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HTTP pipelining is a feature ofHTTP/1.1 that allows multipleHTTP requests to be sent over a singleTCP connection without waiting for the corresponding responses.[1] HTTP/1.1 requires servers to respond to pipelined requests correctly, with non-pipelined but valid responses even if the server does not support HTTP pipelining. Despite this requirement, many legacy HTTP/1.1 servers do not support pipelining correctly, forcing most HTTP clients not to use HTTP pipelining.
The technique was superseded bymultiplexing viaHTTP/2,[2] which is supported by most modernbrowsers.[3]
InHTTP/3, multiplexing is accomplished viaQUIC which replacesTCP. This further reduces loading time, as there is nohead-of-line blocking even if some packets are lost.
The pipelining of requests results in a dramatic improvement[4] in the loading times of HTML pages, especially over highlatency connections such assatellite 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 remainsfirst-in-first-out[1] andHOL blocking can occur.
The asynchronous operations ofHTTP/2 andSPDY are solution for this.[5] By 2017 most browsers supported HTTP/2 by default which uses multiplexing instead.[2]
Non-idempotent requests such asPOST should not be pipelined.[6] Read requests likeGET andHEAD can always be pipelined. A sequence of other idempotent requests likePUT andDELETE 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 produce valid responses to pipelined requests, but may not actually process requests concurrently.[7]
Most pipelining problems happen in HTTP intermediate nodes (hop-by-hop), i.e. inproxy servers, especially in transparent proxy servers (if one of them along the HTTP chain does not handle pipelined requests properly then nothing works as it should).[8]
Using pipelining with HTTP proxy servers is usually not recommended also because the HOL blocking problem may really slow down proxy server responses (as the server responses must be in the same order of the received requests).[1][9]
Example: if a client sends 4 pipelined GET requests to a proxy through a single connection and the first one is not in its cache then the proxy has to forward that request to the destination web server; if the following three requests are instead found in its cache, the proxy has to wait for the web server response, then it has to send it to the client and only then it can send the threecached responses too.
If instead a client opens 4 connections to a proxy and sends 1 GET request per connection (without using pipelining) the proxy can send the three cached responses to client in parallel before the response from server is received, decreasing the overall completion time (because requests are served in parallel with no head-of-line blocking problem).[10] The same advantage exists in HTTP/2 multiplexed streams.
Pipelining was introduced in HTTP/1.1 and was not present in HTTP/1.0.[11]
Of all the major browsers, onlyOpera had a fully working implementation that was enabled by default. In other browsers HTTP pipelining was disabled or not implemented.[5]
Most HTTP proxies do not pipeline outgoing requests.[19]
Some HTTP proxies, including transparent HTTP proxies, may manage pipelined requests very badly (e.g. by mixing up the order of pipelined responses).[20]
Some versions of theSquid 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".[21] Squid supports multiple requests from clients.
ThePolipo proxy pipelines outgoing requests.[22]
Tempesta FW, an open sourceapplication delivery controller,[23] also pipelines requests to backend servers.[24]
Thelibwwwlibrary made by theWorld Wide Web Consortium (W3C), supports pipelining since version 5.1 released at 18 February 1997.[25]
Other application development libraries that support HTTP pipelining include:
System.Net.HttpWebRequest.[27]QNetworkRequest, introduced in 4.4.[28]Some other applications currently exploiting pipelining are:
Testing tools which support HTTP pipelining include:
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