ABSTRACT
This section defines semantics that allow for signaling the decoding dependency (DDP) of different media descriptions with the same media type in the Session Description Protocol (SDP) specified in Request for Comments (RFC) 5583. If multiple “m=” lines exist indicating the same media type, a receiver cannot identify a specific relationship between those media. The relationship between different layers of coding for the same media type needs to be defined. For example, if media data is separated and transported in different network streams due to the use of a layered or multiple descriptive media coding process, the receiver should have a mechanism to know the dependency between different SDP media streams for each media type. A new grouping type “DDP” is defined, to be used in conjunction with RFC 3388 (that is made obsolete by RFC 5888, see Section 7.1), which defines SDP attributes such as “group” and “mid” in addition to others. Further, an attribute is specified describing the relationship of the media streams in a “DDP” group indicated by media identification attribute(s) and media format description(s). The layered dependencies between different coding layers are aimed at increasing the coding efficiency, and SDP is enhanced to carry this information.
