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In order to communicate with the IMS, an item of UE has to know at least one IP address of the P-CSCF. The mechanism by which the UE retrieves these addresses is called “P-CSCF discovery”. Two dynamic mechanisms for P-CSCF discovery have been standardized in 3GPP: the Dynamic Host Configuration Protocol's (DHCP) DNS procedure and the GPRS procedure. Additionally, it is possible to configure either the P-CSCF name or the IP address of the P-CSCF in the UE (see Section 3.6).

In the GPRS procedure (Figure 3.8), the UE includes the P-CSCF address request flag in the PDP context activation request (or secondary PDP c...


Cover of THE IMS: IP Multimedia Concepts and Services, 3rd Edition