[dns-operations] Prime TTL values for TLD and root server delegations.

[Colm MacCárthaigh]

No doubt, this question has been asked before - but I can't seem to
find a good, documented, answer for it;

Why aren't the TTLs on NS delegations served by root servers and TLD
operators prime numbers?

colmmacc at caer (~) $ dig www.example.com @k.root-servers.net
...
com.			172800	IN	NS	a.gtld-servers.net.

colmmacc at colmmacc (~) $ dig www.example.com @a.gtld-servers.net.
...
example.com.		172800	IN	NS	a.iana-servers.net.

As-is, this configuration means that when primed by a typical query,
the caches expire at the same time. If they were different, ideally
prime, numbers then this coincidence of expiration-time would be
minimised (ideally each NS down any given delegation chain would have
a different, unique, prime number) - meaning that real-world requests
would have to deal with fewer cache misses.

I realise that resolvers will typically prime "." asynchronously on
start-up, mitigating the problem slightly, but there is still
sub-optimality. Forgive my naivety, but is there a mechanism by which
these values were reached? Is there a recommendation document
anywhere?

-- 
Colm