http://www.rfc-editor.org/rfc/rfc5583.txt
RFC 5583: Signaling Media Decoding Dependency in the Session Description Protocol (SDP), T. Schierl, S. Wenger2009-07-01T23:00:00-00:00A new grouping type "DDP" -- decoding dependency -- is defined, to be used in conjunction with RFC 3388 entitled "Grouping of Media Lines in the Session Description Protocol". In addition, 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). [STANDARDS-TRACK]