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RFC 7799

Informational
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Active and Passive Metrics and Methods (with Hybrid Types In-Between)

 


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Internet Engineering Task Force (IETF)                         A. Morton
Request for Comments: 7799                                     AT&T Labs
Category: Informational                                         May 2016
ISSN: 2070-1721


                 Active and Passive Metrics and Methods
                     (with Hybrid Types In-Between)

Abstract

   This memo provides clear definitions for Active and Passive
   performance assessment.  The construction of Metrics and Methods can
   be described as either "Active" or "Passive".  Some methods may use a
   subset of both Active and Passive attributes, and we refer to these
   as "Hybrid Methods".  This memo also describes multiple dimensions to
   help evaluate new methods as they emerge.

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 a candidate for any level of Internet
   Standard; see Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc7799.

Copyright Notice

   Copyright (c) 2016 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
   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.

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Table of Contents

   1. Introduction ....................................................2
      1.1. Requirements Language ......................................3
   2. Purpose and Scope ...............................................3
   3. Terms and Definitions ...........................................3
      3.1. Performance Metric .........................................3
      3.2. Method of Measurement ......................................4
      3.3. Observation Point ..........................................4
      3.4. Active Methods .............................................4
      3.5. Active Metric ..............................................5
      3.6. Passive Methods ............................................5
      3.7. Passive Metric .............................................6
      3.8. Hybrid Methods and Metrics .................................6
   4. Discussion ......................................................8
      4.1. Graphical Representation ...................................8
      4.2. Discussion of PDM .........................................10
      4.3. Discussion of "Coloring" Method ...........................11
      4.4. Brief Discussion of OAM Methods ...........................11
   5. Security Considerations ........................................12
   6. References .....................................................12
      6.1. Normative References ......................................12
      6.2. Informative References ....................................13
   Acknowledgements ..................................................14
   Author's Address ..................................................14

1.  Introduction

   The adjectives "Active" and "Passive" have been used for many years
   to distinguish between two different classes of Internet performance
   assessment.  The first Passive and Active Measurement (PAM)
   Conference was held in 2000, but the earliest proceedings available
   online are from the second PAM conference in 2001
   <https://www.ripe.net/ripe/meetings/pam-2001>.

   The notions of "Active" and "Passive" are well-established.  In
   general:

   o  An Active Metric or Method depends on a dedicated measurement
      packet stream and observations of the stream.

   o  A Passive Metric or Method depends *solely* on observation of one
      or more existing packet streams.  The streams only serve
      measurement when they are observed for that purpose, and are
      present whether or not measurements take place.

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   As new techniques for assessment emerge, it is helpful to have clear
   definitions of these notions.  This memo provides more-detailed
   definitions, defines a new category for combinations of traditional
   Active and Passive techniques, and discusses dimensions to evaluate
   new techniques as they emerge.

   This memo provides definitions for Active and Passive Metrics and
   Methods based on long usage in the Internet measurement community,
   and especially the Internet Engineering Task Force (IETF).  This memo
   also describes the combination of fundamental Active and Passive
   categories that are called Hybrid Methods and Metrics.

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 [RFC2119].

2.  Purpose and Scope

   The scope of this memo is to define and describe Active and Passive
   versions of metrics and methods that are consistent with the long-
   time usage of these adjectives in the Internet measurement community
   and especially the IETF.  Since the science of measurement is
   expanding, we provide a category for combinations of the traditional
   extremes, treating Active and Passive as a continuum and designating
   combinations of their attributes as Hybrid Methods.

   Further, this memo's purpose includes describing multiple dimensions
   to evaluate new methods as they emerge.

3.  Terms and Definitions

   This section defines the key terms of the memo.  Some definitions use
   the notion of "stream of interest", which is synonymous with
   "population of interest" defined in clause 6.1.1 of ITU-T
   Recommendation Y.1540 [Y.1540].  These definitions will be useful for
   any work in progress, such as [PASSIVE] (with which there is already
   good consistency).

3.1.  Performance Metric

   The standard definition of a quantity, produced in an assessment of
   performance and/or reliability of the network, which has an intended
   utility and is carefully specified to convey the exact meaning of a
   measured value.  (This definition is consistent with that of
   Performance Metric in [RFC2330] and [RFC6390]).

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3.2.  Method of Measurement

   The procedure or set of operations having the object of determining a
   Measured Value or Measurement Result.

3.3.  Observation Point

   See Section 2 of [RFC7011] for the definition of Observation Point (a
   location in the network where packets can be observed), and related
   definitions.  The comparable term defined in IETF literature on
   Active measurement is "Measurement Point" (see Section 4.1 of
   [RFC5835]).  Both of these terms have come into use describing
   similar actions at the identified point in the network path.

3.4.  Active Methods

   Active Methods of Measurement have the following attributes:

   o  Active Methods generate packet streams.  Commonly, the packet
      stream of interest is generated as the basis of measurement.
      Sometimes, the adjective "synthetic" is used to categorize Active
      measurement streams [Y.1731].  An accompanying packet stream or
      streams may be generated to increase overall traffic load, though
      the loading stream(s) may not be measured.

   o  The packets in the stream of interest have fields or field values
      (or are augmented or modified to include fields or field values)
      that are dedicated to measurement.  Since measurement usually
      requires determining the corresponding packets at multiple
      measurement points, a sequence number is the most common
      information dedicated to measurement, and it is often combined
      with a timestamp.

   o  The Source and Destination of the packet stream of interest are
      usually known a priori.

   o  The characteristics of the packet stream of interest are known at
      the Source (at least), and may be communicated to the Destination
      as part of the method.  Note that some packet characteristics will
      normally change during packet forwarding.  Other changes along the
      path are possible, see [STDFORM].

   When adding traffic to the network for measurement, Active Methods
   influence the quantities measured to some degree, and those
   performing tests should take steps to quantify the effect(s) and/or
   minimize such effects.

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3.5.  Active Metric

   An Active Metric incorporates one or more of the aspects of Active
   Methods in the metric definition.

   For example, IETF metrics for IP performance (developed according to
   the framework described in [RFC2330]) include the Source-packet
   stream characteristics as metric-input parameters, and also specify
   the packet characteristics (Type-P) and Source and Destination IP
   addresses (with their implications on both stream treatment and
   interfaces associated with measurement points).

3.6.  Passive Methods

   Passive Methods of Measurement are:

   o  based solely on observations of an undisturbed and unmodified
      packet stream of interest (in other words, the method of
      measurement MUST NOT add, change, or remove packets or fields or
      change field values anywhere along the path).

   o  dependent on the existence of one or more packet streams to supply
      the stream of interest.

   o  dependent on the presence of the packet stream of interest at one
      or more designated Observation Points.

   Some Passive Methods simply observe and collect information on all
   packets that pass Observation Point(s), while others filter the
   packets as a first step and only collect information on packets that
   match the filter criteria, and thereby narrow the stream of interest.

   It is common that Passive Methods are conducted at one or more
   Observation Points.  Passive Methods to assess Performance Metrics
   often require multiple Observation Points, e.g., to assess the
   latency of packet transfer across a network path between two
   Observation Points.  In this case, the observed packets must include
   enough information to determine the corresponding packets at
   different Observation Points.

   Communication of the observations (in some form) to a collector is an
   essential aspect of Passive Methods.  In some configurations, the
   traffic load generated when communicating (or exporting) the Passive
   Method results to a collector may itself influence the measured
   network's performance.  However, the collection of results is not
   unique to Passive Methods, and the load from management and
   operations of measurement systems must always be considered for
   potential effects on the measured values.

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3.7.  Passive Metric

   Passive Metrics apply to observations of packet traffic (traffic
   flows in [RFC7011]).

   Passive performance metrics are assessed independently of the packets
   or traffic flows, and solely through observation.  Some refer to such
   assessments as "out of band".

   One example of Passive Performance Metrics for IP packet transfer can
   be found in ITU-T Recommendation Y.1540 [Y.1540], where the metrics
   are defined on the basis of reference events generated as packets
   pass reference points.  The metrics are agnostic to the distinction
   between Active and Passive when the necessary packet correspondence
   can be derived from the observed stream of interest as required.

3.8.  Hybrid Methods and Metrics

   Hybrid Methods are Methods of Measurement that use a combination of
   Active Methods and Passive Methods, to assess Active Metrics, Passive
   Metrics, or new metrics derived from the a priori knowledge and
   observations of the stream of interest.  ITU-T Recommendation Y.1540
   [Y.1540] defines metrics that are also applicable to the hybrid
   categories, since packet correspondence at different observation/
   reference points could be derived from "fields or field values which
   are dedicated to measurement", but otherwise the methods are Passive.

   There are several types of Hybrid Methods, as categorized below.

   With respect to a *single* stream of interest, Hybrid Type I methods
   fit in the continuum as follows, in terms of what happens at the
   Source (or Observation Point nearby):

   o  Generation of the stream of interest => Active

   o  Augmentation or modification of the stream of interest, or
      employment of methods that modify the treatment of the stream =>
      Hybrid Type I

   o  Observation of a stream of interest => Passive

   As an example, consider the case where the method generates traffic
   load stream(s), and observes an existing stream of interest according
   to the criteria for Passive Methods.  Since loading streams are an
   aspect of Active Methods, the stream of interest is not "solely
   observed", and the measurements involve a single stream of interest
   whose treatment has been modified by the presence of the load.
   Therefore, this is a Hybrid Type I method.

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   We define Hybrid Type II as follows: Methods that employ two or more
   different streams of interest with some degree of mutual coordination
   (e.g., one or more Active streams and one or more undisturbed and
   unmodified packet streams) to collect both Active and Passive Metrics
   and enable enhanced characterization from additional joint analysis.
   [HYBRID] presents a problem statement for Hybrid Type II Methods and
   Metrics.  Note that one or more Hybrid Type I streams could be
   substituted for the Active streams or undisturbed streams in the
   mutually coordinated set.  It is the Type II Methods where unique
   Hybrid Metrics are anticipated to emerge.

   Methods based on a combination of a single (generated) Active stream
   and Passive observations applied to the stream of interest at
   intermediate Observation Points are also Hybrid Methods.  However,
   [RFC5644] already defines these as Spatial Metrics and Methods.  It
   is possible to replace the Active stream of [RFC5644] with a Hybrid
   Type I stream and measure Spatial Metrics (but this was unanticipated
   when [RFC5644] was developed).

   The table below illustrates the categorization of methods (where
   "Synthesis" refers to a combination of Active and Passive Method
   attributes).

                       | Single Stream          | Multiple Simultaneous
                       | of Interest            | Streams of Interest
                       |                        | from Different Methods
   ====================================================================
   Single Fundamental  | Active or Passive      |
   Method              |                        |

   Synthesis of        | Hybrid Type I          |
   Fundamental Methods |                        |

   Multiple Methods    | Spatial Metrics        | Hybrid Type II
                       | [RFC5644]              |

   There may be circumstances where results measured with Hybrid Methods
   can be considered equivalent to those measured with Passive Methods.
   This notion references the possibility of a "class C" where packets
   of different Type-P are treated equally in network implementation, as
   described in Section 13 of [RFC2330] and using the terminology for
   paths from Section 5 of [RFC2330]:

      Hybrid Methods of measurement that augment or modify packets of a
      "class C" in a host should produce results equivalent to Passive
      Methods of Measurement when hosts accessing and links transporting
      these packets along the path (other than those performing
      augmentation/modification) treat packets from both categories of

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      methods (with and without the augmentation/modification) as the
      same "class C".  The Passive Methods of Measurement represent the
      Ground Truth when comparing results between Passive and Hybrid
      Methods, and this comparison should be conducted to confirm the
      "class C" treatment.

4.  Discussion

   This section illustrates the definitions and presents some examples.

4.1.  Graphical Representation

   If we compare the Active and Passive Methods, there are at least two
   dimensions on which methods can be evaluated.  This evaluation space
   may be useful when a method is a combination of the two alternative
   methods.

   The two dimensions (initially chosen) are:

   Y-Axis:  "Effect of the measured stream on network conditions".  The
      degree to which the stream of interest biases overall network
      conditions experienced by that stream and other streams.  This is
      a key dimension for Active measurement error analysis.  (Comment:
      There is also the notion of time averages -- a measurement stream
      may have significant effect while it is present, but the stream is
      only generated 0.1% of the time.  On the other hand, observations
      alone have no effect on network performance.  To keep these
      dimensions simple, we consider the stream effect only when it is
      present, but note that reactive networks defined in [RFC7312] may
      exhibit bias for some time beyond the life of a stream.)

   X-Axis:  "a priori Stream Knowledge".  The degree to which stream
      characteristics are known a priori.  There are methodological
      advantages of knowing the source stream characteristics, and
      having complete control of the stream characteristics.  For
      example, knowing the number of packets in a stream allows more-
      efficient operation of the measurement receiver, and so is an
      asset for Active Methods of Measurement.  Passive Methods (with no
      sample filter) have few clues available to anticipate what
      protocol the first packet observed will use or how many packets
      will comprise the flow; once the standard protocol of a flow is
      known, the possibilities narrow (for some compliant flows).
      Therefore, this is a key dimension for Passive measurement error
      analysis.

   There are a few examples we can plot on a two-dimensional space.  We
   can anchor the dimensions with reference point descriptions.

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   Y-Axis:Effect of the measured stream on network conditions
   ^ Max
   |* Active using max capacity stream
   |
   |
   |
   |
   |* Active using stream with load of typical user
   |
   |
   |
   |* Active using extremely sparse, randomized stream
   |                             * PDM                        Passive
   | Min                                                            *
   +----------------------------------------------------------------|
   |                                                                |
   Stream          X-Axis: a priori Stream Knowledge        No Stream
   Characteristics                                    Characteristics
   Completely                                                   Known
   Known

   (In the graph above, "PDM" refers to [PDMOPTION], an IPv6 Option
   Header for Performance and Diagnostic Measurements, described in
   Section 4.2.)

   We recognize that method categorization could be based on additional
   dimensions, but this would require a different graphical approach.

   For example, "effect of stream of interest on network conditions"
   could easily be further qualified into:

   1.  effect on the performance of the stream of interest itself: for
       example, choosing a packet marking or Differentiated Services
       Code Point (DSCP) resulting in domain treatment as a real-time
       stream (as opposed to default/best-effort marking).

   2.  effect on unmeasured streams that share the path and/or
       bottlenecks: for example, an extremely sparse measured stream of
       minimal size packets typically has little effect on other flows
       (and itself), while a stream designed to characterize path
       capacity may affect all other flows passing through the capacity
       bottleneck (including itself).

   3.  effect on network conditions resulting in network adaptation: for
       example, a network monitoring load and congestion conditions
       might change routing, placing some flows on alternate paths to
       mitigate the congestion.

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   We have combined 1 and 2 on the Y-axis, as examination of examples
   indicates strong correlation of the effects in this pair, and network
   adaptation is not addressed.

   It is apparent that different methods of IP network measurement can
   produce different results, even when measuring the same path at the
   same time.  The two dimensions of the graph help us to understand how
   the results might change with the method chosen.  For example, an
   Active Method to assess throughput adds some amount of traffic to the
   network, which might result in lower throughput for all streams.
   However, a Passive Method to assess throughput can also err on the
   low side due to unknown limitations of the hosts providing traffic,
   competition for host resources, limitations of the network interface,
   or private sub-networks that are not an intentional part of the path,
   etc.  Hybrid Methods could easily suffer from both forms of error.
   Another example of potential errors stems from the pitfalls of using
   an Active stream with known a bias, such as a periodic stream defined
   in [RFC3432].  The strength of modeling periodic streams (like Voice
   over IP (VoIP)) is a potential weakness when extending the measured
   results to other application whose streams are non-periodic.  The
   solutions are to model the application streams more exactly with an
   Active Method or to accept the risks and potential errors with the
   Passive Method discussed above.

4.2.  Discussion of PDM

   In [PDMOPTION], an IPv6 Option Header for Performance and Diagnostic
   Measurements (PDM) is described which, when added to the stream of
   interest at strategic interfaces, supports performance measurements.
   This method processes a user traffic stream and adds "fields which
   are dedicated to measurement" (the measurement intent is made clear
   in the title of this option).  Thus:

   o  The method intends to have a minor effect on the measured stream
      and other streams in the network.  There are conditions where this
      intent may not be realized.

   o  The measured stream has unknown characteristics until it is
      processed to add the PDM Option header.  Note that if the packet
      MTU is exceeded after adding the header, the intent to have a
      minor effect will not be realized.

   We conclude that this is a Hybrid Type I method, having at least one
   characteristic of both Active and Passive Methods for a single stream
   of interest.

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4.3.  Discussion of "Coloring" Method

   [OPSAWG], proposed to color packets by re-writing a field of the
   stream at strategic interfaces to support performance measurements
   (noting that this is a difficult operation at an intermediate point
   on an encrypted Virtual Private Network).  This method processes a
   user traffic stream and inserts "fields or values which are dedicated
   to measurement".  Thus:

   o  The method intends to have a minor effect on the measured stream
      and other streams in the network (less than PDM above).  There are
      conditions where this intent may not be realized.

   o  The measured stream has unknown characteristics until it is
      processed to add the coloring in the header, and the stream could
      be measured and time-stamped during that process.

   We note that [COLORING] proposes a method similar to [OPSAWG], as
   discussion on the IPPM mailing list revealed.

   We conclude that this is a Hybrid Type I method, having at least one
   characteristic of both Active and Passive Methods for a single stream
   of interest.

4.4.  Brief Discussion of OAM Methods

   Many Operations, Administration, and Management (OAM) methods exist
   beyond the IP layer.  For example, [Y.1731] defines several different
   measurement methods that we would classify as follows:

   o  Loss Measurement (LM) occasionally injects frames with a count of
      previous frames since the last LM message.  We conclude LM is
      Hybrid Type I, because this method processes a user traffic stream
      and augments the stream of interest with frames having "fields
      which are dedicated to measurement".

   o  Synthetic Loss Measurement (SLM) and Delay Measurement (DM)
      methods both inject dedicated measurement frames, so the "stream
      of interest is generated as the basis of measurement".  We
      conclude that SLM and DM methods are Active Methods.

   We also recognize the existence of alternate terminology used in OAM
   at layers other than IP.  Readers are encouraged to consult [RFC6374]
   for MPLS Loss and Delay measurement terminology, for example.

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5.  Security Considerations

   When considering the security and privacy of those involved in
   measurement or those whose traffic is measured, there is sensitive
   information communicated and observed at observation and measurement
   points described above, and protocol issues to consider.  We refer
   the reader to the security and privacy considerations described in
   the Large-Scale Measurement of Broadband Performance (LMAP) Framework
   [RFC7594], which covers Active and Passive measurement techniques and
   supporting material on measurement context.

6.  References

6.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <http://www.rfc-editor.org/info/rfc2119>.

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

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

   [RFC5644]  Stephan, E., Liang, L., and A. Morton, "IP Performance
              Metrics (IPPM): Spatial and Multicast", RFC 5644,
              DOI 10.17487/RFC5644, October 2009,
              <http://www.rfc-editor.org/info/rfc5644>.

   [RFC5835]  Morton, A., Ed. and S. Van den Berghe, Ed., "Framework for
              Metric Composition", RFC 5835, DOI 10.17487/RFC5835, April
              2010, <http://www.rfc-editor.org/info/rfc5835>.

   [RFC6390]  Clark, A. and B. Claise, "Guidelines for Considering New
              Performance Metric Development", BCP 170, RFC 6390,
              DOI 10.17487/RFC6390, October 2011,
              <http://www.rfc-editor.org/info/rfc6390>.

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   [RFC7011]  Claise, B., Ed., Trammell, B., Ed., and P. Aitken,
              "Specification of the IP Flow Information Export (IPFIX)
              Protocol for the Exchange of Flow Information", STD 77,
              RFC 7011, DOI 10.17487/RFC7011, September 2013,
              <http://www.rfc-editor.org/info/rfc7011>.

   [RFC7312]  Fabini, J. and A. Morton, "Advanced Stream and Sampling
              Framework for IP Performance Metrics (IPPM)", RFC 7312,
              DOI 10.17487/RFC7312, August 2014,
              <http://www.rfc-editor.org/info/rfc7312>.

   [RFC7594]  Eardley, P., Morton, A., Bagnulo, M., Burbridge, T.,
              Aitken, P., and A. Akhter, "A Framework for Large-Scale
              Measurement of Broadband Performance (LMAP)", RFC 7594,
              DOI 10.17487/RFC7594, September 2015,
              <http://www.rfc-editor.org/info/rfc7594>.

6.2.  Informative References

   [COLORING]
              Chen, M., Ed., Zheng, L., Ed., Mirsky, G., Ed., Fioccola,
              G., Ed., and T. Mizrahi, Ed., "IP Flow Performance
              Measurement Framework", Work in Progress, draft-chen-ippm-
              coloring-based-ipfpm-framework-06, March 2016.

   [HYBRID]   Trammell, B., Zheng, L., Berenguer, S., and M. Bagnulo,
              "Hybrid Measurement using IPPM Metrics", Work in Progress,
              draft-trammell-ippm-hybrid-ps-01, February 2014.

   [OPSAWG]   Capello, A., Cociglio, M., Castaldelli, L., and A. Bonda,
              "A packet based method for passive performance
              monitoring", Work in Progress, draft-tempia-opsawg-p3m-04,
              February 2014.

   [PASSIVE]  Zheng, L., Elkins, N., Lingli, D., Ackermann, M., and G.
              Mirsky, "Framework for IP Passive Performance
              Measurements", Work in Progress, draft-zheng-ippm-
              framework-passive-03, February 2015.

   [PDMOPTION]
              Elkins, N. and M. Ackermann, "IPv6 Performance and
              Diagnostic Metrics (PDM) Destination Option", Work in
              Progress, draft-ietf-ippm-6man-pdm-option-02, April 2016.

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   [RFC6374]  Frost, D. and S. Bryant, "Packet Loss and Delay
              Measurement for MPLS Networks", RFC 6374,
              DOI 10.17487/RFC6374, September 2011,
              <http://www.rfc-editor.org/info/rfc6374>.

   [STDFORM]  Morton, A., Fabini, J., Elkins, N., Ackermann, M., and V.
              Hegde, "Updates for IPPM's Active Metric Framework:
              Packets of Type-P and Standard-Formed Packets", Work in
              Progress, draft-morton-ippm-2330-stdform-typep-02,
              December 2015.

   [Y.1540]   ITU-T, "Internet protocol data communication service - IP
              packet transfer and availability performance parameters",
              March 2011,
              <https://www.itu.int/rec/T-REC-Y.1540-201103-I/en>.

   [Y.1731]   ITU-T, "Operation, administration and management (OAM)
              functions and mechanisms for Ethernet-based networks",
              August 2015,
              <https://www.itu.int/rec/T-REC-G.8013-201508-I/en>.

Acknowledgements

   Thanks to Mike Ackermann for asking the right question, and for
   several suggestions on terminology.  Brian Trammell provided key
   terms and references for the Passive category, and suggested ways to
   expand the Hybrid description and types.  Phil Eardley suggested some
   hybrid scenarios for categorization as part of his review.  Tiziano
   Ionta reviewed the document and suggested the classification for the
   "coloring" Method of Measurement.  Nalini Elkins identified several
   areas for clarification following her review.  Bill Jouris, Stenio
   Fernandes, and Spencer Dawkins suggested several editorial
   improvements.  Tal Mizrahi, Joachim Fabini, Greg Mirsky, and Mike
   Ackermann raised many key considerations in their Working Group Last
   Call (WGLC) reviews, based on their broad measurement experience.

Author's Address

   Al Morton
   AT&T Labs
   200 Laurel Avenue South
   Middletown, NJ
   United States

   Email: acmorton@att.com