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HTTP pipelining is a feature of HTTP/1.1 which allows multiple HTTP requests to be sent over a single TCP (transmission control protocol) connection without waiting for the corresponding responses. HTTP/1.1 specification requires servers to respond to pipelined requests correctly, sending back non-pipelined but valid responses even if server does not support HTTP pipelining. Despite this requirement, many legacy HTTP/1.1 servers do not support pipelining correctly, forcing most HTTP clients to not use HTTP pipelining in practice.
Motivation and limitations
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 HTTP/2 and SPDY are solutions for this. Browsers ultimately did not enable pipelining by default, and by 2017 most browsers supported HTTP/2 by default which used multiplexing instead.
Non-idempotent requests, like those using
POST, should not be pipelined. Sequences of
HEAD requests can always be pipelined. A sequence of other idempotent requests like
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 was introduced in HTTP/1.1 and was not present in HTTP/1.0.
It looks like that, strangely, implementing HTTP pipelining properly has never been an easy task for some developers, maybe excepted for web server developers. There have always been complains about browsers, proxies, web caches, etc. not working well when using pipelined requests / responses, up to the point that for many years (at least till 2011) software developers, engineers, etc. tried to summarize the various kind of problems they noted, to fix things and to give advices about how to deal with pipelining on the Open Web.
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
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 was disabled or not implemented.
- Internet Explorer 8 does not pipeline requests, due to concerns regarding buggy proxies and head-of-line blocking.
- Internet Explorer 11 does not support pipelining.
- 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. Support for H1 Pipeline was removed from Mozilla Firefox in Version 54.
- Konqueror 2.0 supports pipelining, but it is disabled by default.
- Google Chrome previously supported pipelining, but it has been disabled due to bugs and problems with poorly behaving servers.
- Pale Moon (web browser) supports pipelining, and is enabled by default.
Implementation in web proxies
Most HTTP proxies do not pipeline outgoing requests.
Some HTTP proxies, including transparent HTTP proxies, may manage pipelined requests very badly (i.e. by mixing up the order of pipelined responses, etc.).
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.
- 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 previously had limited support for pipelining using the CURLMOPT_PIPELINING option, but this support was removed in version 7.65.0
- portsnap (a FreeBSD ports tree distribution system)
- Advanced Packaging Tool (APT) supports 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.
Testing tools which support HTTP pipelining include:
- "Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing". ietf.org. Retrieved 2014-07-24.
- "Revision 1330814 | Connection management in HTTP/1.x | MDN". MDN Web Docs. Retrieved 2018-03-19.
- "HTTP2 browser support". Retrieved March 9, 2017.
- 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'".
- "Archived copy". Archived from the original on 2016-04-24. Retrieved 2016-04-16.CS1 maint: archived copy as title (link)
- Mark Nottingham (March 14, 2011). "Making HTTP Pipelining Usable on the Open Web". Retrieved October 16, 2021.
- "Wayback link of 'Windows Internet Explorer 8 Expert Zone Chat (August 14, 2008)'". Microsoft. August 14, 2008. Archived from the original on December 4, 2010. Retrieved May 10, 2012.
- "Internet Explorer and Connection Limits". IEBlog. Retrieved 2016-11-14.
- Pipelining Network MozillaZine
- Cheah Chu Yeow. Firefox secrets. p. 180. ISBN 0-9752402-4-2.
- "Bug 264354: Enable HTTP pipelining by default". Mozilla. Retrieved September 16, 2011.
- "Source code – nsHttpConnection.cpp". Firefox source code. Mozilla. May 7, 2010. Retrieved December 5, 2010.
- "Bug 1340655: Remove H1 Pipeline Support". Mozilla. Retrieved March 22, 2017.
- Emir Arian. Internet Communication: Protocols and related subjects. Retrieved 2021-10-16.
- HTTP Pipelining - The Chromium Projects
- "HTTP/1 Pipelining support has been removed in Firefox 54 - Pale Moon forum". forum.palemoon.org. Retrieved 2018-06-07.
- Mark Nottingham (June 20, 2007). "The State of Proxy Caching". Retrieved May 16, 2009.
- Mark Nottingham (July 11, 2011). "What proxies must do". Retrieved October 16, 2021.
- "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.
- "Tempesta FW — a Linux Application Delivery Controller". GitHub. Retrieved March 29, 2018.
- "Servers: Tempesta's side - tempesta-tech/tempesta Wiki". Tempesta Technologies INC. August 1, 2017. Retrieved March 29, 2018.
- 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)" (PDF). Archived from the original (PDF) on 2012-03-10. Retrieved 2010-08-03.
- System.Net.HttpWebRequest & pipelining
- QNetworkRequest Class Reference Archived 2009-12-22 at the Wayback Machine, Nokia QT documentation
- Pipelined HTTP GET utility
- Curl pipelining explanation Archived 2012-06-27 at the Wayback Machine, Curl developer documentation
- Curl pipelining removal announcementArchived 2021-02-05 at the Wayback Machine
- 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
- "HTTP Website". Archived from the original on 2012-06-08. Retrieved 2010-10-01.
- RFC 7230 "Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing". ietf.org. Retrieved 2014-07-24.
- HTTP/1.1 Pipelining FAQ at mozilla.org
- "Network Performance Effects of HTTP/1.1, CSS1, and PNG" at w3.org
- "Optimizing Page Load Times" article
- serf C library