Re: [Ntp] NTPv5 Loop Detection without Stratum - Why do we want this?

[Miroslav Lichvar <[e-mail]>]

On Tue, Sep 06, 2022 at 07:29:18PM -0400, Danny Mayer wrote:
> On 9/6/22 6:43 AM, Miroslav Lichvar wrote:
> > There will be a loop, but it will terminate after several rounds due
> > to some server reaching the maximum stratum (currently 16), which will
> > not be acceptable by the client.
> You won't get that far. The server tries to select the best quality source
> of timestamps and uses that until another better quality source becomes
> preferred.

If stratum comes from the best sourcea and the loop is not over the
best source, then stratum won't terminate the loop.

If stratum came from the maximum from all selected sources, it would
always terminate the loop, but I think it would lead to unstable
selection. We can experiment with that.

> > Stratum cannot prevent loops unless we don't allow clients to select a
> > server with a higher stratum (as it was in NTPv3). It can only
> > terminate them and it doesn't prevent them from happening again.
> That's a bad idea. Symmetric peers may be at the same stratum and that's
> okay especially if you get to orphan state.

In orphan mode stratum is interpreted differently. Symmetric mode or
client/server mode makes no difference.

> > I'd not agree with that. Stratum doesn't indicate accuracy. It depends
> > on the reference clocks used by primary servers and network paths between
> > the servers.
> 
> I'm not sure what the stratum-1 servers have to do with this. The network is
> far more important when choosing a preferred source.

If a local stratum-1 server had a poor reference clock (e.g. a GPS
receiver without PPS), it would be better to use a distant server with
higher stratum, i.e. stratum doesn't say much about accuracy.

-- 
Miroslav Lichvar