACE security considerations"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/110>: "Clarify rules for determining what entities a response carries"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/140>: "update note citing RFC 1945 and 2068"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/182>: "update note about redirect limit"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/191>: "Location header field ABNF should use 'URI'"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/192>: "fragments in Location vs status 303"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/198>: "move IANA registrations for optional status codes"

Partly resolved issues: ¶

• <http://tools.ietf.org/wg/httpbis/trac/ticket/171>: "Are OPTIONS and TRACE safe?"

Closed issues: ¶

• <http://tools.ietf.org/wg/httpbis/trac/ticket/13>: "Updated reference for language tags"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/110>: "Clarify rules for determining what entities a response carries"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/154>: "Content-Location base-setting problems"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/155>: "Content Sniffing"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/188>: "pick IANA policy (RFC5226) for Transfer Coding / Content Coding"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/189>: "move definitions of gzip/deflate/compress to part 1"

Partly resolved issues: ¶

• <http://tools.ietf.org/wg/httpbis/trac/ticket/148>: "update IANA requirements wrt Transfer-Coding values" (add the IANA Considerations subsection)
• <http://tools.ietf.org/wg/httpbis/trac/ticket/149>: "update IANA requirements wrt Content-Coding values" (add the IANA Considerations subsection)

Closed issues: ¶

• <http://tools.ietf.org/wg/httpbis/trac/ticket/10>: "Safe Methods vs Redirection" (we missed the introduction to the 3xx status codes when fixing this previously)

Closed issues: ¶

• <http://tools.ietf.org/wg/httpbis/trac/ticket/81>: "Content Negotiation for media types"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/181>: "Accept-Language: which RFC4647 filtering?"

Closed issues: ¶

• <http://tools.ietf.org/wg/httpbis/trac/ticket/43>: "Fragment combination / precedence during redirects"

Partly resolved issues: ¶

• <http://tools.ietf.org/wg/httpbis/trac/ticket/185>: "Location header field payload handling"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/196>: "Term for the requested resource's URI"

Closed issues: ¶

• <http://tools.ietf.org/wg/httpbis/trac/ticket/122>: "MIME-Version not listed in P1, general header fields"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/143>: "IANA registry for content/transfer encodings"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/155>: "Content Sniffing"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/200>: "use of term "word" when talking about header field structure"

Partly resolved issues: ¶

• <http://tools.ietf.org/wg/httpbis/trac/ticket/196>: "Term for the requested resource's URI"

Closed issues: ¶

• <http://tools.ietf.org/wg/httpbis/trac/ticket/69>: "Clarify 'Requested Variant'"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/109>: "Clarify entity / representation / variant terminology"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/139>: "Methods and Caching"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/190>: "OPTIONS vs Max-Forwards"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/199>: "Status codes and caching"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/220>: "consider removing the 'changes from 2068' sections"

Closed issues: ¶

• <http://tools.ietf.org/wg/httpbis/trac/ticket/69>: "Clarify 'Requested Variant'"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/80>: "Content-Location isn't special"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/90>: "Delimiting messages with multipart/byteranges"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/109>: "Clarify entity / representation / variant terminology"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/136>: "confusing req. language for Content-Location"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/167>: "Content-Location on 304 responses"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/183>: "'requested resource' in content-encoding definition"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/220>: "consider removing the 'changes from 2068' sections"

Partly resolved issues: ¶

• <http://tools.ietf.org/wg/httpbis/trac/ticket/178>: "Content-MD5 and partial responses"

Closed issues: ¶

• <http://tools.ietf.org/wg/httpbis/trac/ticket/229>: "Considerations for new status codes"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/230>: "Considerations for new methods"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/232>: "User-Agent guidelines" (relating to the 'User-Agent' header field)

Closed issues: ¶

• <http://tools.ietf.org/wg/httpbis/trac/ticket/123>: "Factor out Content-Disposition"

Closed issues: ¶

• <http://tools.ietf.org/wg/httpbis/trac/ticket/43>: "Fragment combination / precedence during redirects" (added warning about having a fragid on the redirect might cause inconvenience in some cases)
• <http://tools.ietf.org/wg/httpbis/trac/ticket/79>: "Content-* vs. PUT"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/88>: "205 Bodies"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/102>: "Understanding Content-* on non-PUT requests"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/103>: "Content-*"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/104>: "Header field type defaulting"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/112>: "PUT - 'store under' vs 'store at'"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/137>: "duplicate ABNF for reason-phrase"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/180>: "Note special status of Content-* prefix in header field registration procedures"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/203>: "Max-Forwards vs extension methods"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/213>: "What is the value space of HTTP status codes?" (actually fixed in draft-ietf-httpbis-p2-semantics-11)
• <http://tools.ietf.org/wg/httpbis/trac/ticket/224>: "Header Field Classification"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/225>: "PUT side effect: invalidation or just stale?"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/226>: "proxies not supporting certain methods"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/239>: "Migrate CONNECT from RFC2817 to p2"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/240>: "Migrate Upgrade details from RFC2817"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/267>: "clarify PUT semantics'"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/275>: "duplicate ABNF for 'Method'"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/276>: "untangle ABNFs for header fields"

Closed issues: ¶

• <http://tools.ietf.org/wg/httpbis/trac/ticket/224>: "Header Field Classification"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/276>: "untangle ABNFs for header fields"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/277>: "potentially misleading MAY in media-type def"

Closed issues: ¶

• <http://tools.ietf.org/wg/httpbis/trac/ticket/276>: "untangle ABNFs for header fields"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/251>: "message body in CONNECT request"

Closed issues: ¶

• <http://tools.ietf.org/wg/httpbis/trac/ticket/20>: "Default charsets for text media types"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/178>: "Content-MD5 and partial responses"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/276>: "untangle ABNFs for header fields"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/281>: "confusing undefined parameter in media range example"

Closed issues: ¶

• <http://tools.ietf.org/wg/httpbis/trac/ticket/255>: "Clarify status code for rate limiting"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/294>: "clarify 403 forbidden"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/296>: "Clarify 203 Non-Authoritative Information"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/298>: "update default reason phrase for 413"

None.¶

Closed issues: ¶

• <http://tools.ietf.org/wg/httpbis/trac/ticket/285>: "Strength of requirements on Accept re: 406"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/303>: "400 response isn't generic"

Closed issues: ¶

• <http://tools.ietf.org/wg/httpbis/trac/ticket/285>: "Strength of requirements on Accept re: 406"

Closed issues: ¶

• <http://tools.ietf.org/wg/httpbis/trac/ticket/160>: "Redirects and non-GET methods"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/186>: "Document HTTP's error-handling philosophy"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/231>: "Considerations for new header fields"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/310>: "clarify 303 redirect on HEAD"

Closed issues: ¶

• <http://tools.ietf.org/wg/httpbis/trac/ticket/186>: "Document HTTP's error-handling philosophy"

Closed issues: ¶

• <http://tools.ietf.org/wg/httpbis/trac/ticket/185>: "Location header field payload handling"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/255>: "Clarify status code for rate limiting" (change backed out because a new status code is being defined for this purpose)
• <http://tools.ietf.org/wg/httpbis/trac/ticket/312>: "should there be a permanent variant of 307"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/325>: "When are Location's semantics triggered?"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/327>: "'expect' grammar missing OWS"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/329>: "header field considerations: quoted-string vs use of double quotes"

Closed issues: ¶

• <http://tools.ietf.org/wg/httpbis/trac/ticket/323>: "intended maturity level vs normative references"

Closed issues: ¶

• <http://tools.ietf.org/wg/httpbis/trac/ticket/227>: "Combining HEAD responses"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/238>: "Requirements for user intervention during redirects"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/250>: "message-body in CONNECT response"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/295>: "Applying original fragment to 'plain' redirected URI"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/302>: "Misplaced text on connection handling in p2"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/331>: "clarify that 201 doesn't require Location header fields"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/332>: "relax requirements on hypertext in 3/4/5xx error responses"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/333>: "example for 426 response should have a payload"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/336>: "drop indirection entries for status codes"

Closed issues: ¶

• <http://tools.ietf.org/wg/httpbis/trac/ticket/330>: "is ETag a representation header field?"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/338>: "Content-Location doesn't constrain the cardinality of representations"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/346>: "make IANA policy definitions consistent"

Closed issues: ¶

• <http://tools.ietf.org/wg/httpbis/trac/ticket/312>: "should there be a permanent variant of 307"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/347>: "clarify that 201 can imply *multiple* resources were created"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/351>: "merge P2 and P3"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/361>: "ABNF requirements for recipients"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/364>: "Capturing more information in the method registry"
• <http://tools.ietf.org/wg/httpbis/trac/ticket/368>: "note introduction of new IANA registries as normative changes"