netdev - Re: [PATCH v3 net 4/5] tcp: prevent bogus undos when SACK is not enabled

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Message-ID: <alpine.DEB.2.20.1804041311030.29120@whs-18.cs.helsinki.fi>
Date:   Wed, 4 Apr 2018 13:23:56 +0300 (EEST)
From:   Ilpo Järvinen <ilpo.jarvinen@...sinki.fi>
To:     Yuchung Cheng <ycheng@...gle.com>
cc:     netdev <netdev@...r.kernel.org>,
        Neal Cardwell <ncardwell@...gle.com>,
        Eric Dumazet <edumazet@...gle.com>,
        Sergei Shtylyov <sergei.shtylyov@...entembedded.com>
Subject: Re: [PATCH v3 net 4/5] tcp: prevent bogus undos when SACK is not
 enabled

On Wed, 28 Mar 2018, Yuchung Cheng wrote:

> On Tue, Mar 13, 2018 at 3:25 AM, Ilpo Järvinen
> <ilpo.jarvinen@...sinki.fi> wrote:
> > When a cumulative ACK lands to high_seq at the end of loss
> > recovery and SACK is not enabled, the sender needs to avoid
> > false fast retransmits (RFC6582). The avoidance mechanisms is
> > implemented by remaining in the loss recovery CA state until
> > one additional cumulative ACK arrives. During the operation of
> > this avoidance mechanism, there is internal transient in the
> > use of state variables which will always trigger a bogus undo.
>
> Do we have to make undo in non-sack perfect? can we consider a much
> simpler but imperfect fix of
> 
> diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
> index 8d480542aa07..95225d9de0af 100644
> --- a/net/ipv4/tcp_input.c
> +++ b/net/ipv4/tcp_input.c
> @@ -2356,6 +2356,7 @@ static bool tcp_try_undo_recovery(struct sock *sk)
>                  * fast retransmits (RFC2582). SACK TCP is safe. */
>                 if (!tcp_any_retrans_done(sk))
>                         tp->retrans_stamp = 0;
> +               tp->undo_marker = 0;
>                 return true;
>         }

Yes, that's of course a possible and would workaround the issue too. 
In fact, I initially did that kind of fix for myself (I put it into a 
block with tp->retrans_stamp = 0 though). But then I realized that it is 
not that complicated to make the fix locally into tcp_packet_delayed()
(except the annoyance of passing all the necessary state parameters
through the deep static call-chain but that should pose no big challenge 
for the compiler to handle I guess).

BTW, do you know under what circumstances that tcp_any_retrans_done(sk) 
would return non-zero here (snd_una == high_seq so those rexmit 
would need to be above high_seq)?




-- 
 i.