3. Examples of SDP Usage
3.1. Session Initiation
The Session Initiation Protocol (SIP) [RFC3261] is an application-layer control protocol for creating, modifying, and terminating sessions such as Internet multimedia conferences, Internet telephone calls, and multimedia distribution. The SIP messages used to create sessions carry session descriptions that allow participants to agree on a set of compatible media types [RFC6838]. These session descriptions are commonly formatted using SDP. When used with SIP, the offer/answer model [RFC3264] provides a limited framework for negotiation using SDP.
3.2. Streaming Media
The Real-Time Streaming Protocol (RTSP) [RFC7826], is an application-level protocol for control over the delivery of data with real-time properties. RTSP provides an extensible framework to enable controlled, on-demand delivery of real-time data, such as audio and video. An RTSP client and server negotiate an appropriate set of parameters for media delivery, partially using SDP syntax to describe those parameters.
3.3. Email and the World Wide Web
Alternative means of conveying session descriptions include electronic mail and the World Wide Web (WWW). For both email and WWW distribution, the media type "application/sdp" is used. This enables the automatic launching of applications for participation in the session from the WWW client or mail reader in a standard manner.
Note that descriptions of multicast sessions sent only via email or the WWW do not have the property that the receiver of a session description can necessarily receive the session because the multicast sessions may be restricted in scope, and access to the WWW server or reception of email is possibly outside this scope.
3.4. Multicast Session Announcement
In order to assist the advertisement of multicast multimedia conferences and other multicast sessions, and to communicate the relevant session setup information to prospective participants, a distributed session directory may be used. An instance of such a session directory periodically sends packets containing a description of the session to a well-known multicast group. These advertisements are received by other session directories such that potential remote participants can use the session description to start the tools required to participate in the session.
One protocol used to implement such a distributed directory is the SAP [RFC2974]. SDP provides the recommended session description format for such session announcements.