S1. IMS Session Initiation signalling flow —
mobile origination and termination roaming
with different network operators
S1-1b. SIP INVITE request (UE1 to UE2):
from P-CSCF1 to S-CSCF1
(3 of 70)
P-CSCF1 adds itself to the Record-Route header and Via header. As the request is forwarded to an interface that is not compressed, the own P-CSCF SIP URI does not contain the "comp=sigcomp" parameter.
P-CSCF1 removes the Security-Verify header and associated "sec-agree" option-tags prior to forwarding the request. As the Proxy-Require header is empty, it removes this header completely.
: P-CSCF1 inserts the SIP URI in the P-Asserted-Identity header field and it also removes P-Preferred-Identity header field.
: P-CSCF1 inserts this header and populates the icid parameters with a globally unique value.
S-CSCF#1 validates the service profile of this subscriber and evaluates the initial filter criterias. For this example, assume no Application Server involvement.