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

[Danny Mayer <[e-mail]>]

On 9/6/22 6:43 AM, Miroslav Lichvar wrote:
> On Tue, Sep 06, 2022 at 12:15:46PM +0200, kristof.teichel=[e-mail] wrote:
>> If everyone does use stratum consistently (and in particular sees to it
>> that their own stratum is higher than the max of those of its own
>> sources), then there will never be a loop.
> 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.
> When the client unselects that server, its stratum will go back to
> some small value, which will cause the server to decrease its stratum
> and it will become selectable for the client again. The loop is back.
>
> 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.
> The Bloom filter proposed for NTPv5 should allow the network to reach
> a stable state where the loops are broken in a single point.
>
>> 1b) Wouldn't tighter requirements on stratum use make things even easier?
>> I assume that everyone agrees that using higher stratum servers is never
>> beneficial for accuracy - and often detrimental.
> 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.

Danny

>