HTTP pipelining

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Time diagram 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 or SPDY could be a solution for this.[3]

Non-idempotent requests, like those using POST, should not be pipelined.[4] Sequences of GET and HEAD requests can always be pipelined. A sequence of other idempotent requests like PUT and DELETE can be pipelined or not depending on whether requests in the sequence depend on the effect of others.[1]

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.[5]

Implementation status[edit]

Pipelining was introduced in HTTP/1.1 and was not present in HTTP/1.0.[6]

Implementation in web servers[edit]

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[edit]

Of all the major browsers, only Opera based on Presto layout engine had a fully working implementation that was enabled by default. In all other browsers HTTP pipelining is disabled or not implemented.[3]

Implementation in web proxies[edit]

Most HTTP proxies do not pipeline outgoing requests.[14]

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."[15] Squid supports multiple requests from clients.

The Polipo proxy pipelines outgoing requests.[16]

Other implementations[edit]

The libwww library made by the World Wide Web Consortium (W3C), supports pipelining since version 5.1 released at 18 February 1997.[17]

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.[18]
  • 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.[19]
  • Qt class QNetworkRequest, introduced in 4.4.[20]

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[edit]


  1. ^ a b c "Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing". Retrieved 2014-07-24. 
  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". 
  4. ^ "Connections". 
  5. ^ "HTTP/1.1 Pipelining FAQ'". 
  6. ^
  7. ^ "Wayback link of 'Windows Internet Explorer 8 Expert Zone Chat (August 14, 2008)'". Microsoft. August 14, 2008. Retrieved May 10, 2012. 
  8. ^ "Internet Explorer and Connection Limits". IEBlog. Retrieved 2016-11-14. 
  9. ^ Pipelining Network MozillaZine
  10. ^ Cheah Chu Yeow. Firefox secrets. p. 180. ISBN 0-9752402-4-2. 
  11. ^ "Bug 264354: Enable HTTP pipelining by default". Mozilla. Retrieved September 16, 2011. 
  12. ^ "Source code – nsHttpConnection.cpp". Firefox source code. Mozilla. May 7, 2010. Retrieved December 5, 2010. 
  13. ^ HTTP Pipelining - The Chromium Projects
  14. ^ Mark Nottingham (June 20, 2007). "The State of Proxy Caching". Retrieved May 16, 2009. 
  15. ^ "squid : pipeline_prefetch configuration directive". Squid. November 9, 2009. Retrieved December 1, 2009. 
  16. ^ "Polipo — a caching web proxy". Juliusz Chroboczek. September 18, 2009. Retrieved November 12, 2009. 
  17. ^ Kahan, José (June 7, 2002). "Change History of libwww". World Wide Web Consortium. Retrieved August 3, 2010. 
  18. ^ Using HTTP::Async for Parallel HTTP Requests (Colin Bradford)
  19. ^ System.Net.HttpWebRequest & pipelining
  20. ^ QNetworkRequest Class Reference, Nokia QT documentation
  21. ^ Pipelined HTTP GET utility
  22. ^ Curl pipelining explanation, Curl developer documentation
  23. ^ C. Michael Pilato; Ben Collins-Sussman; Brian W. Fitzpatrick (2008). Version Control with Subversion. O'Reilly Media. p. 238. ISBN 0-596-51033-0. 
  24. ^ Justin R. Erenkrantz (2007). "Subversion: Powerful New Toys" (PDF). 
  25. ^ "HTTP/HTTPS messages". Microsoft TechNet. January 21, 2005. 
  26. ^ How CICS Web support handles pipelining
  27. ^ HTTP Website

External links[edit]