Suppose that the local DNS server caches all information coming in from all root, TLD$,$ and authoritative

DNS servers for $20$ time units. $($Thus$,$ for example, when a root server returns the name and address of a

TLD server for $.$ca$,$ the cache remembers that this is the TLD server to use to resolve a $.$ca name$).$ Assume

also that the local cache is initially empty, that iterative DNS queries are always used, that DNS requests

are just for name$-$to$-$IP$-$address translation, that $1$ time unit is needed for each server$-$to$-$server or hostto$-$server $($one way$)$ request or response, and that there is only one authoritative name server $($each$)$ for

any $.$ca domain