[Freeswitch-users] codec negotiation error

Brian May brian at linuxpenguins.xyz
Mon Aug 8 04:32:55 UTC 2022

Piotr Gregor <piotr at dataandsignal.com> writes:

> You can start from checking if these packets are sent at all, and if you
> get those packets at all.
> Use wireshark or tcpdump.
> Do the same on both machines, you will see if and where RTP is sent/recved.
> If all RTP is sent but it cannot reach your FreeSWITCH, then check ports
> are allowed in the firewall.

But how do I know if the remote server which I don't have access to is
actually sending the RTP? I don't appear to see anything. This was a big
unknown. Is it sending? Is it sending to the correct address? etc.

I think I have it working now. Although somewhat confused, because I
tried this same setup earlier - or I thought I had - and it didn't work.

My theory:

* I could see outgoing RTP packets but not incoming RTP packets.

* Outboard RTP packets were being blocked by my firewall (EdgeRouter),
  because its stateful connection tracking doesn't seem to be working

* Remote server decided "seeing as I am not receiving any RTP packets I
  am not going to bother sending any either". Actually this bit seems a
  bit suspicious.

* By whitelisting 1024-65535 UDP outgoing (incoming was already
  whitelisted), it works. Which isn't great solution, but maybe the best
  I can do for now.

Result: It looks like incoming packets is the problem, but in actual
fact outgoing packets is the problem.

Also curious, when I run tcpdump on the EdgeRouter, it doesn't show any
of these UDP packets, even when there must be some because I have two
way audio communications. So the one really good tool I have to debug
firewall issues is actually very misleading.

There are some points in this theory that don't entirely add up (such as
the bandwidth graphs for the Internet port on the Internet side of the
firewall seemed to increase when blocked voice session in progress), but
I think the important part is that I do have this working again.
Brian May <brian at linuxpenguins.xyz>

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