Accept-Encoding header

The HTTP Accept-Encoding request and response header indicates the content encoding (usually a compression algorithm) that the sender can understand. In requests, the server uses content negotiation to select one of the encoding proposals from the client and informs the client of that choice with the Content-Encoding response header. In responses, it provides information about which content encodings the server can understand in messages to the requested resource, so that the encoding can be used in subsequent requests to the resource. For example, Accept-Encoding is included in a 415 Unsupported Media Type response if a request to a resource (e.g., PUT) used an unsupported encoding.

Even if both the client and the server support the same compression algorithms, the server may choose not to compress the body of a response if the identity value is also acceptable. This happens in two common cases:

1. The data is already compressed, meaning a second round of compression will not reduce the transmitted data size, and may actually increase the size of the content in some cases. This is true for pre-compressed image formats (JPEG, for instance).
2. The server is overloaded and cannot allocate computing resources to perform the compression. For example, Microsoft recommends not to compress if a server uses more than 80% of its computational power.

As long as the identity;q=0 or *;q=0 directives do not explicitly forbid the identity value that means no encoding, the server must never return a 406 Not Acceptable error.