netdev - Re: [PATCH net-next v2] tcp: avoid reducing cwnd when ACK+DSACK is received

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Message-ID: <1420729656.5947.50.camel@edumazet-glaptop2.roam.corp.google.com>
Date:	Thu, 08 Jan 2015 07:07:36 -0800
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Sébastien Barré <sebastien.barre@...ouvain.be>
Cc:	David Miller <davem@...emloft.net>, netdev@...r.kernel.org,
	Gregory Detal <gregory.detal@...ouvain.be>,
	Nandita Dukkipati <nanditad@...gle.com>,
	Yuchung Cheng <ycheng@...gle.com>,
	Neal Cardwell <ncardwell@...gle.com>
Subject: Re: [PATCH net-next v2] tcp: avoid reducing cwnd when ACK+DSACK is
 received

On Thu, 2015-01-08 at 13:20 +0100, Sébastien Barré 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.


The most probable cause would not be a delayed ack (rto should be much
bigger), but a lost ACK.


>  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.

Do you have at hand a packetdrill test to demonstrate that the patch
works ?

Thanks !


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