Individual Submission J. Snell
Internet-Draft October 25, 2011
Intended status: Informational
Expires: April 27, 2012
Prefer Header for HTTP
draft-snell-http-prefer-04
Abstract
This specification defines an HTTP header that can be used by a user-
agent to request that certain behaviors be implemented by a server
while processing a request.
Status of this Memo
This Internet-Draft is submitted to IETF in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on April 27, 2012.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. The Prefer Request Header . . . . . . . . . . . . . . . . . . 3
3. The "return-accepted" Preference . . . . . . . . . . . . . . . 4
4. The "return-content" Preference . . . . . . . . . . . . . . . 4
5. The "return-status" Preference . . . . . . . . . . . . . . . . 5
6. The "return-no-content" Preference . . . . . . . . . . . . . . 5
7. The "wait" Preference . . . . . . . . . . . . . . . . . . . . 6
8. The "priority" Preference . . . . . . . . . . . . . . . . . . 6
9. The "strict" and "lenient" Processing Preferences . . . . . . 7
10. The "detail" Preference . . . . . . . . . . . . . . . . . . . 7
11. Registered Preferences . . . . . . . . . . . . . . . . . . . . 8
12. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
12.1. The Registry of Preferences . . . . . . . . . . . . . . . 8
12.1.1. Initial Registry Contents . . . . . . . . . . . . . . 9
13. Security Considerations . . . . . . . . . . . . . . . . . . . 11
14. Normative References . . . . . . . . . . . . . . . . . . . . . 11
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 11
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1. Introduction
This specification defines a new HTTP request header that may be used
by user-agents 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].
2. The Prefer Request Header
The Prefer request-header is used to indicate that particular server
behaviors are preferred by the user-agent, but not required for
successful completion of the request. Prefer is similar in nature to
the Expect header defined by [RFC2616] with the exception that
servers are allowed to ignore stated preferences.
Prefer = "Prefer" ":" 1#preference
preference = (return-accepted /
return-no-content /
return-content /
return-status /
wait /
priority /
handling /
detail /
preference-extension)
*prefer-params
preference-value = token / quoted-string
preference-token = token [ "=" preference-value ]
preference-extension = preference-token
prefer-params = ";" preference-token
This header 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 values in the Prefer header of a request MUST ignore those
values and MUST NOT stop processing or signal an error.
An optional, arbitrary collection of "prefer-params" MAY be specified
for any of the defined preference tokens as well as any preference-
extensions. The meaning and application of such parameters is
dependent on the definition of each preference directive and the
server's implementation thereof.
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Comparison of preference tokens is case-insensitive for unquoted
tokens and is case-sensitive for quoted-string preference-extensions
and prefer-params values.
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 SHOULD
be included listing the Prefer header as one of the selecting header
fields.
The Prefer request header 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
directives 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.
3. The "return-accepted" Preference
The "return-accepted" preference indicates that the user-agent
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 user-agent to indicate to a server it's capability and preference
for handling 202 Accepted responses.
4. The "return-content" Preference
The "return-content" preference indicates that the user-agent prefers
that the server include an entity representing the current state of
the resource in the response to a successful request.
return-content = "return-content"
When honoring the "return-content" preference, the server MUST
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include a Content-Location header specifying the URI of the resource
representation being returned. Per section 6.1 of
[TODO:draft-ietf-httpbis-p2-semantics], the presence of the Content-
Location header in the response asserts that the payload is a
representation of the resource identified by the Content-Location
URI.
The "return-content" preference is intended primarily to provide a
means of optimizing communication between the user-agent 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-content" preference -- along with the "return-
status" and "return-no-content" directives defined below -- allow the
server to take the user-agent's preferences into consideration while
constructing the response.
5. The "return-status" Preference
The "return-status" preference indicates that the user-agent 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 in the response.
6. The "return-no-content" Preference
The "return-no-content" preference indicates that the user-agent
wishes the server to not include an entity in the response to a
successful request. Typically, such responses would use the 204 No
Content status, but other codes MAY be used as appropriate.
Regardless of the status returned, when honoring the "return-no-
content" preference, the server MUST NOT include an entity within the
response.
return-no-content = "return-no-content"
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The "return-no-content" preference is intended to provide a means of
optimizing communication between the user-agent and server by
reducing the amount of data the server is required to return to the
user-agent following a modification request. This can be
particularly useful, for instance, when communicating with limited-
bandwidth mobile devices or when the user-agent simply does not
require any further information about the result of a request beyond
knowing if it was successfully processed.
7. The "wait" Preference
The "wait" preference can be used to establish an upper bound on the
length of time, in seconds, the user-agent is willing to wait for a
response, after which the user-agent 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" "=" delta-seconds
8. The "priority" Preference
ED NOTE: This preference directive is currently exploratory in
nature. I've added it to solicit feedback as to it's general
utility. It is possible that I may pull this back out.
The "priority" preference can be used to indicate the priority a
user-agent wishes the server or proxy to assign to processing the
request relative to other requests that may be concurrently received.
The application and assignment of a priority value to requests is
entirely at the discretion of the server or proxy. Priority values
are specified as non-negative integers within the range 0-100,
inclusive, where the value 0 indicates that the user-agent wishes to
have a server-determined default priority assigned to the request,
and all other values indicate a strictly decreasing priority as the
integer value increases.
priority = "priority" "=" "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 user-agent provide a value
outside the acceptable range. In such cases, the server SHOULD
either ignore the preference or apply a reasonable default value.
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9. The "strict" and "lenient" Processing Preferences
ED NOTE: This preference directive is currently exploratory in
nature. I've added it to solicit feedback as to it's general
utility. It is possible that I may pull this back out.
The "strict" and "lenient" preferences are mutually-exclusive
directives indicating, at the servers discretion, how the user-agent
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 user-agent to
indicate that, in such conditions, it would prefer that the server
reject the request, while the "lenient" preference indicates that the
user-agent 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"
10. The "detail" Preference
ED NOTE: This preference directive is currently exploratory in
nature. I've added it to solicit feedback as to it's general
utility. It is possible that I may pull this back out.
The "detail" preference specifies, at the servers discretion, the
level of detail the user-agent 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
values specify a strictly decreasing level of detail as the integer
value increases.
detail = "detail" "=" "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 user-agent provide a value
outside the acceptable range. In such cases, the server SHOULD
either ignore the preference or apply a reasonable default value.
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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).
11. Registered Preferences
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, user-agents 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.
12. IANA Considerations
The 'Prefer' header should be added to the permanent registry (see
[RFC3864]).
Header field name: Prefer
Applicable Protocol: HTTP
Status:
Author/Change controller: IETF
Specification document: this specification
12.1. The Registry of Preferences
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]).
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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:
o Preference: (A value for the Prefer request header that conforms
to the syntax rule given in Section 2)
o Description:
o Reference:
o Notes: [optional]
o Application Data: [optional]
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.
12.1.1. Initial Registry Contents
The Preferences Registry's initial contents are:
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o Preference: return-accepted
o Description: Indicates that the user-agent prefers the server to
respond with a 202 Accepted status as described by Section 3
o Reference: [this specification]
o Preference: return-no-content
o Description: Indicates that the user-agent prefers the server not
to include a payload in response to a request as described by
Section 6
o Reference: [this specification]
o Preference: return-content
o Description: Indicates that the user-agent prefers the server to
include a representation of the current state of the resource in
response to a request as described by Section 4
o Reference: [this specification]
o Preference: return-status
o Description: Indicates that the user-agent prefers the server to
return an entity describing the current state of a resource in
response to a request as described by Section 5
o Reference: [this specification]
o Preference: wait
o Description: Indicates an upper bound to the lenght of time the
user-agent is willing to wait for a response, after which the
request may be aborted.
o Reference: [this specification]
o Preference: priority
o Description: Indicates the priority a client wishes the server to
assign to the processing of a request relative to other
concurrently processed requests.
o Reference: [this specification]
o Preference: strict
o Description: Indicates that the client wishes the server to apply
strict validation and error handling to the processing of a
request.
o Reference: [this specification]
o Preference: lenient
o Description: Indicates that the client wishes the server to apply
lenient validation and error handling to the processing of a
request.
o Reference: [this specification]
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o Preference: detail
o Description: Indicates the user-agent's preference as to the
amount of detail the server should include in responses to a
request.
o Reference: [this specification]
13. Security Considerations
Specific preferences requested by a client can introduce security
considerations and concerns beyond those discussed in [RFC2616].
Implementors must refer to the specifications and descriptions of
those preferences to determine the security considerations relevant
to each.
14. Normative References
[RFC2026] Bradner, S., "The Internet Standards Process -- Revision
3", BCP 9, RFC 2026, October 1996.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2434] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 2434,
October 1998.
[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.
[RFC3864] Klyne, G., Nottingham, M., and J. Mogul, "Registration
Procedures for Message Header Fields", BCP 90, RFC 3864,
September 2004.
[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 5226,
May 2008.
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Author's Address
James M Snell
Phone:
Email: jasnell@gmail.com
URI:
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