[Cake] suggest not to use udp with iperf3 versions prior to 3.1.5

[Pete Heist]

Just spent an hour chasing down a strange problem with gradually increasing UDP packet loss with cake (but not fq_codel), which happens when testing with iperf3 3.1.3, but not iperf or irtt. After compiling iperf3 from HEAD, poof, the problem disappeared. It’s not just iperf3’s -l flag as I know it’s default was too high in 3.1.3 and causing fragmentation, but some other UDP handling issue with iperf3.

I still don’t know the reason why it would only happen with cake+iperf3 3.1.3 and not fq_codel+iperf3 3.1.3 (and go away when ‘cake interplanetary’ is used), but I’ll rather leave it alone now and suggest not to use iperf3 < 3.1.5 for UDP...

Pete