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

[Miroslav Lichvar <[e-mail]>]

On Wed, Sep 07, 2022 at 02:50:23PM -0400, Danny Mayer wrote:
> On 9/7/22 2:19 AM, Miroslav Lichvar wrote:
> > 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.
> Only one source is selected at a time so this makes no sense.

An NTP client can select multiple sources at the same time for its
synchronization. Look for "combine" in the NTP RFCs. With the "ntpq
peers" command you would see sources marked with the * and + symbols.

The best source (in terms of root distance) determines stratum and
root delay/dispersion of the client. That other sources don't.
 
> > In orphan mode stratum is interpreted differently. Symmetric mode or
> > client/server mode makes no difference.
> There is no symmetric mode for client/server. A client is by definition one
> stratum value larger than the server. Peers can have the same stratum.

This is about the onwire protocol, not about source selection or
synchronization. A client polling a server can have a smaller, equal,
or higher stratum than the server.

The difference between the client/server and symmetric mode is that in
symmetric mode a message is a request and response at the same time.
A client/server association provides measurements (offset, delay) only
for the client side. In symmetric mode the association provides
measurements for both sides.

Two client/server associations in opposite directions provide the same
functionality as a symmetric association, except the number of
exchanged messages is doubled, it's much easier to implement and
understand, and there are no replay attacks.

> > 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.
> 
> Then it wouldn't be a stratum-1 server any more.

What it would be?

-- 
Miroslav Lichvar