Section: 13a Interworking with IMS 13a.1 General
Content: Interworking with the IP Multimedia Core Network Subsystem (IMS) puts additional requirements on the GGSN/P-GW. When the MS/UE connects to the IP Multimedia Core Network Subsystem (IMS), specific parameters in Session Management messages may be handled. The IMS specific parameters are: IMS signalling flag, P-CSCF address request, P-CSCF restoration support, returned P-CSCF address(es) and flow identifier(s). For interworking with the IMS, the Gx interface (see 3GPP TS 29.212[ Policy and Charging Control (PCC); Reference points ]  [75]) is used to correlate the session (SIP/SDP) and the bearer (PDP Contexts). The mechanisms in GGSN/P-GW to support IMS shall be: - P-CSCF discovery. - Dedicated signalling bearer (e.g. PDP contexts) (with or without enhanced QoS) for IMS signalling; with associated packet filters to permit signalling to/from designated servers. - Gx interface for policy and charging control of bearer (e.g. PDP contexts) for IMS media flows. - P-CSCF restoration. These mechanisms are however not restricted to the IMS and could be used for other services that could benefit from these mechanisms. The P-GW shall not modify the fields Type of Service (IPv4) and Traffic Class (IPv6) of the received downlink packets. NOTE: The P-CSCF can support paging policy differentiation for different traffic or service types over LTE by marking the fields Type of Service (IPv4) and Traffic Class (IPv6) (see clause L.3.2.4 of 3GPP TS 24.229[ IP multimedia call control protocol based on Session Initiation Protocol (SIP) and Session Description Protocol (SDP); Stage 3 ]  [47]).