



OPSAWG                                                           X. Song
Internet-Draft                                                    Y. Liu
Intended status: Standards Track                               ZTE Corp.
Expires: 8 August 2026                                   4 February 2026


        Export of L4S ECN in IP Flow Information Export (IPFIX)
                     draft-song-opsawg-ipfix-ecn-01

Abstract

   This document defines a set of IP Flow Information Export (IPFIX)
   Information Elements for monitoring the Low Latency, Low Loss, and
   Scalable throughput (L4S) service.  Specially, these elements enable
   network operators to monitor the Explicit Congestion Notification
   (ECN) information of L4S deployment and performance of traffic.

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 8 August 2026.

Copyright Notice

   Copyright (c) 2026 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
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   provided without warranty as described in the Revised BSD License.




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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
     2.1.  Terms Used in This Document . . . . . . . . . . . . . . .   3
     2.2.  Requirements Language . . . . . . . . . . . . . . . . . .   3
   3.  Overview of ECN Format  . . . . . . . . . . . . . . . . . . .   3
     3.1.  IPv4 ECN Field  . . . . . . . . . . . . . . . . . . . . .   3
     3.2.  IPv6 ECN Field  . . . . . . . . . . . . . . . . . . . . .   4
   4.  IPFIX Information Elements for L4S ECN Monitoring . . . . . .   4
     4.1.  ipv4HeaderEcn . . . . . . . . . . . . . . . . . . . . . .   4
     4.2.  ipv6HeaderEcn . . . . . . . . . . . . . . . . . . . . . .   5
     4.3.  mplsHeaderEcn . . . . . . . . . . . . . . . . . . . . . .   5
     4.4.  ipsecSaEcnMode  . . . . . . . . . . . . . . . . . . . . .   6
     4.5.  l2tpEcnNego . . . . . . . . . . . . . . . . . . . . . . .   6
     4.6.  notEctPacketDeltaCount  . . . . . . . . . . . . . . . . .   7
     4.7.  notEctPacketTotalCount  . . . . . . . . . . . . . . . . .   7
     4.8.  ect0PacketDeltaCount  . . . . . . . . . . . . . . . . . .   8
     4.9.  ect0PacketTotalCount  . . . . . . . . . . . . . . . . . .   8
     4.10. ect1PacketDeltaCount  . . . . . . . . . . . . . . . . . .   8
     4.11. ect1PacketTotalCount  . . . . . . . . . . . . . . . . . .   9
     4.12. cePacketDeltaCount  . . . . . . . . . . . . . . . . . . .   9
     4.13. cePacketTotalCount  . . . . . . . . . . . . . . . . . . .   9
   5.  Operational Considerations  . . . . . . . . . . . . . . . . .  10
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  11
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  11
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  12
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .  12
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  13
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  14

1.  Introduction

   The Low Latency, Low Loss, and Scalable throughput (L4S) service,
   defined in [RFC9331], introduces a new network service that enables
   low latency and high throughput for traffic using Scalable congestion
   controls.  To deploy and operate L4S effectively, network operators
   need visibility into L4S traffic patterns, performance metrics, and
   interoperability with existing traffic.

   IP Flow Information Export (IPFIX) [RFC7011] provides a standard
   protocol for exporting flow information from network devices.  This
   document defines a set of IPFIX Information Elements specifically
   designed for monitoring L4S ECN traffic.







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   These Information Elements are particularly useful during the
   experimental phase of L4S deployment as specified in [RFC9331],
   allowing operators to gather data to examine performance and identify
   nodes where remediation may be necessary to provide the best
   performance.

2.  Terminology

2.1.  Terms Used in This Document

   This document makes use of the terms defined in [RFC9331], [RFC9330]
   and [RFC7011].

   IPFIX: IP Flow Information Export

   IPFIX Information Elements

   Observation Point

   L4S: Low Latency, Low Loss, and Scalable throughput (L4S) service

   ECN: Explicit Congestion Notification

   ECT: ECN-capable Transport

   Not-ECT: Not ECN-capable transport

   CE: Congestion Experienced

2.2.  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.  Overview of ECN Format

3.1.  IPv4 ECN Field

   For IPv4 packets, the ECN field is located in the Type of Service
   (TOS) byte of the IP header, specifically in bits 6 to 7.  The
   definition for the IPv4 TOS octet (see [RFC791]) has been superseded
   by the six-bit DS (Differentiated Services) Field (see [RFC2474],
   [RFC2780]).  Bits 6 and 7 are listed in [RFC2474] as Currently
   Unused, and are specified in [RFC2780] as approved for experimental
   use for ECN.  The ECN field in IPv4 header is showed as follows.



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            0     1     2     3     4     5     6     7
         +-----+-----+-----+-----+-----+-----+-----+-----+
         |          DS Field, DSCP           | ECN Field |
         +-----+-----+-----+-----+-----+-----+-----+-----+


                        Figure 1: ECN Fields in IPv4

   ECN Codepoint values:

   00: Not ECT

   01: ECT(0)

   10: ECT(1)

   11: CE

3.2.  IPv6 ECN Field

   For IPv6 packets, the ECN field is located in bits 6 to 7 of IPv6
   Traffic Class.  The definition for the IPv6 Traffic Class octet (see
   [RFC791]) has been superseded by the six-bit DS (Differentiated
   Services) Field (see [RFC2474], [RFC2780]).  Bits 6 and 7 are listed
   in [RFC2474] as Currently Unused, and are specified in [RFC2780] as
   approved for experimental use for ECN.  The ECN field in IPv6 header
   is showed as follows.


      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |Version| Traffic Class |           Flow Label                  |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


                        Figure 2: ECN Fields in IPv6

4.  IPFIX Information Elements for L4S ECN Monitoring

   This section defines the Information Elements for L4S ECN.  These
   elements are intended for experimental use in L4S monitoring.

4.1.  ipv4HeaderEcn

   Name:  ipv4HeaderEcn

   ElementID:  TBD1

   Description:  This element is used for capturing the complete ECN



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      state of each packet, enabling detailed analysis of congestion
      notification.  The ECN field for IPv4 header is introduced in
      section 3.1 of this draft.  L4S traffic is identified by the
      ECT(1) as specified in [RFC9331].

      The Information Element encodes only these 2 bits.  Therefore, its
      value may range from 0 to 3.

   Abstract Data Type:  unsigned8

   Data Type Semantics:  identifier

   Additional Information:  See [RFC2474] and [RFC2780] for IPv4 ECN
      field defintion.

   Reference:  [RFC3168], [RFC9331], this document.

4.2.  ipv6HeaderEcn

   Name:  ipv6HeaderEcn

   ElementID:  TBD2

   Description:  This element is used for capturing the complete ECN
      state of each packet, enabling detailed analysis of congestion
      notification.  The ECN field format is introduced in section 3.2
      of this draft.  L4S traffic is identified by the ECT(1) codepoint
      as specified in [RFC9331].

      The Information Element encodes only these 2 bits.  Therefore, its
      value may range from 0 to 3.

   Abstract Data Type:  unsigned8

   Data Type Semantics:  identifier

   Additional Information:  See [RFC2474] and [RFC2780] for IPv6 ECN
      field defintion.

   Reference:  [RFC3168], [RFC9331], this document.

4.3.  mplsHeaderEcn

   Name:  mplsHeaderEcn

   ElementID:  TBD3

   Description:  The EXP field of the MPLS label header is used for



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      carring ECN information in the MPLS domain.  As recommended in
      [RFC5129], explicit congestion notification in MPLS should use
      codepoints in the EXP field.

      The Information Element encodes only these 3 bits.  Therefore, its
      value may range from 0 to 7.

      It is noted that the information extraction of this information
      element is only used when the MPLS domain has ECN support.

   Abstract Data Type:  unsigned8

   Data Type Semantics:  identifier

   Additional Information:  see [RFC5129] for detailed information for
      MPLS ECN tunnel negotiation.

   Reference:  [RFC5129], this document.

4.4.  ipsecSaEcnMode

   Name:  ipsecSaEcnMode

   ElementID:  TBD4

   Description:  The information element indicates whether ECN
      functionality is allowed for an IPsec Security Association (SA) in
      tunnel encapsulation mode.  The IPsec SA Attribute value 10 is
      defined for ECN tunnel negotiation as defined in section 9.2.1 of
      [RFC3168].  The negotiation value includes allowed (value set 1)
      and forbidden (value set 2) attribute.  The allowed value enables
      ECN congestion notifications and the forbidden value disables such
      notifications.

   Abstract Data Type:  unsigned8

   Data Type Semantics:  identifier

   Additional Information:  See [RFC3168] for detailed information for
      IPsec tunnel ECN negotiation.

   Reference:  [RFC5129], this document.

4.5.  l2tpEcnNego

   Name:  l2tpEcnNego

   ElementID:  TBD5



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   Description:  For L2TP tunnels, ECN processing is performed at the
      L2TP encapsulation layer.  [RFC9601] defines an ECN Capability AVP
      (Type 103) for negotiation between L2TP Control Connection
      Endpoints.  The presence of this AVP indicates support for ECN
      propagation.

   Abstract Data Type:  unsigned16

   Data Type Semantics:  identifier

   Additional Information:  See [RFC9601] for detailed information for
      L2TP tunnel ECN negotiation.

   Reference:  [RFC5129], this document.

4.6.  notEctPacketDeltaCount

   Name:  notEctPacketDeltaCount

   ElementID:  TBD6

   Description:  The number of packets since the previous report (if
      any) in this Flow with ECN codepoint set to Not-ECT (binary 00).

   Abstract Data Type:  unsigned64

   Data Type Semantics:  deltaCounter

   Additional Information:  Refer to [RFC9331].

   Reference:  [RFC3168], [RFC9331], this document.

4.7.  notEctPacketTotalCount

   Name:  notEctPacketTotalCount

   ElementID:  TBD7

   Description:  The total number of packets of this Flow with ECN
      codepoint set to Not-ECT at the Observation Point since the
      Metering Process (re-)initialization for this Observation Point.

   Abstract Data Type:  unsigned64

   Data Type Semantics:  totalCounter

   Additional Information:  Refer to [RFC9331].




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   Reference:  [RFC3168], [RFC9331], this document.

4.8.  ect0PacketDeltaCount

   Name:  ect0PacketDeltaCount

   ElementID:  TBD8

   Description:  The number of packets since the previous report (if
      any) in this Flow with ECN codepoint set to ECT(0) (binary 01).

   Abstract Data Type:  unsigned64

   Data Type Semantics:  deltaCounter

   Additional Information:  Refer to [RFC3168].

   Reference:  [RFC3168], [RFC9331], this document.

4.9.  ect0PacketTotalCount

   Name:  ect0PacketTotalCount

   ElementID:  TBD9

   Description:  The total number of packets of this Flow with ECN
      codepoint set to ECT(0) at the Observation Point since the
      Metering Process (re-)initialization for this Observation Point.

   Abstract Data Type:  unsigned64

   Data Type Semantics:  totalCounter

   Additional Information:  Refer to [RFC3168].

   Reference:  [RFC3168], [RFC9331], this document.

4.10.  ect1PacketDeltaCount

   Name:  ect1PacketDeltaCount

   ElementID:  TBD10

   Description:  The number of packets since the previous report (if
      any) in this Flow with ECN codepoint set to ECT(1) (binary 10).

   Abstract Data Type:  unsigned64




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   Data Type Semantics:  deltaCounter

   Additional Information:  Refer to [RFC3168].

   Reference:  [RFC3168], [RFC9331], this document.

4.11.  ect1PacketTotalCount

   Name:  ect1PacketTotalCount

   ElementID:  TBD11

   Description:  The total number of packets of this Flow with ECN
      codepoint set to ECT(1) at the Observation Point since the
      Metering Process (re-)initialization for this Observation Point.

   Abstract Data Type:  unsigned64

   Data Type Semantics:  totalCounter

   Additional Information:  Refer to [RFC9331].

   Reference:  [RFC3168], [RFC9331], this document.

4.12.  cePacketDeltaCount

   Name:  cePacketDeltaCount

   ElementID:  TBD12

   Description:  The number of packets since the previous report (if
      any) in this Flow with ECN codepoint set to CE (Congestion
      Experienced, binary 11).

   Abstract Data Type:  unsigned64

   Data Type Semantics:  deltaCounter

   Additional Information:  Refer to [RFC9331].

   Reference:  [RFC3168], [RFC9331], this document.

4.13.  cePacketTotalCount

   Name:  cePacketTotalCount

   ElementID:  TBD13




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   Description:  The total number of packets of this Flow with ECN
      codepoint set to CE at the Observation Point since the Metering
      Process (re-)initialization for this Observation Point.

   Abstract Data Type:  unsigned64

   Data Type Semantics:  totalCounter

   Additional Information:  Refer to [RFC9331].

   Reference:  [RFC3168], [RFC9331], this document.

5.  Operational Considerations

   The IPFIX IEs defined in this draft may have their information
   extraction positions adjusted based on different ECN monitoring
   purposes in the network.  Among them, the basic ECN field elements
   are used to reflect the ECN codepoints carried in the IPv4 header,
   the IPv6 header, or the MPLS header.  These fields can be flexibly
   extracted at any node along the path that has IPFIX export
   capability.  For tunnel ECN negotiation status IEs, the IPFIX data
   can only be provided by the specific tunnel endpoints that
   participate in the negotiation.  For cumulative statistics IEs, the
   statistical data may be processed with a higher priority at traffic
   aggregation or egress nodes.

   In L4S deployments, CE marking may be either probabilistic or
   deterministic, as determined by the specific AQM implementation.  The
   IEs on the statiscal count of CE-mark packets is defined to enable
   operators to perfom quantitative monitoring and management of L4S
   service performance, typically based on flow data that may be
   acquired via packet or flow sampling.  Implementations should employ
   sampling methods (see [RFC5475]) that preserve the statistical
   representativeness of these IEs and reduce bias risk of sampling
   results.

   For IPsec tunnels, monitoring ECN requires exporting both outer and
   inner IP header ECN fields (ipHeaderOuterEcn and ipHeaderInnerEcn),
   along with ipsecSaEcnMode (see section 4.4 of this draft).  Relying
   solely on the outer IP header ECN field may be insufficient, as it
   could be set to Not ECT due to tunnel mode restrictions.  Similarly,
   for L2TP tunnels, ECN monitoring should be verified the l2tpEcnNego
   element (see section 4.5) except the ECN information extraction from
   tunnel outer header and inner header of packets.

   For MPLS tunnels, the ECN handling mechanism differs fundamentally
   from IP based tunnels.  ECN information is not carried in a dedicated
   IP header field but is encoded within the MPLS label stack using the



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   EXP field, as defined in [RFC5129].  Therefore, monitoring ECN over
   MPLS requires exporting the mplsHeaderEcn element defined in
   Section 4.3 of this draft.  This element captures the congestion
   indication as conveyed within the MPLS domain, which is independent
   of the inner IP packet's ECN field.

   The calculation of derived metrics (e.g., L4S CE marking ratios) from
   the base counters defined in the section 4 of this draft is an
   implementation issue for the IPFIX Collector.  Operators can utilize
   the per-flow counts such as ect1PacketTotalCount and
   cePacketTotalCount for such purposes.  The caculation strategy is out
   of the scope of this draft.

6.  Security Considerations

   The security considerations for IPFIX [RFC7011] apply to this
   document.  The elements for ECN reveal information about endpoint ECN
   capabilities.  Although the information may generally be not
   sensitive, operators should consider applicable privacy regulations.
   IPFIX records containing L4S monitoring information SHOULD be
   transported using secure protocols such as TLS or DTLS and satisfy
   the mutual authentication between IPFIX Exporting Processes and IPFIX
   Collecting Processes as specified in [RFC7011].

7.  IANA Considerations

   IANA is requested to assign the following Information Elements in the
   IPFIX Information Elements registry.























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          +============+========================+===============+
          | Element ID | Name                   | Reference     |
          +============+========================+===============+
          | TBD1       | IPv4HeaderEcn          | This document |
          +------------+------------------------+---------------+
          | TBD2       | IPv6HeaderEcn          | This document |
          +------------+------------------------+---------------+
          | TBD3       | MPLSHeaderEcn          | This document |
          +------------+------------------------+---------------+
          | TBD4       | ipsecSaEcnMode         | This document |
          +------------+------------------------+---------------+
          | TBD5       | l2tpEcnNego            | This document |
          +------------+------------------------+---------------+
          | TBD6       | notEctPacketDeltaCount | This document |
          +------------+------------------------+---------------+
          | TBD7       | notEctPacketTotalCount | This document |
          +------------+------------------------+---------------+
          | TBD8       | ect0PacketDeltaCount   | This document |
          +------------+------------------------+---------------+
          | TBD9       | ect0PacketTotalCount   | This document |
          +------------+------------------------+---------------+
          | TBD10      | ect1PacketDeltaCount   | This document |
          +------------+------------------------+---------------+
          | TBD11      | ect1PacketTotalCount   | This document |
          +------------+------------------------+---------------+
          | TBD12      | cePacketDeltaCount     | This document |
          +------------+------------------------+---------------+
          | TBD13      | cePacketTotalCount     | This document |
          +------------+------------------------+---------------+

                  Table 1: New IPFIX Information Elements

8.  References

8.1.  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>.






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   [RFC7011]  Claise, B., Ed., Trammell, B., Ed., and P. Aitken,
              "Specification of the IP Flow Information Export (IPFIX)
              Protocol for the Exchange of Flow Information", STD 77,
              RFC 7011, DOI 10.17487/RFC7011, September 2013,
              <https://www.rfc-editor.org/info/rfc7011>.

   [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>.

8.2.  Informative References

   [RFC2474]  Nichols, K., Blake, S., Baker, F., and D. Black,
              "Definition of the Differentiated Services Field (DS
              Field) in the IPv4 and IPv6 Headers", RFC 2474,
              DOI 10.17487/RFC2474, December 1998,
              <https://www.rfc-editor.org/info/rfc2474>.

   [RFC2780]  Bradner, S. and V. Paxson, "IANA Allocation Guidelines For
              Values In the Internet Protocol and Related Headers",
              BCP 37, RFC 2780, DOI 10.17487/RFC2780, March 2000,
              <https://www.rfc-editor.org/info/rfc2780>.

   [RFC5129]  Davie, B., Briscoe, B., and J. Tay, "Explicit Congestion
              Marking in MPLS", RFC 5129, DOI 10.17487/RFC5129, January
              2008, <https://www.rfc-editor.org/info/rfc5129>.

   [RFC5475]  Zseby, T., Molina, M., Duffield, N., Niccolini, S., and F.
              Raspall, "Sampling and Filtering Techniques for IP Packet
              Selection", RFC 5475, DOI 10.17487/RFC5475, March 2009,
              <https://www.rfc-editor.org/info/rfc5475>.

   [RFC791]   Postel, J., "Internet Protocol", STD 5, RFC 791,
              DOI 10.17487/RFC0791, September 1981,
              <https://www.rfc-editor.org/info/rfc791>.

   [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>.




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   [RFC9601]  Briscoe, B., "Propagating Explicit Congestion Notification
              across IP Tunnel Headers Separated by a Shim", RFC 9601,
              DOI 10.17487/RFC9601, August 2024,
              <https://www.rfc-editor.org/info/rfc9601>.

Authors' Addresses

   Xueyan Song
   ZTE Corp.
   Email: song.xueyan2@zte.com.cn


   Yao Liu
   ZTE Corp.
   Email: liu.yao71@zte.com.cn




































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