Re: [RTG-DIR] [Detnet] Review of draft-ietf-detnet-pof-04 for the RTGDIR

[Balázs Varga A <[e-mail]>]

Hi Henning,

Thanks for further clarifications. I think that highlights the somewhat different
mindset deterministic communication related functions follow. The per packet
replication and elimination are used for applications those require extreme 
parameters on loss, i.e., practically lossless communication.
Replication (PRF) and Elimination (PEF) are doing their job to ensure this lossless 
communication. POF is intended to correct the rare side-effect (i.e., out-of-order 
delivery) resulted from PRF/PEF. 

With those in mind, POF should never drop any packet as it would harm the application
(requiring lossless communication). If POF configured incorrectly (like in your example), 
then it will not correct out-of-order delivery. However dropping packets would make the 
situation even worst for the application. Dropping out-of-order packets is a task for the
application, if it decides to do so for too late or out-of-order packets.

I hope these clarifies your concerns.

Many thanks
Bala'zs

-----Original Message-----
From: Henning Rogge <[e-mail]> 
Sent: Wednesday, November 8, 2023 9:24 AM
To: Balázs Varga A <[e-mail]>
Cc: [e-mail]; [e-mail]; Daniam Henriques <[e-mail]>
Subject: Re: [Detnet] Review of draft-ietf-detnet-pof-04 for the RTGDIR

On Tue, Nov 7, 2023 at 11:59 AM Balázs Varga A <[e-mail]> wrote:
>
> Hi Henning,
> 2, “why POFLastSent is always set to seq_num”
>
> Yes, this is an interesting question. Both POF algorithm assume, that 
> the POFMaxDelay parameter applied on a buffered packet is correctly 
> designed. It means, that a packet is removed from the buffer only if 
> there is no chance anymore to receive an earlier (smaller SeqNum) 
> packet over the slowest path. (In practice that means that POFMaxDelay 
> > latency difference of the fastest and the slowest paths.) Therefore “if seq_num 10 already has been forwarded and now seq_num 5 is being forwarded” is a broken scenario, with correctly designed POFMaxDelay arrival of seq_num 5 is an invalid event or happened due to multiple network failures.
> State and initialization of the basic POF algorithm are described on page 7, so I would propose no change to the text.

Maybe a short "packages with a sequence number of less than POFLastSent MUST/SHOULD be silently dropped" could be added to make implementations more robust to these situations?

Henning Rogge