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A Scalable and Topology-Aware MPLS Data-Plane Monitoring System
RFC 8403

Document Type RFC - Informational (July 2018)
Authors Ruediger Geib , Clarence Filsfils , Carlos Pignataro , Nagendra Kumar Nainar
Last updated 2018-07-24
RFC stream Internet Engineering Task Force (IETF)
Additional resources Mailing list discussion
IESG Responsible AD Alvaro Retana
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RFC 8403
Internet Engineering Task Force (IETF)                      R. Geib, Ed.
Request for Comments: 8403                              Deutsche Telekom
Category: Informational                                      C. Filsfils
ISSN: 2070-1721                                        C. Pignataro, Ed.
                                                                N. Kumar
                                                     Cisco Systems, Inc.
                                                               July 2018

    A Scalable and Topology-Aware MPLS Data-Plane Monitoring System


   This document describes features of an MPLS path monitoring system
   and related use cases.  Segment-based routing enables a scalable and
   simple method to monitor data-plane liveliness of the complete set of
   paths belonging to a single domain.  The MPLS monitoring system adds
   features to the traditional MPLS ping and Label Switched Path (LSP)
   trace, in a very complementary way.  MPLS topology awareness reduces
   management and control-plane involvement of Operations,
   Administration, and Maintenance (OAM) measurements while enabling new
   OAM features.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for informational purposes.

   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).  Not all documents
   approved by the IESG are candidates for any level of Internet
   Standard; see 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

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Copyright Notice

   Copyright (c) 2018 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
   ( in effect on the date of
   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 Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Terminology and Abbreviations . . . . . . . . . . . . . . . .   5
     2.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   5
     2.2.  Abbreviations . . . . . . . . . . . . . . . . . . . . . .   6
   3.  An MPLS Topology-Aware Path Monitoring System . . . . . . . .   6
   4.  Illustration of an SR-Based Path Monitoring Use Case  . . . .   8
     4.1.  Use Case 1: LSP Data-Plane Monitoring . . . . . . . . . .   8
     4.2.  Use Case 2: Monitoring a Remote Bundle  . . . . . . . . .  11
     4.3.  Use Case 3: Fault Localization  . . . . . . . . . . . . .  12
   5.  Path Trace and Failure Notification . . . . . . . . . . . . .  12
   6.  Applying SR to Monitoring LSPs That Are Not SR Based (LDP and
       Possibly RSVP-TE) . . . . . . . . . . . . . . . . . . . . . .  13
   7.  PMS Monitoring of Different Segment ID Types  . . . . . . . .  14
   8.  Connectivity Verification Using PMS . . . . . . . . . . . . .  14
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  15
   10. Security Considerations . . . . . . . . . . . . . . . . . . .  15
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  17
     11.1.  Normative References . . . . . . . . . . . . . . . . . .  17
     11.2.  Informative References . . . . . . . . . . . . . . . . .  17
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  19
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  19

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1.  Introduction

   Network operators need to be able to monitor the forwarding paths
   used to transport user packets.  Monitoring packets are expected to
   be forwarded in the data plane in a similar way to user packets.
   Segment Routing (SR) enables forwarding of packets along predefined
   paths and segments; thus, an SR monitoring packet can stay in the
   data plane while passing along one or more segments to be monitored.

   This document describes a system as a functional component called
   (MPLS) Path Monitoring System or PMS.  The PMS uses capabilities for
   MPLS data-plane path monitoring.  The use cases introduced here are
   limited to a single Interior Gateway Protocol (IGP) MPLS domain.  The
   use cases of this document refer to the PMS realized as a separate
   node.  Although many use cases depict the PMS as a physical node, no
   assumption should be made, and the node could be virtual.  This
   system is defined as a functional component abstracted to have many
   realizations.  The terms "PMS" and "system" are used interchangeably

   The system applies to the monitoring of non-SR LSPs like Label
   Distribution Protocol (LDP) as well as to the monitoring of SR LSPs
   (Section 7 offers some more information).  As compared to non-SR
   approaches, SR is expected to simplify such a monitoring system by
   enabling MPLS topology detection based on IGP-signaled segments.  The
   MPLS topology should be detected and correlated with the IGP
   topology, which is also detected by IGP signaling.  Thus, a
   centralized and MPLS-topology-aware monitoring unit can be realized
   in an SR domain.  This topology awareness can be used for Operation,
   Administration, and Maintenance (OAM) purposes as described by this

   Benefits offered by the system:

   o  The ability to set up an SR-domain-wide centralized connectivity
      validation.  Many operators of large networks regard a centralized
      monitoring system as useful.

   o  The MPLS ping (or continuity check) packets never leave the MPLS
      user data plane.

   o  SR allows the transport of MPLS path trace or connectivity
      validation packets for every LSP to all nodes of an SR domain.
      This use case doesn't describe new path-trace features.  The
      system described here allows for the set up of an SR-domain-wide
      centralized connectivity validation, which is useful in large
      network operator domains.

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   o  The system sending the monitoring packet is also receiving it.
      The payload of the monitoring packet may be chosen freely.  This
      allows probing packets to be sent that represent customer traffic,
      possibly from multiple services (e.g., small Voice over IP
      packets, larger HTTP packets), and allows the embedding of useful
      monitoring data (e.g., accurate timestamps since both sender and
      receiver have the same clock and sequence numbers to ease the

   o  Set up of a flexible MPLS monitoring system in terms of
      deployment: from one single centralized one to a set of
      distributed systems (e.g., on a per-region or service basis), and
      in terms of redundancy from 1+1 to N+1.

   In addition to monitoring paths, problem localization is required.
   Topology awareness is an important feature of link-state IGPs
   deployed by operators of large networks.  MPLS topology awareness
   combined with IGP topology awareness enables a simple and scalable
   data-plane-based monitoring mechanism.  Faults can be localized:

   o  by capturing the IGP topology and analyzing IGP messages
      indicating changes of it.

   o  by correlation between different SR-based monitoring probes.

   o  by setting up an MPLS traceroute packet for a path (or segment) to
      be tested and transporting it to a node to validate path
      connectivity from that node on.

   MPLS OAM offers flexible traceroute (connectivity verification)
   features to detect and execute data paths of an MPLS domain.  By
   utilizing the ECMP-related tool set offered, e.g., by RFC 8029
   [RFC8029], an SR-based MPLS monitoring system can be enabled to:

   o  detect how to route packets along different ECMP-routed paths.

   o  Construct ping packets that can be steered along a single path or
      ECMP towards a particular LER/LSR whose connectivity is to be

   o  limit the MPLS label stack of such a ping packet, checking
      continuity of every single IGP segment to the maximum number of 3
      labels.  A smaller label stack may also be helpful, if any router
      interprets a limited number of packet header bytes to determine an
      ECMP along which to route a packet.

   Alternatively, any path may be executed by building suitable label
   stacks.  This allows path execution without ECMP awareness.

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   The MPLS PMS may be any server residing at a single interface of the
   domain to be monitored.  The PMS doesn't need to support the complete
   MPLS routing or control plane.  It needs to be capable of learning
   and maintaining an accurate MPLS and IGP topology.  MPLS ping and
   traceroute packets need to be set up and sent with the correct
   segment stack.  The PMS must further be able to receive and decode
   returning ping or traceroute packets.  Packets from a variety of
   protocols can be used to check continuity.  These include Internet
   Control Message Protocol (ICMP) [RFC0792] [RFC4443] [RFC4884]
   [RFC4950], Bidirectional Forwarding Detection (BFD) [RFC5884],
   Seamless Bidirectional Forwarding Detection (S-BFD) [RFC7880]
   [RFC7881] (see Section 3.4 of [RFC7882]), and MPLS LSP ping
   [RFC8029].  They can also have any other OAM format supported by the
   PMS.  As long as the packet used to check continuity returns to the
   server while no IGP change is detected, the monitored path can be
   considered as validated.  If monitoring requires pushing a large
   label stack, a software-based implementation is usually more flexible
   than a hardware-based one.  Hence, router label stack depth and label
   composition limitations don't limit MPLS OAM choices.

   RFC 8287 [RFC8287] discusses SR OAM applicability and MPLS traceroute
   enhancements adding functionality to the use cases described by this

   The document describes both use cases and a standalone monitoring
   framework.  The monitoring system reuses existing IETF OAM protocols
   and leverage Segment Routing (Source Routing) to allow a single
   device to send, have exercised, and receive its own probing packets.
   As a consequence, there are no new interoperability considerations.
   A Standards Track RFC is not required; Informational status for this
   document is appropriate

2.  Terminology and Abbreviations

2.1.  Terminology

   Continuity Check

       See Section 2.2.7 of RFC 7276 [RFC7276].

   Connectivity Verification

       See Section 2.2.7 of RFC 7276 [RFC7276].

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   MPLS topology

       The MPLS topology of an MPLS domain is the complete set of MPLS-
       and IP-address information and all routing and data-plane
       information required to address and utilize every MPLS path
       within this domain from an MPLS PMS attached to this MPLS domain
       at an arbitrary access.  This document assumes availability of
       the MPLS topology (which can be detected with available protocols
       and interfaces).  None of the use cases will describe how to set
       it up.

   This document further adopts the terminology and framework described
   in [RFC8402].

2.2.  Abbreviations

   ECMP    Equal-Cost Multipath

   IGP     Interior Gateway Protocol

   LER     Label Edge Router

   LSP     Label Switched Path

   LSR     Label Switching Router

   OAM     Operations, Administration, and Maintenance

   PMS     Path Monitoring System

   RSVP-TE Resource Reservation Protocol - Traffic Engineering

   SID     Segment Identifier

   SR      Segment Routing

   SRGB    Segment Routing Global Block

3.  An MPLS Topology-Aware Path Monitoring System

   Any node at least listening to the IGP of an SR domain is MPLS
   topology aware (the node knows all related IP addresses, SR SIDs and
   MPLS labels).  An MPLS PMS that is able to learn the IGP Link State
   Database (LSDB) (including the SIDs) is able to execute arbitrary
   chains of LSPs.  To monitor an MPLS SR domain, a PMS needs to set up
   a topology database of the MPLS SR domain to be monitored.  It may be
   used to send ping-type packets to only check continuity along such a
   path chain based only on the topology information.  In addition, the

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   PMS can be used to trace MPLS LSP and, thus, verify their
   connectivity and correspondence between control and data planes,
   respectively.  The PMS can direct suitable MPLS traceroute packets to
   any node along a path segment.

   Let us describe how the PMS constructs a label stack to transport a
   packet to LER i, monitor its path to LER j, and then receive the
   packet back.

   The PMS may do so by sending packets carrying the following MPLS
   label stack information:

   o  Top Label: a path from PMS to LER i, which is expressed as Node-
      SID of LER i.

   o  Next Label: the path that needs to be monitored from LER i to LER
      j.  If this path is a single physical interface (or a bundle of
      connected interfaces), it can be expressed by the related Adj-SID.
      If the shortest path from LER i to LER j is supposed to be
      monitored, the Node-SID (LER j) can be used.  Another option is to
      insert a list of segments expressing the desired path (hop by hop
      as an extreme case).  If LER i pushes a stack of labels based on
      an SR policy decision and this stack of LSPs is to be monitored,
      the PMS needs an interface to collect the information enabling it
      to address this SR-created path.

   o  Next Label or address: the path back to the PMS.  Likely, no
      further segment/label is required here.  Indeed, once the packet
      reaches LER j, the 'steering' part of the solution is done, and
      the probe just needs to return to the PMS.  This is best achieved
      by popping the MPLS stack and revealing a probe packet with PMS as
      destination address (note that in this case, the source and
      destination addresses could be the same).  If an IP address is
      applied, no SID/label has to be assigned to the PMS (if it is a
      host/server residing in an IP subnet outside the MPLS domain).

   The PMS should be physically connected to a router that is part of
   the SR domain.  It must be able to send and receive MPLS packets via
   this interface.  As mentioned above, the routing protocol support
   isn't required, and the PMS itself doesn't have to be involved in IGP
   or MPLS routing.  A static route will do.  The option to connect a
   PMS to an MPLS domain by a tunnel may be attractive to some
   operators.  So far, MPLS separates networks securely by avoiding
   tunnel access to MPLS domains.  Tunnel-based access of a PMS to an
   MPLS domain is out of scope of this document, as it implies
   additional security aspects.

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4.  Illustration of an SR-Based Path Monitoring Use Case

4.1.  Use Case 1: LSP Data-Plane Monitoring

   Figure 1 shows an example of this functional component as a system,
   which can be physical or virtual.

                  +---+     +----+     +-----+
                  |PMS|     |LSR1|-----|LER i|
                  +---+     +----+     +-----+
                     |      /      \    /
                     |     /        \__/
                   +-----+/           /|
                   |LER m|           / |
                   +-----+\         /  \
                           \       /    \
                            \+----+     +-----+
                             |LSR2|-----|LER j|
                             +----+     +-----+

        Figure 1: Example of a PMS-Based LSP Data-Plane Monitoring

   For the sake of simplicity, let's assume that all the nodes are
   configured with the same SRGB [RFC8402].

   Let's assign the following Node-SIDs to the nodes of the figure:
   PMS = 10, LER i = 20, LER j = 30.

   The aim is to set up a continuity check of the path between LER i and
   LER j.  As has been said, the monitoring packets are to be sent and
   received by the PMS.  Let's assume the design aim is to be able to
   work with the smallest possible SR label stack.  In the given
   topology, a fairly simple option is to perform an MPLS path trace, as
   specified by RFC 8029 [RFC8029] (using the Downstream (Detailed)
   Mapping information resulting from a path trace).  The starting point
   for the path trace is LER i and the PMS sends the MPLS path trace
   packet to LER i.  The MPLS echo reply of LER i should be sent to the
   PMS.  As a result, the IP destination address choices are detected,
   which are then used to target any one of the ECMP-routed paths
   between LER i and LER j by the MPLS ping packets to later check path
   continuity.  The label stack of these ping packets doesn't need to
   consist of more than 3 labels.  Finally, the PMS sets up and sends
   packets to monitor connectivity of the ECMP routed paths.  The PMS
   does this by creating a measurement packet with the following label
   stack (top to bottom): 20 - 30 - 10.  The ping packets reliably use
   the monitored path, if the IP-address information that has been

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   detected by the MPLS traceroute is used as the IP destination address
   (note that this IP address isn't used or required for any IP

   LER m forwards the packet received from the PMS to LSR1.  Assuming
   Penultimate Hop Popping is deployed, LSR1 pops the top label and
   forwards the packet to LER i.  There the top label has a value 30 and
   LER i forwards it to LER j.  This will be done transmitting the
   packet via LSR1 or LSR2.  The LSR will again pop the top label.  LER
   j will forward the packet now carrying the top label 10 to the PMS
   (and it will pass a LSR and LER m).

   A few observations on the example given in Figure 1:

   o  The path from PMS to LER i must be available (i.e., a continuity
      check along the path to LER i must succeed).  If desired, an MPLS
      traceroute may be used to exactly detect the data-plane path taken
      for this MPLS segment.  It is usually sufficient to just apply any
      of the existing Shortest Path routed paths.

   o  If ECMP is deployed, separate continuity checks monitoring all
      possible paths that a packet may use between LER i and LER j may
      be desired.  This can be done by applying an MPLS traceroute
      between LER i and LER j.  Another option is to use SR, but this
      will likely require additional label information within the label
      stack of the ping packet.  Further, if multiple links are deployed
      between two nodes, SR methods to address each individual path
      require an Adj-SID to be assigned to each single interface.  This
      method is based on control-plane information -- a connectivity
      verification based on MPLS traceroute seems to be a fairly good
      option to deal with ECMP and validation of correlation between
      control and data planes.

   o  The path LER j to PMS must be available (i.e., a continuity check
      only along the path from LER j to PMS must succeed).  If desired,
      an MPLS traceroute may be used to exactly detect the data-plane
      path taken for this MPLS segment.  It is usually sufficient to
      just apply any of the existing Shortest Path routed paths.

   Once the MPLS paths (Node-SIDs) and the required information to deal
   with ECMP have been detected, the path continuity between LER i and
   LER j can be monitored by the PMS.  Path continuity monitoring by
   ping packets does not require the MPLS OAM functionality described in
   RFC 8029 [RFC8029].  All monitoring packets stay on the data plane;
   hence, path continuity monitoring does not require control-plane
   interaction in any LER or LSR of the domain.  To ensure consistent
   interpretation of the results, the PMS should be aware of any changes
   in IGP or MPLS topology or ECMP routing.  While this document

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   describes path connectivity checking as a basic application,
   additional monitoring (like checking continuity of underlying
   physical infrastructure or performing delay measurements) may be
   desired.  A change in ECMP routing that is not caused by an IGP or
   MPLS topology change may not be desirable for connectivity checks and
   delay measurements.  Therefore, a PMS should also periodically verify
   connectivity of the SR paths that are monitored for continuity.

   Determining a path to be executed prior to a measurement may also be
   done by setting up a label stack including all Node-SIDs along that
   path (if LSR1 has Node-SID 40 in the example and it should be passed
   between LER i and LER j, the label stack is 20 - 40 - 30 - 10).  The
   advantage of this method is that it does not involve connectivity
   verification as specified in RFC 8029 [RFC8029] and, if there's only
   one physical connection between all nodes, the approach is
   independent of ECMP functionalities.  The method still is able to
   monitor all link combinations of all paths of an MPLS domain.  If
   correct forwarding along the desired paths has to be checked, or
   multiple physical connections exist between any two nodes, all Adj-
   SIDs along that path should be part of the label stack.

   While a single PMS can detect the complete MPLS control- and data-
   plane topology, a reliable deployment requires two separated PMSs.
   Scalable permanent surveillance of a set of LSPs could require
   deployment of several PMSs.  The PMS may be a router, but could also
   be a dedicated monitoring system.  If measurement system reliability
   is an issue, more than a single PMS may be connected to the MPLS

   Monitoring an MPLS domain by a PMS based on SR offers the option of
   monitoring complete MPLS domains with limited effort and a unique
   possibility to scale a flexible monitoring solution as required by
   the operator (the number of PMSs deployed is independent of the
   locations of the origin and destination of the monitored paths).  The
   PMS can be enabled to send MPLS OAM packets with the label stacks and
   address information identical to those of the monitoring packets to
   any node of the MPLS domain.  The routers of the monitored domain
   should support MPLS LSP ping RFC 8029 [RFC8029].  They may also
   incorporate the additional enhancements defined in RFC 8287 [RFC8287]
   to incorporate further MPLS traceroute features.  ICMP-ping-based
   continuity checks don't require router-control-plane activity.  Prior
   to monitoring a path, MPLS OAM may be used to detect ECMP-dependent
   forwarding of a packet.  A PMS may be designed to learn the IP
   address information required to execute a particular ECMP-routed path
   and interfaces along that path.  This allows for the monitoring of
   these paths with label stacks reduced to a limited number of Node-

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   SIDs resulting from Shortest Path First (SPF) routing.  The PMS does
   not require access to information about LSR/LER management or data
   planes to do so.

4.2.  Use Case 2: Monitoring a Remote Bundle

               +---+    _   +--+                    +-------+
               |   |   { }  |  |---991---L1---662---|       |
               |PMS|--{   }-|R1|---992---L2---663---|R2 (72)|
               |   |   {_}  |  |---993---L3---664---|       |
               +---+        +--+                    +-------+

      Figure 2: SR-Based Probing of All the Links of a Remote Bundle

   In the figure, R1 addresses Link "x" Lx by the Adj-SID 99x, while R2
   addresses Link Lx by the Adj-SID 66(x+1).

   In the above figure, the PMS needs to assess the data-plane
   availability of all the links within a remote bundle connected to
   routers R1 and R2.

   The monitoring system retrieves the SID/label information from the
   IGP LSDB and appends the following segment list/label stack: {72,
   662, 992, 664} on its IP probe (whose source and destination
   addresses are the address of the PMS).

   The PMS sends the probe to its connected router.  The MPLS/SR domain
   then forwards the probe to R2 (72 is the Node-SID of R2).  R2
   forwards the probe to R1 over link L1 (Adj-SID 662).  R1 forwards the
   probe to R2 over link L2 (Adj-SID 992).  R2 forwards the probe to R1
   over link L3 (Adj-SID 664).  R1 then forwards the IP probe to the PMS
   as per classic IP forwarding.

   As was mentioned in Section 4.1, the PMS must be able to monitor the
   continuity of the path PMS to R2 (Node-SID 72) as well as the
   continuity from R1 to the PMS.  If both are given and packets are
   lost, forwarding on one of the three interfaces connecting R1 to R2
   must be disturbed.

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4.3.  Use Case 3: Fault Localization

   In the previous example, a unidirectional fault on the middle link in
   direction of R2 to R1 would be localized by sending the following two
   probes with respective segment lists:

   o  72, 662, 992, 664

   o  72, 663, 992, 664

   The first probe would succeed while the second would fail.
   Correlation of the measurements reveals that the only difference is
   using the Adj-SID 663 of the middle link from R2 to R1 in the
   unsuccessful measurement.  Assuming the second probe has been routed
   correctly, the problem is that, for some (possibly unknown) reason,
   SR packets to be forwarded from R2 via the interface identified by
   Adj-SID 663 are lost.

   The example above only illustrates a method to localize a fault by
   correlated continuity checks.  Any operational deployment requires
   well-designed engineering to allow for the desired unambiguous
   diagnosis on the monitored section of the SR network.  'Section' here
   could be a path, a single physical interface, the set of all links of
   a bundle, or an adjacency of two nodes (just to name a few).

5.  Path Trace and Failure Notification

   Sometimes, forwarding along a single path doesn't work, even though
   the control-plane information is healthy.  Such a situation may occur
   after maintenance work within a domain.  An operator may perform on-
   demand tests, but execution of automated PMS path trace checks may be
   set up as well (scope may be limited to a subset of important end-to-
   end paths crossing the router or network section after completion of
   the maintenance work there).  Upon detection of a path that can't be
   used, the operator needs to be notified.  A check ensuring that a re-
   routing event is differed from a path facing whose forwarding
   behavior doesn't correspond to the control-plane information is
   necessary (but out of scope of this document).

   Adding an automated problem solution to the PMS features only makes
   sense if the root cause of the symptom appears often, can be assumed
   to be unambiguous by its symptoms, can be solved by a predetermined
   chain of commands, is not collaterally damaged by the automated PMS
   reaction.  A closer analysis is out of scope of this document.

   The PMS is expected to check control-plane liveliness after a path
   repair effort was executed.  It doesn't matter whether the path
   repair was triggered manually or by an automated system.

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6.  Applying SR to Monitoring LSPs That Are Not SR Based (LDP and
    Possibly RSVP-TE)

   The MPLS PMS described by this document can be realized with
   technology that is not SR based.  Making such a monitoring system
   that is not SR MPLS based aware of a domain's complete MPLS topology
   requires, e.g., management-plane access to the routers of the domain
   to be monitored or set up of a dedicated tLDP tunnel per router to
   set up an LDP adjacency.  To avoid the use of stale MPLS label
   information, the IGP must be monitored and MPLS topology must be
   aligned with IGP topology in a timely manner.  Enhancing IGPs to the
   exchange of MPLS-topology information as done by SR significantly
   simplifies and stabilizes such an MPLS PMS.

   An SR-based PMS connected to an MPLS domain consisting of LER and
   LSRs supporting SR and LDP or RSVP-TE in parallel in all nodes may
   use SR paths to transmit packets to and from the start and endpoints
   of LSPs that are not SR based to be monitored.  In the example given
   in Figure 1, the label stack top to bottom may be as follows, when
   sent by the PMS:

   o  Top: SR-based Node-SID of LER i at LER m.

   o  Next: LDP or RSVP-TE label identifying the path or tunnel,
      respectively, from LER i to LER j (at LER i).

   o  Bottom: SR-based Node-SID identifying the path to the PMS at LER

   While the mixed operation shown here still requires the PMS to be
   aware of the LER LDP-MPLS topology, the PMS may learn the SR MPLS
   topology by the IGP and use this information.

   An implementation report on a PMS operating in an LDP domain is given
   in [MPLS-PMS-REPORT].  In addition, this report compares delays
   measured with a single PMS to the results measured by systems that
   are conformant with IP Performance Metrics (IPPM) connected to the
   MPLS domain at three sites (see [RFC6808] for IPPM conformance).  The
   delay measurements of the PMS and the IPPM Measurement Agents were
   compared based on a statistical test in [RFC6576].  The Anderson
   Darling k-sample test showed that the PMS round-trip delay
   measurements are equal to those captured by an IPPM-conformant IP
   measurement system for 64 Byte measurement packets with 95%

   The authors are not aware of similar deployment for RSVP-TE.
   Identification of tunnel entry- and transit-nodes may add complexity.
   They are not within scope of this document.

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7.  PMS Monitoring of Different Segment ID Types

   MPLS SR topology awareness should allow the PMS to monitor liveliness
   of SIDs related to interfaces within the SR and IGP domain,
   respectively.  Tracing a path where an SR-capable node assigns an
   Adj-SID for a node that is not SR capable may fail.  This and other
   backward compatibility with non-SR devices are discussed by RFC 8287

   To match control-plane information with data-plane information for
   all relevant types of Segment IDs, RFC 8287 [RFC8287] enhances MPLS
   OAM functions defined by RFC 8029 [RFC8029].

8.  Connectivity Verification Using PMS

   While the PMS-based use cases explained in Section 5 are sufficient
   to provide continuity checks between LER i and LER j, they may not
   help perform connectivity verification.

                      +----+     +----+     +-----+
                      |LSRa|-----|LSR1|-----|LER i|
                      +----+     +----+     +-----+
                         |      /      \    /
                         |     /        \__/
                       +-----+/           /|
                       |LER m|           / |
                       +-----+\         /  \
                               \       /    \
                                \+----+     +-----+
                                 |LSR2|     |LER j|
                                 +----+     +-----+

              Figure 3: Connectivity Verification with a PMS

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   Let's assign the following Node-SIDs to the nodes of the figure:
   PMS = 10, LER i = 20, LER j = 30, LER m = 40.  The PMS is intended to
   validate the path between LER m and LER j.  In order to validate this
   path, the PMS will send the probe packet with a label stack of (top
   to bottom): {40} {30} {10}.  Imagine any of the below forwarding
   entry misprogrammed situation:

   o  LSRa receiving any packet with top label 40 will POP and forwards
      to LSR1 instead of LER m.

   o  LSR1 receiving any packet with top label 30 will pop and forward
      to LER i instead of LER j.

   In either of the above situations, the probe packet will be delivered
   back to the PMS leading to a falsified path liveliness indication by
   the PMS.

   Connectivity Verification functions help us to verify if the probe is
   taking the expected path.  For example, the PMS can intermittently
   send the probe packet with a label stack of (top to bottom):
   {40;ttl=255} {30;ttl=1} {10;ttl=255}.  The probe packet may carry
   information about LER m, which could be carried in the Target FEC
   Stack in case of an MPLS Echo Request or Discriminator in the case of
   Seamless BFD.  When LER m receives the packet, it will punt due to
   Time-To-Live (TTL) expiry and send a positive response.  In the
   above-mentioned misprogramming situation, LSRa will forward to LSR1,
   which will send a negative response to the PMS as the information in
   probe does not match the local node.  The PMS can do the same for
   bottom label as well.  This will help perform connectivity
   verification and ensure that the path between LER m and LER j is
   working as expected.

9.  IANA Considerations

   This document has no IANA actions.

10.  Security Considerations

   The PMS builds packets with the intent of performing OAM tasks.  It
   uses address information based on topology information rather than a

   The PMS allows the insertion of traffic into non-SR domains.  This
   may be required in the case of an LDP domain attached to the SR
   domain, but it can be used to maliciously insert traffic in the case
   of external IP domains and MPLS-based VPNs.

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   To prevent a PMS from inserting traffic into an MPLS VPN domain, one
   or more sets of label ranges may be reserved for service labels
   within an SR domain.  The PMS should be configured to reject usage of
   these service label values.  In the same way, misuse of IP
   destination addresses is blocked if only IP destination address
   values conforming to RFC 8029 [RFC8029] are settable by the PMS.

   To limit potential misuse, access to a PMS needs to be authorized and
   should be logged.  OAM supported by a PMS requires skilled personnel;
   hence, only experts requiring PMS access should be allowed to access
   such a system.  It is recommended to directly attach a PMS to an SR
   domain.  Connecting a PMS to an SR domain by a tunnel is technically
   possible, but adds further security issues.  A tunnel-based access of
   a PMS to an SR domain is not recommended.

   Use of stale MPLS or IGP routing information could cause a PMS-
   monitoring packet to leave the domain where it originated.  PMS-
   monitoring packets should not be sent using stale MPLS- or IGP-
   routing information.  To carry out a desired measurement properly,
   the PMS must be aware of and respect the actual route changes,
   convergence events, as well as the assignment of Segment IDs relevant
   for measurements.  At a minimum, the PMS must be able to listen to
   IGP topology changes or pull routing and segment information from
   routers signaling topology changes.

   Traffic insertion by a PMS may be unintended, especially if the IGP
   or MPLS topology stored locally is in stale state.  As soon as the
   PMS has an indication that its IGP or MPLS topology are stale, it
   should stop operations involving network sections whose topology may
   not be accurate.  However, note that it is the task of an OAM system
   to discover and locate network sections where forwarding behavior is
   not matching control-plane state.  As soon as a PMS or an operator of
   a PMS has the impression that the PMS topology information is stale,
   measures need to be taken to refresh the topology information.  These
   measures should be part of the PMS design.  Matching forwarding and
   control-plane state by periodically automated execution of the
   mechanisms described in RFC 8029 [RFC8029] may be such a feature.
   Whenever network maintenance tasks are performed by operators, the
   PMS topology discovery should be started asynchronously after network
   maintenance has been finished.

   A PMS that is losing network connectivity or crashing must remove all
   IGP- and MPLS-topology information prior to restarting operation.

   A PMS may operate routine measurements on a large scale.  Care must
   be taken to avoid unintended traffic insertion after topology changes
   that result in, e.g., changes of label assignments to routes or
   interfaces within a domain.  If the labels concerned are part of the

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   label stack composed by the PMS for any measurement packet and their
   state is stale, the measurement initially needs to be stopped.  Setup
   and operation of routine measurements may be automated.  Secure
   automated PMS operation requires a working automated detection and
   recognition of stale routing state.

11.  References

11.1.  Normative References

   [RFC7276]  Mizrahi, T., Sprecher, N., Bellagamba, E., and Y.
              Weingarten, "An Overview of Operations, Administration,
              and Maintenance (OAM) Tools", RFC 7276,
              DOI 10.17487/RFC7276, June 2014,

   [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, <>.

11.2.  Informative References

              Leipnitz, R., Ed. and R. Geib, "A scalable and topology
              aware MPLS data plane monitoring system", Work in
              Progress, draft-leipnitz-spring-pms-implementation-
              report-00, June 2016.

   [RFC0792]  Postel, J., "Internet Control Message Protocol", STD 5,
              RFC 792, DOI 10.17487/RFC0792, September 1981,

   [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,

   [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,

   [RFC4950]  Bonica, R., Gan, D., Tappan, D., and C. Pignataro, "ICMP
              Extensions for Multiprotocol Label Switching", RFC 4950,
              DOI 10.17487/RFC4950, August 2007,

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RFC 8403                SR MPLS Monitoring System              July 2018

   [RFC5884]  Aggarwal, R., Kompella, K., Nadeau, T., and G. Swallow,
              "Bidirectional Forwarding Detection (BFD) for MPLS Label
              Switched Paths (LSPs)", RFC 5884, DOI 10.17487/RFC5884,
              June 2010, <>.

   [RFC6576]  Geib, R., Ed., Morton, A., Fardid, R., and A. Steinmitz,
              "IP Performance Metrics (IPPM) Standard Advancement
              Testing", BCP 176, RFC 6576, DOI 10.17487/RFC6576, March
              2012, <>.

   [RFC6808]  Ciavattone, L., Geib, R., Morton, A., and M. Wieser, "Test
              Plan and Results Supporting Advancement of RFC 2679 on the
              Standards Track", RFC 6808, DOI 10.17487/RFC6808, December
              2012, <>.

   [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,

   [RFC7881]  Pignataro, C., Ward, D., and N. Akiya, "Seamless
              Bidirectional Forwarding Detection (S-BFD) for IPv4, IPv6,
              and MPLS", RFC 7881, DOI 10.17487/RFC7881, July 2016,

   [RFC7882]  Aldrin, S., Pignataro, C., Mirsky, G., and N. Kumar,
              "Seamless Bidirectional Forwarding Detection (S-BFD) Use
              Cases", RFC 7882, DOI 10.17487/RFC7882, July 2016,

   [RFC8029]  Kompella, K., Swallow, G., Pignataro, C., Ed., Kumar, N.,
              Aldrin, S., and M. Chen, "Detecting Multiprotocol Label
              Switched (MPLS) Data-Plane Failures", RFC 8029,
              DOI 10.17487/RFC8029, March 2017,

   [RFC8287]  Kumar, N., Ed., Pignataro, C., Ed., Swallow, G., Akiya,
              N., Kini, S., and M. Chen, "Label Switched Path (LSP)
              Ping/Traceroute for Segment Routing (SR) IGP-Prefix and
              IGP-Adjacency Segment Identifiers (SIDs) with MPLS Data
              Planes", RFC 8287, DOI 10.17487/RFC8287, December 2017,

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RFC 8403                SR MPLS Monitoring System              July 2018


   The authors would like to thank Nobo Akiya for his contribution.
   Raik Leipnitz kindly provided an editorial review.  The authors would
   also like to thank Faisal Iqbal for an insightful review and a useful
   set of comments and suggestions.  Finally, Bruno Decraene's Document
   Shepherd review led to a clarified document.

Authors' Addresses

   Ruediger Geib (editor)
   Deutsche Telekom
   Heinrich Hertz Str. 3-7
   Darmstadt  64295

   Phone: +49 6151 5812747

   Clarence Filsfils
   Cisco Systems, Inc.


   Carlos Pignataro (editor)
   Cisco Systems, Inc.
   7200 Kit Creek Road
   Research Triangle Park, NC  27709-4987
   United States of America


   Nagendra Kumar
   Cisco Systems, Inc.
   7200 Kit Creek Road
   Research Triangle Park, NC  27709-4987
   United States of America


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