Explicit Congestion Notification Using MPLS Network Actions

Introduction Network throughput (also referred to as bandwidth) has increased exponentially over the last several decades, enabling a variety of new features for end users, including video streaming, online gaming, and mixed reality implementations, as well as more advanced applications such as remote teleoperation and autonomous services. The focus on network throughput, however, underestimates the fact that users are primarily interested in application responsiveness rather than the number of bits they can transfer per second. As a result, user experience improvements for time-critical applications have not necessarily increased at the same pace as network throughput (i.e., the increased bandwidth does not result in a corresponding increase in the user experience). Optimizing network latency rather than throughput can lead to a significantly improved user experience for time-critical applications. Low Latency, Low Loss, and Scalable Throughput (L4S) technology, introduced in , uses Explicit Congestion Notification (ECN) () bits in IP or TCP by marking packets suffering potential congestion in the network (). L4S operates as a congestion-control mechanism, using ECN markers within the data packets to detect and promptly respond to congestion conditions. This feedback loop enables devices (e.g., endpoints such as client devices and server devices) to adjust data flow in real-time, preventing bottlenecks and ensuring smoother transmission. describes a mechanism for supporting ECN in the Multiprotocol Label Switching (MPLS) data plane. That mechanism is based on the codepoints that take part in the Traffic Class (TC) field and, thus, prevent the use of the TC field for traffic differentiation. This document defines how ECN can be supported in the MPLS data plane using the MPLS Network Actions technology () while preserving the existing MPLS traffic class capability.

Conventions Used in this Document

ECN: Explicit Congestion Notification ISD: In-Stack Data L4S: Low Latency, Low Loss, and Scalable Throughput LER: Label Edge Router LSR: Label Switching Router MPLS: Multiprotocol Label Switching MNA: MPLS Network Action NAS: Network Action Sub-stack PSD: Post-Stack Data TC: Traffic Class

Requirements Language The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 when, and only when, they appear in all capitals, as shown here.

Realization of Explicit Congestion Notification as an MPLS Network Action MNA, according to , can support ECN by an MNA Sub-stack using either In-Stack Data (ISD) or Post-Stack Data (PSD) for ancillary data.

ECN Encapsulation Using In-Stack MPLS Network Actions displays the encoding of ECN using ISD MNA.

Format of the ECN MNA Using ISD MNA Where fields are defined as follows:

Opcode ECN is seven-bit field. IANA is requested to assign (TBA1) value .
Data1 is 14-bit field. It MUST be zeroed on transmission and ignored on reception and processing of the Opcode ECN.
ECN is two-bit field.
S is the Bottom of Stack one-bit flag, as defined in
U is a one-bit Unknown Network Action Handling field as defined in .
Data2 is four-bit field. It MUST be zeroed on transmission and ignored on reception and processing of the Opcode ECN.
NAL is three-bit Network Action Length as defined in .

ECN Encoding Using Post-Stack MPLS Network Actions displays the encoding of ECN using PSD MNA.

Format of the ECN MNA Using PSD MNA Where the enclosed elements are defined as follows:

ECN PS-MNA-OP is seven-bit field. IANA is requested to assign (TBA2) value ().
R - two Reserved bits as defined in .
PS-NAL is seven-bit field as defined in .
ECN is two-bit field.
Post-Stack Data is 14-bit field. It MUST be zeroed on transmission and ignored on reception and processing of the ECN PS-MNA-OP.

Theory of Operation If an ingress Label Edge Router (LER) that supports this specification is enabled to use ECN MNA receives a packet with the indication that the endpoints of the transport protocol are ECN-capable or the packet has experienced congestion, it MUST copy the value of the ECN field in the received IP packet into the ECN field of the ECN MNA using ISD MNA () or of the ECN MNA using PSD MNA (). If the incoming packet is marked as non-ECN capable, the encapsulator SHOULD NOT use the ECN MNA. If, for some reason, the local policy requires the ECN MNA to be used, it MUST set the ECN value to Not-ECT. This corresponds to the normal mode behavior defined in . If a transit Label Switching Router (LSR) that supports this specification receives an MPLS packet with ECN MNA and determines that the egress interface for the received packet experiences congestion although its buffer is not full so that the packet can be transmitted (see for the detailed explanation of the incipient congestion ), the LSR MUST set the ECN field to the Congestion Experienced (0b11) value. If an egress LER that supports this specification, receives an MPLS packet with ECN MNA, it MUST follow the decapsulator's behavior defined in Section 4.2 of . In some scenarios, the Readable Label Depth of the constructed path through the MPLS domain may require the placement of more than one ECN MNA Network Action Sub-stacks (NASes). If that is the case, then the Label Switching Router that exposes the NAS with ECN MNA MUST copy the value of the ECN field in that NAS into the ECN field in the next NAS that includes ECN MNA.

IANA Considerations

ECN as an MPLS Network Action Opcode IANA is requested to assign an ECN codepoint (TBA1) from its Network Action Opcodes registry (creation requested in ) as specified in . ECN as MPLS Network Action Opcode

Opcode	Description	Reference
TBA1	ECN as MPLS Network Action Indicator	This document

ECN as an Post-Stack Network Action Opcode IANA is requested to assign an ECN codepoint (TBA1) from its Post-Stack Network Action Opcodes registry (creation requested in ) as specified in . ECN as MPLS Network Action Opcode

Opcode	Description	Reference
TBA2	ECN as Post-Stack Network Action Opcode	This document

Security Considerations Security considerations discussed in , , and apply to this document.

Acknowledgments TBA