[SIPForum-discussion] What is the significance of "a:nortpproxy:yes" attribute in SIP SDP message.

Naveen Gujje gujjenaveen at gmail.com
Mon Feb 19 12:18:33 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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