# Technical Summary of S4-260255: Application-Layer Approaches for Multi-Access Media Delivery

## Document Overview

This document is a partial Change Request (pCR) for 3GPP TR 26.804 that proposes updates to enable multi-access media delivery using application-layer approaches, specifically focusing on Common Media Client Framework (CMMF). It is intended to be merged with CR 0036 as part of the FS_AMD_Ph2 feasibility study.

## Main Technical Contributions

### 1. Application-Layer Multi-Access Using CMMF (Clause 5.18.1.2)

#### 1.1 Introduction to Upper Layer Approaches

The document introduces two application-layer methods for multi-access media delivery without requiring lower-layer support like ATSSS:

- **Multipath transport protocols** (MPTCP, MPQUIC) - requires implementation on both UE and Application Server
- **CMMF-based approach** - allows transport layer, network layer, and Application Server to remain agnostic of multi-access usage

Both approaches use multiple TCP/QUIC connections bound to different UE network interfaces, with traffic steering performed using existing UE and network functionality.

#### 1.2 CMMF-Specific Implementation

**Key characteristics:**
- Media encoded within CMMF bitstreams/transport objects made available at one or more service locations
- No changes required to CMMF bitstream/transport object creation or network provisioning
- No network/transport layer support (multipath protocols, ATSSS) required
- Clients only need capability to communicate over multiple access networks and steer traffic
- Operation over one access network largely independent of others

**Deployment scenarios:**
- **Scenario 1**: Multiple service locations, each serving requests over specific access network
- **Scenario 2**: Single service location delivering CMMF-encoded media across both access networks

In both scenarios, different CMMF-encoded representations (e.g., CMMF-A and CMMF-B) are requested over different access networks.

**Traffic steering mechanism:**
- Multiple HTTP clients (one per access network) bound to different network interfaces
- HTTP requests/responses automatically routed over bound interface/access network

#### 1.3 Performance Results

The document includes experimental results showing:
- Stable playback bitrate maintained despite WLAN degradation
- Healthy playback buffer maintained throughout session
- Seamless transition from WLAN to 5G cellular as WLAN quality degraded
- Dynamic adaptation of download distribution across access networks based on connection quality

### 2. Architecture Mapping (Clause 5.18.3.1)

**Updates to architecture mapping:**
- Expands upon architecture mappings from clause 5.19.3
- Media Access Client has capability to steer network traffic to different UE network interfaces/access networks
- Can switch between access networks or use both simultaneously
- When using CMMF, different CMMF representation/variation retrieved over each access network
- CMMF decoder (sub-component of Media Access Client) recovers requested media resource

**Key architectural point:**
- Reference point M4d defined over both 3GPP and non-3GPP access networks
- Media Access Client architecture not normatively specified in 5GMS System
- **No changes to 5GMS architecture anticipated**

### 3. High-Level Call Flow (Clause 5.18.4.1)

**Procedure assumptions:**
- Media Player has functionality to stream from multiple service locations and/or across multiple access networks
- Capabilities include: switching between service locations, concurrent use of multiple service locations, steering network traffic, concurrent use of multiple access networks
- Configuration information available via Media Player Entry document or referenced documents
- Content may be hosted at multiple service locations (inside or outside 5GMS System)
- Content may be streamed over two or more access networks

**Key procedure updates:**
- **Step 1**: Content available from two or more service locations
- **Step 11**: Media Player determines multiple service location configuration and method of access
- **Step 15**: Transport sessions established over different access networks based on UE connection and Media Player capabilities
- **Steps 17-18**: Initialization information and media segments may be obtained over one or more access networks

### 4. Gap Analysis (Clause 5.18.5.1)

**Identified open issues:**

1. Existing downlink media streaming procedures in TS 26.501 don't address multi-access downlink media streaming using application-layer approaches
2. Media Access Client capability to use multiple access networks without lower-layer multipath support not addressed in 5GMS stage-3 specifications
3. CMMF procedures, protocols, and formats not defined for downlink media streaming over multiple access networks in TS 26.512

### 5. Candidate Solutions (Clause 5.18.6.1)

#### 5.1 General Use Cases

Application-layer approaches may be used when:
- 5GMSd Application Provider wants to influence how connections to multiple access networks are used
- Using multiple access networks to access media hosted at different service locations
- Transport/network-layer multi-access protocols (MPTCP, ATSSS, etc.) not supported by 5GMSd Client or AS

#### 5.2 CMMF-Based Solution

**Approach:**
- 5GMSd Client accesses/downloads CMMF-encoded media objects over multiple access networks simultaneously from single 5GMSd AS
- Multiple different CMMF-encoded bitstreams/objects (representations/stripes) stored/cached within single logical 5GMSd AS
- Different CMMF-encoded representation downloaded over each available access network
- CMMF decoder yields original source content once enough information received

**Key differentiator from other multi-access technologies:**
- Responsibility for setup, request, and steering rests with application layer (Media Player)
- Multiple HTTP connections in parallel, each bound to different network interface
- HTTP responses routed appropriately following standard network-layer/IP routing

**Traffic steering policies:**
- **Best-effort policy**: Download as much CMMF-encoded content from each access network until decoder can successfully decode
- **Preference policy**: Scheduler throttles requests over one access network to preference another

**Proposed changes:**

1. Define mapping of application-layer approaches into 5GMS procedures in TS 26.501 for multi-access network delivery using CMMF
2. Define procedures, protocols, and formats in Annex H of TS 26.512:
   - Specify media player requirements/recommendations for multi-access delivery using CMMF (including capability to select and steer network traffic)
   - Specify use of CMMF delivery conformance profile
   - Provide recommendations for constructing CMMF Media Player Entries for multiple access networks

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

**Recommendations:**

1. **For TS 26.501** - Add informative annex documenting:
   - Brief description of multi-access media delivery
   - Mapping of ATSSS architecture into 5GMS architecture
   - Mapping of application-layer approaches into 5GMS procedures for CMMF-based multi-access delivery

2. **For TS 26.510** - Implement changes to Configuration Settings API and Dynamic Status Information API for application configuration and status information exchange

3. **Future study** should examine:
   - Whether future ATSSS work supports traffic splitting for GBR QoS Flows
   - Impact on splitting M4 media flows if transported as GBR QoS Flows
   - Impact on UE multi-path management, Dynamic Policy, Network Assistance, and network slicing procedures
   - Closer alignment with study on media delivery from multiple service endpoints/locations

4. **For TS 26.512** - Update Annex H procedures, protocols, and formats to enable application-layer multi-access media delivery using CMMF

### 7. Overall Conclusions (Clause 6.18)

**Stage-2 recommendations:**
- Add informative annex to TS 26.501 with multi-access media delivery description, ATSSS architecture mapping, and application-layer approaches mapping

**Stage-3 recommendations:**
- Implement API changes in TS 26.510
- Update Annex H of TS 26.512 for CMMF-based multi-access delivery

**Future work:**
- Monitor ATSSS specification work in TS 23.501 and TS 23.502
- Study impact on UE multi-access path management, Dynamic Policies, Network Assistance, and network slicing procedures