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

[Danny Mayer <[e-mail]>]

On 9/7/22 2:19 AM, Miroslav Lichvar wrote:
> 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.
Only one source is selected at a time so this makes no sense.
>
>>> 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.
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.
>
>>> 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.

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

Danny

>