[ntp:questions] Re: Refids

David L. Mills mills at udel.edu
Sun Nov 27 15:58:50 UTC 2005


I've said very many times the reference ID is solely and exclusively 
intended to detect timing loops. A server can itself be synchronized to 
only one source and that source is revealed in the reference ID. There 
is no need nor no intent to interpret that field for any other purpose. 
The same reference ID goes out to all clients, regardless of the number 
of interfaces. A client of this server doesn't know whether this is an 
IPv4 or IPv6 address. The well-spoken client can check whether the field 
coincides with (any of) its IPv4 addresses or hashes to (any of) its 
IPv6 addresses, birthdays notwithstanding. There is no need to change 
any aspect of the reference ID as specified in rfc2030 and now implemented.


Danny Mayer wrote:
> David L. Mills wrote:
>>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.
> Danny
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