Delegation
The actual DNS records for host names and servers, and so on, for a domain
are the responsibility of, and under the complete control of, the administrator
of that domain.
For example, the Address records (or A records) that contain the IP
addresses of servers within the domain “example.com” (such as
mail.example.com, http://www.example.com, ftp.example.com, and so on)
are maintained there. The domain “example.com” is said to be authoritative
for the IP addresses, names, local resources, and delegations further down
within its own domain. The creation and deletion of names is fully distributed
and delegated to lower levels of the DNS in this manner.
Figure 4.1 shows an example with several leaves of the “golden tree” of the
DNS. The name “golden tree” is because of the clear structure of delegation for
authority within the DNS so as to avoid any errors caused by incorrect dupli-
cation of data. This clear structure of the DNS is a key architectural principle of
the Internet and is the technical explanation why the Internet has one and only
authority for assigning names and numbers—the Internet Authority for
Assigned Names and Numbers (IANA).
Caching
Without caching, every DNS query would have to begin at the root DNS
server, continue downward to the top-level domain server, and then end at the
authoritative DNS server. However, very efficient caching schemes employed
in DNS make this the rare exception rather than the rule. Most DNS queries
only traverse one or two DNS servers. The price paid for this efficient caching
is that DNS changes (updates) do not happen in real time but take significant
time to propagate throughout the Internet. As a result, DNS is not suited for
roaming and other mobility services where the IP address may change rapidly.
Figure 4.1 Example of the DNS “golden tree” leaves
COM NET EDU BIZ ARPASMU TAMU COLUMBIA ISI MITDNS and ENUM 59