Section: 13a.2.1 IMS Specific Configuration in the GGSN/P-GW
Content: The GGSN/P-GW shall have a list of preconfigured addresses of signalling servers (P-CSCF servers). This list shall be provided to MSs/UEs on request. The list shall be possible to preconfigure per APN. The GGSN/P-GW shall have locally preconfigured packet filters, and/or applicable PCC rules, as specified in 3GPP TS 29.212[ Policy and Charging Control (PCC); Reference points ]  [75] to be applied on the dedicated signalling bearer (e.g. PDP context). The packet filters shall filter up-link and down-link packets and only allow traffic to/from the signalling servers and to DNS and DHCP servers. The locally preconfigured packet filters shall be possible to pre-configure per APN. It shall be possible to enable/disable the use of the Gx interface per APN. When Gx is enabled the GGSN/P-GW shall handle IP-CAN Bearer Establishment (e.g. Create PDP Context Requests) as specified in 3GPP TS 29.212[ Policy and Charging Control (PCC); Reference points ]  [75]. The GGSN/P-GW shall support IPv4 and/or IPv6 addresses and protocol for IMS signalling and IMS bearers. The methods for an MS/UE to discover P-CSCF address(es) may vary depending on the access technology that the MS/UE is on. The details of the P-CSCF discovery mechanisms for various accesses are specified in 3GPP TS 23.228[ IP Multimedia Subsystem (IMS); Stage 2 ]  [52] and 3GPP TS 24.229[ IP multimedia call control protocol based on Session Initiation Protocol (SIP) and Session Description Protocol (SDP); Stage 3 ]  [47]. For example, for an MS/UE accesing to IMS via GPRS in GERAN and UTRAN, the GGSN shall provide support for P-CSCF discovery in three different ways (see 3GPP TS 23.228[ IP Multimedia Subsystem (IMS); Stage 2 ]  [52] and 3GPP TS 24.229[ IP multimedia call control protocol based on Session Initiation Protocol (SIP) and Session Description Protocol (SDP); Stage 3 ]  [47]): - GPRS procedure for P-CSCF discovery, i.e. request and provision of P-CSCF address(es) within the PCO IE in GPRS Session Management procedures (see 3GPP TS 24.008[ Mobile radio interface Layer 3 specification; Core network protocols; Stage 3 ]  [54]). - Via DHCP servers i.e. the GGSN shall provide the functionality of a DHCP relay agent - If the MS/UE has a P-CSCF FQDN locally configured and request the DNS IP address(es) from the GGSN (via PCO or DHCP procedures), the GGSN shall be able to provide DNS IP address(es) to the MS/UE. Similarly, the PGW shall have similar functional support depending on the P-CSCF discovery methods supported by the UE on the access technology. For example, for a UE in 3GPP access network (i.e. GERAN, UTRAN, EUTRAN), the P-GW shall support same functions as the GGSN that are specified above except that the P-GW does not act as a DHCP relay agent between the MS/UE and the external DHCP server. In other words, as specified in clause 13 of this document, the P-GW shall have DHCP server function towards the MS/UE while acting as a DHCP client towards external DHCP server, if the P-GW is configured to request DNS and/or P-CSCF IP addresses from external DHCP servers. The GGSN/P-GW shall be able to deliver DNS and/or P-CSCF addresses to the MS/UE if requested by the MS/UE via PCO (see 3GPP TS 24.008[ Mobile radio interface Layer 3 specification; Core network protocols; Stage 3 ]  [54] and 3GPP TS 24.301[ Non-Access-Stratum (NAS) protocol for Evolved Packet System (EPS); Stage 3 ]  [84]) or via DHCP procedures using the relevant DHCP options for IPv4/IPv6 DNS and SIP servers (see RFC 2132 [85], RFC 3361 [86], RFC 3646 [87] and RFC 3319 [88]). On APNs providing IMS services, the information advertised in Router Advertisements from GGSN/P-GW to MSs/UEs shall be configured in the same manner as for other APNs providing IPv6 services (see subclause 11.2.1.3.4), except that the "O-flag" shall be set. NOTE: The "O- flag" shall be set in IPv6 Router Advertisement messages sent by the GGSN/P-GW for APNs used for IMS services. This will trigger a DHCP capable MS/UE to start a DHCPv6 session to retrieve server addresses and other configuration parameters. An MS/UE which doesn’t support DHCP will simply ignore the "O-flag". An MS/UE, which doesn’t support DHCP, shall request IMS specific configuration (e.g. P-CSCF address) via the other discovery methods supported in the MS/UE (i.e. via locally configured P-CSCF address/FQDN in the MS/UE or via PCO procedure, if applicable). The GGSN/P-GW shall have configurable policy rules for controlling bearers (e.g. PDP contexts) used for signalling as specified in section 13a.2.2.2.