Section: 13a.2.2.2 Establishment of a PDP Context/EPS Bearer for Signalling
Content: The following applies for establishing a PDP context/EPS bearer for IMS signalling in the GGSN/P-GW as per 3GPP TS 23.228[ IP Multimedia Subsystem (IMS); Stage 2 ]  [52]: I. The GGSN/P-GW shall allow IMS signalling on a "general-purpose PDP context"/default EPS bearer, in which case the IMS signalling shall be provided like any other transparent services provided by the packet domain. II. The GGSN/P-GW may (dependent on operator policy) also support PDP Contexts/EPS Bearers dedicated for IMS services. If the the bearer establishment selected for the APN is controlled by the MS/UE, the MS/UE may request a dedicated PDP context/EPS Bearer for IMS signalling (see 3GPP TS 24.229[ IP multimedia call control protocol based on Session Initiation Protocol (SIP) and Session Description Protocol (SDP); Stage 3 ]  [47]) by setting the IM CN Subsystem signalling flag in the PCO IE. Specifically, the GGSN/P-GW may receive the IM CN Subsystem signalling flag in the PCO IE in the PDP context activation or Secondary PDP context activation for GERAN and UTRAN accesses; in the Attach, UE-requested PDN connectivity request or UE-requested bearer resource modification procedures for E-UTRAN access. If the bearer establishment selected for the APN is controlled by the network, depending on the operator policy, the GGSN or P-GW may also activate a PDP context or dedicated EPS bearer for IMS signalling via network-initiated Secondary PDP context activation or network-initiated Dedicated Bearer activation procedures, respectively. If dedicated PDP contexts/EPS Bearers for IMS signalling are not supported, the GGSN/P-GW will reset the signalling flag in the response to the MS/UE. For P-GW, if the APN offers non-IMS services, the default bearer shall not be allowed to be set up as a dedicated IM CN signalling bearer. If the PDP context/EPS bearer can be established as a dedicated bearer for IMS signalling, the GGSN/P-GW can provide a set of UL filters for the PDP context/EPS bearer used for IMS. The UL filters provide the MS/UE with the rules and restrictions applied by the GGSN/P-GW for the dedicated PDP context/EPS bearer for IMS signalling. The GGSN/P-GW can in addition provide the IMS signalling flag to explicitly indicate to the MS/UE the intention of using the PDP context/EPS bearer for IMS related signalling. In both cases, I and II, - The GGSN may receive the Signalling Indication parameter in the QoS IE. This indicates a request for prioritised handling over the radio interface. The GGSN shall be able to downgrade the QoS (dependent on operator policy) by resetting the Signalling Indication according to the normal procedures for QoS negotiation, see 3GPP TS 23.060[ General Packet Radio Service (GPRS); Service description; Stage 2 ]  [3]. - During the (default or dedicated) EPS bearer activation procedure, the P-GW may receive the request for appropriate QCI for IMS signalling traffic as specified in 3GPP TS 23.203[ Policy and charging control architecture ]  [90] in EPS bearer QoS IE that indicates the prioritised handling of the EPS bearer over the radio interface. The P-GW may either accept or reject the request based on the operator policy configured at the P-GW. IM CN subsystem signalling flag and Signalling Indication or appropriate QCI for IMS signalling in the QoS IE may be used independently of each other. However, based on the operator policy, the network may honor the Signalling Indication or appropriate QCI for IMS signalling if the IM CN Subsystem signalling flag is present in PCO IE. The operator may provide other properties to the PDP contexts/EPS bearers dedicated for IMS signalling, e.g. special charging. It is out of the current scope of this TS to further specify these properties. For a PDP Context/EPS bearer marked as a dedicated for IMS signalling, the GGSN/P-GW shall apply the applicable PCC rules, as specified in 3GPP TS 29.212[ Policy and Charging Control (PCC); Reference points ]  [75], and/or locally preconfigured packet filters, which shall only allow packets to be sent to and from a set of signalling servers, such as P-CSCF(s), DHCP server(s) and DNS server(s). The TFT filters on the dedicated signalling bearer shall have a precedence value so that they precede any other TFT filters. This will secure the use of the correct PDP context/EPS bearer for the signalling traffic, and that only authorized traffic uses the signalling PDP context. The locally preconfigured packet filters shall be defined in the GGSN/P-GW by the operator.