Reading for Lecture Nov 4

Development of the Domain Name System

The large distribution cost of the HOSTS.TXT system prompts the genesis of DNS. The design assumptions for the DNS are: 1) provide at least all the same information as HOSTS.TXT; 2) Allow the database to be maintained in a distributed manner, 3) Have no obvious size limits for the names, name components, data associated with a name, 4) Interoperate across the DARPA Internet and in many other environment as possible, 5) Provide tolerable performance.

DNS has two major types of active components: name servers and resolvers. Name servers are repositories of information answering queries using whatever information they process. Resolvers interface to clients programs.

The DNS internal name space is a variable-depth tree where each node in the tree has an associated label. The domain name of a node is the concatenation of all labels on the path from the node to the root of the tree. The name space searching operation is done in a case-insensitive manner.

An organization can gets control of a zone of the name space by persuading a parent organization to delegate a subzone consisting of a single node. A caching mechanism for latter queries controlled by the TTL filed attached to each RR.

The search algorithm for the DNS is a downward search from domains that it can access already. Resolvers are typically configured with hints pointing at servers for the root node and the top of the local domain.

DNS Performance and the Effectiveness of Caching

The two widely believed most contributors to the scalability of DNS is hierarchy design and the aggressively use of caching. Both factors seek to reduce the load on the root servers at the top of the name space hierarchy while successful caching helps limit the client-perceived delays and wide area network usage.

The major questions would be: 1) What performance, in term of latency and failure, do DNS clients have? 2) How does varying the TTL and degree of cache sharing impact caching effectiveness? These questions are answered by using methods of analyzing the traces of TCP traffic along with related DNS traffic of three network traces.

One surprising result is that over a third of all lookups are not successfully answered. The DNS servers also appear to retransmit overly aggressively. The DNS usage patterns and performance are also observed to change. The capture of TCP traffic helps perform trace-driven simulations to investigate two important factors that effect caching effectiveness.

The root of the hierarchy is centrally administered and served from thirteen root servers. Sub-domains are delegated to other servers that are authoritative of the portion of their name space. To perform the analyzing, both DNS traffic and its driving workload are collected.