Network Working Group M. Nottingham
Internet-Draft March 13, 2011
Intended status: Informational
Expires: September 14, 2011
Making HTTP Pipelining Usable on the Open Web
draft-nottingham-http-pipeline-01
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.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
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This Internet-Draft will expire on September 14, 2011.
Copyright Notice
Copyright (c) 2011 IETF Trust and the persons identified as the
document authors. All rights reserved.
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described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Requirements . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. HTTP Pipelining Issues . . . . . . . . . . . . . . . . . . . . 4
4. Blacklisting Origin Servers . . . . . . . . . . . . . . . . . 4
5. Discovering Faulty Proxies . . . . . . . . . . . . . . . . . . 4
6. Correlating Responses . . . . . . . . . . . . . . . . . . . . 5
7. Hinting Pipelinable Content . . . . . . . . . . . . . . . . . 7
8. Indicating Blocking Responses . . . . . . . . . . . . . . . . 7
9. Handling Pipelining Problems . . . . . . . . . . . . . . . . . 8
10. Security Considerations . . . . . . . . . . . . . . . . . . . 8
11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 9
12.1. Normative References . . . . . . . . . . . . . . . . . . 9
12.2. Informative References . . . . . . . . . . . . . . . . . 9
Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 9
Appendix B. Frequently Asked Questions . . . . . . . . . . . . . 9
Appendix C. Changes . . . . . . . . . . . . . . . . . . . . . . . 10
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 10
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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 (POST 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 mechanisms that are designed to make
its use more reliable and safe for browsers.
Note that this memo does not suggest drastic changes to HTTP, nor
does it require that intermediaries change to better support
pipelining. Instead, it takes the position that removing the
responsibility for making pipelining decisions from browsers, as well
as reduce associated risks for browsers, we make it more likely that
browsers will support it.
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. Reviewers
are encouraged to pay particular attention to items marked FEEDBACK.
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].
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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. Balking Servers - server implementations can stall pipelined
requests, or close their connection when the client attempts to
pipeline. This is one of the most commonly cited problems.
2. Confused Servers - a few server implementations can respond to
pipelined requests in the wrong order. Even fewer might corrupt
the actual responses. Because this has security implications
(consider multiple users behind such a proxy, or multiple tabs in
a browser), this is very concerning.
3. Head-of-Line Blocking - 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, including so-called "transparent" proxies (also known
as intercepting proxies).
The remainder of this memo proposes mechanisms that can be used to
mitigate one or more of these problems; not all of them will survive
discussion and implementation.
4. Blacklisting Origin Servers
To address balking and confused origin servers, a client SHOULD
maintain a blacklist of origins that it does not attempt pipelining
with.
Such a blacklist MAY be populated by external information (e.g., a
list of known-bad servers maintained by the browser vendor), and when
pipelining has been detected to fail to the origin.
5. Discovering Faulty Proxies
When a balking or confused server is a proxy, pipelining won't work
for any requests sent through it. Therefore, clients SHOULD test the
network for such proxies periodically.
This can be done by sending pipelined requests to a known server, and
examining the responses for errors.
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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.
RECOMMENDED measures in such tests include:
o Sending a non-trivial number of pipelined requests (e.g., 10)
o Sending multiple pipelined requests in the same packet
o Inserting request bodies with various sizes
o Assuring that caching is disabled, so that requests are end-to-end
o Sending a variety of responses types that includes 100 and 304
responses
o Examining responses to assure that they all appear in the correct
order
o Examining received requests and responses to assure that they
aren't unduly modified
These tests SHOULD be performed by clients (both user agent and
proxy) upon startup, as well as periodically afterwards to assure
that a new intercepting proxy hasn't been interposed. They MAY be
performed after a pipelining problem is detected, to determine
whether the issue is proxy- related.
See <https://github.com/mnot/pipeline-surveyor> for an example
implementation.
6. Correlating 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 confused origin and proxy servers (as well as
serve other potential use cases), this memo introduces the "Assoc-
Req" response header field.
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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.
A client wishing 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 it believes it to be associated with (based upon
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,
and the origin server (identified by its (host, port) tuple) SHOULD
be blacklisted.
A client MAY choose to blacklist any origin server that does not send
the Assoc-Req header.
FEEDBACK: Omitting the URI scheme and authority (i.e., just making it
the path and query components) would make the header easier to
generate and avoid some false positives (e.g., when a "reverse proxy"
or other URI rewriter is present), but may fail to identify cases
where two requests are confused (consider requests for
"http://example.com/style.css" and
"https://foo.example.net/style.css").
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7. Hinting Pipelinable Content
It's necessary to assist clients in determining what requests are
suitable for pipelining, so that the sole responsibility for deciding
what and when to pipeline isn't theirs. This can be done through
origin server hinting.
Such 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 connection afterwards.
There are several possible ways that content could be hinted,
including:
o The "quick" link relation type can appear on individual HTML
elements such as "img", "script" and "link" to indicate that the
link they contain has low overhead.
o A similar link relation could also be used in the HTTP link header
to indicate "quick" links within the response in a format-neutral
way.
o A server can indicate that all its resources are suitable for
pipelining by returning a successful response status code (2xx) to
requests for the path "/.well-known/pipeline". In the future, a
format available at this location could give more fine-grained
information.
FEEDBACK: thoughts on the suitability of these hinting mechanisms is
encouraged, so that the list can eventually be narrowed down.
A user agent MAY have a policy of only pipelining to hinted
resources.
8. Indicating Blocking Responses
An alternate way to avoid head-of-line blocking is for the origin
server to aggressively indicate when a request would block.
This can be done by using a new HTTP status code, 430 WOULD BLOCK.
The meaning of "would block" is defined by the server; e.g., it may
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return this code when the response is known to be over a certain
size, or when the code to generate the response is known to take a
long time to execute.
When a client (user agent or intermediary) receives a 430 WOULD
BLOCK, it SHOULD resubmit the associated request on a new or idle
connection.
An origin server MUST NOT send a 430 WOULD BLOCK status code to
clients that do not include a "PWB: 1" (mnemonic: Pipelining Would
Block) request header. User Agents that support the status code
SHOULD send this header, and intermediaries that are willing to
handle its processing MAY append it to requests that do not already
include it.
A cache MUST NOT store a 430 WOULD BLOCK response, and origin servers
SHOULD mark them as explicitly uncacheable (e.g., with Cache-Control:
no-store).
FEEDBACK: This is a relatively new idea; thoughts? In some ways it's
easier to deploy, but it does add a certain amount of latency to
requests that block. Theoretically, a Location header could be added
to redirect the client to a place where the generated response will
be waiting (if the blocking is caused by server-side think time), but
this may be impractical to implement.
9. Handling Pipelining Problems
Upon encountering an indication of pipelining problems with a
particular response (e.g., an incorrect Assoc-Req field-value, a
pipelined response that stalls), user agents SHOULD discard the
response in question, all subsequent responses on the same
connection, and 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 (see
"Blacklisting Origin Servers").
10. Security Considerations
TBD
11. IANA Considerations
TBD
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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., and J. Reschke,
"HTTP/1.1, part 1: URIs, Connections, and Message
Parsing", draft-ietf-httpbis-p1-messaging-12 (work in
progress), October 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., and J. Reschke,
"HTTP/1.1, part 6: Caching",
draft-ietf-httpbis-p6-cache-12 (work in progress),
October 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 Ilya Grigorik, Anirban Kundu, Patrick McManus and Julian
Reschke. 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.
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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.
Appendix C. Changes
draft -00 to draft -01:
o Add guidelines for blacklisting
o Remove advice on signature checking (for now)
o Clarified problem statement
o Rearranged
o Added 430 WOULD BLOCK
Author's Address
Mark Nottingham
Email: mnot@mnot.net
URI: http://www.mnot.net/
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