Network Working Group J. Snell
Internet-Draft October 12, 2012
Intended status: Standards Track
Expires: April 15, 2013
Prefer Header for HTTP
draft-snell-http-prefer-15
Abstract
This specification defines an HTTP header field that can be used by a
client to request that certain behaviors be employed 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 15, 2013.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Syntax Notation . . . . . . . . . . . . . . . . . . . . . 4
2. The Prefer Request Header Field . . . . . . . . . . . . . . . 4
2.1. Examples . . . . . . . . . . . . . . . . . . . . . . . . . 6
3. The Preference-Applied Response Header Field . . . . . . . . . 7
4. Preference Definitions . . . . . . . . . . . . . . . . . . . . 8
4.1. The "return-asynch" Preference . . . . . . . . . . . . . . 8
4.2. The "return-representation" and "return-minimal"
Preferences . . . . . . . . . . . . . . . . . . . . . . . 9
4.3. The "wait" Preference . . . . . . . . . . . . . . . . . . 11
4.4. The "strict" and "lenient" Processing Preferences . . . . 12
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 12
5.1. The Registry of Preferences . . . . . . . . . . . . . . . 13
5.2. Initial Registry Contents . . . . . . . . . . . . . . . . 14
6. Security Considerations . . . . . . . . . . . . . . . . . . . 14
7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 15
7.1. Normative References . . . . . . . . . . . . . . . . . . . 15
7.2. Informative References . . . . . . . . . . . . . . . . . . 15
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 in 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 [RFC5023] --
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 3.2.1 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 6.1.2 of
[I-D.ietf-httpbis-p2-semantics] with the exception that servers are
allowed to ignore stated preferences.
ABNF:
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 5.1).
A server that does not recognize or is unable to comply with
particular preference tokens in the Prefer header field of a request
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MUST ignore those tokens and continue processing instead of
signalling an error.
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.
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 header field is end-to-end and SHOULD be forwarded by a
proxy if the request is forwarded unless Prefer is explicitly
identified as being hop-by-hop using the Connection header field
defined by [I-D.ietf-httpbis-p1-messaging], Section 6.1.
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.
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.
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
To avoid possible ambiguity, individual preference tokens SHOULD NOT
appear multiple times within a single request. If any preference is
specified more than once, only the first instance is to be
considered. All subsequent occurrences SHOULD be ignored without
signaling an error or otherwise altering the processing of the
request. This is the only case in which the ordering of preferences
within a request is considered to be significant.
Due to the inherent complexities involved with properly implementing
server-driven content negotiation, effective caching, and the
application of optional preferences, implementors are urged to
exercise caution when using preferences in a way that impacts the
caching of a response and SHOULD NOT use the Prefer header mechanism
for content negotiation. If a server supports the optional
application of a preference that might 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.
2.1. 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:
Prefer: return-minimal; foo="some parameter"
3. The Preference-Applied Response Header Field
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 a request.
ABNF:
Preference-Applied = "Preference-Applied" ":" 1#token
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 only necessary when it is not
readily and obviously apparent that a server applied a given
preference and such ambiguity might have an impact on the client's
handling of the response. For instance, when using either the
"return-representation" or "return-minimal" preferences, a client
application might not be capable of reliably determining that the
preference was applied simply by examining the payload of the
response. In such case the Preference-Applied header field can be
used.
Request:
PATCH /my-document HTTP/1.1
Host: example.org
Content-Type: application/json-patch
Prefer: return-representation
[{"op": "add", "path": "/a", "value": 1}]
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Response:
HTTP/1.1 200 OK
Content-Type: application/json
Preference-Applied: return-representation
Content-Location: /my-document
{"a": 1}
4. 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 5.1).
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.
4.1. 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 a "202 Accepted"
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
While the "202 Accepted" response status is defined by
[I-D.ietf-httpbis-p2-semantics], little guidance is given on how and
when to use the response code and the process for determining the
subsequent final result of the operation is left entirely undefined.
Therefore, whether and how any given server supports asynchronous
responses is an implementation specific detail that is considered to
be out of the scope of this specification.
4.2. The "return-representation" and "return-minimal" Preferences
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.
The "return-minimal" preference, on the other hand, indicates that
the client wishes the server to return only 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-representation = "return-representation"
return-minimal = "return-minimal"
When honoring the "return-representation" preference, the returned
representation might not be a representation of the effective request
URI when the request is affecting another resource. In such cases,
the Content-Location header can be used to identify the URI of the
returned representation.
The "return-representation" preference is intended to provide a means
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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
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:
PATCH /item/123 HTTP/1.1
Host: example.org
Content-Type: text/patch
Prefer: return-representation
1c1
< ABCDEFGHIJKLMNOPQRSTUVWXYZ
---
> BCDFGHJKLMNPQRSTVWXYZ
An example response containing the resource representation:
HTTP/1.1 200 OK
Content-Location: http://example.org/item/123
Preference-Applied: return-representation
Content-Type: text/plain
ETag: "d3b07384d113edec49eaa6238ad5ff00"
BCDFGHJKLMNPQRSTVWXYZ
In contrast, the "return-minimal" preference can reduce 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.
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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
The "return-minimal" and "return-representation" preferences are
mutually exclusive directives. A request that contains both
preferences can be treated as though neither were specified.
4.3. The "wait" Preference
The "wait" preference can be used to establish an upper bound on the
length of time, in seconds, the client expects it will take the
server to process the request once it has been received. In the case
that generating a response will take longer than the time specified,
the server, or proxy, can choose to utilize an asynchronous
processing model by returning -- for example -- a "202 Accepted"
response.
ABNF:
wait = "wait" BWS "=" BWS delta-seconds
It is important to consider that HTTP messages spend some time
traversing the network and being processed by intermediaries. This
increases the length of time that a client will wait for a response
in addition to the time the server takes to process the request. A
client that has strict timing requirements can estimate these factors
and adjust the wait value accordingly.
As with other preferences, the "wait" preference could be ignored.
Clients can abandon requests that take longer than they are prepared
to wait.
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For example, a server receiving the following request might choose to
respond asynchronously if processing 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
{Data}
4.4. 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 to indicate that,
while any particular error may be recoverable, the client would
prefer that the server reject the request. The "lenient" preference,
on the other hand, indicates that the client wishes the server to
attempt to process the request.
ABNF:
handling = "strict" / "lenient"
An example request specifying the "strict" preference:
POST /collection HTTP/1.1
Host: example.org
Content-Type: text/plain
Prefer: strict
5. IANA Considerations
The 'Prefer' and 'Preference-Applied' header fields 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
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Applicable Protocol: HTTP
Status: Standard
Author: James M Snell <jasnell@gmail.com>
Change controller: IETF
Specification document: this specification
Header field name: Preference-Applied
Applicable Protocol: HTTP
Status: Standard
Author: James M Snell <jasnell@gmail.com>
Change controller: IETF
Specification document: this specification
5.1. 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 4.
Registration requests consist of the completed registration template
below, typically published in the required specification. However,
to allow for the allocation of values prior to publication, the
Designated Expert can approve registration based on a separately
submitted template once they are satisfied that a specification will
be published. 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.
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.
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5.2. 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 4.1
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 4.2
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 4.2
o Preference: wait
o Description: Indicates an upper bound to the length of time the
client expects it will take the server to process the request once
it has been received.
o Reference: [this specification], Section 4.3
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], Section 4.4
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 4.4
6. Security Considerations
Specific preferences requested by a client can introduce security
considerations and concerns beyond those discussed within HTTP/1.1
[I-D.ietf-httpbis-p1-messaging] and it's additional associated
specification documents. Implementers need to refer to the
specifications and descriptions of each preference to determine the
security considerations relevant to each.
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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.
7. References
7.1. Normative References
[I-D.ietf-httpbis-p1-messaging]
Fielding, R. and J. Reschke, "Hypertext Transfer Protocol
(HTTP/1.1): Message Syntax and Routing",
draft-ietf-httpbis-p1-messaging-21 (work in progress),
October 2012.
[I-D.ietf-httpbis-p2-semantics]
Fielding, R. and J. Reschke, "Hypertext Transfer Protocol
(HTTP/1.1): Semantics and Content",
draft-ietf-httpbis-p2-semantics-21 (work in progress),
October 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
Specifications: ABNF", STD 68, RFC 5234, January 2008.
7.2. Informative References
[RFC5023] Gregorio, J. and B. de hOra, "The Atom Publishing
Protocol", RFC 5023, October 2007.
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Author's Address
James M Snell
Email: jasnell@gmail.com
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