A registration lasts as long as the subscriber device remains in the service area. For
example, in a wireless network, as long as the subscriber device continues to receive
service from the same cell site, the registration remains alive. However, as soon as the
subscriber moves to another cell site, the registration is changed (because the cell site
address is changed).
In a wireline model, as long as the subscriber device maintains the same IP address,
the registration remains alive. When the subscriber is assigned a new IP address, then
the subscriber device must register its new IP address.
Of course, there is a timer associated with all registrations. Each registration has an
expiration, and once the expiration is reached, the registration will be terminated by the
S-CSCF and HSS. This prevents a subscriber device from registering with the network
and then disconnecting without canceling the registration. That would of course provide
an opportunity for a hacker to come along and take advantage of the registration.
To prevent exposure of network topology (the number of S-CSCFs, addresses of internal
entities, etc.) to non-trusted networks, the I-CSCF provides a function known
as topology hiding. Topology hiding entails removal of certain SIP headers to prevent
other networks from learning about the network.
For example, the ROUTE header contains addresses of all the IMS entities that were
used to route the message.
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