Messages in this thread | | | Subject | Re: [RFC] tcp: Delay sending non-probes for RFC4821 mtu probing | From | Leonard Crestez <> | Date | Mon, 26 Apr 2021 20:09:13 +0300 |
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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?
-- Regards, Leonard
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