10.3. P-CSCF discovery
The P-CSCF is the single entry point for all SIP messages that are sent from Tobias's UE to the IMS. Therefore, the P-CSCF address needs to be known by the UE before the first SIP message is sent. As this address is not pre-configured in our example, it needs to be discovered by the UE.
In the case of the GPRS the UE can request the addresses of a P-CSCF during the establishment of the general or signalling PDP context. The GGSN will then return the IPv6 prefix of an P-CSCF in response to the activate PDP context request.
Alternatively, the UE can choose to use Dynamic Host Configuration Protocol for IPv6 (DHCPv6) in order to discover the P-CSCF. If the P-CSCF address is returned from DHCP as a Fully Qualified Domain Name (FQDN) rather than an IP address, then the P-CSCF address will be resolved via the Domain Name System (DNS) as the address of any other SIP server. The related procedures are described in Chapter 12.
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