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

Mobility

The registration function of SIP is very similar to registration in cell phones. In

a registration message, a user sends a proxy server the URI for which it wishes

to receive calls. This built-in support of mobility is an extremely useful feature

of SIP and is one of the most often cited benefits of the protocol over others. It

is also this support of mobility that has lead to the application of the protocol

in many new applications and its proposed used for call control in third gen-

eration wireless networks.

The SIPREGISTERrequest is used to accomplish this function. The request

contains Contactheaders, which are the URIs being registered by the user.

For example, a successful user agent registration is shown in Figure 6.10. The

user initially registers his office SIP phone by sending a REGISTERmessage to

the Registrar server. The Registrar updates the user’s record in the location

service and returns a 200 OKconfirmation of the registration. Later in the day,

the user leaves his office for home where he cancels his office phone registra-

tion and registers his SIP home phone. (A mobile phone registration during the

commute could also be envisaged). Note that the protocol used to upload the

registration to the location service or other database is not SIP. Incoming calls

to the users URI will now be routed to the IP address of his SIP home phone.

Also note that the home phone need not be SIP for this pre-call mobility. The

user could also register a PSTN phone using web access, e-mail, or having the

registration preprogrammed for certain times.

Figure 6.10 Mobility Example using REGISTER

Phone at work

Phone at home

1 REGISTER

Location Server

Register Server uploads

registration information

to a Location Service so

that other Servers in

the domain can use the

registration information.

User moves from Office

to Home and clears all

registrations.

User re-REGISTERs at

Home Phone location.

User moves

from office to

home and

re-REGISTERs

Registrar Server

2 Update (non-SIP)

10 Update (non-SIP)

6 Update (non-SIP)

7 Response (non-SIP)

3 Response (non-SIP)

11 Response (non-SIP)

5 REGISTER

8 200 OK

12 200 OK

9 REGISTER

4 200 OK

124 Chapter 6