[SIPForum-discussion] Change of contact field when behind NAT

Vijay Swami Vijay.Swami at matrixcomsec.com
Thu Oct 17 06:34:41 UTC 2013

Hi Rahul,


It seems that one of the router is giving symmetric behavior. Please check
the mapped port for which STUN query is made and the actual mapped port
while sending RTP.

If both are same then there is no symmetric behavior of router and some
other issue is there but if they are not same then Router is giving
symmetric behavior and it will not work with STUN. 

STUN do not work with Symmetric NAT.



Vijay Swami 


From: discussion-bounces at sipforum.org
[mailto:discussion-bounces at sipforum.org] On Behalf Of rahul sankanur
Sent: Friday, October 11, 2013 3:01 PM
To: discussion at sipforum.org
Subject: Re: [SIPForum-discussion] Change of contact field when behind NAT


Hi all, 


Mohit and Vijay ,thank you very much for the information....


I have a Belkin and a Dlink router..I have been seeing something very
strange in both...

I have two devices behind NAT and STUN is enabled in both ..Therefore in
their SDPs, the devices will be their respective mapped IP and ports..

But i see that the both the routers are NOT forwarding the media packets to
the other side..Both the end devices send to the  mapped IP and mapped ports
though...I get ICMP packets sometimes with destination port unreachable (but
media STUN keepalive is ON).


But when one device is inside NAT and other is outside , router is
forwarding properly...


So is there any other specific setting to be done in the routers or both the
routers' routing tables are flawed???


This problem arises only when 2 devices are in same NAT and media is flowing
through the mapped port of the routers....



Thanks and regards,

Rahul Sankanur


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