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

samuel samu60 at gmail.com
Mon Feb 19 12:57:51 UTC 2007

SIP Proxy detects that both client are behind the same IP and the
associated NAT transversal solution (RTPProxy) is not needed, that is
why there is such attribute in the SDP. (no rtp proxy = yes).

It is expected that the router takes care of the media of the clients
connected in the same LAN but some of them does not do it properly.
The "technical" term for this behaviour is that the router must
support hairpining (if I recall properly).

You've got 2 options:
1)either does not check wether SIP proxy clients are behind the same
public IP and  use RTPproxy also in these situations.
2)Configure/Change the NAT Router for one supporting hairpining.

If you go for option 1 you'll provide a more reliable solution (you
will avoid more similar cases) at the expenses of increasing RTP proxy
use and therefore costs...it's up to you.

Hope it helps,

2007/2/19, Naveen Gujje <gujjenaveen at gmail.com>:
> 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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