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

[Naveen Gujje]

Hi Samuel,

Now i understood for what this nortpproxy:yes is used. Thanks for the
explanation.

My router supports the so-called "hairpining" concept. And i am
implementing SIP ALG in this router.

Here the problem observed is,

1) The connection information (c:) part of SDP is modified with some
server ip by iptel.org before forwarding INVITE/200OK to the other
side. And this ip is same in both INVITE and 200OK.
2). And a new field is added as a:nortpproxy:yes

If the server doesn't change the connection information part of SDP,
then packet reaches other client with Nat ip and everything works
fine.

So, how should i avoid this problem. Is there any way to modify the
packet so that server doesn't alter it? OR, is there any way to make
the server do proxying instead of router?

Thanks in advance,
Naveen G.

On 2/19/07, samuel <samu60 at gmail.com> wrote:
> 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,
> Samuel.
>
> 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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