Description
ATSSS-LL (Access Traffic Steering, Switching and Splitting - Low-Layer) is a core component of the 3GPP's ATSSS framework, standardized from Release 16 onward. It operates at the data link (Layer 2) and network (Layer 3) layers to manage user plane traffic for a single Protocol Data Unit (PDU) Session that is simultaneously connected to both a 3GPP access network (like 5G New Radio) and a non-3GPP access network (like trusted Wi-Fi). The primary architectural entity implementing ATSSS-LL functionality is the User Plane Function (UPF), which acts as a multi-access PDU Session Anchor. The UPF is equipped with an ATSSS-LL capability that includes classifiers and a steering function. The classifier, configured via control plane policies from the Session Management Function (SMF), inspects incoming uplink or downlink packets and assigns them to a specific access leg (3GPP, non-3GPP, or both) based on filters such as 5-tuple (source/destination IP/port, protocol). The steering function then executes the decided action: steering (sending all packets of a flow over one selected access), switching (moving an ongoing flow from one access to another), or splitting (distributing packets of a single flow across multiple accesses). For splitting, ATSSS-LL supports mechanisms like Multi-Path Transmission Control Protocol (MPTCP) at the transport layer or a generic 3GPP-defined Adaptation Layer (ATSSS-LL-A) that sits between the IP layer and the underlying access-specific link layers, adding sequence numbers for packet reordering. The UPF and the User Equipment (UE) must both support the same ATSSS-LL mechanisms to coordinate these operations. Performance measurements (e.g., latency, loss) from both accesses can be fed back to the control plane to dynamically adjust steering policies, optimizing for the best application performance. This low-layer integration allows for rapid, network-assisted decisions that are transparent to most higher-layer applications, providing a robust and efficient multi-access data plane.
Purpose & Motivation
ATSSS-LL was created to address the growing need for seamless and efficient utilization of multiple concurrent access technologies, a cornerstone of 5G system design. Prior to ATSSS, devices could connect to multiple networks (e.g., cellular and Wi-Fi) but typically used them in a mutually exclusive or static manner (like IP flow mobility), often relying on higher-layer, end-to-end solutions like MPTCP which required application support and were not network-controlled. This led to suboptimal resource use, inability to react quickly to network changes, and a fragmented user experience. The purpose of ATSSS-LL is to give the 3GPP network core (specifically the 5GC) direct, low-layer control over how a user's traffic is distributed across available access paths. This solves key problems: it improves aggregate throughput and capacity by bonding access links, enhances connection reliability and resilience through instant failover (switching) between accesses, and reduces latency by steering sensitive traffic to the best-performing path. It was motivated by the vision of Always-Best-Connected services and the need to support demanding use cases like ultra-reliable low-latency communications (URLLC) and enhanced mobile broadband (eMBB) where a single access might be insufficient. By operating at low layers and being managed by network policies, ATSSS-LL provides a standardized, optimized, and transparent multi-access service that is superior to pre-standard or purely client-based implementations.
Classification
Detected Changes Across Releases
from 3GPP Change RequestsSpecific changes extracted from the „Change history“ tables of 3GPP specifications (322 CRs across 6 releases). Complements the general historical overview above with the evidence-based evolution of this function.
In Release 15, the ATSSS-LL function introduced foundational procedures for Access Traffic Steering, Switching, and Splitting between a 3GPP access and a non-3GPP access. It established the core mechanisms for traffic detection and routing based on Application Function (AF) influence, including clarifications for traffic steering control during interworking. The release also defined the handling of traffic mapping information and provided corrections to the AF influence procedures for ensuring proper traffic routing.
- Support for Traffic Segregation TS 24.501CR0476
- Update on Traffic Detection Information TS 23.501CR0026
- Correction to AF influence on traffic routing TS 23.501CR0037
- Clarifications to AF influence on traffic routing TS 23.501CR0038
- Traffic mapping information that disallows UL packets TS 23.501CR0053
- Updates to AF influence on traffic routing TS 23.501CR0150
+ 34 more changes
In Release 16, the ATSSS-LL function introduced new low-layer capabilities for Access Traffic Steering, Switching, and Splitting between 3GPP and non-3GPP accesses. This included specific support for MA PDU session modification to update ATSSS parameters and the definition of procedures for access traffic steering, switching, and splitting at the service data flow level. Furthermore, the release enhanced PCC rule support and usage monitoring control specifically for managing this split traffic.
- Introduction of ATSSS Support TS 23.501CR0735
- Support of Steering Functions for ATSSS TS 23.501CR0740
- ATSSS-SMF and UPF selection TS 23.501CR0761
- Updating 5.8.2.11 for N4 Rules to support ATSSS TS 23.501CR0785
- Control of traffic forwarding in 5G-LAN TS 23.501CR0909
- Transfer of N4 information for local traffic switching from SMF to I-SMF TS 23.501CR1050
+ 104 more changes
In Release 17, the ATSSS-LL function was enhanced with new capabilities including the application of thresholds to its Load-Balancing steering mode for more granular traffic management and enabling traffic steering control specifically for 5G-LAN type of services. It also introduced a mechanism for a partial ATSSS rule update by using an ATSSS Rule ID, allowing for more efficient modifications. Furthermore, the release extended the PCC rule definition for ATSSS and provided updates to AF requests to influence traffic routing.
- Applying thresholds to Load-Balancing steering mode in ATSSS TS 23.501CR2590
- Partial ATSSS rule update by using ATSSS rule ID TS 23.501CR2886
- Updates to AF requests to influence traffic routing TS 23.501CR2913
- Notification to upper layer upper layer for MMTEL video call when T3346 or T3525 running TS 24.501CR2811
- Triggering Service Request procedure due to lower layers request for ProSe layer-2 UE-to-network relay TS 24.501CR3683
- IPv6 prefix delegation via DHCPv6 for 5G ProSe layer-3 UE-to-network relay TS 24.501CR3775
+ 39 more changes
In Release 18, key ATSSS-LL enhancements included the introduction of non-3GPP access path switching capability indications for the UE, AMF, and SMF, and the formal clarification that the redundant steering mode is not applicable for ATSSS-LL functionality. Furthermore, the release specified new procedures for PMFP suspend and resume traffic duplication and expanded support for determining ATSSS capabilities when the UE supports MPQUIC.
- KI#4 23.501 AF traffic influence for common EAS, DNAI selection TS 23.501CR3788
- Support for non-3GPP access path switching TS 23.501CR3878
- KI#4 AF traffic influence for common EAS, DNAI selection TS 23.501CR3987
- KI#3: provisioning of traffic characteristics and monitoring of performance characteristics TS 23.501CR4087
- Delivery of Traffic Influence information for Home Routed-Session Breakout (HR-SBO) support TS 23.501CR4160
- Support of extra traffic characteristics for alternative QoS profile TS 23.501CR4183
+ 79 more changes
In Release 19, the ATSSS-LL function was enhanced to support new steering functionalities for MPQUIC-based proxies and MPQUIC-UDP, and to enable the support of these MPQUIC-based functionalities within the ATSSS capabilities framework. The release also introduced mechanisms for identifying PDU Set information for end-to-end encrypted traffic using connect-UDP and Media over QUIC (MoQ), and added the inclusion of ATSSS status in related session management messages.
- Support PDU Set information identification based on MoQ for encrypted XRM traffic TS 23.501CR5632
- PDU Set Information Identification for end-to-end encrypted traffic using connect-UDP - architecture part TS 23.501CR5728
- KI#3: Enhancement for AF influence on traffic routing with Energy related information TS 23.501CR5713
- HSBO_roaming traffic offloading via session breakout in HPLMN TS 23.501CR5748
- Support of QoS differentiation of traffic for N3GPP device behind UE or 5G-RG TS 24.501CR6618
- Allow configurable 5G registration retries for some lower layer failures TS 24.501CR6677
+ 35 more changes
In Release 20, the ATSSS-LL function introduced new mitigation actions based on abnormal user plane traffic analytics. This enhancement allows the system to proactively respond to anomalous traffic patterns detected within the low-layer steering, switching, and splitting procedures.
- Mitigation actions based on New Abnormal user plane traffic Analytics TS 23.501CR6507
Explore further
Broader topics and technologies where ATSSS-LL plays a role.
Defining Specifications
3GPP specifications that define or reference ATSSS-LL, with the latest known release. Sourced from the 3GPP document catalog — see methodology.
| Specification | Title | Release |
|---|---|---|
| TS 23.501 vk00 | 5G System Architecture Stage 2 | Rel-20 |
| TS 24.193 vj50 | ATSSS Procedures Specification | Rel-19 |
| TS 24.501 vj50 | 5G NAS Protocols Specification | Rel-19 |
| TS 29.244 vj40 | PFCP Specification for Control/User Plane Separation | Rel-19 |
| TS 29.512 vj40 | 5G Session Management Policy Control Service | Rel-19 |