6a) Before the 9/11 attacks, the top-level domain (TLD) server for South Africa was located in New York City.  Explain why the physical destruction on 9/11 disrupted Internet communication within South Africa (e.g., for a Web user in South Africa accessing a Web site in South Africa)?

(6b) Following up on question 6a, explain why the effects in South Africa took place gradually, disrupting progressively more communication within the country in the hours (and even days) after connectivity to NYC was lost.

(6c) How do the local DNS servers know the identity of the root servers?  Why are most of the host-to-address queries seen by the root DNS servers for bogus or malformed names (like graceland.elvis or numeric top-level domains)?

(6d) Who determines the value of the time-to-live field that determines how long DNS servers cache a name-to-address mapping?  What are the pros and cons of using a small value?

(6e) A local DNS server typically discards cached name-to-address mappings when the time-to-live expires.  Alternatively, the local DNS server could optimistically issue a new query for the cached domain name.  Given one advantage and one disadvantage of that approach.