Network Working Group J. Snell
Internet-Draft August 23, 2012
Intended status: Standards Track
Expires: February 24, 2013
Prefer Header for HTTP
draft-snell-http-prefer-14
Abstract
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.
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 February 24, 2013.
Copyright Notice
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Syntax Notation . . . . . . . . . . . . . . . . . . . . . 4
2. The Prefer Request Header Field . . . . . . . . . . . . . . . 4
2.1. Content Negotiation and Cache Considerations . . . . . . . 6
2.2. Examples . . . . . . . . . . . . . . . . . . . . . . . . . 6
3. Preference Definitions . . . . . . . . . . . . . . . . . . . . 7
3.1. The "return-asynch" Preference . . . . . . . . . . . . . . 7
3.2. The "return-representation" Preference . . . . . . . . . . 8
3.3. The "return-minimal" Preference . . . . . . . . . . . . . 9
3.4. The "wait" Preference . . . . . . . . . . . . . . . . . . 10
3.5. The "strict" and "lenient" Processing Preferences . . . . 11
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 12
4.1. The Registry of Preferences . . . . . . . . . . . . . . . 12
4.2. Initial Registry Contents . . . . . . . . . . . . . . . . 13
5. Security Considerations . . . . . . . . . . . . . . . . . . . 14
6. Normative References . . . . . . . . . . . . . . . . . . . . . 14
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 16
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1. Introduction
Within the course of processing an HTTP request there are typically a
range of required and optional behaviors that a server or
intermediary can employ. These often manifest is a variety of subtle
and not-so-subtle ways within the response.
For example, when using the HTTP PUT method to modify a resource --
similar to that defined for the Atom Publishing Protocol [RFC 5023]
-- the server is given the option of returning either a complete
representation of a modified resource or a minimal response that
indicates only the successful completion of the operation. The
selection of which type of response to return to the client generally
has no bearing on the successful processing of the request but could,
for instance, have an impact on what actions the client must take
after receiving the response. That is, returning a representation of
the modified resource within the response can allow the client to
avoid sending an additional subsequent GET request.
Similarly, servers that process requests are often faced with
decisions about how to process requests that may be technically
invalid or incorrect but are still understandable. It might be the
case that the server is able to overlook the technical errors in the
request but still successfully process the request. Depending on the
specific requirements of the application and the nature of the
request being made, the client might or might not consider such
lenient processing of its request to be appropriate.
While the decision of exactly which behaviors to apply in these cases
lies with the server processing the request, the server might wish to
defer to the client to specify which optional behavior is preferred.
Currently, HTTP offers no explicitly defined means of expressing the
client's preferences regarding the optional aspects of handling of a
given request. While HTTP does provide the Expect header -- which
can be used to identify mandatory expectations for the processing of
a request -- use of the field to communicate optional preferences is
problematic:
1. The semantics of the Expect header field are such that
intermediaries and servers are required to reject any request
that states unrecognized or unsupported expectations.
2. While the Expect header field is end-to-end, the HTTP
specification requires that the header be processed hop-by-hop.
That is, every interceding intermediary that handles a request
between the client and the origin server is required to process
an expectation and determine whether it is capable of
appropriately handling it.
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The rigid, must-understand semantics of the Expect header, therefore,
make it a poor choice for the general expression of optional
preferences that may be specific to an individual application and are
therefore unknown to an intermediary or are otherwise irrelevant to
the intermediaries successful handling of the request and response.
Another option available to clients is to utilize Request URI query-
string parameters to express preferences. Doing so, however, results
in a variety of issues affecting the cacheability of responses.
As an alternative, this specification defines a new HTTP request
header field that can be used by clients to request that optional
behaviors be applied by a server during the processing the request.
Additionally, a handful of initial preference tokens for use with the
new header are defined.
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].
1.1. Syntax Notation
This specification uses the Augmented Backus-Naur Form (ABNF)
notation of [RFC5234] and includes, by reference, the "token",
"word", "OWS", "BWS" rules and the #rule extension as defined within
Sections 1.2 and 3.2.4 of [I-D.ietf-httpbis-p1-messaging].
2. The Prefer Request Header Field
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 Section 9.3 of
[I-D.ietf-httpbis-p2-semantics] with the exception that servers are
allowed to ignore stated preferences.
Prefer = "Prefer" ":" 1#preference
preference = token [ BWS "=" BWS word ]
*( OWS ";" [ OWS parameter ] )
parameter = token [ BWS "=" BWS word ]
This header field is defined with an extensible syntax to allow for
future values included in the Registry of Preferences (Section 4.1).
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.
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A preference token can contain 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 set of parameters can 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.
The Prefer request header field is end-to-end and MUST be forwarded
by a proxy if the request is forwarded.
In various situations, a proxy might determine that it is capable of
honoring a preference independently of the server to which the
request has been directed. For instance, an intervening proxy might
be capable of providing asynchronous handling of a request using 202
Accepted responses independently of the origin server. Such proxies
can choose to honor the "return-asynch" preference on their own
despite whether the origin is capable or willing to do so. In such
cases, however, the proxy is still required to forward the Prefer
header on to the origin server.
Individual preference tokens MAY define their own requirements and
restrictions as to whether and how intermediaries can apply the
preference to a request independently of the origin server.
As per Section 3.2 of [I-D.ietf-httpbis-p1-messaging],
Implementations MUST support 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 field. The following
examples are equivalent:
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Multiple Prefer Header Fields:
POST /foo HTTP/1.1
Host: example.org
Prefer: return-asynch
Prefer: wait=100
Date: Tue, 20 Dec 2011 12:34:56 GMT
Single Prefer Header Field:
POST /foo HTTP/1.1
Host: example.org
Prefer: wait=100, return-asynch
Date: Tue, 20 Dec 2011 12:34:56 GMT
No significance is given to the order in which preference tokens
appear within a request.
2.1. Content Negotiation and Cache Considerations
Note that while the Prefer header field is not intended to be used as
content negotiation mechanism, the application of a preference
potentially could affect the caching characteristics of a response.
Specifically, if a server supports the optional application of a
preference that could even just potentially result in a variance to a
cache's handling of a response entity, a Vary header field MUST be
included with the response listing the Prefer header field regardless
of whether the client actually used Prefer in the request.
Because of the inherent complexities involved with properly
implementing server-driven content negotiation, effective caching,
and the application of optional preferences, implementors must
exercise caution when utilizing preferences in such a way as to
impact the caching of a response and SHOULD NOT use the Prefer header
mechanism for content negotiation.
2.2. Examples
The following examples illustrate the use of various preferences
defined by this specification, as well as undefined extensions for
strictly illustrative purposes:
1. 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-asynch, wait=10;
Prefer: priority=5;
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2. Use lenient processing:
Prefer: Lenient
3. 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
3. Preference Definitions
The following subsections define an initial set of preferences.
Additional 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 4.1).
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
remain 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.
3.1. The "return-asynch" Preference
The "return-asynch" preference indicates that the client prefers the
server to respond asynchronously to a response. For instance, in the
case when the length of time it takes to generate a response will
exceed some arbitrary threshold established by the server, the server
can honor the return-asynch preference by returning either a "202
Accepted" or "303 See Other" response.
ABNF:
return-asynch = "return-asynch"
The key motivation for the "return-asynch" preference is to
facilitate the operation of asynchronous request handling by allowing
the client to indicate to a server its capability and preference for
handling asynchronous responses.
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An example request specifying the "return-asynch" preference:
POST /collection HTTP/1.1
Host: example.org
Content-Type: text/plain
Prefer: return-asynch
{Data}
An example asynchronous response using "202 Accepted":
HTTP/1.1 202 Accepted
Location: http://example.org/collection/123
An alternative asynchronous response using "303 See Other":
HTTP/1.1 303 See Other
Location: http://example.org/collection/123
Retry-After: 10
3.2. The "return-representation" Preference
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.
ABNF:
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 can 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
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entity to return, if any at all, is solely at the discretion of the
server, the "return-representation" preference -- along with the
"return-minimal" preference defined below -- allow the server to take
the client's preferences into consideration while constructing the
response.
An example request specifying the "return-representation" preference:
PUT /collection/123 HTTP/1.1
Host: example.org
Content-Type: text/plain
Prefer: return-representation
{Data}
An example response containing the resource representation:
HTTP/1.1 200 OK
Content-Location: http://example.org/collection/123
Content-Type: text/plain
ETag: "d3b07384d113edec49eaa6238ad5ff00"
{Data}
The "return-minimal" and "return-representation" preferences are
mutually exclusive directives that MUST NOT be used in combination
within a single request. If a server receives a request containing
both the "return-minimal" and "return-representation" preferences, it
MAY choose to ignore either or both of the stated preferences but
MUST NOT signal an error or fail to process the request solely on the
basis of those preferences.
3.3. The "return-minimal" Preference
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.
ABNF:
return-minimal = "return-minimal"
The "return-minimal" preference is intended to provide a means of
optimizing communication between the client and server by reducing
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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.
An example request specifying the "return-minimal" preference:
POST /collection HTTP/1.1
Host: example.org
Content-Type: text/plain
Prefer: return-minimal
{Data}
An example minimal response:
HTTP/1.1 201 Created
Location: http://example.org/collection/123
Content-Length: 0
The "return-minimal" and "return-representation" preferences are
mutually exclusive directives that MUST NOT be used in combination
within a single request. If a server receives a request containing
both the "return-minimal" and "return-representation" preferences, it
MAY choose to ignore either or both of the stated preferences but
MUST NOT signal an error or fail to process the request solely on the
basis of those preferences.
3.4. The "wait" Preference
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 might choose to abandon the request.
In the case generating a response will take longer than the time
specified, the server, or proxy, MAY choose to utilize an
asynchronous processing model by returning, for example, "202
Accepted" or "303 See Other" responses.
ABNF:
wait = "wait" BWS "=" BWS delta-seconds
Clients specifying the "wait" preference SHOULD also use the Date
header field, as specified in Section 9.2 of
[I-D.ietf-httpbis-p2-semantics], 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
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require the server to use the instant it received or began processing
the request as the baseline for determining how long the client has
been waiting which could yield unintended results.
The lack of a Date header in the request, or poor clock
synchronization between the client and server makes it impossible to
determine the exact length of time the client has already been
waiting when the request is received by the server. The only
reliable information conveyed by the wait preference is that the
client is not expecting the server to spend more than the specified
time on request processing and can terminate the transaction at any
time.
An example request specifying the "wait" and "return-asynch"
preferences to indicate that the client wishes the server to respond
asynchronously if processing of the request will take longer than 10
seconds:
POST /collection HTTP/1.1
Host: example.org
Content-Type: text/plain
Prefer: return-asynch, wait=10
Date: Tue, 20 Dec 2011 12:34:56 GMT
{Data}
3.5. The "strict" and "lenient" Processing Preferences
The "strict" and "lenient" preferences are mutually-exclusive
directives indicating, at the server's discretion, how the client
wishes the server to handle potential error conditions that can 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 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.
ABNF:
handling = "strict" / "lenient"
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An example request specifying the "strict" preference:
POST /collection HTTP/1.1
Host: example.org
Content-Type: text/plain
Prefer: strict
An example request specifying the "lenient" preference:
POST /collection HTTP/1.1
Host: example.org
Content-Type: text/plain
Prefer: lenient
4. IANA Considerations
The 'Prefer' header field should be added to the Permanent Message
Header Fields registry defined in [RFC3864]
(http://www.iana.org/assignments/message-headers/perm-headers.html).
Header field name: Prefer
Applicable Protocol: HTTP
Status:
Author: James M Snell <jasnell@gmail.com>
Change controller: IETF
Specification document: this specification
4.1. The Registry of Preferences
IANA is asked to create a new registry, "HTTP Preferences", under the
Hypertext Transfer Protocol (HTTP) Parameters group. New
registrations will use the Specification Required policy [RFC5226].
The requirements for registered preferences are described in
Section 3.
Registration requests consist of the completed registration template
below, typically published in an RFC or Open Standard (in the sense
described by Section 7 of [RFC2026]). However, to allow for the
allocation of values prior to publication, the Designated Expert can
approve registration once they are satisfied that a specification
will be published.
Note that preferences can be registered by third parties, if the
Designated Expert determines that an unregistered preference is
widely deployed and not likely to be registered in a timely manner.
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The registration template is:
o Preference: (A value for the Prefer request header field that
conforms to the syntax rule given in Section 2)
o Description:
o Reference:
o Notes: [optional]
Registration requests should be sent to the ietf-http-wg@w3.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.
4.2. Initial Registry Contents
The Preferences Registry's initial contents are:
o Preference: return-asynch
o Description: Indicates that the client prefers the server to
respond asynchronously to a request.
o Reference: [this specification], Section 3.1
o Preference: return-minimal
o Description: Indicates that the client prefers the server return a
minimal response to a request.
o Reference: [this specification], Section 3.3
o Preference: return-representation
o Description: Indicates that the client prefers the server to
include a representation of the current state of the resource in
response to a request.
o Reference: [this specification], Section 3.2
o Preference: wait
o Description: Indicates an upper bound to the lenght of time the
client is willing to wait for a response, after which the request
can be aborted.
o Reference: [this specification], Section 3.4
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.
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o Reference: [this specification], Section 3.5
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], Section 3.5
5. Security Considerations
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.
A server could incur greater costs in attempting to comply with a
particular preference (for instance, the cost of providing a
representation in a response that would not ordinarily contain one;
or the commitment of resources necessary to track state for an
asynchronous response). Unconditional compliance from a server could
allow the use of preferences for denial of service. A server can
ignore an expressed preference to avoid expending resources that it
does not wish to commit.
6. Normative References
[I-D.ietf-httpbis-p1-messaging]
Fielding, R., Gettys, J., Mogul, J., Nielsen, H.,
Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., and
J. Reschke, "HTTP/1.1, part 1: URIs, Connections, and
Message Parsing", draft-ietf-httpbis-p1-messaging-18 (work
in progress), January 2012.
[I-D.ietf-httpbis-p2-semantics]
Fielding, R., Gettys, J., Mogul, J., Nielsen, H.,
Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., and
J. Reschke, "HTTP/1.1, part 2: Message Semantics",
draft-ietf-httpbis-p2-semantics-18 (work in progress),
January 2012.
[I-D.ietf-httpbis-p3-payload]
Fielding, R., Gettys, J., Mogul, J., Nielsen, H.,
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Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., and
J. Reschke, "HTTP/1.1, part 3: Message Payload and Content
Negotiation", draft-ietf-httpbis-p3-payload-18 (work in
progress), January 2012.
[I-D.ietf-httpbis-p4-conditional]
Fielding, R., Gettys, J., Mogul, J., Nielsen, H.,
Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., and
J. Reschke, "HTTP/1.1, part 4: Conditional Requests",
draft-ietf-httpbis-p4-conditional-18 (work in progress),
January 2012.
[I-D.ietf-httpbis-p5-range]
Fielding, R., Gettys, J., Mogul, J., Nielsen, H.,
Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., and
J. Reschke, "HTTP/1.1, part 5: Range Requests and Partial
Responses", draft-ietf-httpbis-p5-range-18 (work in
progress), January 2012.
[I-D.ietf-httpbis-p6-cache]
Fielding, R., Gettys, J., Mogul, J., Nielsen, H.,
Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y.,
Nottingham, M., and J. Reschke, "HTTP/1.1, part 6:
Caching", draft-ietf-httpbis-p6-cache-18 (work in
progress), January 2012.
[I-D.ietf-httpbis-p7-auth]
Fielding, R., Gettys, J., Mogul, J., Nielsen, H.,
Masinter, L., Leach, P., Berners-Lee, T., Lafon, Y., and
J. Reschke, "HTTP/1.1, part 7: Authentication",
draft-ietf-httpbis-p7-auth-18 (work in progress),
January 2012.
[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.
[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.
[RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax
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Specifications: ABNF", STD 68, RFC 5234, January 2008.
Author's Address
James M Snell
Email: jasnell@gmail.com
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