# 3GPP TR 26.804 Change Request - Network Assistance for Multi-Access Media Delivery

## CR Details
- **CR Number**: 0036 rev 4
- **Current Version**: 19.1.0
- **Category**: B (addition of feature)
- **Release**: Rel-20
- **Work Item**: FS_AMD_Ph2

## Summary
This CR addresses gaps in network assistance for multi-access media delivery scenarios, particularly for uplink streaming use cases when using ATSSS (Access Traffic Steering, Switching and Splitting). It extends the Rel-19 TR 26.804 by adding key issue descriptions, candidate solutions, and addressing identified gaps.

## Main Technical Contributions

### 1. References and Abbreviations Updates

**New References Added:**
- TS 26.512: 5G Media Streaming (5GMS) Protocols
- TS 28.405: Quality of Experience (QoE) measurement collection
- TS 24.193: ATSSS specifications

**New Abbreviations:**
- PMF: Performance Measurement Functionality
- RTT: Round-Trip Time

### 2. ATSSS Architecture Background (Clause 5.18.1.3.1)

Provides comprehensive summary of ATSSS architecture from TS 23.501, including:

**Key Principles:**
- Multi-Access PDU Connectivity Service enabling simultaneous use of 3GPP and non-3GPP access networks
- MA PDU Session with independent N3/N9 tunnels between PSA UPF and RAN/AN
- Application flows at M4 and M5 reference points can utilize two different access networks

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

**QoS Support:**
- Same 5G QoS model applies to MA PDU Sessions
- QoS Flow is access-agnostic with same network QoS across different access networks
- QoS rules provided via one access apply to both 3GPP and non-3GPP access

**Traffic Steering Mechanisms:**
- **MPTCP (Multipath TCP)**: UPF provides MPTCP proxy functionality
- **MPQUIC (Multipath QUIC)**: UPF provides MPQUIC proxy functionality  
- **ATSSS-LL (Low-Layer)**: Steering based on IP layer and below

**Steering Modes:**
- Active-Standby: Primary/backup access selection
- Smallest Delay: Route based on lowest RTT
- Load-Balancing: Split traffic across both accesses
- Priority-based: Route to higher priority access
- Redundant: Duplicate packets on both accesses

**IP Address Allocation:**
- One IP address for MA PDU Session
- Two MPTCP link-specific multipath addresses (one per access)
- Two MPQUIC link-specific multipath addresses (one per access)

### 3. Performance Measurement Functionality (PMF) - New Clause 5.18.1.3.1A

Detailed description of PMF protocol from TS 24.193 for measurement assistance:

**Measurement Assistance Information Includes:**
- IP address for PMF functionality in UPF
- UDP ports for 3GPP and non-3GPP access measurements
- List of QoS Flows for per-flow measurements

**PMF Protocol Messages:**

**RTT Measurements:**
- PMF-Echo Request/Response messages exchanged between UE and UPF
- Independent RTT measurements at both UE and UPF
- Average RTT calculation per access type and QoS Flow

**Packet Loss Rate (PLR) Measurements:**
- Downlink PLR: UPF sends PMF-PLR Count Request, counts transmitted packets, UE counts received packets, UPF calculates loss ratio
- Uplink PLR: Reverse procedure initiated by UE
- Average PLR calculation per QoS Flow and access type

**Access Network Availability Reporting:**
- UE detects and reports access network availability/unavailability to UPF

**UE Assistance Data:**
- Upon PCF authorization, UE can override ATSSS rule split percentages
- PMF-UAD message informs UPF of UE-chosen uplink split
- UPF may align downlink traffic distribution accordingly
- PMF-UAT message terminates UE assistance operation

**Traffic Duplication Control:**
- PMF-Suspend Duplication: UPF requests UE to stop redundant transmission (e.g., during congestion)
- PMF-Resume Duplication: UPF requests UE to restart duplication

### 4. Key Issue Description - Network Assistance with Multi-Access (New Clause 5.18.1.4A)

**Current Network Assistance Methods:**
- **AF-based Network Assistance**: Media Client requests assistance from Media AF, which interacts with PCF
- **ANBR-based Network Assistance**: Based on UE modem and RAN signaling

**Available Facilities:**
- **Bit rate recommendation**: Media Client requests estimated achievable bit rate from Media AF
- **Delivery boost**: Media Client requests temporary bit rate increase

**Identified Issues for Multi-Access:**
- Current procedures assume single access network
- Unclear if assistance information is useful/sufficient for MA PDU Sessions
- Lack of per-access detailed metrics in all cases
- Configuration-dependent measurement/reporting coverage

### 5. Key Issue Objectives (Clause 5.18.1.5.2)

**Dynamic Policy Questions:**
- Meaning of "activate dynamic policy with QoS requirements" for multi-access M4 flows
- Feasibility of requesting QoS for specific access path subsets
- Need for ApplicationFlowDescription enhancements to identify M4 flows over multiple accesses

**Network Assistance Questions:**
- Sufficiency of current bit rate recommendation and delivery boost for multi-access
- How Media Client uses bit rate recommendations to distribute M4 flows
- Ability to request/receive assistance for specific access networks
- Need for enhanced assistance information including:
  - Split recommendations across access networks (e.g., 30%/70%)
  - Split recommendations by media type (audio/video on different accesses)
  - Split recommendations by stream priority
  - Initial throughput with latency per access
  - Upcoming network conditions and handover information

### 6. Collaboration Scenario (New Clause 5.18.2.3)

Describes how UE determines use of MA PDU Sessions during media delivery through AF-based Network Assistance to improve QoS and QoE while leveraging multiple access paths.

### 7. High-Level Call Flow - Access-Specific Network Assistance (New Clause 5.18.4.3)

Comprehensive call flow with following phases:

**Phase 1: Initial Media Delivery Over Single Access**
1. Provisioning of 5G Media Streaming Session
2. Media-aware Application selects Media Player Entry
3. Acquire Service Access Information
4. Media Streaming over single access
5. QoE metrics collection and reporting

**Phase 2: Multi-Access Activation**
6. Media-aware Application subscribes to OS events for multiple access availability
7. UE OS/modem detects multiple access networks (using access availability reports per TS 23.501)
8. UE OS informs Media-aware Application
9. Media-aware Application expresses preference for multi-access at M4 (via Client API in TS 26.512 for downlink; no equivalent for uplink/RTC in Rel-19)
10-11. Media Access Function checks OS for multiple access availability
12. Media Access Function activates multi-access mode
13. Media Access Function notifies Media-aware Application of multi-access status (using Dynamic Status Information per TS 26.512 for downlink; no equivalent for uplink/RTC)
14. (Optional) Media Access Function notifies Media Session Handler

**Phase 3: AF-Based Network Assistance Activation**
15. Media-aware Application requests network assistance for initial bit rate recommendation, may include access type identification
16. Media Session Handler creates Network Assistance session with Media AF (M5)
17-18. Media Session Handler requests bit rate recommendation, includes access type information; Media AF calculates per-access recommendations
19. Media Access Function receives bit rate recommendations per access (e.g., 5 Mbps LTE, 8 Mbps Wi-Fi)
20. Media Access Function chooses optimal bit rate
21-22. Media delivery starts using multi-access with network assistance
23. MA PDU Session with AF-based Network Assistance established

**Phase 4: Subscription to Bit Rate Recommendations**
24. Media Session Handler subscribes for ongoing bit rate recommendations from Media AF (M5), may be access-specific
25. Media AF sends updated recommendations via MQTT notification (per TS 26.510)
26. Media AF notifies Media Access Function with BIT_RATE_RECOMMENDATION, embellished with access-specific information
27. Media Access Function adapts media delivery according to new bit rate
28. Media streaming continues with adapted bit rate

**Phase 5: Delivery Boost Request**
29. Media-aware Application detects degraded QoE (comparing with historic metrics using QMC framework per TS 28.405)
30. Media-aware Application requests delivery boost
31. Media Access Function sends boost request to Media Session Handler (using requestDeliveryBoost() per TS 26.510 for downlink; no equivalent for uplink/RTC)
32. Media Session Handler determines which access network needs boost
33-34. Media Session Handler requests Media AF for boost over specific access; Media AF invokes requestDeliveryBoost() API
35. Media Access Function receives boost confirmation (Boolean success/failure)
36. Media Access Function adjusts bit rate temporarily

**Phase 6: Session Termination**
37. Media Access Function ends Network Assistance Session
38. Media Session Handler destroys session (destroyNetworkAssistanceSession() operation)
39. Session continues without Network Assistance or ends

### 8. PMF-Based Network Assistance Call Flow (New Clause 5.18.4.4)

Alternative approach using PMF for network assistance:

**Key Facilities:**
- Measurement of RTT and PLR per QoS Flow per access network
- Configuration of uplink/downlink traffic distribution using PMF UE Assistance Data

**Call Flow Steps:**

**Initial Setup:**
1. 5G Media Delivery session established over MA PDU Session
2. Media-Aware Application requests PMF-based network assistance
3. Media Access Function forwards request to Media Session Handler
4. Media Session Handler requests UE OS to activate PMF
5. UE OS activates PMF Functionality

**PMF Measurements:**
6. PMF Functionality in UE and UPF perform calculations (RTT, PLR)
7-9. Calculated measurements reported through UE OS to Media Session Handler
10. Media Session Handler forwards to Media Access Function
11. Media Access Function forwards to Media-aware Application

**Traffic Split Configuration:**
12. Media-aware Application configures traffic split (percentage across 3GPP/non-3GPP for UL/DL)
13. Media Access Function requests UL traffic split at UE ATSSS Steering Functionality
14. UE ATSSS confirms successful configuration
15. Media Access Function requests DL traffic split via UE PMF Functionality using PMF UE Assistance
16. UE PMF requests DL distribution from UPF PMF
17. UPF PMF configures UPF ATSSS Steering Functionality
18. UPF ATSSS confirms to UPF PMF
19. UPF PMF confirms to UE PMF

**Media Delivery with Configured Split:**
20. Media Access Function forwards UL M4 content to UE ATSSS
21. UE ATSSS applies configured UL split
22. UE ATSSS delivers split content to UPF ATSSS across both accesses; UPF combines and forwards to Media AS
23. Media AS sends DL M4 content to UPF ATSSS
24. UPF ATSSS applies configured DL split
25. UPF ATSSS delivers split content to UE ATSSS
26. UE ATSSS combines DL content and forwards to Media Access Function

### 9. Gap Analysis (Clause 5.18.5.2.5)

**Identified Gaps in AF-Based Network Assistance:**

When using multi-access PDU sessions with ATSSS, current specifications (TS 26.501, TS 26.512, TS 26.510) do not clarify:

1. **Access-Specific Assistance**: Whether Media Client can request/receive Network Assistance specific to a given access network (e.g., 3GPP NR vs. Wi-Fi)

2. **Per-Access Bit Rate Recommendations**: How bit rate recommendations are obtained or boosts applied for different access networks when session spans multiple links

3. **ATSSS Policy Interaction**: How Network Assistance interacts with ATSSS steering/switching rules managed by PCF

**Resulting Issues:**
- Media Client receives only session-level (aggregate) bit rate recommendation, not per-access insight
- Delivery boosts applied at PDU Session level, not selectively to one access leg
- Uplink media delivery cannot dynamically exploit multiple paths
- No standardized signaling linkage between AF-based Network Assistance (M5) and ATSSS policy control (Npcf → Nsmf)
- Full path-level detailed per-access metrics not provided in all cases

### 10. Candidate Solutions

#### 10.1 AF-Based Network Assistance Solution (Clause 5.18.6.3)

**Proposed Enhancements:**

1. **Media-Aware Application Level:**
   - Subscribe to OS events for multiple access network availability
   - Receive information from UE OS about available access networks
   - Include access identification and type when requesting initial bit rate recommendation (M6)

2. **Media Session Handler Level:**
   - Include access identification and type in bit rate recommendation requests to Media AF (M5)
   - Subscribe to access-specific bit rate recommendations from Media AF (M5)
   - Determine which access network needs delivery boost upon request from Media-Aware Application (M6)
   - Request Media AF for network boost over specific access network (M5)

3. **Media AF Level:**
   - Report recommended bit rate parameters for each access network type to Media Session Handler (M5)
   - Provide notifications about bit rate recommendations over specific access networks (M5)
   - Notify Media Session Handler of network boost result for specific access network (M5)

4. **Media Access Function Level:**
   - Receive bit rate recommendations for different access networks (M11)
   - Choose optimum bit rate and operation point based on per-access recommendations

#### 10.2 PMF-Based Network Assistance Solution (Clause 5.18.6.4)

**Proposed Enhancements:**

1. **Request and Measurement Flow:**
   - Media-Aware Application requests PMF-based network assistance via Media Access Function (M6/M7)
   - Media Session Handler receives PMF measurements from UE ATSSS Steering Functionality
   - Media Session Handler shares PMF measurements to Media-Aware Application via Media Access Function

2. **Traffic Split Configuration:**
   - Media-Aware Application configures traffic split (UL/DL) over different access networks at Media Access Function (M6)
   - Media Access Function configures UL traffic split at UE ATSSS Steering Functionality
   - Media Access Function requests DL traffic split configuration to UE PMF Functionality

### 11. Summary and Conclusions (Clause 5.18.7)

**Key Findings:**
- Multi-access media delivery enables efficient content access over multiple access networks
- ATSSS architecture impact on 5GMS examined, including application awareness and dynamic policy enhancements
- MPTCP and MPQUIC link-specific multipath IP addresses not routable via N6 in current release
- Traffic splitting for GBR QoS Flows not supported; if M4 flows use GBR QoS, ATSSS splitting not supported

**Release 19 Recommendations (Already Adopted):**
1. Informative annex added to TS 26.501 documenting multi-access media delivery and ATSSS-to-5GMS architecture mapping
2. Changes to Configuration Settings API and Dynamic Status Information API in TS 26.510 for multi-access configuration and status exchange

**Release 20 Recommendations (From This CR):**
3. Modifications to 3GPP specifications based on candidate solutions in clause 5.18.6.3 (AF-based) and 5.18.6.4 (PMF-based)

## Affected Clauses
- Clause 2 (References)
- Clause 3.3 (Abbreviations)
- Clause 5.18.1.3.1 (ATSSS Architecture Background)
- Clause 5.18.1.3.1A (NEW - PMF Measurement Assistance)
- Clause 5.18.1.4A (NEW - Network Assistance Key Issue)
- Clause 5.18.1.5.2 (Key Issue Objectives)
- Clause 5.18.2.3 (NEW - Collaboration Scenario)
- Clause 5.18.4.3 (NEW - AF-based Call Flow)
- Clause 5.18.4.4 (NEW - PMF-based Call Flow)
- Clause 5.18.5.2.5 (NEW - Gap Analysis)
- Clause 5.18.6.3 (NEW - AF-based Candidate Solution)
- Clause 5.18.6.4 (NEW - PMF-based Candidate Solution)
- Clause 5.18.7 (Summary and Conclusions)