netdev - Re: [PATCH net 4/5] tcp: prevent bogus undos when SACK is not enabled

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Message-ID: <alpine.DEB.2.20.1803131201160.29120@whs-18.cs.helsinki.fi>
Date:   Tue, 13 Mar 2018 12:24:50 +0200 (EET)
From:   Ilpo Järvinen <ilpo.jarvinen@...sinki.fi>
To:     David Miller <davem@...emloft.net>
cc:     Yuchung Cheng <ycheng@...gle.com>, ncardwell@...gle.com,
        Netdev <netdev@...r.kernel.org>, edumazet@...gle.com
Subject: Re: [PATCH net 4/5] tcp: prevent bogus undos when SACK is not
 enabled

On Fri, 9 Mar 2018, David Miller wrote:

> From: Ilpo Järvinen <ilpo.jarvinen@...sinki.fi>
> Date: Fri, 9 Mar 2018 16:11:47 +0200 (EET)
> 
> > Unfortunately I don't have now permission to publish the time-seq
> > graph about it but I've tried to improve the changelog messages so
> > that you can better understand under which conditions the problem
> > occurs.
> 
> It is indeed extremely unfortunate that you wish to justify a change
> for which you cannot provide the supporting data at all.

Here is the time-seqno graph about the issue:

https://www.cs.helsinki.fi/u/ijjarvin/linux/nonsackbugs/recovery_undo_bug.pdf

First the correct CC action (wnd reduction) occurs; then bogus undo 
causes bursting back to the window with which the congestion losses 
occurred earlier; because of the burst, some packets get lost due to 
congestion again.

The sender is actually somewhat lucky here: If only one packet would get 
lost instead of three, the same process would repeat for the next recovery 
(as cumulative ACK to high_seq condition would reoccur).

-- 
 i.