Subject: [iccrg] cwnd 4 in BBRv2?

From: Dave Taht <[email-masked]>
Date: Thu, 28 Mar 2019 10:09:48 -0700

I just finished watching the sessions in ietf iccrg...

and although I have many questions about BBRv2, I am curious if the
cwnd in it is capped to a minimum of 4, not 2, still? BBRv1 is...

https://github.com/torvalds/linux/blob/master/net/ipv4/tcp_bbr.c#L385

-- 

Dave Täht
CTO, TekLibre, LLC
http://www.teklibre.com
Tel: 1-831-205-9740

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From: Neal Cardwell <[email-masked]>
Date: Fri, 29 Mar 2019 01:03:05 +0100

> https://github.com/torvalds/linux/blob/master/net/ipv4/tcp_bbr.c#L385

So far that part of the algorithm has not changed in BBRv2. So, except
for certain corner cases in loss recovery (e.g. cwnd=1 on RTO), the
min cwnd is generally 4.

But keep in mind that BBR generally tries to be pacing-limited, rather
than cwnd-limited. And below a cwnd of 4 delayed ACKs can inhibit
pipelining, causing stop-and-wait-type problems that significantly
hurt throughput.

best,
neal