Symmetric NAT Traversal

The Symmetric NAT Traversal 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 2020 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 2020 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).



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.


Symmetric NAT Traversal Mode

When Symmetric NAT Traversal is enabled, (in the IP Bearer Profile pane) this specifies the setup role for the 2020 IMG. The role can either be Passive or Active. See below for more information on these roles.



Used when the 2020 IMG is on a public network and the 2020 IMG will allow a call to be made to a distant endpoint that is behind a NAT. The 2020 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.




This applies to SIP only and is used when the 2020 IMG is behind a NAT and is configured to send or accept SIP INVITES. The 2020 IMG will  convey that it is behind a NAT to the distant endpoint in an SDP message. The 2020 IMG then 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 message.