The visited network
may access its own domain name server to learn the IP address for the I-CSCF of the
subscriber??™s home network, but it would not query a DNS to learn the IP address for
the S-CSCF in the subscriber home network. This is left to the I-CSCF in the home
network.
For subscribers, the use of a domain name server is not as important, because the domain
of the service provider is all that is needed. To locate the address of any individual
subscriber, networks would first query the registrar in the home network, which is the
S-CSCF. The S-CSCF stores the IP address of each subscriber during the registration
process.
As a subscriber moves about the network (and changes his or her IP address) the
S-CSCF is updated with the user??™s identities and associated IP addresses. Note that a
device may have multiple IP addresses assigned to multiple user identities as well.
User Identities
User identities are used for two purposes in the IMS. First and foremost, routing of
requests and responses are directed to a user identity, which is associated with an IP
address (disclosed during the registration process). This same identity cannot be used
for billing, however, as this would pose a security threat.
For this reason a separate identity is used for billing and accounting within the IMS.
This identity is not disclosed outside of the network and is not visible to end users of
IMS services.
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