SDP is an IETF standard that allows a multimedia device to describe the kinds of media that has to offer or that it wishes to accept. As part of this description, the device will indicate the type of media (audio, video, text, etc.), the IP ports used, the protocols used (e.g., T.120), and other information necessary for a device to receive the specified media and understand how to handle that media.

SDP has been published by the IETF as RFC 4566. There are additional RFCs that document extensions or enhancements to SDP.

How it Works

The owner of a conference advertises it over a network by sending multicast messages which contain a description of the session e.g. the name of the owner, the name of the session, the coding, the timing, etc. SDP does not provide the content of the media form itself but simply provides a negotiation between two end points to allow them to agree on a media type and format.

The recipients of the SDP message then make a decision about participating in the session. SDP is generally contained in the body part of Session Initiation Protocol popularly called SIP.

SDP started off as a component of SAP, but found other uses in conjunction with RTP, SIP, and as a standalone format as described above.

There are five terms related to SDP:

  1. Conference: Two or more communicating users who utilize communication devices to meet rather than meeting in person.
  2. Session: The flowing stream of data between an open multimedia sender and receiver.
  3. Session Announcement: A session announcement is a session description conveyed to users who may or may not expect the announcement.
  4. Session Advertisement: same as session announcement.
  5. Session Description: The information included in the session announcement or advertisement.

Basic Usage

SDP is ideal to inform business partners, clients, and other large groups of interconnected individuals and groups about upcoming events. But, like with any software, there is a learning and usability curve. The SDP offer/answer model is where most SIP interoperability issues occur, and following the RFC may or may not resolve your issues. The problems in session advertising also may be concerned with endpoint types and various control protocols.

Within the context of VoIP architecture, it is seen that there are a number of different media endpoints which may be controlled by MGCP, H248 or SIP. For all combinations of endpoints on a given connection, there is a need to ensure that CODEC negotiation takes place and that the differing uses of SDP are reconciled.

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