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Internet Engineering Task Force (IETF)                         A. MortonRequest for Comments: 7799                                     AT&T LabsCategory: Informational                                         May 2016ISSN: 2070-1721Active 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; seeSection 2 of RFC 5741.   Information about the current status of this document, any errata,   and how to provide feedback on it may be obtained athttp://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 toBCP 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.Morton                        Informational                     [Page 1]

RFC 7799               Active, Passive and Hybrid               May 2016Table of Contents1. Introduction ....................................................21.1. Requirements Language ......................................32. Purpose and Scope ...............................................33. Terms and Definitions ...........................................33.1. Performance Metric .........................................33.2. Method of Measurement ......................................43.3. Observation Point ..........................................43.4. Active Methods .............................................43.5. Active Metric ..............................................53.6. Passive Methods ............................................53.7. Passive Metric .............................................63.8. Hybrid Methods and Metrics .................................64. Discussion ......................................................84.1. Graphical Representation ...................................84.2. Discussion of PDM .........................................104.3. Discussion of "Coloring" Method ...........................114.4. Brief Discussion of OAM Methods ...........................115. Security Considerations ........................................126. References .....................................................126.1. Normative References ......................................126.2. Informative References ....................................13   Acknowledgements ..................................................14   Author's Address ..................................................141.  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.Morton                        Informational                     [Page 2]

RFC 7799               Active, Passive and Hybrid               May 2016   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]).Morton                        Informational                     [Page 3]

RFC 7799               Active, Passive and Hybrid               May 20163.2.  Method of Measurement   The procedure or set of operations having the object of determining a   Measured Value or Measurement Result.3.3.  Observation Point   SeeSection 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" (seeSection 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.Morton                        Informational                     [Page 4]

RFC 7799               Active, Passive and Hybrid               May 20163.5.  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.Morton                        Informational                     [Page 5]

RFC 7799               Active, Passive and Hybrid               May 20163.7.  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.Morton                        Informational                     [Page 6]

RFC 7799               Active, Passive and Hybrid               May 2016   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 inSection 13 of [RFC2330] and using the terminology for   paths fromSection 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 ofMorton                        Informational                     [Page 7]

RFC 7799               Active, Passive and Hybrid               May 2016      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.Morton                        Informational                     [Page 8]

RFC 7799               Active, Passive and Hybrid               May 2016   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 inSection 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.Morton                        Informational                     [Page 9]

RFC 7799               Active, Passive and Hybrid               May 2016   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.Morton                        Informational                    [Page 10]

RFC 7799               Active, Passive and Hybrid               May 20164.3.  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.Morton                        Informational                    [Page 11]

RFC 7799               Active, Passive and Hybrid               May 20165.  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.  References6.1.  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>.Morton                        Informational                    [Page 12]

RFC 7799               Active, Passive and Hybrid               May 2016   [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.Morton                        Informational                    [Page 13]

RFC 7799               Active, Passive and Hybrid               May 2016   [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.comMorton                        Informational                    [Page 14]