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

[Leonard Crestez]

On 26.04.2021 18:59, Neal Cardwell wrote:
> On Sun, Apr 25, 2021 at 10:34 PM Leonard Crestez <lcrestez@xxxxxxxxxxxxx> wrote:
> > On 4/21/21 3:47 PM, Neal Cardwell wrote:
> > > On Wed, Apr 21, 2021 at 6:21 AM Leonard Crestez <cdleonard@xxxxxxxxx> 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?

--
Regards,
Leonard