| Individual Submission | J. Snell |
| Internet-Draft | December 7, 2011 |
| Intended status: Informational | |
| Expires: June 9, 2012 |
This specification defines an HTTP header field that can be used by a client to request that certain behaviors be implemented by a server while processing a request.¶
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This specification defines a new HTTP request header field that may be used by clients to request optional behaviors be applied by a server during the processing the request.¶
In this document, the key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" are to be interpreted as described in [RFC2119].¶
This specification uses the Augmented Backus-Naur Form (ABNF) notation of [RFC5234] and includes, by reference, the "token", "quoted-string", "OWS", "BWS" rules and the #rule extension as defined within [I-D.ietf-httpbis-p1-messaging] Section 1.2.¶
The Prefer request-header field is used to indicate that particular server behaviors are preferred by the client, but not required for successful completion of the request. Prefer is similar in nature to the Expect header field defined by [I-D.ietf-httpbis-p2-semantics] Section 9.3 with the exception that servers are allowed to ignore stated preferences.¶
Prefer = "Prefer" ":" 1#preference
preference = token [ BWS "=" BWS value ]
*( OWS ";" [ OWS parameter ] )
parameter = token [ BWS "=" BWS value ]
value = token / quoted-string
This header field is defined with an extensible syntax to allow for future values included in the Registry of Preferences (Section 12.1)). A server that does not recognize or is unable to comply with particular preference tokens in the Prefer header field of a request MUST ignore those tokens and MUST NOT stop processing or signal an error.¶
A preference token MAY specify a value. Empty, or zero length values on both the preference token and within parameters are equivalent to no value being specified at all. The following, then, are equivalent:¶
Prefer: foo; bar=""
Prefer: foo=; bar
Prefer: foo=""; bar=
An optional, arbitrary collection of parameters MAY be specified for any preference token. The meaning and application of such parameters is dependent on the definition of each preference token and the server's implementation thereof.¶
If a particular preference token or parameter is specified multiple times, repeated occurrences MUST be ignored without signaling an error or otherwise altering the processing of the request.¶
Comparison of preference token names is case-insensitive while values are case-sensitive regardless of whether token or quoted-string values are used.¶
Note that the application of a preference by the server MAY affect the caching characteristics of the response. Specifically, should the application of a preference result in a variance to the representation returned by a cacheable response, a Vary header field MUST be included listing the Prefer header field as one of the selecting header fields.¶
The Prefer request header field MUST be forwarded by the proxy if the request is forwarded. In various situations, A proxy may determine that it is capable of honoring a preference independently of the server to which the request is directed. For instance, an intervening proxy may be capable of transparently providing asynchronous handling of a request using a 202 Accepted responses independently of the origin server. Such proxies could choose to honor the "return-accepted" preference. Individual preference tokens MAY define their own requirements and restrictions as to whether and how proxies may apply the preference to a request independently of the origin server.¶
As per [I-D.ietf-httpbis-p1-messaging], Section 3.2, Implementations MUST be capable of supporting either multiple instances of the Prefer header field in a single message as well as multiple preference tokens separated by commas in a single Prefer header, for instance, the following examples are equivalent:¶
# Multiple Prefer Header Fields
POST /foo HTTP/1.1
Host: example.org
Prefer: return-accepted
Prefer: wait=100
# Single Prefer Header Field
POST /foo HTTP/1.1
Host: example.org
Prefer: return-accepted, wait=100
The following examples illustrate the use of various Preferences defined by this specification, as well as undefined extensions for strictly illustrative purposes:¶
# Return a 202 Accepted response for asynchronous processing
# if the response cannot be processed within 10 seconds.
# An undefined "priority" preference is also specified.
Prefer: return-accepted;
Prefer: wait=10;
Prefer: priority=5;
# Use lenient processing, a reporting detail level of 10,
# and return an entity describing the status of the request
# rather than a representation of the resource
Prefer: Lenient, Detail=10, Return-Status
# Use of an optional, undefined parameter on the
# return-minimal preference requesting a response
# status code of 204 for a successful response.
Prefer: return-minimal; status=204
The Preference-Applied response header MAY be included within a response message as an indication as to which Prefer tokens were honored by the server and applied to the processing of the request.¶
Preference-Applied = "Preference-Applied" ":" 1#token
Note that the syntax of the Preference-Applied header differs from that of the Prefer header in that token values and parameters are not included.¶
Use of the Preference-Applied header is not required in all cases. For instance, when using the return-accepted, return-minimal or wait preferences, as defined below, the application of the preference will be apparent based on the context and nature of the response. However, there are cases in which the application of a preference cannot be easily determined by the client. For instance, when posting a resource to a server using either the return-representation or return-status preferences, a client cannot be certain in all cases whether the returned entity is a representation of the modified resource or a representation of the request status. In such situations, where the nature of the response is ambiguous and a clear preference has been stated by the client using the Prefer request header field, the Preference-Applied response header field SHOULD be used.¶
For example, here a request is sent to the server requesting the return of the resource representation.¶
#Request
POST /collection HTTP/1.1
Host: example.org
Content-Type: application/json
Prefer: return-representation
{...}
# Response
HTTP/1.1 201 Created
Content-Type: application/json
Preference-Applied: return-representation
Location: /collection/1
{...}
Whereas in this example, the request is sent asking for the return of a status representation.¶
#Request
POST /collection HTTP/1.1
Host: example.org
Content-Type: application/json
Prefer: return-status
{...}
# Response
HTTP/1.1 201 Created
Content-Type: application/json
Preference-Applied: return-status
Location: /collection/1
{...}
Use of the Preference-Applied response header allows the client to unambiguously determine whether the requested preference was applied.¶
The "return-accepted" preference indicates that the client prefers the server to respond with a 202 Accepted status in the case where the length of time it takes to generate a response will exceed some arbitrary threshold established by the server.¶
return-accepted = "return-accepted"
The key motivation for the "return-accepted" preference is to facilitate the operation of asynchronous request handling by allowing the client to indicate to a server it's capability and preference for handling 202 Accepted responses.¶
The "return-representation" preference indicates that the client prefers that the server include an entity representing the current state of the resource in the response to a successful request.¶
return-representation = "return-representation"
When honoring the "return-representation" preference, the server MUST include a Content-Location header field specifying the URI of the resource representation being returned. Per section 6.1 of [I-D.ietf-httpbis-p2-semantics], the presence of the Content-Location header field in the response asserts that the payload is a representation of the resource identified by the Content-Location URI.¶
The "return-representation" preference is intended primarily to provide a means of optimizing communication between the client and server by eliminating the need for a subsequent GET request to retrieve the current representation of the resource following a modification.¶
Currently, after successfully processing a modification request such as a POST or PUT, a server may choose to return either an entity describing the status of the operation or a representation of the modified resource itself. While the selection of which type of entity to return, if any at all, is solely at the discretion of the server, the "return-representation" preference -- along with the "return-status" and "return-minimal" directives defined below -- allow the server to take the client's preferences into consideration while constructing the response.¶
The "return-status" preference indicates that the client prefers the server to include an entity describing the status of the request in the response as opposed to returning a representation of the current state of the resource.¶
return-status = "return-status"
When honoring the "return-status" preference, the server SHOULD NOT include a Content-Location header field in the response.¶
The "return-minimal" preference indicates that the client wishes the server to return a minimal response to a successful request. Typically, such responses would utilize the 204 No Content status, but other codes MAY be used as appropriate, such as a 200 status with a zero-length response entity. The determination of what constitutes an appropriate minimal response is solely at the discretion of the server.¶
return-minimal = "return-minimal"
The "return-minimal" preference is intended to provide a means of optimizing communication between the client and server by reducing the amount of data the server is required to return to the client following a request. This can be particularly useful, for instance, when communicating with limited-bandwidth mobile devices or when the client simply does not require any further information about the result of a request beyond knowing if it was successfully processed.¶
The "wait" preference can be used to establish an upper bound on the length of time, in seconds, the client is willing to wait for a response, after which the client may choose to abandon the request. In the case generating a response will take longer than the time specified, the server, or proxy, can choose to either return a 202 Accepted response, cancel processing, or continue attempting to complete the request.¶
wait = "wait" BWS "=" BWS delta-seconds
Clients specifying the "wait" Preference SHOULD also use the Date header field, as specified in [I-D.ietf-httpbis-p2-semantics] Section 9.2, within the request to establish the time at which the client began waiting for the completion of the request. Failing to include a Date header field in the request would require the server to use the instant it received and began processing the request as the baseline for determining how long the client has been waiting which could yield unintended results depending on how out of synch the client and server clocks are.¶
The "strict" and "lenient" preferences are mutually-exclusive directives indicating, at the servers discretion, how the client wishes the server to handle potential error conditions that may arise in the processing of a request. For instance, if the payload of a request contains various minor syntactical or semantic errors, but the server is still capable of comprehending and successfully processing the request, a decision must be made to either reject the request with an appropriate 4xx error response or to go ahead with processing. The "strict" preference can be used by the client to indicate that, in such conditions, it would prefer that the server reject the request, while the "lenient" preference indicates that the client would prefer the server to attempt to process the request. The specific meaning and application of the "strict" and "lenient" directives is specific to each type of resource, the request method and the operation of the server.¶
handling = "strict" / "lenient"
The "detail" preference specifies the level of detail the client wishes the server to provide within a response to an operation. This preference is akin to specifying the level of verbose output an operation should generate or to specifying the trace level within a debug log. The detail level is specified as a non-negative integer in the range 0-100, where the value 0 indicates a server-determined default detail level and all other integer values specify a strictly decreasing level of detail as the integer value increases.¶
detail = "detail" BWS "=" BWS ("100" / 1*2DIGIT )
Implementations are free to apply additional constraints on the range of acceptable values for this directive but MUST NOT signal an error or fail to process the request should the client provide a value outside the acceptable range. In such cases, the server SHOULD either ignore the preference or apply a reasonable default value.¶
One example of a potential use for the application of the "detail" preference would be in deciding the amount of detailed error information a server includes in the payload of a 4xx or 5xx response. Solely at the discretion of the server, an error response to a request specifying a higher detail level (e.g., detail=1) may included significantly more detailed information about the error condition than an error response specifying a much lower detail level (e.g., detail=10).¶
Well-defined preferences can be registered for convenience and/or to promote reuse by other applications. This specification establishes an IANA registry of such relation types see Section Section 12.1.¶
Registered preference names MUST conform to the token rule, and MUST be compared character-by-character in a case-insensitive fashion. They SHOULD be appropriate to the specificity of the preference; i.e., if the semantics are highly specific to a particular application, the name should reflect that, so that more general names are available for less specific use.¶
Registered preferences MUST NOT constrain servers, clients or any intermediaries involved in the exchange and processing of a request to any behavior required for successful processing. The use and application of a preference within a given request MUST be optional on the part of all participants.¶
Header field name: Prefer
Applicable Protocol: HTTP
Status:
Author/Change controller: IETF
Specification document: this specification
Preferences are registered on the advice of a Designated Expert (appointed by the IESG or their delegate), with a Specification Required (using terminology from [RFC5226]).¶
The requirements for registered preferences are described in Section 11¶
Registration requests consist of the completed registration template below, typically published in an RFC or Open Standard (in the sense described by [RFC2026], Section 7). However, to allow for the allocation of values prior to publication, the Designated Expert may approve registration once they are satisfied that a specification will be published.¶
Note that relation types can be registered by third parties, if the Designated Expert determines that an unregistered relation type is widely deployed and not likely to be registered in a timely manner.¶
The registration template is:¶
Registration requests should be sent to the preferences@ietf.org mailing list, marked clearly in the subject line (e.g., "NEW PREFERENCE - example" to register an "example" preference).¶
Within at most 14 days of the request, the Designated Expert(s) will either approve or deny the registration request, communicating this decision to the review list and IANA. Denials should include an explanation and, if applicable, suggestions as to how to make the request successful.¶
Decisions (or lack thereof) made by the Designated Expert can be first appealed to Application Area Directors (contactable using app-ads@tools.ietf.org email address or directly by looking up their email addresses on http://www.iesg.org/ website) and, if the appellant is not satisfied with the response, to the full IESG (using the iesg@iesg.org mailing list).¶
IANA should only accept registry updates from the Designated Expert(s), and should direct all requests for registration to the review mailing list.¶
The Preferences Registry's initial contents are:¶
Specific preferences requested by a client can introduce security considerations and concerns beyond those discussed in HTTP/1.1 Parts 1 [I-D.ietf-httpbis-p1-messaging], 2 [I-D.ietf-httpbis-p2-semantics], 3 [I-D.ietf-httpbis-p3-payload], 4 [I-D.ietf-httpbis-p4-conditional], 5 [I-D.ietf-httpbis-p5-range], 6 [I-D.ietf-httpbis-p6-cache], and 7 [I-D.ietf-httpbis-p7-auth]. Implementors must refer to the specifications and descriptions of each preference to determine the security considerations relevant to each.¶