Re: [RFC] tcp: Delay sending non-probes for RFC4821 mtu probing

[Neal Cardwell <ncardwell@google.com>]

On Mon, Apr 26, 2021 at 1:09 PM Leonard Crestez <lcrestez@drivenets.com> wrote:
>
> On 26.04.2021 18:59, Neal Cardwell wrote:
> > On Sun, Apr 25, 2021 at 10:34 PM Leonard Crestez <lcrestez@drivenets.com> wrote:
> >> On 4/21/21 3:47 PM, Neal Cardwell wrote:
> >>> On Wed, Apr 21, 2021 at 6:21 AM Leonard Crestez <cdleonard@gmail.com> wrote:
>
> >>> If the goal is to increase the frequency of PMTU probes, which seems
> >>> like a valid goal, I would suggest that we rethink the Linux heuristic
> >>> for triggering PMTU probes in the light of the fact that the loss
> >>> detection mechanism is now RACK-TLP, which provides quick recovery in
> >>> a much wider variety of scenarios.
> >>
> >>> You mention:
> >>>> Linux waits for probe_size + (1 + retries) * mss_cache to be available
> >>>
> >>> The code in question seems to be:
> >>>
> >>>     size_needed = probe_size + (tp->reordering + 1) * tp->mss_cache;
> >>> How about just changing this to:
> >>>
> >>>     size_needed = probe_size + tp->mss_cache;
> >>>
> >>> The rationale would be that if that amount of data is available, then
> >>> the sender can send one probe and one following current-mss-size
> >>> packet. If the path MTU has not increased to allow the probe of size
> >>> probe_size to pass through the network, then the following
> >>> current-mss-size packet will likely pass through the network, generate
> >>> a SACK, and trigger a RACK fast recovery 1/4*min_rtt later, when the
> >>> RACK reorder timer fires.
> >>
> >> This appears to almost work except it stalls after a while. I spend some
> >> time investigating it and it seems that cwnd is shrunk on mss increases
> >> and does not go back up. This causes probes to be skipped because of a
> >> "snd_cwnd < 11" condition.
> >>
> >> I don't undestand where that magical "11" comes from, could that be
> >> shrunk. Maybe it's meant to only send probes when the cwnd is above the
> >> default of 10? Then maybe mtu_probe_success shouldn't shrink mss below
> >> what is required for an additional probe, or at least round-up.
> >>
> >> The shrinkage of cwnd is a problem with this "short probes" approach
> >> because tcp_is_cwnd_limited returns false because tp->max_packets_out is
> >> smaller (4). With longer probes tp->max_packets_out is larger (6) so
> >> tcp_is_cwnd_limited returns true even for a cwnd of 10.
> >>
> >> I'm testing using namespace-to-namespace loopback so my delays are close
> >> to zero. I tried to introduce an artificial delay of 30ms (using tc
> >> netem) and it works but 20ms does not.
> >
> > I agree the magic 11 seems outdated and unnecessarily high, given RACK-TLP.
> >
> > I think it would be fine to change the magic 11 to a magic
> > (TCP_FASTRETRANS_THRESH+1), aka 3+1=4:
> >
> >    - tp->snd_cwnd < 11 ||
> >    + p->snd_cwnd < (TCP_FASTRETRANS_THRESH + 1) ||
> >
> > As long as the cwnd is >= TCP_FASTRETRANS_THRESH+1 then the sender
> > should usually be able to send the 1 probe packet and then 3
> > additional packets beyond the probe, and in the common case (with no
> > reordering) then with failed probes this should allow the sender to
> > quickly receive 3 SACKed segments and enter fast recovery quickly.
> > Even if the sender doesn't have 3 additional packets, or if reordering
> > has been detected, then RACK-TLP should be able to start recovery
> > quickly (5/4*RTT if there is at least one SACK, or 2*RTT for a TLP if
> > there is no SACK).
>
> As far as I understand tp->reordering is a dynamic evaluation of the
> fastretrans threshold to deal with environments with lots of reordering.
> Your suggestion seems equivalent to the current size_needed calculation
> except using packets instead of bytes.
>
> Wouldn't it be easier to drop the "11" check and just verify that
> size_needed fits into cwnd as bytes?

Yes, that sounds good to me (dropping the "11" check in favor of
verifying that size_needed fits into the cwnd).

neal