|This article is outdated. (March 2015)|
The pipelining of requests results in a dramatic improvement 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 and HOL blocking can occur. The asynchronous operation of the upcoming HTTP/2 or SPDY could be a solution for this.
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.
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.
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 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.
- Internet Explorer 8 does not pipeline requests, due to concerns regarding buggy proxies and head-of-line blocking.
- Mozilla browsers (such as Mozilla Firefox, SeaMonkey and Camino) support pipelining; however, it is disabled by default. Pipelining is disabled by default to avoid issues with misbehaving servers. When pipelining is enabled, Mozilla browsers use some heuristics, especially to turn pipelining off for older IIS servers.
- Konqueror 2.0 supports pipelining, but it's disabled by default.
- Google Chrome previously supported pipelining, but it has been disabled due to bugs and problems with poorly behaving servers. 
Implementation in web proxies
Most HTTP proxies do not pipeline outgoing requests.
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." Squid supports multiple requests from clients.
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.
- Apache Foundation project HttpComponents provides pipelining support in the HttpCore NIO extensions.
- The Microsoft .NET Framework 3.5 supports HTTP pipelining in the module
- Qt class
QNetworkRequest, introduced in 4.4.
Some other applications currently exploiting pipelining are:
- IceBreak application server since BUILD389
- phttpget from FreeBSD (a minimalist pipelined HTTP client)
- libcurl has limited support for pipelining using the CURLMOPT_PIPELINING option.
- portsnap (a FreeBSD ports tree distribution system)
- Advanced Packaging Tool (APT) support pipelining.
- Subversion (SVN) has optional support for HTTP pipelining with the serf WebDAV access module (the default module, neon does not have pipelining support).
- Microsoft Message Queuing on Windows Server 2003 utilises pipelining on HTTP by default, and can be configured to use it on HTTPS.
- IBM CICS 3.1 supports HTTP pipelining within its client.
- Pipelined::HttpClient is a C++ client library based on pipelining.
Testing tools which support HTTP pipelining include:
- "Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing". ietf.org. Retrieved 2014-07-24.
- 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.
- Willis, Nathan (18 November 2009). "Reducing HTTP latency with SPDY". LWN.net.
- "Connections". w3.org.
- "HTTP/1.1 Pipelining FAQ'".
- "Wayback link of 'Windows Internet Explorer 8 Expert Zone Chat (August 14, 2008)'". Microsoft. August 14, 2008. Retrieved May 10, 2012.
- Pipelining Network MozillaZine
- Cheah Chu Yeow. Firefox secrets. p. 180. ISBN 0-9752402-4-2.
- "https://bugzilla.mozilla.org/show_bug.cgi?id=264354". Mozilla. Retrieved September 16, 2011.
- "Source code – nsHttpConnection.cpp". Firefox source code. Mozilla. May 7, 2010. Retrieved December 5, 2010.
- HTTP Pipelining - The Chromium Projects
- Mark Nottingham (June 20, 2007). "The State of Proxy Caching". Retrieved May 16, 2009.
- "squid : pipeline_prefetch configuration directive". Squid. November 9, 2009. Retrieved December 1, 2009.
- "Polipo — a caching web proxy". Juliusz Chroboczek. September 18, 2009. Retrieved November 12, 2009.
- Kahan, José (June 7, 2002). "Change History of libwww". World Wide Web Consortium. Retrieved August 3, 2010.
- Using HTTP::Async for Parallel HTTP Requests (Colin Bradford)
- System.Net.HttpWebRequest & pipelining
- QNetworkRequest Class Reference, Nokia QT documentation
- Pipelined HTTP GET utility
- Curl pipelining explanation, Curl developer documentation
- C. Michael Pilato, Ben Collins-Sussman, Brian W. Fitzpatrick (2008). Version Control with Subversion. O'Reilly Media. p. 238. ISBN 0-596-51033-0.
- Justin R. Erenkrantz (2007). "Subversion: Powerful New Toys" (PDF).
- "HTTP/HTTPS messages". Microsoft TechNet. January 21, 2005.
- How CICS Web support handles pipelining
- "Pipelined HTTP Client".
- HTTP Website