HTTP pipelining
HTTP pipelining is a technique in which multiple HTTP requests are written out to a single socket without waiting for the corresponding responses. Pipelining is only supported in HTTP/1.1, not in 1.0.
The pipelining of requests results in a dramatic improvement in page loading times, especially over high latency connections such as satellite Internet connections.
Since it is usually possible to fit several HTTP requests in the same TCP packet, HTTP pipelining allows fewer TCP packets to be sent over the network, reducing network load.
Only idempotent requests should be pipelined, such as GET and HEAD requests. POST requests should not be pipelined. Requests on a new connection should not be pipelined either, since it has not yet been determined if the origin server (or proxy server) supports HTTP/1.1. Hence, pipelining can only be done when reusing an existing (keepalive) persistent connection.
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
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.
Exceptions include IIS 4 and reportedly 5.
Implementation in web browsers
Internet Explorer as of version 7 doesn't support pipelining.
Konqueror 2.0 supports pipelining, but it's disabled by default. Instructions for enabling it can be found at Konqueror: Tips & Tricks.
Opera has pipelining enabled by default. It uses heuristics to control the level of pipelining employed depending on the connected server. [1]
Implementation in web proxies
Most HTTP proxies do not pipeline outgoing requests, as demonstrated by this report.
Some versions of the Squid web proxy will pipeline up to two outgoing requests. This functionality has been disabled in the latest versions.
The Polipo proxy pipelines outgoing requests.
Other implementations
Some other applications currently exploiting pipelining are phttpget from FreeBSD (a minimalist pipelined HTTP client), portsnap (a FreeBSD ports tree distribution system) and lftp (a sophisticated file transfer program).
External links
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