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

[Leonard Crestez <lcrestez@drivenets.com>]

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