[ntp:questions] Refids

Danny Mayer mayer at ntp.isc.org
Sat Nov 26 21:36:36 UTC 2005

David L. Mills wrote:
> Guys,
> I'm answering Danny's message only because it is short. Bear in mind:
> 1. The protocol design originated over 25 years ago. There are estimates
> of over 25 million clients on the planet now. Changing the IPv4 refid
> compatibility is simply not in the cards. Certainly it is possible to
> accidently configure two associations with the same server and I have
> done that for test. It may be duplicative, but it is not fatal and does
> not impair good timekeeping.

Dave, the reason to change the way it currently is defined is that you
frequently have multiple IP addresses and this is even more true with
IPv6 in addition to IPv4 becoming the norm. Choosing any 32-bit number
will do the same job including the first non-loopback IP address as long
as the same ID goes out on all interfaces. There is nothing sacred about
the refid as long as we just treat it as a 32-bit number. Such a number
will interoperate with all previous versions of NTP since it cannot
distinguish an IPv4 address from a hole-in-the-wall otherwise the MD5
hash of IPv6 addresses would never have worked. By making it unique
across all interfaces and addresses that a system services you have
better opportunities to ensure that you are preventing loops. It's the
only real use that I'm aware of.


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