Coupling ECN Marking Thresholds with Dynamic Buffer Allocation
draft-yang-dynamic-ecn-threshold-00

Transport Area Working Group                                     J. Yang
Internet-Draft                                                  W. Cheng
Intended status: Informational                                   Y. Tian
Expires: 2 September 2026                                   China Mobile
                                                                 J. Wang
                                                                G. Zhang
                                                                  Centec
                                                            1 March 2026

     Coupling ECN Marking Thresholds with Dynamic Buffer Allocation
                  draft-yang-dynamic-ecn-threshold-00

Abstract

   Explicit Congestion Notification (ECN) marking thresholds are
   typically configured statically.  In modern network devices that
   employ dynamic buffer allocation -- where the maximum buffer
   available to a queue fluctuates dynamically based on the number of
   active queues and the remaining shared buffer pool -- a static ECN
   threshold can frequently become misaligned with the actual
   instantaneous buffering capacity.

   This misalignment can lead to pathological behaviors: either
   premature marking (which underutilizes available buffers and
   throttles throughput) or late marking (which provides no advance
   warning before tail drop occurs).  This document describes an
   operational framework and a deterministic reference algorithm for
   dynamically coupling the ECN marking threshold with the dynamic
   buffer allocation limit.  By maintaining an adaptive relationship
   through configurable parameters, this mechanism ensures robust
   congestion signaling across varying load conditions without requiring
   complex external machine-learning models or per-flow tracking.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on 2 September 2026.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
     2.1.  Requirements Language . . . . . . . . . . . . . . . . . .   4
   3.  Applicability Statement . . . . . . . . . . . . . . . . . . .   4
   4.  Problem Statement . . . . . . . . . . . . . . . . . . . . . .   5
   5.  Dynamic Coupling Architecture . . . . . . . . . . . . . . . .   5
     5.1.  Prerequisite: Buffer State Awareness  . . . . . . . . . .   5
     5.2.  Reference Algorithm for ECN Threshold . . . . . . . . . .   6
     5.3.  Architectural Invariants  . . . . . . . . . . . . . . . .   7
   6.  Operational Considerations  . . . . . . . . . . . . . . . . .   7
     6.1.  Update Synchronization  . . . . . . . . . . . . . . . . .   7
     6.2.  Tuning the Offset Parameter . . . . . . . . . . . . . . .   8
     6.3.  Tuning the ECN_Floor Parameter  . . . . . . . . . . . . .   8
   7.  Implementation Status . . . . . . . . . . . . . . . . . . . .   8
   8.  Related Work  . . . . . . . . . . . . . . . . . . . . . . . .   9
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .   9
   10. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  10
   11. Normative References  . . . . . . . . . . . . . . . . . . . .  10
   12. Informative References  . . . . . . . . . . . . . . . . . . .  10
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  11

1.  Introduction

   Explicit Congestion Notification (ECN) [RFC3168] enables network
   devices to signal incipient congestion to endpoints without resorting
   to packet drops.  A device marks a packet's IP header with the
   Congestion Experienced (CE) codepoint when a specific queue metric
   exceeds a configured Active Queue Management (AQM) threshold.  The
   sender, upon learning of the CE mark through transport-layer
   feedback, proactively reduces its sending rate.

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   Conventionally, the ECN marking threshold is established as a static
   value chosen by the network operator.  This static approach functions
   adequately when the maximum buffer available to a given queue is also
   static and predictable.  However, the architecture of modern data
   center switches heavily relies on dynamic buffer allocation.  In such
   architectures, the maximum buffer a queue is permitted to consume
   (Buf_Thrd) fluctuates significantly based on the total available
   shared buffer and the instantaneous number of active queues drawing
   from it.  Dynamic buffer allocation schemes, such as those utilizing
   the alpha parameter model, are widely deployed in commodity switching
   silicon to maximize memory utilization.

   When Buf_Thrd shrinks (e.g., due to an incast event activating many
   queues), a static ECN threshold originally positioned well below the
   nominal buffer limit may suddenly be equal to or greater than the
   current Buf_Thrd.  In this scenario, the device is forced into tail
   drop before the queue occupancy ever reaches the ECN threshold.  The
   ECN mechanism effectively fails, yielding severe packet loss and
   higher tail latency rather than graceful rate reduction.

   Conversely, when the network load decreases and Buf_Thrd expands, the
   static threshold may sit far below the actual buffer capacity.  This
   drastically underutilizes available buffering, generating premature
   congestion signals that trigger unnecessary rate reduction and
   diminish overall link utilization.

   Unlike sojourn-time based AQM algorithms (such as CoDel [RFC8289] or
   PIE [RFC8033]), which inherently adapt to buffer size variations by
   measuring delay rather than bytes, queue-depth based marking
   mechanisms (e.g., standard step-marking in DCTCP [RFC8257] or RoCEv2
   environments) are highly vulnerable to dynamic buffer fluctuations.

   This document specifies an operational mechanism that continually
   derives the ECN marking threshold (ECN_Thrd) from the instantaneous
   value of Buf_Thrd.  The computation introduces two operator-
   configurable parameters to maintain predictable headroom.  The
   approach offers a deterministic, hardware-friendly solution to
   maintain a consistent relationship between ECN marking and buffer
   availability.

2.  Terminology

   In the context of this document, a "queue" typically refers to a per-
   port, per-traffic-class transmission queue within a forwarding
   device.

   Buf_Thrd (Buffer Threshold):  The dynamic buffer allocation limit for

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      a specific queue.  This represents the maximum amount of shared
      buffer memory that the queue is currently authorized to occupy.
      Buf_Thrd is periodically or event-driven recomputed by the
      device's buffer management subsystem.

   ECN_Thrd (ECN Threshold):  The active ECN marking threshold for a
      queue.  When the instantaneous or averaged queue occupancy meets
      or exceeds ECN_Thrd, the device applies the CE codepoint to
      arriving ECN-capable packets.

   Offset:  A configurable parameter dictating the desired buffer
      headroom (typically measured in bytes or cells) maintained between
      Buf_Thrd and ECN_Thrd.  The Offset acts as a shock absorber for
      packets already in-flight during the control loop feedback delay.

   ECN_Floor:  A configurable parameter establishing the minimum
      permissible boundary for ECN_Thrd.  It acts as a safeguard against
      ECN_Thrd collapsing to excessively low values (e.g., below a
      single MTU), which would cause catastrophic throughput degradation
      via aggressive continuous marking.

2.1.  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 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

3.  Applicability Statement

   This document is explicitly applicable to network forwarding devices
   utilizing queue-depth based ECN marking mechanisms in conjunction
   with a dynamic buffer allocation scheme.  It is primarily targeted at
   Data Center Networks (DCN) and high-speed interconnects where
   instantaneous queue length or average queue length is evaluated
   against a byte-based or cell-based threshold.

   This specification does NOT target devices employing sojourn-time
   based AQMs (e.g., [RFC8289], [RFC8033]), as time-based algorithms
   naturally abstract away the physical buffer size and are generally
   immune to the dynamic shared buffer problem described herein.

   The operational logic defined here is strictly internal to the
   network device.  It does not alter the ECN wire protocol, IP-layer
   ECN codepoint semantics, or the transport-layer negotiation
   standardized in [RFC3168].

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   The method is compatible with Classic ECN marking as well as modern
   scalable congestion controls (e.g., the L4S architecture [RFC9330]
   and its ECN protocol [RFC9331]).  In a DualQ Coupled AQM
   [I-D.ietf-tsvwg-aqm-dualq-coupled] architecture, the dynamically
   computed ECN_Thrd may serve as the target threshold for the Classic
   queue, leaving the L4S queue's specialized marking behavior
   independent.

4.  Problem Statement

   To formalize the problem context, consider a Top-of-Rack (ToR) switch
   equipped with a 12 MB shared buffer pool and 48 egress ports.  Under
   light traffic conditions with only 4 queues active, the dynamic
   buffer management may assign a Buf_Thrd of 3 MB to each active queue.
   Assuming a network operator statically configures an ECN threshold of
   200 KB, the system operates with 2.8 MB of effective headroom,
   providing ample shock absorption.

   However, during a coordinated incast event where all 48 ports become
   heavily congested, the shared buffer is fractured, and the dynamic
   Buf_Thrd for each queue plummets to 250 KB.  The statically
   configured 200 KB ECN threshold now yields a mere 50 KB of headroom.
   In high-speed environments (e.g., 100Gbps+), 50 KB is significantly
   smaller than the Bandwidth-Delay Product (BDP) of the control loop.
   Consequently, the queue will hit the tail drop limit (Buf_Thrd)
   before the transport sender has time to react to the CE marks,
   inducing severe retransmission timeouts and latency spikes.

   Conversely, if the operator statically configures the ECN threshold
   to 2 MB to optimize for high throughput under light load, the ECN
   mechanism will completely fail during the incast event because the
   static ECN threshold (2 MB) heavily exceeds the active Buf_Thrd (250
   KB).

   A deterministic, dynamic coupling between Buf_Thrd and ECN_Thrd is
   necessary to resolve these dual failure modes without relying on
   static compromises.

5.  Dynamic Coupling Architecture

5.1.  Prerequisite: Buffer State Awareness

   The foundation of this architecture requires the device's forwarding
   plane to expose the current Buf_Thrd value to the AQM/ECN marking
   engine.  The specific memory management algorithm (e.g., alpha-based
   proportional allocation) calculating Buf_Thrd is outside the scope of
   this document.  The sole prerequisite is that Buf_Thrd is
   continuously updated and accessible with low latency.

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5.2.  Reference Algorithm for ECN Threshold

   Network devices SHOULD compute ECN_Thrd continuously based on
   Buf_Thrd, Offset, and ECN_Floor.  To ensure stability across all load
   extremes, the logic is segmented into three distinct operational
   regions:

   Region A -- Sufficient Buffer (Nominal State):

   Condition: (Buf_Thrd - Offset) > ECN_Floor.  The buffer allocation is
   generous enough to accommodate the full requested headroom (Offset).
   Here, ECN_Thrd = Buf_Thrd - Offset.  The ECN threshold securely
   tracks the dynamic buffer limit, guaranteeing precisely the
   configured absorption capacity.

   Region B -- Constrained Buffer (Congested State):

   Condition: (Buf_Thrd - Offset) <= ECN_Floor AND Buf_Thrd > ECN_Floor.
   The shared buffer is highly constrained.  Enforcing the full Offset
   would depress ECN_Thrd below the critical ECN_Floor, risking
   excessive marking and severe throughput collapse.  To mitigate this,
   the threshold is clamped: ECN_Thrd = ECN_Floor.  The available
   headroom compresses to (Buf_Thrd - ECN_Floor), prioritizing
   reasonable throughput over optimal packet absorption.

   Region C -- Critical Buffer (Exhaustion State):

   Condition: Buf_Thrd <= ECN_Floor.  The queue's buffer allocation has
   collapsed to or below the minimum floor.  In this critical state,
   clamping ECN_Thrd to ECN_Floor would result in ECN_Thrd >= Buf_Thrd,
   rendering ECN useless (tail drops would occur silently).  Thus,
   ECN_Thrd = Buf_Thrd.  While zero headroom remains, the device marks
   packets exactly at the tail drop boundary, ensuring the network still
   transmits explicit congestion signals.

   The reference logic is expressed as follows:

   function compute_ecn_threshold(Buf_Thrd, Offset, ECN_Floor):
       IF (Buf_Thrd - Offset) > ECN_Floor:
           RETURN Buf_Thrd - Offset          // Region A: Optimal tracking
       ELSE IF Buf_Thrd > ECN_Floor:
           RETURN ECN_Floor                  // Region B: Floor clamped
       ELSE:
           RETURN Buf_Thrd                   // Region C: Drop boundary

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                        Buf_Thrd Update Event
                              |
                              v
                 +---------------------------+
                 | (Buf_Thrd - Offset) >     |
                 |        ECN_Floor?         |
                 +-------------+-------------+
                 | YES         | NO
                 v             v
       ECN_Thrd =     +------------------+
       Buf_Thrd -     | Buf_Thrd >       |
       Offset         |    ECN_Floor?    |
       [Region A]     +--------+---------+
                      | YES    | NO
                      v        v
            ECN_Thrd =   ECN_Thrd =
            ECN_Floor    Buf_Thrd
            [Region B]   [Region C]

            Figure 1: State Transition of Dynamic ECN Threshold

   This algorithm requires minimal logic gates (two comparators and one
   subtractor), ensuring it can be evaluated in standard Application-
   Specific Integrated Circuit (ASIC) pipelines with nominal nanosecond
   latency.

5.3.  Architectural Invariants

   Implementations conforming to this framework SHOULD validate the
   following invariants to prevent anomalous traffic handling:

   1.  ECN_Thrd MUST NOT exceed Buf_Thrd (ECN_Thrd <= Buf_Thrd).  This
   mathematically guarantees ECN marking is always attempted prior to or
   simultaneously with queue tail drop.

   2.  ECN_Thrd MUST NOT fall below ECN_Floor, UNLESS the maximum
   physical buffer limit (Buf_Thrd) has itself fallen below ECN_Floor.

6.  Operational Considerations

6.1.  Update Synchronization

   ECN_Thrd MUST be inherently recomputed concurrently with any
   transition in Buf_Thrd.  Event-driven synchronization is highly
   RECOMMENDED over periodic polling.  Polling introduces phase-delay,
   leaving the ECN_Thrd stale during the most critical microsecond
   inflection points of transient congestion.  If atomic hardware
   updates are impossible, implementations SHOULD bias the asynchronous

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   race condition to temporarily favor a lower ECN_Thrd (causing a
   premature mark) over a higher ECN_Thrd (causing an unnotified drop).

6.2.  Tuning the Offset Parameter

   The Offset represents the network's required "shock absorber."
   Operators SHOULD calibrate the Offset to slightly exceed the expected
   Bandwidth-Delay Product (BDP) of the typical congestion control
   feedback loop:

   Offset ≈ Link_Rate * RTT

   In contemporary intra-data-center fabrics (RTT ~20-50 microseconds,
   400 Gbps links), Offset values ranging from 1 MB to 2.5 MB are
   operationally appropriate.  Oversizing the Offset prematurely
   throttles flows; undersizing it invites high tail-drop rates despite
   ECN capability.

6.3.  Tuning the ECN_Floor Parameter

   ECN_Floor establishes the maximum throttling severity.  It MUST NOT
   be configured smaller than the Maximum Transmission Unit (MTU) of the
   link (e.g., 9000 bytes).  For environments executing Data Center TCP
   (DCTCP) [RFC8257], ECN_Floor SHOULD typically mirror the static
   thresholds recommended for shallow buffering (e.g., 30 KB to 100 KB),
   preventing the queue from emptying completely while maintaining
   ultra-low queuing delay.

7.  Implementation Status

   [RFC Editor: Please remove this section before publication.]

   This section records the status of known implementations of the
   protocol defined by this specification at the time of posting of this
   Internet-Draft, and is based on a proposal described in RFC 7942.
   The description of implementations in this section is intended to
   assist the IETF in its decision processes in progressing drafts to
   RFCs.

   The dynamic ECN threshold coupling mechanism described in this
   document has been implemented and validated in the data plane of
   Centec Networks' switching silicon, specifically designed to mitigate
   micro-bursts and incast congestion in large-scale RDMA over Converged
   Ethernet (RoCEv2) deployments by China Mobile.

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8.  Related Work

   AQM recommendations generalized in [RFC7567] outline the complexities
   of parameter tuning.  While this document aligns with the intent of
   [RFC7567], it specifically isolates and resolves the intersection of
   AQM and dynamic shared buffering, a domain not fully explored in
   legacy AQM guidelines.

   The AI-based ECN approach proposed in
   [I-D.zhuang-tsvwg-ai-ecn-for-dcn] targets similar parameter
   adaptation via machine learning.  The framework in this document,
   conversely, advocates for a mathematically deterministic data-path
   calculation, demanding no training data, no external control-plane
   telemetry loop, and zero inference latency.

   TCP Alternative Backoff with ECN (ABE) [RFC8511] optimizes how
   endpoints react to CE marks.  ABE is strictly complementary; it
   refines the sender response, whereas this architecture ensures the
   network device generates those marks at structurally correct moments.

9.  Security Considerations

   This specification introduces an automated internal parameter
   coupling within the network forwarding plane.  It does not exchange
   new protocol messages across the wire, thus introducing no new
   cryptographic or protocol-level attack surfaces.

   Operational Degradation via Misconfiguration: Invalid configuration
   of Offset or ECN_Floor can initiate self-inflicted Denial of Service
   (DoS) behaviors.  For instance, an immensely inflated Offset might
   universally push the system into Region C, effectively disabling
   early congestion warning.  Implementations SHOULD validate parameter
   inputs through management interfaces and emit warnings if Offset
   exceeds typical physical buffer allocations.

   Internal Signaling Integrity: The architectural dependency between
   the memory management unit (MMU) and the ECN marking engine requires
   deterministic internal signaling.  If the internal update of Buf_Thrd
   is delayed or corrupted under heavy system load, the ECN_Thrd
   calculation will be based on stale memory constraints, leading to
   temporary periods of over-marking or under-marking.  Hardware designs
   SHOULD prioritize this internal signaling path.

   Buffer Exhaustion Vectors: Malicious, non-responsive flows could
   intentionally occupy massive allocations of the shared buffer pool.
   In dynamic buffer architectures, this action compresses the Buf_Thrd
   for all other benign queues, plunging them into Region B or Region C.
   This is an inherent vulnerability of shared memory switches, not

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   generated by this ECN algorithm.  Operators MUST utilize per-queue
   maximum caps, port-level QoS scheduling, and admission control to
   insulate queues from cross-traffic buffer starvation.

10.  IANA Considerations

   This document has no IANA actions.

11.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC3168]  Ramakrishnan, K., Floyd, S., and D. Black, "The Addition
              of Explicit Congestion Notification (ECN) to IP",
              RFC 3168, DOI 10.17487/RFC3168, September 2001,
              <https://www.rfc-editor.org/info/rfc3168>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

12.  Informative References

   [RFC7567]  Baker, F., Ed. and G. Fairhurst, Ed., "IETF
              Recommendations Regarding Active Queue Management",
              BCP 197, RFC 7567, DOI 10.17487/RFC7567, July 2015,
              <https://www.rfc-editor.org/info/rfc7567>.

   [RFC8033]  Pan, R., Natarajan, P., Baker, F., and G. White,
              "Proportional Integral Controller Enhanced (PIE): A
              Lightweight Control Scheme to Address the Bufferbloat
              Problem", RFC 8033, DOI 10.17487/RFC8033, February 2017,
              <https://www.rfc-editor.org/info/rfc8033>.

   [RFC8257]  Bensley, S., Thaler, D., Balasubramanian, P., Eggert, L.,
              and G. Judd, "Data Center TCP (DCTCP): TCP Congestion
              Control for Data Centers", RFC 8257, DOI 10.17487/RFC8257,
              October 2017, <https://www.rfc-editor.org/info/rfc8257>.

   [RFC8289]  Nichols, K., Jacobson, V., McGregor, A., Ed., and J.
              Iyengar, Ed., "Controlled Delay Active Queue Management",
              RFC 8289, DOI 10.17487/RFC8289, January 2018,
              <https://www.rfc-editor.org/info/rfc8289>.

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   [RFC8511]  Khademi, N., Welzl, M., Armitage, G., and G. Fairhurst,
              "TCP Alternative Backoff with ECN (ABE)", RFC 8511,
              DOI 10.17487/RFC8511, December 2018,
              <https://www.rfc-editor.org/info/rfc8511>.

   [RFC9330]  Briscoe, B., Ed., De Schepper, K., Bagnulo, M., and G.
              White, "Low Latency, Low Loss, and Scalable Throughput
              (L4S) Internet Service: Architecture", RFC 9330,
              DOI 10.17487/RFC9330, January 2023,
              <https://www.rfc-editor.org/info/rfc9330>.

   [RFC9331]  De Schepper, K. and B. Briscoe, Ed., "The Explicit
              Congestion Notification (ECN) Protocol for Low Latency,
              Low Loss, and Scalable Throughput (L4S)", RFC 9331,
              DOI 10.17487/RFC9331, January 2023,
              <https://www.rfc-editor.org/info/rfc9331>.

   [I-D.zhuang-tsvwg-ai-ecn-for-dcn]
              Zhuang, Y., Zhang, B., and H. Pan, "Artificial
              Intelligence (AI) based ECN adaptive reconfiguration for
              datacenter networks", Work in Progress, Internet-Draft,
              draft-zhuang-tsvwg-ai-ecn-for-dcn-00, October 2019,
              <https://datatracker.ietf.org/doc/draft-zhuang-tsvwg-ai-
              ecn-for-dcn/>.

   [I-D.ietf-tsvwg-aqm-dualq-coupled]
              De Schepper, K. and B. Briscoe, Ed., "DualQ Coupled AQMs
              for Low Latency, Low Loss and Scalable Throughput (L4S)",
              Work in Progress, Internet-Draft, draft-ietf-tsvwg-aqm-
              dualq-coupled-24, 2024, <https://datatracker.ietf.org/doc/
              draft-ietf-tsvwg-aqm-dualq-coupled/>.

Authors' Addresses

   Jin Yang
   China Mobile
   Beijing
   100053
   China
   Email: yangjinwl@chinamobile.com

   Weiqiang Cheng
   China Mobile
   Beijing
   100053
   China
   Email: chengweiqiang@chinamobile.com

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   Yuchi Tian
   China Mobile
   Beijing
   100053
   China
   Email: tianyuchi@chinamobile.com

   Junjie Wang
   Centec
   Suzhou
   215000
   China
   Email: wangjj@centec.com

   Guoying Zhang
   Centec
   Suzhou
   215000
   China
   Email: zhanggy@centec.com

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