path: root/doc/rfc/rfc9259.txt

Internet Engineering Task Force (IETF)                            Z. Ali
Request for Comments: 9259                                   C. Filsfils
Category: Standards Track                                  Cisco Systems
ISSN: 2070-1721                                            S. Matsushima
                                                                Softbank
                                                                D. Voyer
                                                             Bell Canada
                                                                 M. Chen
                                                                  Huawei
                                                               June 2022


  Operations, Administration, and Maintenance (OAM) in Segment Routing
                            over IPv6 (SRv6)

Abstract

   This document describes how the existing IPv6 mechanisms for ping and
   traceroute can be used in a Segment Routing over IPv6 (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 is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for 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 this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc9259.

Copyright Notice

   Copyright (c) 2022 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
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   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Revised BSD License text as described in Section 4.e of the
   Trust Legal Provisions and are provided without warranty as described
   in the Revised BSD License.

Table of Contents

   1.  Introduction
     1.1.  Requirements Language
     1.2.  Abbreviations
     1.3.  Terminology and Reference Topology
   2.  OAM Mechanisms
     2.1.  OAM Flag in the Segment Routing Header
       2.1.1.  OAM Flag Processing
     2.2.  OAM Operations
   3.  Security Considerations
   4.  Privacy Considerations
   5.  IANA Considerations
   6.  References
     6.1.  Normative References
     6.2.  Informative References
   Appendix A.  Illustrations
     A.1.  Ping in SRv6 Networks
       A.1.1.  Pinging an IPv6 Address via a Segment List
       A.1.2.  Pinging a SID
     A.2.  Traceroute in SRv6 Networks
       A.2.1.  Traceroute to an IPv6 Address via a Segment List
       A.2.2.  Traceroute to a SID
     A.3.  Hybrid OAM Using the OAM Flag
     A.4.  Monitoring of SRv6 Paths
   Acknowledgements
   Contributors
   Authors' Addresses

1.  Introduction

   As Segment Routing over IPv6 (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 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.

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

   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 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 Identifier

   SL:  Segments Left

   SR:  Segment Routing

   SRH:  Segment Routing Header [RFC8754]

   SRv6:  Segment Routing over IPv6 [RFC8402]

   PSP:  Penultimate Segment Pop [RFC8986]

   USP:  Ultimate Segment Pop [RFC8986]

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

   IS-IS:  Intermediate System to Intermediate System

   OSPF:  Open Shortest Path First [RFC2328]

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

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

1.3.  Terminology and Reference Topology

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

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

                        Figure 1: Reference Topology

   In the reference topology:

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

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

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

   *  CE1 and CE2 are Customer Edge devices of any data plane capability
      (e.g., IPv4, IPv6, 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 between nodes i and j at the i
      side is represented as 2001:db8:i:j:in::. For example, in
      Figure 1, the IPv6 address of link6 (the second link between nodes
      N3 and N4) at node N3 is 2001:db8:3:4:32::. Similarly, the IPv6
      address of link5 (the first link between nodes 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 between nodes i
      and j.  For example, 2001:db8:K:2:X31:: represents End.X at node
      N2 towards node N3 via link3 (the first link between nodes N2 and
      N3).  Similarly, 2001:db8:K:4:X52:: represents the End.X at node
      N4 towards node N5 via link10 (the second link between nodes N4
      and N5).  Please refer to [RFC8986] for a description of End.X
      SID.

   *  A SID list is represented as <S1, S2, S3>, where S1 is the first
      SID to visit, S2 is the second SID to 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 address DA, and
         SRH as 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, 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 payload of the packet.

2.  OAM Mechanisms

   This section defines OAM enhancements for SRv6 networks.

2.1.  OAM Flag in the Segment Routing Header

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

   The O-flag in the SRH is used as a marking bit in user packets to
   trigger 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 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 it simply ignores it upon reception.  If a node
   supports the O-flag, it can optionally advertise its potential via
   control plane protocol(s).

   The following is appended to line S01 of the pseudocode associated
   with the SID S (as defined in Section 4.3.1.1 of [RFC8754]) when N
   receives a packet destined to S, S is a local SID, and the O-flag is
   processed.

      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,
                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 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, line S01.1 of
   the pseudocode specified in this document is inserted between lines
   S01 and S02 of the pseudocode defined in 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 Packet Sampling
   (PSAMP) [RFC5476] for exporting the requested sections of the
   mirrored packets.

   If the penultimate segment of a segment list is a 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, 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 impacting
   the performance of the OAM and telemetry collection processes.
   Failure to implement the rate limit can lead to a denial-of-service
   attack, as detailed in 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 this document.  Appendix A illustrates use of the O-flag for
   implementing a hybrid OAM mechanism, where the "hybrid"
   classification is based on [RFC7799].

2.2.  OAM Operations

   IPv6 OAM operations can be performed for any SRv6 SID whose behavior
   allows 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 ping and UDP-based
   traceroute mechanisms for ping and traceroute to an SRv6 SID.
   Although this document only illustrates ICMPv6-based ping and UDP-
   based traceroute to an SRv6 SID, the procedures are equally
   applicable to other OAM mechanisms that probe an SRv6 SID (e.g.,
   Bidirectional Forwarding Detection (BFD) [RFC5880], Seamless BFD
   (S-BFD) [RFC7880], and Simple Two-way Active Measurement Protocol
   (STAMP) probe message processing [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.  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 SID
   manipulation, O-flag manipulation is not considered a threat within
   the SR domain.  Procedures for securing an SR domain are defined in
   Sections 5.1 and 7 of [RFC8754].

   As noted in 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 2.1.1 is not susceptible to that
   denial-of-service attack.  Additionally, SRH flags are protected by
   the Hashed Message Authentication Code (HMAC) TLV, as described in
   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], and [RFC8986].

4.  Privacy Considerations

   The per-packet marking capabilities of the O-flag 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.

5.  IANA Considerations

   IANA has registered the following in the "Segment Routing Header
   Flags" subregistry in the "Internet Protocol Version 6 (IPv6)
   Parameters" registry:

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

                                  Table 1

6.  References

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>.

6.2.  Informative References

   [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 or non-SRv6-capable) can initiate, transit, and
   egress a ping packet.

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

A.1.1.  Pinging an IPv6 Address via a 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.  The user issues a ping
   from node N1 to a loopback of node N5 via segment list
   <2001:db8:K:2:X31::, 2001:db8:K:4:X52::>.  The SID behavior used in
   the example is 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 segment list
   <2001:db8:K:2:X31::, 2001:db8:K:4:X52::>.

       > ping 2001:db8:L:5:: via 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: Sample Ping Output at an 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 example in
   Figure 2:

   *  Node N1 initiates an ICMPv6 ping packet with the SRH as 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).

   *  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 node N3.

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

   *  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
      node N5.  If 2001:db8:K:4:X52:: is a PSP SID, the penultimate node
      (node N4) does not, should 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.

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

A.1.2.  Pinging a SID

   The ping mechanism described above can also be used to perform SID
   reachability checks and to validate that the SID is locally
   programmed at the target node.  This is explained in the following
   example.  The example uses ping to an 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:

   *  Node N1 initiates an ICMPv6 ping packet with the SRH as 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).

   *  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.

   *  Node N3, which is a 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.

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

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

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

A.2.  Traceroute in SRv6 Networks

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

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

A.2.1.  Traceroute to an IPv6 Address via a 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>.  The user issues a traceroute from node N1 to a
   loopback of node N5 via segment list <2001:db8:K:2:X31::,
   2001:db8:K:4:X52::>.  The SID behavior used in the example is 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 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: Sample Traceroute Output at an 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 node N3, which is a non-SRv6-capable node.  Nonetheless,
   the ingress node is able to display SR header contents as the packet
   travels through the non-SRv6-capable node.  This is because the "Time
   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 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 node 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 node.  Similarly, both
   SRv6-capable and 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, the IP
   next hop to which the datagram would have been forwarded, the IP
   interface upon which the datagram arrived, and the sub-IP component
   of an IP interface upon which 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 nodes N2 and N4, respectively.  Specifically, the information
   displayed for the second hop contains the incoming interface address
   2001:db8:2:3:31:: at node N3.  This matches 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 node N5.  This matches 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 via a segment list
   described in the previous section can also be used to traceroute a
   remote SID behavior, as explained in the following example.  The
   example uses traceroute to an 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:

   *  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 value is incremented by 1 for each subsequent traceroute
      probe.

   *  When node N2 receives the packet with hop-count = 1, it processes
      the hop-count expiry.  Specifically, node N2 responds with the
      ICMPv6 message with type "Time Exceeded" and code "hop limit
      exceeded in transit".  The ICMPv6 response is IP routed.

   *  When 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.

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

   *  When node N3, which is a 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.

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

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

   *  If the target SID (2001:db8:K:4::) is locally instantiated or
      represents a local interface, the node processes the Upper-Layer
      header.  As part of the Upper-Layer header processing, node N4
      responds with the ICMPv6 message with type "Destination
      Unreachable" and code "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 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: Sample Output for Hop-by-Hop Traceroute to a SID

A.3.  Hybrid OAM Using the OAM Flag

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

   This illustration is different from the "in situ OAM" defined in
   [RFC9197].  This is because in situ OAM records operational and
   telemetry information in the packet as the packet traverses a path
   between two points in the network [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 SR Policy P
   installed at node N1.  To exercise a 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 node N7 associated with VPN 999
   is 2001:db8:K:7:DT999::.  2001:db8:K:7:DT999:: is a USP SID.  Nodes
   N1, N4, and N7 are capable of processing the O-flag, but node N2 is
   not capable of processing the O-flag.  Node N100 is the centralized
   controller capable of processing and correlating the copy of the
   packets sent from nodes N1, N4, and N7.  Node N100 is aware of O-flag
   processing capabilities.  Node N100, with help from nodes N1, N4, and
   N7, implements a hybrid OAM mechanism using the O-flag as follows:

   *  A packet P1 is sent from CE1 to node N1.  The packet is:

      P1: (IPv4 header)(payload)

   *  Node N1 steers packet P1 through the SR Policy P.  Based on local
      configuration, node N1 also implements logic to sample traffic
      steered through SR 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 SR Policy P.  As part
      of setting the O-flag, node N1 also sends a timestamped copy of
      packet P1 to a local OAM process.  The packet is:

      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)

      The local OAM process sends a full or partial copy of packet P1 to
      node N100.  The OAM process includes the recorded timestamp,
      additional OAM information (like incoming and outgoing interface),
      and any applicable metadata.  Node N1 forwards the original packet
      towards the next segment 2001:db8:K:2:X31::.

   *  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
      [RFC8986] indicated by the 2001:db8:K:2:X31:: SID and forwards
      packet P1 over link3 towards node N3.  The packet is:

      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)

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

   *  When node N4 receives packet P1, it processes the O-flag.  The
      packet is:

      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)

      As part of processing the O-flag, it sends a timestamped copy of
      the packet to a local OAM process.  Based on local configuration,
      the local OAM process sends a full or partial copy of packet P1 to
      node N100.  The OAM process includes the recorded timestamp,
      additional OAM information (like incoming and outgoing interface,
      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 packet P1 over link10 towards
      node N5.  The packet is:

      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)

   *  When node N5, which is a non-SRv6-capable node, receives 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.

   *  When node N7 receives packet P1, it processes the O-flag.  The
      packet is:

      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)

      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 packet P1 to node N100.  The OAM process
      includes the recorded timestamp, additional OAM information (like
      incoming and outgoing interface, 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, based
      on lookup in table 100, forwards packet P1 towards CE2.  The
      packet is:

      P1: (IPv4 header)(payload)

   *  Node N100 processes and correlates the copy of the packets sent
      from nodes N1, 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 clocks
      are synchronized across the SR domain.

   *  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, [RFC8403]
   describes a procedure that can be used to perform path continuity
   checks between any nodes within an SR domain from a centralized
   monitoring system.  However, while [RFC8403] focuses on SR networks
   with MPLS data 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, node N100 uses an IGP protocol
   like OSPF or IS-IS to get a view of the topology within the IGP
   domain.  Node 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.

   Node N100 advertises an 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 N100.  To
   distinguish between a failure in the monitored path and loss of
   connectivity between the controller and the network, node N100 runs a
   suitable mechanism to monitor its connectivity to the monitored
   network.

   The following example illustrates loopback probes in which node N100
   needs to verify a segment list <2001:db8:K:2:X31::,
   2001:db8:K:4:X52::>:

   *  Node 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::.

   *  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 node N3.

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

   *  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 node N5.

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

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

Acknowledgements

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

Contributors

   The following people 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.
   Email: rgandhi@cisco.com


   Frank Brockners
   Cisco Systems, Inc.
   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(Guoyi) Chen
   Huawei
   Email: mach.chen@huawei.com