[ntp:questions] zeroconf for ntpd?

Danny Mayer mayer at ntp.isc.org
Sun May 27 19:28:42 UTC 2007

Wolfgang S. Rupprecht wrote:
> mayer at ntp.isc.org (Danny Mayer) writes:
>> Wolfgang S. Rupprecht wrote:
>>>  If I understand
>>> things correctly that would require transmitting on ff0e::101, the
>>> global multicast address with the 0x0101 ntp group id.
>> The trouble is that only site-local (ff05::101) is implemented in the
>> network routers. I don't believe that there is any router hardware that
>> supports global multicasting.
> Danny, thanks for the low-down.  This is a bit disheartening.  I take
> it site-local is implemented using some simplistic flooding (or
> similar) that is inappropriate for the global multicast.

It's a bit more complicated than that. From my understanding there is no
agreement on global multicasting and the router/switch vendors are not
going to put something in unless there is a demand for it. At the global
level these things need to be agreed upon at an international level and
possibly by ICANN, IANA, IETF, etc. I don't think there is any agreement
 on it. In any case I think there are diminishing returns when you go
beyond site. I don't know enough about the routing protocols to talk
about how complex it is to implement nor if the vendors have all the
information that they would need to make this practical.

A much better method might be to go to SRV records which have some
advantage over A records in the DNS to find an NTP server. This is
basically what zeroconf (at least some implementations do) by having the
NTP server add an SRV record to announce it's availability. Of course
that requires a dynamic DNS zone. A drawback to that is if the NTP
server goes down, or the system it's on goes down the SRV record doesn't
get removed.


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