3. Representations

Considering that a resource could be anything, and that the uniform interface provided by HTTP is similar to a window through which one can observe and act upon such a thing only through the communication of messages to some independent actor on the other side, an abstraction is needed to represent ("take the place of") the current or desired state of that thing in our communications. That abstraction is called a representation [REST].

For the purposes of HTTP, a "representation" is information that is intended to reflect a past, current, or desired state of a given resource, in a format that can be readily communicated via the protocol, and that consists of a set of representation metadata and a potentially unbounded stream of representation data.

An origin server might be provided with, or be capable of generating, multiple representations that are each intended to reflect the current state of a target resource. In such cases, some algorithm is used by the origin server to select one of those representations as most applicable to a given request, usually based on content negotiation. This "selected representation" is used to provide the data and metadata for evaluating conditional requests [RFC7232] and constructing the payload for 200 (OK) and 304 (Not Modified) responses to GET [Section 4.3.1].

3.1. Representation Metadata

Representation header fields provide metadata about the representation. When a message includes a payload body, the representation header fields describe how to interpret the representation data enclosed in the payload body. In a response to a HEAD request, the representation header fields describe the representation data that would have been enclosed in the payload body if the same request had been a GET.

3.1.1. Processing Representation Data

The purpose of representation metadata is to allow recipients to know what format they are dealing with, what language or languages the representation uses, and what transformations have been applied to the representation data by intervening intermediaries.

3.1.1.1. Media Type

HTTP uses Internet media types [RFC2046] in the Content-Type (Section 3.1.1.5) and Accept (Section 5.3.2) header fields in order to provide open and extensible data typing and type negotiation. Media types define both a data format and various processing models: how to process that data in accordance with each context in which it is received.

media-type = type "/" subtype *( OWS ";" OWS parameter )
type       = token
subtype    = token

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

parameter      = token "=" ( token / quoted-string )

The type, subtype, and parameter name tokens 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 either as a token or within a quoted-string. The quoted and unquoted values are equivalent. For example, the following examples are all equivalent, but the first is preferred for consistency:

text/html;charset=utf-8
text/html;charset=UTF-8
text/HTML;charset="utf-8"
text/html; charset="utf-8"

Internet media types ought to be registered with IANA according to the procedures defined in [BCP13].

3.1.1.2. Charset

HTTP uses charset names to indicate or negotiate the character encoding scheme of a textual representation [RFC6365]. A charset is identified by a case-insensitive token.

charset = token

Charset names ought to be registered in the IANA "Character Sets" registry (http://www.iana.org/assignments/character-sets) according to the procedures defined in [RFC2978].

3.1.1.3. Canonicalization and Text Defaults

Internet media types are registered with a canonical form in order to be interoperable among systems with varying native encoding formats. Representations selected or transferred via HTTP ought to be in canonical form, for many of the same reasons described by the Multipurpose Internet Mail Extensions (MIME) [RFC2045]. However, the performance characteristics of email deployments (i.e., store and forward messages to peers) are significantly different from those common to HTTP and the Web (server-based information services). Furthermore, MIME's constraints for the sake of compatibility with older mail transfer protocols do not apply to HTTP (see Appendix A).

MIME's canonical form requires that media subtypes of the "text" type use CRLF as the text line break. HTTP allows the transfer of text media with plain CR or LF alone representing a line break, when such line breaks are consistent for an entire representation. An HTTP sender MAY generate, and a recipient MUST be able to parse, line breaks in text media that consist of CRLF, bare CR, or bare LF. In addition, text media in HTTP is not limited to charsets that use octets 13 and 10 for CR and LF, respectively. This flexibility regarding line breaks applies only to text within a representation that has been assigned a "text" media type; it does not apply to "multipart" types or HTTP elements outside the payload body (e.g., header fields).

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

3.1.1.4. 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 include a boundary parameter as part of the media type value. The message body is itself a protocol element; a sender MUST generate only CRLF to represent line breaks between body parts.

HTTP message framing does not use the multipart boundary as an indicator of message body length, though it might be used by implementations that generate or process the payload. For example, the "multipart/byteranges" type is defined by HTTP for use in some 206 (Partial Content) responses [RFC7233].

3.1.1.5. Content-Type

The Content-Type header field indicates the media type of the associated representation: either the representation enclosed in the message payload or the selected representation, as determined by the message semantics. The indicated media type defines both the data format and how that data is intended to be processed by a recipient, within the scope of the received message semantics, after any content codings indicated by Content-Encoding are decoded.

Content-Type = media-type

Media types are defined in Section 3.1.1.1. An example of the field is:

Content-Type: text/html; charset=ISO-8859-4

A sender that generates a message containing a payload body SHOULD generate a Content-Type header field in that message unless the intended media type of the enclosed representation is unknown to the sender. If a Content-Type header field is not present, the recipient MAY either assume a media type of "application/octet-stream" ([RFC2046] Section 4.5.1) or examine the data 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 payload'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.

3.1.2. Encoding for Compression or Integrity

3.1.2.1. 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.

content-coding   = token

All content-coding values are case-insensitive and ought to be registered within the "HTTP Content Coding Registry", as defined in Section 8.4. They are used in the Accept-Encoding (Section 5.3.4) and Content-Encoding (Section 3.1.2.2) header fields.

The following content-coding values are defined by this specification:

• compress (and x-compress): UNIX "compress" data format [Welch]. Historical; not recommended for use in new applications.

• deflate: The "zlib" format [RFC1950] in combination with the "deflate" compression mechanism [RFC1951].

• gzip (and x-gzip): LZ77 coding with a 32-bit CRC [RFC1952].

• identity: The "identity" coding (no transformation). This coding is used only in the Accept-Encoding header field, and SHOULD NOT be used in the Content-Encoding header field.

3.1.2.2. Content-Encoding

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 data in the media type referenced by the Content-Type header field. Content-Encoding is primarily used to allow a representation's data to be compressed without losing the identity of its underlying media type.

Content-Encoding = 1#content-coding

An example of its use is:

Content-Encoding: gzip

If one or more encodings have been applied to a representation, the sender that applied the encodings MUST generate a Content-Encoding header field that lists the content codings in the order in which they were applied. Additional information about the encoding parameters can be provided by other header fields not defined by this specification.

Unlike Transfer-Encoding (Section 3.3 of [RFC7230]), the codings listed in Content-Encoding are a characteristic of the representation; the representation is defined in terms of the coded form, and all other metadata about the representation is about the coded form unless otherwise noted in the metadata definition. Typically, the representation is only decoded just prior to rendering or analogous usage.

If the media type includes an inherent encoding, such as a data format that is always compressed, then that encoding would not be restated in 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 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).

An origin server MAY respond with a status code of 415 (Unsupported Media Type) if a representation in the request message has a content coding that is not acceptable.

3.1.3. Audience Language

3.1.3.1. 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 header fields. Accept-Language uses the broader language-range production defined in Section 5.3.5, whereas Content-Language uses the language-tag production defined below.

language-tag = <Language-Tag, see [RFC5646], Section 2.1>

A language tag is a sequence of one or more case-insensitive subtags, each separated by a hyphen character ("-", %x2D). In most cases, a language tag consists of a primary language subtag that identifies a broad family of related languages (e.g., "en" = English), which is optionally followed by a series of subtags that refine or narrow that language's range (e.g., "en-CA" = the variety of English as communicated in Canada). Whitespace is not allowed within a language tag. Example tags include:

en, en-US, en-cockney, i-cherokee, x-pig-latin

See [RFC5646] for further information.

3.1.3.2. Content-Language

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.

Content-Language = 1#language-tag

Language tags are defined in Section 3.1.3.1. The primary purpose of Content-Language is to allow a user to identify and differentiate representations according to the users' own preferred language. Thus, if the content is intended only for a Danish-literate audience, the appropriate field is:

Content-Language: da

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:

Content-Language: mi, en

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.

3.1.4. Identification

3.1.4.1. Identifying a Representation

When a complete or partial representation is transferred in a message payload, it is often desirable for the sender to supply, or the recipient to determine, an identifier for a resource corresponding to that representation.

For a request message:

• If the request has a Content-Location header field, then the sender asserts that the payload is a representation of the resource identified by the Content-Location field-value. However, such an assertion cannot be trusted unless it can be verified by other means (not defined by this specification). The information might still be useful for revision history links.

• Otherwise, the payload is unidentified.

For a response message, the following rules are applied in order until a match is found:

1. If the request method is GET or HEAD and the response status code is 200 (OK), 204 (No Content), 206 (Partial Content), or 304 (Not Modified), the payload is a representation of the resource identified by the effective request URI (Section 5.5 of [RFC7230]).

2. If the request method is GET or HEAD and the response status code is 203 (Non-Authoritative Information), the payload is a potentially modified or enhanced representation of the target resource as provided by an intermediary.

3. If the response has a Content-Location header field and its field-value is a reference to the same URI as the effective request URI, the payload is a representation of the resource identified by the effective request URI.

4. If the response has a Content-Location header field and its field-value is a reference to a URI different from the effective request URI, then the sender asserts that the payload is a representation of the resource identified by the Content-Location field-value. However, such an assertion cannot be trusted unless it can be verified by other means (not defined by this specification).

5. Otherwise, the payload is unidentified.

3.1.4.2. Content-Location

The Content-Location header field references a URI that can be used as an identifier for a specific resource corresponding to the representation in this message's payload. In other words, if one were to perform a GET request 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.

Content-Location = absolute-URI / partial-URI

The Content-Location value is not a replacement for the effective request URI (Section 5.5 of [RFC7230]). 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 2xx (Successful) response message and its value refers (after conversion to absolute form) to a URI that is the same as the effective request URI, then the recipient MAY consider the payload to be a current representation of that resource at the time indicated by the message origination date. For a GET (Section 4.3.1) or HEAD (Section 4.3.2) 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 (Section 4.3.4) or POST (Section 4.3.3), it implies that the server's response contains the new representation of that resource, thereby distinguishing it from representations that might only report on 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 2xx (Successful) response message and its field-value refers to a URI that differs from the effective request URI, then the origin server claims that the URI is an identifier for a different resource corresponding to the enclosed representation. Such a claim can only be trusted if both identifiers share the same resource owner, which cannot be programmatically determined via HTTP.

• For a response to a GET or HEAD request, this is an indication that the effective request URI refers to a resource that is subject to content negotiation and the Content-Location field-value is a more specific identifier for the selected representation.

• For a 201 (Created) response to a POST request, the Content-Location field-value is a reference to a resource that corresponds to the submitted data (i.e., a distinct resource from the one identified by the effective request URI).

• Otherwise, such a Content-Location indicates that this payload is a representation of some resource other than the target of the request message, and it has no other significance for HTTP.

If Content-Location is included in a response message and the representation consists of multiple parts, as indicated by a Content-Type of "multipart/*", then each part MAY have its own Content-Location to indicate the resource corresponding to that part. Otherwise, the Content-Location applies only to the payload as a whole.

3.2. Representation Data

The representation data 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 ) )

3.3. Payload Semantics

Some HTTP messages transfer a complete or partial representation as the message "payload". In some cases, a payload might contain only the associated representation's header fields (e.g., responses to HEAD) or only some part(s) of the representation data (e.g., the 206 (Partial Content) status code).

The purpose of a payload in a request is defined by the method semantics. For example, a representation in the payload of a PUT request (Section 4.3.4) represents the desired state of the target resource if the request is successfully applied, whereas a representation in the payload of a POST request (Section 4.3.3) represents information to be processed by the target resource.

In a response, the payload's purpose is defined by both the request method and the response status code. For example, the payload of a 200 (OK) response to GET (Section 4.3.1) represents the current state of the target resource, as observed at the time of the message origination date (Section 7.1.1.2), whereas the payload of the same status code in a response to POST might represent either the processing result or the new state of the target resource after applying the processing.

A payload is considered "complete" in the following cases:

• the entire header section has been received and the indicated payload body length is zero;

• a chunked payload is complete when the chunked-body part is complete (as indicated by the zero-length chunk and terminating CRLF);

• an unknown payload length is complete when the connection is closed.

3.4. Content Negotiation

When responses convey payload information, whether indicating a success or an error, the origin server often has different ways of representing that information; for example, in different formats, languages, or encodings. Likewise, different users or user agents might have differing capabilities, characteristics, or preferences that could influence which representation, among those available, would be best to deliver. For this reason, HTTP provides mechanisms for content negotiation.

This specification defines three patterns of content negotiation that can be made visible within the protocol: "proactive", where the server selects the representation based upon the user agent's stated preferences; "reactive", where the server provides a list of representations for the user agent to choose from; and "request content", where the user agent selects the representation for a request payload. Other patterns of content negotiation include "conditional content", where the representation consists of multiple parts that are selectively rendered based on user agent parameters, "active content", where the representation contains a script that makes additional (more specific) requests based on the user agent characteristics, and "Transparent Content Negotiation" ([RFC2295]), where content selection is performed by an intermediary. These patterns are not mutually exclusive, and each has trade-offs in applicability and practicality.

Note that, in all cases, HTTP is not aware of the resource semantics. The consistency with which an origin server responds to requests, over time and over the varying dimensions of content negotiation, and thus the "sameness" of a resource's observed representations over time, is determined entirely by whatever entity or algorithm selects or generates those responses. HTTP pays no attention to the man behind the curtain.

3.4.1. Proactive Negotiation

When content negotiation preferences are sent by the user agent in a request to encourage an algorithm located at the server to select the preferred representation, it is called proactive negotiation (a.k.a., server-driven negotiation). Selection is based on the available representations for a response (the dimensions over which it might vary, such as language, content-coding, etc.) compared to various information supplied in the request, including both the explicit negotiation fields of Section 5.3 and implicit characteristics, such as the client's network address or parts of the User-Agent field.

Proactive negotiation is advantageous when the algorithm for selecting from among the available representations is difficult to describe to a user agent, or when the server desires to send its "best guess" to the user agent 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, a user agent MAY send request header fields that describe its preferences.

Proactive negotiation has serious disadvantages:

• 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?);

• 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 risk to the user's privacy;

• It complicates the implementation of an origin server and the algorithms for generating responses to a request; and,

• It limits the reusability of responses for shared caching.

A user agent cannot rely on proactive negotiation preferences being consistently honored, since the origin server might not implement proactive negotiation for the requested resource or might decide that sending a response that doesn't conform to the user agent's preferences is better than sending a 406 (Not Acceptable) response.

A Vary header field (Section 7.1.4) is often sent in a response subject to proactive negotiation to indicate what parts of the request information were used in the selection algorithm.

3.4.2. Reactive Negotiation

With reactive negotiation (a.k.a., agent-driven negotiation), selection of the best response representation (regardless of the status code) is performed by the user agent after receiving an initial response from the origin server that contains a list of resources for alternative representations. If the user agent is not satisfied by the initial response representation, it can perform a GET request on one or more of the alternative resources, selected based on metadata included in the list, to obtain a different form of representation for that response. Selection of alternatives might be performed automatically by the user agent or manually by the user selecting from a generated (possibly hypertext) menu.

Note that the above refers to representations of the response, in general, not representations of the resource. The alternative representations are not necessarily representations of the same resource, though they might be.

A server might choose not to send an initial representation, other than the list of alternatives, and thereby indicate that reactive negotiation by the user agent is preferred. For example, the alternatives listed in responses with the 300 (Multiple Choices) and 406 (Not Acceptable) status codes include information about the available representations so that the user or user agent can react by making a selection.

Reactive 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.

Reactive negotiation suffers from the disadvantages of transmitting a list of alternatives to the user agent, which degrades user-perceived latency if transmitted in the header section, and needing a second request to obtain an alternate representation. Furthermore, this specification does not define a mechanism for supporting automatic selection, though it does not prevent such a mechanism from being developed as an extension.

3.4.3. Request Content Negotiation

When content negotiation preferences are sent in a server's response to a client, the client can use those preferences to determine which representation is best for subsequent requests to that resource. This is called request content negotiation (a.k.a., "content negotiation as a quality of service").

For example, an Accept header field in a 415 (Unsupported Media Type) response might indicate what media types are accepted in a PUT request to that resource, or the Accept-Language field of a response might indicate the language preferences of the service.

---