6man

Internet Engineering Task Force (IETF)                            Z. Ali
Internet-Draft
Request for Comments: 9259                                   C. Filsfils
Intended status:
Category: Standards Track                                  Cisco Systems
Expires: July 27, 2022
ISSN: 2070-1721                                            S. Matsushima
                                                                Softbank
                                                                D. Voyer
                                                             Bell Canada
                                                                 M. Chen
                                                                  Huawei
                                                        January 23,
                                                                May 2022

  Operations, Administration, and Maintenance (OAM) in Segment Routing
                  Networks with IPv6 Data plane Plane (SRv6)
                   draft-ietf-6man-spring-srv6-oam-13

Abstract

   This document describes how the existing IPv6 mechanisms for ping and
   traceroute can be used in an SRv6 network.  The document also
   specifies the OAM flag (O-flag) in the Segment Routing Header (SRH)
   for performing controllable and predictable flow sampling from
   segment endpoints.  In addition, the document describes how a
   centralized monitoring system performs a path continuity check
   between any nodes within an SRv6 domain.

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 an Internet Standards Track document.

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   Internet-Drafts are draft documents valid the IETF community.  It has
   received public review and has been approved for a maximum publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 7841.

   Information about the current status of six months this document, any errata,
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   This Internet-Draft will expire on July 27, 2022.
   https://www.rfc-editor.org/info/rfc9259.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   3
     1.2.  Abbreviations . . . . . . . . . . . . . . . . . . . . . .   3
     1.3.  Terminology and Reference Topology  . . . . . . . . . . .   4
   2.  OAM Mechanisms  . . . . . . . . . . . . . . . . . . . . . . .   5
     2.1.  O-flag  OAM Flag in the Segment Routing Header  . . . . . . . . . . . .   5
       2.1.1.  O-flag  OAM Flag Processing . . . . . . . . . . . . . . . . . .   6
     2.2.  OAM Operations  . . . . . . . . . . . . . . . . . . . . .   8
   3.  Implementation Status . . . . . . . . . . . . . . . . . . . .   8
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .   9
   5.
   4.  Privacy Considerations  . . . . . . . . . . . . . . . . . . .   9
   6.
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   9
   7.
   6.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  10
     7.1.
     6.1.  Normative References  . . . . . . . . . . . . . . . . . .  10
     7.2.
     6.2.  Informative References  . . . . . . . . . . . . . . . . .  10
   Appendix A.  Illustrations  . . . . . . . . . . . . . . . . . . .  12
     A.1.  Ping in SRv6 Networks . . . . . . . . . . . . . . . . . .  12
       A.1.1.  Pinging an IPv6 Address via a Segment-list  . . . . .  13 Segment List
       A.1.2.  Pinging a SID . . . . . . . . . . . . . . . . . . . .  14
     A.2.  Traceroute  . . . . . . . . . . . . . . . . . . . . . . .  15
       A.2.1.  Traceroute to an IPv6 Address via a Segment-list  . .  15 Segment List
       A.2.2.  Traceroute to a SID . . . . . . . . . . . . . . . . .  17
     A.3.  A  Hybrid OAM Using O-flag . . . . . . . . . . . . . . . .  18 the OAM Flag
     A.4.  Monitoring of SRv6 Paths  . . . . . . . . . . . . . . . .  21
   Appendix B.
   Acknowledgements . . . . . . . . . . . . . . . . . .  22
   Appendix C.
   Contributors . . . . . . . . . . . . . . . . . . . .  22
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  23

1.  Introduction

   As Segment Routing with IPv6 data plane (SRv6) [RFC8402] simply adds
   a new type of Routing Extension Header, existing IPv6 OAM mechanisms
   can be used in an SRv6 network.  This document describes how the
   existing IPv6 mechanisms for ping and traceroute can be used in an
   SRv6 network.  This includes illustrations of pinging an SRv6 SID Segment
   Identifier (SID) to verify that the SID is reachable and is locally
   programmed at the target node.  This also includes illustrations for
   tracerouting to an SRv6 SID for hop-by-hop fault localization as well
   as path tracing to a SID.

   The

   This document also introduces enhancements for the OAM mechanism for
   SRv6 networks for performing controllable and predictable flow
   sampling from segment endpoints using, e.g., the IP Flow Information
   Export (IPFIX) protocol [RFC7011].  Specifically, the document
   specifies the O-flag OAM flag (O-flag) in the SRH as a marking-bit marking bit in the
   user packets to trigger the telemetry data collection and export at the
   segment endpoints.

   The

   This document also outlines how the centralized OAM technique in
   [RFC8403] can be extended for SRv6 to perform a path continuity check
   between any nodes within an SRv6 domain.  Specifically, the document
   illustrates how a centralized monitoring system can monitor arbitrary
   SRv6 paths by creating the loopback probes that originate and terminate
   at the centralized monitoring system.

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.

1.2.  Abbreviations

   The following abbreviations are used in this document:

   SID:  Segment ID. Identifier

   SL:  Segments Left. Left

   SR:  Segment Routing. Routing

   SRH:  Segment Routing Header [RFC8754]. [RFC8754]

   SRv6:  Segment Routing with IPv6 Data plane. data plane

   PSP:  Penultimate Segment Pop of the SRH [RFC8986]. [RFC8986]

   USP:  Ultimate Segment Pop of the SRH [RFC8986]. [RFC8986]

   ICMPv6: ICMPv6 Specification [RFC4443].  Internet Control Message Protocol for the Internet Protocol
      version 6 [RFC4443]

   IS-IS:  Intermediate System to Intermediate System

   OSPF:  Open Shortest Path First protocol [RFC2328]

   IGP:  Interior Gateway Protocols Protocol (e.g., OSPF, IS-IS). OSPF and IS-IS)

   BGP-LS:  Border Gateway Protocol - Link State Extensions [RFC8571]

1.3.  Terminology and Reference Topology

   Throughout the document, the following

   The terminology and simple topology is in this section are used for illustration.
   illustration throughout the document.

   +--------------------------| N100 |---------------------------------+
   |                                                                   |
   |  ====== link1====== link3------ link5====== link9------   ======  |
      ||N1||------||N2||------| N3 |------||N4||------| N5 |---||N7||
      ||  ||------||  ||------|    |------||  ||------|    |---||  ||
      ====== link2====== link4------ link6======link10------   ======
         |            |                      |                   |
      ---+--          |       ------         |                 --+---
      |CE 1|
      |CE1 |          +-------| N6 |---------+                 |CE 2|                 |CE2 |
      ------            link7 |    | link8                     ------
                              ------

                        Figure 1 1: Reference Topology

   In the reference topology:

   *  Node j has a an IPv6 loopback address 2001:db8:L:j::/128.

   *  Nodes N1, N2, N4 N4, and N7 are SRv6-capable nodes.

   *  Nodes N3, N5 N5, and N6 are IPv6 nodes that are not SRv6-capable. SRv6-capable
      nodes.  Such nodes are referred to as non-SRv6 capable nodes. "non-SRv6-capable nodes".

   *  CE1 and CE2 are Customer Edge devices of any data plane capability
      (e.g., IPv4, IPv6, L2, etc.). and L2).

   *  A SID at node j with locator block 2001:db8:K::/48 and function U
      is represented by 2001:db8:K:j:U::.

   *  Node N100 is a controller.

   *  The IPv6 address of the nth Link link between node i and j at the i
      side is represented as 2001:db8:i:j:in::, e.g., 2001:db8:i:j:in::. For example, in
      Figure 1, the IPv6 address of link6 (the 2nd second link between N3
      and N4) at N3 in Figure 1 is 2001:db8:3:4:32::. Similarly, the IPv6 address of
      link5 (the 1st first link between N3 and N4) at node N3 is
      2001:db8:3:4:31::.

   *  2001:db8:K:j:Xin:: is explicitly allocated as the End.X SID at
      node j towards neighbor node i via the nth Link link between node i and
      node j.  e.g.,  For example, 2001:db8:K:2:X31:: represents End.X at N2
      towards N3 via link3 (the 1st first link between N2 and N3).
      Similarly, 2001:db8:K:4:X52:: represents the End.X at N4 towards
      N5 via link10 (the 2nd second link between N4 and N5).  Please refer
      to [RFC8986] for a description of End.X SID.

   *  A SID list is represented as <S1, S2, S3> S3>, where S1 is the first
      SID to visit, S2 is the second SID to visit visit, and S3 is the last
      SID to visit along the SR path.

   *  (SA,DA) (S3, S2, S1; SL)(payload) represents an IPv6 packet with:

      *

      -  IPv6 header with source address SA, destination addresses DA DA,
         and SRH as next-header

      * the next header

      -  SRH with SID list <S1, S2, S3> with SegmentsLeft = SL

      *

         Note the difference between the < > and () symbols: <S1, S2,
         S3> represents a SID list where S1 is the first SID and S3 is
         the last SID to traverse.  (S3, S2, S1; SL) represents the same
         SID list but encoded in the SRH format where the rightmost SID
         in the SRH is the first SID and the leftmost SID in the SRH is
         the last SID.  When referring to an SR policy in a high-level
         use-case,
         use case, it is simpler to use the <S1, S2, S3> notation.  When
         referring to an illustration of the detailed packet behavior,
         the (S3, S2, S1; SL) notation is more convenient.

      *

      -  (payload) represents the the payload of the packet.

2.  OAM Mechanisms

   This section defines OAM enhancement enhancements for the SRv6 networks.

2.1.  O-flag  OAM Flag in the Segment Routing Header

   [RFC8754] describes the Segment Routing Header (SRH) and how SR SR-
   capable nodes use it.  The SRH contains an 8-bit "Flags" Flags field.

   This document defines the following bit in the SRH Flags field to
   carry the O-flag:

                  0 1 2 3 4 5 6 7
                 +-+-+-+-+-+-+-+-+
                 |   |O|         |
                 +-+-+-+-+-+-+-+-+

   Where:

   O-flag:  OAM flag in the SRH Flags field defined in [RFC8754].

2.1.1.  O-flag  OAM Flag Processing

   The O-flag in the SRH is used as a marking-bit marking bit in the user packets to
   trigger the telemetry data collection and export at the segment
   endpoints.

   An SR domain ingress edge node encapsulates packets traversing the SR
   domain as defined in [RFC8754].  The SR domain ingress edge node MAY
   use the O-flag in the SRH for marking the packet to trigger the
   telemetry data collection and export at the segment endpoints.  Based
   on a local configuration, the SR domain ingress edge node may implement
   a classification and sampling mechanism to mark a packet with the
   O-flag in the SRH.  Specification of the classification and sampling
   method is outside the scope of this document.

   This document does not specify the data elements that need to be
   exported and the associated configurations.  Similarly, this document
   does not define any formats for exporting the data elements.
   Nonetheless, without the loss of generality, this document assumes
   that the IP Flow Information Export (IPFIX) protocol [RFC7011] is
   used for exporting the traffic flow information from the network
   devices to a controller for monitoring and analytics.  Similarly,
   without the loss of generality, this document assumes that requested
   information elements are configured by the management plane through
   data set templates (e.g., as in IPFIX [RFC7012]).

   Implementation of the O-flag is OPTIONAL.  If a node does not support
   the O-flag, then upon reception it simply ignores it. it upon reception.  If a node
   supports the O-flag, it can optionally advertise its potential via
   control plane protocol(s).

   When N receives a packet destined to S and S is a local SID, the line S01
   of the pseudo-code pseudocode associated with the SID S, as S (as defined in
   section
   Section 4.3.1.1 of [RFC8754], [RFC8754]) is appended to as follows for the O-flag
   processing.

      S01.1. IF the O-flag is set and local configuration permits
             O-flag processing {
                a. Make a copy of the packet.
                b. Send the copied packet, along with a timestamp timestamp,
                to the OAM process for telemetry data collection
                and export.      ;; Ref1
                }
      Ref1: To provide an accurate timestamp, an implementation should
      copy and record the timestamp as soon as possible during packet
      processing. Timestamp and any other metadata is are not carried in
      the packet forwarded to the next hop.

   Please note that the O-flag processing happens before execution of
   regular processing of the local SID S.  Specifically, the line S01.1 of
   the pseudo-code pseudocode specified in this document is inserted between
   line lines
   S01 and S02 of the pseudo-code pseudocode defined in section Section 4.3.1.1 of
   [RFC8754].

   Based on the requested information elements configured by the
   management plane through data set templates [RFC7012], the OAM
   process exports the requested information elements.  The information
   elements include parts of the packet header and/or parts of the
   packet payload for flow identification.  The OAM process uses
   information elements defined in IPFIX [RFC7011] and PSAMP Packet Sampling
   (PSAMP) [RFC5476] for exporting the requested sections of the
   mirrored packets.

   If the penultimate segment of a segment-list segment list is a Penultimate Segment
   Pop (PSP) PSP SID, telemetry
   data from the ultimate segment cannot be requested.  This is because,
   when the penultimate segment is a PSP SID, the SRH is removed at the
   penultimate segment segment, and the O-flag is not processed at the ultimate
   segment.

   The processing node MUST rate-limit the number of packets punted to
   the OAM process to a configurable rate.  This is to avoid hitting any
   performance impact on the OAM and the telemetry collection processes.
   Failure in implementing to implement the rate limit can lead to a denial-of-
   service denial-of-service
   attack, as detailed in section 4. Section 3.

   The OAM process MUST NOT process the copy of the packet or respond to
   any upper-layer header (like ICMP, UDP, etc.) payload to prevent
   multiple evaluations of the datagram.

   The OAM process is expected to be located on the routing node
   processing the packet.  Although the specification of the OAM process
   or the external controller operations are beyond the scope of this
   document, the OAM process SHOULD NOT be topologically distant from
   the routing node, as this is likely to create significant security
   and congestion issues.  How to correlate the data collected from
   different nodes at an external controller is also outside the scope
   of the this document.  Appendix A illustrates use of the O-flag for
   implementing a hybrid OAM mechanism, where the "hybrid"
   classification is based on RFC7799 [RFC7799].

2.2.  OAM Operations

   IPv6 OAM operations can be performed for any SRv6 SID whose behavior
   allows Upper Layer Upper-Layer Header processing for an applicable OAM payload
   (e.g., ICMP, UDP).

   Ping to an SRv6 SID is used to verify that the SID is reachable and
   is locally programmed at the target node.  Traceroute to a SID is
   used for hop-by-hop fault localization as well as path tracing to a
   SID.  Appendix A illustrates the ICMPv6 based ICMPv6-based ping and the UDP based UDP-based
   traceroute mechanisms for ping and traceroute to an SRv6 SID.
   Although this document only illustrates ICMPv6 ICMPv6-based ping and UDP UDP-
   based traceroute to an SRv6 SID, the procedures are equally
   applicable to other IPv6 OAM probing to an SRv6 SID (e.g.,
   Bidirectional Forwarding Detection (BFD) [RFC5880], Seamless BFD (SBFD)
   (S-BFD) [RFC7880], STAMP and Simple Two-way Active Measurement Protocol
   (STAMP) probe message processing [I-D.gandhi-spring-stamp-srpm], etc.). [STAMP-SR]).  Specifically, as long
   as local configuration allows the Upper-layer Header processing of
   the applicable OAM payload for SRv6 SIDs, the existing IPv6 OAM
   techniques can be used to target a probe to a (remote) SID.

   IPv6 OAM operations can be performed with the target SID in the IPv6
   destination address without an SRH or with an SRH where the target
   SID is the last segment.  In general, OAM operations to a target SID
   may not exercise all of its processing depending on its behavior
   definition.  For example, ping to an End.X SID [RFC8986] only
   validates the SID is locally programmed at the target node and does
   not validate switching to the correct outgoing interface.  To
   exercise the behavior of a target SID, the OAM operation should
   construct the probe in a manner similar to a data packet that
   exercises the SID behavior, i.e. to include that SID as a transit SID
   in either an SRH or IPv6 DA of an outer IPv6 header or as appropriate
   based on the definition of the SID behavior.

3.  Implementation Status

   This section is to be removed prior to publishing as an RFC.

   See [I-D.matsushima-spring-srv6-deployment-status] for updated
   deployment and interoperability reports.

4.  Security Considerations

   [RFC8754] defines the notion of an SR domain and use of the SRH
   within the SR domain.  The use of OAM procedures described in this
   document is restricted to an SR domain.  For example, similar to the SID
   manipulation, O-flag manipulation is not considered as a threat within
   the SR domain.  Procedures for securing an SR domain are defined the section in
   Sections 5.1 and section 7 of [RFC8754].

   As noted in section Section 7.1 of [RFC8754], compromised nodes within the SR
   domain may mount attacks.  The O-flag may be set by an attacking node
   attempting a denial-of-service attack on the OAM process at the
   segment endpoint node.  An implementation correctly implementing the
   rate limiting described in section Section 2.1.1 is not susceptible to that denial-of-
   service
   denial-of-service attack.  Additionally, SRH Flags flags are protected by
   the HMAC Hashed Message Authentication Code (HMAC) TLV, as described in section
   Section 2.1.2.1 of [RFC8754].  Once an HMAC is generated for a
   segment list with the O-flag set, it can be used for an arbitrary
   amount of traffic using that segment list with the O-flag set.

   The security properties of the channel used to send exported packets
   marked by the O-flag will depend on the specific OAM processes used.
   An on-path attacker able to observe this OAM channel could conduct
   traffic analysis, or potentially eavesdropping (depending on the OAM
   configuration), of this telemetry for the entire SR domain from such
   a vantage point.

   This document does not impose any additional security challenges to
   be considered beyond the security threats described in [RFC4884],
   [RFC4443], [RFC0792], [RFC8754] [RFC8754], and [RFC8986].

5.

4.  Privacy Considerations

   The per-packet marking capabilities of the O-flag provides provide a granular
   mechanism to collect telemetry.  When this collection is deployed by
   an operator with the knowledge and consent of the users, it will
   enable a variety of diagnostics and monitoring to support the OAM and
   security operations use cases needed for resilient network
   operations.  However, this collection mechanism will also provide an
   explicit protocol mechanism to operators for surveillance and
   pervasive monitoring use cases done contrary to the user's consent.

6.

5.  IANA Considerations

   This document requests that

   IANA allocate has registered the following registration in the "Segment Routing Header
   Flags" sub-registry for subregistry in the "Internet Protocol Version 6 (IPv6)
   Parameters" registry maintained by IANA:

            +-------+------------------------------+---------------+ registry:

                     +=====+=============+===========+
                     | Bit | Description | Reference |
            +=======+==============================+===============+
                     +=====+=============+===========+
                     | 2   | O-flag      | This document RFC 9259  |
            +-------+------------------------------+---------------+

7.
                     +-----+-------------+-----------+

                                  Table 1

6.  References

7.1.

6.1.  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>.

   [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>.

   [RFC8754]  Filsfils, C., Ed., Dukes, D., Ed., Previdi, S., Leddy, J.,
              Matsushima, S., and D. Voyer, "IPv6 Segment Routing Header
              (SRH)", RFC 8754, DOI 10.17487/RFC8754, March 2020,
              <https://www.rfc-editor.org/info/rfc8754>.

   [RFC8986]  Filsfils, C., Ed., Camarillo, P., Ed., Leddy, J., Voyer,
              D., Matsushima, S., and Z. Li, "Segment Routing over IPv6
              (SRv6) Network Programming", RFC 8986,
              DOI 10.17487/RFC8986, February 2021,
              <https://www.rfc-editor.org/info/rfc8986>.

7.2.

6.2.  Informative References

   [I-D.gandhi-spring-stamp-srpm]
              Gandhi, R., Filsfils, C., Voyer, D., Chen, M., Janssens,
              B., and R. Foote, "Performance Measurement Using Simple
              TWAMP (STAMP) for Segment Routing Networks", draft-gandhi-
              spring-stamp-srpm-07 (work in progress), July 2021.

   [I-D.ietf-ippm-ioam-data]
              Brockners, F., Bhandari, S., and T. Mizrahi, "Data Fields
              for In-situ OAM", draft-ietf-ippm-ioam-data-11 (work in
              progress), November 2020.

   [I-D.matsushima-spring-srv6-deployment-status]
              Matsushima, S., Filsfils, C., Ali, Z., Li, Z., and K.
              Rajaraman, "SRv6 Implementation and Deployment Status",
              draft-matsushima-spring-srv6-deployment-status-11 (work in
              progress), February 2021.

   [RFC0792]  Postel, J., "Internet Control Message Protocol", STD 5,
              RFC 792, DOI 10.17487/RFC0792, September 1981,
              <https://www.rfc-editor.org/info/rfc792>.

   [RFC2328]  Moy, J., "OSPF Version 2", STD 54, RFC 2328,
              DOI 10.17487/RFC2328, April 1998,
              <https://www.rfc-editor.org/info/rfc2328>.

   [RFC4443]  Conta, A., Deering, S., and M. Gupta, Ed., "Internet
              Control Message Protocol (ICMPv6) for the Internet
              Protocol Version 6 (IPv6) Specification", STD 89,
              RFC 4443, DOI 10.17487/RFC4443, March 2006,
              <https://www.rfc-editor.org/info/rfc4443>.

   [RFC4884]  Bonica, R., Gan, D., Tappan, D., and C. Pignataro,
              "Extended ICMP to Support Multi-Part Messages", RFC 4884,
              DOI 10.17487/RFC4884, April 2007,
              <https://www.rfc-editor.org/info/rfc4884>.

   [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>.

   [RFC5837]  Atlas, A., Ed., Bonica, R., Ed., Pignataro, C., Ed., Shen,
              N., and JR. Rivers, "Extending ICMP for Interface and
              Next-Hop Identification", RFC 5837, DOI 10.17487/RFC5837,
              April 2010, <https://www.rfc-editor.org/info/rfc5837>.

   [RFC5880]  Katz, D. and D. Ward, "Bidirectional Forwarding Detection
              (BFD)", RFC 5880, DOI 10.17487/RFC5880, June 2010,
              <https://www.rfc-editor.org/info/rfc5880>.

   [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>.

   [RFC7012]  Claise, B., Ed. and B. Trammell, Ed., "Information Model
              for IP Flow Information Export (IPFIX)", RFC 7012,
              DOI 10.17487/RFC7012, September 2013,
              <https://www.rfc-editor.org/info/rfc7012>.

   [RFC7799]  Morton, A., "Active and Passive Metrics and Methods (with
              Hybrid Types In-Between)", RFC 7799, DOI 10.17487/RFC7799,
              May 2016, <https://www.rfc-editor.org/info/rfc7799>.

   [RFC7880]  Pignataro, C., Ward, D., Akiya, N., Bhatia, M., and S.
              Pallagatti, "Seamless Bidirectional Forwarding Detection
              (S-BFD)", RFC 7880, DOI 10.17487/RFC7880, July 2016,
              <https://www.rfc-editor.org/info/rfc7880>.

   [RFC8402]  Filsfils, C., Ed., Previdi, S., Ed., Ginsberg, L.,
              Decraene, B., Litkowski, S., and R. Shakir, "Segment
              Routing Architecture", RFC 8402, DOI 10.17487/RFC8402,
              July 2018, <https://www.rfc-editor.org/info/rfc8402>.

   [RFC8403]  Geib, R., Ed., Filsfils, C., Pignataro, C., Ed., and N.
              Kumar, "A Scalable and Topology-Aware MPLS Data-Plane
              Monitoring System", RFC 8403, DOI 10.17487/RFC8403, July
              2018, <https://www.rfc-editor.org/info/rfc8403>.

   [RFC8571]  Ginsberg, L., Ed., Previdi, S., Wu, Q., Tantsura, J., and
              C. Filsfils, "BGP - Link State (BGP-LS) Advertisement of
              IGP Traffic Engineering Performance Metric Extensions",
              RFC 8571, DOI 10.17487/RFC8571, March 2019,
              <https://www.rfc-editor.org/info/rfc8571>.

   [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>.

   [STAMP-SR] Gandhi, R., Ed., Filsfils, C., Voyer, D., Chen, M.,
              Janssens, B., and R. Foote, "Performance Measurement Using
              Simple TWAMP (STAMP) for Segment Routing Networks", Work
              in Progress, Internet-Draft, draft-ietf-spring-stamp-srpm-
              03, 1 February 2022,
              <https://datatracker.ietf.org/doc/html/draft-ietf-spring-
              stamp-srpm-03>.

Appendix A.  Illustrations

   This appendix shows how some of the existing IPv6 OAM mechanisms can
   be used in an SRv6 network.  It also illustrates an OAM mechanism for
   performing controllable and predictable flow sampling from segment
   endpoints.  How the centralized OAM technique in [RFC8403] can be
   extended for SRv6 is also described in this appendix.

A.1.  Ping in SRv6 Networks

   The existing mechanism to perform the reachability checks, along the
   shortest path, continues to work without any modification.  Any IPv6
   node (SRv6 capable (SRv6-capable or a non-SRv6 capable) non-SRv6-capable) can initiate, transit, and
   egress a ping packet.

   The following subsections outline some additional use cases of the ICMPv6
   ping in the SRv6 networks.

A.1.1.  Pinging an IPv6 Address via a Segment-list Segment List

   If an SRv6-capable ingress node wants to ping an IPv6 address via an
   arbitrary segment list <S1, S2, S3>, it needs to initiate an ICMPv6
   ping with an SR header containing the SID list <S1, S2, S3>.  This is
   illustrated using the topology in Figure 1.  User  The user issues a ping
   from node N1 to a loopback of node N5, N5 via segment list
   <2001:db8:K:2:X31::, 2001:db8:K:4:X52::>.  The SID behavior used in
   the example is End.X SID, End.X, as described in [RFC8986], but the procedure is
   equally applicable to any other (transit) SID type.

   Figure 2 contains sample output for a ping request initiated at node
   N1 to a loopback address of node N5 via a segment list
   <2001:db8:K:2:X31::, 2001:db8:K:4:X52::>.

       > ping 2001:db8:L:5:: via segment-list segment list 2001:db8:K:2:X31::,
              2001:db8:K:4:X52::

       Sending 5, 100-byte ICMPv6 Echos to B5::, timeout is 2 seconds:
       !!!!!
       Success rate is 100 percent (5/5), round-trip min/avg/max = 0.625
       /0.749/0.931 ms

            Figure 2 A sample ping output 2: Sample Ping Output at an SRv6-capable node SRv6-Capable Node

   All transit nodes process the echo request message like any other
   data packet carrying an SR header and hence do not require any
   change.  Similarly, the egress node does not require any change to
   process the ICMPv6 echo request.  For example, in the ping example of in
   Figure 2:

   o

   *  Node N1 initiates an ICMPv6 ping packet with the SRH as follows follows:
      (2001:db8:L:1::, 2001:db8:K:2:X31::) (2001:db8:L:5::,
      2001:db8:K:4:X52::, 2001:db8:K:2:X31::, SL=2, NH = ICMPv6)(ICMPv6
      Echo Request).

   o

   *  Node N2, which is an SRv6-capable node, performs the standard SRH
      processing.  Specifically, it executes the End.X behavior
      indicated by the 2001:db8:K:2:X31:: SID and forwards the packet on
      link3 to N3.

   o

   *  Node N3, which is a non-SRv6 capable non-SRv6-capable node, performs the standard
      IPv6 processing.  Specifically, it forwards the echo request based
      on the DA 2001:db8:K:4:X52:: in the IPv6 header.

   o

   *  Node N4, which is an SRv6-capable node, performs the standard SRH
      processing.  Specifically, it observes the End.X behavior
      (2001:db8:K:4:X52::) and forwards the packet on link10 towards N5.
      If 2001:db8:K:4:X52:: is a PSP SID, the penultimate node (Node (node N4)
      does not, should not not, and cannot differentiate between the data
      packets and OAM probes.  Specifically, if 2001:db8:K:4:X52:: is a
      PSP SID, node N4 executes the SID like any other data packet with
      DA = 2001:db8:K:4:X52:: and removes the SRH.

   o

   *  The echo request packet at N5 arrives as an IPv6 packet with or
      without an SRH.  If N5 receives the packet with an SRH, it skips
      SRH processing (SL=0).  In either case, Node node N5 performs the
      standard ICMPv6 processing on the echo request and responds with
      the echo reply message to N1.  The echo reply message is IP
      routed.

A.1.2.  Pinging a SID

   The ping mechanism described above applies equally to perform SID
   reachability check and to validate the SID is locally programmed at
   the target node.  This is explained using an example in the
   following. following example.  The
   example uses ping to an END End SID, as described in [RFC8986], but the
   procedure is equally applicable to ping any other SID behaviors.

   Consider the example where the user wants to ping a remote SID
   2001:db8:K:4::, via 2001:db8:K:2:X31::, from node N1.  The ICMPv6
   echo request is processed at the individual nodes along the path as
   follows:

   o

   *  Node N1 initiates an ICMPv6 ping packet with the SRH as follows follows:
      (2001:db8:L:1::, 2001:db8:K:2:X31::) (2001:db8:K:4::,
      2001:db8:K:2:X31::; SL=1; NH=ICMPv6)(ICMPv6 Echo Request).

   o

   *  Node N2, which is an SRv6-capable node, performs the standard SRH
      processing.  Specifically, it executes the End.X behavior
      indicated by the 2001:db8:K:2:X31:: SID on the echo request
      packet.  If 2001:db8:K:2:X31:: is a PSP SID, node N4 executes the
      SID like any other data packet with DA = 2001:db8:K:2:X31:: and
      removes the SRH.

   o

   *  Node N3, which is a non-SRv6 capable non-SRv6-capable node, performs the standard
      IPv6 processing.  Specifically, it forwards the echo request based
      on DA = 2001:db8:K:4:: in the IPv6 header.

   o

   *  When node N4 receives the packet, it processes the target SID
      (2001:db8:K:4::).

   o

   *  If the target SID (2001:db8:K:4::) is not locally instantiated and
      does not represent a local interface, the packet is discarded
   o

   *  If the target SID (2001:db8:K:4::) is locally instantiated or
      represents a local interface, the node processes the upper layer upper-layer
      header.  As part of the upper layer upper-layer header processing processing, node N4
      respond
      responds to the ICMPv6 echo request message and responds with the an echo reply
      message.  The echo reply message is IP routed.

A.2.  Traceroute

   The existing traceroute mechanisms, along the shortest path,
   continues continue
   to work without any modification.  Any IPv6 node (SRv6
   capable (SRv6-capable or a non-SRv6 capable)
   non-SRv6-capable) can initiate, transit, and egress a traceroute
   probe.

   The following subsections outline some additional use cases of the
   traceroute in the SRv6 networks.

A.2.1.  Traceroute to an IPv6 Address via a Segment-list Segment List

   If an SRv6-capable ingress node wants to traceroute to an IPv6
   address via an arbitrary segment list <S1, S2, S3>, it needs to
   initiate a traceroute probe with an SR header containing the SID list
   <S1, S2, S3>.  User  The user issues a traceroute from node N1 to a
   loopback of node N5, N5 via segment list <2001:db8:K:2:X31::,
   2001:db8:K:4:X52::>.  The SID behavior used in the example is End.X SID, End.X,
   as described in [RFC8986], but the procedure is equally applicable to
   any other (transit) SID type.  Figure 3 contains sample output for
   the traceroute request.

   > traceroute 2001:db8:L:5:: via segment-list segment list 2001:db8:K:2:X31::,
                2001:db8:K:4:X52::

   Tracing the route to 2001:db8:L:5::
   1  2001:db8:2:1:21:: 0.512 msec 0.425 msec 0.374 msec
      DA: 2001:db8:K:2:X31::,
      SRH:(2001:db8:L:5::, 2001:db8:K:4:X52::, 2001:db8:K:2:X31::, SL=2)
   2  2001:db8:3:2:31:: 0.721 msec 0.810 msec 0.795 msec
      DA: 2001:db8:K:4:X52::,
      SRH:(2001:db8:L:5::, 2001:db8:K:4:X52::, 2001:db8:K:2:X31::, SL=1)
   3  2001:db8:4:3::41:: 0.921 msec 0.816 msec 0.759 msec
      DA: 2001:db8:K:4:X52::,
      SRH:(2001:db8:L:5::, 2001:db8:K:4:X52::, 2001:db8:K:2:X31::, SL=1)
   4  2001:db8:5:4::52:: 0.879 msec 0.916 msec 1.024 msec
      DA: 2001:db8:L:5::

         Figure 3 A sample traceroute output 3: Sample Traceroute Output at an SRv6-capable node SRv6-Capable Node

   In the sample traceroute output, the information displayed at each
   hop is obtained using the contents of the "Time Exceeded" or
   "Destination Unreachable" ICMPv6 responses.  These ICMPv6 responses
   are IP routed.

   In the sample traceroute output, the information for link3 is
   returned by N3, which is a non-SRv6 capable non-SRv6-capable node.  Nonetheless, the
   ingress node is able to display SR header contents as the packet
   travels through the non-SRv6 capable non-SRv6-capable node.  This is because the "Time
   Exceeded Message"
   Exceeded" ICMPv6 message can contain as much of the invoking packet
   as possible without the ICMPv6 packet exceeding the minimum IPv6 MTU
   [RFC4443].  The SR header is included in these ICMPv6 messages
   initiated by the non-SRv6 capable non-SRv6-capable transit nodes that are not running
   SRv6 software.  Specifically, a node generating an ICMPv6 message
   containing a copy of the invoking packet does not need to understand
   the extension header(s) in the invoking packet.

   The segment list information returned for the first hop is returned
   by N2, which is an SRv6-capable node.  Just like for the second hop,
   the ingress node is able to display SR header contents for the first
   hop.

   There is no difference in processing of the traceroute probe at an
   SRv6-capable and a non-SRv6 capable non-SRv6-capable node.  Similarly, both
   SRv6-capable and non-SRv6 capable non-SRv6-capable nodes may use the address of the
   interface on which probe was received as the source address in the
   ICMPv6 response.  ICMPv6 extensions defined in [RFC5837] can be used
   to display information about the IP interface through which the
   datagram would have been forwarded had it been forwardable, and the IP
   next hop to which the datagram would have been forwarded, the IP
   interface upon which a the datagram arrived, and the sub-IP component
   of an IP interface upon which a the datagram arrived.

   The IP address of the interface on which the traceroute probe was
   received is useful.  This information can also be used to verify if
   SIDs 2001:db8:K:2:X31:: and 2001:db8:K:4:X52:: are executed correctly
   by N2 and N4, respectively.  Specifically, the information displayed
   for the second hop contains the incoming interface address
   2001:db8:2:3:31:: at N3.  This matches with the expected interface bound
   to End.X behavior 2001:db8:K:2:X31:: (link3).  Similarly, the
   information displayed for the fourth hop contains the incoming
   interface address 2001:db8:4:5::52:: at N5.  This matches with the
   expected interface bound to the End.X behavior 2001:db8:K:4:X52::
   (link10).

A.2.2.  Traceroute to a SID

   The mechanism to traceroute an IPv6 Address address via a Segment-list segment list
   described in the previous section applies equally to traceroute a
   remote SID behavior, as explained using an example in the following. following example.  The
   example uses traceroute to an END End SID, as described in [RFC8986], but
   the procedure is equally applicable to tracerouting any other SID
   behaviors.

   Please note that traceroute to a SID is exemplified using UDP probes.
   However, the procedure is equally applicable to other implementations
   of traceroute mechanism.  The UDP encoded message to traceroute a SID
   would use the UDP ports assigned by IANA for "traceroute use".

   Consider the example where the user wants to traceroute a remote SID
   2001:db8:K:4::, via 2001:db8:K:2:X31::, from node N1.  The traceroute
   probe is processed at the individual nodes along the path as follows:

   o

   *  Node N1 initiates a traceroute probe packet as follows
      (2001:db8:L:1::, 2001:db8:K:2:X31::) (2001:db8:K:4::,
      2001:db8:K:2:X31::; SL=1; NH=UDP)(Traceroute probe).  The first
      traceroute probe is sent with the hop-count value set to 1.  The hop-
      count
      hop-count value is incremented by 1 for each following subsequent traceroute
      probes.

   o
      probe.

   *  When node N2 receives the packet with hop-count = 1, it processes
      the hop-count expiry.  Specifically, the node N2 responds with the
      ICMPv6 message (Type: "Time Exceeded", Code: "Hop "hop limit exceeded
      in transit").  The ICMPv6 response is IP routed.

   o

   *  When Node node N2 receives the packet with hop-count > 1, it performs
      the standard SRH processing.  Specifically, it executes the End.X
      behavior indicated by the 2001:db8:K:2:X31:: SID on the traceroute
      probe.  If 2001:db8:K:2:X31:: is a PSP SID, node N2 executes the
      SID like any other data packet with DA = 2001:db8:K:2:X31:: and
      removes the SRH.

   o

   *  When node N3, which is a non-SRv6 capable non-SRv6-capable node, receives the
      packet with hop-count = 1, it processes the hop-count expiry.
      Specifically, the node N3 responds with the ICMPv6 message (Type:
      "Time Exceeded", Code: "Hop limit exceeded in Transit"). transit").  The
      ICMPv6 response is IP routed.

   o

   *  When node N3, which is a non-SRv6 capable non-SRv6-capable node, receives the
      packet with hop-count > 1, it performs the standard IPv6
      processing.  Specifically, it forwards the traceroute probe based
      on DA 2001:db8:K:4:: in the IPv6 header.

   o

   *  When node N4 receives the packet with DA set to the local SID
      2001:db8:K:4::, it processes the END End SID.

   o

   *  If the target SID (2001:db8:K:4::) is not locally instantiated and
      does not represent a local interface, the packet is discarded.

   o

   *  If the target SID (2001:db8:K:4::) is locally instantiated or
      represents a local interface, the node processes the upper layer upper-layer
      header.  As part of the upper layer upper-layer header processing processing, node N4
      responds with the ICMPv6 message (Type: Destination unreachable, "Destination Unreachable",
      Code: Port Unreachable). "Port Unreachable").  The ICMPv6 response is IP routed.

   Figure 4 displays a sample traceroute output for this example.

     > traceroute 2001:db8:K:4:X52:: via segment-list segment list 2001:db8:K:2:X31::

     Tracing the route to SID 2001:db8:K:4:X52::
     1  2001:db8:2:1:21:: 0.512 msec 0.425 msec 0.374 msec
        DA: 2001:db8:K:2:X31::,
        SRH:(2001:db8:K:4:X52::, 2001:db8:K:2:X31::; SL=1)
     2  2001:db8:3:2:21:: 0.721 msec 0.810 msec 0.795 msec
        DA: 2001:db8:K:4:X52::,
        SRH:(2001:db8:K:4:X52::, 2001:db8:K:2:X31::; SL=0)
     3  2001:db8:4:3:41:: 0.921 msec 0.816 msec 0.759 msec
        DA: 2001:db8:K:4:X52::,
        SRH:(2001:db8:K:4:X52::, 2001:db8:K:2:X31::; SL=0)

         Figure 4 A sample output 4: Sample Output for hop-by-hop traceroute Hop-by-Hop Traceroute to a SID

A.3.  A  Hybrid OAM Using O-flag the OAM Flag

   This section illustrates a hybrid OAM mechanism using the the O-flag.
   Without loss of the generality, the illustration assumes N100 is a
   centralized controller.

   The

   This illustration is different than from the In-situ OAM "in situ OAM" defined in [I.D-
   draft-ietf-ippm-ioam-data].
   [RFC9197].  This is because In-situ in situ OAM records operational and
   telemetry information in the packet as the packet traverses a path
   between two points in the network [I.D-draft-ietf-
   ippm-ioam-data]. [RFC9197].  The illustration in
   this subsection does not require the recording of OAM data in the
   packet.

   The illustration does not assume any formats for exporting the data
   elements or the data elements that need to be exported.  The
   illustration assumes system clocks among all nodes in the SR domain
   are synchronized.

   Consider the example where the user wants to monitor sampled IPv4 VPN
   999 traffic going from CE1 to CE2 via a low latency low-latency SR policy P
   installed at Node node N1.  To exercise a low latency low-latency path, the SR Policy
   P forces the packet via segments 2001:db8:K:2:X31:: and
   2001:db8:K:4:X52::.  The VPN SID at N7 associated with VPN 999 is
   2001:db8:K:7:DT999::.  2001:db8:K:7:DT999:: is a USP SID.  N1, N4,
   and N7 are capable of processing O-flag the O-flag, but N2 is not capable of
   processing the O-flag.  N100 is the centralized controller capable of
   processing and correlating the copy of the packets sent from nodes
   N1, N4, and N7.  N100 is aware of O-flag processing capabilities.
   Controller N100 N100, with the help from nodes N1, N4, N7 and N7, implements a
   hybrid OAM mechanism using the O-flag as follows:

   o

   *  A packet P1:(IPv4 header)(payload) is sent from CE1 to Node node N1.

   o

   *  Node N1 steers the packet P1 through the Policy P.  Based on a local
      configuration, Node node N1 also implements logic to sample traffic
      steered through policy P for hybrid OAM purposes.  Specification
      for the sampling logic is beyond the scope of this document.
      Consider the case where packet P1 is classified as a packet to be
      monitored via the hybrid OAM.  Node N1 sets the O-flag during the
      encapsulation required by policy P.  As part of setting the
      O-flag, node N1 also sends a timestamped copy of the packet P1:
      (2001:db8:L:1::, 2001:db8:K:2:X31::) (2001:db8:K:7:DT999::,
      2001:db8:K:4:X52::, 2001:db8:K:2:X31::; SL=2; O-flag=1;
      NH=IPv4)(IPv4 header)(payload) to a local OAM process.  The local
      OAM process sends a full or partial copy of the packet P1 to the
      controller N100.  The OAM process includes the recorded timestamp,
      additional OAM information like (like incoming and outgoing interface,
      etc. along with interface),
      and any applicable metadata.  Node N1 forwards the original packet
      towards the next segment 2001:db8:K:2:X31::.

   o

   *  When node N2 receives the packet with the O-flag set, it ignores
      the O-flag.  This is because node N2 is not capable of processing
      the O-flag.  Node N2 performs the standard SRv6 SID and SRH
      processing.  Specifically, it executes the End.X behavior
      indicated by the 2001:db8:K:2:X31:: SID as described in [RFC8986]
      and forwards the packet P1 (2001:db8:L:1::, 2001:db8:K:4:X52::)
      (2001:db8:K:7:DT999::, 2001:db8:K:4:X52::, 2001:db8:K:2:X31::;
      SL=1; O-flag=1; NH=IPv4)(IPv4 header)(payload) over link 3 link3 towards
      Node
      node N3.

   o

   *  When node N3, which is a non-SRv6 capable non-SRv6-capable node, receives the packet P1 ,
      P1, it performs the standard IPv6 processing.  Specifically, it
      forwards the packet P1 based on DA 2001:db8:K:4:X52:: in the IPv6
      header.

   o

   *  When node N4 receives the packet P1 (2001:db8:L:1::,
      2001:db8:K:4:X52::) (2001:db8:K:7:DT999::, 2001:db8:K:4:X52::,
      2001:db8:K:2:X31::; SL=1; O-flag=1; NH=IPv4)(IPv4
      header)(payload), it processes the O-flag.  As part of processing
      the O-flag, it sends a timestamped copy of the packet to a local
      OAM process.  Based on a local configuration, the local OAM process
      sends a full or partial copy of the packet P1 to the controller N100.
      The OAM process includes the recorded timestamp, additional OAM
      information like (like incoming and outgoing interface,
      etc. along with etc.), and any
      applicable metadata.  Node N4 performs the standard SRv6 SID and
      SRH processing on the original packet P1.  Specifically, it
      executes the End.X behavior indicated by the 2001:db8:K:4:X52::
      SID and forwards the packet P1 (2001:db8:L:1::, 2001:db8:K:7:DT999::)
      (2001:db8:K:7:DT999::, 2001:db8:K:4:X52::, 2001:db8:K:2:X31::;
      SL=0; O-flag=1; NH=IPv4)(IPv4 header)(payload) over link 10 link10 towards Node
      node N5.

   o

   *  When node N5, which is a non-SRv6 capable non-SRv6-capable node, receives the packet
      P1, it performs the standard IPv6 processing.  Specifically, it
      forwards the packet based on DA 2001:db8:K:7:DT999:: in the IPv6
      header.

   o

   *  When node N7 receives the packet P1 (2001:db8:L:1::,
      2001:db8:K:7:DT999::) (2001:db8:K:7:DT999::, 2001:db8:K:4:X52::,
      2001:db8:K:2:X31::; SL=0; O-flag=1; NH=IPv4)(IPv4
      header)(payload), it processes the O-flag.  As part of processing
      the O-flag, it sends a timestamped copy of the packet to a local
      OAM process.  The local OAM process sends a full or partial copy
      of the packet P1 to the controller N100.  The OAM process includes the
      recorded timestamp, additional OAM information like (like incoming and
      outgoing interface, etc. along with etc.), and any applicable metadata.  Node N7
      performs the standard SRv6 SID and SRH processing on the original
      packet P1.  Specifically, it executes the VPN SID indicated by the
      2001:db8:K:7:DT999:: SID and and, based on lookup in table 100 100,
      forwards the packet P1 (IPv4 header)(payload) towards CE
      2.

   o CE2.

   *  The controller N100 processes and correlates the copy of the
      packets sent from nodes N1, N4 N4, and N7 to find segment-by-segment
      delays and provide other hybrid OAM information related to packet
      P1.  For segment-by-segment delay computation, it is assumed that
      clock
      clocks are synchronized time across the SR domain.

   o

   *  The process continues for any other sampled packets.

A.4.  Monitoring of SRv6 Paths

   In the recent past, network operators demonstrated interest in
   performing network OAM functions in a centralized manner.  [RFC8403]
   describes such a centralized OAM mechanism.  Specifically, the
   document [RFC8403]
   describes a procedure that can be used to perform path continuity check
   checks between any nodes within an SR domain from a centralized
   monitoring system.  However, the document while [RFC8403] focuses on SR networks
   with MPLS data plane.  This plane, this document describes how the concept can be
   used to perform path monitoring in an SRv6 network from a centralized
   controller.

   In the reference topology in Figure 1, N100 uses an IGP protocol like
   OSPF or IS-IS to get a view of the topology view within the IGP domain.
   N100 can also use BGP-LS to get the complete view of an inter-domain
   topology.  The controller leverages the visibility of the topology to
   monitor the paths between the various endpoints.

   The controller N100 advertises an END End SID [RFC8986]
   2001:db8:K:100:1::. To monitor any arbitrary SRv6 paths, the
   controller can create a loopback probe that originates and terminates
   on Node node N100.  To distinguish between a failure in the monitored path
   and loss of connectivity between the controller and the network, Node node
   N100 runs a suitable mechanism to monitor its connectivity to the
   monitored network.

   The following example illustrates loopback probes are exemplified using an example where in which controller
   N100 needs to verify a segment list <2001:db8:K:2:X31::,
   2001:db8:K:4:X52::>:

   o

   *  N100 generates an OAM packet (2001:db8:L:100::,
      2001:db8:K:2:X31::)(2001:db8:K:100:1::, 2001:db8:K:4:X52::,
      2001:db8:K:2:X31::, SL=2)(OAM Payload).  The controller routes the
      probe packet towards the first segment, which is
      2001:db8:K:2:X31::.

   o

   *  Node N2 executes the End.X behavior indicated by the
      2001:db8:K:2:X31:: SID and forwards the packet (2001:db8:L:100::,
      2001:db8:K:4:X52::)(2001:db8:K:100:1::, 2001:db8:K:4:X52::,
      2001:db8:K:2:X31::, SL=1)(OAM Payload) on link3 to N3.

   o

   *  Node N3, which is a non-SRv6 capable non-SRv6-capable node, performs the standard
      IPv6 processing.  Specifically, it forwards the packet based on
      the DA
      2001:db8:K:4:X52:: in the IPv6 header.

   o

   *  Node N4 executes the End.X behavior indicated by the
      2001:db8:K:4:X52:: SID and forwards the packet (2001:db8:L:100::,
      2001:db8:K:100:1::)(2001:db8:K:100:1::, 2001:db8:K:4:X52::,
      2001:db8:K:2:X31::, SL=0)(OAM Payload) on link10 to N5.

   o

   *  Node N5, which is a non-SRv6 capable non-SRv6-capable node, performs the standard
      IPv6 processing.  Specifically, it forwards the packet based on
      the DA
      2001:db8:K:100:1:: in the IPv6 header.

   o

   *  Node N100 executes the standard SRv6 END behavior.  It
      decapsulates the header and consume consumes the probe for OAM processing.
      The information in the OAM payload is used to detect any missing
      probes, round trip round-trip delay, etc.

   The OAM payload type or the information carried in the OAM probe is a
   local implementation decision at the controller and is outside the
   scope of this document.

Appendix B.

Acknowledgements

   The authors would like to thank Joel M. Halpern, Greg Mirsky, Bob
   Hinden, Loa Andersson, Gaurav Naik, Ketan Talaulikar Talaulikar, and Haoyu Song
   for their review comments.

Appendix C.

Contributors

   The following people have contributed to this document:

   Robert Raszuk
   Bloomberg LP
   Email: robert@raszuk.net

   John Leddy
   Individual
   Email: john@leddy.net

   Gaurav Dawra
   LinkedIn
   Email: gdawra.ietf@gmail.com

   Bart Peirens
   Proximus
   Email: bart.peirens@proximus.com

   Nagendra Kumar
   Cisco Systems, Inc.
   Email: naikumar@cisco.com

   Carlos Pignataro
   Cisco Systems, Inc.
   Email: cpignata@cisco.com

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

   Frank Brockners
   Cisco Systems, Inc.
      Germany
   Email: fbrockne@cisco.com

   Darren Dukes
   Cisco Systems, Inc.
   Email: ddukes@cisco.com

   Cheng Li
   Huawei
   Email: chengli13@huawei.com

   Faisal Iqbal
   Individual
   Email: faisal.ietf@gmail.com

Authors' Addresses

   Zafar Ali
   Cisco Systems
   Email: zali@cisco.com

   Clarence Filsfils
   Cisco Systems
   Email: cfilsfil@cisco.com

   Satoru Matsushima
   Softbank
   Email: satoru.matsushima@g.softbank.co.jp

   Daniel Voyer
   Bell Canada
   Email: daniel.voyer@bell.ca

   Mach

   Mach(Guoyi) Chen
   Huawei
   Email: mach.chen@huawei.com

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