(Session Description Protocol) An IETF protocol that defines a text-based message format for describing a multimedia session. Data such as version number, contact information, broadcast times and audio and video encoding types are included in the message. Designed originally for describing multicast transmissions in the Mbone, SDP is also used by the RTSP streaming protocol and SIP and MGCP IP telephony protocols. See SDP, Mbone, RTSP, SIP and MGCP/MEGACO.