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

[Randall Stewart <[e-mail]>]

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] <mailto:[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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------
Randall Stewart
[e-mail]