R4. IMS Registration signalling flow —
P-CSCF subscription for registration state event package
R4-1. SIP SUBSCRIBE request
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This example describes the subscription procedure for the network initiated deregistration event, whereby the P-CSCF
requests to be notified by the S-CSCF when the event has occurred.
It is assumed that the user has registered prior to initiating subscription of an event.
For this example the trigger point at the P-CSCF for sending out the SUBSCRIBE
request is the 200 (OK) response of the user's registration.
P-CSCF1 performs the DNS queries to locate the I-CSCF in the home network. The look up in the
DNS is based on the address specified in the Request URI.