http://www.rfc-editor.org/rfc/rfc4569.txt
RFC 4569: Internet Assigned Number Authority (IANA) Registration of the Message Media Feature Tag, G. Camarillo2006-07-01T23:00:00-00:00This document registers with the IANA a new media feature tag associated with the 'message' media type. This media feature tag indicates that a particular device supports 'message' as a streaming media type. Media feature tags can be used to route calls to devices that support certain features. This memo provides information for the Internet community.