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SIP NAT traversal confirm

hello, just to confirm that I understand this correctly:

My understanding is that after a SIP session is established, when SDP is used to negotiate media capabilities etc., the internal LAN address of the user agent is written into the SIP packet (within the relevant SDP fields) for the purpose of establishing an RTP connection. This causes a problem with the ADSL router NAT system as this IP address is embedded within the SDP packet. It is therefore not detected by NAT and is not converted to a workable external NAT address/port combination.

Therefore most modern ADSL routers have the capability to 'deep' inspect SIP packets and rewrite the SDP IP address/port values in order to overcome this NAT traversal issue.

Is this correct ?

Thanks for confirmation.


SIP NAT traversal confirm

Your basic idea is correct. Whether any ADSL modems do deep-packet inspect for SIP NAT, though, is another matter...


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