netdev - Re: [PATCH net-next] tcp: remove bad timeout logic in fast recovery

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Date:	Sun, 19 May 2013 23:51:48 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	nanditad@...gle.com
Cc:	ncardwell@...gle.com, eric.dumazet@...il.com, ycheng@...gle.com,
	edumazet@...gle.com, ilpo.jarvinen@...helsinki.fi,
	netdev@...r.kernel.org
Subject: Re: [PATCH net-next] tcp: remove bad timeout logic in fast recovery

From: Nandita Dukkipati <nanditad@...gle.com>
Date: Fri, 17 May 2013 22:52:34 -0700

> On Fri, May 17, 2013 at 8:17 PM, Neal Cardwell <ncardwell@...gle.com> wrote:
>> On Fri, May 17, 2013 at 9:46 PM, Eric Dumazet <eric.dumazet@...il.com> wrote:
>>> On Fri, 2013-05-17 at 16:45 -0700, Yuchung Cheng wrote:
>>>> tcp_timeout_skb() was intended to trigger fast recovery on timeout,
>>>> unfortunately in reality it often causes spurious retransmission
>>>> storms during fast recovery. The particular sign is fast retransmit
>>>> over highest sacked sequence (SND.FACK).
>>>>
>>>> Currently the RTO timer re-arming (as in RFC6298) offers a nice cushion
>>>> to avoid spurious timeout: when SND.UNA advances the sender re-arms
>>>> RTO and extends the timeout by icsk_rto. The sender does not offset
>>>> the time elapsed since the packet at SND.UNA was sent.
>>>>
>>>> But if the next (DUP)ACK arrives later than ~RTTVAR and triggers
>>>> tcp_fastretrans_alert(), then tcp_timeout_skb() will mark any packet
>>>> sent before icsk_rto interval lost, including one that's above the
>>>> highest sacked sequence. Most likely a large part of scorebard will
>>>> be marked.
>>>>
>>>> If most packets are not lost then the subsequence DUPACKs with new
>>>> SACK blockes will cause the sender to continue retransmit packets
>>>> beyond SND.FACK spuriously right. Even only one packet is lost the
>>>> sender may falsely retransmit almost the entire window.
>>>>
>>>> The situation becomes common in the world of bufferbloat: the RTT
>>>> continues to grow as the queue builds up but RTTVAR remains small and
>>>> close to the minimum 200ms. If a data packet is lost and the DUPACK
>>>> triggered by the next data packet is slightly delayed, then a spurious
>>>> retransmission storm forms.
>>>>
>>>> As the original comment on tcp_timeout_skb() suggests: the usefulness
>>>> of this feature is questionable. It also wastes cycles walking the
>>>> sack scoreboard and is actually harmful because of the false recovery.
>>>> It's time to remove this.
>>>>
>>>> Signed-off-by: Yuchung Cheng <ycheng@...gle.com>
>>>> ---
>>>>  include/linux/tcp.h  |  1 -
>>>>  include/net/tcp.h    |  1 -
>>>>  net/ipv4/tcp_input.c | 65 +---------------------------------------------------
>>>>  3 files changed, 1 insertion(+), 66 deletions(-)
>>>
>>> Acked-by: Eric Dumazet <edumazet@...gle.com>
>>
>> Acked-by: Neal Cardwell <ncardwell@...gle.com>
> 
> Acked-by: Nandita Dukkipati <nanditad@...gle.com>

Applied, thanks everyone.
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