R4. IMS Registration signalling flow —
P-CSCF subscription for registration state event package
R4-1a. SIP SUBSCRIBE request:
from P-CSCF1 to I-CSCF1
(2 of 10)
: This header is populated with the SIP URI that identifies the P-CSCF.
: This is where the NOTIFY requests for this subscription will be sent.
: This field is set to the value 'reg' to specify the use of the reg event package.
I-CSCF1 sends a query to the HSS to find out the S-CSCF of the called user. The HSS responds with the address of the current S-CSCF for the terminating subscriber.