netdev - Re: [PATCH net-next] Avoid reducing cwnd when ACK+DSACK is received

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Date:	Fri, 12 Dec 2014 10:54:11 -0500
From:	Neal Cardwell <ncardwell@...gle.com>
To:	Sébastien Barré <sebastien.barre@...ouvain.be>
Cc:	David Miller <davem@...emloft.net>,
	Netdev <netdev@...r.kernel.org>,
	Gregory Detal <gregory.detal@...ouvain.be>,
	Nandita Dukkipati <nanditad@...gle.com>,
	Yuchung Cheng <ycheng@...gle.com>
Subject: Re: [PATCH net-next] Avoid reducing cwnd when ACK+DSACK is received

On Thu, Dec 11, 2014 at 2:58 PM, Sébastien Barré
<sebastien.barre@...ouvain.be> wrote:
> When the peer has delayed ack enabled, it may reply to a probe with an
> ACK+D-SACK, with ack value set to tlp_high_seq. In the current code,
> such ACK+DSACK will be missed and only at next, higher ack will the TLP
> episode be considered done. Since the DSACK is not present anymore,
> this will cost a cwnd reduction.
>
> This patch ensures that this scenario does not cause a cwnd reduction, since
> receiving an ACK+DSACK indicates that both the initial segment and the probe
> have been received by the peer.
>
> Cc: Gregory Detal <gregory.detal@...ouvain.be>
> Cc: Nandita Dukkipati <nanditad@...gle.com>
> Cc: Yuchung Cheng <ycheng@...gle.com>
> Signed-off-by: Sébastien Barré <sebastien.barre@...ouvain.be>

BTW, I like this idea, and I'll test it.

Two suggestions for the next iteration:

(1) for TCP patch style, I'd suggest a commit first-line like:

  tcp: avoid reducing cwnd when ACK+DSACK is received

(2) Your patch is essentially a reworking of the is_tlp_dupack
computation. So rather than splitting the logic between the
is_tlp_dupack computation and the new expression inside the if
condition, I'd suggest moving and centralizing all the logic in the if
condition. There is no real need for the is_tlp_dupack variable.

neal
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