[SR-Users] Need help understanding/separating signaling from media

[Grant Bagdasarian]

Alex wrote:

The Kamailio 'rtpproxy' module goes out to the rtpproxy service[1] and asks it to engage the call, saying: "Can you please allocate and provide me with a pair of ports for each end of this call?"  The rtpproxy process answers, and Kamailio then writes the IP and ports provided by rtpproxy into the SDP of both the requests and replies involved in setting up the call.



What if both endpoints are on different networks?



Endpoint 1 -------------- Kamailio -------------- Endpoint 2

         |                                      |                                   |

         |                                      |                                   |

         |                                      |                                   |

         +-------------------- RTPProxy -----------------+



Please correct me if I'm wrong:

1)      Endpoint 1 (10.0.0.1) sends an INVITE containing the offer to Kamailio (10.0.0.2).

2)      Kamailio asks RTPProxy to engage the call and allocate two ports.

3)      Kamailio writes the IP and port of the RTPProxy (10.0.0.3:1000) in the SDP of the 100 Trying and replies back to Endpoint 1.

4)      Kamailio (20.0.0.2) now sends the INVITE with offer to Endpoint 2 (20.0.0.1), but with modified SDP (20.0.0.3:1001).

5)      .....

6)      Endpoint 1 sends its media to 10.0.0.3:1000 and Endpoint 2 sends its media to 20.0.0.3:1001.

7)      RTPProxy bridges both ends.



What makes it so difficult for me to understand is that both Kamailio and RTPProxy are connected to two different networks. A call between endpoint 1 and 2 must be bridged by Kamailio and RTPProxy.

How does Kamailio know it has to tell RTPProxy to engage a call on network 10.0.0.0 and 20.0.0.0 and allocate two ports, 1 for each network?



Regards,



Grant



-----Original Message-----
From: sr-users-bounces at lists.sip-router.org [mailto:sr-users-bounces at lists.sip-router.org] On Behalf Of Alex Balashov
Sent: Monday, January 28, 2013 5:11 PM
To: sr-users at lists.sip-router.org
Subject: Re: [SR-Users] Need help understanding/separating signaling from media



Hi Grant,



On 01/28/2013 11:03 AM, Grant Bagdasarian wrote:



> I think that the RTP Proxy module does precisely this, but what I

> still don't understand is how the rtp stream is passed between these

> two agents for each call.



The Kamailio 'rtpproxy' module goes out to the rtpproxy service[1] and asks it to engage the call, saying: "Can you please allocate and provide me with a pair of ports for each end of this call?"  The rtpproxy process answers, and Kamailio then writes the IP and ports provided by rtpproxy into the SDP of both the requests and replies involved in setting up the call.



The result is that as far as both UAs are concerned, rtpproxy is their endpoint, and they will send media to it.  Kamailio instructs rtpproxy to bridge both streams.



When the call is torn down, Kamailio tells rtpproxy to disengage and deallocate the bridge mapping for those streams.



-- Alex



[1] Which may be running on the same host, or may be running on another host, in a distributed fashion.  Kamailio talks to rtpproxy through rtpproxy's UDP control socket, and more than one rtpproxy may be used, both for failover and round-robin load distribution.  This is readily baked into the rtpproxy module.



--

Alex Balashov - Principal

Evariste Systems LLC

235 E Ponce de Leon Ave

Suite 106

Decatur, GA 30030

United States

Tel: +1-678-954-0670

Web: http://www.evaristesys.com/, http://www.alexbalashov.com/



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