Internet Communications Using SIP : Delivering VoIP and Multimedia Services With Session Initiation Protocol {2Nd Ed.}

sends a set of ENUM queries in step 7 to DNS to find the corresponding URI
address, which is returned and used as sip:[email protected]
Step 9. The request is then routed to user B at that IP address, who returns a
successful SIP response 200 OKin step 10 to the proxy server. The proxy server
forwards the success response 200 OKin step 11 back to caller A.
The address resolution process in SIP is dynamic—a proxy can use any
header present in a request and many other factors in routing decisions,
including the following:

■■ Time of day

■■ Fromheader

■■ Various request header fields for load sharing or automatic call distrib-

utor (ACD) applications

Usually, this process of address resolution only occurs once at the start of a
session. The results of the initial address resolution are cached and used in
future requests between user agents.

Figure 6.2 Request address resolution example using location service and DNS

SIP User Agent
[email protected]

SIP User Agent

DNS Server [email protected]

1 SRV Query

Proxy Server Location Service DNS Server

5 Query

7 DNS Query

8 A Record

9 SIP Request

10 200 OK

6 Response

3 SIP Request

4 100 Trying

11 200 OK

2 SRV Record

SIP Overview 109