- Based on the user's URI, P-CSCF1 determines that UE1 is registering from a visiting domain and performs the DNS queries to locate the I-CSCF in the home network. When forwarding the REGISTER request P-CSCF1 needs to specify the protocol, port number and IP address of the I-CSCF server in the home network to which to send the REGISTER request. The DNS records are retrieved according to RFC 3263.
- P-CSCF1 performs an NAPTR query for the domain specified in the Request-URI.
- Based on the order and preference of the NAPTR record and the local preference, UDP is preferred and P-CSCF1 finds the I-CSCF by a DNS SRV lookup according to RFC 2782.
- In the Answer field of the query-response each I-CSCF is identified by its host domain name. The returned SRV Resource Records (RRs) are merged and ordered, and the selection technique (employing the Priority and Weight parameters returned in the RRs) as specified in RFC 2782 is used to select the I-CSCF (i.e. "icscf1_p.home1.net"). Since the Additional Data field of the query-response also contains the IP address of the selected I-CSCF (i.e. "5555::aba:dab:aaa:daa"), a new query to the DNS is not required. P-CSCF1 will forward the REGISTER request to the IP address "5555::aba:dab:aaa:daa" using the UDP protocol and port number 5060.