The procedure specified in this clause enables to restore in the EPC the PDN connections affected by an PGW-C/SMF failure with or without restart or scale-in operation, and thus to maintain the UE connectivity to the PDN and corresponding services with minimum service interruption and minimal signalling in the network (e.g. no signalling wih the UE). The procedure specified in this clause is optional to support for the MME, ePDG supporting S2b over GTPv2, SGW and PGW-C/SMF. This procedure applies for combo PGW-C/SMF that are deployed in an PGW-C/SMF set (i.e. a set of PGW-C/SMF instances that are functionally equivalent and inter-changeable and that share the same contexts, see clause 5.21.3 of TS 23.501). In scenarios where a PGW-C/SMF becomes no longer available (e.g. PGW-C/SMF failure without restart, scale-in operation causing a PGW-C/SMF to be de-instantiated from the PGW-C/SMF set) or that require to change the PGW-C/SMF of a PDN connection, other PGW-C/SMFs from the same PGW-C/SMF set may take over the control of the PDN connections that were served by the PGW-C/SMF that is no longer available. When a PGW-C/SMF fails with restart, the restarted PGW-C/SMF should continue supporting the same PDN connections, if possible. When a PGW-C/SMF of a PGW-C/SMF set fails with or without restart, the Recovery (Restart Counter) shall not be incremented over S5/S8 (assuming the PDN connection context is still available in the PGW-C/SMF set). The restoration of a PDN connection may be triggered by the MME (e.g. when the SGW detects and reports to the MME that the PGW-C has failed) or by the ePDG supporting S2b over GTPv2, or by the PGW-C/SMF (e.g. scale-in operation or PGW-C/SMF reselection by another network function such as PCF). The procedure supports the restoration of Home Routed PDN connections, if the VPLMN and HPLMN support this procedure. If the VPLMN or HPLMN does not support this procedure, the existing behaviour applies, e.g. the MME and SGW clears all PDN connections of the PGW-C/SMF when detecting the failure or restart of the PGW-C/SMF and the MME may request UEs to release and reactivate some PDN connections (e.g. IMS PDN connections). Clause 31.6 defines procedures to enable to restore a set of PDN connections affected by a PGW-C/SMF failure with or without restart, or scale-in operation, associated with specific FQ-CSIDs, Group IDs or PGW-C/SMF IP addresses.

The PDN connection shall be established as defined in TS 23.401 with the following additions.

Likewise, a PDU session shall be moved from 5GS to EPS with the N26 interface as defined in clause 4.11 of TS 23.502 with the following additions:

• the MME may signal that it supports this procedure by setting the Restoration of PDN connections after an PGW-C/SMF Change Support Indication in the Indication Flags IE in:
  • the Create Session Request during 5GS to EPS Idle mode mobility using N26 interface (see clause 4.11.1.3.2 of TS 23.502); and
  • in the Modify Bearer Request during an 5GS to EPS handover using N26 interface (see clause 4.11.1.2.1 of TS 23.502);
• If the SGW also supports this feature, it shall relay this indication in the Modify Bearer Request towards the PGW;
• If the Modify Bearer Request received by the PGW-C/SMF indicates support of this procedure and the PGW supports the Restoration of PDN connections after a PGW-C/SMF change procedure specified in clause 31, the PGW-C/SMF shall return a PGW Change Info IE (with the same content as described above) in the Modify Bearer Response, that the SGW shall relay transparently towards the MME. This provides the same indication as described above to the MME.

The PDN connection shall be established as defined in clause 7.2.4 of TS 23.402 with the following additions.

Likewise, a PDU session shall be moved from 5GS to EPC/ePDG as defined in clause 4.11.4 of TS 23.502 with the following additions:

• the ePDG may signal that it supports this procedure by setting the Restoration of PDN connections after an PGW-C/SMF Change Support Indication in the Indication Flags IE in the Create Session Request during handover from 5GS to EPC/ePDG (see clause 4.11.4.2 of TS 23.502); and
• If the Create Session Request received by the PGW-C/SMF indicates support of this procedure and the PGW supports the Restoration of PDN connections after a PGW-C/SMF change procedure specified in clause 31, the PGW-C/SMF shall return a PGW Change Info IE (with the same content as described above) in the Create Session Response to provides the same indication as described above to the ePDG.