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HTTP pipelining is a transfer method of HTTP/1.1 for improving performance, when multiple HTTP requests are written out to a socket together without waiting for the corresponding responses. Responses are then expected in order they were requested. Servers can also send multiple responses without closing their sockets. This results in fewer round trips and faster load times.

The act of pipelining the requests can result 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 can be pipelined, such as GET and HEAD requests. POST and PUT requests should not be pipelined. Requests on a new connection shouldn't 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 to not fail if a client chooses to pipeline requests.

Some applications currently exploiting pipelining are phttpget from FreeBSD (a minimalist pipelined HTTP client) and portsnap (a FreeBSD ports tree distribution system).

Use in web browsers

Internet Explorer as of version 7 doesn't support pipelining.

Mozilla Firefox 2.0 supports pipelining, but it's disabled by default. It uses some heuristics, especially to turn pipelining off for IIS servers.

Opera has pipelining enabled by default. It uses heuristics to control the level of pipelining employed depending on the connected server.

External links

• 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

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