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Versions: (draft-geib-spring-oam-usecase) 00 01 02 03 04 05 06

spring                                                      R. Geib, Ed.
Internet-Draft                                          Deutsche Telekom
Intended status: Informational                               C. Filsfils
Expires: August 25, 2017                               C. Pignataro, Ed.
                                                                N. Kumar
                                                     Cisco Systems, Inc.
                                                       February 21, 2017


     A Scalable and Topology-Aware MPLS Dataplane Monitoring System
                    draft-ietf-spring-oam-usecase-06

Abstract

   This document describes features of a 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 LSP path trace, in a very
   complementary way.  MPLS topology awareness reduces management and
   control plane involvement of OAM measurements while enabling new OAM
   features.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on August 25, 2017.

Copyright Notice

   Copyright (c) 2017 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
   (http://trustee.ietf.org/license-info) in effect on the date of



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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 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.  Acronyms  . . . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   5
   4.  An MPLS Topology-Aware Path Monitoring System . . . . . . . .   6
   5.  SR-based Path Monitoring Use Case Illustration  . . . . . . .   7
     5.1.  Use Case 1 - LSP Dataplane Monitoring . . . . . . . . . .   7
     5.2.  Use Case 2 - Monitoring a Remote Bundle . . . . . . . . .  10
     5.3.  Use Case 3 - Fault Localization . . . . . . . . . . . . .  11
   6.  Failure Notification from PMS to LERi . . . . . . . . . . . .  11
   7.  Applying SR to Monitoring non-SR based LSPs (LDP and possibly
       RSVP-TE)  . . . . . . . . . . . . . . . . . . . . . . . . . .  11
   8.  PMS Monitoring of Different Segment ID Types  . . . . . . . .  12
   9.  Connectivity Verification Using PMS . . . . . . . . . . . . .  13
   10. Extensions of Specifications Relevant to this Use Case  . . .  13
   11. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  13
   12. Security Considerations . . . . . . . . . . . . . . . . . . .  13
   13. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  14
   14. References  . . . . . . . . . . . . . . . . . . . . . . . . .  14
     14.1.  Normative References . . . . . . . . . . . . . . . . . .  14
     14.2.  Informative References . . . . . . . . . . . . . . . . .  14
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  15

1.  Acronyms

   ECMP    Equal-Cost Multi-Path

   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



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   SID     Segment Identifier

   SR      Segment Routing

   SRGB    Segment Routing Global Block

2.  Introduction

   It is essential for a network operator to monitor all the forwarding
   paths observed by the transported user packets.  Monitoring packets
   are expected to be forwarded in dataplane in a similar way as user
   packets.  Segment Routing enables forwarding of packets along pre-
   defined paths and segments and thus a Segment Routed monitoring
   packet can stay in dataplane while passing along one or more segments
   to be monitored.

   This document describes a system using MPLS data plane path
   monitoring capabilities.  The use cases introduced here are limited
   to a single Interior Gateway Protocol (IGP) MPLS domain.

   The system applies to monitoring of pre Segment Routing LSP's ( like
   LDP) as well as to monitoring of Segment Routed LSP's (section 7
   offers some more information).  As compared to pre Segment Routing
   approaches, Segment Routing is expected to simplify such a monitoring
   system by enabling MPLS topology detection based on IGP signaled
   segments as specified by specified by
   [I-D.ietf-isis-segment-routing-extensions],
   [I-D.ietf-ospf-segment-routing-extensions] and
   [I-D.ietf-idr-bgp-ls-segment-routing-ext].  Thus a centralised and
   MPLS topology aware monitoring unit can be realized in a Segment
   Routed domain.  This topology awareness can be used for OAM purposes
   as described by this document.

   The system offers several benefits for network monitoring:

   o  A single centralized MPLS monitoring system which is able to
      perform a continuity check (ping) along all Label Switched Paths
      of the SR domain.

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

   o  SR allows to transport MPLS path trace or connectivity validation
      packets for any existing Label Switched Path to all nodes of an SR
      domain.  This use case doesn't describe any new path trace
      features, but the system described here allows to set up an SR
      domain wide centralised connectivity validation.




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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 sending probing packets representing customer traffic,
      possibly from multiple services (e.g., small VoIP packet, larger
      HTTP packets) and embedding of useful monitoring data (e.g.,
      accurate time stamps since both sender and receiver have the same
      clock, sequence numbers to ease the measurement...).

   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 base), and
      in terms of redundancy from 1+1 to N+1.

   In addition to monitoring paths, problem localization is required.
   Faults can be localized:

   o  by capturing the Interior Gateway Protocol (IGP) topology and
      analysing 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.

   Topology awareness is an essential part of link state IGPs.  Adding
   MPLS topology awareness to an IGP speaking device hence enables a
   simple and scalable data plane based monitoring mechanism.

   MPLS OAM offers flexible traceroute (connectivity verification)
   features to recognise and execute data paths of an MPLS domain.  By
   utilising the ECMP related tool set offered, e.g., by RFC 4379
   [RFC4379], a SR based MPLS monitoring system can be enabled to:

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

   o  construct ping packets, which can be precisely steered to paths
      whose connectivity is to be checked, also if ECMP is present.

   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 path 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 Path Monitoring System 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 to learn and maintain 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 further must be able to receive and
   decode returning ping or traceroute packets.  Packets used to check
   continuity could have BFD or LSP Ping format, or have any other OAM
   format supported by the PMS.  As long as the packet used to check
   continuity returns back 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 an hardware based one.
   Hence router label stack depth and label composition limitations
   don't limit MPLS OAM choices.

   Documents discussing SR OAM requirements and MPLS traceroute
   enhancements adding functionality to the use cases described by this
   document are in work within IETF, see
   [I-D.ietf-spring-sr-oam-requirement] and
   [I-D.draft-ietf-mpls-spring-lsp-ping].

3.  Terminology

   Continuity Check

       RFC 7276 [RFC7276] defines Continuity Checks to be used to verify
       that a destination is reachable, and are typically sent
       proactively, though they can be invoked on-demand as well.
       Segment Routing allows to realise a continuity check along any
       given SR domain path within data plane.

   Connectivity Verification

       RFC 7276 [RFC7276] defines Connectivity Verification as a
       mechanism to check connectivity between two nodes by checking
       whether a path between both can be used.  RFC 4379 [RFC4379]
       specifies a Connectivity Verification for MPLS domains.  As RFC
       7276 states, Connectivity Verification and Continuity Checks are
       considered complementary mechanisms and are often used in
       conjunction with each other.  This document proposes the use of
       SR based network monitoring as a new Continuity Check method.  In
       some special cases, it also covers some limited Connectivity
       Verification.  When applicable, this is indicated in the
       description of the use case.

   MPLS topology




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       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 utilise every MPLS path
       within this domain from an MPLS Path Monitoring System 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 [I-D.ietf-spring-segment-routing].

4.  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 which is able to learn the IGP LSDB
   (including the SID's) is able to execute arbitrary chains of label
   switched paths.  To monitor an MPLS SR domain, a PMS needs to set up
   a topology data base 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 on the topology information only.  In addition,
   the PMS can be used to trace MPLS Label Switched Path and thus verify
   their connectivity and correspondence between control and data plane,
   respectively.  The PMS can direct suitable MPLS traceroute packets to
   any node along a path segment.

   Let us describe how the PMS constructs a labels 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
      Adjacency-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 a 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.




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   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 which is part of
   the SR domain.  It must be able to send and receive MPLS packets via
   this interface.  As mentioned above, 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.  Further options, like
   deployment of a PMS connecting to the MPLS domain by a tunnel only
   require more thought, as this implies security aspects.  MPLS so far
   separates networks securely by avoiding tunnel access to MPLS
   domains.

5.  SR-based Path Monitoring Use Case Illustration

5.1.  Use Case 1 - LSP Dataplane Monitoring

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

   Example of a PMS based LSP dataplane monitoring

                                 Figure 1

   For the sake of simplicity, let's assume that all the nodes are
   configured with the same SRGB [I-D.ietf-spring-segment-routing].

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





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   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 RFC4379 (using the Downstream (Detailed) Mapping
   information resulting from a "tree trace", see [RFC4379]).  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, 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 which has been
   detected by the MPLS trace route is used as the IP destination
   address (note that this IP address isn't used or required for any IP
   routing).

   LER m forwards the packet received from the PMS to LSR1.  Assuming
   Pen-ultimate Hop Popping to be 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 PMS to LER i must be available (i.e., a continuity check
      only along the path to LER i must succeed).  If desired, an MPLS
      trace route 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 which a packet may use between LER i and LER j may
      be desired.  This can be done by applying an MPLS trace route
      between LER i and LER j.  Another option is to use SR routing, 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



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      option to deal with ECMP and validation of control and data plane
      correlation.

   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 trace route 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 RFC4379 MPLS OAM functionality.  All
   monitoring packets stay on dataplane, 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 the description given here
   pronouncing path connectivity checking as a simple basic application,
   others like checking continuity of underlying physical infrastructure
   or delay measurements may be desired.  In both cases, a change in
   ECMP routing which is not caused by an IGP or MPLS topology change
   may not be desirable.  A PMS therefore should also periodically
   verify connectivity of the SR paths which 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 RFC 4379
   connectivity verification 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.

   In theory at least, a single PMS is able to monitor data plane
   availability of all LSPs in the domain.  The PMS may be a router, but
   could also be dedicated monitoring system.  If measurement system
   reliability is an issue, more than a single PMS may be connected to
   the MPLS domain.

   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



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   the operator (the number of PMS 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 RFC 4379 and its standardised extensions to allow for
   MPLS trace route.  Ping based continuity checks don't require router
   control plane activity.  Prior to monitoring a path, MPLS OAM may be
   used to detect ECMP dependant 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 to monitor these paths with label stacks reduced to a limited
   number of Node-SIDs resulting from SPF routing.  The PMS does not
   require access to LSR / LER management- or data-plane information to
   do so.

5.2.  Use Case 2 - Monitoring a Remote Bundle


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


   SR based probing of all the links of a remote bundle

                                 Figure 2

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

   In the above figure, the PMS needs to assess the dataplane
   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).

   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 (Adjacency SID 662).  R1 forwards the probe
   to R2 over link L2 (Adjacency SID 992).  R2 forwards the probe to R1
   over link L3 (Adjacency SID 664).  R1 then forwards the IP probe to
   PMS as per classic IP forwarding.



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   As has been mentioned in section 5.1, the PMS must be able monitor
   continuity of the path PMS to R2 (Node-SID 72) as well as 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.

5.3.  Use Case 3 - Fault Localization

   In the previous example, a uni-directional 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 Adjacency SID 663 of the middle link from R2 to R1 in the
   non successful measurement.  Assuming the second probe has been
   routed correctly, the fault must have been occurring in R2 which
   didn't forward the packet to the interface identified by its
   Adjacency SID 663.

   The example above only illustrates a method to localise a fault by
   correlated continuity checks.  Any operational deployment requires a
   well designed engineering to allow for the desired non ambiguous
   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.  Such a
   design is not within scope of this document.

6.  Failure Notification from PMS to LERi

   PMS on detecting any failure in the path liveliness may use any out-
   of-band mechanism to signal the failure to LER i.  This document does
   not propose any specific mechanism and operators can choose any
   existing or new approach.

   Alternately, the Operator may log the failure in local monitoring
   system and take necessary action by manual intervention.

7.  Applying SR to Monitoring non-SR based LSPs (LDP and possibly RSVP-
    TE)

   The MPLS path monitoring system described by this document can be
   realised with pre-Segment Routing (SR) based technology.  Making such
   a pre-SR MPLS monitoring system aware of a domain's complete MPLS



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   topology requires, e.g., management plane access to the routers of
   the domain to be monitored or set up of a dedicated T-LDP 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 timely aligned with IGP topology.  Obviously, enhancing IGPs to
   exchange of MPLS topology information as done by SR significantly
   simplifies and stabilises such an MPLS path monitoring system.

   A SR based PMS connected to a MPLS domain consisting of LER and LSR
   supporting SR and LDP or RSVP-TE in parallel in all nodes may use SR
   paths to transmit packets to and from start and end points of non-SR
   based LSP paths to be monitored.  In the above example, 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 j

   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 IGP and use this information.

   An implementation report on a PMS operating in an LDP domain is given
   in [I-D.leipnitz-spring-pms-implementation-report].  In addition,
   this report compares delays measured with a single PMS to the results
   measured by three IP Performance Measurement Work Group (IPPM WG)
   standard conformant Measurement Agents (connected to an MPLS domain
   at three different sites).  The delay measurements of the PMS and the
   IPPM Measurement Agents were compared based on a statistical test
   published by the IPPM WG[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% confidence.

   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.

8.  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 non-SR-capable node may fail.  This and other backward




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   compatibility with non Segment Routing devices are discussed by
   [I-D.draft-ietf-mpls-spring-lsp-ping].

   To match control plane information with data plane information for
   all relevant types of Segment IDs,
   [I-D.draft-ietf-mpls-spring-lsp-ping]enhances MPLS OAM functions
   defined by RFC 4379 [RFC4379].

9.  Connectivity Verification Using PMS

   While the PMS based use cases explained in Section 5 are sufficient
   to provide continuity check between LER i and LER j, it may not help
   perform connectivity verification.  So in some cases like data plane
   programming corruption, it is possible that a transit node between
   LER i and LER j erroneously removes the top segment ID and forwards a
   monitoring packet to the PMS based on the bottom segment ID leading
   to a falsified path liveliness indication by the PMS.

   There are various method to perform basic connectivity verification
   like intermittently setting the TTL to 1 in bottom label so LER j
   selectively perform connectivity verification.  Other methods are
   possible and may be added when requirements and solutions are
   specified.

10.  Extensions of Specifications Relevant to this Use Case

   The following activities are welcome enhancements supporting this use
   case, but they are not part of it:

   RFC4379 [RFC4379] functions should be extended to support Flow- and
   Entropy Label based ECMP.

11.  IANA Considerations

   This memo includes no request to IANA.

12.  Security Considerations

   As mentioned in the introduction, a PMS monitoring packet should
   never leave the domain where it originated.  It therefore should
   never use stale MPLS or IGP routing information.  Further, assigning
   different label ranges for different purposes may be useful.  A well
   known global service level range may be excluded for utilisation
   within PMS measurement packets.  These ideas shouldn't start a
   discussion.  They rather should point out, that such a discussion is
   required when SR based OAM mechanisms like a SR are standardised.





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   Should the approach of a PMS connected to an SR domain by a tunnel be
   picked up, some fundamental MPLS security properties need to be
   discussed.  MPLS domains so far allow to separate the MPLS network
   from an IP network by allowing no tunneled MPLS access to an MPLS
   domain.

13.  Acknowledgements

   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 shepherd
   review led to a clarified document.

14.  References

14.1.  Normative References

   [RFC4379]  Kompella, K. and G. Swallow, "Detecting Multi-Protocol
              Label Switched (MPLS) Data Plane Failures", RFC 4379,
              DOI 10.17487/RFC4379, February 2006,
              <http://www.rfc-editor.org/info/rfc4379>.

   [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,
              <http://www.rfc-editor.org/info/rfc7276>.

14.2.  Informative References

   [I-D.draft-ietf-mpls-spring-lsp-ping]
              IETF, "Label Switched Path (LSP) Ping/Trace for Segment
              Routing Networks Using MPLS Dataplane", IETF,
              https://datatracker.ietf.org/doc/draft-ietf-mpls-spring-
              lsp-ping/, 2016.

   [I-D.ietf-idr-bgp-ls-segment-routing-ext]
              IETF, "BGP Link-State extensions for Segment Routing",
              IETF,  https://datatracker.ietf.org/doc/draft-ietf-idr-
              bgp-ls-segment-routing-ext/, 2016.

   [I-D.ietf-isis-segment-routing-extensions]
              IETF, "IS-IS Extensions for Segment Routing", IETF,
              https://datatracker.ietf.org/doc/draft-ietf-isis-segment-
              routing-extensions/, 2016.





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   [I-D.ietf-ospf-segment-routing-extensions]
              IETF, "OSPF Extensions for Segment Routing", IETF,
              https://datatracker.ietf.org/doc/draft-ietf-ospf-segment-
              routing-extensions/, 2016.

   [I-D.ietf-spring-segment-routing]
              IETF, "Segment Routing Architecture", IETF,
              https://datatracker.ietf.org/doc/draft-ietf-spring-
              segment-routing/, 2016.

   [I-D.ietf-spring-sr-oam-requirement]
              IETF, "OAM Requirements for Segment Routing Network",
              IETF,  https://datatracker.ietf.org/doc/draft-ietf-spring-
              sr-oam-requirement/, 2016.

   [I-D.leipnitz-spring-pms-implementation-report]
              Leipnitz, R. and R. Geib, "A scalable and topology aware
              MPLS data plane monitoring system", IETF,  draft-leipnitz-
              spring-pms-implementation-report-00, 2016.

   [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, <http://www.rfc-editor.org/info/rfc6576>.

Authors' Addresses

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

   Phone: +49 6151 5812747
   Email: Ruediger.Geib@telekom.de


   Clarence Filsfils
   Cisco Systems, Inc.
   Brussels
   Belgium

   Email: cfilsfil@cisco.com








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

   Email: cpignata@cisco.com


   Nagendra Kumar
   Cisco Systems, Inc.
   7200 Kit Creek Road
   Research Triangle Park, NC  27709
   US

   Email: naikumar@cisco.com



































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