Section: 7.2.1 Create Session Request
Content: The direction of this message shall be from MME/S4-SGSN to SGW and from SGW to PGW, and from ePDG/TWAN to the PGW (see Table 6.1-1). The Create Session Request message shall be sent on the S11 interface by the MME to the SGW, and on the S5/S8 interface by the SGW to the PGW as part of the procedures: - E-UTRAN Initial Attach when a PDN connection needs to be established through the SGW and PGW - Handover from Trusted or Untrusted Non-3GPP IP Access to E-UTRAN with GTP on S5/S8 interface (see clauses 8.2, 8.6 and 16.11 of 3GPP TS 23.402[ Architecture enhancements for non-3GPP accesses ]  [45]) - UE requested PDN connectivity when a PDN connection needs to be established through the SGW and PGW - Addition of a 3GPP access of NBIFOM procedure as specified by 3GPP TS 23.161[ Network-Based IP Flow Mobility (NBIFOM); Stage 2 ]  [71] - Restoration of PDN connections after an PGW-C/SMF change as specified in 3GPP TS 23.007[ Restoration procedures ]  [17] The message shall also be sent on S4 interface by the SGSN to the SGW, and on the S5/S8 interface by the SGW to the PGW as part of the procedures: - PDP Context Activation - Handover from Trusted or Untrusted Non-3GPP IP Access to UTRAN/GERAN with GTP on S5/S8 interface (see clauses 8.2, 8.6 and 16.11 of 3GPP TS 23.402[ Architecture enhancements for non-3GPP accesses ]  [45]) - Addition of a 3GPP access of the NBIFOM procedure as specified by 3GPP TS 23.161[ Network-Based IP Flow Mobility (NBIFOM); Stage 2 ]  [71] The message shall also be sent on the S11 interface by the MME to the SGW as part of the procedures: - Tracking Area Update procedure with Serving GW change - S1/X2-based handover with SGW change - UTRAN Iu mode to E-UTRAN Inter RAT handover with SGW change - GERAN A/Gb mode to E-UTRAN Inter RAT handover with SGW change - 3G Gn/Gp SGSN to MME combined hard handover and SRNS relocation procedure - Gn/Gp SGSN to MME Tracking Area Update procedure - Restoration of PDN connections after an SGW failure if the MME and PGW support these procedures as specified in 3GPP TS 23.007[ Restoration procedures ]  [17] - MME triggered Serving GW relocation - Handover from Trusted or Untrusted Non-3GPP IP Access to E-UTRAN with PMIP on S5/S8 interface (see clauses 8.2 and 16.11 of 3GPP TS 23.402[ Architecture enhancements for non-3GPP accesses ]  [45]) and on the S4 interface by the SGSN to the SGW as part of the procedures: - Routing Area Update with MME interaction and with SGW change - Gn/Gp SGSN to S4 SGSN Routing Area Update - Inter SGSN Routeing Area Update Procedure and Combined Inter SGSN RA / LA Update using S4 with SGW change - Iu mode RA Update Procedure using S4 with SGW change - E-UTRAN to UTRAN Iu mode Inter RAT handover with SGW change - E-UTRAN to GERAN A/Gb mode Inter RAT handover with SGW change - Serving RNS relocation using S4 with SGW change - Combined hard handover and SRNS relocation using S4 with SGW change - Combined Cell / URA update and SRNS relocation using S4 with SGW change - Enhanced serving RNS relocation with SGW relocation - Restoration of PDN connections after an SGW failure if the SGSN and PGW support these procedures as specified in 3GPP TS 23.007[ Restoration procedures ]  [17] - S4-SGSN triggered Serving GW relocation - Handover from Trusted or Untrusted Non-3GPP IP Access to UTRAN/GERAN with PMIP on S5/S8 interface (see clauses 8.2 and 16.11 of 3GPP TS 23.402[ Architecture enhancements for non-3GPP accesses ]  [45]) and on the S2b interface by the ePDG to the PGW as part of the procedures: - Initial Attach with GTP on S2b - UE initiated Connectivity to Additional PDN with GTP on S2b - Handover to Untrusted Non-3GPP IP Access with GTP on S2b (See clause 8.6 of 3GPP TS 23.402[ Architecture enhancements for non-3GPP accesses ]  [45]) - Initial Attach for emergency session (GTP on S2b) - Addition of an access using S2b of NBIFOM procedure as specified by 3GPP TS 23.161[ Network-Based IP Flow Mobility (NBIFOM); Stage 2 ]  [71] and on the S2a interface by the TWAN to the PGW as part of the procedure: - Initial Attach in WLAN on GTP S2a - Initial Attach in WLAN for Emergency Service on GTP S2a - UE initiated Connectivity to Additional PDN with GTP on S2a - Handover to TWAN with GTP on S2a (See clause 16.10 of 3GPP TS 23.402[ Architecture enhancements for non-3GPP accesses ]  [45]) - Addition of an access using S2a of NBIFOM procedure as specified by 3GPP TS 23.161[ Network-Based IP Flow Mobility (NBIFOM); Stage 2 ]  [71]. If the new Create Session Request received by the SGW collides with an existing active PDN connection context (the existing PDN connection context is identified with the tuple [IMSI, EPS Bearer ID], where IMSI shall be replaced by TAC and SNR part of ME Identity for emergency or RLOS attached UE without UICC or authenticated IMSI), this Create Session Request shall be treated as a request for a new session. Before creating the new session, the SGW should delete: - the existing PDN connection context locally, if the Create Session Request is received with the TEID set to zero in the header, or if it is received with a TEID not set to zero in the header and it collides with the default bearer of an existing PDN connection context; - the existing dedicated bearer context locally, if the Create Session Request collides with an existing dedicated bearer context and the message is received with a TEID not set to zero in the header. In the former case, if the PGW S5/S8 IP address for control plane received in the new Create Session Request is different from the PGW S5/S8 IP address for control plane of the existing PDN connection, the SGW should also delete the existing PDN connection in the corresponding PGW by sending a Delete Session Request message. NOTE 1: The SGW can send the Create Session Request and Delete Session Request over S5/S8 asynchronously, e.g. the SGW can send the Delete Session Request and then the Create Session Request without having to wait for the Delete Session Response. It does not matter if the PGW happens to receive the Delete Session Request after the Create Session Request since the PGW assigns a different S5/S8 F-TEID for control plane to the new PDN connection. In some network deployment, e.g. when 5G Network Slice is deployed and the combined PGW-C/SMFs are connected to the UDM, the PGW may select another PGW supporting the network slice for which the UE has subscription and then forward the Create Session Request to that PGW. When forwarding the Create Session Request message, the PGW shall forward the Create Session Request message as received from the SGW but with the following modifications: - the destination IP address of the message shall be set to the selected PGW IP address; - the CSRMFI flag shall be set to "1"; - the source IP address and UDP port of the message shall be set to the IP address and port of the forwarding PGW. NOTE 2: The Create Session Response message is sent back to the forwarding PGW that forwards it to the SGW. It is assumed that GTPv2/UDP/IP connectivity between the source PGW/SMF and the target PGW/SMF which are in different slices is allowed in such network deployment; otherwise, if there is no GTPv2/UDP/IP connectivity between the source PGW/SMF and the target PGW/SMF, or if the source PGW/SMF does not support forwarding the request to the target PGW/SMF, the source PGW/SMF can proceed as specified in clause 7.2.2. If the new Create Session Request received by the PGW collides with an existing PDN connection context (the existing PDN connection context is identified with the triplet [IMSI, EPS Bearer ID, Interface type], where applicable Interface type here is S2a TWAN GTP-C interface or S2b ePDG GTP-C interface or S5/S8 SGW GTP-C interface, and where IMSI shall be replaced by TAC and SNR part of ME Identity for emergency or RLOS attached UE without UICC or authenticated IMSI), this Create Session Request shall be treated as a request for a new session. Before creating the new session, the PGW should delete: - the existing PDN connection context, if the Create Session Request collides with the default bearer of an existing PDN connection context; - the existing dedicated bearer context, if the Create Session Request collides with a dedicated bearer of an existing PDN connection context. The PGW shall allocate a new PGW S5/S8 F-TEID for control plane to the new PDN connection, i.e. not the same F-TEID value as the one which was assigned to the existing PDN connection. NOTE 3: With GTP based S2a and S2b, the EPS Bearer IDs assigned for specific UE over S2a between the TWAN and PGW and over S2b between an ePDG and PGW are independent of the EPS Bearer IDs assigned for the same UE over S5/S8 and may overlap in value (see 3GPP TS 23.402[ Architecture enhancements for non-3GPP accesses ]  [45] clause 4.6.2). NOTE 4: Only the TAC and SNR part of the ME Identity is used to identify an emergency or RLOS attached UE without UICC or authenticated IMSI. Table -1: Information Elements in a Create Session Request Table -2: Bearer Context to be created within Create Session Request Table -3: Bearer Context to be removed within Create Session Request Table -4: Overload Control Information within Create Session Request Table 7.2.1-5: Remote UE Context Connected within Create Session Request