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

[Danny Mayer <[e-mail]>]

On 9/8/22 2:24 AM, Miroslav Lichvar wrote:
> 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.
Only is chosen (marked with a *). The + ones are possible candidates.
> The best source (in terms of root distance) determines stratum and
> root delay/dispersion of the client. That other sources don't.

Those are not the only criteria.


>>> 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.
That's irrelevant. You are polling a server for time, what you do with 
it depends on the analysis done on the packets.
>
> 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.
That's a naive assumption. You can send any kind of replay no matter 
what the mode of the packet. Why do you think that you are sending twice 
as many packets in a symmetric association?
>
>>> 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?

I can only assume you have not read Section 3 of RFC5905 which tells you 
the answer.

Danny