3CX and SDP changes

[aliziad]

I have built a SIP Client/VOIP softphone into one of our products. The application softphone feature needs to dial out and accept voip calls via SIP. Our product would run alongside a SIP server so we reviewed some windows SIP servers and came across 3CX which seems featureful and easy to use.

However, I am having a serious issue with a 3CX behavior that appears to be configuration related but not sure how to configure differently.

While establishing a call in SIP the communication endpoints exchange a session description protocol (SDP) message which defines the IP and Port numbers to where to send RTP media to.

If I run 3CX on the same host as my in-house softphone, the SDP's appear to get modified by 3CX. While testing against a linksys phone, the SDP IP comes in as the local loopback address 127.0.0.1! which cannot be right and I cannot send audio to it. The port numbers appear to be modified too, I am explicitly setting the port numbers and would like them to stay as configured by my application but again they get remapped somehow.

If I run 3CX on another host then everything works fine. For our final product I would not want to require a second physical server and I would love to get around this configuration issue.

Is 3CX really modifying the SDP's and is there a way to turn that off ?

Thanks,
Ali Ziad

 

[archie]

Sorry for question, but why you can not send to 127.0.0.1? Any softphone can.

 

[aliziad]

Because I want to send my audio out to the other person, not talk to myself I am getting 127.0.0.1 instead of the remote peer's IP address.

Can you elaborate on the logic used for modifying the IP/Port numebrs?

Whats the relationship the RTP port modifications have to do with "Allow non-sequential ports" in the configuration screen?

Are there any configuration options I should be aware of that deal with firewalls that alter SDP contents?

thanks,
-ali

 

[archie]

Because I want to send my audio out to the other person, not talk to myself I am getting 127.0.0.1 instead of the remote peer's IP address.

You're running your phone on the same box with 3CX PBX. So, sending to 127.0.0.1 can also mean sending to 3CX. Have you noticed that port in SDP is not the port you're listening on?

Can you elaborate on the logic used for modifying the IP/Port numebrs?

When PBX decides to pass audio thru 3CX Media Server - it modifies SDP.

 

[aliziad]

Yes I noticed the ports and IPs are altered by the proxy which renders it useless for us if we cannot disable this behavior.