netdev - Re: [PATCH net] tcp: avoid multiple ssthresh reductions in on retransmit window

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Date:	Mon, 16 Jun 2014 17:25:30 -0700
From:	Yuchung Cheng <ycheng@...gle.com>
To:	Neal Cardwell <ncardwell@...gle.com>
Cc:	Michal Kubecek <mkubecek@...e.cz>,
	"David S. Miller" <davem@...emloft.net>,
	netdev <netdev@...r.kernel.org>,
	Alexey Kuznetsov <kuznet@....inr.ac.ru>,
	James Morris <jmorris@...ei.org>,
	Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
	Patrick McHardy <kaber@...sh.net>
Subject: Re: [PATCH net] tcp: avoid multiple ssthresh reductions in on
 retransmit window

On Mon, Jun 16, 2014 at 4:42 PM, Neal Cardwell <ncardwell@...gle.com> wrote:
> On Mon, Jun 16, 2014 at 6:39 PM, Yuchung Cheng <ycheng@...gle.com> wrote:
>> On Mon, Jun 16, 2014 at 2:19 PM, Michal Kubecek <mkubecek@...e.cz> wrote:
>>> RFC 5681 says that ssthresh reduction in response to RTO should
>>> be done only once and should not be repeated until all packets
>>> from the first loss are retransmitted. RFC 6582 (as well as its
>>> predecessor RFC 3782) is even more specific and says that when
>>> loss is detected, one should mark current SND.NXT and ssthresh
>>> shouldn't be reduced again due to a loss until SND.UNA reaches
>>> this remembered value.
>>>
>>> In Linux implementation, this is done in tcp_enter_loss() but an
>>> additional condition
>>>
>>>   (icsk->icsk_ca_state == TCP_CA_Loss && !icsk->icsk_retransmits)
>>>
>>> allows to further reduce ssthresh before snd_una reaches the
>>> high_seq (the snd_nxt value at the previous loss) as
>>> icsk_retransmits is reset as soon as snd_una moves forward. As a
>>> result, if a retransmit timeout ouccurs early in the retransmit
>>> phase, we can adjust snd_ssthresh based on very low value of
>>> cwnd. This can be especially harmful for reno congestion control
>>> with slow linear cwnd growth in congestion avoidance phase.
>>>
>>> The patch removes the condition above so that snd_ssthresh is
>>> not reduced again until snd_una reaches high_seq as described in
>>> RFC 5681 and 6582.
>>>
>>> Signed-off-by: Michal Kubecek <mkubecek@...e.cz>
>
> AFAICT this commit description is arguing from a mis-reading of the
> RFCs.
>
> RFC 6582 and RFC 3782 are only about Fast Recovery, and not relevant
> to the timeout recovery we're dealing with in tcp_enter_loss().
>
> RFC 5681, Section 4.3 says:
>
>    Loss in two successive windows of data, or the loss of a
>    retransmission, should be taken as two indications of congestion and,
>    therefore, cwnd (and ssthresh) MUST be lowered twice in this case.
>
> So if we're in TCP_CA_Loss snd_una advances (FLAG_DATA_ACKED is set
> and icsk_retransmits is zero), but snd_una does not advance above
> high_seq, then if we subsequently suffer an RTO (and call
> tcp_enter_loss()) then that indicates a retransmission is lost, which
> this passage from sec 4.3 indicates should be taken as a second
> indication of congestion.
That's right. I should have checked the RFC more thoroughly. Sorry
please ignore my Acked-by.

However Linux is inconsistent on the loss of a retransmission. It
reduces ssthresh (and cwnd) if this happens on a timeout, but not in
fast recovery (tcp_mark_lost_retrans). We should fix that and that
should help dealing with traffic policers.


>
>> - (icsk->icsk_ca_state == TCP_CA_Loss && !icsk->icsk_retransmits)) {
>
> AFAICT this existing code is a faithful implementation of, RFC 5681,
> Section 7:
>
>    The treatment of ssthresh on retransmission timeout was clarified.
>    In particular, ssthresh must be set to half the FlightSize on the
>    first retransmission of a given segment and then is held constant on
>    subsequent retransmissions of the same segment.
>
> That is, if snd_una advances (FLAG_DATA_ACKED is set and
> icsk_retransmits is zero), if we subsequently suffer an RTO and call
> tcp_enter_loss() then we will be sending a "first retransmission" at
> the segment pointed to by the new/higher snd_una. So this is the first
> retransmission of that new segment, so we should reduce ssthresh.
>
> And from first principles, the current Linux code and RFCs seem
> sensible on this matter, AFAICT. Suppose we suffer an RTO, and then
> over the following RTTs in TCP_CA_Loss we grow cwnd exponentially
> again. If we suffer another RTO in this cwnd growth process, then it
> seems like a good idea to remember the reduced ssthresh inferred from
> this smaller cwnd at which we suffered a loss.
>
> So AFAICT the existing code is sensible and complies with the RFC.
>
> Now, I agree the linear growth of Reno in such situations is
> problematic, but I think it's a somewhat separate issue. Or at least
> if we're going to change the behavior here then we should justify it
> by using data, and not by reference to RFCs. :-)
>
> neal
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