S1. IMS Session Initiation signalling flow —
mobile origination and termination roaming
with different network operators
S1-5a. SIP UPDATE request (UE1 to UE2):
from UE1 to P-CSCF1
(28 of 70)
When the resource reservation is completed, UE1 sends the UPDATE request to the terminating endpoint, via the signalling path established by the INVITE request.
: Takes the value of the Contact header of the received 183 Session Progress response.
: Takes the value of either the IP address or FQDN of the originating UE.
: Takes a higher value than that in the previous request.
: Contains the security agreement as represented by the received Security-Server header.