One document matched: draft-snell-http-prefer-13.xml
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<rfc category="std" ipr="trust200811" docName="draft-snell-http-prefer-13">
<front>
<title abbrev="HTTP Prefer">
Prefer Header for HTTP
</title>
<author initials="J.M." surname="Snell" fullname="James M Snell">
<address>
<email>jasnell@gmail.com</email>
</address>
</author>
<date month="August" year="2012" />
<area>Applications</area>
<keyword>I-D</keyword>
<keyword>http</keyword>
<keyword>prefer</keyword>
<abstract>
<t>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.</t>
</abstract>
</front>
<middle>
<section anchor="intro" title="Introduction">
<t>This specification defines a new HTTP request header field that can be
used by clients to request optional behaviors be applied
by a server during the processing the request.</t>
<t>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 <xref target="RFC2119" />.</t>
<section title="Syntax Notation">
<t>This specification uses the Augmented Backus-Naur Form (ABNF)
notation of <xref target="RFC5234"/> and includes, by reference,
the "word", "OWS", "BWS" rules and the #rule
extension as defined within Sections 1.2 and 3.2.4 of
<xref target="I-D.ietf-httpbis-p1-messaging"/>.</t>
</section>
</section>
<section title="The Prefer Request Header Field" anchor="prefer">
<t>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 <xref target="I-D.ietf-httpbis-p2-semantics"/>
with the exception that servers are allowed to ignore stated preferences.</t>
<figure><artwork>
Prefer = "Prefer" ":" 1#preference
preference = token [ BWS "=" BWS word ]
*( OWS ";" [ OWS parameter ] )
parameter = token [ BWS "=" BWS word ]
</artwork></figure>
<t>This header field is defined with an extensible syntax to allow for future
values included in the <xref target="registry">Registry of Preferences</xref>.
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.</t>
<t>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:</t>
<figure><artwork>
Prefer: foo; bar
Prefer: foo; bar=""
Prefer: foo=""; bar
</artwork></figure>
<t>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.</t>
<t>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. </t>
<t>Comparison of preference token names is case-insensitive while values
are case-sensitive regardless of whether token or quoted-string values
are used.</t>
<t>The Prefer request header field MUST be forwarded by a proxy if the
request is forwarded. In various situations, a proxy can determine
that it is capable of honoring a preference independently of the server
to which the request is directed. For instance, an intervening proxy
can 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-asynch" preference.
Individual preference tokens MAY define their own requirements and
restrictions as to whether and how proxies can apply the preference to
a request independently of the origin server.</t>
<t>As per Section 3.2 of <xref target="I-D.ietf-httpbis-p1-messaging"/>,
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:</t>
<figure><preamble>Multiple Prefer Header Fields:</preamble><artwork>
POST /foo HTTP/1.1
Host: example.org
Prefer: return-asynch
Prefer: wait=100
Date: Tue, 20 Dec 2011 12:34:56 GMT
</artwork></figure>
<figure><preamble>Single Prefer Header Field:</preamble><artwork>
POST /foo HTTP/1.1
Host: example.org
Prefer: return-asynch, wait=100
Date: Tue, 20 Dec 2011 12:34:56 GMT
</artwork></figure>
<section title="Content Negotiation and Cache Considerations">
<t>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 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
uses Prefer in the request.</t>
<t>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 avoid using the Prefer
header mechanism for content negotiation.</t>
</section>
<section title="Examples">
<t>The following examples illustrate the use of various Preferences
defined by this specification, as well as undefined extensions for
strictly illustrative purposes:</t>
<figure><preamble>Return a 202 Accepted response for asynchronous
processing if the response cannot be processed within 10 seconds. An
undefined "priority" preference is also specified:</preamble><artwork>
Prefer: return-asynch, wait=10;
Prefer: priority=5;
</artwork></figure>
<figure><preamble>Use lenient processing:</preamble><artwork>
Prefer: Lenient
</artwork></figure>
<figure><preamble>Use of an optional, undefined parameter on the
return-minimal preference requesting a response status code of
204 for a successful response:</preamble><artwork>
Prefer: return-minimal; status=204
</artwork></figure>
</section>
</section>
<section title="The "return-asynch" Preference" anchor="return-asynch">
<t>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.</t>
<figure><artwork>
return-asynch = "return-asynch"
</artwork></figure>
<t>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 it's capability and preference for
handling asynchronous responses.</t>
<figure><preamble>An example request specifying the "return-asynch" preference:</preamble>
<artwork>
POST /collection HTTP/1.1
Host: example.org
Content-Type: text/plain
Prefer: return-asynch
{Data}
</artwork></figure>
<figure><preamble>An example asynchronous response using 202 Accepted:</preamble>
<artwork>
HTTP/1.1 202 Accepted
Location: http://example.org/collection/123
</artwork></figure>
<figure><preamble>An alternative asynchronous response using 303 See Other:</preamble>
<artwork>
HTTP/1.1 303 See Other
Location: http://example.org/collection/123
Retry-After: 10
</artwork>
</figure>
</section>
<section title="The "return-representation" Preference" anchor="return-representation">
<t>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.</t>
<figure><artwork>
return-representation = "return-representation"
</artwork></figure>
<t>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
<xref target="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.</t>
<t>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.</t>
<t>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.</t>
<figure><preamble>An example request specifying the "return-representation" preference:</preamble>
<artwork>
PUT /collection/123 HTTP/1.1
Host: example.org
Content-Type: text/plain
Prefer: return-representation
{Data}
</artwork>
</figure>
<figure><preamble>An example response containing the resource representation:</preamble>
<artwork>
HTTP/1.1 200 OK
Content-Location: http://example.org/collection/123
Content-Type: text/plain
ETag: "d3b07384d113edec49eaa6238ad5ff00"
{Data}
</artwork>
</figure>
</section>
<section title="The "return-minimal" Preference" anchor="return-minimal">
<t>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.</t>
<figure><artwork>
return-minimal = "return-minimal"
</artwork></figure>
<t>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.</t>
<figure><preamble>An example request specifying the "return-minimal" preference:</preamble>
<artwork>
POST /collection HTTP/1.1
Host: example.org
Content-Type: text/plain
Prefer: return-minimal
{Data}
</artwork>
</figure>
<figure><preamble>An example minimal response:</preamble>
<artwork>
HTTP/1.1 201 Created
Location: http://example.org/collection/123
Content-Length: 0
</artwork>
</figure>
<t>The "return-minimal" and "return-representation" preferences are
mutually exclusive directives that SHOULD NOT be used in combination
within a single request.</t>
</section>
<section title="The "wait" Preference" anchor="wait">
<t>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 can 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.</t>
<figure><artwork>
wait = "wait" BWS "=" BWS delta-seconds
</artwork></figure>
<t>Clients specifying the "wait" Preference SHOULD also use the
Date header field, as specified in Section 9.2 of <xref target="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 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.</t>
<t>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.</t>
<figure><preamble>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:</preamble>
<artwork>
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}
</artwork></figure>
</section>
<section title="The "strict" and "lenient" Processing Preferences" anchor="handling">
<t>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 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 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.</t>
<figure><artwork>
handling = "strict" / "lenient"
</artwork></figure>
<figure><preamble>An example request specifying the "strict" preference:</preamble>
<artwork>
POST /collection HTTP/1.1
Host: example.org
Content-Type: text/plain
Prefer: strict
</artwork>
</figure>
<figure><preamble>An example request specifying the "lenient" preference:</preamble>
<artwork>
POST /collection HTTP/1.1
Host: example.org
Content-Type: text/plain
Prefer: lenient
</artwork>
</figure>
</section>
<section title="Registered Preferences" anchor="requirements">
<t>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
<xref target="registry" />).</t>
<t>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.</t>
<t>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.</t>
</section>
<section title="IANA Considerations">
<t>The 'Prefer' header field should be added to
the permanent registry (see <xref target="RFC3864" />).</t>
<t><list>
<t>Header field name: Prefer</t>
<t>Applicable Protocol: HTTP</t>
<t>Status:</t>
<t><![CDATA[Author: James M Snell <jasnell@gmail.com>]]></t>
<t>Change controller: IETF</t>
<t>Specification document: this specification</t>
</list></t>
<section title="The Registry of Preferences" anchor="registry">
<t>Preferences are registered on the advice of a Designated Expert
(appointed by the IESG or their delegate), with a Specification
Required (using terminology from <xref target="RFC5226" />).</t>
<t>The requirements for registered preferences are described in
<xref target="requirements" />.</t>
<t>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 <xref target="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.</t>
<t>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.</t>
<t>The registration template is:</t>
<t>
<list style="symbols">
<t>Preference: (A value for the Prefer request header field that conforms to the
syntax rule given in <xref target="prefer"/>)</t>
<t>Description:</t>
<t>Reference:</t>
<t>Notes: [optional]</t>
</list>
</t>
<t>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).</t>
<t>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.</t>
<t>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).</t>
<t>IANA should only accept registry updates from the Designated
Expert(s), and should direct all requests for registration to the
review mailing list.</t>
<section title="Initial Registry Contents">
<t>The Preferences Registry's initial contents are:</t>
<t>
<list style="symbols">
<t>Preference: return-asynch</t>
<t>Description: Indicates that the client prefers
the server to respond asynchronously to a request
as described by <xref target="return-asynch" /></t>
<t>Reference: [this specification]</t>
</list>
</t>
<t>
<list style="symbols">
<t>Preference: return-minimal</t>
<t>Description: Indicates that the client prefers
the server return a minimal response to a
request as described by <xref target="return-minimal" /></t>
<t>Reference: [this specification]</t>
</list>
</t>
<t>
<list style="symbols">
<t>Preference: return-representation</t>
<t>Description: Indicates that the client prefers
the server to include a representation of the current state
of the resource in response to a request as described by
<xref target="return-representation"/></t>
<t>Reference: [this specification]</t>
</list>
</t>
<t>
<list style="symbols">
<t>Preference: wait</t>
<t>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.</t>
<t>Reference: [this specification]</t>
</list>
</t>
<t>
<list style="symbols">
<t>Preference: strict</t>
<t>Description: Indicates that the client wishes the server
to apply strict validation and error handling to the processing
of a request.</t>
<t>Reference: [this specification]</t>
</list>
</t>
<t>
<list style="symbols">
<t>Preference: lenient</t>
<t>Description: Indicates that the client wishes the server
to apply lenient validation and error handling to the processing
of a request.</t>
<t>Reference: [this specification]</t>
</list>
</t>
</section>
</section>
</section>
<section title="Security Considerations">
<t>Specific preferences requested by a client can introduce security
considerations and concerns beyond those discussed in HTTP/1.1 Parts
<xref target="I-D.ietf-httpbis-p1-messaging">1</xref>,
<xref target="I-D.ietf-httpbis-p2-semantics">2</xref>,
<xref target="I-D.ietf-httpbis-p3-payload">3</xref>,
<xref target="I-D.ietf-httpbis-p4-conditional">4</xref>,
<xref target="I-D.ietf-httpbis-p5-range">5</xref>,
<xref target="I-D.ietf-httpbis-p6-cache">6</xref>, and
<xref target="I-D.ietf-httpbis-p7-auth">7</xref>.
Implementors must refer to the specifications and descriptions of each
preference to determine the security considerations relevant to each.</t>
<t>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.</t>
</section>
</middle>
<back>
<references title="Normative References">
&rfc2119;
&rfc3864;
&rfc5226;
&rfc2026;
&rfc5234;
&part1;
&part2;
&part3;
&part4;
&part5;
&part6;
&part7;
</references>
</back>
</rfc>
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