HTTP pipelining is a technique in which multiple HTTP requests are sent on a single TCP connection
without waiting for the corresponding responses.
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.
Implementation status 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 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 supports pipelining for HTTP in the stable release as a non-default option.
There is no support for pipelining HTTPS yet.
As of version 26, the flag to enable HTTP pipelining in Chrome has been disabled.
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.
The Polipo proxy pipelines outgoing requests.
Other implementations The libwww library made by the World Wide
Web Consortium, 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
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 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
libcurl has limited support for pipelining using the CURLMOPT_PIPELINING option.
portsnap Advanced Packaging Tool support pipelining.
Subversion has optional support for HTTP pipelining with the serf WebDAV access module.
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.
Multipart XHR is implementation of pipelining done purely in JavaScript in combination with
server-side scripting.
Testing tools which support HTTP pipelining include:
httperf See also
HTTP persistent connection WebSocket
SPDY References
External links RFC 7230 "Hypertext Transfer Protocol: 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
phttpget serf C library