14.36.2 Range Retrieval Requests

HTTP retrieval requests using conditional or unconditional GET methods may request one or more sub-ranges of the entity, instead of the entire entity, using the Range request header, which applies to the entity returned as the result of the request:

         Range = "Range" ":" ranges-specifier

A server MAY ignore the Range header. However, HTTP/1.1 origin servers and intermediate caches SHOULD support byte ranges when possible, since Range supports efficient recovery from partially failed transfers, and supports efficient partial retrieval of large entities.

If the server supports the Range header and the specified range or ranges are appropriate for the entity:

• The presence of a Range header in an unconditional GET modifies what is returned if the GET is otherwise successful. In other words, the response carries a status code of 206 (Partial Content) instead of 200 (OK).

• The presence of a Range header in a conditional GET (a request using one or both of If-Modified-Since and If-None-Match, or one or both of If-Unmodified-Since and If-Match) modifies what is returned if the GET is otherwise successful and the condition is true. It does not affect the 304 (Not Modified) response returned if the conditional is false.

In some cases, it may be more appropriate to use the If-Range header (see section 14.27) in addition to the Range header.

If a proxy that supports ranges receives a Range request, forwards the request to an inbound server, and receives an entire entity in reply, it SHOULD only return the requested range to its client. It SHOULD store the entire received response in its cache, if that is consistent with its cache allocation policies.