HTTPBIS M. Thomson
Internet-Draft Microsoft
Intended status: Standards Track August 15, 2013
Expires: February 16, 2014
Marking HTTP Requests as Unimportant
draft-thomson-http-nice-00
Abstract
An HTTP "Nice" header field is defined that marks a request as low
priority. Intermediaries can choose to discard the request or serve
it from cache rather than forwarding it to an origin server. This
enables constrained origin servers, such as those that rely on
battery power, to avoid expending limited resources on serving
requests.
Status of This Memo
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Copyright Notice
Copyright (c) 2013 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Conventions and Terminology . . . . . . . . . . . . . . . 3
2. The Nice Header Field . . . . . . . . . . . . . . . . . . . . 3
2.1. Policies for Treatment of Nice Requests . . . . . . . . . 4
2.2. Polling with Nice Requests . . . . . . . . . . . . . . . 5
3. Security Considerations . . . . . . . . . . . . . . . . . . . 6
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6
5. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 7
6. References . . . . . . . . . . . . . . . . . . . . . . . . . 7
6.1. Normative References . . . . . . . . . . . . . . . . . . 7
6.2. Informative References . . . . . . . . . . . . . . . . . 7
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 7
1. Introduction
HTTP [RFC2616] servers are beginning to appear as the interface to a
wide array of devices. Management interfaces in many devices have
classically been provided as HTTP servers, but this trend now extends
to HTTP APIs on a range of devices, including constrained devices.
Constrained devices are those with limited processing power, network
connectivity or battery capacity.
The Constrained Application Protocol (CoAP) [I-D.ietf-core-coap] in
particular is designed to provide devices with extremely limited
capabilities a way to provide an HTTP-compatible interface to the
information and services they provide. A CoAP-HTTP gateway
[I-D.ietf-core-http-mapping] provides HTTP-capable clients a means of
accessing these devices.
For a device that operates based on a battery, it is often crucial
that the device remain dormant for extended periods. Radio
communication in particular consumes a significant amount of power.
Frequent communication limits the length of time that the device can
operate. It is often the case that communication can be initiated,
but this could require a significant expenditure of stored energy.
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Many constrained devices rely on intermediaries such as the CoAP-HTTP
gateway to terminate requests and mediate access. Clients that
access the services provided by such limited devices can be unaware
of the limited nature of the device serving the request, since they
actually interact with the intermediary. Even when the client is
aware of these limitatons, it is not always possible for clients to
learn whether any given request would cause significant expenditure
of resources at the constrained device.
This document defines an HTTP header field, "Nice" that can be used
by clients to indicate that a request is not urgent or important
enough to cause a constrained server to expend special effort to
serve. An intermediary that is aware that the origin server is
unable to handle the request can instead terminate the request. The
request is forwarded as normal to an origin server that is available.
An intermediary can generate an error response in response to a nice
request, avoiding the need to contact the constrained origin server.
Alternatively, the intermediary could delay the request until the
origin server becomes available or serve a response from cache if
that is possible.
No specific mechanism is defined for an origin server to inform
intermediaries of absence or other indisposition.
1.1. Conventions and Terminology
At times, this document falls back on shorthands for establishing
interoperability requirements on implementations: the capitalized
words "MUST", "SHOULD" and "MAY". The meaning of these is described
in [RFC2119].
2. The Nice Header Field
The "Nice" header field indicates that a request is less important
than a request that doesn't bear this header.
The value of the header field is a decimal number between 0 and 3
inclusive. Values greater than zero indicate increasing levels of
unimportance. A lower value indicates greater urgency; for example,
a value of 3 is less urgent or important than a value of 1. A value
of 0 (or an absent "Nice" header field) indicates that the request is
to be forwarded as normal.
Nice = "Nice" ":" ("3" / "2" / "1" / "0")
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Multiple values for the header field MUST NOT be included. If
multiple values are present, an intermediary MAY choose to treat the
request in any way it chooses.
For example, the following request indicates that it is not urgent:
GET /m HTTP/1.1
Host: device9710.example.net:11453
Nice: 2
An intermediary might reject this request, indicating that the origin
server is not available using a 503 status code.
HTTP/1.1 503 Service Unavailable
Date: Fri, 26 Jul 2013
Content-Type: text/plain;charset=utf8
Content-Length: 63
The server is asleep, don't disturb it unless it's urgent.
A key characteristic of this header field is that intermediaries and
clients that do not understand its semantics treat requests so marked
no different to any other requests. An intermediary that has no
special information about the availability of the origin server will
also forward the request. That means that requests from a client
that does not include this header will always reach the origin
server.
2.1. Policies for Treatment of Nice Requests
An origin server or intermediary might use several inputs in
determing the threshold at which a request is forwarded to the origin
server. An origin server might either directly instruct the
intermediary about the threshold, or it might be provide specific
information that can be used, in conjunction with knowledge the
intermediary has of the origin server, as input to an algorithm for
determining the threshold. Potential inputs include:
o The relative cost of awakening a dormant server. Depending on the
server, this cost may be assessed in different ways, including
monetary, battery or time.
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o The last time that the server was in active communication.
Typically, wireless devices have a period of heightened
availability just after sending or receiving data. During this
period activation and communication can be significantly more
efficient.
o Application preferences or context. For example, a server might
be configured to be more highly responsive to requests during
certain times.
The following describes a potential set of policies regarding
selection and treatment of "Nice" header field value:
nice: 1 = The client regards the request as relatively urgent, but
not critically so. An intermediary might use a heuristic with a
moderate risk of false positives in determining whether the server
is available. An intermediary might also forward a request so
marked to a dormant device that has a relatively low activation
cost.
nice: 2 = The client regards the request as not urgent. An
intermediary might attempt to minimize the probability that it
awakens a server, if it uses a heuristic in determining whether to
forward requests.
nice: 3 = The client regards the request as being of trivial
importance. An intermediary might avoid forwarding requests
unless there is strong indication that the origin server is
available and willing to communicate.
Many different policies can be applied to the selection of a value
for the "Nice" header field, as well as to the treatment of requests
so marked. Specific applications might define a means for providing
more specific policies.
2.2. Polling with Nice Requests
Marking a request as nice is quite useful for requests that do not
require immediate action. Clients might wish to have the request
fulfilled, but are willing to wait until the origin server is
present. Such requests might be sent periodically until they
succeed.
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In some cases, origin server availability is predictable and known to
the intermediary. Some devices have predictable cycles of
availability, which are used for brief bursts of communication. If
the next time that the origin server is available is known, an
intermediary can include a "Retry-After" header field in a generated
error response.
For example:
HTTP/1.1 503 Service Unavailable
Date: Fri, 26 Jul 2013 03:34:19 GMT
Retry-After: 4
Alternatively, requests can be held for an amount of time by the
intermediary to be forwarded when the origin server becomes
available. Including a "Prefer" header field [I-D.snell-http-prefer]
with the "wait" tag provides the intermediary information about how
long the client is prepared to await a response. This could allow
the intermediary to reject the request immediately if the device is
known to be unreachable for the entire duration.
3. Security Considerations
Lowering the priority with which a request is handled is unlikely to
cause any special concern with respect to security.
Intermediaries that do not support the "Nice" header field might
erroneously cache a response from an intermediary that handles the
request without forwarding to the origin server. Intermediaries MUST
NOT generate cacheable responses to requests containing an "Nice"
header field. Intermediaries MAY however provide cached responses
originally provided by the origin server.
4. IANA Considerations
The permanent message header field registry (see [RFC3864]) has been
updated with the following registration:
Header field name: Nice
Applicable protocol: http
Status: standard
Author/Change controller: IETF
Specification document: this specification (Section 2)
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5. Acknowledgements
The original idea for this header field was devised by Matthew
Kaufman and Bruce Lowekamp, who realized the importance of making the
header a negative rather than positive expression of priority.
6. References
6.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[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.
6.2. Informative References
[I-D.ietf-core-coap]
Shelby, Z., Hartke, K., and C. Bormann, "Constrained
Application Protocol (CoAP)", draft-ietf-core-coap-18
(work in progress), June 2013.
[I-D.ietf-core-http-mapping]
Castellani, A., Loreto, S., Rahman, A., Fossati, T., and
E. Dijk, "Best Practices for HTTP-CoAP Mapping
Implementation", draft-ietf-core-http-mapping-01 (work in
progress), July 2013.
[I-D.snell-http-prefer]
Snell, J., "Prefer Header for HTTP", draft-snell-http-
prefer-18 (work in progress), January 2013.
Author's Address
Martin Thomson
Microsoft
3210 Porter Drive
Palo Alto, CA 94304
US
Email: martin.thomson@skype.net
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