Re: [Ntp] NTPv5 and anycasting

[Paul Gear <[e-mail]>]

On 3/12/20 3:48 am, Philip Prindeville wrote:
>> On Dec 2, 2020, at 1:08 AM, Miroslav Lichvar <[e-mail]> wrote:
>>
>> On Tue, Dec 01, 2020 at 10:41:01PM -0700, Philip Prindeville wrote:
>>> Sorry if this has been discussed already, but do we want to have a specific prohibition against the use of anycasting with NTPv5?
>>>
>>> I can’t see the point in sending packets non-deterministically to one of possibly many servers with different clock values, RTT’s, etc.
>> There is a section on anycast in the NTP BCP document. It can be
>> useful. I don't see a reason why NTPv5 specifically should prohibit
>> use of anycast. 
>>
>
> What’s the scenario where non-determinism is a good thing?

Hi Philip,

I agree, and I'm curious to hear Miroslav's answer to this as well.

I can think of two scenarios where anycast might be useful, but neither
is really non-deterministic under normal conditions:

 1. Public services where the same well-known IP (or set of IPs) is
    anycast simultaneously from geographically dispersed locations and
    only moves when its prefix is withdrawn from BGP due to maintenance
    or faults.  (Similar to the various DNS services from Cloudflare,
    Google, etc.)  I would argue that the existing DNS-based public pool
    is a better solution than this for most use cases.
 2. A corporate WAN where clients are configured with a single NTP
    server IP, which is anycast from the local branch router.  My
    argument against using anycast here would be that an internal DNS
    pool is simpler and more robust, but I can see some organisations
    wanting to use anycast due to a mix of platform limitations,
    management software, and available skills.

I think RFC8633 section 7 was rather too gentle in its downplaying of
anycast, and would support wording stating that anycast SHOULD not be
used with NTP.  Of course, that's not really within scope of Miroslav's
v5 proposal.

Regards,
Paul