One document matched: draft-nottingham-http-pipeline-01.xml
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<rfc ipr="trust200902" docName="draft-nottingham-http-pipeline-01" category="info">
<front>
<title abbrev="HTTP Pipelining Enhancements">Making HTTP Pipelining Usable on the Open Web</title>
<author initials="M." surname="Nottingham" fullname="Mark Nottingham">
<organization></organization>
<address>
<email>mnot@mnot.net</email>
<uri>http://www.mnot.net/</uri>
</address>
</author>
<date year="2011"/>
<abstract>
<t>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.</t>
</abstract>
</front>
<middle>
<section title="Introduction">
<t>HTTP/1.1 <xref target="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).</t>
<t>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.</t>
<t>Indeed, in constrained applications of HTTP such as Subversion,
pipelining has been shown to improve end-user perceived latency
considerably.</t>
<t>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.</t>
<t>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.</t>
<t>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.</t>
<t>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.</t>
</section>
<section title="Requirements">
<t>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 <xref
target="RFC2119"/>.</t>
</section>
<section title="HTTP Pipelining Issues">
<t>Anecdotal evidence suggests there are a number of reasons why
clients don't use HTTP pipelining by default. Briefly, they are:
<list style="numbers">
<t>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.</t>
<t>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.</t>
<t>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.</t>
</list>
</t>
<t>Note that here, "servers" can also include proxies and other
intermediaries, including so-called "transparent" proxies (also
known as intercepting proxies).</t>
<t>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.</t>
</section>
<section title="Blacklisting Origin Servers">
<t>To address balking and confused origin servers, a client
SHOULD maintain a blacklist of origins that it does not attempt
pipelining with.</t>
<t>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.</t>
</section>
<section title="Discovering Faulty Proxies">
<t>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.</t>
<t>This can be done by sending pipelined requests to a
known server, and examining the responses for errors.</t>
<t>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.</t>
<t>RECOMMENDED measures in such tests include:</t>
<t><list style="symbols">
<t>Sending a non-trivial number of pipelined requests (e.g., 10)</t>
<t>Sending multiple pipelined requests in the same packet</t>
<t>Inserting request bodies with various sizes</t>
<t>Assuring that caching is disabled, so that requests are end-to-end</t>
<t>Sending a variety of responses types that includes 100 and 304 responses</t>
<t>Examining responses to assure that they all appear in the correct order</t>
<t>Examining received requests and responses to assure that they aren't unduly modified</t>
</list></t>
<t>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.</t>
<t>See <eref target="https://github.com/mnot/pipeline-surveyor" />
for an example implementation.</t>
</section>
<section title="Correlating Responses">
<t>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.</t>
<t>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.</t>
<t>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.</t>
<figure><artwork>
Assoc-Req = "Assoc-Req" ":" OWS Assoc-Req-v
Assoc-Req-v = Method SP absolute-URI
</artwork></figure>
<t>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 <xref
target="I-D.ietf-httpbis-p1-messaging"/>. The header field MUST NOT
be generated by proxies.</t>
<t>For example, given the following request over port 80:</t>
<figure><artwork>
GET /foo?it HTTP/1.1
Host: www.example.com
</artwork></figure>
<t>the appropriate Assoc-Req header field would be:</t>
<figure><artwork>
Assoc-Req: GET http://www.example.com/foo?it
</artwork></figure>
<t>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
<xref target="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.</t>
<t>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 <xref
target="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.</t>
<t>A client MAY choose to blacklist any origin server that does not
send the Assoc-Req header.</t>
<t>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").</t>
</section>
<section title="Hinting Pipelinable Content">
<t>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.</t>
<t>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.</t>
<t>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.</t>
<t>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.</t>
<t>There are several possible ways that content could be hinted,
including:
<list style="symbols">
<t>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.
</t>
<t>
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.</t>
<t>
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.</t>
</list>
</t>
<t>FEEDBACK: thoughts on the suitability of these hinting mechanisms is
encouraged, so that the list can eventually be narrowed down.</t>
<t>A user agent MAY have a policy of only pipelining to hinted resources.</t>
</section>
<section title="Indicating Blocking Responses">
<t>An alternate way to avoid head-of-line blocking is for the origin
server to aggressively indicate when a request would block.</t>
<t>This can be done by using a new HTTP status code, 430 WOULD BLOCK.</t>
<t>The meaning of "would block" is defined by the server; e.g., it
may 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.</t>
<t>When a client (user agent or intermediary) receives a 430 WOULD
BLOCK, it SHOULD resubmit the associated request on a new or idle
connection.</t>
<t>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.</t>
<t>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).
</t>
<t>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.</t>
</section>
<section title="Handling Pipelining Problems">
<t>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").</t>
</section>
<section title="Security Considerations">
<t>TBD</t>
</section>
<section title="IANA Considerations">
<t>TBD</t>
</section>
</middle>
<back>
<references title="Normative References">
&rfc2119; &p1-messaging;
</references>
<references title="Informative References">
&rfc2616; &p6-cache;
</references>
<section title="Acknowledgements">
<t>Thanks to
Ilya Grigorik,
Anirban Kundu,
Patrick McManus and
Julian Reschke.
The author takes all responsibility for errors and
omissions.</t>
</section>
<section title="Frequently Asked Questions">
<t>Isn't full multiplexing better?</t>
<t>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.</t>
<t>Why not have the client generate a unique request identifier?</t>
<t>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.</t>
</section>
<section title="Changes">
<t>draft -00 to draft -01:</t>
<t><list style="symbols">
<t>Add guidelines for blacklisting</t>
<t>Remove advice on signature checking (for now)</t>
<t>Clarified problem statement</t>
<t>Rearranged</t>
<t>Added 430 WOULD BLOCK</t>
</list></t>
</section>
</back>
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