IETF RFC 5688 PDF

The caller preferences specification for the Session Initiation Protocol (SIP) allows a caller to express preferences that the call be routed to a User Agent (UA) with particular capabilities. Similarly, a specification exists to allow a UA to indicate its capabilities in a registration. Amongst those capabilities are the type of media streams the agent supports, described as top-level MIME types. The 'application' MIME type is used to describe a broad range of stream types, and it provides insufficient granularity as a capability. This specification allows a UA to indicate which application subtypes the agent supports.