[ntpwg] [dhcwg] Re: Network Time Protocol (NTP) Optionsfor DHCPv6

[Ted Lemon]

On Nov 25, 2007, at 7:28 PM, Brian Utterback wrote:
> So, the advantage of serving a FQDN is that the client is forced to
> resolve the host and get the IP
> addresses from DNS and removal of an IP from the DNS name system will
> cause all the traffic
> to that IP to eventually disappear.

The client is not *forced* to do anything.   Whether or not it does  
what you describe depends on the implementation.   On a practical  
level, there is no difference between refreshing information from the  
DNS and refreshing information from DHCP.

> We can specify in the DHCP protocol that when the DHCP lease is  
> renewed,
> that the
> NTP client must accept a new IP address and if different from the old
> one, cease using
> the old one.  We can specify that the DHCP client may only serve site
> local addresses
> unless the IP addresses served are themselves the result of periodic  
> DNS
> lookup if
> FQDN's. We can specify that the FQDN used may not be hard coded and  
> must be
> configurable.

You can specify things until you're blue in the face, but no  
specification that you write forces the implementor to do anything.    
As a practical matter, specifying things like this isn't really going  
to accomplish your goals.

It's probably good to write a requirement that the NTP server MUST  
process updates from the DHCP client each time the client refreshes  
the NTP server addresses option, but the reason this will get  
implemented is that clients that don't implement it won't work reliably.

I would expect the DHC working group to choose between using IP  
addresses or using FQDNs.   We don't like to have flags that switch  
between these, because they create implementation complexity and cause  
interoperability problems.

I am in favor of using IP addresses because it uses less space in the  
packet, because it simplifies client-side implementations, and because  
every DHCP server of which I'm aware will take either an IP address or  
a domain name in any configuration field where an IP address is called  
for, and will look the IP address up in the DNS if a domain name was  
provided.   So you get the exact functionality you want, without any  
complexity on the part of the client.