IMS R1-3d. SIP REGISTER request, with credentials: between I-CSCF1 and HSS

IMS R1 signalling flow - Registration: User not registered - IMS R1-3d. SIP REGISTER request, with credentials: between I-CSCF1 and HSS

I-CSCF1 requests information related to the Subscriber registration status by sending the private user identity, public user identity and visited domain name to the HSS. The HSS returns the S-CSCF name which was previously selected in step 1d.