Description
Explicit Congestion Notification (ECN) is a congestion control mechanism defined in IETF RFC 3168 and adopted by 3GPP for use in mobile packet core networks. It enables network nodes (e.g., routers, gateways) to notify endpoints of congestion by setting ECN bits in the IP header of packets, instead of relying solely on packet drops as implicit signals. In 3GPP architectures, ECN is integrated into the Evolved Packet Core (EPC) and 5G Core (5GC) to manage traffic flows, particularly over GTP tunnels and SGi/N6 interfaces. The process involves two bits in the IP header: the ECN-Capable Transport (ECT) bit indicates endpoint support, and the Congestion Experienced (CE) bit is set by congested routers to signal congestion. Endpoints, upon receiving CE-marked packets, reduce their transmission rates proactively, mitigating congestion before it leads to packet loss.
Architecturally, ECN operates across multiple layers in 3GPP systems. At the IP layer, it interacts with transport protocols like TCP and QUIC, which must be ECN-aware to respond to congestion notifications. In the mobile core, elements such as the PGW/UPF, TDF, or PCEF may implement ECN marking based on policy controls or real-time congestion detection. Key components include the ECN field in IPv4 or IPv6 headers, congestion detection algorithms in network nodes (e.g., queue management like RED or CoDel), and endpoint congestion response mechanisms. ECN's role is to enhance Quality of Service (QoS) by reducing packet loss and jitter, which is critical for delay-sensitive applications like voice, video, and interactive gaming in mobile networks.
How ECN works in a 3GPP context involves several steps. First, endpoints negotiate ECN capability during transport session setup. As packets traverse the network, routers monitor queue lengths; if congestion is imminent, they mark packets with CE instead of dropping them, provided the packets are ECN-capable. In mobile networks, this marking can occur at bottlenecks like the SGi interface between the PGW and the internet, or within the core during high traffic loads. The marked packets are delivered to the receiver, which echoes the congestion signal back to the sender via transport-layer acknowledgments. The sender then throttles its transmission rate, easing congestion. 3GPP specifications extend ECN to support differentiated services and integration with QoS frameworks like PCC (Policy and Charging Control), allowing operators to apply ECN policies based on subscriber profiles or service types.
Purpose & Motivation
ECN was created to address the inefficiencies of traditional congestion control, which relies on packet loss as an implicit signal of network congestion. In mobile networks, packet loss can be particularly detrimental due to radio variability and limited bandwidth, causing increased latency and reduced throughput for applications. ECN solves this by providing explicit, early congestion notifications, allowing endpoints to react before loss occurs, thus improving overall network efficiency and user experience.
The historical context traces back to IETF efforts in the late 1990s to enhance Internet congestion management, leading to RFC 3168. 3GPP adopted ECN starting in Release 7 to optimize packet-switched services in UMTS and later LTE/5G networks. Prior approaches, like tail-drop or RED without ECN, led to unnecessary packet drops and TCP timeouts, degrading performance for real-time applications. ECN offered a proactive alternative, aligning with 3GPP's goals for enhanced QoS and support for multimedia services.
Motivations for ECN in 3GPP include reducing latency for low-latency applications, conserving radio resources by minimizing retransmissions, and enabling better traffic management in congested scenarios. It addresses limitations of earlier mobile data systems that lacked sophisticated congestion signaling, supporting the evolution toward all-IP networks and rich media services. ECN also facilitates compliance with regulatory requirements for network neutrality and efficient resource utilization.
Classification
Detected Changes Across Releases
from 3GPP Change RequestsSpecific changes extracted from the „Change history“ tables of 3GPP specifications (187 CRs across 6 releases). Complements the general historical overview above with the evidence-based evolution of this function.
Studied in Rel-7, normative work from Rel-15.
In Release 15, the ECN function was enhanced with new subscription and notification mechanisms for resource allocation outcomes and out-of-credit events, including corresponding data models and service procedures. It also introduced clarifications and corrections for congestion control mechanisms, specifically for NAS-level mobility management, 5GSM, and S-NSSAI-based congestion control across various procedures like PDU session modification. Furthermore, updates were made to QoS Notification Control and the handling of uplink data for mobile-terminated notifications.
- Subscription and notification of resources allocation outcome, data model TS 29.514CR0007
- Notification of resource allocation outcome, service procedures TS 29.514CR0009
- Subscription and notification of out of credit events, data model TS 29.514CR0010
- Subscription to out of credit notification, service procedures TS 29.514CR0011
- Out of credit notification, service procedures TS 29.514CR0012
- Updates of QoS Notification Control description and data model TS 29.514CR0028
+ 32 more changes
In Release 16, the ECN function was enhanced with new congestion control mechanisms specifically for CP data transport in the 5GS and for the UL NAS TRANSPORT message. It also introduced procedures to associate S-NSSAI-based congestion backoff timers with the S-NSSAI provided during PDU session establishment. Furthermore, the release defined conditions for congestion control in the AMF and specified UE behavior for the NOTIFICATION message for additional states and sub-states.
- Congestion control for CP data transport in 5GS TS 24.501CR0990
- Support of “Access Network Charging Correlation Information” notification TS 29.514CR0109
- Support of “Out of credit” notification TS 29.514CR0110
- Support of “Access Network Information Notification” TS 29.514CR0112
- Subscription to notification of Signalling Path Status TS 29.514CR0121
- P-CSCF procedures to support Access Type Change notification TS 29.514CR0138
+ 27 more changes
In Release 17, ECN enhancements introduced notifications for PDU session established/terminated events to the PCF and expanded subscription capabilities for application detection notifications within a PDU session. The release also clarified and corrected several congestion control procedures, including UE reachability after a NOTIFICATION RESPONSE and the handling of slice-based (S-NSSAI) and 5GSM non-congestion back-off timers. Furthermore, it added support for congestion handling in specific scenarios like priority-based steering mode, onboarding in SNPN, and for CIoT containers.
- Notification to upper layer upper layer for MMTEL video call when T3346 or T3525 running TS 24.501CR2811
- NOTIFICATION RESPONSE message indicating failure TS 24.501CR3350
- Duplicated notification TS 29.512CR0806
- Congestion handling for priority-based steering mode TS 29.512CR0819
- Indication of request of notification PDU session established/terminated events TS 29.512CR0856
- Support of subscription to application detection notification for a PDU session TS 29.514CR0295
+ 28 more changes
In Release 18, the ECN function was enhanced with new capabilities for direct event notification of Time-Sensitive Communication (TSC) management information and for the measurement and reporting of congestion information. It also introduced specific procedures for handling notifications when a UE is unavailable and for event notifications for AF-requested QoS for UEs or groups not identified by a UE address. Furthermore, interactions between ECN marking for L4S (Low Latency, Low Loss, Scalable Throughput) and Congestion Monitoring were defined.
- Network notifications for EPS interworking related events TS 23.289CR0032
- Equivalent SNPNs usage for congestion control TS 24.501CR4838
- UE unavailability period reporting overrides mobility management congestion control TS 24.501CR5549
- Handling NOTIFICATION when UE unavailable TS 24.501CR6001
- Support of the direct event notification of TSC management information TS 29.512CR1071
- Support of the congestion information measurement and reporting TS 29.512CR1127
+ 45 more changes
In Release 19, the ECN function was enhanced to support Low Latency Low Loss Scalable throughput (L4S) mechanisms for new service types, specifically for MCVideo push, 5G-RG, and VAL applications over HTTP and CoAP. Furthermore, the release introduced QoS notification control with explicit uplink and downlink direction information and refined connection status reporting for SEALDD with new sleeping duration attributes. These updates were accompanied by necessary corrections and clarifications on ECN marking for L4S and related congestion control procedures.
- Support of exposing network congestion information for MC services TS 23.289CR0125
- MCVideo push enhancement via ECN marking for L4S TS 23.289CR0131
- Support of ECN marking for L4S for 5G-RG TS 24.501CR6626
- SEALDD connection status reporting configuration notification - HTTP TS 24.543CR0038
- SEALDD connection status reporting configuration notification - CoAP TS 24.543CR0039
- Updates for SEALDD enabled congestion control for VAL application by supporting L4S mechanism for HTTP TS 24.543CR0027
+ 23 more changes
In Release 20, the specification for the ECN function was updated to enhance management capabilities. This included an enhancement to the PreDefinedPccRule specifically for ECN marking, along with the addition of missing notifications within the management framework.
Explore further
Broader topics and technologies where ECN plays a role.
Defining Specifications
3GPP specifications that define or reference ECN, with the latest known release. Sourced from the 3GPP document catalog — see methodology.
| Specification | Title | Release |
|---|---|---|
| TS 22.495 v1700 | NGN Requirements for IMS Services | Rel-7 |
| TS 23.228 vj50 | IMS Stage-2 Service Description | Rel-19 |
| TS 23.289 vk10 | Mission Critical services over 5G System | Rel-20 |
| TS 23.333 vj00 | MRFC-MRFP Mp Interface Requirements | Rel-19 |
| TS 23.334 vj00 | IMS-ALG to IMS-AGW Interface (Iq) Stage 2 | Rel-19 |
| TS 23.401 vj50 | Evolved Packet System (EPS) Stage 2 Description | Rel-19 |
| TS 23.802 v1700 | Enhanced End-to-End QoS Architecture | Rel-7 |
| TS 23.860 va00 | Codec Rate Adaptation Enhancements Study | Rel-10 |
| TS 24.229 vj50 | IMS call control protocol based on SIP and SDP | Rel-19 |
| TS 24.501 vj50 | 5G NAS Protocols Specification | Rel-19 |
| TS 24.543 vj50 | SEAL Data Delivery Management Protocol | Rel-19 |
| TS 26.114 vj10 | IMS Multimedia Telephony Media Handling | Rel-19 |
| TS 26.501 vj30 | 5G Media Streaming (5GMS) Architecture | Rel-19 |
| TS 26.510 vj10 | Media Delivery APIs for 5GMS and RTC Systems | Rel-19 |
| TS 26.512 vj10 | 5G Media Streaming Protocols & APIs | Rel-19 |
| TS 26.804 vj10 | 5G Media Streaming Extensions Study | Rel-19 |
| TR 26.919 vj00 | Study on 5G Conversational Media Handling | Rel-19 |
| TS 28.541 vk00 | 5G Network Resource Model (NRM) Stage 2/3 | Rel-20 |
| TS 29.162 vj00 | IMS-IP Network Interworking | Rel-19 |
| TS 29.163 vj00 | Interworking between 3GPP IM CN and CS networks | Rel-19 |
| TS 29.232 vj00 | Mc Interface Protocol Profile | Rel-19 |
| TS 29.238 vj00 | H.248 Profile for IBCF-TrGW Interface | Rel-19 |
| TS 29.292 vj00 | IMS Centralized Services (ICS) Interworking | Rel-19 |
| TS 29.332 vj00 | MGCF-IM-MGW Interface Protocol (Mn) | Rel-19 |
| TS 29.333 vj00 | MRFC-MRFP Mp Interface Protocol | Rel-19 |
| TS 29.334 vj00 | IMS-ALG to IMS-AGW Interface Protocol | Rel-19 |
| TS 29.512 vj40 | 5G Session Management Policy Control Service | Rel-19 |
| TS 29.514 vj40 | 5G System; Policy Authorization Service; Stage 3 | Rel-19 |
| TS 36.750 ve10 | Study on enhancement of VoLTE | Rel-14 |
| TS 38.415 vj10 | PDU Session User Plane Protocol | Rel-19 |