http://www.rfc-editor.org/rfc/rfc3890.txt
RFC 3890: A Transport Independent Bandwidth Modifier for the Session Description Protocol (SDP), M. Westerlund2004-09-01T23:00:00-00:00 The existing SDP bandwidth modifiers and their values include the bandwidth needed for the transport and IP layers. When using SDP with protocols like the Session Announcement Protocol (SAP), the Session Initiation Protocol (SIP), and the Real-Time Streaming Protocol (RTSP), and when the involved hosts has different transport overhead, for example due to different IP versions, the interpretation of what lower layer bandwidths are included is not clear. [STANDARDS-TRACK]