This is based on timers and the EXPIRES parameter
found in the REGISTER message. The device will re-register 600 seconds prior to the
expiration time if the initial expiration time was greater than 1200 seconds. If not, the
device will re-register when the expiration timer reaches half its initial value. Should
a subscriber fail to update their registration prior to expiration, the S-CSCF will deregister
the subscriber from the network.
Re-registration follows the same procedures we have already described, for the exception
of routing. Since the routing is already established for all messages from the
Establishing a Session in the IMS 115
device to the S-CSCF (and recorded via SIP headers by each network entity during the
registration process), the I-CSCF does not pay any attention to the P-CSCF address.
The S-CSCF can also forego the queries to the HSS during re-registration, although
this is left to the operators to decide.
When re-registering with the network, the subscriber??™s device sends the REGISTER
message with the following headers and parameters:
?– AUTHORIZATION populated with the private user identity
?– FROM populated with the public user identity to be registered
?– TO populated with the public user identity to be registered
?– CONTACT populated with the SIP, URI, and IP address of the subscriber
?– VIA populated with the IP address of the subscriber device
?– EXPIRES set to 600,000 seconds
?– REQUEST-URI populated with the domain name of the home network
?– SECURITY-CLIENT populated with the supported security mechanisms
Device P-CSCF I-CSCF HSS S-CSCF
1.
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