netdev - Re: [PATCH net 5/5] tcp: send real dupACKs by locking advertized window for non-SACK flows

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Date:   Wed, 07 Mar 2018 07:58:22 -0800
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Ilpo Järvinen <ilpo.jarvinen@...sinki.fi>,
        netdev@...r.kernel.org
Subject: Re: [PATCH net 5/5] tcp: send real dupACKs by locking advertized
 window for non-SACK flows

On Wed, 2018-03-07 at 14:59 +0200, Ilpo Järvinen wrote:
> Currently, the TCP code is overly eager to update window on
> every ACK. It makes some of the ACKs that the receiver should
> sent as dupACKs look like they update window update that are
> not considered real dupACKs by the non-SACK sender-side code.
> 
> Make sure that when an ofo segment is received, no change to
> window is applied if we are going to send a dupACK. It's ok
> to change the window for non-dupACKs (such as the first ACK
> after ofo arrivals start if that ACK was using delayed ACKs).

This looks dangerous to me.

We certainly want to lower the window at some point, or risk expensive
pruning and/or drops.

It is not clear by reading your changelog/patch, but it looks like some
malicious peers could hurt us.

By current standards, non TCP sack flows are not worth any potential
issues.