[Cake] Cake diffserv3 stalling when all ingress marked with CS1

[Jonathan Foulkes]

Hi Everyone, 

It looks like the little issue with some ISPs re-marking all inbound (to the modem) traffic with CS1 is cropping up randomly.

This is causing what seems to be a bug in Cake with a multi-tier config (diffserv3 in this case).

I have some users on lines that had been working fine for years with various iterations of Cake, but all of the sudden, in the past two weeks, started showing strange issues with large delays every couple of seconds and even lockups on traffic flowing on the LAN (the router UI was not reachable).
Turns out that’s about when the ISP started remarking everything with CS1.

Switching cake to best-effort mode (single-tin) fixes the issue, as does disabling QoS altogether.

We can also make difserv3 work by enabling the ‘strip dscp on inbound’ (wash) option.

My question is: why does cake lock up or have regular hiccups if all inbound traffic is CS1?

Platform is OpenWRT 19.07.6 on a MT7621/7603 router with 512MB of RAM.

Jonathan Foulkes