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

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Date:	Tue, 8 Jul 2014 10:33:46 -0700
From:	Yuchung Cheng <ycheng@...gle.com>
To:	David Miller <davem@...emloft.net>
Cc:	Neal Cardwell <ncardwell@...gle.com>,
	netdev <netdev@...r.kernel.org>
Subject: Re: [PATCH net] tcp: fix false undo corner cases

On Mon, Jul 7, 2014 at 9:41 PM, David Miller <davem@...emloft.net> wrote:
> 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?
Yes this bugfix fits -stable. Thanks.
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