Re: [Ntp] comments on draft-mlichvar-ntp-ntpv5-03 / Extension fields

[Miroslav Lichvar <[e-mail]>]

On Thu, Dec 02, 2021 at 05:20:16PM +0000, Doug Arnold wrote:
> Just to be clear: PTP does not require that all switches are PTP capable.  The ITU-T G.8265.1 and G.8275.2 profiles explicitly assume that there is no or only partial timing support in the network path.  Financial data centers often do not need on-path support since their networks are designed for minimum latency.

I guess I should say "required to perform well".

Yes, technically speaking, PTP will work in a network without full PTP
support, but it was not designed for that (it breaks the assumption
about delay being mostly constant) and generally will not work as well
as protocols that were designed for real computer networks like NTP.

Also, the PTP support in switches is not just about being present or
not. It must be reasonably well implemented. I know that in some cases
it had to be disabled as it actually made things worse.

> Practically speaking, in long network paths, it would take only one busy router with no timing capability to render the correction field irrelevant. The Correction proposed for ntpv5 would probably be most useful for site local use cases, at least at first.  But if it became common for switch and router vendors to implement this feature, then WAN timing would gradually become more precise.

Right.

-- 
Miroslav Lichvar