[Ecn-sane] BBRv3 ecn_low per-route feature and the CE confusion

[Sebastian Moeller]

Hi Pete,


> On Nov 28, 2023, at 13:57, Pete Heist via Ecn-sane <ecn-sane at lists.bufferbloat.net> wrote:
> 
> Regarding the per-route ecn_low feature in BBRv3:
> https://github.com/google/bbr/blob/v3/README.md#introducing-the-ecn_low-per-route-feature
> 
> When set, any incoming CE marks from an ecn_low flagged route will be
> treated as an L4S/RFC9331 CE, while CE marks from routes without that
> flag will be treated as a regular RFC3168 CE.

	I am probably blind, but looking at https://github.com/google/bbr/blob/v3/net/ipv4/tcp_bbr.c
I do not see this fall back to a rfc3168 compliant response, as far as I read this this is DCTCP or nothing... 
Or did you just mean that BBRv3 will treat CE just like BBRv1 and silently ignore it?



> Obviously this is
> problematic, as the two CE marks mean very different things, and
> outside of closed environments, there's no reliable way to know which
> type of CE you're receiving. The consequences of confusing them range
> from massive self-induced bloat, to driving competing traffic in the
> same queue down to minimum cwnd. The solution here is to just punt, so
> at least tests and demos can be made to work.

	The per-route feature looks like a desirable safety... as does the default 5ms rtt "low-pass", where IIUC ECN will only be evaluated if the RTT is below 5ms...


> This isn't a problem with BBR. The problem is, we've started an
> experiment [RFC9331] that redefines the meaning of CE in a way that's
> incompatible with existing RFC3168 middleboxes. This feature in BBR is
> just a reflection of that. CCA developers are now tasked with somehow
> deciding which type of CE they're seeing.
> 
> Speaking of ecn "sane", does anyone else see this as not? :) and as a
> problem that needs solving?

	Yes, however I assume this will solve it self, as is this usage of CE is over-promising and under-delivering, so I do not see this "winning the internet" in spite of the amount of thrust is put behind it...

Regards
	Sebastian


> 
> Pete
> 
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