Overview:
This feature allows the passage of VoIP packets between endpoints when one (but not both) of the endpoints resides on a private network behind a NAT (Network Address Translation) device. This allows the IMG to check the media source of incoming Realtime Transport Protocol (RTP) packets, and allows the endpoint to advertise its presence inside or outside of Network Address Translation (NAT). Using this enables symmetric NAT traversal by supporting the capability to modify and update an existing RTP session remote address and port.
When Symmetric NAT Traversal is enabled, the IMG will look for incoming packets from the distant end and use the source address information in those packets to define the IP address and UDP port information that must be sent back out via RTP to the distant end.
The Comedia feature is based on the Comedia IETF draft version 04 (draft-ietf-mmusic-sdp-comedia).
SIP
Symmetric NAT Traversal for SIP signaling provides the following:
Ability to specify the connection role of the IMG when acting as a UAC or UAS.
Enable interoperability in networks where NAT devices are unaware of SIP or SDP signaling. SIP endpoints may both be outside NAT or one inside and the other outside.
H.323
H323 uses the Symmetric NAT Traversal information internally; no Symmetric NAT Traversal information is shared with other gateways or endpoints.
Symmetric NAT Traversal Mode
When you enable Symmetric NAT Traversal (in the IMG 1010 - IP Bearer Profile pane) you specify the setup role for the IMG.
Passive
Used when the IMG is on a public network and you want the IMG to allow calls to a distant endpoint that is behind a NAT. The IMG uses the Source IP and Ports of the incoming RTP, RTCP, and T38 packets as the Destination IP and Port of the outgoing RTP, RTCP, and T38 packets.
Active
This applies to SIP only. If used when the IMG is behind a NAT and you want the IMG to send or accept SIP INVITES. The IMG will convey that it is behind a NAT to the distant endpoint in the SDP. The IMG advertises to other gateways to change their RTP port and IP address to map with the NAT by sending the direction attribute a=direction<active> in the SIP SDP.
Troubleshooting Notes
If both the IMG and the remote GW is set to passive you will have no voice path.
If neither side is set to passive and you are connecting through a NAT you may not have voice path.