Session Description Protocol (SDP) Capability Negotiation

Abstract

The Session Description Protocol (SDP) was intended to describe multimedia sessions for the purposes of session announcement, session invitation, and other forms of multimedia session initiation. SDP was not intended to provide capability indication or capability negotiation; however, over the years, SDP has seen widespread adoption and as a result it has been gradually extended to provide limited support for these, notably in the form of the offer/answer model defined in RFC 3264. SDP does not define how to negotiate one or more alternative transport protocols (e.g., RTP profiles) or attributes. This makes it difficult to deploy new RTP profiles such as Secure RTP or RTP with RTCP-based feedback, negotiate use of different security keying mechanisms, etc. It also presents problems for some forms of media negotiation.

RTP Clock Source Signalling

“…While negotiation of reference clock and media clock attributes is not defined in this document, negotiation MAY be accomplished using the capabilities negotiation procedures defined in [RFC5939].…”

UDP Transport Layer (UDPTL) over Datagram Transport Layer Security (DTLS)

“…If a user prefers fax to be transported securely using UDPTL over DTLS but is willing to transport the fax insecurely in case the remote user does not support UDPTL over DTLS, then the SDP Capability Negotiation mechanism [RFC5939] can be used to offer both UDPTL over DTLS and UDPTL over UDP. Alternatively, if the remote user rejects an SDP offer for UDPTL over DTLS, a new SDP offer for a UDPTL-over-UDP media stream can be sent.…”

DCCP-UDP: A Datagram Congestion Control Protocol UDP Encapsulation for NAT Traversal

“…The SDP Capability Negotiation framework [RFC5939] may be more suitable. Section 3.7 of RFC 5939 specifies how to provide attributes and transport protocols as capabilities and negotiate them using the framework.…”