21.2. NETWORK MONITORING 663
server is responsible for the root zone of the domain name space. Specifically, root servers
keep a database of the authoritative name servers for the top-level domains (e.g., .com, .edu,
.net, .org, etc.). Since the number of root servers is small and their role is very important
in resolving domain names, each of them is implemented via a number of computers, called
instances, that provide efficiency and resilience for that root server. Hence, when a name
server is trying to answer a query, its request is sent to one of the instances depending
on the status of the routing. Migration happens when the same name server is served by
different instances of the same root server across time.
The authors of [DSN12] describe an