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

[Henning Rogge <[e-mail]>]

On Wed, Nov 8, 2023 at 10:07 AM Balázs Varga A
<[e-mail]> wrote:
>
> 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.

Sorry, I have my problem with this point of view...

If out-of-order packets arriving too late are an invalid state, you
should not modify your protocol control variable, regardless of the
decision if you want to forward them or not. Protocols should be
robust against failures of communication partners.

Imagine the following sequence of packets.

seq 1: arriving in time
seq 10: arriving too late (invalid state?)
seq 8: arriving out-of-order AND too late (also invalid state?), this
will set POFLastSent to 8
seq 11: arriving in time, but will be delayed by buffer because
POFLastSent is 8.

regardless of the decision if sequence number 10 and 8 are delivered
or not (which should be defined in the document), sequence number 11
should not be buffered.

Henning Rogge