HTTP compression

HTTP compression is a capability that can be built into web servers and web clients to improve transfer speed and bandwidth utilization.

HTTP data is compressed before it is sent from the server: compliant browsers will announce what methods are supported to the server before downloading the correct format; browsers that do not support compliant compression method will download uncompressed data. The most common compression schemes include gzip and Deflate, however a full list of available schemes is maintained by the IANA. Additionally, third parties develop new methods and include them in their products, for example the Google Shared Dictionary Compression for HTTP (SDCH) scheme implemented in the Google Chrome browser and used on Google servers.

There are two different ways compression can be done in HTTP. At a lower level, a Transfer-Encoding header field may indicate the payload of a HTTP message is compressed. At a higher level, a Content-Encoding header field may indicate that a resource being transferred, cached, or otherwise referenced is compressed. Compression using Content-Encoding is more widely supported than Transfer-Encoding, and some browsers do not advertise support for Transfer-Encoding compression to avoid triggering bugs in servers.

In most cases, excluding the SDCH, the negotiation is done in two steps, described in RFC 2616:

1. The web client advertises which compression schemes it supports by including a list of tokens in the HTTP request. For Content-Encoding, the list in a field called Accept-Encoding; for Transfer-Encoding, the field is called TE.

2. If the server supports one or more compression schemes, the outgoing data may be compressed by one or more methods supported by both parties. If this is the case, the server will add a Content-Encoding or Transfer-Encoding field in the HTTP response with the used schemes, separated by commas.

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