[Ecn-sane] ECN on Android and webOS

[Pete Heist <pete@heistp.net>]

Yesterday, I observed CE marks and ECE flags in a production
environment with an OM2P-HS WiFi AP (has the ath9k driver with the
airtime fairness changes signaling with ECN). This is the first time I
got a chance to look at this with WiFi in the wild.

There were two notable devices sending ECEs while streaming content:

1) DHCP client ID: "Galaxy-S21-Ultra-5G"

This is presumably a Samsung of that model, which was released earlier
this year and ships with Android 11.

2) DHCP client ID: "LGwebOSTV"

This apparently runs webOS, a Linux based smart TV OS that doesn't seem
to be based on Android.

Does anyone have confirmation that either Samsung or LG has enabled ECN
in some of their recent devices?

If you want to watch for CE marks or ECE flags, this worked for me:

tcpdump -ev -i <dev> -n \
	'(ip and (ip[1]&0x03 == 0x03))' or \
	'(tcp and (tcp[13] & tcp-syn == 0) and (tcp[13] & 0x40 != 0))'

It's also possible to watch for CWR (tcp[13] & 0x80).

My vantage point was upstream from the AP, where I usually saw no CE
marks (except occasional ones from WAN ingress), but I'd see ECE in
response, with the WiFi AP being the main apparent source of CE.

Pete