Re: [Ntp] NTP over PTP

Miroslav Lichvar <mlichvar@redhat.com> Tue, 29 June 2021 06:52 UTC

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Date: Tue, 29 Jun 2021 08:52:11 +0200
From: Miroslav Lichvar <mlichvar@redhat.com>
To: Heiko Gerstung <heiko.gerstung@meinberg.de>
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Subject: Re: [Ntp] NTP over PTP
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On Mon, Jun 28, 2021 at 05:02:43PM +0200, Heiko Gerstung wrote:
> > The unicast mode seems to be intended for networks with partial
> > on-path hardware support, where requirements on accuracy are less
> > strict, and I think this might already be better supported by NTP.
> They might be less strict but that does not mean they are worse/equal compared to NTP.

How so?

You said unicast transparent clocks don't really exist. That would be
the only advantage of unicast PTP. Boundary clocks could fully support
hardware-timestamped NTP if someone was actually interested in
implementing that.

Without full on-path support NTP should generally perform better than
PTP as it doesn't assume network has a constant delay.

In the context of the drafts we are discussing here, I think it might
be easier for an existing PTP implementation to add support for
NTP+NTS than add NTS4UPTP.

> > If there is some hardware that can timestamp only fixed-length PTP
> > messages with no TLVs, then that will not work with any form of
> > NTS4UPTP either, right? There has to be some TLV to authenticate the
> > message and the hardware needs to accept that.
> 
> No, that's not correct. The hardware timestamping is only required during phase 3 (packet transmission), which does not require an NTS TLV as it relies completely on the AUTHENTICATION_TLV and the integrated PTP security. See 3.3 of my draft. The NTS TLV is required in phase 2 where client and server only exchange messages to negotiate the transmission, and those are not hw timestamped. 

I don't see how that is relevant to hardware not timestamping PTP
messages with TLVs. If a TLV present in an event message breaks the
timestamping, the hardware is unusable for any security mechanism
discussed here, except the Daniel's idea to protect unauthenticated
PTP by authenticated NTP.

-- 
Miroslav Lichvar