



Network Working Group                                            W. Yang
Internet-Draft                                                    Y. Ren
Intended status: Standards Track                                 X. Zhou
Expires: 4 May 2026                                                 CNIC
                                                                   Q. Wu
                                                                     ICT
                                                                  G. Xie
                                                                    CNIC
                                                         31 October 2025


    A Packet Marking Policy for Low Latency, Low Loss, and Scalable
                            Throughput (L4S)
                draft-yang-l4s-packet-marking-policy-00

Abstract

   Low Latency, Low Loss, and Scalable Throughput (L4S)[RFC9330] is a
   technology designed to mitigate queueing delays while maintaining
   high throughput for IP traffic.  In real-time communication (RTC)
   applications over 5G networks, rapidly fluctuating wireless link
   conditions impose strict requirements on congestion control
   algorithms, which must simultaneously ensure low latency and high
   bandwidth utilization.  This document proposes a packet marking
   policy for L4S in 5G networks, where intermediate base station
   devices compute a link load factor and use it as a probabilistic
   signal to mark packets in L4S flows.

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 4 May 2026.







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Copyright Notice

   Copyright (c) 2025 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents (https://trustee.ietf.org/
   license-info) in effect on the date of publication of this document.
   Please review these documents carefully, as they describe your rights
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   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  A New L4S Marking Process . . . . . . . . . . . . . . . . . .   3
     3.1.  Deployment Challenges in 5G . . . . . . . . . . . . . . .   4
     3.2.  Link Load Factor Marking Strategy . . . . . . . . . . . .   4
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   5
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   5
   6.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   5
   7.  Normative References  . . . . . . . . . . . . . . . . . . . .   5
   Appendix A.  Historical Note  . . . . . . . . . . . . . . . . . .   6
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   6

1.  Introduction

   Low-latency real-time communication (RTC) applications, such as
   interactive video, cloud gaming, and AR/VR, require efficient
   congestion control mechanisms that can react quickly to changes in
   network conditions.  Accurate and timely congestion signals from
   intermediate network elements are essential to prevent buffer build-
   up and to maintain both low latency and high throughput.

   The Low Latency, Low Loss, and Scalable Throughput (L4S) architecture
   RFC9330 [RFC9331] provides a framework for reducing queuing delays by
   using Explicit Congestion Notification (ECN) as an end-to-end
   congestion signal.  However, existing L4S marking approaches are
   primarily based on queuing delay measurements and do not fully
   capture the dynamics of wireless access networks.  In particular, 5G
   access networks introduce rapid fluctuations in link capacity, where
   queuing delay alone is not a sufficient indicator of congestion.  As
   a result, conventional L4S marking methods may provide delayed or
   imprecise feedback, limiting their effectiveness for RTC
   applications.



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   This document proposes a packet marking strategy for L4S tailored to
   5G environments.  In this design, base stations compute a link load
   factor and use it as a probabilistic signal to mark packets in L4S
   flows.  The link load information is encoded into packets at the 5G
   base station and conveyed to the sender via ACK-based feedback.  On
   the sender side, a dynamic send rate adaptation algorithm adjusts the
   transmission rate based on the reported link load factor, preventing
   network congestion while balancing throughput and latency.

2.  Terminology

   RTC: real-time communication.

   L4S: Low Latency, Low Loss, and Scalable Throughput (L4S) as defined
   in [RFC9330] and [RFC9331].

   ECN: Explicit Congestion Notification, defined in [RFC3168],
   [RFC9330], and [RFC9331], used to signal congestion without dropping
   packets.

   gNB: Next-generation Node B, the 5G base station.

   MAC: Medium Access Control layer in the 5G protocol stack,
   responsible for multiplexing and scheduling data flows.

   RLC: Radio Link Control layer in the 5G protocol stack, responsible
   for segmentation, retransmission, and queue management.

   PDCP: Packet Data Convergence Protocol layer in the 5G protocol
   stack, which handles header compression, security, and delivery of IP
   packets.  In this document, the PDCP layer is considered for applying
   ECN markings, as IP headers remain visible before encryption.

   ACK: Acknowledgment message, used in feedback to the sender to convey
   delivery status and congestion information.

   The keywords "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119] and
   [RFC8174].

3.  A New L4S Marking Process









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3.1.  Deployment Challenges in 5G

   Low Latency, Low Loss, and Scalable Throughput (L4S) is a congestion
   control framework designed to reduce queuing delay in general-purpose
   IP networks.  It uses Explicit Congestion Notification (ECN) bits in
   the IP header to provide end-to-end congestion signaling, enabling
   applications to adjust their sending rates dynamically without
   incurring excessive delay or packet loss.


                               L4S Feedback
+----------->------------------>------------------->------------------------+
                                5G-gNB
+--------+              +----------------------------+             +--------+
|        |              |                            |             |        |
| Client +--------------+-----MAC-----RLC-----PDCP---+-------------+ Server |
|        |              |                      ^     |             |        |
+--------+              |  +------------+      |     |             +--------+
                        |  | R_Length   |      |     |
                        |  | Bandwidth  |---> P_Mark |
                        |  | T_Size     |            |
                        |  +------------+            |
                        |                            |
                        +----------------------------+
+-----------<------------------<-------------------<------------------------+
                                L4S Flow

   Deploying L4S within the 5G protocol stack introduces specific
   challenges.  In a typical 5G setup, L4S may be realized by
   maintaining queuing at the RLC layer and applying ECN markings at the
   PDCP layer, with flow identification based on ECN bits in compliance
   with existing standards.  Although the L4S specification does not
   prescribe a packet marking policy, most current implementations rely
   on queuing delay measurements to probabilistically mark packets.  In
   5G networks, the air interface and wired backhaul mismatch often
   makes the RLC layer a congestion bottleneck.  Ideally, marking should
   occur at the RLC layer to reflect actual buffer conditions.  However,
   since RLC data is encrypted, modifying ECN bits at that layer is
   infeasible.  A practical alternative is to apply marking at the PDCP
   layer, where the IP header is still accessible before encryption.

3.2.  Link Load Factor Marking Strategy

   To address the above limitations, this document specifies a
   probabilistic marking strategy based on a periodically computed _link
   load factor_ (P_Mark):

   P_Mark = (T_Size + R_Length) / (Bandwidth × T_Interval)



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   The parameter P_Mark represents the ratio between the traffic offered
   to the wireless link and the link capacity available over a given
   interval.  It indicates how heavily the link is loaded relative to
   its service capability.

   Where:

   *  T_Interval: The measurement interval.

   *  T_Size: The amount of new traffic arriving during T_Interval.

   *  R_Length: RLC real-time queue length before T_Interval

   *  Bandwidth: The average wireless bandwidth allocated by the base
      station to the current user within the T_Interval

   If P_Mark > 1, all packets are marked; if 0 < P_Mark < 1, packets are
   marked with probability P_Mark.  The marking is performed at the PDCP
   layer, and the L4S marking information is then fed back to the sender
   through ACK, allowing the sender to implement the corresponding
   adaptive rate control.

   This strategy provides a practical and compatible path for deploying
   L4S in 5G systems without altering the core structure of existing
   base stations.

4.  IANA Considerations

   This memo includes no request to IANA.

5.  Security Considerations

   For further study.

6.  Acknowledgements

   Thanks to Wenji Du and Baosen Zhao for discussions and comments on
   the design of this draft.

7.  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>.






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   [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>.

   [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>.

Appendix A.  Historical Note

Authors' Addresses

   Wanghong Yang
   CNIC
   Beijing
   China
   Email: yangwanghong@cnic.cn


   Yongmao Ren
   CNIC
   Beijing
   China
   Email: renyongmao@cstnet.cn


   Xu Zhou
   CNIC
   Beijing
   China
   Email: zhouxu@cstnet.cn







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   Qinghua Wu
   ICT
   Beijing
   China
   Email: wuqinghua@ict.ac.cn


   Gaogang Xie
   CNIC
   Beijing
   China
   Email: xie@cnic.cn







































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