It seems the twamp server always tries to use the port proposed by the client. This is a problem if only testports range is allowed through the firewall. The ports used can be seen in the twping output:
$ twping 192.168.122.100
Approximately 13.0 seconds until results available
Directional delays may be inaccurate due to out of sync clocks!
--- twping statistics from [192.168.122.100]:8939 to [jessie-amd64]:8939 ---
SID: c0a87a64de2ff44d4097618a4044ac29
first: 2018-02-15T12:58:06.746
...
In this example the server used port 8939 although the testports was set to 18760-19960.
vvidic
commented
6 years ago
It seems the twamp server always tries to use the port proposed by the client. This is a problem if only
testportsrange is allowed through the firewall. The ports used can be seen in thetwpingoutput:In this example the server used port
8939although thetestportswas set to18760-19960.