Following is the legend for the items in the NAPTR response:
■■ IN—Internet class
■■ NAPTR—Query type
■■ 50 —Ordering for targeting by the client, RFC3403
■■ 50 —Preference (is a server selection mechanism)
■■ “s”—Flag to indicate the next lookup will be for a service record
■■ “SIPS+D2T”—DNS service field for TLS (D2T), required for SIPSThe result for the NAPTR query performed in the domain altlanta.com
is as follows:_sips._tls. altlanta.com (the underscore “_” prevents name collisions)The SRV query is now (query #2):_sips._tls.-altlanta.comThe DNS with return the domain name and IP address of the server that
provides the answer (for example, dns3.example1.com, 1.2.3.4) and also
SRV locations:sip1.-altlanta.comsip2.-altlanta.comsip3.-altlanta.comThe DNS resolver will now make the query #3 for an A record, that is to find
the IP address of one of the three SIP servers. The pseudorandom mechanism in
the client for load sharing could select sip3.example1.comfor the A type
DNS query. The returned IP address could be 1.2.3.14. Now the SIP UA has
the IP address of an outgoing SIP proxy to register for service, and the resulting
SIP transaction is shown by the dotted double arrow on the left in Figure 4.2.Finding an Incoming SIP Server in the ENUM Case
Suppose that [email protected] not know the SIP URI for Bob, but
has only Bob’s telephone number: +1-404-555-1234. This is where ENUM
comes into play.
Either the SIP UA of Alice or the outgoing SIP server for the domain
atlanta.comneeds to find the URI of Bob when it has only the telephone
number for Bob. The queries shown in this example can be performed either
by the SIP UA of Alice or by the outgoing SIP proxy for the domain
atlanta.com shown in Figure 4.3. We would prefer to give Alice the64 Chapter 4