Network Working Group M. Nottingham
Internet-Draft August 10, 2010
Intended status: Informational
Expires: February 11, 2011
Making HTTP Pipelining Usable on the Open Web
draft-nottingham-http-pipeline-00
Abstract
Pipelining was added to HTTP/1.1 as a means of improving the
performance of persistent connections in common cases. While it is
deployed in some limited circumstances, it is not widely used by
clients on the open Internet. This memo suggests some measures
designed to make it more possible for clients to reliably and safely
use HTTP pipelining in these situations.
This memo should be discussed on the ietf-http-wg@w3.org mailing
list, although it is not a work item of the HTTPbis WG.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
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This Internet-Draft will expire on February 11, 2011.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Requirements . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. HTTP Pipelining Issues . . . . . . . . . . . . . . . . . . . . 3
4. Discovering Faulty Proxies . . . . . . . . . . . . . . . . . . 4
5. Identifying Responses . . . . . . . . . . . . . . . . . . . . . 5
6. Signing Content for Integrity . . . . . . . . . . . . . . . . . 6
7. Hinting Pipelinable Content . . . . . . . . . . . . . . . . . . 6
8. Origin Server Considerations for Pipelining . . . . . . . . . . 7
9. User Agent Considerations for Pipelining . . . . . . . . . . . 7
10. Security Considerations . . . . . . . . . . . . . . . . . . . . 8
11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 8
12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 8
12.1. Normative References . . . . . . . . . . . . . . . . . . . 8
12.2. Informative References . . . . . . . . . . . . . . . . . . 8
Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . . 8
Appendix B. Frequently Asked Questions . . . . . . . . . . . . . . 9
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 9
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1. Introduction
HTTP/1.1 [RFC2616] added pipelining -- that is, the ability to have
more than one outstanding request on a connection at a particular
time -- to improve performance when many requests need to be made
(e.g., when an HTML page references several images).
Although not usable in all circumstances (e.g., POST, PUT and other
non-idempotent requests cannot be pipelined), for the common case of
Web browsing, pipelining seems at first like a broadly useful
improvement -- especially since the number of TCP connections
browsers and servers can use for a given interaction is limited, and
especially where there is noticeable latency present.
Indeed, in constrained applications of HTTP such as Subversion,
pipelining has been shown to improve end-user perceived latency
considerably.
However, pipelining is not broadly used on the Web today; while most
(but not all) servers and intermediaries support pipelining (to
varying degrees), only one major Web browser uses it in its default
configuration, and that implementation is reported to use a number of
proprietary heuristics to determine when it is safe to pipeline.
This memo characterises issues currently encountered in the use of
HTTP pipelining, and suggests the use of mechanisms that, when used
in concert, are designed to make its use more reliable and safe for
browsers. It does not propose large protocol changes (e.g., out-of-
order messages), but rather incremental improvements that can be
deployed within the confines of existing infrastructure.
2. Requirements
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
3. HTTP Pipelining Issues
Anecdotal evidence suggests there are a number of reasons why clients
don't use HTTP pipelining by default. Briefly, they are:
1. Server implementations may stall pipelined requests, or close
their connection. This is one of the most commonly cited
problems.
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2. Server implementations may pipeline responses in the wrong order.
Some implementations mix up the order of pipelined responses;
e.g., when they hit an error state but don't "fill" the response
pipeline with a corresponding representation.
3. A few server implementations may corrupt pipelined responses.
It's been said that a very small number of implementations
actually interleave pipelined responses so that part of response
A appears in response B, which is both a security and
interoperability problem.
4. Clients don't have enough information about what is useful to
pipeline. A given response may take an inordinate amount of time
to generate, and/or be large enough to block subsequent
responses. Clients who pipeline may face worse performance if
they stack requests behind such an expensive request.
Note that here, "servers" can also include proxies and other
intermediaries.
The remainder of this memo proposes mechanisms that, together, can be
used to mitigate these issues.
4. Discovering Faulty Proxies
Issues specific to proxies are limited to the network infrastructure
currently used by the client, and it is reasonable to assume that
testing the infrastructure at the beginning of a session will
indicate how safe it is to pipeline while that infrastructure is in
use.
Such issues can be detected by sending pipelined requests to a known
server, and examining the responses for errors.
For example, if the ExampleBrowser implementation wishes to probe for
faulty proxies, it can send a series of requests to
"http://browser.example.com/pipeline-test/" and subresources. If the
bodies of the resulting responses deviate from those it expects in
any way, it is reasonable to assume that a faulty proxy is present,
and pipelining SHOULD NOT be used through it.
Typically, user agents will do this upon startup and changes in the
network, although they might periodically test to assure that a new
proxy hasn't been interposed.
Note that proxies aren't always configured explicitly.
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5. Identifying Responses
HTTP relies on the context of the connection to associate a given
request with its intended response. In HTTP/1.0, this was a
reasonable assumption, since only one request could be outstanding at
a given time, and each request had exactly one response.
HTTP/1.1 made associating requests and responses in a given
connection more complex (and therefore fault-prone). Not only does
pipelining mean that multiple requests can be outstanding, but also
the 1xx series of response status codes introduce the possibility of
multiple response messages (syntactically) being associated with a
single request.
To improve the client's ability to correlate responses with their
requests and identify responses that are out of order (as well as
serve other potential use cases), this memo introduces the "Assoc-
Req" response header field.
Assoc-Req = "Assoc-Req" ":" OWS Assoc-Req-v
Assoc-Req-v = Method SP absolute-URI
The field-value of the Assoc-Req header field is the method and
effective request URI of the request associated with the response
that it appears in. The URI used MUST be generated using the
algorithm for finding the Effective Request URI in
[I-D.ietf-httpbis-p1-messaging]. The header field MUST NOT be
generated by proxies.
For example, given the following request over port 80:
GET /foo?it HTTP/1.1
Host: www.example.com
the appropriate Assoc-Req header field would be:
Assoc-Req: GET http://www.example.com/foo?it
Note that the Assoc-Req header field is not a perfectly reliable
identifier for the request associated with a response; for example,
it does not incorporate the selecting headers for content negotiation
[I-D.ietf-httpbis-p6-cache], nor does it differentiate request
bodies, when present. However, for the purposes of making pipelining
more reliable, it is sufficient.
Clients who wish to use the Assoc-Req response header field to aid in
identifying problems in pipelining can compare its values to those of
the request that they believe it to be associated with (based upon
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HTTP's message parsing rules, defined in
[I-D.ietf-httpbis-p1-messaging]). If either the method or the URI
differ, it indicates that there may be a pipelining-related issue.
6. Signing Content for Integrity
Another means of protecting against server issues (whether proxy or
origin server) is to sign the response content for integrity, so that
any corruption becomes apparent.
One existing way to do this is to use the Content-MD5 header field
[I-D.ietf-httpbis-p3-payload].
Clients who wish to use the Content-MD5 response header field to aid
in identifying corrupted content due to pipelining issues can compare
the hash to a calculated hash of the content.
In some circumstances, it may be impractical for the server to buffer
the response content in order to calculate a hash before sending it.
In these cases, the Content-MD5 response header can be send in an
HTTP trailer, provided that the connection is HTTP/1.1 from end to
end, and the client is able to process trailers.
Additional means of verifying HTTP response integrity may become
available in time.
7. Hinting Pipelinable Content
Finally, to assist clients in determining what requests are suitable
for pipelining, we define extensions to allow hinting by origin
servers.
Each of these hints indicates URLs that, when dereferenced, will
likely not incur significant latency on the server in generating the
response, nor significant latency on the network in transferring the
response.
What is "significant" is determined by the server. Clients will use
these hints to determine what request(s) it is safe to pipeline
something else after.
For example, if "http://example.com/a" is hinted, a client can be
more confident pipelining another request (e.g., to
"http://example.com/b") on the same request afterwards.
To allow flexibility and ease of administration, different kinds of
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hints are defined:
o The "quick" link relation type [TBD: ref] can appear on individual
HTML elements such as "img", "script" and "link" to indicate that
the link they contain has low overhead. Additionally, it can be
used in the HTTP link header to indicate links within the response
in a format-neutral way.
o A document can indicate that all links from the indicated elements
have low overhead by using the HTML META "quick" element, with the
content indicating the element names that are "quick". For
example, "<META name='quick' content='img script link'/>".
o [DISCUSS: is it worthwhile to define a /.well-known lookup
mechanism for quick links?]
8. Origin Server Considerations for Pipelining
To maximise the potential for request pipelining from clients that
support this specification, origin servers:
o SHOULD send the Assoc-Req response header field in all potentially
pipelinable responses (keeping in mind that downstream caches
might be serving responses in the future).
o SHOULD send the Content-MD5 response header (or trailer) field in
potentially pipelinable responses.
o SHOULD hint potentially pipelinable requests as outlined above.
9. User Agent Considerations for Pipelining
To take advantage of the server-side mechanisms defined in this
specification, user agents:
o SHOULD ascertain whether any proxies present (either configured or
interposed by interception) support pipelining by following the
protocol described above. If they do not, pipelining SHOULD NOT
be used.
o SHOULD check the Assoc-Req response header field-value, when
present, on all pipelined responses.
o SHOULD check the Content-MD5 response header (or trailer) field-
value, when present, on all pipelined responses.
o MAY use content hints for pipelining to assist in determining
whether to pipeline a given request.
Upon encountering an indication of pipelining problems with a
particular response (e.g., an incorrect Assoc-Req field-value, an
incorrect Content-MD5 field-value), user agents SHOULD discard the
response in question, all subsequent responses on the same
connection, close the connection. Unsatisfied requests can be
resubmitted, without pipelining, and the implementation can choose
not to use pipelining to the same server in the future.
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10. Security Considerations
TBD
11. IANA Considerations
TBD
12. References
12.1. 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-11 (work
in progress), August 2010.
[I-D.ietf-httpbis-p3-payload]
Fielding, R., Gettys, J., Mogul, J., Nielsen, H.,
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-11 (work in
progress), August 2010.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
12.2. Informative References
[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-11 (work in
progress), August 2010.
[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.
Appendix A. Acknowledgements
Thanks to Julian Reschke for help in defining the Assoc-Req field-
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value. The author takes all responsibility for errors and omissions.
Appendix B. Frequently Asked Questions
Isn't full multiplexing better?
While "full" multiplexing is theoretically better, pipelining -- once
usable -- is adequate for almost all common Web browsing cases.
Since the browser needs to download HTML first, it has an opportunity
to receive hints about subsequent requests and pipeline them
appropriately. Likewise, by far the most common case for
multiplexing on the Web is when a large number of images and other
page assets need to be fetched with GET; a perfect use of pipelining,
provided that the client has enough information to avoid head-of-line
blocking.
Why not have the client generate a unique request identifier?
While in some ways this would be easier than the approach that the
Assoc-Req header takes, it would be more difficult to deploy, because
existing caching proxies wouldn't be able to serve the correct
identifier when using a cached response.
Author's Address
Mark Nottingham
Email: mnot@mnot.net
URI: http://www.mnot.net/
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