netdev - Re: [PATCH net-next] tcp: improve undo on timeout

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Date:	Fri, 22 Aug 2014 21:28:18 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	ycheng@...gle.com
Cc:	ncardwell@...gle.com, netdev@...r.kernel.org
Subject: Re: [PATCH net-next] tcp: improve undo on timeout

From: Yuchung Cheng <ycheng@...gle.com>
Date: Fri, 22 Aug 2014 14:15:22 -0700

> Upon timeout, undo (via both timestamps/Eifel and DSACKs) was
> disabled if any retransmits were still in flight.  The concern was
> perhaps that spurious retransmission sent in a previous recovery
> episode may trigger DSACKs to falsely undo the current recovery.
> 
> However, this inadvertently misses undo opportunities (using either
> TCP timestamps or DSACKs) when timeout occurs during a loss episode,
> i.e.  recurring timeouts or timeout during fast recovery. In these
> cases some retransmissions will be in flight but we should allow
> undo. Furthermore, we should only reset undo_marker and undo_retrans
> upon timeout if we are starting a new recovery episode. Finally,
> when we do reset our undo state, we now do so in a manner similar
> to tcp_enter_recovery(), so that we require a DSACK for each of
> the outstsanding retransmissions. This will achieve the original
> goal by requiring that we receive the same number of DSACKs as
> retransmissions.
> 
> This patch increases the undo events by 50% on Google servers.
> 
> Signed-off-by: Yuchung Cheng <ycheng@...gle.com>
> Signed-off-by: Neal Cardwell <ncardwell@...gle.com>

Looks good, applied, thanks!
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