ABSTRACT
This section describes the Session Description Protocol (SDP) offer/answer model based on Request for Comments (RFC) 3264. The SDP provides a negotiations mechanism used by communicating entities to arrive at a common view of a multimedia session. In the offer/answer negotiation model that SDP uses, one participant offers the other a description of the desired session from his/her perspective, and the other answers with the desired session from his/her perspective. This offer/answer model is most useful in unicast sessions where information from both participants is needed for the complete view of the session. However, some simple examples of SDP offer/answer model are provided in Section 18.1 following RFC 4317 and using Session Initiation Protocol (SIP). By the same token, additional examples of SDP offer/answer model using web real-time communication are provided in Section 18.2 following Internet Engineering Task Force drafts. These examples are related to codec negotiation and selection, hold and resume, and addition and deletion of media streams, multiple media types, bidirectional, unidirectional, inactive streams, and dynamic payload types, and Third Party Call Control using the offer/answer model. In addition, this section describes a protocol using RFC 5245 for network address translator traversal for User Datagram Protocol-based multimedia sessions established with the offer/answer model. This protocol is called Interactive Connectivity Establishment (ICE). ICE makes use of the simple traversal of UDP around NAT protocol and its extension, traversal using relays around NAT. ICE can be used by any protocol utilizing the offer/answer model, such as the SIP.
