First Incoming Packet Mechanism

In scenarios where the remote user agent (UA) resides behind a NAT server, it’s possible that the device, if not configured for NAT traversal, will send the media (RTP, RTCP and T.38) streams to an invalid IP address and UDP port. In other words, it will send the media to the private IP address:port of the UA and not the public address (of the NAT server) and therefore, the media will not reach the UA. When the UA is located behind NAT, although the UA sends its private IP address:port in the original SIP message (INVITE), the device receives the media packets with a source address of a public IP address:port (i.e., allocated by the NAT server). Therefore, to ensure that the media reaches the UA, the device must send it to the public address.

The device identifies whether the UA is located behind NAT by comparing the source IP address of the first received media packet with the IP address and UDP port of the first received SIP message (INVITE) when the SIP session was started. This is done for each media type--RTP, RTCP and T.38--and therefore, they can have different destination IP addresses and UDP ports than one another.

The device supports various NAT traversal methods, which you can configure using the 'NAT Traversal' (NATMode) parameter. For more information on the different options provided by this parameter, see NAT and STUN Parameters.

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To enable NAT resolution using the First Incoming Packet mechanism:

Open the Media Settings page (Setup menu > Signaling & Media tab > Media folder > Media Settings).

From the 'NAT Traversal' drop-down list (NATMode), select the required NAT option.

Click Apply.