R4. IMS Registration signalling flow —
P-CSCF subscription for registration state event package
R4-2a. SIP 200 OK response:
from S-CSCF1 to I-CSCF1
(5 of 10)
: If the value of the Expires header in SUBSCRIBE request is different from the one received in REGISTER method, then the value of Expires header in the 200 (OK) response is set to match the value of Expires header in REGISTER method.