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Internet Engineering Task Force (IETF) J. Fabini Request for Comments: 7312 Vienna University of Technology Updates: 2330 A. Morton Category: Informational AT&T Labs ISSN: 2070-1721 August 2014

               Advanced Stream and Sampling Framework
                 for IP Performance Metrics (IPPM)


 To obtain repeatable results in modern networks, test descriptions
 need an expanded stream parameter framework that also augments
 aspects specified as Type-P for test packets.  This memo updates the
 IP Performance Metrics (IPPM) Framework, RFC 2330, with advanced
 considerations for measurement methodology and testing.  The existing
 framework mostly assumes deterministic connectivity, and that a
 single test stream will represent the characteristics of the path
 when it is aggregated with other flows.  Networks have evolved and
 test stream descriptions must evolve with them; otherwise, unexpected
 network features may dominate the measured performance.  This memo
 describes new stream parameters for both network characterization and
 support of application design using IPPM metrics.

Status of This Memo

 This document is not an Internet Standards Track specification; it is
 published for informational purposes.
 This document is a product of the Internet Engineering Task Force
 (IETF).  It represents the consensus of the IETF community.  It has
 received public review and has been approved for publication by the
 Internet Engineering Steering Group (IESG).  Not all documents
 approved by the IESG are a candidate for any level of Internet
 Standard; see Section 2 of RFC 5741.
 Information about the current status of this document, any errata,
 and how to provide feedback on it may be obtained at

Fabini & Morton Informational [Page 1] RFC 7312 Advanced Sampling August 2014

Copyright Notice

 Copyright (c) 2014 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
 ( 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.  Definition: Reactive Path Behavior  . . . . . . . . . . .   4
   1.2.  Requirements Language . . . . . . . . . . . . . . . . . .   5
 2.  Scope . . . . . . . . . . . . . . . . . . . . . . . . . . . .   5
 3.  New or Revised Stream Parameters  . . . . . . . . . . . . . .   5
   3.1.  Test Packet Type-P  . . . . . . . . . . . . . . . . . . .   6
     3.1.1.  Multiple Test Packet Lengths  . . . . . . . . . . . .   7
     3.1.2.  Test Packet Payload Content Optimization  . . . . . .   7
   3.2.  Packet History  . . . . . . . . . . . . . . . . . . . . .   8
   3.3.  Access Technology Change  . . . . . . . . . . . . . . . .   8
   3.4.  Time-Slotted Randomness Cancellation  . . . . . . . . . .   9
 4.  Quality of Metrics and Methodologies  . . . . . . . . . . . .  10
   4.1.  Revised Definition of Repeatability . . . . . . . . . . .  10
   4.2.  Continuity No Longer an Alternative Repeatability
         Criterion . . . . . . . . . . . . . . . . . . . . . . . .  11
   4.3.  Metrics Should Be Actionable  . . . . . . . . . . . . . .  12
   4.4.  It May Not Be Possible To Be Conservative . . . . . . . .  13
   4.5.  Spatial and Temporal Composition Support Unbiased
         Sampling  . . . . . . . . . . . . . . . . . . . . . . . .  13
   4.6.  When to Truncate the Poisson Sampling Distribution  . . .  13
 5.  Conclusions . . . . . . . . . . . . . . . . . . . . . . . . .  14
 6.  Security Considerations . . . . . . . . . . . . . . . . . . .  14
 7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  14
 8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  15
   8.1.  Normative References  . . . . . . . . . . . . . . . . . .  15
   8.2.  Informative References  . . . . . . . . . . . . . . . . .  16

Fabini & Morton Informational [Page 2] RFC 7312 Advanced Sampling August 2014

1. Introduction

 The IETF IPPM working group first created a framework for metric
 development in [RFC2330].  This framework has stood the test of time
 and enabled development of many fundamental metrics, while only being
 updated once in a specific area [RFC5835].
 The IPPM framework [RFC2330] generally relies on several assumptions,
 one of which is not explicitly stated but assumed: lightly loaded
 paths conform to the linear "serialization delay = packet size /
 capacity" equation, and they are state-less or history-less (with
 some exceptions, e.g., firewalls are mentioned).  However, this does
 not hold true for many modern network technologies, such as reactive
 paths (those with demand-driven resource allocation) and links with
 time-slotted operation.  Per-flow state can be observed on test
 packet streams, and such treatment will influence network
 characterization if it is not taken into account.  Flow history will
 also affect the performance of applications and be perceived by their
 Moreover, Sections 4 and 6.2 of [RFC2330] explicitly recommend
 repeatable measurement metrics and methodologies.  Measurements in
 today's access networks illustrate that methodological guidelines of
 [RFC2330] must be extended to capture the reactive nature of these
 networks.  There are proposed extensions to allow methodologies to
 fulfill the continuity requirement stated in Section 6.2 of
 [RFC2330], but it is impossible to guarantee they can do so.
 Practical measurements confirm that some link types exhibit distinct
 responses to repeated measurements with identical stimulus, i.e.,
 identical traffic patterns.  If feasible, appropriate fine-tuning of
 measurement traffic patterns can improve measurement continuity and
 repeatability for these link types as shown in [IBD].
 This memo updates the IPPM framework [RFC2330] with advanced
 considerations for measurement methodology and testing.  We note that
 the scope of IPPM work at the time of the publication of [RFC2330]
 (and during more than a decade that followed) was limited to active
 techniques or those that generate packet streams that are dedicated
 to measurement and do not monitor user traffic.  This memo retains
 that same scope.
 We stress that this update of [RFC2330] does not invalidate or
 require changes to the analytic metric definitions prepared in the
 IPPM working group to date.  Rather, it adds considerations for
 active measurement methodologies and expands the importance of
 existing conventions and notions in [RFC2330], such as "packets of

Fabini & Morton Informational [Page 3] RFC 7312 Advanced Sampling August 2014

 Among the evolutionary networking changes is a phenomenon we call
 "reactive behavior", as defined below.

1.1. Definition: Reactive Path Behavior

 Reactive path behavior will be observable by the test packet stream
 as a repeatable phenomenon where packet transfer performance
 characteristics *change* according to prior observations of the
 packet flow of interest (at the reactive host or link).  Therefore,
 reactive path behavior is nominally deterministic with respect to the
 flow of interest.  Other flows or traffic load conditions may result
 in additional performance-affecting reactions, but these are external
 to the characteristics of the flow of interest.
 In practice, a sender may not have absolute control of the ingress
 packet stream characteristics at a reactive host or link, but this
 does not change the deterministic reactions present there.  If we
 measure a path, the arrival characteristics at the reactive host/link
 are determined by the sending characteristics and the transfer
 characteristics of intervening hosts and links.  Identical traffic
 patterns at the sending host might generate different patterns at the
 input of the reactive host/link due to impairments in the
 intermediate subpath.  The reactive host/link is expected to provide
 a deterministic response on identical input patterns (composed of all
 flows, including the flow of interest).
 Other than the size of the payload at the layer of interest and the
 header itself, packet content does not influence the measurement.
 Reactive behavior at the IP layer is not influenced by the TCP ports
 in use, for example.  Therefore, the indication of reactive behavior
 must include the layer at which measurements are instituted.
 Examples include links with Active/Inactive state detectors, and
 hosts or links that revise their traffic serving and forwarding rates
 (up or down) based on packet arrival history.
 Although difficult to handle from a measurement point of view,
 reactive paths' entities are usually designed to improve overall
 network performance and user experience, for example, by making
 capacity available to an active user.  Reactive behavior may be an
 artifact of solutions to allocate scarce resources according to the
 demands of users; thus, it is an important problem to solve for
 measurement and other disciplines, such as application design.

Fabini & Morton Informational [Page 4] RFC 7312 Advanced Sampling August 2014

1.2. Requirements Language

 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
 document are to be interpreted as described in RFC 2119 [RFC2119].

2. Scope

 The purpose of this memo is to foster repeatable measurement results
 in modern networks by highlighting the key aspects of test streams
 and packets and making them part of the IPPM framework.
 The scope is to update key sections of [RFC2330], adding
 considerations that will aid the development of new measurement
 methodologies intended for today's IP networks.  Specifically, this
 memo describes useful stream parameters that complement the
 parameters discussed in Section 11.1 of [RFC2330] and the parameters
 described in Section 4.2 of [RFC3432] for periodic streams.
 The memo also provides new considerations to update the criteria for
 metrics in Section 4 of [RFC2330], the measurement methodology in
 Section 6.2 of [RFC2330], and other topics related to the quality of
 metrics and methods (see Section 4).
 Other topics in [RFC2330] that might be updated or augmented are
 deferred to future work.  This includes the topics of passive and
 various forms of hybrid active/passive measurements.

3. New or Revised Stream Parameters

 There are several areas where measurement methodology definition and
 test result interpretation will benefit from an increased
 understanding of the stream characteristics and the (possibly
 unknown) network conditions that influence the measured metrics.
 1.  Network treatment depends on the fullest extent on the "packet of
     Type-P" definition in [RFC2330], and has for some time.
  • State is often maintained on the per-flow basis at various

points in the path, where "flows" are determined by IP and

        other layers.  Significant treatment differences occur with
        the simplest of Type-P parameters: packet length.  Use of
        multiple lengths is RECOMMENDED.
  • Payload content optimization (compression or format

conversion) in intermediate segments breaks the convention of

        payload correspondence when correlating measurements are made
        at different points in a path.

Fabini & Morton Informational [Page 5] RFC 7312 Advanced Sampling August 2014

 2.  Packet history (instantaneous or recent test rate or inactivity,
     also for non-test traffic) profoundly influences measured
     performance, in addition to all the Type-P parameters described
     in [RFC2330].
 3.  Access technology may change during testing.  A range of transfer
     capacities and access methods may be encountered during a test
     session.  When different interfaces are used, the host seeking
     access will be aware of the technology change, which
     differentiates this form of path change from other changes in
     network state.  Section 14 of [RFC2330] addresses the possibility
     that a host may have more than one attachment to the network, and
     also that assessment of the measurement path (route) is valid for
     some length of time (in Sections 5 and 7 of [RFC2330]).  Here, we
     combine these two considerations under the assumption that
     changes may be more frequent and possibly have greater
     consequences on performance metrics.
 4.  Paths including links or nodes with time-slotted service
     opportunities represent several challenges to measurement (when
     the service time period is appreciable):
  • Random/unbiased sampling is not possible beyond one such link

in the path.

  • The above encourages a segmented approach to end-to-end

measurement, as described in [RFC6049] for Network

        Characterization (as defined in [RFC6703]), to understand the
        full range of delay and delay variation on the path.
        Alternatively, if application performance estimation is the
        goal (also defined in [RFC6703]), then a stream with unbiased
        or known-bias properties [RFC3432] may be sufficient.
  • Multi-modal delay variation makes central statistics

unimportant; others must be used instead.

 Each of these topics is treated in detail below.

3.1. Test Packet Type-P

 We recommend two Type-P parameters to be added to the factors that
 have impact on path performance measurements, namely packet length
 and payload type.  Carefully choosing these parameters can improve
 measurement methodologies in their continuity and repeatability when
 deployed in reactive paths.

Fabini & Morton Informational [Page 6] RFC 7312 Advanced Sampling August 2014

3.1.1. Multiple Test Packet Lengths

 Many instances of network characterization using IPPM metrics have
 relied on a single test packet length.  When testing to assess
 application performance or an aggregate of traffic, benchmarking
 methods have used a range of fixed lengths and frequently augmented
 fixed-size tests with a mixture of sizes, or Internet Mix (IMIX) as
 described in [RFC6985].
 Test packet length influences delay measurements, in that the IPPM
 one-way delay metric [RFC2679] includes serialization time in its
 first-bit to last-bit timestamping requirements.  However, different
 sizes can have a larger influence on link delay and link delay
 variation than serialization would explain alone.  This effect can be
 non-linear and change the instantaneous network performance when a
 different size is used, or the performance of packets following the
 size change.
 Repeatability is a main measurement methodology goal as stated in
 Section 6.2 of [RFC2330].  To eliminate packet length as a potential
 measurement uncertainty factor, successive measurements must use
 identical traffic patterns.  In practice, a combination of random
 payload and random start time can yield representative results as
 illustrated in [IRR].

3.1.2. Test Packet Payload Content Optimization

 The aim for efficient network resource use has resulted in deployment
 of server-only or client-server lossless or lossy payload compression
 techniques on some links or paths.  These optimizers attempt to
 compress high-volume traffic in order to reduce network load.  Files
 are analyzed by application-layer parsers, and parts (like comments)
 might be dropped.  Although typically acting on HTTP or JPEG files,
 compression might affect measurement packets, too.  In particular,
 measurement packets are qualified for efficient compression when they
 use standard plain-text payload.  We note that use of transport-layer
 encryption will counteract the deployment of network-based analysis
 and may reduce the adoption of payload optimizations, however.
 IPPM-conforming measurements should add packet payload content as a
 Type-P parameter, which can help to improve measurement determinism.
 Some packet payloads are more susceptible to compression than others,
 but optimizers in the measurement path can be out ruled by using
 incompressible packet payload.  This payload content could be
 supplied by a pseudo-random sequence generator or by using part of a
 compressed file (e.g., a part of a ZIP compressed archive).

Fabini & Morton Informational [Page 7] RFC 7312 Advanced Sampling August 2014

 Optimization can go beyond the scope of one single data or
 measurement stream.  Many more client- or network-centric
 optimization technologies have been proposed or standardized so far,
 including Robust Header Compression (ROHC) and Voice over IP
 aggregation as presented, for instance, in [EEAW].  Where
 optimization is feasible and valuable, many more of these
 technologies may follow.  As a general observation, the more
 concurrent flows an intermediate host treats and the longer the paths
 shared by flows are, the higher becomes the incentive of hosts to
 aggregate flows belonging to distinct sources.  Measurements should
 consider this potential additional source of uncertainty with respect
 to repeatability.  Aggregation of flows in networking devices can,
 for instance, result in reciprocal timing and performance influence
 of these flows, which may exceed typical reciprocal queueing effects
 by orders of magnitude.

3.2. Packet History

 Recent packet history and instantaneous data rate influence
 measurement results for reactive links supporting on-demand capacity
 allocation.  Measurement uncertainty may be reduced by knowledge of
 measurement packet history and total host load.  Additionally, small
 changes in history, e.g., because of lost packets along the path, can
 be the cause of large performance variations.
 For instance, delay in reactive 3G networks like High Speed Packet
 Access (HSPA) depends to a large extent on the test traffic data
 rate.  The reactive resource allocation strategy in these networks
 affects the uplink direction in particular.  Small changes in data
 rate can be the reason of more than a 200% increase in delay,
 depending on the specific packet size.  A detailed theoretical and
 practical analysis of Radio Resource Control (RRC) link transitions,
 which can cause such behavior in Universal Mobile Terrestrial System
 (UMTS) networks, is presented, e.g., in [RRC].

3.3. Access Technology Change

 [RFC2330] discussed the scenario of multi-homed hosts.  If hosts
 become aware of access technology changes (e.g., because of IP
 address changes or lower-layer information) and make this information
 available, measurement methodologies can use this information to
 improve measurement representativeness and relevance.
 However, today's various access network technologies can present the
 same physical interface to the host.  A host may or may not become
 aware when its access technology changes on such an interface.
 Measurements for paths that support on-demand capacity allocation
 are, therefore, challenging in that it is difficult to differentiate

Fabini & Morton Informational [Page 8] RFC 7312 Advanced Sampling August 2014

 between access technology changes (e.g., because of mobility) and
 reactive path behavior (e.g., because of data rate change).

3.4. Time-Slotted Randomness Cancellation

 Time-slotted operation of path entities -- interfaces, routers, or
 links -- in a network path is a particular challenge for
 measurements, especially if the time-slot period is substantial.  The
 central observation as an extension to Poisson stream sampling in
 [RFC2330] is that the first such time-slotted component cancels
 unbiased measurement stream sampling.  In the worst case, time-
 slotted operation converts an unbiased, random measurement packet
 stream into a periodic packet stream.  Being heavily biased, these
 packets may interact with periodic behavior of subsequent time-
 slotted network entities [TSRC].
 Time-slotted randomness cancellation (TSRC) sources can be found in
 virtually any system, network component or path, their impact on
 measurements being a matter of the order of magnitude when compared
 to the metric under observation.  Examples of TSRC sources include,
 but are not limited to, system clock resolution, operating system
 ticks, time-slotted component or network operation, etc.  The amount
 of measurement bias is determined by the particular measurement
 stream, relative offset between allocated time slots in subsequent
 path entities, delay variation in these paths, and other sources of
 variation.  Measurement results might change over time, depending on
 how accurately the sending host, receiving host, and time-slotted
 components in the measurement path are synchronized to each other and
 to global time.  If path segments maintain flow state, flow parameter
 change or flow reallocations can cause substantial variation in
 measurement results.
 Practical measurements confirm that such interference limits delay
 measurement variation to a subset of theoretical value range.
 Measurement samples for such cases can aggregate on artificial
 limits, generating multi-modal distributions as demonstrated in
 [IRR].  In this context, the desirable measurement sample statistics
 differentiate between multi-modal delay distributions caused by
 reactive path behavior and the ones due to time-slotted interference.
 Measurement methodology selection for time-slotted paths depends to a
 large extent on the respective viewpoint.  End-to-end metrics can
 provide accurate measurement results for short-term sessions and low
 likelihood of flow state modifications.  Applications or services
 that aim at approximating path performance for a short time interval
 (in the order of minutes) and expect stable path conditions should,

Fabini & Morton Informational [Page 9] RFC 7312 Advanced Sampling August 2014

 therefore, prefer end-to-end metrics.  Here, stable path conditions
 refer to any kind of global knowledge concerning measurement path
 flow state and flow parameters.
 However, if long-term forecast of time-slotted path performance is
 the main measurement goal, a segmented approach relying on
 measurement of subpath metrics is preferred.  Regenerating unbiased
 measurement traffic at any hop can help to reveal the true range of
 path performance for all path segments.

4. Quality of Metrics and Methodologies

 [RFC6808] proposes repeatability and continuity as one of the metric
 and methodology properties to infer on measurement quality.
 Depending mainly on the set of controlled measurement parameters,
 measurements repeated for a specific network path using a specific
 methodology may or may not yield repeatable results.  Challenging
 measurement scenarios for adequate parameter control include
 wireless, reactive, or time-slotted networks as discussed earlier in
 this document.  This section presents an expanded definition of
 "repeatability" beyond the definition in [RFC2330] and an expanded
 examination of the concept of "continuity" in [RFC2330] and its
 limited applicability.

4.1. Revised Definition of Repeatability

 [RFC2330] defines repeatability in a general way:
 "A methodology for a metric should have the property that it is
 repeatable: if the methodology is used multiple times under identical
 conditions, the same measurements should result in the same
 The challenge is to develop this definition further, such that it
 becomes an objective measurable criterion (and does not depend on the
 concept of continuity discussed below).  Fortunately, this topic has
 been treated in other IPPM work.  In BCP 176 [RFC6576], the criteria
 of equivalent results was agreed as the surrogate for
 interoperability when assessing metric RFCs for Standards Track
 advancement.  The criteria of equivalence were expressed as objective
 statistical requirements for comparison across the same
 implementations and independent implementations in the test plans
 specific to each RFC evaluated ([RFC2679] in the test plan of
 The tests of [RFC6808] rely on nearly identical conditions to be
 present for analysis and accept that these conditions cannot be
 exactly identical in the production network paths used.  The test

Fabini & Morton Informational [Page 10] RFC 7312 Advanced Sampling August 2014

 plans allow some correction factors to be applied (some statistical
 tests are hyper-sensitive to differences in the mean of
 distributions) and recognize the original findings of [RFC2330]
 regarding excess sample sizes.
 One way to view the reliance on identical conditions is to view it as
 a challenge: How few parameters and path conditions need to be
 controlled and still produce repeatable methods/measurements?
 Although the test plan in [RFC6808] documented numerical criteria for
 equivalence, we cannot specify the exact numerical criteria for
 repeatability *in general*.  The process in the BCP [RFC6576] and
 statistics in [RFC6808] have been used successfully, and the
 numerical criteria to declare a metric repeatable should be agreed by
 all interested parties prior to measurement.
 We revise the definition slightly, as follows:
    A methodology for a metric should have the property that it is
    repeatable: if the methodology is used multiple times under
    identical conditions, the methods should produce equivalent
    measurement results.

4.2. Continuity No Longer an Alternative Repeatability Criterion

 In the original framework [RFC2330], the concept of continuity was
 introduced to provide a relaxed criteria for judging repeatability
 and was described in Section 6.2 of [RFC2330] as follows:
 "...a methodology for a given metric exhibits continuity if, for
 small variations in conditions, it results in small variations in the
 resulting measurements."
 Although there are conditions where metrics may exhibit continuity,
 there are others where this criteria would fail for both user traffic
 and active measurement traffic.  Consider link fragmentation and the
 non-linear increase in delay when we increase packet size just beyond
 the limit of a single fragment.  An active measurement packet would
 see the same delay increase when exceeding the fragment size.
 The Bulk Transfer Capacity (BTC) [RFC3148] gives another example in
 Section 1, bottom of page 2:
    There is also evidence that most TCP implementations exhibit non-
    linear performance over some portion of their operating region.
    It is possible to construct simple simulation examples where
    incremental improvements to a path (such as raising the link data
    rate) results in lower overall TCP throughput (or BTC) [Mat98].

Fabini & Morton Informational [Page 11] RFC 7312 Advanced Sampling August 2014

 Clearly, the time-slotted network elements described in Section 3.4
 of this document also qualify as a new exception to the ideal of
    Therefore, we deprecate continuity as an alternate criterion on
    metrics and prefer the more exact evaluation of repeatability

4.3. Metrics Should Be Actionable

 The IP Performance Metrics Framework [RFC2330] includes usefulness as
 a metric criterion:
 "...The metrics must be useful to users and providers in
 understanding the performance they experience or provide...".
 When considering measurements as part of a maintenance process,
 evaluation of measurement results for a path under observation can
 draw attention to potential performance problems "somewhere" on the
 path.  Anomaly detection is, therefore, an important phase and first
 step that already satisfies the usefulness criterion for many
 This concept of usefulness can be extended, becoming a subset of what
 we refer to as "actionable" criterion in the following.  We note that
 this is not the term from law.
 Central to maintenance is the isolation of the root cause of reported
 anomalies down to a specific subpath, link or host, and metrics
 should support this second step as well.  While detection of path
 anomaly may be the result of an on-going monitoring process, the
 second step of cause isolation consists of specific, directed on-
 demand measurements on components and subpaths.  Metrics must support
 users in this directed search, becoming actionable:
    Metrics must enable users and operators to understand path
    performance and SHOULD help to direct corrective actions when
    warranted, based on the measurement results.
 Besides characterizing metrics, usefulness and actionable properties
 are also applicable to methodologies and measurements.

Fabini & Morton Informational [Page 12] RFC 7312 Advanced Sampling August 2014

4.4. It May Not Be Possible To Be Conservative

 [RFC2330] adopts the term "conservative" for measurement
 methodologies for which:
 "... the act of measurement does not modify, or only slightly
 modifies, the value of the performance metric the methodology
 attempts to measure."
 It should be noted that this definition of "conservative" in the
 sense of [RFC2330] depends to a large extent on the measurement
 path's technology and characteristics.  In particular, when deployed
 on reactive paths, subpaths, links or hosts conforming to the
 definition in Section 1.1 of this document, measurement packets can
 originate capacity (re)allocations.  In addition, small measurement
 flow variations can result in other users on the same path perceiving
 significant variations in measurement results.  Therefore:
    It is not always possible for the method to be conservative.

4.5. Spatial and Temporal Composition Support Unbiased Sampling

 Concepts related to temporal and spatial composition of metrics in
 Section 9 of [RFC2330] have been extended in [RFC5835].  [RFC5835]
 defines multiple new types of metrics, including Spatial Composition,
 Temporal Aggregation, and Spatial Aggregation.  So far, only the
 metrics for Spatial Composition have been standardized [RFC6049],
 providing the ability to estimate the performance of a complete path
 from subpath metrics.  Spatial Composition aligns with the finding of
 [TSRC] that unbiased sampling is not possible beyond the first time-
 slotted link within a measurement path.
    In cases where unbiased measurement for all segments of a path is
    not feasible due to the presence of a time-slotted link, restoring
    randomness of measurement samples when necessary is recommended as
    presented in [TSRC], in combination with Spatial Composition

4.6. When to Truncate the Poisson Sampling Distribution

 Section 11.1.1 of [RFC2330] describes Poisson sampling, where the
 inter-packet send times have a Poisson distribution.  A path element
 with reactive behavior sensitive to flow inactivity could change
 state if the random inter-packet time is too long.
    It is recommended to truncate the tail of Poisson distribution
    when needed to avoid reactive element state changes.

Fabini & Morton Informational [Page 13] RFC 7312 Advanced Sampling August 2014

 Tail truncation has been used without issue to ensure that minimum
 sample sizes can be attained in a fixed-test interval.

5. Conclusions

 Safeguarding repeatability as a key property of measurement
 methodologies is highly challenging and sometimes impossible in
 reactive paths.  Measurements in paths with demand-driven allocation
 strategies must use a prototypical application packet stream to infer
 a specific application's performance.  Measurement repetition with
 unbiased network and flow states (e.g., by rebooting measurement
 hosts) can help to avoid interference with periodic network behavior,
 with randomness being a mandatory feature for avoiding correlation
 with network timing.
 Inferring the path performance between one measurement session or
 packet stream and other sessions/streams with alternate
 characteristics is generally discouraged with reactive paths because
 of the huge set of global parameters that have influence on
 instantaneous path performance.

6. Security Considerations

 The security considerations that apply to any active measurement of
 live paths are relevant here as well.  See [RFC4656] and [RFC5357].
 When considering privacy of those involved in measurement or those
 whose traffic is measured, the sensitive information available to
 potential observers is greatly reduced when using active techniques
 that are within this scope of work.  Passive observations of user
 traffic for measurement purposes raise many privacy issues.  We refer
 the reader to the privacy considerations described in the Large Scale
 Measurement of Broadband Performance (LMAP) Framework [LMAP], which
 covers active and passive techniques.

7. Acknowledgements

 The authors thank Rudiger Geib, Matt Mathis, Konstantinos
 Pentikousis, and Robert Sparks for their helpful comments on this
 memo, Alissa Cooper and Kathleen Moriarty for suggesting ways to
 "update the update" for heightened privacy awareness and its
 consequences, and Ann Cerveny for her editorial review and comments
 that helped to improve readability overall.

Fabini & Morton Informational [Page 14] RFC 7312 Advanced Sampling August 2014

8. References

8.1. Normative References

 [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
            Requirement Levels", BCP 14, RFC 2119, March 1997.
 [RFC2330]  Paxson, V., Almes, G., Mahdavi, J., and M. Mathis,
            "Framework for IP Performance Metrics", RFC 2330, May
 [RFC2679]  Almes, G., Kalidindi, S., and M. Zekauskas, "A One-way
            Delay Metric for IPPM", RFC 2679, September 1999.
 [RFC3432]  Raisanen, V., Grotefeld, G., and A. Morton, "Network
            performance measurement with periodic streams", RFC 3432,
            November 2002.
 [RFC4656]  Shalunov, S., Teitelbaum, B., Karp, A., Boote, J., and M.
            Zekauskas, "A One-way Active Measurement Protocol
            (OWAMP)", RFC 4656, September 2006.
 [RFC5357]  Hedayat, K., Krzanowski, R., Morton, A., Yum, K., and J.
            Babiarz, "A Two-Way Active Measurement Protocol (TWAMP)",
            RFC 5357, October 2008.
 [RFC5835]  Morton, A. and S. Van den Berghe, "Framework for Metric
            Composition", RFC 5835, April 2010.
 [RFC6049]  Morton, A. and E. Stephan, "Spatial Composition of
            Metrics", RFC 6049, January 2011.
 [RFC6576]  Geib, R., Morton, A., Fardid, R., and A. Steinmitz, "IP
            Performance Metrics (IPPM) Standard Advancement Testing",
            BCP 176, RFC 6576, March 2012.
 [RFC6703]  Morton, A., Ramachandran, G., and G. Maguluri, "Reporting
            IP Network Performance Metrics: Different Points of View",
            RFC 6703, August 2012.

Fabini & Morton Informational [Page 15] RFC 7312 Advanced Sampling August 2014

8.2. Informative References

 [EEAW]     Pentikousis, K., Piri, E., Pinola, J., Fitzek, F.,
            Nissilae, T., and I. Harjula, "Empirical Evaluation of
            VoIP Aggregation over a Fixed WiMAX Testbed", Proceedings
            of the 4th International Conference on Testbeds and
            research infrastructures for the development of networks
            and communities (TridentCom '08), Article No. 19, March
            2008, <>.
 [IBD]      Fabini, J., Karner, W., Wallentin, L., and T. Baumgartner,
            "The Illusion of Being Deterministic - Application-Level
            Considerations on Delay in 3G HSPA Networks", Lecture
            Notes in Computer Science, Volume 5550, pp. 301-312 , May
 [IRR]      Fabini, J., Wallentin, L., and P. Reichl, "The Importance
            of Being Really Random: Methodological Aspects of IP-Layer
            2G and 3G Network Delay Assessment", ICC'09 Proceedings of
            the 2009 IEEE International Conference on Communications,
            doi: 10.1109/ICC.2009.5199514, June 2009.
 [LMAP]     Eardley, P., Morton, A., Bagnulo, M., Burbridge, T.,
            Aitken, P., and A. Akhter, "A framework for large-scale
            measurement platforms (LMAP)", Work in Progress, June
 [Mat98]    Mathis, M., "Empirical Bulk Transfer Capacity", IP
            Performance Metrics Working Group report in Proceedings of
            the Forty-Third Internet Engineering Task Force, Orlando,
            FL, December 1998,
 [RFC3148]  Mathis, M. and M. Allman, "A Framework for Defining
            Empirical Bulk Transfer Capacity Metrics", RFC 3148, July
 [RFC6808]  Ciavattone, L., Geib, R., Morton, A., and M. Wieser, "Test
            Plan and Results Supporting Advancement of RFC 2679 on the
            Standards Track", RFC 6808, December 2012.
 [RFC6985]  Morton, A., "IMIX Genome: Specification of Variable Packet
            Sizes for Additional Testing", RFC 6985, July 2013.

Fabini & Morton Informational [Page 16] RFC 7312 Advanced Sampling August 2014

 [RRC]      Peraelae, P., Barbuzzi, A., Boggia, G., and K.
            Pentikousis, "Theory and Practice of RRC State Transitions
            in UMTS Networks", IEEE Globecom 2009 Workshops, doi:
            10.1109/GLOCOMW.2009.5360763, November 2009.
 [TSRC]     Fabini, J. and M. Abmayer, "Delay Measurement Methodology
            Revisited: Time-slotted Randomness Cancellation", IEEE
            Transactions on Instrumentation and Measurement, Volume
            62, Issue 10, doi:10.1109/TIM.2013.2263914, October 2013.

Authors' Addresses

 Joachim Fabini
 Vienna University of Technology
 Gusshausstrasse 25/E389
 Vienna  1040
 Phone: +43 1 58801 38813
 Fax:   +43 1 58801 38898
 Al Morton
 AT&T Labs
 200 Laurel Avenue South
 Middletown, NJ  07748
 Phone: +1 732 420 1571
 Fax:   +1 732 368 1192

Fabini & Morton Informational [Page 17]

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