Addressing
IP communications use SIP Uniform Resource Identifiers (URIs) for address-
ing similar to e-mail, where the form of the URI resembles an e-mail address in
mailto:, such as user@domain. A more detailed discussion of URIs is pro-
vided in Chapter 4, “DNS and ENUM.”
SIP URIs can have various forms and include telephone numbers. For
example,
sip:[email protected]Example: “Dialing” an address refers to the PC of Henry in the domain
pulver.com. (See Chapter 4, “DNS and ENUM,” for more about domain
names.)
Here are some examples of using URIs for SIP:
sip:[email protected]; user=phoneis a phone number that can be reached via a gateway (note that visual separa-
tors within a telephone number, such as dashes and dots, are optional and are
ignored by the protocol);
sip:[email protected]; user=phone; phone-context=VNETis a phone number in the internal network “VNET” of pulver.comand
sip:[email protected]is the address of the laptop of a guest plugged into the LAN of a conference
room in the pulver.comdomain.
The support for both telephony and web-type addressing enables Internet
communications to bridge in a seamless way the telephone network and the
Internet. Users on either network can reach any point either on the PSTN or on
the Internet without giving up existing devices or the accustomed conven-
iences of either. For example, a user of the telephone network can make a call
to a device on the Internet or to any other device on any other network (mobile
voice, paging, data networks) just by dialing a number, as will be explained
further. The ENUM technology allows users to have a single URl or phone
number, if they so prefer, on their business card for contact information.
SIP in a Nutshell
Table 2.3 explains the core SIP functions in a nutshell and Figure 2.1 shows the
flow diagram for the message exchange between to SIP endpoints.
Internet Communications Enabled by SIP 15