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RTP Symmetric
stream.
How to set it up is defined in the SDP part of a SIP Invite. An IETF Internet draft for connection-oriented
media setup with SDP (COMEDIA) describes how to defines the role of the two endpoints wanting to setup communication. Although thet Draft describes TCP sessions, these methods are used in Symmetric RTP to handle communication over NAT.
Quote from the Comedia draft:
An important attribute of connection-oriented protocols is the setup procedure. One endpoint needs to initiate the connection and the other endpoint needs to accept the connection. The direction attribute is used to describe these roles, and the syntax is as follows:
a=direction:<role>
The <role> is one of the following:
- passive: The endpoint will accept an incoming connection.
- active: The endpoint will initiate an outgoing connection.
- both: The endpoint will both accept an incoming connection and will initiate an outgoing connection.
Recommended reading
- Cisco.com: SIP: Connection-Oriented Media Enhancements for SIP
- IETF Draft: Connection-Oriented Media Transport in SDP
- For an summary of the problems encountered in getting voice and multimedia to traverse NAT devices, symmetric NAT included, see the white paper: NAT Traversal for Multimedia over IP Services
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