Session Description Protocol (SDP)

 

A session description expressed in SDP is a short structured textual description of the name and purpose of the session, and the media, protocols, codec formats, timing and transport information that are required to decide whether a session is likely to be of interest and to know how to start media tools to participate in the session.

SDP is purely a format for session description - it does not incorporate a transport protocol, and is intended to use different transport protocols as appropriate, including the Session Announcement Protocol (see section 6.4, Session Initiation Protocol (section 6.5), Real-Time Streaming Protocol (see chapter ), electronic mail using the MIME extensions, and the Hyper-Text Transport Protocol (HTTP).

Although SDP is not intended to be read by humans, it is relatively easily understood. An example session description is shown in figure 6.1.

  

Figure 6.1: Annotated SDP Session Description

Even without the annotation, it would be easy to guess that this session is entitled ``Multimedia Seminar'', that it will use audio, video and an application called ``wb'', and that someone called ``Mark Handley'' is responsible for the session.

However this SDP also indicates the precise timing of the session (the ``t='' line gives start and stop times), that the session is multicast to group 224.2.17.12 with a TTL of 127, the audio is 8KHz $\mu$law carried by RTP to UDP port 49170, the video is H.261 encoded, also over RTP but to port 51372, and the whiteboard program should be started up in portrait mode using port 32416.

Thus SDP includes the session name and a description of its purpose, the times the session is active, the media comprising the session, and information to receive those media (addresses, ports, formats and so on).

As resources necessary to participate in a session may be limited, some additional information may also be desirable, including information about the bandwidth to be used by the conference and contact information for the person responsible for the session.

In general, SDP must convey sufficient information to be able to join a session (with the possible exception of encryption keys) and to announce the resources to be used to non-participants that may need to know.