# 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**: 
   - M5 reference point (Media Session Handler to Media AF) may use two different access networks
   - M4 reference point (Media Access Client to Media AS) may use two different access networks

3. **Policy Rules Distribution**:
   - UE receives ATSSS rules for uplink traffic distribution
   - UPF receives N4 rules for downlink traffic distribution
   - SMF configures both rule sets, mapping from PCF PCC rules

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

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

6. **QoS Support**: 
   - Same 5G QoS model applies to MA PDU Sessions
   - QoS Flow is access-agnostic with same network QoS across different access networks
   - Application flows at M5/M4 maintain similar network QoS as 3GPP-only access

7. **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