[SIPForum-discussion] For what purpose a:nortpproxy:yes is used???

Naveen Gujje gujjenaveen at gmail.com
Tue Feb 13 15:06:01 UTC 2007

Hi All, go through the problem below and suggest me your view.

The setup is as follows.

                          |                           |
           H1 ----------|                           |-------{ SIP Proxy }
                          |---- NAT Router ----|           ( Iptel )
                          |                           |
           H2 ----------|

 Private Network                                   Public Network [internet]

H1 & H2 are registering with external SIP Proxy server.

When H1 is calling H2, it is sending INVITE request which contains H1's
Private IP in the connection information part of SDP message to SIP Proxy.
Before sending it out, NAT router replaces this ip with Nat Ip. And before
SIP Proxy forwarding this INVITE to H2, it is replacing the Nat Ip in
connection information part of SDP message with some server ip and also
adding a new media attribute "a:nortpproxy:yes".

This behaviour of SIP Proxy is also observed in case of 200 OK message which
is sent by H2 as a response to INVITE of H1.

And in this case, the IP in the connection information part of SDP message,
which is replaced by SIP Proxy, is same in INVITE & 200 OK.

Due to this, both H1 & H2 are sending RTP traffic to the new ip which is
replaced by SIP Proxy and the SIP Proxy itself is not doing any kind of
proxying. With this voice flow is failing.

So, can anyone explain me the reason for this type of behaviour by SIP
Proxy. And also, what is the purpose of media attribute a:nortpproxy:yes ?

The SIP Proxy is IPTEL and Clients are SJPhones.

Looking forward for your responses,
Naveen G.
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