[Ntp] Antw: [EXT] Re: NTPv5 Loop Detection without Stratum

[Ulrich Windl <[e-mail]>]

>>> Miroslav Lichvar <[e-mail]> schrieb am 24.08.2022 um 15:31 in
Nachricht <YwYoRGQNMno+c0+G@localhost>:
> On Wed, Aug 24, 2022 at 08:35:39AM +1000, Paul Gear wrote:
>> I'm far from an expert on bloom filters, but they can have false
positives,
>> which would result in a source which was not part of a client's upstreams
>> being erroneously flagged as being so.  Would this not require a trace
(like
>> what Heiko has described above) to detect?
> 
> I'm not an expert either. To me it seems that it shouldn't be a
> problem unless we expect the number of IDs to grow to several
> hundreds.
> 
> With the proposed filter (m=4096 and k=10) the chances of a collision
> are about:
> ‑ 1e‑12 with 26 IDs
> ‑ 1e‑10 with 43 IDs
> ‑ 1e‑8 with 70 IDs
> ‑ 1e‑6 with 118 IDs
> ‑ 1e‑4 with 207 IDs
> ‑ 1e‑2 with 408 IDs
> ‑ 1e‑1 with 648 IDs
> 
> I guess we could get away even with a smaller filter, but this is
> going to be difficult to estimate.
> 
>> In some cases, false positives would be inconsequential (e.g. when the
>> client is using a well‑served zone in the public pool and can easily ask
for
>> another source), but in private networks where the number of sources may
be
>> more limited discarding an upstream source may be more problematic.
> 
> Would those clients in a private network be expected to operate as a
> server? If not, they don't need to check for loops.

A typical scenario for an Intranet would be:
Have One or two local stratum-1 servers, plus a few external ones, possibly
hard-wired (not pools).
And then have a stratum-2 layer that will actually server the Intranet
clients.
Such an architecture makes it easier to replace the startum-1 sources while
the clients can continue to use the local stratum-2 servers.
The priorities are typically:
1) Have a common time in the Intranet
2) Have the correct time in the Intranet

Regards,
Ulrich



> 
> ‑‑ 
> Miroslav Lichvar
> 
> _______________________________________________
> ntp mailing list
> [e-mail] 
> https://www.ietf.org/mailman/listinfo/ntp