There are cases when a device is sending
a re-register that the S-CSCF would not have to query the HSS again because the
device is already known to be trusted.
Only one registration can be executed at a time. The device cannot send a REGISTER
while waiting for a response from a previous REGISTER. One registration must be
completed before starting another one.
Registration must be changed whenever the device changes locations. For example,
when a mobile subscriber is roaming, that subscriber will have to register each time
he or she roams into a new service area (or changes cell tower connections). Remember
that the registration process is for informing the HSS and the S-CSCF of the present
location (physical address) for the device so that requests and responses can be correctly
routed to the device.
Registration is also a means for authenticating subscribers and verifying their credentials.
This is accomplished by equipping devices with an authentication key that
can then be stored in the Home Subscriber Server (HSS). Each time the subscriber
registers, the device will be challenged by the registrar (the S-CSCF) to provide the
authentication keys. Chapter 5 discusses more about the authentication process and
overall security in the IMS.
One final note about authentication. The AUTHORIZATION header in the REGISTER
message may contain the parameter INTEGRITY-PROTECTED=YES.
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