From: Damien Miller <djm@mindrot.org>
Subject: Re: sysctl: enable ECN by default?
To: Tim Leslie <tleslie@protonmail.com>
Cc: "tech@openbsd.org" <tech@openbsd.org>
Date: Mon, 14 Jul 2025 04:41:31 +1000

On Sat, 12 Jul 2025, Tim Leslie wrote:

> Trivial patch makes ECN (RFC 3168) enabled by default in OpenBSD. ECN
> has been in-tree since 2002 but remains off by default; early rollout
> challenges in a few middle-boxes slowed adoption.
>
> Enabling ECN end-to-end typically reduces queuing delay and packet
> loss when both endpoints signal support. Network infrastructure today
> generally supports marking instead of drop. Most TCP stacks will
> honor CE marks when they receive them, even if they didn’t originate
> ECT. CPU overhead is negligible, and we fall back automatically to
> loss-based TCP on non-ECN peers.

Sure, it's supported but does any network infrastructure enable it
by default? AFAIK ECN is only a cost if nothing on-path is actually
doint congestion marking.