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UDP/RTP PL96 in a routed network

Good day, I have deployed an intercom like phone system in a company wide network with subnets. There is central dispatcher who can talk to any remote site through a UDP/RTP stream with opus encoded payload. The voice transmission works perfectly well, no matter how I set the payload type (either 0 or 96).

Now I wanted to add an "in service quality monitoring" feature to capture UDP/RTP on the embedded Linux MCU's at the remote sites. So I have installed Tshark at the remote site MCU's.

In my laboratory tests I had only one subnet. No matter how I was setting the PL type (0 or 96) I could always capture the UDP/RTP packets at the remote end.

After rolling out the new SW in the customers network I observe this:

  • encoding/decoding in the same subnet: capture works for both PL types (0 and 96)
  • encoding/decoding in different subnets: capture only works if PL type is 0 (zero)
  • encoding/decoding in different subnets: no capture for PL 96 but voice streaming is ok (packets are received in the destination subnet)

For me that is mysterious?

For me PL 96 would be a nice feature because I found an article how to decode captured *.pcap to an audio file.

In that case the admin of the system would be able not only to analyze the stream, but also to listen to the stream how it is received by the user at the remote end.

I would be happy to receive an advice.