Re: [tcpm] draft-ietf-tcpm-prr-rfc6937bis-03: set cwnd to ssthresh exiting fast recovery?

[Yoshifumi Nishida <[e-mail]>]

Hi Yuchung,

Thanks for the response.
I just would like to check one thing.
In my understanding, Neal's suggestion here was to adjust cwnd to ssthresh
at the end of recovery.
But, I cannot find the statement or logic for such adjustment. Does this
mean we decided there's no adjustment at the end of recovery? Or, am I
missing something?

Thanks,
--
Yoshi

On Tue, Aug 8, 2023 at 2:34 PM Yuchung Cheng <[e-mail]> wrote:

> Hi Yoshifumi,
>
> That part is how the "RecoverFS" state variable is calculated in the
> draft. See the diff of 03/04 on Section 5 and 6 regarding "RecoverFS" state
> variable definition and computation.
> https://author-tools.ietf.org/iddiff?url2=draft-ietf-tcpm-prr-rfc6937bis-04
>
> Does that make sense?
>
> On Tue, Aug 8, 2023 at 12:01 AM Yoshifumi Nishida <[e-mail]>
> wrote:
>
>> Hi Yuchung,
>>
>> I think you have already updated the draft on the following point from
>> the discussions in the last WG meeting.
>> Could you point out which part has been updated? I'm just checking..
>> Thanks,
>> --
>> Yoshi
>>
>> On Fri, May 5, 2023 at 11:51 AM Yoshifumi Nishida <[e-mail]>
>> wrote:
>>
>>> Hi Neal,
>>>
>>> Yes, I think I understand your point.
>>> I prefer the current logic in some ways as it's more conservative as I
>>> think we cannot always presume that queue has been drained at the end of
>>> recovery.
>>> But, I also think it may look too conservative.
>>> I am expecting that the authors provide some insights on this point.
>>> --
>>> Yoshi
>>>
>>>
>>> On Tue, May 2, 2023 at 11:31 AM Neal Cardwell <[e-mail]>
>>> wrote:
>>>
>>>> Hi Yoshi,
>>>>
>>>> You are right that because PRR always sets cwnd to ssthresh at the end
>>>> of recovery, there will be some cases where with PRR cwnd jumps up
>>>> drastically at the end of the recovery.
>>>>
>>>> However, AFAIK cwnd jumping up drastically, per se, is not a problem.
>>>> Big bursts of packets going into the network is a problem. And given the
>>>> dynamics of the alternative loss recovery algorithms (RFC6675 and PRR),
>>>> both can allow bursts of packets; just in different circumstances:
>>>>
>>>> (1) RFC6675: Because RFC6675 sets cwnd once at the start of fast
>>>> recovery, using (4.2) from RFC6675:
>>>>
>>>> ssthresh = cwnd = (FlightSize / 2)
>>>>
>>>> ...that means RFC6675 allows big bursts at the moment any loss is
>>>> detected: any time L packets are lost, the sender can burst L more packets.
>>>>
>>>> (2) PRR: PRR is specifically designed to avoid big bursts in response
>>>> to packet losses; no matter the structure or timing of the losses, PRR only
>>>> allows a big burst at the end of Fast Recovery after all holes have been
>>>> plugged, and the algorithm sets cwnd to ssthresh.
>>>>
>>>> So in your example ("For example, many packets were lost before
>>>> entering recovery"), AFAICT RFC6675 can allow a big burst at the beginning
>>>> of recovery, when the lost packets are detected. AFAICT in this case PRR
>>>> can allow a burst of packets at the end of recovery when it sets cwnd to
>>>> ssthresh, but at least at this point the bottleneck queue has potentially
>>>> drained somewhat.
>>>>
>>>> Please let me know if that analysis misses something important. :-)
>>>>
>>>> Thanks!
>>>> neal
>>>>
>>>>
>>>> On Mon, May 1, 2023 at 5:22 PM Yoshifumi Nishida <[e-mail]>
>>>> wrote:
>>>>
>>>>> Hi Randall,
>>>>>
>>>>> I might miss something, but here's what I've thought..
>>>>> If we lost many packets in a RTT such as the Figure 5 in the 6937bis
>>>>> draft, I think the window growth during the recovery period will be bound
>>>>> by PRR-CRB or PRR-SSRB.
>>>>> Hence, I think the cwnd at the end of recovery can be smaller than we
>>>>> expect as shown in figure 5.
>>>>> --
>>>>> Yoshi
>>>>>
>>>>> On Mon, May 1, 2023 at 4:17 AM Randall Stewart <[e-mail]>
>>>>> wrote:
>>>>>
>>>>>> Hi Neal and Yoshi:
>>>>>>
>>>>>> Neal: So the FreeBSD implementation in rack, like linux, does the
>>>>>> same exact thing set cwnd to ssthresh at
>>>>>> exit from recovery.
>>>>>>
>>>>>> Yoshi: I don’t see how this would cause cwnd to be larger, since at
>>>>>> the entry to recovery you set ssthresh = cwnd *  Beta. But
>>>>>>           maybe I am missing something, can you give an example like
>>>>>> Neal did below?
>>>>>>
>>>>>>
>>>>>> Thanks
>>>>>>
>>>>>> R
>>>>>>
>>>>>> On May 1, 2023, at 5:32 AM, Yoshifumi Nishida <[e-mail]>
>>>>>> wrote:
>>>>>>
>>>>>> Hi Neal,
>>>>>>
>>>>>> If we always set cwnd to ssthresh at the end of recovery, I am
>>>>>> guessing there will be some cases where cwnd jumps up drastically at the
>>>>>> end of the recovery. For example, many packets were lost before entering
>>>>>> recovery.  Or, am I missing something?
>>>>>> --
>>>>>> Yoshi
>>>>>>
>>>>>> On Wed, Apr 19, 2023 at 7:37 PM Neal Cardwell <ncardwell=
>>>>>> [e-mail]> wrote:
>>>>>>
>>>>>>> Working through examples for the "draft-ietf-tcpm-prr-rfc6937bis-03
>>>>>>> and RecoverFS initialization" thread this evening, I ran into another
>>>>>>> potential issue.
>>>>>>>
>>>>>>> The Linux TCP implementation of PRR explicitly/directly sets cwnd to
>>>>>>> ssthresh at the end of fast recovery (in tcp_end_cwnd_reduction()). But
>>>>>>> this behavior is not in the algorithm in the PRR RFC or draft, at least in
>>>>>>> the figures in section 6, Algorithms. Maybe it is in the prose somewhere
>>>>>>> and I missed it; but in that case I'd argue strongly to put this in the
>>>>>>> figures in section 6, Algorithms.
>>>>>>>
>>>>>>> AFAICT in some cases this is strictly necessary to get cwnd to grow
>>>>>>> to reach ssthresh. Without such a direct step, cwnd could end up far below
>>>>>>> ssthresh at the end of recovery. Here's an example to illustrate:
>>>>>>>
>>>>>>> CC = CUBIC
>>>>>>>
>>>>>>> cwnd = 10
>>>>>>>
>>>>>>> The reordering degree was estimated to be large, so the connection
>>>>>>> will wait for more than 3 packets to be SACKed before entering fast
>>>>>>> recovery.
>>>>>>>
>>>>>>> --- Application writes 10*MSS.
>>>>>>>
>>>>>>> TCP sends packets P1 .. P10.
>>>>>>> pipe = 10 packets in flight (P1 .. P10)
>>>>>>>
>>>>>>> --- P2..P9 SACKed  -> do nothing
>>>>>>>
>>>>>>> (Because the reordering degree was previously estimated to be large.)
>>>>>>>
>>>>>>> --- P10 SACKed -> mark P1 as lost and enter fast recovery
>>>>>>>
>>>>>>> PRR:
>>>>>>> ssthresh = CongCtrlAlg() = 7 packets // CUBIC
>>>>>>> prr_delivered = 0
>>>>>>> prr_out = 0
>>>>>>> RecoverFS = snd.nxt - snd.una = 10 packets (P1..P10)
>>>>>>>
>>>>>>> DeliveredData = 1  (P10 was SACKed)
>>>>>>>
>>>>>>> prr_delivered += DeliveredData   ==> prr_delivered = 1
>>>>>>>
>>>>>>> pipe =  0  (all packets are SACKed or lost; P1 is lost, rest are
>>>>>>> SACKed)
>>>>>>>
>>>>>>> safeACK = false (snd.una did not advance)
>>>>>>>
>>>>>>> if (pipe > ssthresh) => if (0 > 7) => false
>>>>>>> else
>>>>>>>   // PRR-CRB by default
>>>>>>>   sndcnt = MAX(prr_delivered - prr_out, DeliveredData)
>>>>>>>          = MAX(1 - 0, 1)
>>>>>>>          = 1
>>>>>>>
>>>>>>>   sndcnt = MIN(ssthresh - pipe, sndcnt)
>>>>>>>          = MIN(7 - 0, 1)
>>>>>>>          = 1
>>>>>>>
>>>>>>> cwnd = pipe + sndcnt
>>>>>>>      = 0    + 1
>>>>>>>      = 1
>>>>>>>
>>>>>>> retransmit P1
>>>>>>>
>>>>>>> prr_out += 1   ==> prr_out = 1
>>>>>>>
>>>>>>> --- P1 retransmit plugs hole; receive cumulative ACK for P1..P10
>>>>>>>
>>>>>>> DeliveredData = 1  (P1 was newly ACKed)
>>>>>>>
>>>>>>> prr_delivered += DeliveredData   ==> prr_delivered = 2
>>>>>>>
>>>>>>> pipe =  0  (all packets are cumuatively ACKed)
>>>>>>>
>>>>>>> safeACK = (snd.una advances and no further loss indicated)
>>>>>>> safeACK = true
>>>>>>>
>>>>>>> if (pipe > ssthresh) => if (0 > 7) => false
>>>>>>> else
>>>>>>>   // PRR-CRB by default
>>>>>>>   sndcnt = MAX(prr_delivered - prr_out, DeliveredData)
>>>>>>>          = MAX(2 - 1, 1)
>>>>>>>          = 1
>>>>>>>   if (safeACK) => true
>>>>>>>     // PRR-SSRB when recovery is in good progress
>>>>>>>     sndcnt += 1   ==> sndcnt = 2
>>>>>>>
>>>>>>>   sndcnt = MIN(ssthresh - pipe, sndcnt)
>>>>>>>          = MIN(7 - 0, 2)
>>>>>>>          = 2
>>>>>>>
>>>>>>> cwnd = pipe + sndcnt
>>>>>>>      = 0    + 2
>>>>>>>      = 2
>>>>>>>
>>>>>>> So we exit fast recovery with cwnd=2 even though ssthresh is 7.
>>>>>>>
>>>>>>> As noted above, the Linux TCP implementation does not suffer this
>>>>>>> problem because it explicitly/directly sets cwnd to ssthresh at the end of
>>>>>>> fast recovery.
>>>>>>>
>>>>>>> I would recommend including this cwnd=ssthresh step at the end of
>>>>>>> recovery in the draft, to ensure that cwnd reaches ssthresh at the end of
>>>>>>> fast recovery, even in cases like this where there will be insufficient
>>>>>>> delivered data in fast recovery to allow pipe to incrementally grow to
>>>>>>> reach ssthresh using PRR-SSRB.
>>>>>>>
>>>>>>> Best regards,
>>>>>>> neal
>>>>>>>
>>>>>>> _______________________________________________
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>>>>>>>
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>>>>>>
>>>>>>
>>>>>> ------
>>>>>> Randall Stewart
>>>>>> [e-mail]
>>>>>>
>>>>>>
>>>>>>
>>>>>>