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

Figure 4.2 High-level DNS and ENUM service example for SIP

Finding an Outgoing SIP Server

We assume the user Alice has a SIP service subscription in the domain
Atlanta.com. To make use of the SIP service, the SIP user agent (UA) Alice
must first register with the outgoing SIP server in the domain Atlanta.com.
To register, the SIP UA must first:

1. Chose a transport protocol for with adequate security to register; that is
   to discover Transport Layer Security (TLS) if it is available. SIP signal-
   ing using TLS transport is called SIPS in the following.


2. Discover the outgoing SIP servers.

The DNS resolver in the application that also contains the SIP UA will first
decide to use SIPS with TLS transport and then do a NAPTR query (query #1
in Figure 4.3) for the SIP servers in the domain example1.com.

IN NAPTR 50 50 “s” “SIPS+D2T”

SIP UA
[email protected]

1

SIP UA

[email protected]

DNS

Servers

SIP Transactions

DNS

queries

and

responses

Outgoing SIP

Servers

Incoming SIP

Servers

DNS

Servers

2 3 4 5 6 7

SIP Transactions

SIP

Transactions

ENUM

and DNS

queries

and

responses

Domain atlanta.com Domain biloxi.com

DNS and ENUM 63