netdev - Re: TCP NewReno and single retransmit

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Date:	Tue, 04 Nov 2014 11:12:50 -0200
From:	Marcelo Ricardo Leitner <mleitner@...hat.com>
To:	Yuchung Cheng <ycheng@...gle.com>,
	Neal Cardwell <ncardwell@...gle.com>
CC:	netdev <netdev@...r.kernel.org>, Eric Dumazet <edumazet@...gle.com>
Subject: Re: TCP NewReno and single retransmit

On 04-11-2014 05:59, Yuchung Cheng wrote:
> 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.

Yup, agreed.

Thanks,
Marcelo

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