Question: In a Subdomain-based Public Service Identity (PSI) setup, how can an incoming session from an external network be routed to the Application Server (AS) hosting the PSI when the subdomain is not defined in the global DNS?
Option 1: The originating S-CSCF directly queries the HSS for the AS's IP address.
Option 2: The PSI is resolved by the global DNS to an I-CSCF address, which then forwards the request to the AS after resolving it internally.
Option 3: The external network's DNS infrastructure is updated with the subdomain details to directly route to the AS hosting the PSI.
Option 4: The incoming session is rejected due to the lack of global DNS entry for the subdomain.
Explanation: In Subdomain-based PSIs, without a global DNS entry for the subdomain, the PSI resolves to an I-CSCF address via global DNS. The I-CSCF then internally resolves the PSI to the AS's address and forwards the request, ensuring routability without direct global DNS entry for the subdomain.