MPLS On-Path Telemetry Network Action Flag for OAM
draft-ietf-mpls-on-path-telemetry-flag-00

MPLS                                                             H. Song
Internet-Draft                                    Futurewei Technologies
Intended status: Standards Track                             G. Fioccola
Expires: 2 October 2026                              Huawei Technologies
                                                               R. Gandhi
                                                           Cisco Systems
                                                           31 March 2026

           MPLS On-Path Telemetry Network Action Flag for OAM
               draft-ietf-mpls-on-path-telemetry-flag-00

Abstract

   This document describes the postcard-based on-path telemetry with
   packet marking (PBT-M) using an MPLS Network Actions (MNA) flag to
   support OAM in MPLS networks.  The scheme uses a single bit in the
   MNA header opcode dedicated for the flag-based actions.  The document
   provides the solutions to address the requirements for applying PBT-M
   in MPLS networks.

Status of This Memo

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   This Internet-Draft will expire on 2 October 2026.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   3
   2.  PBT-M: Direct Export for On-path Telemetry based on Packet
           Marking . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  New Requirements  . . . . . . . . . . . . . . . . . . . . . .   5
   4.  Design Considerations . . . . . . . . . . . . . . . . . . . .   6
     4.1.  Packet Marking  . . . . . . . . . . . . . . . . . . . . .   6
     4.2.  Flow Path Discovery . . . . . . . . . . . . . . . . . . .   6
     4.3.  Packet Identity for Export Data Correlation . . . . . . .   7
     4.4.  Load Control  . . . . . . . . . . . . . . . . . . . . . .   7
   5.  Implementation Recommendation . . . . . . . . . . . . . . . .   8
     5.1.  Configuration . . . . . . . . . . . . . . . . . . . . . .   8
     5.2.  Data Export . . . . . . . . . . . . . . . . . . . . . . .   8
     5.3.  Use Cases . . . . . . . . . . . . . . . . . . . . . . . .   8
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   8
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   9
   8.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .   9
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   9
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .   9
     9.2.  Informative References  . . . . . . . . . . . . . . . . .   9
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  11

1.  Introduction

   To gain detailed data plane visibility to support effective network
   OAM, it is essential to be able to examine the trace of user packets
   along their forwarding paths.  Such on-path flow data reflect the
   state and status of each user packet's real-time experience and
   provide valuable information for network monitoring, measurement, and
   diagnosis.

   The telemetry data include but not limited to the detailed forwarding
   path, the timestamp/latency at each network node, and, in case of
   packet drop, the drop location as well as the reason.  The emerging
   programmable data plane devices allow user-defined data collection or
   conditional data collection based on trigger events.  Such on-path
   flow data are from and about the live user traffic, which complements
   the data acquired through other passive and active OAM mechanisms
   such as IPFIX [RFC7011] and ICMP [RFC4560].

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   On-path telemetry was developed to cater to the need of collecting
   on-path flow data.  There are two basic modes for on-path telemetry:
   the passport mode (e.g., IOAM trace option [RFC9197]) and the
   postcard mode (e.g., IOAM direct export option (DEX) [RFC9326]).

   In MPLS networks, MPLS Network Action (MNA) [I-D.ietf-mpls-mna-fwk]
   extends the MPLS label stack by supporting extra in-stack network
   actions and ancillary data encoded in stack, the in-stack MNA header
   is described in [I-D.ietf-mpls-mna-hdr].  MNA also extends the MPLS
   payload by supporting extra post-stack network actions and ancillary
   data encoded post-stack, the post-stack MNA header is described in
   [I-D.jags-mpls-ps-mna-hdr].

   This document describes the method to apply a new variation of the
   postcard mode on-path telemetry, PBT-M, to MPLS network using an MNA
   flag only.  PBT-M does not require a telemetry instruction header but
   a single trigger bit in MNA flags.  The similar mechanism has been
   adopted by SRv6 for SRv6 OAM [RFC9259], which uses the O-bit in SRH
   flags as the marking bit to trigger the on-path telemetry.  The key
   benefits of PBT-M are its low overhead and high flexibility.
   However, PBT-M introduces some unique requirements that need to be
   considered.  This document discusses the requirements and their
   solutions in MPLS networks.

1.1.  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.

2.  PBT-M: Direct Export for On-path Telemetry based on Packet Marking

   As the name suggests, PBT-M only needs a marking-bit in the existing
   headers of user packets to trigger the telemetry data collection and
   export.  The sketch of PBT-M is as follows.  If on-path data need to
   be collected, the user packet is marked at the path head node.  At
   each PBT-M-aware node, if the mark is detected and data collection is
   enabled, a postcard packet (i.e., the dedicated OAM packet triggered
   by a marked user packet) is generated and sent to a collector.  The
   postcard contains the data requested by the management plane.  The
   requested data are configured by the management plane.  Once the
   collector receives all the postcards for a single user packet, it can
   infer the packet's forwarding path and analyze the data set.  The
   path end node is configured to un-mark the packets to its original
   format if necessary.

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   The overall architecture of PBT-M is depicted in Figure 1.

                         +------------+        +-----------+
                         | Network    |        | Telemetry |
                         | Management |(-------| Data      |
                         |            |        | Collector |
                         +-----:------+        +-----------+
                               :                     ^
                               :configurations       |postcards
                               :                     |(OAM pkts)
                ...............:.....................|........
                :             :               :      |       :
                :   +---------:---+-----------:---+--+-------:---+
                :   |         :   |           :   |          :   |
                V   |         V   |           V   |          V   |
             +------+-+     +-----+--+     +------+-+     +------+-+
   usr pkts  | Head   |     | Path   |     | Path   |     | End    |
        ====>| Node   |====>| Node   |====>| Node   |====>| Node   |===>
             |        |     | A      |     | B      |     |        |
             +--------+     +--------+     +--------+     +--------+
           mark usr pkts  gen postcards  gen postcards  gen postcards
           gen postcards                                unmark usr pkts

                      Figure 1: Architecture of PBT-M

   The advantages of PBT-M are summarized as follows.

   *  1: PBT-M avoids augmenting user packets with new headers and the
      signaling for telemetry data collection remains in the data plane.

   *  2: PBT-M is extensible for collecting arbitrary new data to
      support possible future use cases.  The data set to be collected
      can be configured through the management plane or control plane.

   *  3: PBT-M can avoid interfering with the normal forwarding.  The
      collected data are free to be transported independently through
      in-band or out-of-band channels.  The data collecting, processing,
      assembly, encapsulation, and transport are, therefore, decoupled
      from the forwarding of the corresponding user packets and can be
      performed in data-plane slow-path if necessary.

   *  4: For PBT-M, the types of data collected from each node can vary
      depending on application requirements and node capability.

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   *  5: PBT-M makes it easy to secure the collected data without
      exposing it to unnecessary entities.  For example, both the
      configuration and the telemetry data can be encrypted and/or
      authenticated before being transported, so passive eavesdropping
      and a man-in-the-middle attack can both be deterred.

   *  6: Even if a user packet under inspection is dropped at some node
      in the network, the postcards collected from the preceding nodes
      are still valid and can be used to diagnose the packet drop
      location and reason.

   *  7: Raw data can be processed or aggregated in data plane to reduce
      the exporting traffic load.

3.  New Requirements

   Although PBT-M has some unique features, it also introduces a few new
   requirements.

   *  Req. 1 (Packet Marking Bit): A user packet needs to be marked to
      trigger the path-associated data collection.  Since PBT-M aims to
      avoid the need to augment user packets with new headers, it needs
      to reserve or reuse a single bit from the existing header fields.

   *  Req. 2 (Configuration): Since the packet header will not carry
      telemetry instructions anymore, the data plane devices need to be
      configured to know what data to collect.  However, in general, the
      forwarding path of a flow packet (due to ECMP or dynamic routing)
      is unknown beforehand (note that there are some notable
      exceptions, such as segment routing).  If the per-flow customized
      data collection is required, configuring the data set for each
      flow at all data plane devices might be expensive in terms of
      configuration load and data plane resources.

   *  Req. 3 (Data Correlation): Due to the variable transport latency,
      the dedicated postcard packets for a single packet may arrive at
      the collector out of order or be dropped in networks for some
      reason.  In order to infer the packet forwarding path, the
      collector needs some information from the postcard packets to
      identify the user packet affiliation and the order of path node
      traversal.

   *  Req. 4 (Overhead and Security): Since each postcard packet has its
      header, the overall network bandwidth overhead of PBT-M can be
      high.  A large number of postcards could add processing pressure
      on data collecting servers.  That can be used as an attack vector
      for DoS.

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4.  Design Considerations

   To address the above requirements, we propose several design details
   for applying PBT-M in MPLS networks.

4.1.  Packet Marking

   To trigger the path-associated data collection, usually, a single bit
   from some header field is sufficient.  The proposed action encoding
   is shown in Figure 2 changed from [I-D.ietf-mpls-mna-hdr].

        0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |      NASI=bSPL                        | TC  |S|    TTL        |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |NAI-Opcode=1 |P|                       |R|IHS|S|U|NASL=0 |NAL=0|
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                         Figure 2: Action Encoding

   In the figure, NAI-Opcode is Network Action Indicator Opcode.  If the
   Opcode has the value of 'one', then the 13 bits following the Opcode
   carries NAI-flags.  P-flag, the first flag bit in the flag-based
   network action field, is used as the PBT indicator.  If the bit is
   set to '1', a node is triggered to collect and export the telemetry
   data as configured by the control plane.

   Note that the in-stack MNA encoding may take different form as the
   header encoding draft evolves, and these flag-based on-path telemetry
   use cases would adapt to the change.

4.2.  Flow Path Discovery

   In case the path that a flow traverses is unknown in advance, all
   PBT-M-aware nodes should be configured to react to the marked packets
   by exporting some basic data, such as node ID and TTL before a data
   set template for that flow is configured.  This way, the management
   plane can learn the flow path dynamically.

   If the management plane wants to collect the on-path data for some
   flow, it configures the head node with a probability or time interval
   for the flow packet marking.  When the first marked packet is
   forwarded in the network, the PBT-M-aware nodes will export the basic
   data set to the collector.  Hence, the flow path is identified.  If
   other data types need to be collected, the management plane can
   further configure the data set's template to the target nodes on the

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   flow's path.  The PBT-M-aware nodes collect and export data
   accordingly if the packet is marked and a data set template is
   present.

   If the flow path is changed for any reason, the new path can be
   quickly learned by the collector.  Consequently, the management plane
   controller can be directed to configure the nodes on the new path.
   The outdated configuration can be automatically timed out or
   explicitly revoked by the management plane controller.

4.3.  Packet Identity for Export Data Correlation

   The collector needs to correlate all the postcard packets for a
   single user packet.  Once this is done, the TTL (or the timestamp, if
   the network time is synchronized) can be used to infer the flow
   forwarding path.  The key issue here is to correlate all the
   postcards for the same user packet.

   The first possible approach includes the flow ID in the OAM packets.
   In case of MPLS, the MPLS label stack can server as the flow ID.  If
   the packet marking interval is large enough, the flow ID is enough to
   identify a user packet.  As a result, it can be assumed that all the
   exported postcard packets for the same flow during a short time
   interval belong to the same user packet.

   Alternatively, if the network is synchronized, then the flow ID plus
   the timestamp at each node can also infer the postcard affiliation.
   However, some errors may occur under some circumstances.  For
   example, two consecutive user packets from the same flows are marked,
   but one exported postcard from a node is lost.  It is difficult for
   the collector to decide to which user packet the remaining postcard
   is related.  In many cases, such a rare error has no catastrophic
   consequence.  Therefore it is tolerable.

4.4.  Load Control

   PBT-M should not be applied to all the packets all the time.  It is
   better to be used in an interactive environment where the network
   telemetry applications dynamically decide which subset of traffic is
   under scrutiny.  The network devices can limit the packet marking
   rate through sampling and metering.  The postcard packets can be
   distributed to different servers to balance the processing load.

   Because PBT-M sends telemetry data by dedicated postcard packets, it
   allows data aggregation and compression.  Each node can process the
   generated raw data according to the configured local data-export
   policies.  Such policies may specify how raw data is used to
   calculate performance metrics, e.g., max, min, mean, percentile, etc.

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5.  Implementation Recommendation

5.1.  Configuration

   Access lists with an optional sampler, [RFC5476], should be
   configured and attached at the ingress of the PBT-M encapsulation
   node's to select the intended flows for PTB-M.

   Based on the requirements and node capability, the flow data could be
   exported at each transit node and at the end edge node with IPFIX
   [RFC7011].

5.2.  Data Export

   The data decomposition can be achieved on the PBT-M-aware node
   exporting the data or on the IPFIX data collection.
   [I-D.spiegel-ippm-ioam-rawexport] describes how data is being
   exported when decomposed at IPFIX data collection.  When being
   decomposed on the PBT-M-aware node the data can be aggregated
   according to section 5 of [RFC7015].

5.3.  Use Cases

   In MPLS networks, MLD is a great concern which limits the MNA size
   and in turn the OAM capability.  Moreover, for SR-MPLS, Maximum SID
   Depth(MSD) as well as PMTU in SR Policy are critical issues for SR
   path instantiation by a controller.  PBT-M can become a critical
   solution to ensure that flexible on-path telemetry can be viable for
   operators by eliminating telemetry data from being carried in-situ in
   the SR-TE policy path.

   This draft provides a critical optimization that fills the gaps with
   IOAM DEX related to packet marking triggers using existing mechanisms
   as well as flow path discovery mechanisms to avoid configuration of
   on path data plane node complexity and helps mitigate SR MSD and PMTU
   issues.

6.  Security Considerations

   Only the ingress node is allowed to set these flag bits.  The other
   on-path nodes can only react to the bit values.  The tampering of
   these flag-based actions would result in DoS attack or unreliable
   measurements.  Therefore, security measures must be taken to ensure
   the proper functioning of these actions.

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7.  IANA Considerations

   This document requires IANA to assign a bit for PBT-M action (TBA1)
   from the MPLS "In-Stack MPLS Network Action Indicator Flags" registry
   created in [I-D.ietf-mpls-mna-hdr].

8.  Acknowledgments

9.  References

9.1.  Normative References

   [I-D.ietf-mpls-mna-hdr]
              Rajamanickam, J., Gandhi, R., Zigler, R., Song, H., and K.
              Kompella, "MPLS Network Action (MNA) Sub-Stack
              Specification including In-Stack Network Actions and
              Data", Work in Progress, Internet-Draft, draft-ietf-mpls-
              mna-hdr-21, 24 February 2026,
              <https://datatracker.ietf.org/doc/html/draft-ietf-mpls-
              mna-hdr-21>.

   [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>.

   [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>.

   [RFC7015]  Trammell, B., Wagner, A., and B. Claise, "Flow Aggregation
              for the IP Flow Information Export (IPFIX) Protocol",
              RFC 7015, DOI 10.17487/RFC7015, September 2013,
              <https://www.rfc-editor.org/info/rfc7015>.

   [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>.

9.2.  Informative References

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   [I-D.ietf-mpls-mna-fwk]
              Andersson, L., Bryant, S., Bocci, M., and T. Li, "MPLS
              Network Actions (MNA) Framework", Work in Progress,
              Internet-Draft, draft-ietf-mpls-mna-fwk-15, 27 December
              2024, <https://datatracker.ietf.org/doc/html/draft-ietf-
              mpls-mna-fwk-15>.

   [I-D.jags-mpls-ps-mna-hdr]
              Rajamanickam, J., Gandhi, R., Zigler, R., Li, T., and J.
              Dong, "Post-Stack MPLS Network Action (MNA) Solution",
              Work in Progress, Internet-Draft, draft-jags-mpls-ps-mna-
              hdr-05, 20 March 2025,
              <https://datatracker.ietf.org/doc/html/draft-jags-mpls-ps-
              mna-hdr-05>.

   [I-D.spiegel-ippm-ioam-rawexport]
              Spiegel, M., Brockners, F., Bhandari, S., and R.
              Sivakolundu, "In-situ OAM raw data export with IPFIX",
              Work in Progress, Internet-Draft, draft-spiegel-ippm-ioam-
              rawexport-07, 12 February 2024,
              <https://datatracker.ietf.org/doc/html/draft-spiegel-ippm-
              ioam-rawexport-07>.

   [RFC4560]  Quittek, J., Ed. and K. White, Ed., "Definitions of
              Managed Objects for Remote Ping, Traceroute, and Lookup
              Operations", RFC 4560, DOI 10.17487/RFC4560, June 2006,
              <https://www.rfc-editor.org/info/rfc4560>.

   [RFC5476]  Claise, B., Ed., Johnson, A., and J. Quittek, "Packet
              Sampling (PSAMP) Protocol Specifications", RFC 5476,
              DOI 10.17487/RFC5476, March 2009,
              <https://www.rfc-editor.org/info/rfc5476>.

   [RFC9197]  Brockners, F., Ed., Bhandari, S., Ed., and T. Mizrahi,
              Ed., "Data Fields for In Situ Operations, Administration,
              and Maintenance (IOAM)", RFC 9197, DOI 10.17487/RFC9197,
              May 2022, <https://www.rfc-editor.org/info/rfc9197>.

   [RFC9259]  Ali, Z., Filsfils, C., Matsushima, S., Voyer, D., and M.
              Chen, "Operations, Administration, and Maintenance (OAM)
              in Segment Routing over IPv6 (SRv6)", RFC 9259,
              DOI 10.17487/RFC9259, June 2022,
              <https://www.rfc-editor.org/info/rfc9259>.

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   [RFC9326]  Song, H., Gafni, B., Brockners, F., Bhandari, S., and T.
              Mizrahi, "In Situ Operations, Administration, and
              Maintenance (IOAM) Direct Exporting", RFC 9326,
              DOI 10.17487/RFC9326, November 2022,
              <https://www.rfc-editor.org/info/rfc9326>.

Authors' Addresses

   Haoyu Song
   Futurewei Technologies
   United States of America
   Email: haoyu.song@futurewei.com

   Giuseppe Fioccola
   Huawei Technologies
   Germany
   Email: giuseppe.fioccola@huawei.com

   Rakesh Gandhi
   Cisco Systems
   Canada
   Email: rgandhi@cisco.com

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