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

Vijay Swami Vijay.Swami at matrixcomsec.com
Thu Oct 10 05:25:35 UTC 2013

Hi Rahul,


There is one more thing to be taken care in addition to what Mohit just

Whenever we are using Router then SIP ALG supported routers change SDP
information and add the actual mapped IP and port in connection info.

In such case UAC need not have to take care of anything and connection info
is automatically altered by the routers.


However, the Contact field behavior is important while doing the call in
reverse. Contact field's IP address is the IP that generally is registered
in most of the Registrar sever. However there are Registrar server which
Registers the UAC based on the real IP detection so that UAC is always


Also there is one more mechanism for Contact in which Registrar server send
the actual(Real detected) IP and port using rport to UAC and UAC then
changes its Contact field as per the rport parameter. This requires UAC to
Register on Server using the received rport parameters. Also UAC is required
to send the Register request using rport field in Via header.



Vijay Swami


From: discussion-bounces at sipforum.org
[mailto:discussion-bounces at sipforum.org] On Behalf Of Mohit Soni
Sent: Tuesday, October 08, 2013 11:02 AM
To: 'rahul sankanur'; discussion at sipforum.org
Subject: Re: [SIPForum-discussion] Change of contact field when behind NAT


Hi Rahul,



In the behind the NAT scenarios, the SIP UAC should change the IP Address in
the Contact, VIA, "c=" and "m=" for successful calls.


However, if the SIP UAS is Outbound Proxy or B2BUA then they have symmetric
signaling and Symmetric RTP implemented to overcome the NAT issues.

So, in that case, the calls will be successful even if the UAC is not
changing the IP Address  and Port in Contact, VIA, "c=" and "m=" fields.



Best regards,

Mohit Soni


From: discussion-bounces at sipforum.org
[mailto:discussion-bounces at sipforum.org] On Behalf Of rahul sankanur
Sent: Tuesday, October 01, 2013 4:30 PM
To: discussion at sipforum.org
Subject: [SIPForum-discussion] Change of contact field when behind NAT


Hello all,


When we are behind NAT, we need to change the 'c=' field and the 'm=' field
for changing the ip address and port respectively.


I wanted to know if we have to change the contact field in the message
header when we are behind NAT..Does the rfc specify the change ?


Best regards,

Rahul Sankanur

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