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Unread 10-06-2009, 04:28 AM   #3
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Recently 'STUN Test Enable' has come to my attention. According to the Linksys ATA manual;
If the STUN Enable feature is enabled and a valid STUN server is available, the Linksys ATA can perform a NAT-type discovery operation when it powers on. It contacts the configured STUN server, and the result of the discovery is reported in a Warning header in all subsequent REGISTER requests. If the Linksys ATA detects symmetric NAT or a symmetric firewall, NAT mapping is disabled.
The interesting part is "If the Linksys ATA detects symmetric NAT or a symmetric firewall, NAT mapping is disabled." I don't have a symmetric NAT router for testing, but it seems that "STUN Test Enable: yes" should work well with Voxalot's "Symmetric NAT Handling" option, which even when set 'yes' is not activated until NAT Mapping is disabled in the ATA. The ATA would then be portable between symmetric and non-symmetric routers without any configuration change. RTP packets would be proxied through Voxalot when necessary (when router is symmetric NAT) but when using a SIP compatible router RTP path would be direct for lower voice latency.

Some users may not know if they are behind symmetric NAT. Rather than asking the novice user to discover if they are behind symmetric NAT and manually turn off "NAT Mapping Enable" it might be better to have the ATA figure this out and automatically make the necessary adjustments.
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