The DNS is designed to support many applications, such as referring to host
addresses or mailbox data, and, as shown here, is also used to locate SIP end-
points and SIP proxies in the network. Address formats differ for various pro-
tocols, and the DNS is designed to support various protocols with their own
notion of an address (such as the addresses for FTP, HTTP, mail, or SIP).
RFC 2219 [11] specifies the protocols and services. Following are some of the
ones of interest:
■■ File Transfer Protocol—FTP (RFC 959)
■■ Lightweight Directory Access Protocol—LDAP (RFC 1777)
■■ Network Time Protocol—NTP (RFC 1305)
■■ Post Office Protocol—POP (RFC 1939)
■■ Session Initiation Protocol—SIP (RFC 3261)
■■ SMTP mail(RFC 821)—SMTP
■■ World Wide Web, HyperText Transport Protocol—HTTP (RFC 1945).Other real-time communication services are shown in Table 4.1.
The DNS is independent of the underlying transport system for the data and
can work equally well with datagrams or with virtual circuits as originally
designed, although it is almost universally used with IP datagrams. The DNS
is also designed to be used by computers of all sizes, from small personal
devices to large computers. RFC 1034 was the initial basis for the DNS system
design. Other RFCs have introduced various improvements. Here, we limit
the discussion of the DNS to the information required for understanding SIP
services that use DNS.
Most data in the DNS system is assumed to change very slowly (for exam-
ple, mailbox addresses or host name–address bindings). Current IETF work
aims at providing faster, dynamic updates. Lower levels of the DNS may also
accommodate faster changes, for example on the order of minutes or even
seconds.A Partial DNS Glossary
Table 4.2 shows a summary overview of some DNS terminology, with special
emphasis on the use of DNS with SIP, where real-time communications reside.60 Chapter 4