For the purposes of the present document, the terms and definitions given in TR 21.905 and the following apply. A term defined in the present document takes precedence over the definition of the same term, if any, in TR 21.905. MPTCP client: MPQUIC client:

1. RFC 9221 for supporting unreliable datagram transport with QUIC; and
2. draft-ietf-quic-multipath [9I] for supporting QUIC connections using multiple paths simultaneously.

For the purposes of the present document, the following terms and definitions given in TS 23.501 apply:

For the purposes of the present document, the abbreviations given in TR 21.905 and the following apply. An abbreviation defined in the present document takes precedence over the definition of the same abbreviation, if any, in TR 21.905. 5G-RG ATSSS ATSSS-LL LADN MA PDU MAI MPTCP MPQUIC PCO PDU PLR PMF QFI RTT SA PDU SDF TDR TDS UAD UAT UPF URSP

ATSSS is an optional feature that can be supported by the UE and the 5GC network to route data traffic across 3GPP access and non-3GPP access networks. An ATSSS capable UE establishes an MA PDU session supporting multi-access connectivity over 3GPP access and non-3GPP access networks as described in clause 4.2. The ATSSS capable UE can support ATSSS-LL MPTCP or MPQUIC steering functionality or any combination of them as described in clause 4.3, with associated steering modes, i.e. active-standby, smallest delay, load balancing, priority based or redundant. The ATSSS capable UE indicates the steering functionality and associated steering modes to the 5GC network. When the ATSSS capable UE registers to a registration area, it receives an indication from the AMF if the network supports the ATSSS. The procedure for how the AMF indicates the UE about its ATSSS support is specified in TS 24.501. The UE capable ATSSS and the network supporting ATSSS exchange access performance measurements as described in clause 4.4. Clause 4.5 describes the traffic distribution over 3GPP access and non-3GPP access networks. Clause 4.6 provides a description for interworking with EPS network. Clause 4.7 describes ATSSS when the ATSSS capable UE is interconnected with EPS. The architecture reference model for ATSSS support is described in clause 4.2.10 of TS 23.501.

A PDU session supporting a multi-access PDU connectivity service is referred to as multi-access PDU (MA PDU) session. An MA PDU session is a PDU session which can use one 3GPP access network or one non-3GPP access network at a time, or simultaneously one 3GPP access network and one non-3GPP access network as defined in TS 23.501. An MA PDU session can be established when the UE is registered to the same PLMN over 3GPP access network and non-3GPP access network or registered to different PLMNs over 3GPP access network and non-3GPP access network respectively. A UE can initiate MA PDU session establishment when the UE is registered to a PLMN over both 3GPP access network and non-3GPP access network, or only registered to one access network. Therefore, at any given time, the MA PDU session can have user-plane resources established on both 3GPP access and non-3GPP access, or on one access only (either 3GPP access or non-3GPP access), or can have no user-plane resources established on any access. An ATSSS capable UE can establish an MA PDU session based on the URSP rules as defined in TS 24.526. The following PDU session types are defined for an MA PDU session: IPv4, IPv6, IPv4v6 and Ethernet. MA PDU sessions for LADN are not supported.

An ATSSS capable UE can use a steering functionality to steer, switch and split the UL traffic across the 3GPP access network and the non-3GPP access network as defined in clause 5.32.6 of TS 23.501. An ATSSS capable network can use the corresponding steering functionality for the DL traffic. The UE and the network can support one or more steering functionalities as defined in clause 5.32.6 of TS 23.501, in the categories of:

1. high-layer steering functionalities i.e.:
   1. the MPTCP steering functionality which operates above the IP layer, where the UE and an associated MPTCP proxy functionality in the UPF can communicate by using the MPTCP protocol; and
   2. the MPQUIC steering functionality which operates above the UDP/IP layer, where the UE and an associated QUIC proxy in the UPF can communicate by using the QUIC protocol; and
2. low-layer steering functionalities, i.e.:
   1. the ATSSS-LL steering functionality which operates below the IP layer as a data switching function.

The ATSSS capable UE can perform access performance measurements to decide how to distribute traffic over 3GPP access and non-3GPP access. The access performance measurements can be performed by using the QoS flow(s) of default QoS rule. Based on the UE capability that the UE has indicated to the network, the access performance measurements can also be performed by using the QoS flows of non-default QoS rules. An ATSSS capable UE receives MAI from the SMF during the PDU session establishment procedure for an MA PDU session as described in clause 5.32.5 of TS 23.501, during a network-requested PDU session modification procedure as specified in clause 6.3.2 of TS 24.501, during UE requested PDN connectivity procedure according to clause 6.5.1 of TS 24.301 or during EPS bearer context modification procedure according to clause 6.4.3 of TS 24.301. The MAI can contain the addressing information of the PMF in the UPF, as well as an indicator on whether access availability/unavailability reports need to be sent to the network. If the UE indicates to the network the capability to perform the access performance measurements by using the QoS flows of non-default QoS rules, the MAI can also indicate to the UE that the performance measurement is for the QoS flows of non-default QoS rules and therefore include a QoS flow list for which, the measurements are to be performed. The encoding of the MAI is specified in clause 6.1.5. An ATSSS capable UE that supports the MPTCP steering functionality can use the measurements available at the MPTCP layer. An ATSSS capable UE that supports the MPQUIC steering functionality can use the measurements available at the MPQUIC layer. The following PMF protocol messages can be exchanged between the PMF in the UE and the PMF in the UPF:

1. messages for RTT measurements, only applicable for the ATSSS-LL steering functionality;
2. messages for reporting access availability/unavailability by the UE to the UPF;
3. messages for PLR measurements, only applicable for the ATSSS-LL steering functionality; or
4. messages for UAD provisioning ;
5. messages for UAT ;
6. messages for TDS; or
7. messages for TDR.

An ATSSS capable UE does not apply the ATSSS rules to the PMF protocol messages. The performance measurement function protocol procedures are specified with following procedures:

1. UE-initiated RTT measurement (see clause 5.4.3);
2. Network-initiated RTT measurement (see clause 5.4.4);
3. UE-initiated PLR measurement (see clause 5.4.6);
4. Network-initiated PLR measurement (see clause 5.4.7);
5. UE assistance data provisioning procedure (see clause 5.4.8);
6. The access availability/unavailability procedures (see clause 5.4.5);
7. UE assistance data termination procedure (see clause 5.4.9);
8. Traffic duplication suspend procedure (see clause 5.4.10); and
9. Traffic duplication resume procedure (see clause 5.4.11).

The UE can receive ATSSS rules during the PDU session establishment procedure for an MA PDU session or network-requested PDU session modification procedure. The ATSSS rule ID and ATSSS rule operation for each rule is used to add a new ATSSS rule, or to delete or update an existing ATSSS rule. The UE can distribute the UL traffic except for the PMF protocol messages across the 3GPP access network and the non-3GPP access network according to the ATSSS rules and other local conditions (such as network interface availability, signal loss conditions, user preferences, etc.).

In the network supporting N26 interface:

1. if the UE established an MA PDU session over non-3GPP access only, no EPS bearer identity can be assigned to any QoS flow of the MA PDU session as specified in TS 23.502;
2. if the UE established an MA PDU session over 3GPP access and non-3GPP access and the user plane of the MA PDU session over 3GPP access is released, the EPS bearer identity assigned for the MA PDU session can be revoked as specified in TS 23.502;
3. for an inter-system change from N1 mode to S1 mode:
   1. if the UE established an MA PDU session over 3GPP access only, the UE follows the procedure as specified in clause 6.1.4.1 of TS 24.501; or
   2. if the UE established an MA PDU session over 3GPP access and non-3GPP access, the UE follows the procedure as specified in clause 6.1.4.1 of TS 24.501, and
      1. if the MA PDU session is transferred to EPS as a PDN connection and the UE did not indicate its support of establishing a PDN connection as the user plane resource of an MA PDU session during the MA PDU session establishment procedure as specified in clause 6.4.1.2 of TS 24.501, the SMF can initiate the network-requested PDU session release procedure over non-3GPP access as specified in clause 6.3.3.2 of TS 24.501 or perform a local release of the MA PDU session. The UE performs a local release of the MA PDU session over 3GPP access and non-3GPP access;
      2. if the MA PDU session is transferred to EPS as a PDN connection and the UE indicated its support of establishing a PDN connection as the user plane resource of an MA PDU session during the MA PDU session establishment procedure as specified in clause 6.4.1.2 of TS 24.501, the SMF can keep the MA PDU session over non-3GPP access; or
      3. if the MA PDU session is not transferred to EPS as a PDN connection and the SMF decides to move the traffic of the MA PDU session from 3GPP access to non-3GPP access, the SMF can initiate the network-requested PDU session modification procedure as specified in clause 6.3.2.2 of TS 24.501; and
   3. if the UE established an MA PDU session with the user-plane resource over 3GPP access and the user-plane resource with the PDN connection over untrusted non-3GPP access network, the UE follows the procedure as specified in clause 6.1.4.1 of TS 24.501, and
      1. if the MA PDU session is transferred to EPS as a PDN connection and:
         • the UE did not indicate its support of establishing a PDN connection as the user plane resource of an MA PDU session during the MA PDU session establishment procedure as specified in clause 6.4.1.2 of TS 24.501, the SMF can initiate the tunnel disconnection procedure over untrusted non-3GPP access network as specified in clause 7.4.3 of TS 24.302 or perform a local release of the MA PDU session . The UE performs a local release of the MA PDU session over 3GPP access and over untrusted non-3GPP access; or
         • the UE indicates its support of establishing a PDN connection as the user plane resource of an MA PDU session during the MA PDU session establishment procedure as specified in clause 6.4.1.2 of TS 24.501, the network can keep the MA PDU session with the user-plane resource with the PDN connection over 3GPP access network and locally release the user-plane resource over untrusted non-3GPP access. The UE performs a local release of the MA PDU session over untrusted non-3GPP access.
      2. if the MA PDU session is not transferred to EPS as a PDN connection and the SMF decides to move the traffic of the MA PDU session from 3GPP access to untrusted non-3GPP access, the SMF can initiate the tunnel modification procedure as specified in clause 7.4.2 of TS 24.302; and
4. for an inter-system change from S1 mode to N1 mode, if the UE requests an MA PDU session or the related URSP or UE local configuration does not mandate that the PDU session is established over a single access when transferring the PDN connection to 3GPP access, the PDN connection can be converted by the network to an MA PDU session via the UE-requested PDU session modification procedure (see clause 5.2.5).

In the network not supporting N26 interface:

5GCN and E-UTRAN EPC as specified in clause 4.22.2.3 of TS 23.502. When establishing a PDN connection via 3GPP access connected to EPS, the UE can indicate that the PDN connection is to be used as a user-plane resource associated with:

1. a new MA PDU session; or
2. an existing MA PDU session established in non-3GPP access connected to 5GCN.

If the UE does not support Ethernet PDN type or the Ethernet PDN type is not supported in EPC, there are following requirements:

1. the UE cannot establish a PDN connection as the user plane resource associated with a new MA PDU session of Ethernet PDU session type; and
2. the UE can establish a PDN connection of non-IP PDN type as the user plane resource associated with an existing MA PDU session of Ethernet PDU session type.

Clause 5.3 specifies the procedure of MA PDU session establishment with 3GPP access connected to EPC and non-3GPP access connected to 5GCN.

A UE can establish an MA PDU session via 3GPP access connected to 5GCN and via untrusted non-3GPP access connected to EPC as specified in clause 4.22.2.4 of TS 23.502. When establishing a PDN connection to EPS via untrusted non-3GPP access network, the UE can indicate that the PDN connection is to be used as a user-plane resource associated with:

1. a new MA PDU session; or
2. an existing MA PDU session established in 3GPP access connected to 5GCN.

Clause 5.3a specifies the session management procedures of MA PDU session established with untrusted non-3GPP access connected to EPC and 3GPP access connected to 5GCN.