netdev - Re: [PATCH net] tcp: fix false undo corner cases

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Date:	Mon, 07 Jul 2014 21:41:17 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	ycheng@...gle.com
Cc:	ncardwell@...gle.com, netdev@...r.kernel.org
Subject: Re: [PATCH net] tcp: fix false undo corner cases

From: Yuchung Cheng <ycheng@...gle.com>
Date: Wed,  2 Jul 2014 12:07:16 -0700

> The undo code assumes that, upon entering loss recovery, TCP
> 1) always retransmit something
> 2) the retransmission never fails locally (e.g., qdisc drop)
> 
> so undo_marker is set in tcp_enter_recovery() and undo_retrans is
> incremented only when tcp_retransmit_skb() is successful.
> 
> When the assumption is broken because TCP's cwnd is too small to
> retransmit or the retransmit fails locally. The next (DUP)ACK
> would incorrectly revert the cwnd and the congestion state in
> tcp_try_undo_dsack() or tcp_may_undo(). Subsequent (DUP)ACKs
> may enter the recovery state. The sender repeatedly enter and
> (incorrectly) exit recovery states if the retransmits continue to
> fail locally while receiving (DUP)ACKs.
> 
> The fix is to initialize undo_retrans to -1 and start counting on
> the first retransmission. Always increment undo_retrans even if the
> retransmissions fail locally because they couldn't cause DSACKs to
> undo the cwnd reduction.
> 
> Signed-off-by: Yuchung Cheng <ycheng@...gle.com>
> Signed-off-by: Neal Cardwell <ncardwell@...gle.com>

Applied, thanks.

Do you guys think this is -stable material?
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