Summary of 3GPP Technical Document S4-260206

Document Information

• Specification: TS 26.804 (Release 20)
• CR Number: 0033 (Revision 02)
• Category: B (Addition of feature)
• Work Item: FS_AMD_Ph2 (Study on Advanced Media Delivery Phase 2)

Main Technical Contribution

This Change Request proposes baseline procedures for establishing multi-access uplink media streaming sessions using ATSSS (Access Traffic Steering, Switching, and Splitting) in 5G Media Streaming (5GMS) systems. The contribution addresses Work Task #8 on multi-access media delivery phase 2, specifically focusing on the impact of multi-access media delivery for uplink streaming.

Technical Content

Background on ATSSS Architecture (Clause 5.18.1.3.1)

The document provides comprehensive background on ATSSS architecture as specified in TS 23.501:

Key ATSSS Principles

1. Multi-Access PDU Connectivity Service: Enables simultaneous use of one 3GPP access network and one non-3GPP access network via independent N3/N9 tunnels between PSA UPF and RAN/AN

2. MA PDU Session Application:

3. M5 reference point (Media Session Handler to Media AF) may use two different access networks

4. M4 reference point (Media Access Client to Media AS) may use two different access networks

5. Policy Rules Distribution:

6. UE receives ATSSS rules for uplink traffic distribution
7. UPF receives N4 rules for downlink traffic distribution

8. SMF configures both rule sets, mapping from PCF PCC rules

9. UE ATSSS Capability Indication: UE indicates ATSSS support (steering functionalities and modes) in PDU Session Establishment Request

10. Network Slicing: Same S-NSSAI allowed to span both access networks

11. QoS Support:

12. Same 5G QoS model applies to MA PDU Sessions
13. QoS Flow is access-agnostic with same network QoS across different access networks

14. Application flows at M5/M4 maintain similar network QoS as 3GPP-only access

15. Measurement Assistance: Network provides measurement assistance information to UE/UPF for packet round-trip time and packet loss rate measurements

Traffic Steering Mechanisms

Three steering mechanisms are supported:

1. MPTCP (Multipath TCP): UPF provides MPTCP proxy functionality
2. MPQUIC (Multipath-enabled QUIC): UPF provides MPQUIC proxy functionality
3. ATSSS-LL (ATSSS Low-Layer): Steering, switching, and splitting based on IP layer and below

Steering Modes

Five steering modes are defined:

• Active-Standby: Primary access with fallback to standby
• Smallest Delay: Route to access with smallest round-trip time
• Load-Balancing: Split traffic between both accesses
• Priority-based: Route to higher priority access
• Redundant: Duplicate packets on both accesses

IP Address Allocation

UE may receive up to five IP addresses:
- One for MA PDU session (always allocated)
- Two MPTCP link-specific multipath addresses (one per access)
- Two MPQUIC link-specific multipath addresses (one per access)

Multi-Access Uplink Media Delivery Procedures (Clause 5.18.4.3)

Architecture Overview

The document describes the control plane architecture:
- PCF generates PCC rules for each Service Data Flow (SDF)
- SMF translates PCC rules into N4 rules for UPF (FARs, PDRs, MARs, ATSSS rules)
- SMF delivers ATSSS rules and URSP rules to UE via AMF over N1 interface
- UE ATSSS functions manage uplink traffic
- UPF ATSSS functions manage downlink traffic

MA PDU Session Setup Options

Three methods for establishing multi-access PDU sessions:

1. Set up single-access PDU Session first, then register over another access and request MA PDU Session
2. Indicate ATSSS capability and request MA PDU Session from the start
3. Request single-access PDU Session but network transparently sets up MA PDU Session

Detailed Call Flow

The document provides a comprehensive baseline procedure (Figure 5.18.4.3-1) with the following phases:

Phase 1: Single Access Initial State (Steps 1-10)
- Media-Aware Application starts uplink streaming
- Media Access Function requests streaming session establishment with Media Session Handler
- Media Session Handler coordinates with Media AF
- Media AF → PCF → SMF establishes transport session using single access
- SMF retrieves ATSSS policy from PCF
- Uplink transport and media streaming sessions established with Media AS

Phase 2: Access Availability Detection (Steps 11-14)
- UE modem detects presence of multiple access networks
- Physical layer performs radio access-specific functions (cell/AP detection, quality measurement)
- UE OS informed of new access availability
- Media Access Function receives notification about multiple access networks including access ID and type (per TS 23.503)
- Media Access Function evaluates eligibility of new access for uplink media streaming
- Media Access Function sends MultiAccessDetected notification to Media Session Handler with:
- Access ID and type
- QoS capability/path metrics (latency, throughput, loss rate)
- Possible IP endpoints

Phase 3: Multi-Access Activation Trigger (Steps 15-16)
- Media Session Handler analyzes event and decides on MA activation
- Media Session Handler notifies Media-Aware Application about multi-access availability
- Media Session Handler sends "Activate MA" request to Media AF via M5u API
- Media AF consults PCF for ATSSS policy rules, delivery boosts, or QoS changes

Phase 4: Network Provisioning (Steps 17-20)
- Media AF contacts PCF/SMF to:
- Authorize dual access usage (PCC policy)
- Allocate new QoS flows for added access path
- Return updated transport/steering instructions
- PCF requests SMF to allocate new QoS flows
- SMF confirms upon acceptable conditions
- Confirmation forwarded through PCF → Media AF → Media Session Handler

Phase 5: Transport Subflow Establishment (Steps 21-23)
- Media Session Handler instructs Media Streamer to open transport subflow on new access (MP-TCP add_addr or MP-QUIC create PATH)
- Media Streamer establishes new subflow based on confirmation
- Media Streamer requests modem to open socket to newly detected access
- Upon confirmation, Media Streamer notifies Media Session Handler that multi-access is active

Phase 6: Media Delivery Status Notification (Steps 24-25)
- Media Access Function decides to switch to media delivery over multiple access networks
- Decision shared with Media Session Handler
- Media Access Function notifies Media Session Handler about multi-access delivery status at M11 reference point
- For downlink streaming, uses Dynamic Status Information per TS 26.512 clause 13.2.6

Phase 7: Multi-Access Session Establishment (Steps 26-32)
- Media Session Handler requests streaming session establishment with Media AF using multiple access
- Media AF → PCF → SMF establishes transport session using multiple access networks
- SMF retrieves policy from PCF and confirms
- Confirmation forwarded through PCF → Media AF → Media Session Handler
- Media Access Function establishes transport sessions with Media AS over multiple access networks at M4
- Media streaming session established between Media Access Function and Media AS over multiple access networks

Phase 8: Final Configuration (Steps 33-34)
- Media Session Handler decides split ratios/steering policy
- Media Session Handler updates Media Streamer and/or transport with scheduling/path selection rules (e.g., round-robin, split, weighted load)
- Media-Aware Application informed that multi-access is active
- Application can adjust encoder settings (e.g., send enhancement layers over Wi-Fi)

Key Assumptions

• 5GMS Client is unaware of UE ATSSS steering functionality
• Uplink 5G Media Streaming session set up over 3GPP access first before switching to MA PDU Session
• Applications typically unaware of multiple access networks unless explicitly informed via OS notification mechanisms
• Physical layer has no awareness of applications and does not provide direct signaling to application entities

Important Notes

• Support for QoS when PDUs conveyed over PDU Session belonging to network slice spanning non-3GPP access is unknown
• Support for PDU Session QoS when PDUs conveyed over non-3GPP access network is unknown
• MPTCP and MPQUIC link-specific multipath addresses may not be routable via N6
• MPTCP and MPQUIC link-specific multipath addresses can be the same
• No equivalent client API notification mechanism exists at M7 reference point for uplink media streaming or RTC as of Release 19