netdev - Re: tcp: multiple ssthresh reductions before all packets are retransmitted

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Date:	Mon, 16 Jun 2014 22:06:40 +0200
From:	Michal Kubecek <mkubecek@...e.cz>
To:	Yuchung Cheng <ycheng@...gle.com>
Cc:	netdev <netdev@...r.kernel.org>
Subject: Re: tcp: multiple ssthresh reductions before all packets are
 retransmitted

On Mon, Jun 16, 2014 at 12:04:37PM -0700, Yuchung Cheng wrote:
> On Mon, Jun 16, 2014 at 10:47 AM, Michal Kubecek <mkubecek@...e.cz> wrote:
> > On Mon, Jun 16, 2014 at 10:02:08AM -0700, Yuchung Cheng wrote:
> >> On Mon, Jun 16, 2014 at 8:35 AM, 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.
> >> >
> >> > Linux implementation does exactly that but in TCP_CA_Loss state,
> >> > tcp_enter_loss() also takes icsk_retransmits into account:
> >> >
> >> >         /* Reduce ssthresh if it has not yet been made inside this window. */
> >> >         if (icsk->icsk_ca_state <= TCP_CA_Disorder ||
> >> >             !after(tp->high_seq, tp->snd_una) ||
> >> >             (icsk->icsk_ca_state == TCP_CA_Loss && !icsk->icsk_retransmits)) {
> >> >                 new_recovery = true;
> >> >                 tp->prior_ssthresh = tcp_current_ssthresh(sk);
> >> >                 tp->snd_ssthresh = icsk->icsk_ca_ops->ssthresh(sk);
> >> >                 tcp_ca_event(sk, CA_EVENT_LOSS);
> >> >         }
> >> >
> >> > This seems correct as icsk_retransmits is supposed to mean "we still
> >> > have packets to retransmit". But icsk_retransmits is reset in
> >> > tcp_process_loss() if one of two conditions is satisfied:
> >>
> >> icsk_retransmits indicates the number of recurring timeouts (of the
> >> same sequence). so it is reset when the recovery is done or SND_UNA is
> >> advanced. the variable name however is confusing.
> >
> > In that case, I suppose the problem would be this part
> >
> >     (icsk->icsk_ca_state == TCP_CA_Loss && !icsk->icsk_retransmits)
> >
> > of the condition above (in tcp_enter_loss()). As that would mean we
> > would allow further ssthresh reduction as soon as SND.UNA moves forward
> > (not when it reaches high_seq).
> 
> Nice catch. That check is incorrect and should be removed. I have used
> packetdrill to verify it fixes the ssthresh problem.
> 
...
> 
> Do you want to submit a one-liner patch? or I am happy to do that.

I'll submit it. Thank you for your help.

                                                          Michal Kubecek

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