HTTP pipelining - Misplaced Pages

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HTTP

Persistence Compression HTTPS QUIC
Request methods
OPTIONS GET HEAD POST PUT DELETE TRACE CONNECT PATCH
Header fields
Cookie ETag Location HTTP referer DNT X-Forwarded-For
Response status codes
301 Moved Permanently 302 Found 303 See Other 403 Forbidden 404 Not Found 451 Unavailable for Legal Reasons
Security access control methods
Basic access authentication Digest access authentication
Security vulnerabilities
HTTP header injection HTTP request smuggling HTTP response splitting HTTP parameter pollution
v t e

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.

The technique was superseded by multiplexing via HTTP/2, which is supported by most modern browsers.

In HTTP/3, the multiplexing is accomplished through the new underlying QUIC transport protocol, which replaces TCP. This further reduces loading time, as there is no head-of-line blocking anymore.

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 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.

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.

Most pipelining problems may happen in HTTP intermediate nodes (hop-by-hop), i.e. mainly in proxy servers (proxies), specially in transparent proxy servers (because they are used anyway without requiring user client configuration, so if only one of them, along the HTTP chain, does not handle pipelined requests properly then nothing works as it should).

Using pipelining with HTTP proxy servers is usually not recommended also because the HOL blocking problem may really slow down a lot proxy server responses (as the server responses must be in the same order of the received requests).

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 server response, then it has to send it to the client and only then it can send the three cached responses too.

If instead a client opens 4 connections to a proxy, the proxy can send the three cached responses to client in parallel before the response from server is received, decreasing a lot the overall completion time (because requests are served in parallel with no head-of-line blocking problem). The same advantage, but with more speed, happens in HTTP/2 multiplexed streams.

Implementation status

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

It looks like that since the beginning, implementing HTTP pipelining properly and / or deploying it has never been an easy task for anybody (excepted for developers of web servers). There have always been complains about browsers, proxy servers, etc. not working well when using pipelined requests / responses, up to the point that for many years (at least till 2011) software developers, engineers, web experts, 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 (that fully implement HTTP/1.1) 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 proxy servers

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.

The Polipo proxy pipelines outgoing requests.

Tempesta FW, an open source application delivery controller, also pipelines requests to backend servers.

Other implementations

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

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 System.Net.HttpWebRequest.
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.