[tsvwg] Disable ECN on VPNs, really?

[Sebastian Moeller <[e-mail]>]

Dear list,

in today's tsvwg session chat, Mirja proposed that home-users could simply disable ECN negotiation for VPN tunnels to accommodate rfc3168 AQMs on the path and L4s flows in the tunnel.
That is certainly an option, as would be to try to disable L4S for all home nodes or trying tp bleach ECT(1) on egress and ingress).
	Currently e.g. SQM (distributed as an OpenWrt installable package) employs an ECN fq_AQM at the ingress of home links to great success. The rationale to use ECN here is, that all packets entering the ingress AQM have already traversed the true bottleneck, so dropping them would simply just waste the "transmit slot" on the bottleneck that they already used-up and it would even delay the initiation of the please-slow-down signaling, as we need a few dupACKs to detect congestion from drop, while CE is more immediate (this is a simplification, sure).
	In short even classic ECN on a home link's ingress has immense value, and that is true for all packets, including packets in a tunnel, as these also consumed transmit slots when the AQM needs to decide whether to drop/mark. And more, that is an already deployed solution out in the field that works pretty well, you might want to try it ;) before declaring it obsolete and exchange it with the yet unproven promises of L4S.

Best Regards
	Sebastian