Content for  TS 23.501  Word version:  18.5.0

When an MA PDU Session is established, the network may provide the UE with Measurement Assistance Information. This information assists the UE in determining which measurements shall be performed over both accesses, as well as whether measurement reports need to be sent to the network. Measurement Assistance Information shall include the addressing information of a Performance Measurement Function (PMF) in the UPF, the UE can send PMF protocol messages to:

• For a PDU Session of IP type, Measurement Assistance Information contains one IP address for the PMF, one UDP port associated with 3GPP access and another UDP port associated with non-3GPP access. PMF messages sent by UE to one of these UDP ports, shall be transmitted to UPF via the QoS Flow associated with the default QoS rule.
• For a PDU Session of Ethernet type, Measurement Assistance Information contains one MAC address associated with 3GPP access and another MAC address associated with non-3GPP access. PMF messages sent by UE to one of these MAC addresses, shall be transmitted to UPF via the QoS Flow associated with the default QoS rule.

If the SMF determines that access performance measurements per QoS Flow shall be applied for the MA PDU Session, then the Measurement Assistance Information shall also include a list of QoS Flows on which access performance measurements may be performed. For each QoS Flow in this list, the following information is included:

• The QFI of the associated QoS Flow.
• For a PDU Session of IP type, one UDP port associated with 3GPP access and another UDP port associated with non-3GPP access. PMF messages sent by UE to one of these UDP ports, shall be transmitted to UPF via the associated QoS Flow.
• For a PDU Session of Ethernet type, one MAC address associated with 3GPP access and another MAC address associated with non-3GPP access. PMF messages sent by UE to one of these MAC addresses, shall be transmitted to UPF via the associated QoS Flow.

The QoS rules and the N4 rules provided by SMF to UE and to UPF respectively shall include information (e.g. packet filters containing the UDP port or the MAC address associated with a QoS Flow), which enables the UE and UPF to route a PMF message to a specific QoS Flow. The UE and the UPF may need to perform access performance measurements in order to estimate the Round-Trip Time (RTT) and/or the Packet Loss Rate (PLR) that an SDF is expected to experience when transmitted on a certain access type. Based on these measurements and the provisioned ATSSS rules in the UE and MAR rules in the UPF, the UE and the UPF decide how to distribute the traffic of an SDF across the two accesses. If the UE and the UPF decide to initiate access performance measurements to estimate the RTT and/or the PLR for an SDF, the access performance measurements shall be performed either

1. using the QoS Flow associated with the default QoS rule; or
2. using the target QoS Flow, which is the QoS Flow that the SDF traffic is transmitted on.

When the access performance measurements are using the target QoS Flow, it is termed that "access performance measurements per QoS Flow" are applied for the MA PDU Session. The UE shall indicate in its ATSSS capabilities that it supports access performance measurements per QoS Flow. Based on this UE capability and other information (such as local policy), the SMF determines whether access performance measurements per QoS Flow shall be applied for the MA PDU Session or not. If the SMF determines that access performance measurements per QoS Flow shall be applied for the MA PDU Session, then:

• The SMF determines a list of QoS Flows over which access performance measurements may be performed and provides this list to the UE (within the Measurement Assistance Information) and to the UPF.
• The UE and the UPF may initiate access performance measurements on one or more of the QoS Flows included in this list. The UE and the UPF shall be able to receive and respond to PMF messages sent on any QoS Flow included in this list.
• The SMF may update the list of QoS Flows over which access performance measurements may be performed during the lifetime of a MA PDU Session, e.g. when a new PCC rule that could benefit from PMF access performance measurements is bound to a QoS Flow.

The UE shall perform access performance measurements per QoS Flow only when this is explicitly indicated in the Measurement Assistance Information, i.e. only when the UE receives the list of QoS Flows over which access performance measurements may be performed. Otherwise, the UE shall perform access performance measurements based on the QoS Flow associated with the default QoS rule. The UPF shall perform access performance measurements per QoS Flow only when this is explicitly indicated by SMF, i.e. only when the UPF receives the list of QoS Flows over which access performance measurements may be performed. Otherwise, the UPF shall perform access performance measurements based on the QoS Flow associated with the default QoS rule. In this case the UPF learns what QoS Flow to use as described in TS 29.244. The UE and the UPF may decide not to initiate access performance measurements using PMF over a certain target QoS Flow, when they already have access performance measurements for another target QoS Flow which they determine can be reused. When access performance measurements for an SDF are performed based on the target QoS Flow, the UE needs to be able to determine the QoS Flow a downlink packet arrives on. In order to enable this, the SMF shall include downlink Packet Filter information in the QoS rule provided to UE matching this SDF, unless Reflective QoS is used for the SDF. The addressing information of the PMF in the UPF is retrieved by the SMF from the UPF during N4 session establishment. If the UPF receives from the SMF, during N4 session establishment or modification procedure, a list of QoS Flows over which access performance measurements may be performed, the UPF allocates different UDP ports per QoS Flow per access for IP PDU sessions, or allocates different MAC addresses per QoS Flow per access for Ethernet PDU sessions. For IP PDU sessions, the UPF sends the PMF IP addressing information and the UDP ports with the QFI of the associated QoS Flow to the SMF. For Ethernet PDU sessions, the UPF sends the MAC addresses with the QFI of the associated QoS Flow to the SMF. The following PMF protocol messages can be exchanged between the UE and the UPF:

• Messages to allow for Round Trip Time (RTT) measurements, i.e. when the "Smallest Delay" steering mode is used or when either "Priority-based", "Load-Balancing" or "Redundant" steering mode is used with RTT threshold value being applied;
• Messages to allow for Packet Loss Rate (PLR) measurements, i.e. when steering mode is used either "Priority-based", "Load-Balancing" or "Redundant" steering mode is used with PLR threshold value being applied;
• Messages for reporting Access availability/unavailability by the UE to the UPF.
• Messages for sending UE-assistance data to UPF. Such messages may be sent from the UE to UPF only when the UE receives the UE-assistance indicator in an ATSSS rule, as specified in clause 5.32.8. Further details are provided in clause 5.32.5.5.
• Messages for sending Suspend Traffic Duplication and Resume Traffic Duplication from UPF to UE to suspend or resume traffic duplication as defined in clause 5.32.5.6.

Since steering modes can be different in up-link and down-link, the UE needs to be able to handle PMF protocol messages for RTT and PLR measurements received from UPF even if it is not using one of the steering modes associated with the RTT and PLR measurements (and vice versa). The PMF protocol is specified in TS 24.193. The PMF protocol messages used for access availability/unavailability reports shall be sent on the QoS Flow associated with the default QoS rule. The PMF protocol messages used for access performance measurements shall be sent either on the QoS Flow associated with the default QoS rule, or on the target QoS Flow, as specified above. The QoS Flow associated with the default QoS rule for MA PDU Session is Non-GBR QoS Flow. The UE shall not apply the ATSSS rules and the UPF shall not apply the MAR rules for the PMF protocol messages. When the UE requests a MA PDU session and indicates it is capable to support:

• the MPTCP functionality with any steering mode and the ATSSS-LL functionality with only the Active-Standby steering mode (as specified in clause 5.32.6.1); or
• the MPQUIC functionality with any steering mode and the ATSSS-LL functionality with only the Active-Standby steering mode (as specified in clause 5.32.6.1); or
• the MPTCP functionality with any steering mode, and the MPQUIC functionality with any steering mode and the ATSSS-LL functionality with only the Active-Standby steering mode (as specified in clause 5.32.6.1);

the network may send Measurement Assistance Information for the UE to send Access availability/unavailability reports to the UPF. In this case, the UE and UPF shall not perform RTT and PLR measurements using PMF as the UE and UPF can use measurements available at the MPTCP layer and/or at the MPQUIC layer.

As described in clause 5.32.5.2, 5.32.5.2a and 5.32.5.3, a UE and UPF may exchange PMF messages to measure access performance and report access availability/unavailability. When the UE and UPF exchanges PMF message, source and destination address of the PMF messages shall be assigned as follows:

1. In the case of a MA PDU Session of IP type:
   • If access performance measurements are performed only over the QoS Flow associated with the default QoS rule, the PMF in the UE sends PMF messages to the PMF in the UPF over UDP/IP. The destination IP address is the IP address contained in the Measurement Assistance Information and the destination UDP port is one of the two UDP ports contained in the Measurement Assistance Information. One UDP port is used for sending PMF messages to UPF over 3GPP access and the other UDP port is used for sending PMF messages to UPF over non-3GPP access. The source IP address is the IP address assigned to UE for the MA PDU Session and the source UDP port is a UDP port that is dynamically allocated by the UE for PMF communication. This source UDP port in the UE remains the same for the entire lifetime of the MA PDU Session.
     If access performance measurements per QoS Flow is performed, the Measurement Assistance Information contains UDP ports, one for each QoS Flow and access combination. When the UE sends PMF message over a QoS Flow of an access, the UE shall set the destination UDP port as the UDP port for the QoS Flow and the access in Measurement Assistance information.
   • If access performance measurements are performed only over the QoS Flow associated with the default QoS rule, the PMF in the UPF sends PMF messages to the PMF in the UE over UDP/IP. The source IP address is the same IP address as the one provided in the Measurement Assistance Information and the source UDP port is one of the two UDP ports as provided in the Measurement Assistance Information. One UDP port is used for sending PMF messages to UE over 3GPP access and the other UDP port is used for sending PMF messages to UE over the non-3GPP access. The destination IPv4 address is the IPv4 address assigned to UE for the MA PDU Session (if any) and the destination IPv6 address is an IPv6 address selected by the UE from the IPv6 prefix assigned for the MA PDU Session (if any). The destination UDP port is the dynamically allocated UDP port in the UE, which is contained in all PMF messages received from the UE.
     If access performance measurements per QoS Flow is performed, when the UPF sends PMF message over a QoS Flow of an access, the UPF shall set the source UDP port with the UDP port for the QoS Flow and the access as the one for the QoS Flow and the access provided in Measurement Assistance information.
   • If the UE receives Measurement Assistance Information, the UE shall inform the network via the user plane about the UE's dynamically allocated UDP port, and the IPv6 address if IPv6 is used for PMF messages, so that it is possible for the UPF to know the UE's IPv6 address (if applicable) and dynamically allocated UDP port as soon as the MA PDU Session has been established.
2. In the case of a MA PDU Session of Ethernet type:
   • The PMF in the UE sends PMF messages to the PMF in the UPF over Ethernet. The Ethertype is the Ethertype contained in the Measurement Assistance Information. If access performance measurements are performed only over the QoS Flow associated with the default QoS rule, the destination MAC address is one of the two MAC addresses contained in the Measurement Assistance Information. One MAC address is used for sending PMF messages to UPF over 3GPP access and the other MAC address is used for sending PMF messages to UPF over non-3GPP access. The source MAC address is a MAC address of the UE, which remains the same for the entire lifetime of the MA PDU Session.
     If access performance measurements per QoS Flow is performed, Measurement Assistance Information contains MAC addresses for each QoS Flow and each access. When the UE sends PMF message over a QoS Flow of an access, the UE shall set the destination MAC address as the MAC address for the QoS Flow and the access in Measurement Assistance information.
   • The PMF in the UPF sends PMF messages to the PMF in the UE over Ethernet. The Ethertype is the same Ethertype as the one provided in the Measurement Assistance Information. If access performance measurements are performed only over the QoS Flow associated with the default QoS rule, the source MAC address is one of the two MAC addresses as provided in the Measurement Assistance Information. One MAC address is used for sending PMF messages to UE over 3GPP access and the other MAC address is used for sending PMF messages to UE over non-3GPP access. The destination MAC address is the MAC address of the UE, which is contained in all PMF messages received from the UE.
     If access performance measurements per QoS Flow is performed, when the UPF sends PMF message over a QoS Flow of an access, the UPF shall set the source MAC address with the MAC address for the QoS Flow and the access as the one for the QoS Flow and the access provided in Measurement Assistance information.
   • If the UE receives Measurement Assistance Information, the UE shall inform the network via the user plane about the UE's MAC address so that it is possible for the UPF to know the UE's MAC address as soon as the MA PDU Session has been established.

RTT measurements can be conducted by the UE and UPF independently. There is no measurement reporting from one side to the other. RTT measurements are defined to support the "Smallest Delay", "Priority-based", "Load Balancing" or "Redundant" steering mode (i.e. when RTT threshold value is applied). The estimation of the RTT by the UE and by the UPF is based on the following mechanism:

1. The PMF in the UE sends over the user plane PMF-Echo Request messages to the PMF in the UPF, and the PMF in the UPF responds to each one with a PMF-Echo Response message. Similarly, the PMF in the UPF sends over the user plane PMF-Echo Request messages to the PMF in the UE, and the PMF in the UE responds to each one with a PMF-Echo Response message.
2. When the UP connection of the MA PDU session is deactivated on an access, no PMF-Echo Request messages are sent on this access. The PMF in the UPF shall not send PMF-Echo Request on this access if the UP connection is not available or after it receives notification from the (H-)SMF to stop sending the PMF-Echo Request on this access.
3. The UE and the UPF derive an estimation of the average RTT over an access type and QoS Flow by averaging the RTT measurements obtained over this access type and QoS Flow.

The UE and the UPF may decide to estimate the Packet Loss Rate (PLR) for an SDF over both accesses. For example, the UE may take this decision when an ATSSS rule in the UE requires the traffic of an SDF to be steered in accordance with a PLR-based threshold condition (e.g. PLR < 2%). The UE and the UPF calculate the PLR for an SDF by exchanging PMF-PLR Report messages, as specified below. A PMF-PLR Report message is sent over 3GPP access or over non-3GPP access, using either the QoS Flow associated with the default QoS rule or a "target" QoS Flow, as specified in clause 5.32.5.1. The calculation of the PLR by the UE and by the UPF is based on the following mechanism. It is assumed that the PLR should be calculated for a target QoS Flow, however, the same mechanism applies when the PLR should be calculated for the QoS Flow associated with the default QoS rule.

• The UE requests from UPF to start counting the number of received UL packets by sending a PMF-PLR Count Request message over the target QoS Flow. The UPF starts counting of the received UL packets over the target QoS Flow and over the access network which the PMF-PLR Count Request message was received from. The UE starts counting the transmitted UL packets over the target QoS Flow and access network when it sends a PMF-PLR Count Request message to UPF.
• The UE stops the counting and requests from UPF to report the number of counted UL packets by sending a PMF-PLR Report Request message over the target QoS Flow. The UPF stops the counting and sends a PMF-PLR Report Response message over the QoS Flow including the number of UL packets counted since it received the last PMF-PLR Count Request message.
• The UE calculates the UL packet loss ratio based on the local counting result of the number of transmitted UL packets and reported number of received UL packets in the UPF.
• The UPF applies the same procedure for calculating the DL PLR, i.e. it sends to UE a PMF-PLR Count Request message on a target QoS Flow to request from UE to start counting the number of DL packets received on this target QoS Flow. As defined in clause 5.32.5.1, the UE determines which DL packets are received on the target QoS Flow by checking the QFI included in the header of DL packets (e.g. in the SDAP header). If no QFI is included in the header of a DL packet, the UE determines the QFI for this DL packet by applying the Packet Filters for downlink in the QoS Rules received from SMF.
• When the UP connection of the MA PDU session is deactivated on an access, no PMF-PLR messages are sent on this access. The PMF in the UPF shall not send PMF-PLR message on this access if the UP connection is not available or after it receives notification from the (H-)SMF to stop sending the PMF-PLR message on this access.
• The UE and the UPF derive an estimation of the average PLR per QoS Flow over an access type by averaging the PLR measurements obtained over this access.

If required by the network in the Measurement Assistance Information, the UE shall provide access availability/unavailability reports to the network. How the UE detects the unavailability and the availability of an access is based on implementation. The CM state of a UE is not a factor when determining whether the 3GPP access is available. When the UE detects the unavailability/availability of an access, it shall:

• build a PMF-Access Report containing the access type and an indication of availability/unavailability of this access;
• send the PMF-Access Report to the UPF via the user plane.

The UPF shall acknowledge the PMF-Access Report received from the UE.

In the case of an MA PDU Session with type Ethernet, the protocol stack over 3GPP access is that same as the one in the above Figure, but the PMF protocol operates on top of Ethernet, instead of UDP/IP.

In the case of an MA PDU Session with type Ethernet, the protocol stack over Untrusted non-3GPP access is the same as the one in the above Figure, but the PMF protocol operates on top of Ethernet, instead of UDP/IP.

In the case of an MA PDU Session with type Ethernet, the protocol stack over Trusted non-3GPP access is the same as the one in the above Figure, but the PMF protocol operates on top of Ethernet, instead of UDP/IP.

When UE-assistance operation is authorized by the PCF in the PCC Rule, the SMF provides an indication for UE-assistance in the ATSSS Rule to the UE, as described in clause 5.32.8, and in the MAR to the UPF, as described in clause 5.8.5.8. If the UE receives the UE-assistance indicator in an ATSSS rule (as specified in clause 5.32.8) and the UE decides to apply a different UL traffic distribution for an SDF than the default UL traffic distribution indicated in the Steering Mode component of the ATSSS rule (e.g. because the UE is running out of battery), then the following applies:

• The UE may apply any split percentages for the UL traffic distribution of an SDF, based on implementation specific criteria.
• The UE may send a PMF-UAD (UE Assistance Data) message to UPF that contains the split percentages that may be used by UPF for all DL traffic that the UE-assistance operation applies. The UPF acknowledges the reception of the PMF-UAD message by sending a PMF-UAD complete message to the UE.
• The UPF may apply the information in a received PMF-UAD message to align the DL traffic distribution for traffic that is allowed to use UE-assistance operation, i.e. traffic where the MAR contains a Steering Mode Indicator set to UE-assistance operation.
• If the UE decides to terminate the UE assistance operation, the UE may send a PMF-UAT (UE Assistance Termination) message to the UPF indicating that the UE assistance operation is terminated and the UE performs the UL traffic distribution according to the split percentages in the ATSSS rule received from the network. If the UPF receives the PMF-UAT message, the UPF acknowledges the reception by sending a PMF-UAT complete message and performs DL traffic distribution by applying the split percentages included in the MAR.

As part of the Redundant Steering Mode, a UPF can decide to suspend traffic duplication for a UE by sending PMF- Suspend Duplication Request message to the UE. How the UPF determines to suspend traffic duplication is implementation specific. The UPF may indicate in the PMF-Suspend Duplication Request message the type of traffic (i.e. GBR or non-GBR) for which traffic duplication is being suspended. The PMF-Suspend Duplication Request message is sent over the user plane of any available access network of the MA PDU Session. Once the UE receives the PMF-Suspend Duplication Request message from the UPF, the UE shall stop duplicating the type of traffic for which traffic duplication is suspended. If the UPF does not provide the type of traffic (GBR or non-GBR) in the PMF-Suspend Duplication Request message, traffic duplication is suspended for all traffic for which traffic duplication is being performed. Once UPF suspended traffic duplication and if no Primary Access is configured, both the UE and the UPF decide, based on their own implementation, which access network to use for sending UL and DL traffic. If the Primary Access is configured, both the UE and the UPF use the Primary Access for sending UL and DL traffic. The UPF may decide to resume traffic duplication for a UE by sending the PMF-Resume Duplication Request message. How the UPF determines to resume traffic duplication is implementation specific. Once the UE receives the PMF-Resume Duplication Request message from the UPF, the UE may restart to duplicate the type of traffic for which traffic duplication is resumed based on the provided Redundant steering mode policies and UE implementation.