Table 4.2 (continued)The failure of one or more servers should not cause
a service failureat all, as long as there are other
servers left. If a server fails during a call setup, that
call setup will fail, however, and a new call setup
request must be made.
The use of SRV records to locate a SIP proxy server
for a particular domain is described in RFC 3263
[13].
Mail Exchange Record (MX) Identifies a mail exchange in the domain.
Name Server (NS) Authoritative name server for the domain.
Pointer (PTR) Pointer to another part of the domain space. These
records are used for backward DNS lookups—
resolving an IP address into a domain name.
Naming Authority Pointer The Naming Authority Pointer (NAPTR) [3] is an RR
(NAPTR) used for ENUM written according to specific rules, so that the
address lookup by the DNS client can be continued
for a new target that is computed from the response.
For example, the E.164 phone number
+1-770-555-1212
can be converted for lookup in the DNS of the
e.164.arpadomain to
2.1.2.1.5.5.5.0.7.7.1.e164.arpa
A query sent to 164.arpamay produce the NAPTR
records for the hosts that can further process this
address:
sip:[email protected]
mailto:[email protected].
Thus, when the phone number +1-770-555-1212 is
dialed, the call will go to the information service of
example.com.DNS and ENUM Usage Example
We will illustrate the DNS and ENUM terminology with an example for real-
time communications based on SIP. Since SIP will be explained in more detail
in the following chapters, the required SIP message exchanges have been sum-
marized in Figure 4.2 as “SIP Transactions,” because they are outside the topic
of DNS.62 Chapter 4