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

[Jim Reid]

On 21 Dec 2009, at 18:24, Colm MacCárthaigh wrote:

> To take the simplest example;
>
>   foo 10 IN NS bar.example.com
>   bar.example.com 20 IN A 192.0.2.0
>
> if foo is a frequently accessed record - e.g. at least 1 query per
> second. Won't the NS and A expire at the same time, always?

Nope. The NS record will expire every 10 seconds and the A record  
every 20, all other things being equal.

> That's two extra resolution queries.

How awful.

BTW it's actually 3 system queries every 20 seconds for the scenario  
you outline, 2 "refreshes" for foo and 1 for bar. In that 20 seconds,  
your resolver has had to answer 20 queries for some name in foo. So  
the cache hit rate is 85%. For simplicity, assume the name clients are  
looking up in foo have a 1 second TTL and don't involve further  
delegation or chasing down CNAME/DNAME chains.

> If the 10 and 20 were instead 11 and 23 ..
> this occurs every 253 seconds, or one in every 23 times compared to
> the former example.

No. If the numbers were 11 and 23, the NS record for foo would expire  
every 11 seconds and the A record every 23: 3 or 4 system queries  
every 23 seconds in the simple scenario you describe. There would be 1  
"refresh" for bar and 2 or 3 for foo every 23 seconds. That's a hit  
rate of 87%.