On the https://portal.plus.net/support/broadband/speed_guide/traffic_management.shtml page it says that plusnet won't correct identify traffic using non-standard ports. However it doesn't actually specify which ports map to which plusnet categories, which isn't terribly helpful. Is there a more useful breakdown somewhere? I'm asking because I'm seeing performance issues with VPN. I suspect it's a traffic management issue, but without knowing the exact port mappings I can't confirm this.
That page doesn't say traffic from applications using non-standard ports *will not* be correctly classified, just that it *can* happen. Wireshark enables you to check. If you are unfamiliar with Wireshark and traffic prioritisation marking see article Using Wireshark for Data Capture.
