netdev - Re: [PATCH net-next 1/7] tcp: apply Kern's check on RTTs used for congestion control

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Date:	Tue, 2 Feb 2016 15:28:54 -0800
From:	Yuchung Cheng <ycheng@...gle.com>
To:	Kenneth Klette Jonassen <kennetkl@....uio.no>
Cc:	David Miller <davem@...emloft.net>,
	netdev <netdev@...r.kernel.org>,
	Neal Cardwell <ncardwell@...gle.com>,
	Eric Dumazet <edumazet@...gle.com>
Subject: Re: [PATCH net-next 1/7] tcp: apply Kern's check on RTTs used for
 congestion control

On Tue, Feb 2, 2016 at 11:30 AM, Kenneth Klette Jonassen
<kennetkl@....uio.no> wrote:
>
> On Sat, Oct 17, 2015 at 6:57 AM, Yuchung Cheng <ycheng@...gle.com> wrote:
> > Currently ca_seq_rtt_us does not use Kern's check. Fix that by
> > checking if any packet acked is a retransmit, for both RTT used
> > for RTT estimation and congestion control.
> >
> *snip*
>
> This patch (commit 9e45a3e) puzzles me, because Karn's check was
> already in effect:
> http://lxr.free-electrons.com/source/net/ipv4/tcp_input.c?v=4.3#L3117
>
> Since first_ackt/last_ackt is only set when non-retransmitted packets
> are ACKed (sequentially), we know them to be unambiguous samples for
> RTTM. Even if a (sequential) ACK covers retransmitted packets, we can
> still make a valid RTTM if that ACK also covers non-retransmitted
> packets. But this patch seems to prevent that?
Perhaps the commit message is not clear. Here is an example: an ACK
acks 2 packets where the 1st packet was retransmitted but the 2nd
packet is not.

Since we don't know if the ACk was caused by the retransmission (plugs
a hole) or by the original, we should not take an RTT sample. In other
words, we should refrain from taking RTT sample as long as the ACK
covers any retransmitted sequence.