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

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Date:   Sun, 23 Jul 2017 22:37:35 -0400
From:   Neal Cardwell <ncardwell@...gle.com>
To:     Yuchung Cheng <ycheng@...gle.com>
Cc:     Lisong Xu <xu@....edu>, Wei Sun <unlcsewsun@...il.com>,
        netdev <netdev@...r.kernel.org>
Subject: Re: A buggy behavior for Linux TCP Reno and HTCP

On Sun, Jul 23, 2017 at 10:36 PM, Neal Cardwell <ncardwell@...gle.com> wrote:
> On Fri, Jul 21, 2017 at 5:16 PM, Yuchung Cheng <ycheng@...gle.com> wrote:
>> On Fri,  Jul 21, 2017 at 1:46 PM, Neal Cardwell <ncardwell@...gle.com> wrote:
>>> 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
>> I would prefer the former b/c loss_cwnd may not be universal TCP
>> state, just like ssthresh carries no meaning in some CC (bbr). It also
>> seems also more consistent with the recent change on undo
>>
>> commit e97991832a4ea4a5f47d65f068a4c966a2eb5730
>> Author: Florian Westphal <fw@...len.de>
>> Date:   Mon Nov 21 14:18:38 2016 +0100
>>
>>     tcp: make undo_cwnd mandatory for congestion modules
>>
>
> You are certainly right that it is more pure to keep a CC detail like
> that inside the CC module.
>
> But it's a bit sad to me that we have 9 separate identical
> implementations of a cwnd undo function, and that approach would add 6
> more.
>
> We do have tp->snd_ssthresh and tp->prior_ssthresh, even though not
> all CC modules use ssthresh.
>
> What if we call the field tp->prior_cwnd? Then at least we'd have some
> nice symmetry:
>
> - tp->snd_cwnd,  which is saved in tp->prior_cwnd  (and restored upon undo)
> - tp->snd_ssthresh,  which is saved in tp-> prior_ssthresh  (and
> restored upon undo)
>
> That sounds appealing to me. WDYT?

And, I should add, if we go with the tp->prior_cwnd approach, then we
can have a single "default"/CUBIC-style undo function, instead of 15
separate but identical implementations...

neal