The X-Features header is used to indicate support for a certain type. The 
format of the X-Features header is as follows:
X-Features: feature/version [, feature/version]

For example:
X-Features: queue/1.0

This header can be used in gnet handshake, http uploads and downloads.
When this header is not present all features are assumed to be 
available.

references:
http://groups.yahoo.com/group/the_gdf/message/14720