A Connectivity Monitoring Metric for IPPM
draft-ietf-ippm-connectivity-monitoring-01

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ippm                                                        R. Geib, Ed.
Internet-Draft                                          Deutsche Telekom
Intended status: Standards Track                       February 22, 2021
Expires: August 26, 2021

               A Connectivity Monitoring Metric for IPPM
               draft-ietf-ippm-connectivity-monitoring-01

Abstract

   Within a Segment Routing domain, segment routed measurement packets
   can be sent along pre-determined paths.  This enables new kinds of
   measurements.  Connectivity monitoring allows to supervise the state
   and performance of a connection or a (sub)path from one or a few
   central monitoring systems.  This document specifies a suitable
   type-P connectivity monitoring metric.

Status of This Memo

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   Copyright (c) 2021 IETF Trust and the persons identified as the
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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   5
   2.  A brief segment routing connectivity monitoring framework . .   5
   3.  Topology and measurement loop set up requirements . . . . . .   9
     3.1.  General network topology requirements . . . . . . . . . .   9
     3.2.  Sub-path Monitoring measurement loop routing requirements  10
     3.3.  Path  . . . . . . . . . . . . . . . . . . . . . . . . . .  11
   4.  Generic Type-P-SR-Path-Periodic-* metric  . . . . . . . . . .  11
     4.1.  Metric Name . . . . . . . . . . . . . . . . . . . . . . .  12
     4.2.  Generic Metric Parameters . . . . . . . . . . . . . . . .  12
     4.3.  Metric Units  . . . . . . . . . . . . . . . . . . . . . .  12
   5.  Singleton Definition for Type-P-SR-Path-Periodic-Delay  . . .  12
     5.1.  Metric Name . . . . . . . . . . . . . . . . . . . . . . .  12
     5.2.  Metric Parameters . . . . . . . . . . . . . . . . . . . .  12
     5.3.  Delay Metric Units  . . . . . . . . . . . . . . . . . . .  12
     5.4.  Definition  . . . . . . . . . . . . . . . . . . . . . . .  13
     5.5.  Discussion  . . . . . . . . . . . . . . . . . . . . . . .  13
     5.6.  Methodologies . . . . . . . . . . . . . . . . . . . . . .  13
     5.7.  Errors and Uncertainties  . . . . . . . . . . . . . . . .  13
     5.8.  Reporting the metric  . . . . . . . . . . . . . . . . . .  13
   6.  Definition of Samples for Type-P-SR-Path-Periodic-Delay . . .  13
     6.1.  Generic Type-P-SR-Path-Periodic-Delay-* metric  . . . . .  13
       6.1.1.  Metric Name . . . . . . . . . . . . . . . . . . . . .  14
       6.1.2.  Metric Parameters . . . . . . . . . . . . . . . . . .  14
       6.1.3.  Metric Units  . . . . . . . . . . . . . . . . . . . .  14
       6.1.4.  Metric Defintion  . . . . . . . . . . . . . . . . . .  14
       6.1.5.  Discussion  . . . . . . . . . . . . . . . . . . . . .  14
       6.1.6.  Errors and uncertainties  . . . . . . . . . . . . . .  14
     6.2.  Definition of Type-P-SR-Path-Periodic-Delay-Stream  . . .  14
       6.2.1.  Metric Name . . . . . . . . . . . . . . . . . . . . .  15
     6.3.  Definition of Type-P-SR-Path-Periodic-Delay-Variation . .  15
       6.3.1.  Metric Name . . . . . . . . . . . . . . . . . . . . .  15
       6.3.2.  Methodologies . . . . . . . . . . . . . . . . . . . .  15
       6.3.3.  Discussion of SRDV  . . . . . . . . . . . . . . . . .  15
       6.3.4.  Errors and uncertainties  . . . . . . . . . . . . . .  15
     6.4.  Definition of Type-P-SR-Path-Periodic-Delay-Variation-
           Stream  . . . . . . . . . . . . . . . . . . . . . . . . .  15
       6.4.1.  Metric Name . . . . . . . . . . . . . . . . . . . . .  15
       6.4.2.  Metric Defintion  . . . . . . . . . . . . . . . . . .  16
   7.  Statistic Definitions for SR-Path-Periodic-*-Stream samples .  16
     7.1.  SR-Path-Periodic-*-Mean . . . . . . . . . . . . . . . . .  16
     7.2.  SR-Path-Periodic-*-Std  . . . . . . . . . . . . . . . . .  16
   8.  Sub-Path monitoring metrics derived from samples captured

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       along the measurement loops . . . . . . . . . . . . . . . . .  17
     8.1.  Baseline measurement  . . . . . . . . . . . . . . . . . .  17
     8.2.  Discussion of the baseline measurement  . . . . . . . . .  18
     8.3.  Definition of SR-Path-Sub-Path-RTD-Estimate . . . . . . .  18
     8.4.  Definition of SR-Path-Sub-Path-*-Changepoint  . . . . . .  19
     8.5.  Discussion of SR-Path-Sub-Path-*-Changepoint  . . . . . .  19
     8.6.  Definition of SR-Path-Sub-Path-Congestion-Location  . . .  20
     8.7.  Discussion of SR-Path-Sub-Path-*-Location . . . . . . . .  21
   9.  Discussion of Temporal Resolution . . . . . . . . . . . . . .  21
   10. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  22
   11. Security Considerations . . . . . . . . . . . . . . . . . . .  22
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .  22
     12.1.  Normative References . . . . . . . . . . . . . . . . . .  22
     12.2.  Informative References . . . . . . . . . . . . . . . . .  23
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  24

1.  Introduction

   Within a Segment Routing domain, measurement packets can be sent
   along pre-determined segment routed paths [RFC8402].  A segment
   routed path may consist of pre-determined sub paths, specific router-
   interfaces or a combination of both.  A measurement path may also
   consist of sub paths spanning multiple routers, given that all
   segments to address a desired path are available and known at the SR
   domain edge interface.

   A Path Monitoring System or PMS (see [RFC8403]) is a dedicated
   central Segment Routing (SR) domain monitoring device (as compared to
   a distributed monitoring approach based on router-data and -functions
   only).  Monitoring individual sub-paths or point-to-point connections
   is executed for different purposes.  IGP exchanges hello messages
   between neighbors to keep alive routing and swiftly adapt routing to
   topology changes.  Network Operators may be interested in monitoring
   connectivity and congestion of interfaces or sub-paths at a timescale
   of seconds, minutes or hours.  In both cases, the periodicity is
   significantly smaller than commodity interface monitoring based on
   router counters, which may be collected on a minute timescale to keep
   the processor- or monitoring data-load low.

   The IPPM architecture was a first step to that direction [RFC2330].
   Commodity IPPM solutions require dedicated measurement systems, a
   large number of measurement agents and synchronised clocks.
   Monitoring a domain from edge to edge by commodity IPPM solutions
   increases scalability of the monitoring system.  But localising the
   site of a detected change in network behaviour may then require
   network tomography methods.

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   The IPPM Metrics for Measuring Connectivity offer generic
   connectivity metrics [RFC2678].  These metrics allow to measure
   connectivity between end nodes without making any assumption on the
   paths between them.  The metric and the type-p packet specified by
   this document follow a different approach: they are designed to
   monitor connectivity and performance of a specific single link or a
   path segment.  The underlying definition of connectivity is partially
   the same: a packet not reaching a destination indicates a loss of
   connectivity.  An IGP re-route may indicate a loss of a link, while
   it might not cause loss of connectivity between end systems.  The
   metric specified here detects a link-loss, if the change in end-to-
   end delay along a new route is differing from that of the original
   path.

   A Segment Routing PMS is part of an SR domain.  The PMS is IGP
   topology aware, covering the IP and (if present) the MPLS layer
   topology [RFC8402].  This allows to steer PMS measurement packets
   along arbitrary pre-determined concatenated sub-paths, identified by
   suitable Segment IDs.  Basically, the SR connectivity metric as
   specified by this document requires set up of a number of
   constrained, overlaid measurement loops (or measurement paths).  The
   delay of the packets sent along each of these measurement loops is
   measured.  A single congested interface or a single loss of
   connectivity of a monitored sub-path cause a delay change on several
   measurement paths.  Any single evnet of that type on one of the
   monitored sub-paths changes delays of a unique subset of measurement
   loops.  The number of measurement loops may be limited to one per
   sub-path (or connection) to be monitored, if a hub-and-spoke like
   sub-path topology as described below is monitored.  In addition to
   information revealed by a commodity ICMP ping measurement, the
   metrics and methods specified here identify the location of a
   congested interface.  To do so, tomography assumptions and methods
   are combined to first plan the overlaid SR measurement loop set up
   and later on to evaluate the captured delay measurements.

   There's another difference as compared with commodity ping: the
   measurement loop packets remain in the data plane of passed routers.
   These need to forward the measurement packets without additional
   processing apart from that.

   It is recommended to consider automated measurement loop set-ups.
   The methods proposed here are error-prone if the topology and
   measurement loop design isn't followed properly.  While details of an
   automated set-up are not within scope of this document, some formal
   defintions of constraints to respected are given.

   This document specifies a type-p metric determining properties of an
   SR path which allows to monitor connectivity and congestion of

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   interfaces and further allows to locate the path or interface which
   caused a change in the reported type-p metric.  This document is
   limited to the Segment Routing MPLS layer, but the methodology may be
   applied within SR domains or MPLS domains in general.

1.1.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [RFC2119].

2.  A brief segment routing connectivity monitoring framework

   The Segment Routing IGP topology information consists of the IP and
   (if present) the MPLS layer topology.  The minimum SR topology
   information consists of Node-Segment-Identifiers (Node-SID),
   identifying an SR router.  The IGP exchange of Adjacency-SIDs
   [RFC8667], which identify local interfaces to adjacent nodes, is
   optional.  It is RECOMMENDED to distribute Adj-SIDs in a domain
   operating a PMS to monitor connectivity as specified below.  If Adj-
   SIDs aren't availbale, [RFC8287] provides methods how to steer
   packets along desired paths by the proper choice of an MPLS Echo-
   request IP-destination address.  A detailed description of [RFC8287]
   methods as a replacement of Adj-SIDs is out of scope of this
   document.

   An active round trip measurement between two adjacent nodes is a
   simple method to monitor connectivity of a connecting link.  If
   multiple links are operational between two adjacent nodes and only a
   single one fails, a single plain round trip measurement may fail to
   notice that or identify which link has failed.  A round trip
   measurement also fails to identify which interface is congested, even
   if only a single link connects two adjacent nodes.

   Segment Routing enables the set-up of extended measurement loops.
   Several different measurement loops can be set up to form a partial
   overlay.  If done properly, any network change impacts more than a
   single measurement loop's round trip delay (or causes drops of
   packets of more than one loop).  Randomly chosen measurement loop
   paths including the interfaces or paths to be monitored may fail to
   produce the desired unique result patterns, hence commodity network
   tomography methods aren't applicable here [CommodityTomography].  The
   approach pursued here uses a pre-specified measurement loop overlay
   design.

   A centralised monitoring approach doesn't require report collection
   and result correlation from two (or more) receivers (the measured
   delays of different measurement loops still need to be correlated).

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   An additional property of the measurement path set-up specified below
   is that it allows to estimate the packet round trip and the one way
   delay of a monitored sub-path.  The delay along a single link is not
   perfectly symmetric.  Packet processing causes small delay
   differences per interface and direction.  These cause an error, which
   can't be quantified or removed by the specified method.  Quantifying
   this error requires a different measurement set-up.  As this will
   introduce additional measurements loops, packets and evaluations, the
   cost in terms of reduced scalability is not felt to be worth the
   benefit in measurement accuracy.  IPPM metrics prefer precision to
   accuracy and the mentioned processing differences are relatively
   stable, resulting in relatively precise delay estimates for each
   monitored sub-path.

   An example hub and spoke network, operated as SR domain, is shown
   below.  The included PMS shown is supposed to monitor the
   connectivity of all 6 links (a very generic kind of sub-path)
   attaching the spoke-nodes L050, L060 and L070 to the hub-nodes L100
   and L200.

      +---+   +----+     +----+
      |PMS|   |L100|-----|L050|
      +---+   +----+\   /+----+
        |    /    \  \_/_____
        |   /      \  /      \+----+
     +----+/        \/_  +----|L060|
     |L300|         /  |/     +----+
     +----+\       /   /\_
            \     /   /   \
             \+----+ /   +----+
              |L200|-----|L070|
              +----+     +----+

   Hub and spoke connectivity verification with a PMS

                                 Figure 1

   The SID values are picked for convenient reading only.  Node-SID: 100
   identifies L100, Node-SID: 300 identifies L300 and so on.  Adj-SID
   10050: Adjacency L100 to L050, Adj-SID 10060: Adjacency L100 to L060,
   Adj-SID 60200: Adjacency L60 to L200 and so on (note that the Adj-SID
   are locally assigned per node interface, meaning two per link).

   Monitoring the 6 links between hub nodes Ln00 (where n=1,2) and spoke
   nodes L0m0 (where m=5,6,7) requires 6 measurement loops, which have
   the following properties:

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   o  Each measurement loop follows a single round trip from one hub
      Ln00 to one spoke L0m0 (e.g., between L100 and L050).

   o  Each measurement loop passes two more links: one between the same
      hub Ln00 and another spoke L0m0 and from there to the alternate
      hub Ln00 (e.g., between L100 and L060 and then from L060 to L200)

   o  Every monitored link is passed by a single round trip measurement
      loop only once and further only once unidirectional by two other
      loops.  These unidirectional mearurement loop sections forward
      packets in opposing direction along the monitored link.  In the
      end, three measurement loops pass each single monitored link (sub-
      path).  In figure 1, e.g., one measurement loop having a round
      trip L100 to L050 and back (M1, see below), a second loop passing
      L100 to L050 only (M3) and a third loop passing L050 to L100 only
      (M6).

   Note that any 6 links connecting two to five nodes can be monitored
   that way too.  Further note that the measurement loop overlay chosen
   is optimised for 6 links and a hub and spoke topology of two to five
   nodes.  The 'one measurement loop per measured sub-path' paradigm
   only works under these conditions.

   The above overlay scheme results in 6 measurement loops for the given
   example.  The start and end of each measurement loop is PMS to L300
   to L100 or L200 and a similar sub-path on the return leg.  These
   parts of the measurement loops are omitted here for brevity (some
   discussion may befound below).  The following delays are measured
   along the SR paths of each measurement loop:

   1.  M1 is the delay along L100 -> L050 -> L100 -> L060 -> L200

   2.  M2 is the delay along L100 -> L060 -> L100 -> L070 -> L200

   3.  M3 is the delay along L100 -> L070 -> L100 -> L050 -> L200

   4.  M4 is the delay along L200 -> L050 -> L200 -> L060 -> L100

   5.  M5 is the delay along L200 -> L060 -> L200 -> L070 -> L100

   6.  M6 is the delay along L200 -> L070 -> L200 -> L050 -> L100

   An example for a stack of a loop consisting of Node-SID segments
   allowing to caprture M1 is (top to bottom): 100 | 050 | 100 | 060 |
   200 | PMS.

   An example for a stack of Adj-SID segments the loop resulting in M1
   is (top to bottom): 100 | 10050 | 50100 | 10060 | 60200 | PMS.  As

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   can be seen, the Node-SIDs 100 and PMS are present at top and bottom
   of the segment stack.  Their purpose is to transport the packet from
   the PMS to the start of the measurement loop at L100 and return it to
   the PMS from its end.

   The Evaluation of the measurement loop Round Trip Delays M1 - M6
   allow to detect the follwing state-changes of the monitored sub-
   paths:

   o  If the loops are set up using Node-SIDs only, any single complete
      loss of connectivity caused by a failing single link between any
      Ln00 and any L0m0 node briefly disturbs (and changes the measured
      delay) of three loops.  The traffic to the Node-SIDs is rerouted
      (in the case of a single links loss, no node is completely
      disconnected in the example network).

   o  If the loops are set up using Adj-SIDs only, any single complete
      loss of connectivity caused by a failing single link between any
      Ln00 and any L0m0 node terminates the traffic along three
      measurement loops.  The packets of all three loops will be
      dropped, until the link gets back into service.  Traffic to Adj-
      SIDs is not rerouted.  Note that Node-SIDs may be used to foward
      the measurement packets from the PMS to the hub node, where the
      first sub-path to be monitored begins and from the hub node,
      receiving the measurement from the last monitored sub path, to the
      PMS.

   o  Any congested single interface between any Ln00 and any L0m0 node
      only impacts the measured delay of two measurement loops.

   o  As an example, the formula for a single link (sub-path) Round Trip
      Delay (RTD) is shown here 4 * RTD_L100-L050-L100 = 3 * M1 + M3 +
      M6 - M2 - M4 - M5.  This formula is reproducible for all other
      links: sum up 3*RTD measured along the loop passing the monitored
      link of interest in round trip fashion, and add the RTDs of the
      two measurement loops passing the link of interest only in a
      single direction.  From this sum subtract the RTD measured on all
      loops not passing the monitored link of interest to get four times
      the RTD of the monitored link of interest.

   A closer look reveals that each single event of interest for the
   proposed metric, which are a loss of connectivity or a case of
   congestion, uniquely only impacts a single set of measurement loops
   which can be determined a-priori.  If, e.g., connectivity is lost
   between L200 and L050, measurement loops (3), (4) and (6) indicate a
   change in the measured delay.

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   As a second example: if the interface L070 to L100 is congested,
   measurement loops (3) and (5) indicate a change in the measured
   delay.  Without listing all events, all cases of single losses of
   connectivity or single events of congestion influence only delay
   measurements of a unique set of measurement loops.

   Assume that the measurement loops are set up while there's no
   congestion.  In that case, the congestion free RTDs of all monitored
   links can be calculated as shown above.  A single congestion event
   adds queuing delay to the RTD measured by two specific measurement
   loops.  The two measurement loops impacted allow to distinct the
   congested interface and calculation of the queue-depth in terms of
   seconds.  As an example, assume a queue of an average depth of 20 ms
   to build up at interface L200 to L070 after the uncongested
   measurement interval T0.  The measurement loops M5 and M6 are the
   only ones passing the interface in that direction.  Both indicate a
   congestion M5 and M6 of + 20 ms during measurement interval T1, while
   M1-4 indicate no change.  The location of the congested interface is
   determined by the combination of the two (and only two) measurement
   loops M5 and M6 showing an increased delay.  The average queue depth
   = ( M5[T1] - M5[T0] + M5[T1] - M5[T0] )/2.

   As mentioned there's a constant delay added for each measurement
   loop, which is the delay of the path transversed from PMS -> L100 +
   L200 -> PMS.  Please note, that this added delay is appearing twice
   in the formula resulting in the monitored link delay estimate of the
   example network.  Then it is the RTD PMS -> L100 + RTD L200 -> PMS.
   Both RTDs can be directly measured by two additional measurements
   Cor1 = RTD ( PMS -> L100 -> PMS) and Cor2 = RTD (PMS -> L200 -> PMS).
   The monitored link RTD formula was linkRTDuncor = 3*Mx + My + Mz - Ms
   - Mt - Mu.  The correct 4*linkRTDx = 4*linkRTDxuncor - Cor1 - Cor2.

   If the interface between PMS and L100/L200 is congested, all
   measurements loops M1-M6 as well as Cor1 and Cor2 will see a change.
   A congested interface of a monitored link doesn't impact the RTDs
   captured by Cor1 and Cor2.

   The measurement loops may also be set up between hub nodes L100 and
   L200, if that's preferred and supported by the nodes.  In that case,
   the above formulas apply without correction.

3.  Topology and measurement loop set up requirements

3.1.  General network topology requirements

   The metric and methods specified below can be applied to monitor
   networks or sub-paths forming a hub and spoke topology.  A single
   sub-path status change of type loss of connectivity or congestion can

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   be detected.  The nodes don't have to act as hubs or spokes, this
   terminology is only chosen to describe a topology requirement.  In
   detail, the topology to be monitored MUST meet the following
   constraints:

   o  The SR domain sub-paths to be monitored create a hub and spoke
      topology with a PMS connected to all hub nodes.  The PMS may
      reside in a hub.

   o  Exactly 6 (six) sub-paths are monitored.

   o  The monitored sub-paths connect at least two and no more than 5
      nodes.

   o  Every spoke node MUST have at least one path to every hub node.

   o  Every spoke node MUST at least be connected to one (or more) hub
      node(s) by two monitored sub-paths.

   o  Sub-paths between spokes can't be monitored and therefore are out
      of scope (the overlay measurement loops can't be set up as
      desired).

   Shared resources, like a Shared Risk Link Group (e.g., a single fiber
   bundle) or a shared queue passed by several logical links need to be
   considered during set up.  Shared resources may either be desired or
   to be avoided.  As an example, if a set of logical links share one
   parental scheduler queue, it is sufficient to monitor a single
   logical connection to monitor the state of that parental scheduler.

3.2.  Sub-path Monitoring measurement loop routing requirements

   The methodologies sepcified by this document REQUIRE a measurement
   loop path overlay of all path delay measurement streams Fi, i in [1,
   2...6] as defined in this section.  In the follwing, a path delay
   measurement stream Fi is called measurement (loop) Fi for brevity.

   o  Define the segment routed Sub-paths SPi, i in [1, 2...6] to be
      monitored.  The Sub-paths SPi SHOULD not share resources, if the
      operator isn't aware of the impact of the shared resources on the
      measurement loops Fi and the methodologies defined below.  The
      Sub-path SPi topology SHOULD respect the general network topology
      requirements as specified above.

   o  Set up i = 1, 2...6 measurement loops Fi thus that measurement Fi
      passes SPi and only SPi bidirectional (or by a round-trip) from
      Hub to Spoke and back.  Note that the correspondance of SPi and Fi

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      isn't strictly required.  Measurement Fi thus however appears in
      all methodologies calculating a metric related to SPi.

   o  Set up the SR path per measurement loops Fj and Fk thus that SPi
      is passed by exactly one other measurement loop Fj unidirectional
      in direction Hub to Spoke and by exactly one other measurement
      loop Fk unidirectional in the opposite direction (Spoke to Hub).
      The measurement loop Fi != Fj != Fk.  As a description, one
      measurement loop Fj pass SPi in "downstream" direction from Hub to
      Spoke, whereas measurement loop Fk passes SPi in "upstream"
      direction from Spoke to Hub.

   o  Set up each segment routed measurement loop path Fi thus that it
      passes SPi bidirectional as specified above, SPj unidirectional
      from Hub to Spoke and SPk unidirectional from Spoke to Hub. The
      monitored Sub-path SPi MUST NOT be equal to SPj and MUST NOT be
      equal to SPk.

   o  The measurement loop set up to monitor all Sub-paths SPi is
      completed, if:

   o

      *  Each Sub-path SPi is passed by exactly three measurements loops
         Fi, Fj and Fk as specified above.

      *  Each segment routed measurement loop path Fi passes exactly
         three concatenated Sub-paths SPi, SPm and SPn as specified
         above (indices m and n are chosen here only to avoid
         misconceptions which may result from picking indices j and k
         already appearing before - equality of j and k with either m
         and n is neither excluded nor required).

3.3.  Path

   This document specifies sub-path monitoring within a closed domain by
   a controlled and pre-designed measurement loop set-up.  The path
   traversed by the packet SHOULD be reported, as detecting data plane
   forwarding in line with the desired measurement loop set-up is
   essential for the metric to enable and verify accurate evaluation.
   See [RFC8287] for SR MPLS OAM and
   [ID.draft-ietf-6man-spring-srv6-oam] for SRv6 OAM.

4.  Generic Type-P-SR-Path-Periodic-* metric

   To reduce the redundant information presented in the detailed metrics
   sections that follow, this section presents the specifications that
   are common to two or more metrics.  The section is organized using

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   the same subsections as the individual metrics, to simplify
   comparisons.

4.1.  Metric Name

   All metrics use the Type-P convention as described in [RFC2330].  The
   rest of the name is unique to each metric.

4.2.  Generic Metric Parameters

   Refer to section 3.2.  Metric Parameters: Type-P-* of [RFC6673].  The
   following parameters are added, enhanced or removed:

      Dst SHOULD be a diagnostic IP address as specified by [RFC8287]
      and [RFC8029], if MPLS OAM is operated to capture the metric.

      Fi, where i in [1, 2...6], a selection function defining
      unambiguously a packet of one particular stream i forming part of
      the monitoring overlay measurement loop set up.

      L, a packet length in bits.  The packets of all Type-P-SR-Path-
      Delay-Periodic-Streams Fi SHOULD all be of the same length.

      MLAi, a stack of Segment IDs determining a monitoring loop Fi.
      The Segment-IDs MUST be chosen so that a singleton type-p packet
      of selection function Fi passes the sub-path i to be monitored.

      No support: lambda (Poisson Streams remain ffs.)

4.3.  Metric Units

   Refer to section 3.4.  Metric Units: Type-P-* of [RFC6673].

5.  Singleton Definition for Type-P-SR-Path-Periodic-Delay

5.1.  Metric Name

   Type-P-SR-Path-Periodic-Delay

5.2.  Metric Parameters

   See section Section 4.2.

5.3.  Delay Metric Units

   A sequence of consecutive time values.  The value of a Type-P-SR-
   Path-Periodic-Delay is either a real number or an undefined

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   (informally, infinite) number of seconds per singleton of each stream
   Fi.

5.4.  Definition

   Section 3.4 of [RFC7679] applies per singleton of each stream Fi.
   The additional information related to singletons of section 4.2.4 of
   [RFC3432] applies too.

5.5.  Discussion

   See section 3.5 of [RFC7679].  One generalisation seems appropriate:
   a global satellite navigation system affords one way to achieve
   synchronization within usec.

5.6.  Methodologies

   Section 3.6 of [RFC7679] applies per stream Fi with one exception: at
   the Src host, select Src and Dst IP addresses, if IP-routing is
   applied, or select the proper functional IP-destination address if an
   [RFC8287] SR MPLS OAM packet format is applied.  Further add the
   appropriate stack of Segment IDs MLAi determining the monitoring loop
   Fi and form a test packet of Type-P with these addresses and the
   segment stack.

5.7.  Errors and Uncertainties

   See section 3.7 of [RFC7679] and section 4.6 of [RFC3432].

5.8.  Reporting the metric

   See section 3.8 of [RFC7679].

6.  Definition of Samples for Type-P-SR-Path-Periodic-Delay

   This sections defines metric samples and metrics derived from
   samples.

6.1.  Generic Type-P-SR-Path-Periodic-Delay-* metric

   To reduce the redundant information presented in the detailed metrics
   sections that follow, this section presents the specifications that
   are common to two or more metrics.  The section is organized using
   the same subsections as the individual metrics, to simplify
   comparisons.

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6.1.1.  Metric Name

   Type-P-SR-Path-Periodic-Delay-*

6.1.2.  Metric Parameters

      Src, the IP address of a host

      Dst, the IP address of a host

      MLAi, a stack of Segment IDs

      T0, a time

      Tf, a time

      incT, a time

6.1.3.  Metric Units

   See section Section 5.3.

6.1.4.  Metric Defintion

   Given T0 and Tf and nominal inter-packet interval incT, those time
   values greater than or equal to T0 and less than or equal to Tf are
   then selected.  At each of the selected times in this process, we
   obtain one value of Type-P-SR-Path-Periodic-Delay.  The value of the
   sample is the sequence made up of the resulting [time, delay] pairs.
   If there are no such pairs, the sequence is of length zero and the
   sample is said to be empty.

6.1.5.  Discussion

   See section 4.4 of [RFC3432].

6.1.6.  Errors and uncertainties

   See section 4.6 of [RFC3432].

6.2.  Definition of Type-P-SR-Path-Periodic-Delay-Stream

   The only definition required for this metric is a unique metric name.

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6.2.1.  Metric Name

   Type-P-SR-Path-Periodic-Delay-Stream

6.3.  Definition of Type-P-SR-Path-Periodic-Delay-Variation

   The smallest sample Type-P-SR-Path-Periodic-Delay-Stream is one of
   two consecutively received values.  These may be used to calculate a
   Segment Routed Path Delay-Variation (SRDV) singleton, defined below.

6.3.1.  Metric Name

   Type-P-SR-Path-Periodic-Delay-Variation

6.3.2.  Methodologies

   SRDV[i,j], for each sample of packets j and j-1 of stream Fi, j > 1,
   the delay variation between successive packets is calculated as:

   SRDV[i,j] = Delay[i,j] - Delay [i,j-1],

   j in [2,3...N] and N the total number of packets received at Dst. If
   one or more of the M packets sent by Src are lost, they are ignored
   for the metric, as no reasonable metric value is defined here.  If N
   > 1, the metric is calculated for every valid packet received and the
   preceding one.

6.3.3.  Discussion of SRDV

   Evaluation statistics of differential SRDV metric samples may help to
   identify issues.

6.3.4.  Errors and uncertainties

   See section 2.7 of [RFC3393].

6.4.  Definition of Type-P-SR-Path-Periodic-Delay-Variation-Stream

   The only definition required for this metric is a unique metric name.

6.4.1.  Metric Name

   Type-P-SR-Path-Periodic-Delay-Variation-Stream

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6.4.2.  Metric Defintion

   Given T0 and Tf, those time values greater than or equal to T0 and
   less than or equal to Tf are then selected.  At each of the selected
   times in this process, we obtain one value of Type-P-SR-Path-
   Periodic-Delay.  The value of the sample is the sequence made up of
   the resulting [time, delay-variation] pairs with time being set to
   the Dst timestamp of the Delay-Variation singleton, for which a valid
   singleton is calculated.  If there are no such pairs, the sequence is
   of length zero and the sample is said to be empty.  If N Delay
   singletons are captured and sampled N-1 Delay-Variation singletons
   are sampled during the same interval

7.  Statistic Definitions for SR-Path-Periodic-*-Stream samples

   Change point detection requires statistical defintions.  These are
   provided below.  The names of the statistics contain an "*"
   placeholder, which may be replaced by "Delay" or "Delay-Variation"
   [Editor note: a "Loss" metric remains tbd].

7.1.  SR-Path-Periodic-*-Mean

   For a type-p metric, the mean is specified by:

   SR-*Mean = (1/N) * Sum(from i=1 to N, value[i])

   o  N sample size

   o  value sample value of a sampled [time, value] pair

7.2.  SR-Path-Periodic-*-Std

   For a type-p metric, the Standard-Deviation Std is specified by:

   SR-*Std = [1/(N-1)] * Sum(from i=1 to N, [SR-*Mean - value[i]]^2 )

   o  N sample size

   o  value sample value of a sampled [time, value] pair

   o  SR-*Mean sample mean of the same metric as defined above

   The definition as given above requires a two-pass calculation per
   sample.  Algorithms estimating the standard-deviation by one-pass
   calculation have been published and might be preferable, if metric
   singletons and samples aren't buffered or calculations need to be
   fast.

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8.  Sub-Path monitoring metrics derived from samples captured along the
    measurement loops

   To produce meaningful sub-path monitoring values, the measurement
   loop metrics are captured during a phase with stable networking
   conditions.  In a backbone network domain, the absence of congestion
   often is a sufficient condition (frequent traffic shifts due to
   changes in routing and traffic engineering aren't expected).  This
   may be different in a network based on a shared medium.  It may be
   outright difficult in networks with frequently changing traffic
   management- and routing-policies.

   In the following, the index CS indicates a statistic captured during
   a mesurement interval with stable routing and no congestion.

8.1.  Baseline measurement

   Capture a sample of delay values Type-P-SR-Path-Periodic-Delay-Stream
   of sample size N for each measurment loop Fi.  As a rule of thumb
   choose N in [30, 100].

   For each measurement loop Fi, calculate the following metrics
   characterising the monitored Sub-Paths during stable and congestion
   free network conditions:

   o  SR-Path-Delay-MeanCSi, the mean delay captured along measurement
      loop Fi

   o  SR-Path-Delay-StdCSi, the standard-deviation of the delay captured
      along measurement loop Fi

   o  SR-Path-Delay-Variation-MeanCSi, the mean delay variation captured
      along measurement loop Fi

   o  SR-Path-Delay-Variation-StdCSi, the standard-deviation of the
      delay variation captured along measurement loop Fi

   A stable and uncongested network should produce rather constant
   delays, resulting in low standard-deviation values and almost zero
   mean delay variation.

   Example data was captured in a lightly loaded Gigabit network. 11
   routers are passed per measurement loop.  The sample size is 30
   packets, more than 200 samples were captured per measurement loop.
   The loops are set up for a different purpose than specified here,
   they are picked due to a high number of passed routers.  Note that
   SR-DV-Mean here refers to an abs(SR-DV-Mean) sample, thus small,
   positive, non-zero means result.  The time unit is microseconds.

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         Metric|Quantile|SR-D-Mean|SR-D-Std|SR-DV-Mean|SR-DV-Std
         ------+--------+---------+--------+----------+---------
         Loop1 |   95%  |  34510  |   40   |    28    |   49
         ------+--------+---------+--------+----------+---------
         Loop2 |   95%  |  35104  |   45   |    34    |   49
         ------+--------+---------+--------+----------+---------
         Loop1 |   50%  |  34495  |   17   |    15    |   13
         ------+--------+---------+--------+----------+---------
         Loop2 |   50%  |  35088  |   15   |    14    |   12
         ------+--------+---------+--------+----------+---------
         Loop1 |    5%  |  34504  |   10   |    12    |   11
         ------+--------+---------+--------+----------+---------
         Loop2 |    5%  |  35080  |   13   |    12    |    9
         ------+--------+---------+--------+----------+---------

   Example baseline metrics for an 11 hop measurement loop

                                 Figure 2

8.2.  Discussion of the baseline measurement

   Delay outliers may occur at any time in any communication network,
   and the measurement system packet processing itself may also produce
   some.  It is fair to expect only single outliers in a stable, not
   congested network.  It may be worth to capture several consecutive
   SR-Path-Periodic-*-Stream samples and compare their statistics,
   before picking reasonable baseline metric values.  Samples showing
   higher standard deviations (compare the 95% quantile values in the
   above figure to the 50% quantile values) may benefit from removing
   the maximum singleton value from the sample.  This will smooth the
   mean and standard-deviation, and if the result then is closer to
   those of the majority of the samples, foster confidence in
   determining the baseline metrics.  Depending on the preferred method
   of data-processing and storing, this may require capturing the sample
   maximum as a separate metric.

8.3.  Definition of SR-Path-Sub-Path-RTD-Estimate

   Within a single evaluation interval of identical Time T0 and Tf, SR-
   Path-Delay-MeanCSi(from now on DMeanCSi)is the mean delay of the
   measurement loop passing the monitored Sub-Path SPi by a round trip.
   Let's keep the indexig applied above, then Fj and Fk with captured
   mean delays DMeanCSj and DMeanCSk pass SPi uniderictional.  Further,
   3 measurement loops Fx, Fy and Fz don't pass Sub-Path at all.  The
   corresponding mean delays are DMeanCSs, DMeanCSt and DMeanCSu.

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   The the SR-Path-Sub-Path-RTD-Estimate of the Round Trip Delay along
   the monitored Sub-Path Fi, RTD_Fi, is

   RTD_Fi=(3*DMeanCSi+DMeanCSj+DMeanCSk-DMeanCSx-DMeanCSy-DMeanCSz)/4

8.4.  Definition of SR-Path-Sub-Path-*-Changepoint

   The asterisk stands for "Interface" as well as "Connectivity" (the
   latter may be indicated by packet drops, but also a change in sub-
   path routes with a change in measurement loop delay may be applied to
   detect and locate this event).

   Network changes are often characterised by a change in the mean delay
   of a monitoring measurement.  CUSUM (cumulative sum ) charts have
   been shown to be efficient in detecting shifts in the mean of a
   process [NIST].  The upper bound CUSUM is defined as:

   Sup(t)-Fi-Delay = max(0,Sup(t-1) + xt - SR-Path-*-MeanCSi - ki)

   with Sh0 = 0, ki = Delta * SR-Path-*-StdCSi (Delta is a dimensionless
   integer number), xt = Type-P-SR-Path-Periodic-* singleton for
   measurement loop Fi at time t.

   The actual SR-Path-Delay-Mean of Measurement Loop Fi is decided to be
   significantly above SR-Path-*-MeanCSi, if:

   Sup(t)-Fi-Delay > h_SP, with h_SP = d*ki (d is a dimensionless
   integer number).

   An analogus CUSUM controls changes to a lower mean delay (which may
   be caused by a re-routing event):

   Slo(t)-Fi-Delay = max(0,Slo(t-1) + SR-Path-*-MeanCSi - xj - k)

   The actual SR-Path-Delay-Mean of Fi is decided to be significantly
   below SR-Path-*-MeanCSi, if:

   Slo(t)-Fi-Delay > h_SP

8.5.  Discussion of SR-Path-Sub-Path-*-Changepoint

   CUSUM chart based changepoint detection is sensible even to small
   changes in the mean.  CUSUM charts offer a limited protection against
   single, isolated outliers.  A cumulated sum only grows, if the
   controled process consistenly changes its mean (or standard
   deviation, respectively).  Assuming constant physical minimum delays
   to characterise wireline communication networks, a change in standard

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   deviation not affecting the mean delay doesn't seem to be caused by a
   change in networking conditions.

   The measured delays will change once a Sub-Path route has changed, or
   once persistent congestion starts to fill a queue.  Both indicate
   changes in the network.  As the Sub-Pathes SPi form an overlay with
   designed properties, every network change affecting a sub-path
   creates correlated SR-Path-* metric changes.  As the correspondance
   of network changes to Sub-Path metrics is known a-priory, detecting
   correlated SR-Path-* metric changes allows to locate the change.

   In the absence of packet re-routing, packet loss is characterising a
   loss of connectivity.  Packet loss requires a time threshold when to
   decide that an active measurement packet was lost, and consecutive
   loss requires receiver awareness, that packets have been sent (this
   argues for the sender to be the receiver, unless both comminicate
   fast and reliable out of band).

   The preferred CUSUM parametrisation will depend on the kind of events
   to detected and on the outlier characteristics.

   ki = Delta * SR-Path-*-StdCSi may be set to a value relevant high
   enough to exclude single outliers to trigger an alert, but low enough
   to indicate persistent changes in delay.  The same holds for the to
   be picked for d.

   A broader discussion on CUSUM parametrisation may be found in
   literature.  Networking skills are required to parametrise CUSUM, as
   well as to interprete the results (notably to differ re-routing from
   congestion).

8.6.  Definition of SR-Path-Sub-Path-Congestion-Location

   An interface along a single monitored Sub-Path SPi whose queue is
   persistently filled adds latency to measurement loop Fi and one of
   the two unidirectional measurement loops Fj and Fk passing Sub-Path
   SPi.  Fj has been defined to pass SPi from Hub to Spoke and Fk pass
   SPI in opposite direction.

      IF Sup(t)_SPi_Periodic-Delay + Sup(t)_SPj_Periodic-Delay > h_SP

      AND h_SP > Sup(t)_SPk_Periodic-Delay

      AND h_SP > Sup(t)_SPx_Periodic-Delay

      AND h_SP > Sup(t)_SPy_Periodic-Delay

      AND h_SP > Sup(t)_SPz_Periodic-Delay

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   Then Sub-Path SPi faces congestion in direction "Hub to Spoke".

      IF Sup(t)_SPi_Periodic-Delay + Sup(t)_SPk_Periodic-Delay > h_SP

      AND h_SP > Sup(t)_SPj_Periodic-Delay

      AND h_SP > Sup(t)_SPx_Periodic-Delay

      AND h_SP > Sup(t)_SPy_Periodic-Delay

      AND h_SP > Sup(t)_SPz_Periodic-Delay

   Then Sub-Path SPi faces congestion in direction "Spoke to Hub".

   Here, h_SP is a universal threshold in unit time to indicate a
   filling queue or a significant change in delay due to a Sub-Path
   reroute or another persistent change in topology (like e.g. automated
   Layer 1 / Layer 2 topology changes).  SPx, SPy and SPz don't pass

8.7.  Discussion of SR-Path-Sub-Path-*-Location

   [Editor Note: Discussion and a suitable connectivity monitoring
   metric Definition+Discussion to be added.]

9.  Discussion of Temporal Resolution

   [Editor Note: requires a review..] The metric reports loss of
   connectivity of monitored sub-path or congestion of an interface and
   identifies the sub-path and the direction of traffic in the case of
   congestion.

   The temporal resolution of the detected events depends on the spacing
   interval of packets transmitted per measurement path.  An identical
   sending interval is chosen for every measurement path.  As a rule of
   thumb, an event is reliably detected if a sample consists of at least
   5 probes indicating the same underlying change in behavior.
   Depending on the underlying event either two or three measurement
   paths are impacted.  At least two consecutively received measurement
   packets per measurement path should suffice to indicate a change.
   The values chosen for an operational network will have to reflect
   scalability constraints of a PMS measurement interface.  As an
   example, a PMS may work reliable if no more than one measurement
   packet is transmitted per millisecond.  Further, measurement is
   configured so that the measurement packets return to the sender
   interface.  Assume always groups of 6 links to be monitored as
   described above by 6 measurements paths.  If one packet is sent per
   measurement path within 500 ms, up to 498 links can be monitored with

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   a reliable temporal resolution of roughly one second per detected
   event.

   Note that per group measurement packet spacing, measurement loop
   delay difference and latency caused by congestion impact the
   reporting interval.  If each measurement path of a single 6 link
   monitoring group is addressed in consecutive milliseconds (within the
   500 ms interval) and the sum of maximum physical delay of the per
   group measurement paths and latency possibly added by congestion is
   below 490 ms, the one second reports reliably capture 4 packets of
   two different measurement paths, if two measurement paths are
   congested, or 6 packets of three different measurement paths, if a
   link is lost.

   A variety of reporting options exist, if scalability issues and
   network properties are respected.

10.  IANA Considerations

   If standardised, the metric will require an entry in the IPPM metric
   registry.

11.  Security Considerations

   This draft specifies how to use methods specified or described within
   [RFC8402] and [RFC8403].  It does not introduce new or additional SR
   features.  The security considerations of both references apply here
   too.

12.  References

12.1.  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>.

   [RFC2678]  Mahdavi, J. and V. Paxson, "IPPM Metrics for Measuring
              Connectivity", RFC 2678, DOI 10.17487/RFC2678, September
              1999, <https://www.rfc-editor.org/info/rfc2678>.

   [RFC3393]  Demichelis, C. and P. Chimento, "IP Packet Delay Variation
              Metric for IP Performance Metrics (IPPM)", RFC 3393,
              DOI 10.17487/RFC3393, November 2002,
              <https://www.rfc-editor.org/info/rfc3393>.

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   [RFC3432]  Raisanen, V., Grotefeld, G., and A. Morton, "Network
              performance measurement with periodic streams", RFC 3432,
              DOI 10.17487/RFC3432, November 2002,
              <https://www.rfc-editor.org/info/rfc3432>.

   [RFC6673]  Morton, A., "Round-Trip Packet Loss Metrics", RFC 6673,
              DOI 10.17487/RFC6673, August 2012,
              <https://www.rfc-editor.org/info/rfc6673>.

   [RFC7679]  Almes, G., Kalidindi, S., Zekauskas, M., and A. Morton,
              Ed., "A One-Way Delay Metric for IP Performance Metrics
              (IPPM)", STD 81, RFC 7679, DOI 10.17487/RFC7679, January
              2016, <https://www.rfc-editor.org/info/rfc7679>.

   [RFC7680]  Almes, G., Kalidindi, S., Zekauskas, M., and A. Morton,
              Ed., "A One-Way Loss Metric for IP Performance Metrics
              (IPPM)", STD 82, RFC 7680, DOI 10.17487/RFC7680, January
              2016, <https://www.rfc-editor.org/info/rfc7680>.

   [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,
              <https://www.rfc-editor.org/info/rfc8029>.

   [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,
              <https://www.rfc-editor.org/info/rfc8287>.

   [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>.

   [RFC8667]  Previdi, S., Ed., Ginsberg, L., Ed., Filsfils, C.,
              Bashandy, A., Gredler, H., and B. Decraene, "IS-IS
              Extensions for Segment Routing", RFC 8667,
              DOI 10.17487/RFC8667, December 2019,
              <https://www.rfc-editor.org/info/rfc8667>.

12.2.  Informative References

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   [CommodityTomography]
              Lakhina, A., Papagiannaki, K., Crovella, M., Diot, C.,
              Kolaczyk, ED., and N. Taft, "Structural analysis of
              network traffic flows", 2004,
              <https://www.cc.gatech.edu/classes/AY2007/cs7260_spring/
              papers/odflows-sigm04.pdf>.

   [ID.draft-ietf-6man-spring-srv6-oam]
              Zafar, A., Filsfils, C., Matsushima, S., Voyer, D., and M.
              Chen, "Operations, Administration, and Maintenance (OAM)
              in Segment Routing Networks with IPv6 Data plane (SRv6)",
              2021.

   [NIST]     NIST, "NIST/SEMATECH e-Handbook of Statistical Methods,
              section CUSUM Control Charts", 2021,
              <http://www.itl.nist.gov/div898/handbook/>.

   [RFC2330]  Paxson, V., Almes, G., Mahdavi, J., and M. Mathis,
              "Framework for IP Performance Metrics", RFC 2330,
              DOI 10.17487/RFC2330, May 1998,
              <https://www.rfc-editor.org/info/rfc2330>.

   [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>.

Author's Address

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

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

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