- S-CSCF1 first authorizes the subscription. As S-CSCF1 can trust the content of the P-Asserted-Identity header and <sip:user1_public1@home1.net> is on the list of the authorized users for the "reg" event package stored by S-CSCF1, therefore S-CSCF1 sends an acknowledgement towards the UE indicating that the subscription was successful. This response will traverse the path that the SUBSCRIBE request took as described in the Via list.