On 2/1/06, Carla Ares <carla.ares@gmail.com> wrote:

Hi all,

I have a problem with audio, when the destination of a call is natted and the other side is public. This problem is cause because the SDP info only is present in the 200 OK and in the ACK message.

I'm using ser ( 8.14) with mediaproxy (1.2.1).

I have an H323 to SIP translator (and SIP to H323 too) connected to the PSTN and to the SIP Proxy (SER).
H323 is the origin of the call (public client), and SIP UAC is the destination (Natted client).
My proxy is working in state full mode and has a public IP.

The H323 side is using slow start, so when it starts the transaction with an INVITE, it will not send the SDP info (it will send it in the ACK message).
The proxy forwards this message to the destination.
SIP UAC, starts ringing. When we hooks off, the SIP UAC send an 200 OK with SDP message to the H323 side.

This is the 200 OK / SDP that the proxy receives from the SIP side.

No.     Time        Source                Destination           Protocol Info
    488 28.488811   200.68.89.2           200.68.89.12          SIP/SDP Status: 200 OK, with session description
    Message body
        Session Description Protocol
            Session Description Protocol Version (v): 0
            Owner/Creator, Session Id (o): 700600 6015 6015 IN IP4 192.168.0.101
                Owner Username: 700600
                Session ID: 6015
                Session Version: 6015
                Owner Network Type: IN
                Owner Address Type: IP4
                Owner Address: 192.168.0.101
            Session Name (s): AddPac Gateway SDP
            Connection Information (c): IN IP4 192.168.0.101
                Connection Network Type: IN
                Connection Address Type: IP4
                Connection Address: 192.168.0.101
            Time Description, active time (t): 0 0
                Session Start Time: 0
                Session Stop Time: 0
            Media Description, name and address (m): audio 23018 RTP/AVP 18 8 0 101
                Media Type: audio
                Media Port: 23018
                Media Proto: RTP/AVP
                Media Format: ITU-T G.729
                Media Format: ITU-T G.711 PCMA
                Media Format: ITU-T G.711 PCMU
                Media Format: 101

Following the ser.cfg, this 200 OK / SDP will make the proxy to use mediaproxy (use_media_proxy()), because the destination user is natted. So the mediaproxy module, will generate a lookup command to the proxydispatcher.py.

proxydispatcher[30535]: command lookup 2281401749@200.68.89.10 192.168.0.101:23018:audio 200.68.89.2 200.68.89.10 remote 200.68.89.10 unknown AddPac=20SIP=20Gateway info=from:1150316660@200.68.89.10, to:1152464490@200.68.89.10,fromtag:3512844671,totag:7b07717a4
proxydispatcher[30535]: warning: trying to lookup session with non-existent id: ' 2281401749@200.68.89.10'

The proxydispatcher does not recognize that command because it has not generate the session before. So, the proxy forwards the 200 OK /SDP message without changing the SDP info. When the H323 sides receives that info, it thinks that it has to send RTP to the private IP.

Then the H323 sends the ACK with SDP.

Then, we cannot ear audio in both sides.

Here is the problem, because, I didn't receive an INVITE with SDP that can create the session into the dispatcher.

A is public, B is natted

side A            Proxy            side B

    INVITE without SDP
    ---------->
    100 Trying
    <----------

                INVITE without SDP
                ---------->

                100 Trying
                <----------
                180 Ringing
                <----------
    180 Ringing
    <----------

                200 OK SDP (B private IP)
                <----------
            *1*
    200 OK SDP (B private IP)
    <----------

    ACK SDP (A public IP)
    ---------->
            *2*
                ACK SDP (mediaproxy public IP)
                ---------->

......................................................
            RTP from B
            <----------------------
    *3*
......................................................

*1* The proxy must replace the private IP of the SDP, with the ip of the mediaproxy. The looku prequest does not work, because no session was found
*2* the proxy generates the session in the mediaproxy. So B thinks that the RTP must go to the mediaproxy. But A never knows the mediaproxy address.
*3* RTP from A goes to anywhere, because A does not know where it is the private adrress of B.

Do you know a solution to this? Is the sequence of the message all right?

Regards.
Carla

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