S1. IMS Session Initiation signalling flow —
mobile origination and termination roaming
with different network operators
S1-2a. SIP 183 Session Progress response:
from UE2 to P-CSCF2
(9 of 70)
: UE2 provides the access-type and access-info, related to the serving access network.
: A tag is added to the To header.
: Contains a SIP URI with the IP address or FQDN of UE2. It includes the comp=sigcomp parameter.
: The SDP contains the set of codecs supported UE2. It requests a confirmation of the QoS preconditions for establishing the session.
P-CSCF2 authorizes the resources necessary for this session. The approval of QoS commitment either happens at this stage or after 200 OK of INVITE based on operator local policy.