Content for TS 23.007 Word version: 19.0.0

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31.3 MME triggered PDN connection restoration 31.3A ePDG triggered PDN connection restoration
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31.3 MME triggered PDN connection restoration p. 107

The MME may trigger a PDN connection restoration to move the PDN connection to a different PGW-C/SMF in the same PGW-C/SMF Set, e.g. when detecting that the PGW-C/SMF currently serving the PDN connection is not reachable, as defined in this clause.

The MME may trigger this procedure for a specific PDN connection, e.g. upon receiving a Modify Bearer Response message for this PDN connection with the cause code set to "Remote peer not responding", or for all PDN connections served by a PGW-C/SMF, e.g. when receiving a PGW Restart Notification from the SGW with the cause "PGW not responding" or when the MME determines that the PGW-C/SMF has failed.

Reproduction of 3GPP TS 23.007, Fig. 31.3-1: MME triggered PDN connection restoration

Figure 31.3-1: MME triggered PDN connection restoration

Step 1.

The PGW-C/SMF fails without restart from the PGW-C/SMF set.

Step 2a-2b.

The MME sends a Modify Bearer Request message to the SGW, e.g., to report a User Location Information change, the SGW shall respond with the Modify Bearer Response message with the cause code set to "Remote peer not responding" when the SGW detects that the PGW-C/SMF is not responding.

Step 2c-2d.

When detecting that the PGW-C/SMF has failed, the SGW-C shall send a PGW Restart Notification to the MME as specified in clause 16.1A.2 and the MME shall respond with a PGW Restart Notification Acknowledge message.

When detecting that a PGW-C/SMF has failed, the SGW-C and MME shall maintain the contexts of the PDN connections served by that PGW-C/SMF for which this procedure is supported.

Step 3.

The MME should select an alternative PGW-C/SMF using the PGW Change Info received earlier from the PGW-C/SMF for the PDN connection (for the scenario described as step 2a-2b) or for all PDN connections associated with the failed PGW (for the scenario described as step 2c).

Step 4.

The MME shall send a Create Session Request towards the newly selected PGW-C/SMF, via the same or a different SGW, as specified in clause 7.2.1 of TS 29.274,including (among others) a PGW Change Indication, S5/S8-U F-TEIDs for dedicated bearers (if dedicated bearers were established before moving the PDN connection to the new PGW-C/SMF) and the Linked EPS Bearer ID identifying the default bearer of the PDN connection, and without setting the Operation Indication to "1". The MME may defer doing so until it needs to send signalling to the SGW or PGW-C. The MME should reuse the same SGW if possible, since the PDN connection restoration remains then transparent to the eNodeB. The PGW Change Indication tells the SGW and PGW-C/SMF that this is a request to move an existing PDN connection to the new PGW-C/SMF.

Step 5.

The new PGW-C/SMF shall identify the PDN connection context using the UE's identity (i.e. IMSI or IMEI) and the Linked EPS Bearer Identity received in the Create Session Request. If it can take over the PDN connection, it shall return a Create Session Response towards the SGW and MME as specified in clause 7.2.2 of TS 29.274, including (among others) the new S5/S8 PGW F-TEID for control plane, its PGW node name, the S5/S8-U PGW F-TEID for the default bearer and S5/S8 PGW F-TEIDs for dedicated bearers (if any). The PGW-C/SMF shall not change the UE's IP address.

The SGW and MME shall update their PDN connection context with the information received from the PGW-C/SMF. Any subsequent control plane procedure between the MME, SGW and PGW shall take place as defined in existing procedures.

If the MME reselected a different SGW, the MME shall update the eNB with the new SGW's S1-U F-TEID using the MME triggered Serving GW relocation procedure (see clause 5.10.4 of TS 23.401).

Step 6.

The MME shall update the PGW identity in the HSS.

Step 7.

The HSS shall respond the Notify Request. Upon detecting that the association between APN and PGW-C/SMF FQDN is changed, if an active subscription from the UDM exists for the UE to be notified on the change of the data, the HSS shall notify the UDM as specified in Figure 5.3.4-5 of clause 5.3.4 of TS 23.632.

31.3A ePDG triggered PDN connection restoration p. 109

The ePDG supporting S2b over GTPv2 may trigger a PDN connection restoration to move the PDN connection to a different PGW-C/SMF in the same PGW-C/SMF Set, e.g. when detecting that the PGW-C/SMF currently serving the PDN connection is not reachable, as defined in this clause.

The ePDG may trigger this procedure for a specific PDN connection, e.g. upon receiving no response to a request message for this PDN connection, or for all PDN connections served by a PGW-C/SMF, e.g. when the ePDG determines that the PGW-C/SMF has failed.

Reproduction of 3GPP TS 23.007, Fig. 31.3A-1: ePDG triggered PDN connection restoration

Figure 31.3A-1: ePDG triggered PDN connection restoration

Step 1.

The PGW-C/SMF fails without restart from the PGW-C/SMF set.

Step 2.

The ePDG detects that the PGW-C/SMF has failed as specified in clause 20.1.

When detecting that a PGW-C/SMF has failed, the ePDG shall maintain the contexts of the PDN connections served by that PGW-C/SMF for which this procedure is supported.

Step 3.

The ePDG should select an alternative PGW-C/SMF using the PGW Change Info received earlier from the PGW-C/SMF.

Step 4.

The ePDG shall send a Create Session Request towards the newly selected PGW-C/SMF, as specified in clause 7.2.1 of TS 29.274, including (among others) a PGW Change Indication, S2b-U ePDG F-TEIDs for dedicated bearers (if dedicated bearers were established before moving the PDN connection to the new PGW-C/SMF) and the Linked EPS Bearer ID identifying the default bearer of the PDN connection. The ePDG may defer doing so until it needs to send signalling to the PGW-C. The PGW Change Indication tells the PGW-C/SMF that this is a request to move an existing PDN connection to the new PGW-C/SMF.

Step 5.

The new PGW-C/SMF shall identify the PDN connection context using the UE's identity (i.e. IMSI or IMEI) and the Linked EPS Bearer Identity received in the Create Session Request. If it can take over the PDN connection, it shall return a Create Session Response towards the ePDG, as specified in clause 7.2.2 of TS 29.274, including (among others) the new S2b PGW F-TEID for control plane, its PGW node name, the S2b-U PGW F-TEID for the default bearer and S2b-U PGW F-TEIDs for dedicated bearers (if any). The PGW-C/SMF shall not change the UE's IP address.

The ePDG shall update their PDN connection context with the information received from the PGW-C/SMF. Any subsequent control plane procedure between the ePDG and PGW shall take place as defined in existing procedures.

Step 6.

The new PGW-C/SMF shall update the PGW identity in the 3GPP AAA Server, and the 3GPP AAA server will further populate the PGW identity to the HSS as specified in clause 12.1.4 of TS 23.402. The new PGW-C/SMF shall update the PGW identity to the UDM as specified in clause 4.11.4.3.6 of TS 23.502 if S6b is not deployed between SMF+PGW-C and 3GPP AAA Server.