netdev - Re: TCP NewReno and single retransmit

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Message-ID: <CAK6E8=cgkznkAWTps7aA+txuETpZ2RNiU3rbQf9WxLjawhgNog@mail.gmail.com>
Date:	Tue, 4 Nov 2014 15:59:03 +0800
From:	Yuchung Cheng <ycheng@...gle.com>
To:	Neal Cardwell <ncardwell@...gle.com>
Cc:	Marcelo Ricardo Leitner <mleitner@...hat.com>,
	netdev <netdev@...r.kernel.org>,
	Eric Dumazet <edumazet@...gle.com>
Subject: Re: TCP NewReno and single retransmit

On Tue, Nov 4, 2014 at 7:17 AM, Neal Cardwell <ncardwell@...gle.com> wrote:
> On Mon, Nov 3, 2014 at 4:35 PM, Marcelo Ricardo Leitner
> <mleitner@...hat.com> wrote:
>> So by sticking with the recovery for a bit longer will help disambiguate the
>> 3 dupacks on high_seq, if they ever happen, and with that avoid re-entering
>> Fast Retransmit if initial (2) happen. It's at the cost of leaving the fast
>> retransmit an ack later but if (2) happens the impact would be much worse,
>> AFAICS.
>
> Yes, that's my sense.
>
>> Cool, thanks Neal. If Yuchung is okay with it, I'll proceed with just
>> zeroing that tstamp as initially proposed.
>
> Yes, that sounds good to me for a short-term fix for the "net" branch,
> as long as it's:
>
> +  if (!tcp_any_retrans_done(sk))
> +    tp->retrans_stamp = 0;
>
> Longer-term ("net-next"?) perhaps it makes sense to remove the hold
> state and protect against this spurious FR situation at the time we
> decide to enter Fast Recovery, which seems to be the model the RFC is
> suggesting.
Thanks for checking. So my suggested fix of removing the hold state is
the "less careful variant" that RFC does not recommend. I would rather
have the proposed 2-liner fix than implementing the section 6
heuristics to detect spurious retransmit. It's not worth the effort.
Everyone should use SACK.

>
> neal
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