OCS Mediation server trying to communicate with Gateway using RTP on port 0 RRS feed

  • Question

  • Hi,


    When I make an outbound call from Communcator to a SIP phone on my IPPBX, the mediation server sends an initial RTP packet to the communicator client, followed by an RTP packet to port 0 of the IP-PBX.


    Packet 1: From Mediation ( to Communicator ( - OK

    Code Block

    Internet Protocol, Src: (, Dst: (

    User Datagram Protocol, Src Port: 63055 (63055), Dst Port: 16896 (16896)

    Real-Time Transport Protocol


    Packet 2: From Mediation ( to Gateway ( - BROKEN

    Code Block

    Internet Protocol, Src: (, Dst: (

    User Datagram Protocol, Src Port: 62980 (62980), Dst Port: 0 (0)

    Real-Time Transport Protocol


    The gateway obviously sends an ICMP Port Unreachable message back to the mediation server after this.


    So there are 2 problems with packet 2:

    1. It's going to the wrong place. The gateway is not the RTP endpoint (which is at

    2. Even if it was going to the correct host, it's going to the wrong port.


    As far as i know, port 0 is reserved, and shouldnt be used. I believe this packet should go to the other SIP phone, on the port specified in the SIP Session description. The IPPBX is not an RTP endpoint.


    It does the same thing with the RTCP packets. The reciever reports are sent to the SIP gateway ( on port, rather than to the other RTP endpoint in the coversation.


    I can't find anything where OCS is whinging about a problem that could be causing this. Debug traces dont reveal anything of interest (in the ones I examined).


    I'm not using one of the 'supported' OCS gateways, but I don't think that should make a difference where it seems OCS is breaking RFC guidelines.


    Does anyone have any ideas as to what is going on or where to look to get more information?


    Thanks in advance




    Sunday, November 11, 2007 1:31 AM

All replies

  • Hi Ryan, did you manage to solve this one as i have a similar problem where OCS is sending the voice to port 0, but only for outbound calls over the PSTN.





    Wednesday, November 21, 2007 3:34 PM
  • Hey Nick,


    Not yet, but i'm still working on it. I have seen it try and send all its RTP packets to port 0 as well, mine seems to have stablised to just the first RTP packet going to port 0 now.


    I think this might be happening all the time, just some endpoints deal with it better than others. Using one of the 'supported' dialogic gateways, it does the same thing, dialogic just seems to be able to compensate for the missing packet.


    Will update this thread when I have more info.




    Thursday, November 22, 2007 9:03 PM
  • Hi,


    I'm curious about your findings as well. In my setup I notice the same behaviour. It happens with two different gateways I'm using. With one, the RTP stream does start flowing, with the other one it doesn't. I suspect that the SDP anwer of the working gateway is accepted by the mediation server, while the response of the other gateway is not. But that's a wild guess.




    Tuesday, November 27, 2007 1:52 PM
  • Hi, Ryan,

    What's "unsupported" IP gateway you are using to test? I also connect mediation server to a linksys SPA3000 gateway and get the same result with yours. Mediation server send a 0 port TCP packet to gateway and gateway reply a ICMP unreachable packet and mediation server report "unable to establish connection to gateway"...

    Thursday, December 27, 2007 2:03 AM

    Hi Jun,


    I'm using Asterisk and sipx. I've since realised that this problem exists even with supported gateways. I took a wireshark capture of comms between OCS and a dialogic gateway, and OCS does the same thing. The dialogic gateway seems to be able to ignore/compensate for the problem.



    Sunday, December 30, 2007 5:13 AM
  • Hi Ryan,

    Same problem here !!

    I have a mediation server connected with an Audiocodes gateway (Mediant 1000 version 5.0).

    When you dial from the client comunicator the SIP session is established correctly (200 OK) but when the RTP flow starts the mediation's destination port is 0 (!!), in both in the gateway and in the mediation the port predefined is 60000.



    Thursday, March 6, 2008 8:28 PM
  • Can someone from Microsoft please address this issue? Its an issue that should be addressed. Offer a patch or update or something.

    Monday, July 7, 2008 3:17 AM