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HTTP pipelining

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HTTP
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Schema of non-pipelined vs. pipelined connection.

HTTP pipelining is a technique in which multiple HTTP requests are sent on a single TCP connection without waiting for the corresponding responses.[1]

The pipelining of requests results in a dramatic improvement[2] in the loading times of HTML pages, especially over high latency connections such as satellite Internet connections. The speedup is less apparent on broadband connections, as the limitation of HTTP 1.1 still applies: the server must send its responses in the same order that the requests were received — so the entire connection remains first-in-first-out[1] and HOL blocking can occur. The asynchronous operation of the upcoming HTTP 2.0 or SPDY could be a solution for this[3].

Since it is usually possible to fit several HTTP requests in the same TCP packet, HTTP pipelining allows fewer TCP packets to be sent over the network, reducing network load.

Non-idempotent methods like POST should not be pipelined. Sequences of GET and HEAD requests can always be pipelined. A sequence of other idempotent requests like GET, HEAD, PUT and DELETE can be pipelined or not depending on whether requests in the sequence depend on the effect of others.[4]

HTTP pipelining requires both the client and the server to support it. HTTP/1.1 conforming servers are required to support pipelining. This does not mean that servers are required to pipeline responses, but that they are required not to fail if a client chooses to pipeline requests.

Implementation status

Pipelining is only supported in HTTP/1.1, not in 1.0.

Implementation in web servers

Implementing pipelining in web servers is a relatively simple matter of making sure that network buffers are not discarded between requests. For that reason, most modern web servers handle pipelining without any problem.

Implementation in web browsers

Out of all the major browsers, only Opera has a fully working implementation that is enabled by default. All other browsers HTTP pipelining is disabled or not implemented.[3]

  • Internet Explorer 8 does not pipeline requests, due to concerns regarding buggy proxies and head-of-line blocking.[5]
  • Mozilla browsers (such as Mozilla Firefox, SeaMonkey and Camino) support pipelining, however it is disabled by default.[6][7] Pipelining is disabled by default to avoid issues with misbehaving servers.[8] When pipelining is enabled, Mozilla browsers use some heuristics, especially to turn pipelining off for older IIS servers.[9]
  • Konqueror 2.0 supports pipelining, but it's disabled by default.[citation needed]
  • Google Chrome supports pipelining http (but not https) in the stable release (from version 18) as a non-default option, will be enabled in version 20 (currently dev) as default[10].

Implementation in web proxies

Most HTTP proxies do not pipeline outgoing requests.[11]

Some versions of the Squid web proxy will pipeline up to two outgoing requests. This functionality has been disabled by default and needs to be manually enabled for "bandwidth management and access logging reasons."[12] Squid supports multiple requests from clients.

The Polipo proxy pipelines outgoing requests.[13]

Other implementations

The libwww library made by the World Wide Web Consortium, supports pipelining since version 5.1 released at 18 February 1997.[14]

Other application development libraries that support HTTP pipelining include:

  • Perl modules providing client support for HTTP pipelining are HTTP::Async and the LWPng (libwww-perl New Generation) library.[15]
  • Apache Foundation project HttpComponents provides pipelining support in the HttpCore NIO extensions.
  • The Microsoft .Net Framework 3.5 supports HTTP pipelining in the module System.Net.HttpWebRequest.[16]
  • Qt class QNetworkRequest, introduced in 4.4, supports HTTP Pipelining.[17]

Some other applications currently exploiting pipelining are:

Multipart XHR is implementation of pipelining (without any browser or web server support) done purely in Javascript in combination with server side scripting.[citation needed]

Testing tools which support HTTP pipelining include:

See also

References

  1. ^ a b "part of Hypertext Transfer Protocol -- HTTP/1.1 Section 8.1.2.2 Pipelining". W3C. June 1999. Retrieved 2011-09-16.
  2. ^ Nielsen, Henrik Frystyk; Gettys, Jim; Baird-Smith, Anselm; Prud'hommeaux, Eric; Lie, Håkon Wium; Lilley, Chris (24 June 1997). "Network Performance Effects of HTTP/1.1, CSS1, and PNG". World Wide Web Consortium. Retrieved 14 January 2010.
  3. ^ a b Willis, Nathan (18 November 2009). "Reducing HTTP latency with SPDY". LWN.net.
  4. ^ "part of Hypertext Transfer Protocol -- HTTP/1.1 Section 9.1.2 Idempotent Methods". W3C. June 1999. Retrieved 2009-05-16.
  5. ^ "Wayback link of 'Windows Internet Explorer 8 Expert Zone Chat (August 14, 2008)'". Microsoft. August 14, 2008. Retrieved 2012-05-10.
  6. ^ http://kb.mozillazine.org/Network.http.pipelining
  7. ^ Cheah Chu Yeow. Firefox secrets. p. 180. ISBN 0975240242.
  8. ^ "https://bugzilla.mozilla.org/show_bug.cgi?id=264354". Mozilla. Retrieved 2011-09-16. {{cite web}}: External link in |title= (help)
  9. ^ "Source code – nsHttpConnection.cpp". Firefox source code. Mozilla. 7 May 2010. Retrieved 5 December 2010.
  10. ^ Peter Beverloo: Sub-pixel layout, Inspecting Web Socket Frames and Seamless Iframes
  11. ^ Mark Nottingham (20 June 2007). "The State of Proxy Caching". Retrieved 2009-05-16.
  12. ^ "squid : pipeline_prefetch configuration directive". Squid. November 9, 2009. Retrieved 2009-12-01.
  13. ^ "Polipo — a caching web proxy". Juliusz Chroboczek. September 18, 2009. Retrieved 2009-11-12.
  14. ^ Kahan, José (7 June 2002). "Change History of libwww". World Wide Web Consortium. Retrieved 3 August 2010.
  15. ^ Using HTTP::Async for Parallel HTTP Requests (Colin Bradford)
  16. ^ System.Net.HttpWebRequest & pipelining
  17. ^ QNetworkRequest Class Reference, Nokia QT documentation
  18. ^ C. Michael Pilato, Ben Collins-Sussman, Brian W. Fitzpatrick (2008). Version Control with Subversion. O'Reilly Media. p. 238. ISBN 0596510330.{{cite book}}: CS1 maint: multiple names: authors list (link)
  19. ^ Justin R. Erenkrantz (2007). "Subversion: Powerful New Toys" (PDF).
  20. ^ "HTTP/HTTPS messages". Microsoft TechNet. January 21, 2005.
  21. ^ How CICS Web support handles pipelining
  22. ^ "Pipelined HTTP Client".
  23. ^ http://www.hpl.hp.com/research/linux/httperf/
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