path: root/doc/rfc/rfc6576.txt

Internet Engineering Task Force (IETF)                      R. Geib, Ed.
Request for Comments: 6576                              Deutsche Telekom
BCP: 176                                                       A. Morton
Category: Best Current Practice                                AT&T Labs
ISSN: 2070-1721                                                R. Fardid
                                                    Cariden Technologies
                                                            A. Steinmitz
                                                        Deutsche Telekom
                                                              March 2012


       IP Performance Metrics (IPPM) Standard Advancement Testing

Abstract

   This document specifies tests to determine if multiple independent
   instantiations of a performance-metric RFC have implemented the
   specifications in the same way.  This is the performance-metric
   equivalent of interoperability, required to advance RFCs along the
   Standards Track.  Results from different implementations of metric
   RFCs will be collected under the same underlying network conditions
   and compared using statistical methods.  The goal is an evaluation of
   the metric RFC itself to determine whether its definitions are clear
   and unambiguous to implementors and therefore a candidate for
   advancement on the IETF Standards Track.  This document is an
   Internet Best Current Practice.

Status of This Memo

   This memo documents an Internet Best Current Practice.

   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).  Further information on
   BCPs is available in 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/rfc6576.











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Copyright Notice

   Copyright (c) 2012 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.

Table of Contents

   1. Introduction ....................................................3
      1.1. Requirements Language ......................................5
   2. Basic Idea ......................................................5
   3. Verification of Conformance to a Metric Specification ...........7
      3.1. Tests of an Individual Implementation against a Metric
           Specification ..............................................8
      3.2. Test Setup Resulting in Identical Live Network
           Testing Conditions .........................................9
      3.3. Tests of Two or More Different Implementations
           against a Metric Specification ............................15
      3.4. Clock Synchronization .....................................16
      3.5. Recommended Metric Verification Measurement Process .......17
      3.6. Proposal to Determine an Equivalence Threshold for
           Each Metric Evaluated .....................................20
   4. Acknowledgements ...............................................21
   5. Contributors ...................................................21
   6. Security Considerations ........................................21
   7. References .....................................................21
      7.1. Normative References ......................................21
      7.2. Informative References ....................................23
   Appendix A.  An Example on a One-Way Delay Metric Validation ......24
     A.1.  Compliance to Metric Specification Requirements ...........24
     A.2.  Examples Related to Statistical Tests for One-Way Delay ...25
   Appendix B.  Anderson-Darling K-sample Reference and 2 Sample
                C++ Code .............................................27
   Appendix C.  Glossary .............................................36








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1.  Introduction

   The Internet Standards Process as updated by RFC 6410 [RFC6410]
   specifies that widespread deployment and use is sufficient to show
   interoperability as a condition for advancement to Internet Standard.
   The previous requirement of interoperability tests prior to advancing
   an RFC to the Standard maturity level specified in RFC 2026 [RFC2026]
   and RFC 5657 [RFC5657] has been removed.  While the modified
   requirement is applicable to protocols, wide deployment of different
   measurement systems does not prove that the implementations measure
   metrics in a standard way.  Section 5.3 of RFC 5657 [RFC5657]
   explicitly mentions the special case of Standards that are not "on-
   the-wire" protocols.  While this special case is not explicitly
   mentioned by RFC 6410 [RFC6410], the four criteria in Section 2.2 of
   RFC 6410 [RFC6410] are augmented by this document for RFCs that
   specify performance metrics.  This document takes the position that
   flexible metric definitions can be proven to be clear and unambiguous
   through tests that compare the results from independent
   implementations.  It describes tests that infer whether metric
   specifications are sufficient using a definition of metric
   "interoperability": measuring equivalent results (in a statistical
   sense) under the same network conditions.  The document expands on
   this problem and its solution.

   In the case of a protocol specification, the notion of
   "interoperability" is reasonably intuitive -- the implementations
   must successfully "talk to each other", while exercising all features
   and options.  To achieve interoperability, two implementors need to
   interpret the protocol specifications in equivalent ways.  In the
   case of IP Performance Metrics (IPPM), this definition of
   interoperability is only useful for test and control protocols like
   the One-Way Active Measurement Protocol (OWAMP) [RFC4656] and the
   Two-Way Active Measurement Protocol (TWAMP) [RFC5357].

   A metric specification RFC describes one or more metric definitions,
   methods of measurement, and a way to report the results of
   measurement.  One example would be a way to test and report the one-
   way delay that data packets incur while being sent from one network
   location to another, using the One-Way Delay Metric.

   In the case of metric specifications, the conditions that satisfy the
   "interoperability" requirement are less obvious, and there is a need
   for IETF agreement on practices to judge metric specification
   "interoperability" in the context of the IETF Standards Process.
   This memo provides methods that should be suitable to evaluate metric
   specifications for Standards Track advancement.  The methods proposed
   here MAY be generally applicable to metric specification RFCs beyond
   those developed under the IPPM Framework [RFC2330].



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   Since many implementations of IP metrics are embedded in measurement
   systems that do not interact with one another (they were built before
   OWAMP and TWAMP), the interoperability evaluation called for in the
   IETF Standards Process cannot be determined by observing that
   independent implementations interact properly for various protocol
   exchanges.  Instead, verifying that different implementations give
   statistically equivalent results under controlled measurement
   conditions takes the place of interoperability observations.  Even
   when evaluating OWAMP and TWAMP RFCs for Standards Track advancement,
   the methods described here are useful to evaluate the measurement
   results because their validity would not be ascertained in protocol
   interoperability testing.

   The Standards advancement process aims at producing confidence that
   the metric definitions and supporting material are clearly worded and
   unambiguous, or reveals ways in which the metric definitions can be
   revised to achieve clarity.  The process also permits identification
   of options that were not implemented, so that they can be removed
   from the advancing specification.  Thus, the product of this process
   is information about the metric specification RFC itself:
   determination of the specifications or definitions that are clear and
   unambiguous and those that are not (as opposed to an evaluation of
   the implementations that assist in the process).

   This document defines a process to verify that implementations (or
   practically, measurement systems) have interpreted the metric
   specifications in equivalent ways and produce equivalent results.

   Testing for statistical equivalence requires ensuring identical test
   setups (or awareness of differences) to the best possible extent.
   Thus, producing identical test conditions is a core goal of this
   memo.  Another important aspect of this process is to test individual
   implementations against specific requirements in the metric
   specifications using customized tests for each requirement.  These
   tests can distinguish equivalent interpretations of each specific
   requirement.

   Conclusions on equivalence are reached by two measures.

   First, implementations are compared against individual metric
   specifications to make sure that differences in implementation are
   minimized or at least known.

   Second, a test setup is proposed ensuring identical networking
   conditions so that unknowns are minimized and comparisons are
   simplified.  The resulting separate data sets may be seen as samples
   taken from the same underlying distribution.  Using statistical
   methods, the equivalence of the results is verified.  To illustrate



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   application of the process and methods defined here, evaluation of
   the One-Way Delay Metric [RFC2679] is provided in Appendix A.  While
   test setups will vary with the metrics to be validated, the general
   methodology of determining equivalent results will not.  Documents
   defining test setups to evaluate other metrics should be developed
   once the process proposed here has been agreed and approved.

   The metric RFC advancement process begins with a request for protocol
   action accompanied by a memo that documents the supporting tests and
   results.  The procedures of [RFC2026] are expanded in [RFC5657],
   including sample implementation and interoperability reports.
   [TESTPLAN] can serve as a template for a metric RFC report that
   accompanies the protocol action request to the Area Director,
   including a description of the test setup, procedures, results for
   each implementation, and conclusions.

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.  Basic Idea

   The implementation of a standard compliant metric is expected to meet
   the requirements of the related metric specification.  So, before
   comparing two metric implementations, each metric implementation is
   individually compared against the metric specification.

   Most metric specifications leave freedom to implementors on non-
   fundamental aspects of an individual metric (or options).  Comparing
   different measurement results using a statistical test with the
   assumption of identical test path and testing conditions requires
   knowledge of all differences in the overall test setup.  Metric
   specification options chosen by implementors have to be documented.
   It is RECOMMENDED to use identical metric options for any test
   proposed here (an exception would be if a variable parameter of the
   metric definition is not configurable in one or more
   implementations).  Calibrations specified by metric standards SHOULD
   be performed to further identify (and possibly reduce) potential
   sources of error in the test setup.

   The IPPM Framework [RFC2330] expects that a "methodology for a metric
   should have the property that it is repeatable: if the methodology is
   used multiple times under identical conditions, it should result in
   consistent measurements".  This means an implementation is expected
   to repeatedly measure a metric with consistent results (repeatability
   with the same result).  Small deviations in the test setup are



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   expected to lead to small deviations in results only.  To
   characterize statistical equivalence in the case of small deviations,
   [RFC2330] and [RFC2679] suggest to apply a 95% confidence interval.
   Quoting RFC 2679, "95 percent was chosen because ... a particular
   confidence level should be specified so that the results of
   independent implementations can be compared".

   Two different implementations are expected to produce statistically
   equivalent results if they both measure a metric under the same
   networking conditions.  Formulating in statistical terms: separate
   metric implementations collect separate samples from the same
   underlying statistical process (the same network conditions).  The
   statistical hypothesis to be tested is the expectation that both
   samples do not expose statistically different properties.  This
   requires careful test design:

   o  The measurement test setup must be self-consistent to the largest
      possible extent.  To minimize the influence of the test and
      measurement setup on the result, network conditions and paths MUST
      be identical for the compared implementations to the largest
      possible degree.  This includes both the stability and non-
      ambiguity of routes taken by the measurement packets.  See
      [RFC2330] for a discussion on self-consistency.

   o  To minimize the influence of implementation options on the result,
      metric implementations SHOULD use identical options and parameters
      for the metric under evaluation.

   o  The sample size must be large enough to minimize its influence on
      the consistency of the test results.  This consideration may be
      especially important if two implementations measure with different
      average packet transmission rates.

   o  The implementation with the lowest average packet transmission
      rate determines the smallest temporal interval for which samples
      can be compared.

   o  Repeat comparisons with several independent metric samples to
      avoid random indications of compatibility (or the lack of it).

   The metric specifications themselves are the primary focus of
   evaluation, rather than the implementations of metrics.  The
   documentation produced by the advancement process should identify
   which metric definitions and supporting material were found to be
   clearly worded and unambiguous, OR it should identify ways in which
   the metric specification text should be revised to achieve clarity
   and unified interpretation.




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   The process should also permit identification of options that were
   not implemented, so that they can be removed from the advancing
   specification (this is an aspect more typical of protocol advancement
   along the Standards Track).

   Note that this document does not propose to base interoperability
   indications of performance-metric implementations on comparisons of
   individual singletons.  Individual singletons may be impacted by many
   statistical effects while they are measured.  Comparing two
   singletons of different implementations may result in failures with
   higher probability than comparing samples.

3.  Verification of Conformance to a Metric Specification

   This section specifies how to verify compliance of two or more IPPM
   implementations against a metric specification.  This document only
   proposes a general methodology.  Compliance criteria to a specific
   metric implementation need to be defined for each individual metric
   specification.  The only exception is the statistical test comparing
   two metric implementations that are simultaneously tested.  This test
   is applicable without metric-specific decision criteria.

   Several testing options exist to compare two or more implementations:

   o  Use a single test lab to compare the implementations and emulate
      the Internet with an impairment generator.

   o  Use a single test lab to compare the implementations and measure
      across the Internet.

   o  Use remotely separated test labs to compare the implementations
      and emulate the Internet with two "identically" configured
      impairment generators.

   o  Use remotely separated test labs to compare the implementations
      and measure across the Internet.

   o  Use remotely separated test labs to compare the implementations,
      measure across the Internet, and include a single impairment
      generator to impact all measurement flows in a non-discriminatory
      way.

   The first two approaches work, but involve higher expenses than the
   others (due to travel and/or shipping plus installation).  For the
   third option, ensuring two identically configured impairment
   generators requires well-defined test cases and possibly identical
   hardware and software.




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   As documented in a test report [TESTPLAN], the last option was
   required to prove compatibility of two delay metric implementations.
   An impairment generator is probably required when testing
   compatibility of most other metrics, and it is therefore RECOMMENDED
   to include an impairment generator in metric test setups.

3.1.  Tests of an Individual Implementation against a Metric
      Specification

   A metric implementation is compliant with a metric specification if
   it supports the requirements classified as "MUST" and "REQUIRED" in
   the related metric specification.  An implementation that implements
   all requirements is fully compliant with the specification, and the
   degree of compliance SHOULD be noted in the conclusions of the
   report.

   Further, supported options of a metric implementation SHOULD be
   documented in sufficient detail to evaluate whether the specification
   was correctly interpreted.  The documentation of chosen options
   should minimize (and recognize) differences in the test setup if two
   metric implementations are compared.  Further, this documentation is
   used to validate or clarify the wording of the metric specification
   option, to remove options that saw no implementation or that are
   badly specified from the metric specification.  This documentation
   SHOULD be included for all implementation-relevant specifications of
   a metric picked for a comparison, even those that are not explicitly
   marked as "MUST" or "REQUIRED" in the RFC text.  This applies for the
   following sections of all metric specifications:

   o  Singleton Definition of the Metric.

   o  Sample Definition of the Metric.

   o  Statistics Definition of the Metric.  As statistics are compared
      by the test specified here, this documentation is required even in
      the case that the metric specification does not contain a
      Statistics Definition.

   o  Timing- and Synchronization-related specification (if relevant for
      the Metric).

   o  Any other technical part present or missing in the metric
      specification, which is relevant for the implementation of the
      Metric.







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   [RFC2330] and [RFC2679] emphasize precision as an aim of IPPM metric
   implementations.  A single IPPM-conforming implementation should
   under otherwise identical network conditions produce precise results
   for repeated measurements of the same metric.

   RFC 2330 prefers the "empirical distribution function" (EDF) to
   describe collections of measurements.  RFC 2330 determines, that
   "unless otherwise stated, IPPM goodness-of-fit tests are done using
   5% significance".  The goodness-of-fit test determines by which
   precision two or more samples of a metric implementation belong to
   the same underlying distribution (of measured network performance
   events).  The goodness-of-fit test suggested for the metric test is
   the Anderson-Darling K sample test (ADK sample test, K stands for the
   number of samples to be compared) [ADK].  Please note that RFC 2330
   and RFC 2679 apply an Anderson-Darling goodness-of-fit test, too.

   The results of a repeated test with a single implementation MUST pass
   an ADK sample test with a confidence level of 95%.  The conditions
   for which the ADK test has been passed with the specified confidence
   level MUST be documented.  To formulate this differently, the
   requirement is to document the set of parameters with the smallest
   deviation at which the results of the tested metric implementation
   pass an ADK test with a confidence level of 95%.  The minimum
   resolution available in the reported results from each implementation
   MUST be taken into account in the ADK test.

   The test conditions to be documented for a passed metric test
   include:

   o  The metric resolution at which a test was passed (e.g., the
      resolution of timestamps).

   o  The parameters modified by an impairment generator.

   o  The impairment generator parameter settings.

3.2.  Test Setup Resulting in Identical Live Network Testing Conditions

   Two major issues complicate tests for metric compliance across live
   networks under identical testing conditions.  One is the general
   point that metric definition implementations cannot be conveniently
   examined in field measurement scenarios.  The other one is more
   broadly described as "parallelism in devices and networks", including
   mechanisms like those that achieve load balancing (see [RFC4928]).

   This section proposes two measures to deal with both issues.
   Tunneling mechanisms can be used to avoid parallel processing of
   different flows in the network.  Measuring by separate parallel probe



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   flows results in repeated collection of data.  If both measures are
   combined, Wide Area Network (WAN) conditions are identical for a
   number of independent measurement flows, no matter what the network
   conditions are in detail.

   Any measurement setup must be made to avoid the probing traffic
   itself to impede the metric measurement.  The created measurement
   load must not result in congestion at the access link connecting the
   measurement implementation to the WAN.  The created measurement load
   must not overload the measurement implementation itself, e.g., by
   causing a high CPU load or by causing timestamp imprecision due to
   unwanted queuing while transmitting or receiving test packets.

   Tunneling multiple flows destined for a single physical port of a
   network element allows transmission of all packets via the same path.
   Applying tunnels to avoid undesired influence of standard routing for
   measurement purposes is a concept known from literature, see e.g.,
   GRE-encapsulated multicast probing [GU-Duffield].  An existing
   IP-in-IP tunnel protocol can be applied to avoid Equal-Cost Multi-
   Path (ECMP) routing of different measurement streams if it meets the
   following criteria:

   o  Inner IP packets from different measurement implementations are
      mapped into a single tunnel with a single outer IP origin and
      destination address as well as origin and destination port numbers
      that are identical for all packets.

   o  An easily accessible tunneling protocol allows for carrying out a
      metric test from more test sites.

   o  A low operational overhead may enable a broader audience to set up
      a metric test with the desired properties.

   o  The tunneling protocol should be reliable and stable in setup and
      operation to avoid disturbances or influence on the test results.

   o  The tunneling protocol should not incur any extra cost for those
      interested in setting up a metric test.

   An illustration of a test setup with two layer 2 tunnels and two
   flows between two linecards of one implementation is given in
   Figure 1.









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           Implementation                   ,---.       +--------+
                               +~~~~~~~~~~~/     \~~~~~~| Remote |
            +------->-----F2->-|          /       \     |->---+  |
            | +---------+      | Tunnel 1(         )    |     |  |
            | | transmit|-F1->-|         (         )    |->+  |  |
            | | LC1     |      +~~~~~~~~~|         |~~~~|  |  |  |
            | | receive |-<--+           (         )    | F1  F2 |
            | +---------+    |           |Internet |    |  |  |  |
            *-------<-----+  F2          |         |    |  |  |  |
              +---------+ |  | +~~~~~~~~~|         |~~~~|  |  |  |
              | transmit|-*  *-|         |         |    |--+<-*  |
              | LC2     |      | Tunnel 2(         )    |  |     |
              | receive |-<-F1-|          \       /     |<-*     |
              +---------+      +~~~~~~~~~~~\     /~~~~~~| Router |
                                            `-+-'       +--------+

     For simplicity, only two linecards of one implementation and two
                      flows F between them are shown.

      Figure 1: Illustration of a Test Setup with Two Layer 2 Tunnels

   Figure 2 shows the network elements required to set up layer 2
   tunnels as shown by Figure 1.

            Implementation

            +-----+                   ,---.
            | LC1 |                  /     \
            +-----+                 /       \              +------+
               |        +-------+  (         )  +-------+  |Remote|
            +--------+  |       |  |         |  |       |  |      |
            |Ethernet|  | Tunnel|  |Internet |  | Tunnel|  |      |
            |Switch  |--| Head  |--|         |--| Head  |--|      |
            +--------+  | Router|  |         |  | Router|  |      |
               |        |       |  (         )  |       |  |Router|
            +-----+     +-------+   \       /   +-------+  +------+
            | LC2 |                  \     /
            +-----+                   `-+-'

   Figure 2: Illustration of a Hardware Setup to Realize the Test Setup
        Illustrated by Figure 1 with Layer 2 Tunnels or Pseudowires










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   The test setup successfully used during a delay metric test
   [TESTPLAN] is given as an example in Figure 3.  Note that the shown
   setup allows a metric test between two remote sites.

           +----+  +----+                                +----+  +----+
           |LC10|  |LC11|           ,---.                |LC20|  |LC21|
           +----+  +----+          /     \    +-------+  +----+  +----+
             | V10  | V11         /       \   | Tunnel|   | V20   |  V21
             |      |            (         )  | Head  |   |       |
            +--------+  +------+ |         |  | Router|__+----------+
            |Ethernet|  |Tunnel| |Internet |  +---B---+  |Ethernet  |
            |Switch  |--|Head  |-|         |      |      |Switch    |
            +-+--+---+  |Router| |         |  +---+---+  +--+--+----+
              |__|      +--A---+ (         )--|Option.|     |__|
                                  \       /   |Impair.|
            Bridge                 \     /    |Gener. |     Bridge
            V20 to V21              `-+-?     +-------+     V10 to V11

     Figure 3: Example of Test Setup Successfully Used during a Delay
                                Metic Test

   In Figure 3, LC10 identifies measurement clients / linecards.  V10
   and the others denote VLANs.  All VLANs are using the same tunnel
   from A to B and in the reverse direction.  The remote site VLANs are
   U-bridged at the local site Ethernet switch.  The measurement packets
   of site 1 travel tunnel A->B first, are U-bridged at site 2, and
   travel tunnel B->A second.  Measurement packets of site 2 travel
   tunnel B->A first, are U-bridged at site 1, and travel tunnel A->B
   second.  So, all measurement packets pass the same tunnel segments,
   but in different segment order.

   If tunneling is applied, two tunnels MUST carry all test traffic in
   between the test site and the remote site.  For example, if 802.1Q
   Virtual LANs (VLANs) are applied and the measurement streams are
   carried in different VLANs, the IP tunnel or pseudowires respectively
   are setup in physical port mode to avoid setup of pseudowires per
   VLAN (which may see different paths due to ECMP routing); see
   [RFC4448].  The remote router and the Ethernet switch shown in
   Figure 3 have to support 802.1Q in this setup.

   The IP packet size of the metric implementation SHOULD be chosen
   small enough to avoid fragmentation due to the added Ethernet and
   tunnel headers.  Otherwise, the impact of tunnel overhead on
   fragmentation and interface MTU size must be understood and taken
   into account (see [RFC4459]).






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   An Ethernet port mode IP tunnel carrying several 802.1Q VLANs each
   containing measurement traffic of a single measurement system was
   successfully applied when testing compatibility of two metric
   implementations [TESTPLAN].  Ethernet over Layer 2 Tunneling Protocol
   Version 3 (L2TPv3) [RFC4719] was picked for this test.

   The following headers may have to be accounted for when calculating
   total packet length, if VLANs and Ethernet over L2TPv3 tunnels are
   applied:

   o  Ethernet 802.1Q: 22 bytes.

   o  L2TPv3 Header: 4-16 bytes for L2TPv3 data messages over IP; 16-28
      bytes for L2TPv3 data messages over UDP.

   o  IPv4 Header (outer IP header): 20 bytes.

   o  MPLS Labels may be added by a carrier.  Each MPLS Label has a
      length of 4 bytes.  At the time of this writing, between 1 and 4
      Labels seems to be a fair guess of what's expected.

   The applicability of one or more of the following tunneling protocols
   may be investigated by interested parties if Ethernet over L2TPv3 is
   felt to be unsuitable: IP in IP [RFC2003] or Generic Routing
   Encapsulation (GRE) [RFC2784].  RFC 4928 [RFC4928] proposes measures
   how to avoid ECMP treatment in MPLS networks.

   L2TP is a commodity tunneling protocol [RFC2661].  At the time of
   this writing, L2TPv3 [RFC3931] is the latest version of L2TP.  If
   L2TPv3 is applied, software-based implementations of this protocol
   are not suitable for the test setup, as such implementations may
   cause incalculable delay shifts.

   Ethernet pseudowires may also be set up on MPLS networks [RFC4448].
   While there is no technical issue with this solution, MPLS interfaces
   are mostly found in the network provider domain.  Hence, not all of
   the above criteria for selecting a tunneling protocol are met.

   Note that setting up a metric test environment is not a plug-and-play
   issue.  Skilled networking engineers should be consulted and involved
   if a setup between remote sites is preferred.

   Passing or failing an ADK test with 2 samples could be a random
   result (note that [RFC2330] defines a sample as a set of singleton
   metric values produced by a measurement stream, and we continue to
   use this terminology here).  The error margin of a statistical test
   is higher if the number of samples it is based on is low (the number
   of samples taken influences the so-called "degree of freedom" of a



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   statistical test, and a higher degree of freedom produces more
   reliable results).  To pass an ADK with higher probability, the
   number of samples collected per implementation under identical
   networking conditions SHOULD be greater than 2.  Hardware and load
   constraints may enforce an upper limit on the number of simultaneous
   measurement streams.  The ADK test allows one to combine different
   samples (see Section 9 of [ADK]) and then to run a 2-sample test
   between combined samples.  At least 4 samples per implementation
   captured under identical networking conditions is RECOMMENDED when
   comparing different metric implementations by a statistical test.

   It is RECOMMENDED that tests be carried out by establishing N
   different parallel measurement flows.  Two or three linecards per
   implementation serving to send or receive measurement flows should be
   sufficient to create 4 or more parallel measurement flows.  Other
   options are to separate flows by DiffServ marks (without deploying
   any Quality of Service (QoS) in the inner or outer tunnel) or to use
   a single Constant Bitrate (CBR) flow and evaluate whether every n-th
   singleton belongs to a specific measurement flow.  Note that a
   practical test indeed showed that ADK passed with 4 samples even if a
   2-sample test failed [TESTPLAN].

   Some additional guidelines to calculate and compare samples to
   perform a metric test are:

   o  Comparing different probes of a common underlying distribution in
      terms of metrics characterizing a communication network requires
      respecting the temporal nature for which the assumption of a
      common underlying distribution may hold.  Any singletons or
      samples to be compared must be captured within the same time
      interval.

   o  If statistical events like rates are used to characterize measured
      metrics of a time interval, a minimum of 5 singletons of a
      relevant metric should be picked to ensure a minimum confidence
      into the reported value.  The error margin of the determined rate
      depends on the number of singletons (refer to statistical
      textbooks on student's t-test).  As an example, any packet loss
      measurement interval to be compared with the results of another
      implementation contains at least five lost packets to have some
      confidence that the observed loss rate wasn't caused by a small
      number of random packet drops.

   o  The minimum number of singletons or samples to be compared by an
      Anderson-Darling test should be 100 per tested metric
      implementation.  Note that the Anderson-Darling test detects small





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      differences in distributions fairly well and will fail for a high
      number of compared results (RFC 2330 mentions an example with 8192
      measurements where an Anderson-Darling test always failed).

   o  Generally, the Anderson-Darling test is sensitive to differences
      in the accuracy or bias associated with varying implementations or
      test conditions.  These dissimilarities may result in differing
      averages of samples to be compared.  An example may be different
      packet sizes, resulting in a constant delay difference between
      compared samples.  Therefore, samples to be compared by an
      Anderson-Darling test MAY be calibrated by the difference of the
      average values of the samples.  Any calibration of this kind MUST
      be documented in the test result.

3.3.  Tests of Two or More Different Implementations against a Metric
      Specification

   [RFC2330] expects that "a methodology for a given metric exhibits
   continuity if, for small variations in conditions, it results in
   small variations in the resulting measurements.  Slightly more
   precisely, for every positive epsilon, there exists a positive delta,
   such that if two sets of conditions are within delta of each other,
   then the resulting measurements will be within epsilon of each
   other".  A small variation in conditions in the context of the metric
   test proposed here can be seen as different implementations measuring
   the same metric along the same path.

   IPPM metric specifications, however, allow for implementor options to
   the largest possible degree.  It cannot be expected that two
   implementors allow 100% identical options in their implementations.
   Testers SHOULD pick the same metric measurement configurations for
   their systems when comparing their implementations by a metric test.

   In some cases, a goodness-of-fit test may not be possible or show
   disappointing results.  To clarify the difficulties arising from
   different metric implementation options, the individual options
   picked for every compared metric implementation should be documented
   as specified in Section 3.5.  If the cause of the failure is a lack
   of specification clarity or multiple legitimate interpretations of
   the definition text, the text should be modified and the resulting
   memo proposed for consensus and (possible) advancement to Internet
   Standard.

   The same statistical test as applicable to quantify precision of a
   single metric implementation must be used to compare metric result
   equivalence for different implementations.  To document





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   compatibility, the smallest measurement resolution at which the
   compared implementations passed the ADK sample test must be
   documented.

   For different implementations of the same metric, "variations in
   conditions" are reasonably expected.  The ADK test comparing samples
   of the different implementations may result in a lower precision than
   the test for precision in the same-implementation comparison.

3.4.  Clock Synchronization

   Clock synchronization effects require special attention.  Accuracy of
   one-way active delay measurements for any metric implementation
   depends on clock synchronization between the source and destination
   of tests.  Ideally, one-way active delay measurement [RFC2679] test
   endpoints either have direct access to independent GPS or CDMA-based
   time sources or indirect access to nearby NTP primary (stratum 1)
   time sources, equipped with GPS receivers.  Access to these time
   sources may not be available at all test locations associated with
   different Internet paths, for a variety of reasons out of scope of
   this document.

   When secondary (stratum 2 and above) time sources are used with NTP
   running across the same network, whose metrics are subject to
   comparative implementation tests, network impairments can affect
   clock synchronization and distort sample one-way values and their
   interval statistics.  Discarding sample one-way delay values for any
   implementation is recommended when one of the following reliability
   conditions is met:

   o  Delay is measured and is finite in one direction but not the
      other.

   o  Absolute value of the difference between the sum of one-way
      measurements in both directions and the round-trip measurement is
      greater than X% of the latter value.

   Examination of the second condition requires round-trip time (RTT)
   measurement for reference, e.g., based on TWAMP [RFC5357] in
   conjunction with one-way delay measurement.

   Specification of X% to strike a balance between identification of
   unreliable one-way delay samples and misidentification of reliable
   samples under a wide range of Internet path RTTs requires further
   study.






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   An IPPM-compliant metric implementation of an RFC that requires
   synchronized clocks is expected to provide precise measurement
   results.

   IF an implementation publishes a specification of its precision, such
   as "a precision of 1 ms (+/- 500 us) with a confidence of 95%", then
   the specification should be met over a useful measurement duration.
   For example, if the metric is measured along an Internet path that is
   stable and not congested, then the precision specification should be
   met over durations of an hour or more.

3.5.  Recommended Metric Verification Measurement Process

   In order to meet their obligations under the IETF Standards Process,
   the IESG must be convinced that each metric specification advanced to
   Internet Standard status is clearly written, that there are a
   sufficient number of verified equivalent implementations, and that
   options that have been implemented are documented.

   In the context of this document, metrics are designed to measure some
   characteristic of a data network.  An aim of any metric definition
   should be that it is specified in a way that can reliably measure the
   specific characteristic in a repeatable way across multiple
   independent implementations.

   Each metric, statistic, or option of those to be validated MUST be
   compared against a reference measurement or another implementation as
   specified in this document.

   Finally, the metric definitions, embodied in the text of the RFCs,
   are the objects that require evaluation and possible revision in
   order to advance to Internet Standard.

   IF two (or more) implementations do not measure an equivalent metric
   as specified by this document,

   AND sources of measurement error do not adequately explain the lack
   of agreement,

   THEN the details of each implementation should be audited along with
   the exact definition text to determine if there is a lack of clarity
   that has caused the implementations to vary in a way that affects the
   correspondence of the results.

   IF there was a lack of clarity or multiple legitimate interpretations
   of the definition text,





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   THEN the text should be modified and the resulting memo proposed for
   consensus and (possible) advancement along the Standards Track.

   Finally, all the findings MUST be documented in a report that can
   support advancement to Internet Standard, as described here (similar
   to the reports described in [RFC5657]).  The list of measurement
   devices used in testing satisfies the implementation requirement,
   while the test results provide information on the quality of each
   specification in the metric RFC (the surrogate for feature
   interoperability).

   The complete process of advancing a metric specification to a
   Standard as defined by this document is illustrated in Figure 4.

      ,---.
     /     \
    ( Start )
     \     /    Implementations
      `-+-'        +-------+
        |         /|   1   `.
    +---+----+   / +-------+ `.-----------+     ,-------.
    |  RFC   |  /             |Check for  |   ,' was RFC `. YES
    |        | /              |Equivalence....  clause x   ------+
    |        |/    +-------+  |under      |   `. clear?  ,'      |
    | Metric \.....|   2   ....relevant   |     `---+---'   +----+-----+
    | Metric |\    +-------+  |identical  |      No |       |Report    |
    | Metric | \              |network    |      +--+----+  |results + |
    |  ...   |  \             |conditions |      |Modify |  |Advance   |
    |        |   \ +-------+  |           |      |Spec   +--+RFC       |
    +--------+    \|   n   |.'+-----------+      +-------+  |request   |
                   +-------+                                +----------+

       Figure 4: Illustration of the Metric Standardization Process

   Any recommendation for the advancement of a metric specification MUST
   be accompanied by an implementation report.  The implementation
   report needs to include the tests performed, the applied test setup,
   the specific metrics in the RFC, and reports of the tests performed
   with two or more implementations.  The test plan needs to specify the
   precision reached for each measured metric and thus define the
   meaning of "statistically equivalent" for the specific metrics being
   tested.

   Ideally, the test plan would co-evolve with the development of the
   metric, since that's when participants have the clearest context in
   their minds regarding the different subtleties that can arise.





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   In particular, the implementation report MUST include the following
   at minimum:

   o  The metric compared and the RFC specifying it.  This includes
      statements as required by Section 3.1 ("Tests of an Individual
      Implementation against a Metric Specification") of this document.

   o  The measurement configuration and setup.

   o  A complete specification of the measurement stream (mean rate,
      statistical distribution of packets, packet size or mean packet
      size, and their distribution), Differentiated Services Code Point
      (DSCP), and any other measurement stream properties that could
      result in deviating results.  Deviations in results can also be
      caused if chosen IP addresses and ports of different
      implementations result in different layer 2 or layer 3 paths due
      to operation of Equal Cost Multi-Path routing in an operational
      network.

   o  The duration of each measurement to be used for a metric
      validation, the number of measurement points collected for each
      metric during each measurement interval (i.e., the probe size),
      and the level of confidence derived from this probe size for each
      measurement interval.

   o  The result of the statistical tests performed for each metric
      validation as required by Section 3.3 ("Tests of Two or More
      Different Implementations against a Metric Specification") of this
      document.

   o  A parameterization of laboratory conditions and applied traffic
      and network conditions allowing reproduction of these laboratory
      conditions for readers of the implementation report.

   o  The documentation helping to improve metric specifications defined
      by this section.

   All of the tests for each set SHOULD be run in a test setup as
   specified in Section 3.2 ("Test Setup Resulting in Identical Live
   Network Testing Conditions".

   If a different test setup is chosen, it is recommended to avoid
   effects falsifying results of validation measurements caused by real
   data networks (like parallelism in devices and networks).  Data
   networks may forward packets differently in the case of:






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   o  Different packet sizes chosen for different metric
      implementations.  A proposed countermeasure is selecting the same
      packet size when validating results of two samples or a sample
      against an original distribution.

   o  Selection of differing IP addresses and ports used by different
      metric implementations during metric validation tests.  If ECMP is
      applied on the IP or MPLS level, different paths can result (note
      that it may be impossible to detect an MPLS ECMP path from an IP
      endpoint).  A proposed countermeasure is to connect the
      measurement equipment to be compared by a NAT device or establish
      a single tunnel to transport all measurement traffic.  The aim is
      to have the same IP addresses and port for all measurement packets
      or to avoid ECMP-based local routing diversion by using a layer 2
      tunnel.

   o  Different IP options.

   o  Different DSCP.

   o  If the N measurements are captured using sequential measurements
      instead of simultaneous ones, then the following factors come into
      play: time varying paths and load conditions.

3.6.  Proposal to Determine an Equivalence Threshold for Each Metric
      Evaluated

   This section describes a proposal for maximum error of equivalence,
   based on performance comparison of identical implementations.  This
   comparison may be useful for both ADK and non-ADK comparisons.

   Each metric is tested by two or more implementations (cross-
   implementation testing).

   Each metric is also tested twice simultaneously by the *same*
   implementation, using different Src/Dst Address pairs and other
   differences such that the connectivity differences of the cross-
   implementation tests are also experienced and measured by the same
   implementation.

   Comparative results for the same implementation represent a bound on
   cross-implementation equivalence.  This should be particularly useful
   when the metric does *not* produce a continuous distribution of
   singleton values, such as with a loss metric or a duplication metric.
   Appendix A indicates how the ADK will work for one-way delay and
   should be likewise applicable to distributions of delay variation.





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   Appendix B discusses two possible ways to perform the ADK analysis:
   the R statistical language [Rtool] with ADK package [Radk] and C++
   code.

   Conclusion: the implementation with the largest difference in
   homogeneous comparison results is the lower bound on the equivalence
   threshold, noting that there may be other systematic errors to
   account for when comparing implementations.

   Thus, when evaluating equivalence in cross-implementation results:

   Maximum_Error = Same_Implementation_Error + Systematic_Error

   and only the systematic error need be decided beforehand.

   In the case of ADK comparison, the largest same-implementation
   resolution of distribution equivalence can be used as a limit on
   cross-implementation resolutions (at the same confidence level).

4.  Acknowledgements

   Gerhard Hasslinger commented a first draft version of this document;
   he suggested statistical tests and the evaluation of time series
   information.  Matthias Wieser's thesis on a metric test resulted in
   new input for this document.  Henk Uijterwaal and Lars Eggert have
   encouraged and helped to organize this work.  Mike Hamilton, Scott
   Bradner, David Mcdysan, and Emile Stephan commented on this document.
   Carol Davids reviewed a version of the document before it became a WG
   item.

5.  Contributors

   Scott Bradner, Vern Paxson, and Allison Mankin drafted [METRICTEST],
   and major parts of it are included in this document.

6.  Security Considerations

   This memo does not raise any specific security issues.

7.  References

7.1.  Normative References

   [RFC2003]      Perkins, C., "IP Encapsulation within IP", RFC 2003,
                  October 1996.

   [RFC2119]      Bradner, S., "Key words for use in RFCs to Indicate
                  Requirement Levels", BCP 14, RFC 2119, March 1997.



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   [RFC2330]      Paxson, V., Almes, G., Mahdavi, J., and M. Mathis,
                  "Framework for IP Performance Metrics", RFC 2330,
                  May 1998.

   [RFC2661]      Townsley, W., Valencia, A., Rubens, A., Pall, G.,
                  Zorn, G., and B. Palter, "Layer Two Tunneling Protocol
                  "L2TP"", RFC 2661, August 1999.

   [RFC2679]      Almes, G., Kalidindi, S., and M. Zekauskas, "A One-way
                  Delay Metric for IPPM", RFC 2679, September 1999.

   [RFC2784]      Farinacci, D., Li, T., Hanks, S., Meyer, D., and P.
                  Traina, "Generic Routing Encapsulation (GRE)",
                  RFC 2784, March 2000.

   [RFC3931]      Lau, J., Townsley, M., and I. Goyret, "Layer Two
                  Tunneling Protocol - Version 3 (L2TPv3)", RFC 3931,
                  March 2005.

   [RFC4448]      Martini, L., Rosen, E., El-Aawar, N., and G. Heron,
                  "Encapsulation Methods for Transport of Ethernet over
                  MPLS Networks", RFC 4448, April 2006.

   [RFC4656]      Shalunov, S., Teitelbaum, B., Karp, A., Boote, J., and
                  M. Zekauskas, "A One-way Active Measurement Protocol
                  (OWAMP)", RFC 4656, September 2006.

   [RFC4719]      Aggarwal, R., Townsley, M., and M. Dos Santos,
                  "Transport of Ethernet Frames over Layer 2 Tunneling
                  Protocol Version 3 (L2TPv3)", RFC 4719, November 2006.

   [RFC4928]      Swallow, G., Bryant, S., and L. Andersson, "Avoiding
                  Equal Cost Multipath Treatment in MPLS Networks",
                  BCP 128, RFC 4928, June 2007.

   [RFC5657]      Dusseault, L. and R. Sparks, "Guidance on
                  Interoperation and Implementation Reports for
                  Advancement to Draft Standard", BCP 9, RFC 5657,
                  September 2009.

   [RFC6410]      Housley, R., Crocker, D., and E. Burger, "Reducing the
                  Standards Track to Two Maturity Levels", BCP 9,
                  RFC 6410, October 2011.








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7.2.  Informative References

   [ADK]          Scholz, F. and M. Stephens, "K-sample Anderson-Darling
                  Tests of Fit, for Continuous and Discrete Cases",
                  University of Washington, Technical Report No. 81,
                  May 1986.

   [GU-Duffield]  Gu, Y., Duffield, N., Breslau, L., and S. Sen, "GRE
                  Encapsulated Multicast Probing: A Scalable Technique
                  for Measuring One-Way Loss", SIGMETRICS'07 San Diego,
                  California, USA, June 2007.

   [METRICTEST]   Bradner, S. and V. Paxson, "Advancement of metrics
                  specifications on the IETF Standards Track", Work
                  in Progress, August 2007.

   [RFC2026]      Bradner, S., "The Internet Standards Process --
                  Revision 3", BCP 9, RFC 2026, October 1996.

   [RFC4459]      Savola, P., "MTU and Fragmentation Issues with In-the-
                  Network Tunneling", RFC 4459, April 2006.

   [RFC5357]      Hedayat, K., Krzanowski, R., Morton, A., Yum, K., and
                  J. Babiarz, "A Two-Way Active Measurement Protocol
                  (TWAMP)", RFC 5357, October 2008.

   [Radk]         Scholz, F., "adk: Anderson-Darling K-Sample Test and
                  Combinations of Such Tests.  R package version 1.0",
                  2008.

   [Rtool]        R Development Core Team, "R: A language and
                  environment for statistical computing.  R Foundation
                  for Statistical Computing, Vienna, Austria.  ISBN
                  3-900051-07-0", 2011, <http://www.R-project.org/>.

   [TESTPLAN]     Ciavattone, L., Geib, R., Morton, A., and M. Wieser,
                  "Test Plan and Results for Advancing RFC 2679 on the
                  Standards Track", Work in Progress, March 2012.













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Appendix A.  An Example on a One-Way Delay Metric Validation

   The text of this appendix is not binding.  It is an example of what
   parts of a One-Way Delay Metric test could look like.

A.1.  Compliance to Metric Specification Requirements

   One-Way Delay, Loss Threshold, RFC 2679

   This test determines if implementations use the same configured
   maximum waiting time delay from one measurement to another under
   different delay conditions and correctly declare packets arriving in
   excess of the waiting time threshold as lost.  See Sections 3.5 (3rd
   bullet point) and 3.8.2 of [RFC2679].

   (1)  Configure a path with 1-second one-way constant delay.

   (2)  Measure one-way delay with 2 or more implementations, using
        identical waiting time thresholds for loss set at 2 seconds.

   (3)  Configure the path with 3-second one-way delay.

   (4)  Repeat measurements.

   (5)  Observe that the increase measured in step 4 caused all packets
        to be declared lost and that all packets that arrive
        successfully in step 2 are assigned a valid one-way delay.

   One-Way Delay, First Bit to Last Bit, RFC 2679

   This test determines if implementations register the same relative
   increase in delay from one measurement to another under different
   delay conditions.  This test tends to cancel the sources of error
   that may be present in an implementation.  See Section 3.7.2 of
   [RFC2679] and Section 10.2 of [RFC2330].

   (1)  Configure a path with X ms one-way constant delay and ideally
        include a low-speed link.

   (2)  Measure one-way delay with 2 or more implementations, using
        identical options and equal size small packets (e.g., 100 octet
        IP payload).

   (3)  Maintain the same path with X ms one-way delay.

   (4)  Measure one-way delay with 2 or more implementations, using
        identical options and equal size large packets (e.g., 1500 octet
        IP payload).



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   (5)  Observe that the increase measured in steps 2 and 4 is
        equivalent to the increase in ms expected due to the larger
        serialization time for each implementation.  Most of the
        measurement errors in each system should cancel, if they are
        stationary.

   One-Way Delay, RFC 2679

   This test determines if implementations register the same relative
   increase in delay from one measurement to another under different
   delay conditions.  This test tends to cancel the sources of error
   that may be present in an implementation.  This test is intended to
   evaluate measurements in Sections 3 and 4 of [RFC2679].

   (1)  Configure a path with X ms one-way constant delay.

   (2)  Measure one-way delay with 2 or more implementations, using
        identical options.

   (3)  Configure the path with X+Y ms one-way delay.

   (4)  Repeat measurements.

   (5)  Observe that the increase measured in steps 2 and 4 is ~Y ms for
        each implementation.  Most of the measurement errors in each
        system should cancel, if they are stationary.

   Error Calibration, RFC 2679

   This is a simple check to determine if an implementation reports the
   error calibration as required in Section 4.8 of [RFC2679].  Note that
   the context (Type-P) must also be reported.

A.2.  Examples Related to Statistical Tests for One-Way Delay

   A one-way delay measurement may pass an ADK test with a timestamp
   result of 1 ms.  The same test may fail if timestamps with a
   resolution of 100 microseconds are evaluated.  The implementation is
   then conforming to the metric specification up to a timestamp
   resolution of 1 ms.

   Let's assume another one-way delay measurement comparison between
   implementation 1 probing with a frequency of 2 probes per second and
   implementation 2 probing at a rate of 2 probes every 3 minutes.  To
   ensure reasonable confidence in results, sample metrics are
   calculated from at least 5 singletons per compared time interval.
   This means that sample delay values are calculated for each system
   for identical 6-minute intervals for the duration of the whole test.



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   Per 6-minute interval, the sample metric is calculated from 720
   singletons for implementation 1 and from 6 singletons for
   implementation 2.  Note that if outliers are not filtered, moving
   averages are an option for an evaluation too.  The minimum move of an
   averaging interval is three minutes in this example.

   The data in Table 1 may result from measuring one-way delay with
   implementation 1 (see column Implemnt_1) and implementation 2 (see
   column Implemnt_2).  Each data point in the table represents a
   (rounded) average of the sampled delay values per interval.  The
   resolution of the clock is one micro-second.  The difference in the
   delay values may result, e.g., from different probe packet sizes.

         +------------+------------+-----------------------------+
         | Implemnt_1 | Implemnt_2 | Implemnt_2 - Delta_Averages |
         +------------+------------+-----------------------------+
         |    5000    |    6549    |             4997            |
         |    5008    |    6555    |             5003            |
         |    5012    |    6564    |             5012            |
         |    5015    |    6565    |             5013            |
         |    5019    |    6568    |             5016            |
         |    5022    |    6570    |             5018            |
         |    5024    |    6573    |             5021            |
         |    5026    |    6575    |             5023            |
         |    5027    |    6577    |             5025            |
         |    5029    |    6580    |             5028            |
         |    5030    |    6585    |             5033            |
         |    5032    |    6586    |             5034            |
         |    5034    |    6587    |             5035            |
         |    5036    |    6588    |             5036            |
         |    5038    |    6589    |             5037            |
         |    5039    |    6591    |             5039            |
         |    5041    |    6592    |             5040            |
         |    5043    |    6599    |             5047            |
         |    5046    |    6606    |             5054            |
         |    5054    |    6612    |             5060            |
         +------------+------------+-----------------------------+

                                  Table 1

   Average values of sample metrics captured during identical time
   intervals are compared.  This excludes random differences caused by
   differing probing intervals or differing temporal distance of
   singletons resulting from their Poisson-distributed sending times.







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   In the example, 20 values have been picked (note that at least 100
   values are recommended for a single run of a real test).  Data must
   be ordered by ascending rank.  The data of Implemnt_1 and Implemnt_2
   as shown in the first two columns of Table 1 clearly fails an ADK
   test with 95% confidence.

   The results of Implemnt_2 are now reduced by the difference of the
   averages of column 2 (rounded to 6581 us) and column 1 (rounded to
   5029 us), which is 1552 us.  The result may be found in column 3 of
   Table 1.  Comparing column 1 and column 3 of the table by an ADK test
   shows that the data contained in these columns passes an ADK test
   with 95% confidence.

   Comment: Extensive averaging was used in this example because of the
   vastly different sampling frequencies.  As a result, the
   distributions compared do not exactly align with a metric in
   [RFC2679] but illustrate the ADK process adequately.

Appendix B.  Anderson-Darling K-sample Reference and 2 Sample C++ Code

   There are many statistical tools available, and this appendix
   describes two that are familiar to the authors.

   The "R tool" is a language and command-line environment for
   statistical computing and plotting [Rtool].  With the optional "adk"
   package installed [Radk], it can perform individual and combined
   sample ADK computations.  The user must consult the package
   documentation and the original paper [ADK] to interpret the results,
   but this is as it should be.

   The C++ code below will perform an AD2-sample comparison when
   compiled and presented with two column vectors in a file (using white
   space as separation).  This version contains modifications made by
   Wes Eddy in Sept 2011 to use the vectors and run as a stand-alone
   module.  The status of the comparison can be checked on the command
   line with "$ echo $?" or the last line can be replaced with a printf
   statement for adk_result instead.

  /*

      Copyright (c) 2012 IETF Trust and the persons identified
      as authors of the code.  All rights reserved.

      Redistribution and use in source and binary forms, with
      or without modification, is permitted pursuant to, and subject
      to the license terms contained in, the Simplified BSD License
      set forth in Section 4.c of the IETF Trust's Legal Provisions
      Relating to IETF Documents (http://trustee.ietf.org/license-info).



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  */

  /* Routines for computing the Anderson-Darling 2 sample
  * test statistic.
  *
  * Implemented based on the description in
  * "Anderson-Darling K Sample Test" Heckert, Alan and
  * Filliben, James, editors, Dataplot Reference Manual,
  * Chapter 15 Auxiliary, NIST, 2004.
  * Official Reference by 2010
  * Heckert, N. A. (2001).  Dataplot website at the
  * National Institute of Standards and Technology:
  * http://www.itl.nist.gov/div898/software/dataplot.html/
  * June 2001.
 */

 #include <iostream>
 #include <fstream>
 #include <vector>
 #include <sstream>

 using namespace std;

 int main() {
    vector<double> vec1, vec2;
    double adk_result;
    static int k, val_st_z_samp1, val_st_z_samp2,
               val_eq_z_samp1, val_eq_z_samp2,
               j, n_total, n_sample1, n_sample2, L,
               max_number_samples, line, maxnumber_z;
    static int column_1, column_2;
    static double adk, n_value, z, sum_adk_samp1,
                  sum_adk_samp2, z_aux;
    static double H_j, F1j, hj, F2j, denom_1_aux, denom_2_aux;
    static bool next_z_sample2, equal_z_both_samples;
    static int stop_loop1, stop_loop2, stop_loop3,old_eq_line2,
               old_eq_line1;

    static double adk_criterium = 1.993;

    /* vec1 and vec2 to be initialized with sample 1 and
     * sample 2 values in ascending order */
    while (!cin.eof()) {
       double f1, f2;
       cin >> f1;
       cin >> f2;
       vec1.push_back(f1);
       vec2.push_back(f2);



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    }

    k = 2;
    n_sample1 = vec1.size() - 1;
    n_sample2 = vec2.size() - 1;

    // -1 because vec[0] is a dummy value
    n_total = n_sample1 + n_sample2;

    /* value equal to the line with a value = zj in sample 1.
     * Here j=1, so the line is 1.
     */
    val_eq_z_samp1 = 1;

    /* value equal to the line with a value = zj in sample 2.
     * Here j=1, so the line is 1.
     */
    val_eq_z_samp2 = 1;

    /* value equal to the last line with a value < zj
     * in sample 1.  Here j=1, so the line is 0.
     */
    val_st_z_samp1 = 0;

    /* value equal to the last line with a value < zj
     * in sample 1.  Here j=1, so the line is 0.
     */
    val_st_z_samp2 = 0;

    sum_adk_samp1 = 0;
    sum_adk_samp2 = 0;
    j = 1;

    // as mentioned above, j=1
    equal_z_both_samples = false;

    next_z_sample2 = false;

    //assuming the next z to be of sample 1
    stop_loop1 = n_sample1 + 1;

    // + 1 because vec[0] is a dummy, see n_sample1 declaration
    stop_loop2 = n_sample2 + 1;
    stop_loop3 = n_total + 1;

    /* The required z values are calculated until all values
     * of both samples have been taken into account.  See the
     * lines above for the stoploop values.  Construct required



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     * to avoid a mathematical operation in the while condition.
     */
    while (((stop_loop1 > val_eq_z_samp1)
           || (stop_loop2 > val_eq_z_samp2)) && stop_loop3 > j)
    {
      if(val_eq_z_samp1 < n_sample1+1)
      {
     /* here, a preliminary zj value is set.
      * See below how to calculate the actual zj.
      */
            z = vec1[val_eq_z_samp1];

     /* this while sequence calculates the number of values
      * equal to z.
      */
            while ((val_eq_z_samp1+1 < n_sample1)
                    && z == vec1[val_eq_z_samp1+1] )
                    {
                    val_eq_z_samp1++;
                    }
            }
            else
            {
            val_eq_z_samp1 = 0;
            val_st_z_samp1 = n_sample1;

    // this should be val_eq_z_samp1 - 1 = n_sample1
            }

    if(val_eq_z_samp2 < n_sample2+1)
            {
            z_aux = vec2[val_eq_z_samp2];;

    /* this while sequence calculates the number of values
     * equal to z_aux
     */

            while ((val_eq_z_samp2+1 < n_sample2)
                    && z_aux == vec2[val_eq_z_samp2+1] )
                    {
                    val_eq_z_samp2++;
                    }

    /* the smaller of the two actual data values is picked
     * as the next zj.
     */

        if(z > z_aux)



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                    {
                    z = z_aux;
                    next_z_sample2 = true;
                    }
             else
                    {
                    if (z == z_aux)
                    {
                    equal_z_both_samples = true;
                    }

    /* This is the case if the last value of column1 is
     * smaller than the remaining values of column2.
     */
                   if (val_eq_z_samp1 == 0)
                    {
                    z = z_aux;
                    next_z_sample2 = true;
                    }
                }
            }
           else
              {
            val_eq_z_samp2 = 0;
            val_st_z_samp2 = n_sample2;

    // this should be val_eq_z_samp2 - 1 = n_sample2

            }

     /* in the following, sum j = 1 to L is calculated for
      * sample 1 and sample 2.
      */
           if (equal_z_both_samples)
              {

              /* hj is the number of values in the combined sample
               * equal to zj
               */
                   hj = val_eq_z_samp1 - val_st_z_samp1
                  + val_eq_z_samp2 - val_st_z_samp2;

              /* H_j is the number of values in the combined sample
               * smaller than zj plus one half the number of
               * values in the combined sample equal to zj
               * (that's hj/2).
               */
                  H_j = val_st_z_samp1 + val_st_z_samp2



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                         + hj / 2;

              /* F1j is the number of values in the 1st sample
               * that are less than zj plus one half the number
               * of values in this sample that are equal to zj.
               */

                  F1j = val_st_z_samp1 + (double)
                      (val_eq_z_samp1 - val_st_z_samp1) / 2;

              /* F2j is the number of values in the 1st sample
               * that are less than zj plus one half the number
               * of values in this sample that are equal to zj.
               */
                  F2j = val_st_z_samp2 + (double)
                     (val_eq_z_samp2 - val_st_z_samp2) / 2;

              /* set the line of values equal to zj to the
               * actual line of the last value picked for zj.
               */
                  val_st_z_samp1 = val_eq_z_samp1;

              /* Set the line of values equal to zj to the actual
               * line of the last value picked for zj of each
               * sample.  This is required as data smaller than zj
               * is accounted differently than values equal to zj.
               */
                  val_st_z_samp2 = val_eq_z_samp2;

              /* next the lines of the next values z, i.e., zj+1
               * are addressed.
               */
                val_eq_z_samp1++;

              /* next the lines of the next values z, i.e.,
               * zj+1 are addressed
               */
                  val_eq_z_samp2++;
                  }
           else
                  {

              /* the smaller z value was contained in sample 2;
               * hence, this value is the zj to base the following
               * calculations on.
               */
                            if (next_z_sample2)
                            {



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              /* hj is the number of values in the combined
               * sample equal to zj; in this case, these are
               * within sample 2 only.
               */
                            hj = val_eq_z_samp2 - val_st_z_samp2;

              /* H_j is the number of values in the combined sample
               * smaller than zj plus one half the number of
               * values in the combined sample equal to zj
               * (that's hj/2).
               */
                                H_j = val_st_z_samp1 + val_st_z_samp2
                              + hj / 2;

              /* F1j is the number of values in the 1st sample that
               * are less than zj plus one half the number of values in
               * this sample that are equal to zj.
               * As val_eq_z_samp2 < val_eq_z_samp1, these are the
               * val_st_z_samp1 only.
               */
                            F1j = val_st_z_samp1;

              /* F2j is the number of values in the 1st sample that
               * are less than zj plus one half the number of values in
               * this sample that are equal to zj.  The latter are from
               * sample 2 only in this case.
               */

                    F2j = val_st_z_samp2 + (double)
                         (val_eq_z_samp2 - val_st_z_samp2) / 2;

              /* Set the line of values equal to zj to the actual line
               * of the last value picked for zj of sample 2 only in
               * this case.
               */
                                val_st_z_samp2 = val_eq_z_samp2;

              /* next the line of the next value z, i.e., zj+1 is
               * addressed.  Here, only sample 2 must be addressed.
               */

                    val_eq_z_samp2++;
                                    if (val_eq_z_samp1 == 0)
                                    {
                                    val_eq_z_samp1 = stop_loop1;
                                    }
                            }




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    /* the smaller z value was contained in sample 2;
     * hence, this value is the zj to base the following
     * calculations on.
     */

                  else
                  {

    /* hj is the number of values in the combined
     * sample equal to zj; in this case, these are
     * within sample 1 only.
     */
                  hj = val_eq_z_samp1 - val_st_z_samp1;

    /* H_j is the number of values in the combined
     * sample smaller than zj plus one half the number
     * of values in the combined sample equal to zj
     * (that's hj/2).
     */

          H_j = val_st_z_samp1 + val_st_z_samp2
                + hj / 2;

    /* F1j is the number of values in the 1st sample that
     * are less than zj plus; in this case, these are within
     * sample 1 only one half the number of values in this
     * sample that are equal to zj.  The latter are from
     * sample 1 only in this case.
     */

          F1j = val_st_z_samp1 + (double)
               (val_eq_z_samp1 - val_st_z_samp1) / 2;

    /* F2j is the number of values in the 1st sample that
     * are less than zj plus one half the number of values
     * in this sample that are equal to zj.  As
     * val_eq_z_samp1 < val_eq_z_samp2, these are the
     * val_st_z_samp2 only.
     */

                  F2j = val_st_z_samp2;

    /* Set the line of values equal to zj to the actual line
     * of the last value picked for zj of sample 1 only in
     * this case.
     */

          val_st_z_samp1 = val_eq_z_samp1;



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    /* next the line of the next value z, i.e., zj+1 is
     * addressed.  Here, only sample 1 must be addressed.
     */
                  val_eq_z_samp1++;

                  if (val_eq_z_samp2 == 0)
                          {
                          val_eq_z_samp2 = stop_loop2;
                          }
                  }
                  }

            denom_1_aux = n_total * F1j - n_sample1 * H_j;
            denom_2_aux = n_total * F2j - n_sample2 * H_j;

            sum_adk_samp1 = sum_adk_samp1 + hj
                    * (denom_1_aux * denom_1_aux) /
                                       (H_j * (n_total - H_j)
                    - n_total * hj / 4);
            sum_adk_samp2 = sum_adk_samp2 + hj
           * (denom_2_aux * denom_2_aux) /
                               (H_j * (n_total - H_j)
          - n_total * hj / 4);

            next_z_sample2 = false;
            equal_z_both_samples = false;

    /* index to count the z.  It is only required to prevent
     * the while slope to execute endless
     */
            j++;
            }

    // calculating the adk value is the final step.
    adk_result = (double) (n_total - 1) / (n_total
           * n_total * (k - 1))
            * (sum_adk_samp1 / n_sample1
            + sum_adk_samp2 / n_sample2);


    /* if(adk_result <= adk_criterium)
     * adk_2_sample test is passed
     */
    return adk_result <= adk_criterium;
 }






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Appendix C.  Glossary

   +-------------+-----------------------------------------------------+
   | ADK         | Anderson-Darling K-Sample test, a test used to      |
   |             | check whether two samples have the same statistical |
   |             | distribution.                                       |
   | ECMP        | Equal Cost Multipath, a load-balancing mechanism    |
   |             | evaluating MPLS Labels stacks, IP addresses, and    |
   |             | ports.                                              |
   | EDF         | The "empirical distribution function" of a set of   |
   |             | scalar measurements is a function F(x), which for   |
   |             | any x gives the fractional proportion of the total  |
   |             | measurements that were smaller than or equal to x.  |
   | Metric      | A measured quantity related to the performance and  |
   |             | reliability of the Internet, expressed by a value.  |
   |             | This could be a singleton (single value), a sample  |
   |             | of single values, or a statistic based on a sample  |
   |             | of singletons.                                      |
   | OWAMP       | One-Way Active Measurement Protocol, a protocol for |
   |             | communication between IPPM measurement systems      |
   |             | specified by IPPM.                                  |
   | OWD         | One-Way Delay, a performance metric specified by    |
   |             | IPPM.                                               |
   | Sample      | A sample metric is derived from a given singleton   |
   | metric      | metric by evaluating a number of distinct instances |
   |             | together.                                           |
   | Singleton   | A singleton metric is, in a sense, one atomic       |
   | metric      | measurement of this metric.                         |
   | Statistical | A 'statistical' metric is derived from a given      |
   | metric      | sample metric by computing some statistic of the    |
   |             | values defined by the singleton metric on the       |
   |             | sample.                                             |
   | TWAMP       | Two-way Active Measurement Protocol, a protocol for |
   |             | communication between IPPM measurement systems      |
   |             | specified by IPPM.                                  |
   +-------------+-----------------------------------------------------+















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Authors' Addresses

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

   Phone: +49 6151 58 12747
   EMail: Ruediger.Geib@telekom.de


   Al Morton
   AT&T Labs
   200 Laurel Avenue South
   Middletown, NJ  07748
   USA

   Phone: +1 732 420 1571
   Fax:   +1 732 368 1192
   EMail: acmorton@att.com
   URI:   http://home.comcast.net/~acmacm/


   Reza Fardid
   Cariden Technologies
   888 Villa Street, Suite 500
   Mountain View, CA  94041
   USA

   Phone:
   EMail: rfardid@cariden.com


   Alexander Steinmitz
   Deutsche Telekom
   Memmelsdorfer Str. 209b
   Bamberg  96052
   Germany

   Phone:
   EMail: Alexander.Steinmitz@telekom.de









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