16.10 Handover procedure from 3GPP access to WLAN on S2a

23.4023GPPArchitecture enhancements for non-3GPP accessesRelease 18TS

Tools: ARFCN - Frequency Conversion for 5G NR/LTE/UMTS/GSM

16.10.1 Handover procedure from 3GPP access to WLAN on S2a in single-connection mode

16.10.1.1 Handover in single-connection mode from 3GPP access to WLAN on GTP S2a

This procedure is used in the single-connection mode to hand over a single PDN Connection from 3GPP access to WLAN. The decision to use the single-connection mode is made during authentication as described in clause 16.2.

Figure 16.10.1.1-1: Handover in single-connection mode from 3GPP access to Trusted WLAN on GTP S2a for roaming and non-roaming scenarios

The home routed roaming, LBO and non-roaming scenarios are depicted in the figure 16.2.1-1:

– In the LBO case, the 3GPP AAA Proxy acts as an intermediary, forwarding messages from the 3GPP AAA Server in the HPLMN to the PDN GW in the VPLMN and vice versa. Messages between the PDN GW in the VPLMN and the hPCRF in the HPLMN are forwarded by the vPCRF in the VPLMN.

– In the home routed roaming and non-roaming cases, the vPCRF is not involved, except for the authentication and authorization in step 2.

– In the non-roaming cases, the 3GPP AAA Proxy is not involved. In home routed roaming case, the 3GPP AAA Proxy is not involved in step 5.

The steps in figure 16.10.1-1 are based on figure 16.2.1-1, with the following changes:

– Step 0. The UE is connected in the 3GPP Access and has a PMIPv6 or GTP tunnel on the S5/S8 interface.

– Step 2. This step is the same as step 2 in 16.2.1 for non-emergency sessions and as step 2 in clause 16.2.1a for emergency sessions with the following addition: If the UE indicates single-connection, then the UE indicates also handover via EAP-AKA’ to 3GPP AAA.

For emergency sessions, following modifications apply:

– During authentication and authorization, the HSS shall provide the AAA server with the "PDN GW currently in use for emergency services", if available, as part of the subscription information, relayed by the AAA server to the TWAN;

– If the UE indicated an handover attach in step 2 is non-roaming and has been successfully authenticated, and based on operator policy, the TWAN may select the "PDN GW currently in use for emergency services" , if available, as anchor PDN GW. Otherwise, e.g. if the UE indicated an handover attach in step 2 and has been authorized but not authenticated, or the UE is roaming ,or based on operator configuration (e.g. the network supports handovers to/from HRPD), the TWAN shall select the PDN GW that is statically configured in the TWAN Emergency Configuration Data.

– Step 3. This step is the same as step 3 in 16.2.1 with the following addition: The handover indication is set in the Create Session Request to allow the PDN GW to re-allocate the same IP address or prefix that was assigned to the UE while it was connected to the 3GPP access and to initiate a PCEF-Initiated IP‑CAN Session Modification Procedure with the PCRF.

– Step 4. The PDN GW executes a PCEF-Initiated IP‑CAN Session Modification Procedure with the PCRF as specified in TS 23.203 [19]. The Event Report indicates the change in Access Type.

If the PDN GW decides to allocate a new IP address/prefix instead of preserving the old IP address/prefix, as described in clause 4.1.3.2.3, the PDN GW executes an IP-CAN session Establishment Procedure with the PCRF instead of a PCEF-Initiated IP-CAN Session Modification Procedure.

– Step 6. The PDN GW responds with a Create Session Response (PDN GW Address for the user plane, PDN GW TEID of the user plane, PDN GW TEID of the control plane, PDN Type, PDN Address, EPS Bearer Identity, EPS Bearer QoS, APN-AMBR, Charging ID, Cause). The Create Session Response contains the IP address and/or the prefix that was assigned to the UE while it was connected to the 3GPP IP access. The Charging Id provided by the PGW is the Charging Id previously assigned to the default bearer of the PDN connection in the 3GPP access. For emergency sessions, the TWAN also includes the PDN GW address obtained in step 4.

Depending upon the active PCC rules, the PDN GW may create dedicated bearers on S2a interface. And in that case, it applies the Charging ID previously in use for the corresponding dedicated bearer(s) while the UE was connected to the 3GPP IP access (i.e. bearer with the same QCI and ARP as in 3GPP access).

– Step 10: This step is the same as step 15 in 16.2.1.

– Step 11. The PDN GW shall initiate the PDN GW Initiated PDN Disconnection procedure in 3GPP access as defined in clause 5.6.2.2 or the PDN GW Initiated Bearer Deactivation procedure as defined in TS 23.401 [4], clause 5.4.4.1.

16.10.1.2 Handover in single-connection mode from 3GPP access to WLAN on PMIP S2a

Figure 16.10.1.2-1: Handover in single-connection mode from 3GPP access to Trusted WLAN on PMIP S2a for roaming and non-roaming scenarios

The procedure is based on clause 16.2.2 with the following differences:

– Step 0. The UE is connected in the 3GPP Access and has a PMIPv6 or GTP tunnel on the S5/S8 interface.

– Step 2. This step is the same as step 2 in 16.2.2 with the following addition: If the UE indicates single-connection, then the UE indicates also handover via EAP-AKA’ to 3GPP AAA. If the UE requests a PDN type, and this PDN type is not possible, then the request is rejected with a relevant authorization failure.

– Step 3. This step is the same as step 3 in 16.2.2 with the following addition: The handover indicator is set in the Proxy Binding Update to allow the PDN GW to re-allocate the same IP address or prefix that was assigned to the UE while it was connected to the 3GPP access and to initiate a PCEF-Initiated IP‑CAN Session Modification Procedure with the PCRF.

– Step 4: The PDN GW executes a PCEF-Initiated IP‑CAN Session Modification Procedure with the PCRF as specified in TS 23.203 [19]. The Event Report indicates the change in Access Type.

– Step 6: Tthe PDN GW sends a Proxy Binding Acknowledgement message. The details of the Proxy Binding Acknowledgement message are described in step 6 in clause 16.2.2. The Proxy Binding Acknowledgement message contains the IP address and/or the prefix that was assigned to the UE while it was connected to the 3GPP IP access.

– Step 10: This step is the same as step 15 in 16.2.2.

16.10.2 Handover procedure from 3GPP access to WLAN on S2a in multi-connection mode

16.10.2.1 Handover in multi-connection mode from 3GPP access to WLAN on GTP S2a

The following procedure is used to handover one or more PDN connections from 3GPP access to the Trusted WLAN.The steps involved in this procedure are depicted below for both the non-roaming and roaming cases. It is assumed that while the UE is served by the 3GPP access, PMIPv6 or GTP tunnel(s) are established between the Serving GW and the PDN GW in the EPC.

Figure 16.10.2.1-1: Handover from 3GPP access to Trusted WLAN on GTP S2a for roaming and non-roaming scenarios

In the Local Breakout case, the 3GPP AAA Proxy forwads messages from the 3GPP AAA Server in the HPLMN to the PDN GW in the VPLMN and vice versa. Messages between the PDN GW in the VPLMN and the hPCRF in the HPLMN are forwarded by the vPCRF in the VPLMN. In the home routed roaming and non-roaming cases, the vPCRF and the 3GPP AAA Proxy are not involved, except for the authentication and authorization in step 2.

For connectivity to multiple PDNs the following applies:

– If the UE is connected to both 3GPP access and TWAN before the handover of PDN connection(s) to Trusted WLAN is triggered, steps 1 to 4 shall be skipped and the UE shall only perform step 5 for each PDN connection that is being transferred from 3GPP access.

– If the UE is connected only to 3GPP access before the handover of PDN connection(s) to Trusted WLAN is triggered, steps 1 to 4 shall be performed. The UE shall then repeat step 5 for each of the PDN connections that is being transferred from 3GPP access.

– Step 6 shall be repeated for each PDN connection that is being transferred from 3GPP access.

Step 5 can occur in parallel for each PDN connection. Other impacts related to the handover for multiple PDNs are described in clause 8.1.

0. The UE is connected in the 3GPP access and has PMIPv6 or GTP tunnel(s) on the S5/S8 interface.

1. The UE discovers the trusted WLAN access and determines to transfer one or more of its active PDN connections from the currently used 3GPP access to the discovered trusted WLAN.

2. The UE performs access authentication and authorization in the trusted WLAN. This step is the same as step 2 of the initial attach procedure in clause 16.2.1 for non-emergency sessions and in clause 16.2.1a for emergency sessions, where in this case Multi-Connection Mode is requested by the UE.

For emergency sessions, following modifications apply:

– If the UE indicated an handover attach in step 2 is non-roaming and has been successfully authenticated, based on operator policy, the TWAN may select the "PDN GW currently in use for emergency services" as anchor PDN GW. Otherwise, e.g. if the UE indicated an handover attach in step 2 and has been authorized but not authenticated), or the UE is roaming, or based on operator configuration (e.g. the network supports handovers to/from HRPD), the TWAN shall select the PDN GW that is statically configured in the TWAN Emergency Configuration Data.

3. TWAN sends EAP success to the UE and completing EAP authentication. This step is the same as step 8 of the initial attach procedure in clause 16.2.1.

4. If NSWO is authorized for the UE in step 2, anytime after step 3 the UE may send a L3 attach request to the TWAN for NSWO and receive the address or prefix of the NSWO from TWAN.

5. The UE performs UE-initiated connectivity request procedure for Multi-Connection Mode according to clause 16.8.1 with the following exception:

a) The UE sends a PDN Connectivity Request message to the TWAN with Request Type indicating "Handover".

b) The UE indicates the APN corresponding to the PDN connection which is being handed over to TWAN in the PDN Connectivity Request message. If the APN is not provided, and the subscription context from HSS/AAA contains a PDN GW identity and APN pair corresponding to the default APN, the TWAN uses the default APN for non-emergency sessions and the emergency APN for emergency sessions.

c) Upon receiving the PDN Connectivity Request message, the TWAN selects the PDN GW as indicated in the subscription data received from the 3GPP AAA Server and sends Create Session Request to the selected PDN GW. Since the Request Type is "Handover", a Handover Indication is included in the Create Session Request. In the case of emergency sessions when the UE has been authorized but not authenticated, the TWAN selects the PDN GW that is statically configured in the TWAN Emergency Configuration Data.

d) Since it is a handover, the IP-CAN Session Establishment is not performed. The PDN GW may execute a PCEF-Initiated IP‑CAN Session Modification Procedure with the PCRF as specified in TS 23.203 [19] to report e.g. change in IP-CAN type.

NOTE: In case there are multiple PDN connections to the same APN the PDN connection is selected by the PDN GW as it is specified in clause 8.1

6. The PDN GW shall initiate the PDN GW Initiated PDN Disconnection procedure in 3GPP access as defined in clause 5.6.2.2 or the PDN GW Initiated Bearer Deactivation procedure as defined in TS 23.401 [4], clause 5.4.4.1.

16.10.2.2 Handover in multi-connection mode from 3GPP access to WLAN on PMIP S2a

This procedure is as in clause 16.10.2.1 with the following differences:

Step 5 c) Upon receiving the PDN Connection Request message, the TWAN selects the PDN GW as indicated in the subscription data received from the 3GPP AAA Server and sends the Proxy Binding Update to the selected PDN GW by setting the Handover Indicatior to indicate handoff between two different interfaces of the UE. The details of the messages and parameters to be used are described in Steps 6 and 8 in 8.2.2. Additionally, the Proxy Binding Update includes the current TWAN Identifier as described in clause 16.1.7 and the UE Time Zone.