4 General description

24.1933GPPAccess Traffic Steering, Switching and SplittingStage 3TS

4.1 Introduction

The ATSSS is an optional feature that can be supported by the UE and the 5GC network.

The architecture reference model for ATSSS support is described in clause 4.2.10 of 3GPP TS 23.501 [2].

This clause provides the descriptions of the functionalities that realise ATSSS in stage 3.

4.2 Multi-access PDU session

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 3GPP TS 23.501 [2].

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 3GPP TS 24.526 [5].

The following PDU session types are defined for an MA PDU session: IPv4, IPv6, IPv4v6 and Ethernet.

NOTE: The unstructured PDU session type is not supported in this release of the specification.

4.3 Steering functionalities

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 3GPP TS 23.501 [2]. An ATSSS capable network can use the corresponding steering functionality for the DL traffic.

The UE and the network can support one or more of the following steering functionalities:

a) the MPTCP steering functionality operates above the IP layer. The UE and an associated MPTCP proxy functionality in the UPF can communicate by using the MPTCP protocol; and

b) the ATSSS-LL steering functionality operates below the IP layer as a data switching function.

4.4 Support of access performance measurements

The ATSSS capable UE can perform access performance measurements to decide how to distribute traffic over 3GPP access and non-3GPP access.

An ATSSS capable UE receives measurement assistance information from the network during the MA PDU session establishment procedure as described in clause 5.32.5 of 3GPP TS 23.501 [2]. The measurement assistance information (MAI) is used by the UE to report the access availability/unavailability to the network. The encoding of the measurement assistance information 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.

The following PMF protocol messages can be exchanged between the UE and the PMF:

a) messages for RTT measurements, only applicable for the ATSSS-LL steering functionality; or

b) messages for reporting access availability/unavailability by the UE to the UPF.

An ATSSS capable UE does not apply the ATSSS rules to the PMF protocol messages.

The access performance measurement procedures are specified in clause 5.3 including the procedures for:

a) UE-initiated RTT measurement; and

b) Network-initiated RTT measurement.

The access availability/unavailability procedures are specified in clause 5.4.

4.5 Distribution of traffic across 3GPP access and non-3GPP access networks

The UE can receive ATSSS rules during the MA PDU session establishment procedure or network-requested PDU session modification procedure, and 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.).

NOTE: On the network side, the SMF configures relevant N4 rules according to the ATSSS control information provided by the PCF for the UPF to distribute DL traffic across two access networks.

4.6 EPS interworking

In this release of specification, with the exception of an MA PDU session established as specified in clause 4.7, the MA PDU session is established in 5GS.

In the network supporting N26 interface:

a) 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 3GPP TS 23.502 [3];

b) 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 3GPP TS 23.502 [3];

c) 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 can follow the procedure as specified in clause 6.1.4.1 of 3GPP TS 24.501 [6]; or

2) if the UE established an MA PDU session over 3GPP access and non-3GPP access,

A) if the MA PDU session is transferred to EPS as a PDN connection, the SMF can initiate the network-requested PDU session release procedure over non-3GPP access as specified in clause 6.3.3.2 of 3GPP TS 24.501 [6]; or

NOTE: The QoS flow(s) with EBI assigned over non-3GPP access is also transferred to the corresponding PDN connection.

B) 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 3GPP TS 24.501 [6]; and

d) 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:

a) for an inter-system change from N1 mode to S1 mode:

1) if the UE intends to transfer the MA PDU session to EPS, the UE can follow the procedure as specified in clause 6.1.4.2 of 3GPP TS 24.501 [6] and the network-requested PDU session release procedure can be performed to release the MA PDU session in 5GS as specified in clause 6.3.3.2 of 3GPP TS 24.501 [6]; or

2) otherwise, the UE can keep the MA PDU Session or release the MA PDU session in 5GS which is UE implementation specific; and

b) for an inter-system change from S1 mode to N1 mode, if the related URSP or UE local configuration does not mandate that the PDU session is established over a single access, the UE can initiate the UE-requested PDU session establishment procedure to request an MA PDU session (see clause 5.2.1) or to allow the PDU session to be upgraded to an MA PDU session (see clause 5.2.6) when transferring the PDN connection to 5GS.

4.7 MA PDU session when 5G-RG is connected to EPS

A 5G-RG can connect to both 5GCN and EPC as specified in clause 4.12 of 3GPP TS 23.316 [4].

When establishing a PDN connection over EPS, the 5G-RG can indicate that the PDN connection is to be used as a user-plane resource associated with:

a) a new MA PDU session; or

b) an existing MA PDU session established in wireline access connected to 5GCN.