netdev - Re: A buggy behavior for Linux TCP Reno and HTCP

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Date:   Fri, 21 Jul 2017 16:49:14 -0400
From:   Neal Cardwell <ncardwell@...gle.com>
To:     Lisong Xu <xu@....edu>
Cc:     Yuchung Cheng <ycheng@...gle.com>, Wei Sun <unlcsewsun@...il.com>,
        netdev <netdev@...r.kernel.org>
Subject: Re: A buggy behavior for Linux TCP Reno and HTCP

On Fri, Jul 21, 2017 at 4:27 PM, Lisong Xu <xu@....edu> wrote:
>
> Hi Yuchung,
>
> This test scenario is only one example to trigger this bug. In general, as long as cwnd <4, the undo function has this bug.

Yes, personally I agree that this seems like an issue that is general
enough to be worth fixing. In the sense that, if cwnd <4, then we may
well be very congested. So we don't want to get hit by this bug
wherein an undo of a loss recovery can cause cwnd to suddenly jump
(from 1, 2, or 3) up to 4.

Seems like any of the several CCs that use tcp_reno_undo_cwnd() have this bug.

I guess in my mind the only question is whether we want to add a
tcp_foo_undo_cwnd() and ca->loss_cwnd to every CC module to handle
this issue (i.e. make every CC module handle it the way CUBIC does),
or (my preference) just add a tp->loss_cwnd field so we can use shared
code in tcp_reno_undo_cwnd() to get this right across all CC modules.

neal