Session Description Protocol – SDP Protocol

VOCAL’s software libraries fully support the Session Description Protocol, or SDP, as defined by RFC 8866 (originally RFC 2327 and then RFC 4566) as well as the SDP offer/answer model as defined by RFC 3264 and RFC 8843.  SDP can be used as a standalone module, or as part of a larger library package, such as VOCAL’s Session Initiation Protocol (SIP) or Voice/Video/Modem/Fax/Radio over IP libraries.

What is SDP ?

The Session Description Protocol was designed for the purpose of describing media sessions. Typically SDP is not a standalone protocol, but rather is used by other signaling protocols such as SIP, RTSP, or MGCP to exchange media information during session setup. For a typical call setup using SIP, the SDP would be used to describe a number of parameters. Some of these parameters may include but are not limited to: an IP address and port at which the user would like the media to be sent, the type of media, and a list of supported codecs for a particular type of media.

A model for using SDP to setup media sessions within a higher level protocol such as SIP is defined by RFC 3264 An Offer/Answer Model with the Session Description Protocol(SDP) which defines a mechanism where two endpoints use Session Description Protocol (SDP) to negotiate media session parameters – such as preferred codecs, and bitrates.

Often, a particular codecs negotiated parameters are described in a standalone RFC.  For instance, RFC 8130 RTP Payload Format for MELPe Codec describes the use of the MELP speech coder, and RFC 8817 RTP Payload Format for Tactical Secure Voice Cryptographic Interoperability Specification (TSVCIS) Codec the TSVCIS coder, with SDP.