Since the built-in DNS client will talk to one of the configured DNS servers
in the local IP protocol stack (DHCP or static configuration), when the DNS
returns an IP address, the browser will then display:Contacting: ietf.orgfollowed byWeb site found. Waiting for reply...The web page content is transferred to the browser after the web server has
responded. The speed of the response may sometimes make it difficult to fol-
low this message sequence. The high speed of the this process, given reason-
ably fast Internet access, shows the power of the global Internet DNS system.
SIP is closely related to web technology and can make similar use of DNS-
based lookup.
The preceding steps in finding a web site illustrate very roughly how the
Internet DNS is used by the browser by performing the following operations:- The local DNS resolver sends a DNS query for ietf.org(Finding
site...) - The DNS returns the IP address for the site.
- The HTTP request from the browser is sent there (Contacting...)
- An initial reply comes from the web site (Web site found...), but
this is not a DNS operation any more.
The DNS operations will be described summarily further in this chapter,
before proceeding further with the SIP topics. All SIP topics presume some
understanding of the underlying DNS operations, as well as ENUM [3], which
is of particular interest to VoIP.
The DNS clientlocated in the application is also called a DNS resolver, and
we will use here these names interchangeably.Addressing on the Internet
Understanding the DNS and directories requires a brief review of Internet and
web addressing that we provide here. Readers familiar with this topic can pro-
ceed directly to the section on DNS.The Universal Resource Identifier (URI)
The Universal Resource Identifier (URI) [4] is a name associated with a universal
set of names in a registered naming space, such as Internet domain names54 Chapter 4