Re: [Ntp] ntp-over-ptp concerns

[Miroslav Lichvar <[e-mail]>]

On Tue, Mar 19, 2024 at 04:14:44PM +0000, Rodney Cummings wrote:
> There’s a very basic statement in 6.6.2 of 1588-2019:
>                 “MASTER: The PTP Port is the source of time on the PTP Communication Path served by the PTP Port.”
> 
> What is the source of time in NTP-over-PTP?

There is no working PTP source of time in NTP over PTP. It's like a
network where all PTP clocks are slaveOnly clocks, waiting for a
master to appear, but that never happens. The PTP clocks are just
sending unicast delay requests transporting NTP messages according to
their configuration. Nothing interesting is happening from the PTP
point of view.

> You seem to be saying “It doesn’t comply with any state in 1588”. If so, that proves my point.

No, I'm not saying that. The UNCALIBRATED state seems perfectly fine
for NTP over PTP. It allows delay requests to be sent by the port. It
doesn't require any messages to be received before that. The unicast
option doesn't require the delay response grant for sending delay
requests.

9.5.11.2 has requirements for timing of delay requests. It starts with
"Unless the unicast option of 16.1 is in use", but 16.1 refers back to
9.5.11.2 with "In addition to these transmission requirements,
Delay_Req messages are also required to comply with the transmission
requirements defined in 9.5.11.2.", i.e. there is a loop in the
specification.

Assuming the intention for timing of unicast delay requests was
to also follow the general requirements specified for multicast delay
requests, we need a different state to stop sending delay requests
when not needed. The PASSIVE state seems like a good option. It
ignores all received messages and doesn't transmit anything.
Alternatively, we can immediately destroy the PTP instance after
sending the delay request and create it again when needed.

The port can reach the UNCALIBRATED and PASSIVE states the usual way
from INITIALIZING through LISTENING using an alternate BMCA, or
directly using the "17.6 Mechanism for external configuration of a PTP
Instance’s PTP Port state" option.

Please let me know if you see any errors in this.

-- 
Miroslav Lichvar