





Internet Engineering Task Force (IETF)                          G. Almes
Request for Comments: 7679                                     Texas A&M
STD: 81                                                     S. Kalidindi
Obsoletes: 2679                                                     Ixia
Category: Standards Track                                   M. Zekauskas
ISSN: 2070-1721                                                Internet2
                                                          A. Morton, Ed.
                                                               AT&T Labs
                                                            January 2016


        A One-Way Delay Metric for IP Performance Metrics (IPPM)

Abstract

   This memo defines a metric for one-way delay of packets across
   Internet paths.  It builds on notions introduced and discussed in the
   IP Performance Metrics (IPPM) Framework document, RFC 2330; the
   reader is assumed to be familiar with that document.  This memo makes
   RFC 2679 obsolete.

Status of This Memo

   This is an Internet Standards Track document.

   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
   Internet Standards 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/rfc7679.

















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Copyright Notice

   Copyright (c) 2016 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
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   (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
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   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.





































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Table of Contents

   1. Introduction ....................................................4
      1.1. Motivation .................................................4
   2. General Issues regarding Time ...................................6
   3. A Singleton Definition for One-Way Delay ........................7
      3.1. Metric Name ................................................7
      3.2. Metric Parameters ..........................................7
      3.3. Metric Units ...............................................7
      3.4. Definition .................................................7
      3.5. Discussion .................................................8
      3.6. Methodologies ..............................................9
      3.7. Errors and Uncertainties ..................................10
           3.7.1. Errors or Uncertainties Related to Clocks ..........10
           3.7.2. Errors or Uncertainties Related to Wire
                  Time vs. Host Time .................................11
           3.7.3. Calibration of Errors and Uncertainties ............12
      3.8. Reporting the Metric ......................................14
           3.8.1. Type-P .............................................14
           3.8.2. Loss Threshold .....................................15
           3.8.3. Calibration Results ................................15
           3.8.4. Path ...............................................15
   4. A Definition for Samples of One-Way Delay ......................15
      4.1. Metric Name ...............................................16
      4.2. Metric Parameters .........................................16
      4.3. Metric Units ..............................................16
      4.4. Definition ................................................17
      4.5. Discussion ................................................17
      4.6. Methodologies .............................................18
      4.7. Errors and Uncertainties ..................................18
      4.8. Reporting the Metric ......................................18
   5. Some Statistics Definitions for One-Way Delay ..................18
      5.1. Type-P-One-way-Delay-Percentile ...........................19
      5.2. Type-P-One-way-Delay-Median ...............................19
      5.3. Type-P-One-way-Delay-Minimum ..............................20
      5.4. Type-P-One-way-Delay-Inverse-Percentile ...................20
   6. Security Considerations ........................................21
   7. Changes from RFC 2679 ..........................................22
   8. References .....................................................24
      8.1. Normative References ......................................24
      8.2. Informative References ....................................25
   Acknowledgements ..................................................26
   Authors' Addresses ................................................27








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1.  Introduction

   This memo defines a metric for one-way delay of packets across
   Internet paths.  It builds on notions introduced and discussed in the
   IPPM Framework document, [RFC2330]; the reader is assumed to be
   familiar with that document and its recent update [RFC7312].

   This memo is intended to be parallel in structure to a companion
   document for Packet Loss ("A One-way Packet Loss Metric for IPPM")
   [RFC7680].

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].  Although
   [RFC2119] was written with protocols in mind, the key words are used
   in this document for similar reasons.  They are used to ensure the
   results of measurements from two different implementations are
   comparable and to note instances when an implementation could perturb
   the network.

   Whenever a technical term from the IPPM Framework document is first
   used in this memo, it will be tagged with a trailing asterisk.  For
   example, "term*" indicates that "term" is defined in the Framework
   document.

   The structure of the memo is as follows:

   o  A 'singleton*' analytic metric, called Type-P-One-way-Delay, will
      be introduced to measure a single observation of one-way delay.

   o  Using this singleton metric, a 'sample*' called Type-P-One-way-
      Delay-Poisson-Stream is introduced to measure a sequence of
      singleton delays sent at times taken from a Poisson process,
      defined in Section 11.1.1 of [RFC2330].

   o  Using this sample, several 'statistics*' of the sample will be
      defined and discussed.  This progression from singleton to sample
      to statistics, with clear separation among them, is important.

1.1.  Motivation

   Understanding one-way delay of a Type-P* packet from a source host*
   to a destination host is useful for several reasons:

   o  Some applications do not perform well (or at all) if end-to-end
      delay between hosts is large relative to some threshold value.





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   o  Erratic variation in delay makes it difficult (or impossible) to
      support many real-time applications.

   o  The larger the value of delay, the more difficult it is for
      transport-layer protocols to sustain high bandwidths.

   o  The minimum value of this metric provides an indication of the
      delay due only to propagation and transmission delay.

   o  The minimum value of this metric provides an indication of the
      delay that will likely be experienced when the path* traversed is
      lightly loaded.

   o  Values of this metric above the minimum provide an indication of
      the congestion present in the path.

   The measurement of one-way delay instead of round-trip delay is
   motivated by the following factors:

   o  In today's Internet, the path from a source to a destination may
      be different than the path from the destination back to the source
      ("asymmetric paths"), such that different sequences of routers are
      used for the forward and reverse paths.  Therefore, round-trip
      measurements actually measure the performance of two distinct
      paths together.  Measuring each path independently highlights the
      performance difference between the two paths that may traverse
      different Internet service providers and even radically different
      types of networks (for example, research versus commodity
      networks, or networks with asymmetric link capacities, or wireless
      versus wireline access).

   o  Even when the two paths are symmetric, they may have radically
      different performance characteristics due to asymmetric queuing.

   o  Performance of an application may depend mostly on the performance
      in one direction.  For example, a TCP-based communication will
      experience reduced throughput if congestion occurs in one
      direction of its communication.  Troubleshooting may be simplified
      if the congested direction of TCP transmission can be identified.

   o  In networks in which quality of service (QoS) is enabled,
      provisioning in one direction may be radically different than
      provisioning in the reverse direction and thus the QoS guarantees
      differ.  Measuring the paths independently allows the verification
      of both guarantees.

   It is outside the scope of this document to say precisely how delay
   metrics would be applied to specific problems.



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2.  General Issues regarding Time

   {Comment: The terminology below differs from that defined by ITU-T
   documents (e.g., G.810, "Definitions and terminology for
   synchronization networks" and I.356, "B-ISDN ATM layer cell transfer
   performance") but is consistent with the IPPM Framework document.  In
   general, these differences derive from the different backgrounds; the
   ITU-T documents historically have a telephony origin, while the
   authors of this document (and the Framework document) have a computer
   systems background.  Although the terms defined below have no direct
   equivalent in the ITU-T definitions, after our definitions we will
   provide a rough mapping.  However, note one potential confusion: our
   definition of "clock" is the computer operating systems definition
   denoting a time-of-day clock, while the ITU-T definition of clock
   denotes a frequency reference.}

   Whenever a time (i.e., a moment in history) is mentioned here, it is
   understood to be measured in seconds (and fractions) relative to UTC.

   As described more fully in the Framework document, there are four
   distinct, but related notions of clock uncertainty:

   synchronization*

   measures the extent to which two clocks agree on what time it is.
   For example, the clock on one host might be 5.4 msec ahead of the
   clock on a second host. {Comment: A rough ITU-T equivalent is "time
   error".}

   accuracy*

   measures the extent to which a given clock agrees with UTC.  For
   example, the clock on a host might be 27.1 msec behind UTC. {Comment:
   A rough ITU-T equivalent is "time error from UTC".}

   resolution*

   specification of the smallest unit by which the clock's time is
   updated.  It gives a lower bound on the clock's uncertainty.  For
   example, the clock on an old Unix host might tick only once every 10
   msec, and thus have a resolution of only 10 msec. {Comment: A very
   rough ITU-T equivalent is "sampling period".}

   skew*

   measures the change of accuracy, or of synchronization, with time.
   For example, the clock on a given host might gain 1.3 msec per hour
   and thus be 27.1 msec behind UTC at one time and only 25.8 msec an



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   hour later.  In this case, we say that the clock of the given host
   has a skew of 1.3 msec per hour relative to UTC, which threatens
   accuracy.  We might also speak of the skew of one clock relative to
   another clock, which threatens synchronization. {Comment: A rough
   ITU-T equivalent is "time drift".}

3.  A Singleton Definition for One-Way Delay

3.1.  Metric Name

   Type-P-One-way-Delay

3.2.  Metric Parameters

   o  Src, the IP address of a host

   o  Dst, the IP address of a host

   o  T, a time

   o  Tmax, a loss threshold waiting time

3.3.  Metric Units

   The value of a Type-P-One-way-Delay is either a real number or an
   undefined (informally, infinite) number of seconds.

3.4.  Definition

   For a real number dT, >>the *Type-P-One-way-Delay* from Src to Dst at
   T is dT<< means that Src sent the first bit of a Type-P packet to Dst
   at wire time* T and that Dst received the last bit of that packet at
   wire time T+dT.

   >>The *Type-P-One-way-Delay* from Src to Dst at T is undefined
   (informally, infinite)<< means that Src sent the first bit of a
   Type-P packet to Dst at wire time T and that Dst did not receive that
   packet (within the loss threshold waiting time, Tmax).

   Suggestions for what to report and metric values appear in
   Section 3.8 after a discussion of the metric, methodologies for
   measuring the metric, and error analysis.









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3.5.  Discussion

   Type-P-One-way-Delay is a relatively simple analytic metric, and one
   that we believe will afford effective methods of measurement.

   The following issues are likely to come up in practice:

   o  Real delay values will be positive.  Therefore, it does not make
      sense to report a negative value as a real delay.  However, an
      individual zero or negative delay value might be useful as part of
      a stream when trying to discover a distribution of a stream of
      delay values.

   o  Since delay values will often be as low as the 100 usec to 10 msec
      range, it will be important for Src and Dst to synchronize very
      closely.  Global Positioning System (GPS) systems afford one way
      to achieve synchronization to within several tens of usec.
      Ordinary application of NTP may allow synchronization to within
      several msec, but this depends on the stability and symmetry of
      delay properties among those NTP agents used, and this delay is
      what we are trying to measure.  A combination of some GPS-based
      NTP servers and a conservatively designed and deployed set of
      other NTP servers should yield good results.  This was tested in
      [RFC6808], where a GPS measurement system's results compared well
      with a GPS-based NTP synchronized system for the same
      intercontinental path.

   o  A given methodology will have to include a way to determine
      whether a delay value is infinite or whether it is merely very
      large (and the packet is yet to arrive at Dst).  As noted by
      Mahdavi and Paxson [RFC2678], simple upper bounds (such as the 255
      seconds theoretical upper bound on the lifetimes of IP packets
      [RFC791]) could be used; but good engineering, including an
      understanding of packet lifetimes, will be needed in practice.
      {Comment: Note that, for many applications of these metrics, the
      harm in treating a large delay as infinite might be zero or very
      small.  A TCP data packet, for example, that arrives only after
      several multiples of the RTT may as well have been lost.  See
      Section 4.1.1 of [RFC6703] for examination of unusual packet
      delays and application performance estimation.}

   o  If the packet is duplicated along the path (or paths) so that
      multiple non-corrupt copies arrive at the destination, then the
      packet is counted as received, and the first copy to arrive
      determines the packet's one-way delay.

   o  If the packet is fragmented and if, for whatever reason,
      reassembly does not occur, then the packet will be deemed lost.



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   o  A given methodology will include a way to determine whether the
      packet is standard-formed, the default criteria for all metric
      definitions defined in Section 15 of [RFC2330], otherwise the
      packet will be deemed lost.  Note: At this time, the definition of
      standard-formed packets only applies to IPv4, but also see
      [IPPM-UPDATES].

3.6.  Methodologies

   As with other Type-P-* metrics, the detailed methodology will depend
   on the Type-P (e.g., protocol number, UDP/TCP port number, size,
   Differentiated Services (DS) Field [RFC2780]).

   Generally, for a given Type-P, the methodology would proceed as
   follows:

   o  Arrange that Src and Dst are synchronized; that is, that they have
      clocks that are very closely synchronized with each other and each
      fairly close to the actual time.

   o  At the Src host, select Src and Dst IP addresses, and form a test
      packet of Type-P with these addresses.  Any 'padding' portion of
      the packet needed only to make the test packet a given size should
      be filled with randomized bits to avoid a situation in which the
      measured delay is lower than it would otherwise be, due to
      compression techniques along the path.  Also, see Section 3.1.2 of
      [RFC7312].

   o  At the Dst host, arrange to receive the packet.

   o  At the Src host, place a timestamp in the prepared Type-P packet,
      and send it towards Dst (ideally minimizing time before sending).

   o  If the packet arrives within a reasonable period of time, take a
      timestamp as soon as possible upon the receipt of the packet.  By
      subtracting the two timestamps, an estimate of one-way delay can
      be computed.  Error analysis of a given implementation of the
      method must take into account the closeness of synchronization
      between Src and Dst.  If the delay between Src's timestamp and the
      actual sending of the packet is known, then the estimate could be
      adjusted by subtracting this amount; uncertainty in this value
      must be taken into account in error analysis.  Similarly, if the
      delay between the actual receipt of the packet and Dst's timestamp
      is known, then the estimate could be adjusted by subtracting this
      amount; uncertainty in this value must be taken into account in
      error analysis.  See "Errors and Uncertainties" (Section 3.7) for
      a more detailed discussion.




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   o  If the packet fails to arrive within a reasonable period of time,
      Tmax, the one-way delay is taken to be undefined (informally,
      infinite).  Note that the threshold of "reasonable" is a parameter
      of the metric.  These points are examined in detail in [RFC6703],
      including analysis preferences to assign undefined delay to
      packets that fail to arrive with the difficulties emerging from
      the informal "infinite delay" assignment, and an estimation of an
      upper bound on waiting time for packets in transit.  Further,
      enforcing a specific constant waiting time on stored singletons of
      one-way delay is compliant with this specification and may allow
      the results to serve more than one reporting audience.

   Issues such as the packet format, the means by which Dst knows when
   to expect the test packet, and the means by which Src and Dst are
   synchronized are outside the scope of this document. {Comment: We
   plan to document the implementation techniques of our work in much
   more detail elsewhere; we encourage others to do so as well.}

3.7.  Errors and Uncertainties

   The description of any specific measurement method should include an
   accounting and analysis of various sources of error or uncertainty.
   The Framework document provides general guidance on this point, but
   we note here the following specifics related to delay metrics:

   o  Errors or uncertainties due to uncertainties in the clocks of the
      Src and Dst hosts.

   o  Errors or uncertainties due to the difference between 'wire time'
      and 'host time'.

   In addition, the loss threshold may affect the results.  Each of
   these are discussed in more detail below, along with a section
   (Section 3.7.3) on accounting for these errors and uncertainties.

3.7.1.  Errors or Uncertainties Related to Clocks

   The uncertainty in a measurement of one-way delay is related, in
   part, to uncertainties in the clocks of the Src and Dst hosts.  In
   the following, we refer to the clock used to measure when the packet
   was sent from Src as the source clock, we refer to the clock used to
   measure when the packet was received by Dst as the destination clock,
   we refer to the observed time when the packet was sent by the source
   clock as Tsource, and we refer to the observed time when the packet
   was received by the destination clock as Tdest.  Alluding to the
   notions of synchronization, accuracy, resolution, and skew mentioned
   in the Introduction, we note the following:




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   o  Any error in the synchronization between the source clock and the
      destination clock will contribute to error in the delay
      measurement.  We say that the source clock and the destination
      clock have a synchronization error of Tsynch if the source clock
      is Tsynch ahead of the destination clock.  Thus, if we know the
      value of Tsynch exactly, we could correct for clock
      synchronization by adding Tsynch to the uncorrected value of
      Tdest-Tsource.

   o  The accuracy of a clock is important only in identifying the time
      at which a given delay was measured.  Accuracy, per se, has no
      importance to the accuracy of the measurement of delay.  When
      computing delays, we are interested only in the differences
      between clock values, not the values themselves.

   o  The resolution of a clock adds to uncertainty about any time
      measured with it.  Thus, if the source clock has a resolution of
      10 msec, then this adds 10 msec of uncertainty to any time value
      measured with it.  We will denote the resolution of the source
      clock and the destination clock as Rsource and Rdest,
      respectively.

   o  The skew of a clock is not so much an additional issue as it is a
      realization of the fact that Tsynch is itself a function of time.
      Thus, if we attempt to measure or to bound Tsynch, this needs to
      be done periodically.  Over some periods of time, this function
      can be approximated as a linear function plus some higher order
      terms; in these cases, one option is to use knowledge of the
      linear component to correct the clock.  Using this correction, the
      residual Tsynch is made smaller but remains a source of
      uncertainty that must be accounted for.  We use the function
      Esynch(t) to denote an upper bound on the uncertainty in
      synchronization.  Thus, |Tsynch(t)| <= Esynch(t).

   Taking these items together, we note that naive computation Tdest-
   Tsource will be off by Tsynch(t) +/- (Rsource + Rdest).  Using the
   notion of Esynch(t), we note that these clock-related problems
   introduce a total uncertainty of Esynch(t)+ Rsource + Rdest.  This
   estimate of total clock-related uncertainty should be included in the
   error/uncertainty analysis of any measurement implementation.

3.7.2.  Errors or Uncertainties Related to Wire Time vs. Host Time

   As we have defined one-way delay, we would like to measure the time
   between when the test packet leaves the network interface of Src and
   when it (completely) arrives at the network interface of Dst: we
   refer to these as "wire times."  If the timings are themselves
   performed by software on Src and Dst, however, then this software can



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   only directly measure the time between when Src grabs a timestamp
   just prior to sending the test packet and when Dst grabs a timestamp
   just after having received the test packet: we refer to these two
   points as "host times".

   We note that some systems perform host time stamping on the network-
   interface hardware, in an attempt to minimize the difference from
   wire times.

   To the extent that the difference between wire time and host time is
   accurately known, this knowledge can be used to correct for host time
   measurements, and the corrected value more accurately estimates the
   desired (wire-time) metric.

   To the extent, however, that the difference between wire time and
   host time is uncertain, this uncertainty must be accounted for in an
   analysis of a given measurement method.  We denote by Hsource an
   upper bound on the uncertainty in the difference between wire time
   and host time on the Src host, and similarly define Hdest for the Dst
   host.  We then note that these problems introduce a total uncertainty
   of Hsource+Hdest.  This estimate of total wire-vs-host uncertainty
   should be included in the error/uncertainty analysis of any
   measurement implementation.

3.7.3.  Calibration of Errors and Uncertainties

   Generally, the measured values can be decomposed as follows:

   measured value = true value + systematic error + random error

   If the systematic error (the constant bias in measured values) can be
   determined, it can be compensated for in the reported results.

   reported value = measured value - systematic error

   therefore:

   reported value = true value + random error

   The goal of calibration is to determine the systematic and random
   error generated by the hosts themselves in as much detail as
   possible.  At a minimum, a bound ("e") should be found such that the
   reported value is in the range (true value - e) to (true value + e)
   at least 95% of the time.  We call "e" the calibration error for the
   measurements.  It represents the degree to which the values produced
   by the measurement host are repeatable; that is, how closely an
   actual delay of 30 ms is reported as 30 ms. {Comment: 95% was chosen
   because (1) some confidence level is desirable to be able to remove



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   outliers, which will be found in measuring any physical property; (2)
   a particular confidence level should be specified so that the results
   of independent implementations can be compared; and (3) even with a
   prototype user-level implementation, 95% was loose enough to exclude
   outliers.}

   From the discussion in the previous two sections, the error in
   measurements could be bounded by determining all the individual
   uncertainties, and adding them together to form:

   Esynch(t) + Rsource + Rdest + Hsource + Hdest.

   However, reasonable bounds on both the clock-related uncertainty
   captured by the first three terms and the host-related uncertainty
   captured by the last two terms should be possible by careful design
   techniques and calibrating the hosts using a known, isolated network
   in a lab.

   For example, the clock-related uncertainties are greatly reduced
   through the use of a GPS time source.  The sum of Esynch(t) + Rsource
   + Rdest is small and is also bounded for the duration of the
   measurement because of the global time source.

   The host-related uncertainties, Hsource + Hdest, could be bounded by
   connecting two hosts back-to-back with a high-speed serial link or
   isolated LAN segment.  In this case, repeated measurements are
   measuring the same one-way delay.

   If the test packets are small, such a network connection has a
   minimal delay that may be approximated by zero.  The measured delay
   therefore contains only systematic and random error in the
   measurement hosts.  The "average value" of repeated measurements is
   the systematic error, and the variation is the random error.

   One way to compute the systematic error, and the random error to a
   95% confidence is to repeat the experiment many times -- at least
   hundreds of tests.  The systematic error would then be the median.
   The random error could then be found by removing the systematic error
   from the measured values.  The 95% confidence interval would be the
   range from the 2.5th percentile to the 97.5th percentile of these
   deviations from the true value.  The calibration error "e" could then
   be taken to be the largest absolute value of these two numbers, plus
   the clock-related uncertainty. {Comment: as described, this bound is
   relatively loose since the uncertainties are added, and the absolute
   value of the largest deviation is used.  As long as the resulting
   value is not a significant fraction of the measured values, it is a





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   reasonable bound.  If the resulting value is a significant fraction
   of the measured values, then more exact methods will be needed to
   compute the calibration error.}

   Note that random error is a function of measurement load.  For
   example, if many paths will be measured by one host, this might
   increase interrupts, process scheduling, and disk I/O (for example,
   recording the measurements), all of which may increase the random
   error in measured singletons.  Therefore, in addition to minimal load
   measurements to find the systematic error, calibration measurements
   should be performed with the same measurement load that the hosts
   will see in the field.

   We wish to reiterate that this statistical treatment refers to the
   calibration of the host; it is used to "calibrate the meter stick"
   and say how well the meter stick reflects reality.

   In addition to calibrating the hosts for finite one-way delay, two
   checks should be made to ensure that packets reported as losses were
   really lost.  First, the threshold for loss should be verified.  In
   particular, ensure the "reasonable" threshold is reasonable: that it
   is very unlikely a packet will arrive after the threshold value, and
   therefore the number of packets lost over an interval is not
   sensitive to the error bound on measurements.  Second, consider the
   possibility that a packet arrives at the network interface, but is
   lost due to congestion on that interface or to other resource
   exhaustion (e.g. buffers) in the host.

3.8.  Reporting the Metric

   The calibration and context in which the metric is measured MUST be
   carefully considered and SHOULD always be reported along with metric
   results.  We now present four items to consider: the Type-P of test
   packets, the threshold of infinite delay (if any), error calibration,
   and the path traversed by the test packets.  This list is not
   exhaustive; any additional information that could be useful in
   interpreting applications of the metrics should also be reported (see
   [RFC6703] for extensive discussion of reporting considerations for
   different audiences).

3.8.1.  Type-P

   As noted in Section 13 of the Framework document [RFC2330], the value
   of the metric may depend on the type of IP packets used to make the
   measurement, or "Type-P".  The value of Type-P-One-way-Delay could
   change if the protocol (UDP or TCP), port number, size, or
   arrangement for special treatment (e.g., IP DS Field [RFC2780],
   Explicit Congestion Notification (ECN) [RFC3168], or RSVP) changes.



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   Additional packet distinctions identified in future extensions of the
   Type-P definition will apply.  The exact Type-P used to make the
   measurements MUST be accurately reported.

3.8.2.  Loss Threshold

   In addition, the threshold (or methodology to distinguish) between a
   large finite delay and loss MUST be reported.

3.8.3.  Calibration Results

   o  If the systematic error can be determined, it SHOULD be removed
      from the measured values.

   o  You SHOULD also report the calibration error, e, such that the
      true value is the reported value plus or minus e, with 95%
      confidence (see the last section.)

   o  If possible, the conditions under which a test packet with finite
      delay is reported as lost due to resource exhaustion on the
      measurement host SHOULD be reported.

3.8.4.  Path

   Finally, the path traversed by the packet SHOULD be reported, if
   possible.  In general, it is impractical to know the precise path a
   given packet takes through the network.  The precise path may be
   known for certain Type-P on short or stable paths.  If Type-P
   includes the record route (or loose-source route) option in the IP
   header, and the path is short enough, and all routers* on the path
   support record (or loose-source) route, then the path will be
   precisely recorded.  This is impractical because the route must be
   short enough, many routers do not support (or are not configured for)
   record route, and use of this feature would often artificially worsen
   the performance observed by removing the packet from common-case
   processing.  However, partial information is still valuable context.
   For example, if a host can choose between two links* (and hence, two
   separate routes from Src to Dst), then the initial link used is
   valuable context. {Comment: For example, with Merit's NetNow setup, a
   Src on one Network Access Point (NAP) can reach a Dst on another NAP
   by either of several different backbone networks.}

4.  A Definition for Samples of One-Way Delay

   Given the singleton metric Type-P-One-way-Delay, we now define one
   particular sample of such singletons.  The idea of the sample is to
   select a particular binding of the parameters Src, Dst, and Type-P,
   then define a sample of values of parameter T.  The means for



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   defining the values of T is to select a beginning time T0, a final
   time Tf, and an average rate lambda, then define a pseudorandom
   Poisson process of rate lambda, whose values fall between T0 and Tf.

   The time interval between successive values of T will then average 1/
   lambda.

   Note that Poisson sampling is only one way of defining a sample.
   Poisson has the advantage of limiting bias, but other methods of
   sampling will be appropriate for different situations.  For example,
   a truncated Poisson distribution may be needed to avoid reactive
   network state changes during intervals of inactivity, see Section 4.6
   of [RFC7312].  Sometimes the goal is sampling with a known bias, and
   [RFC3432] describes a method for periodic sampling with random start
   times.

4.1.  Metric Name

   Type-P-One-way-Delay-Poisson-Stream

4.2.  Metric Parameters

   o  Src, the IP address of a host

   o  Dst, the IP address of a host

   o  T0, a time

   o  Tf, a time

   o  Tmax, a loss threshold waiting time

   o  lambda, a rate in reciprocal seconds (or parameters for another
      distribution)

4.3.  Metric Units

   A sequence of pairs; the elements of each pair are:

   o  T, a time, and

   o  dT, either a real number or an undefined number of seconds.

   The values of T in the sequence are monotonic increasing.  Note that
   T would be a valid parameter to Type-P-One-way-Delay and that dT
   would be a valid value of Type-P-One-way-Delay.





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4.4.  Definition

   Given T0, Tf, and lambda, we compute a pseudorandom Poisson process
   beginning at or before T0, with average arrival rate lambda, and
   ending at or after Tf.  Those time values greater than or equal to T0
   and less than or equal to Tf are then selected.  At each of the
   selected times in this process, we obtain one value of Type-P-One-
   way-Delay.  The value of the sample is the sequence made up of the
   resulting <time, delay> pairs.  If there are no such pairs, the
   sequence is of length zero and the sample is said to be empty.

4.5.  Discussion

   The reader should be familiar with the in-depth discussion of Poisson
   sampling in the Framework document [RFC2330], which includes methods
   to compute and verify the pseudorandom Poisson process.

   We specifically do not constrain the value of lambda except to note
   the extremes.  If the rate is too large, then the measurement traffic
   will perturb the network and itself cause congestion.  If the rate is
   too small, then you might not capture interesting network behavior.
   {Comment: We expect to document our experiences with, and suggestions
   for, lambda elsewhere, culminating in a "Best Current Practice"
   document.}

   Since a pseudorandom number sequence is employed, the sequence of
   times, and hence the value of the sample, is not fully specified.
   Pseudorandom number generators of good quality will be needed to
   achieve the desired qualities.

   The sample is defined in terms of a Poisson process both to avoid the
   effects of self-synchronization and also capture a sample that is
   statistically as unbiased as possible. {Comment: there is, of course,
   no claim that real Internet traffic arrives according to a Poisson
   arrival process.} The Poisson process is used to schedule the delay
   measurements.  The test packets will generally not arrive at Dst
   according to a Poisson distribution, since they are influenced by the
   network.

   All the singleton Type-P-One-way-Delay metrics in the sequence will
   have the same values of Src, Dst, and Type-P.

   Note also that, given one sample that runs from T0 to Tf, and given
   new time values T0' and Tf' such that T0 <= T0' <= Tf' <= Tf, the
   subsequence of the given sample whose time values fall between T0'
   and Tf' are also a valid Type-P-One-way-Delay-Poisson-Stream sample.





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4.6.  Methodologies

   The methodologies follow directly from:

   o  The selection of specific times using the specified Poisson
      arrival process, and

   o  The methodologies discussion already given for the singleton Type-
      P-One-way-Delay metric.

   Care must be given to correctly handle out-of-order arrival of test
   packets; it is possible that the Src could send one test packet at
   TS[i], then send a second one (later) at TS[i+1] while the Dst could
   receive the second test packet at TR[i+1], and then receive the first
   one (later) at TR[i].  Metrics for reordering may be found in
   [RFC4737].

4.7.  Errors and Uncertainties

   In addition to sources of errors and uncertainties associated with
   methods employed to measure the singleton values that make up the
   sample, care must be given to analyze the accuracy of the Poisson
   process with respect to the wire times of the sending of the test
   packets.  Problems with this process could be caused by several
   things, including problems with the pseudorandom number techniques
   used to generate the Poisson arrival process, or with jitter in the
   value of Hsource (mentioned above as uncertainty in the singleton
   delay metric).  The Framework document shows how to use the Anderson-
   Darling test to verify the accuracy of a Poisson process over small
   time frames. {Comment: The goal is to ensure that test packets are
   sent "close enough" to a Poisson schedule and avoid periodic
   behavior.}

4.8.  Reporting the Metric

   The calibration and context for the underlying singletons MUST be
   reported along with the stream.  (See "Reporting the Metric" for
   Type-P-One-way-Delay in Section 3.8.)

5.  Some Statistics Definitions for One-Way Delay

   Given the sample metric Type-P-One-way-Delay-Poisson-Stream, we now
   offer several statistics of that sample.  These statistics are
   offered mostly to illustrate what could be done.  See [RFC6703] for
   additional discussion of statistics that are relevant to different
   audiences.





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5.1.  Type-P-One-way-Delay-Percentile

   Given a Type-P-One-way-Delay-Poisson-Stream and a percent X between
   0% and 100%, the Xth percentile of all the dT values in the stream.
   In computing this percentile, undefined values are treated as
   infinitely large.  Note that this means that the percentile could
   thus be undefined (informally, infinite).  In addition, the Type-P-
   One-way-Delay-Percentile is undefined if the sample is empty.

   For example: suppose we take a sample and the results are as follows:

   Stream1 = <

   <T1, 100 msec>

   <T2, 110 msec>

   <T3, undefined>

   <T4, 90 msec>

   <T5, 500 msec>

   >

   Then, the 50th percentile would be 110 msec, since 90 msec and 100
   msec are smaller and 500 msec and 'undefined' are larger.  See
   Section 11.3 of [RFC2330] for computing percentiles.

   Note that if the possibility that a packet with finite delay is
   reported as lost is significant, then a high percentile (90th or
   95th) might be reported as infinite instead of finite.

5.2.  Type-P-One-way-Delay-Median

   Given a Type-P-One-way-Delay-Poisson-Stream, the median of all the dT
   values in the stream.  In computing the median, undefined values are
   treated as infinitely large.  As with Type-P-One-way-Delay-
   Percentile, Type-P-One-way-Delay-Median is undefined if the sample is
   empty.

   As noted in the Framework document, the median differs from the 50th
   percentile only when the sample contains an even number of values, in
   which case the mean of the two central values is used.







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   For example, suppose we take a sample and the results are as follows:

   Stream2 = <

   <T1, 100 msec>

   <T2, 110 msec>

   <T3, undefined>

   <T4, 90 msec>

   >

   Then, the median would be 105 msec, the mean of 100 msec and 110
   msec, the two central values.

5.3.  Type-P-One-way-Delay-Minimum

   Given a Type-P-One-way-Delay-Poisson-Stream, the minimum of all the
   dT values in the stream.  In computing this, undefined values are
   treated as infinitely large.  Note that this means that the minimum
   could thus be undefined (informally, infinite) if all the dT values
   are undefined.  In addition, the Type-P-One-way-Delay-Minimum is
   undefined if the sample is empty.

   In the above example, the minimum would be 90 msec.

5.4.  Type-P-One-way-Delay-Inverse-Percentile

   Note: This statistic is deprecated in this document because of lack
   of use.

   Given a Type-P-One-way-Delay-Poisson-Stream and a time duration
   threshold, the fraction of all the dT values in the stream less than
   or equal to the threshold.  The result could be as low as 0% (if all
   the dT values exceed threshold) or as high as 100%.  Type-P-One-way-
   Delay-Inverse-Percentile is undefined if the sample is empty.

   In the above example, the Inverse-Percentile of 103 msec would be
   50%.










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6.  Security Considerations

   Conducting Internet measurements raises both security and privacy
   concerns.  This memo does not specify an implementation of the
   metrics, so it does not directly affect the security of the Internet
   nor of applications that run on the Internet.  However,
   implementations of these metrics must be mindful of security and
   privacy concerns.

   There are two types of security concerns: potential harm caused by
   the measurements and potential harm to the measurements.  The
   measurements could cause harm because they are active and inject
   packets into the network.  The measurement parameters MUST be
   carefully selected so that the measurements inject trivial amounts of
   additional traffic into the networks they measure.  If they inject
   "too much" traffic, they can skew the results of the measurement and
   in extreme cases cause congestion and denial of service.

   The measurements themselves could be harmed by routers giving
   measurement traffic a different priority than "normal" traffic or by
   an attacker injecting artificial measurement traffic.  If routers can
   recognize measurement traffic and treat it separately, the
   measurements will not reflect actual user traffic.  Therefore, the
   measurement methodologies SHOULD include appropriate techniques to
   reduce the probability that measurement traffic can be distinguished
   from "normal" traffic.

   If an attacker injects packets emulating traffic that are accepted as
   legitimate, the loss ratio or other measured values could be
   corrupted.  Authentication techniques, such as digital signatures,
   may be used where appropriate to guard against injected traffic
   attacks.

   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 [RFC7594],
   which covers active and passive techniques.

   Collecting measurements or using measurement results for
   reconnaissance to assist in subsequent system attacks is quite
   common.  Access to measurement results, or control of the measurement
   systems to perform reconnaissance should be guarded against.  See





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   Section 7 of [RFC7594] (Security Considerations of the LMAP
   Framework) for system requirements that help to avoid measurement
   system compromise.

7.  Changes from RFC 2679

   The text above constitutes a revision to RFC 2769, which is now an
   Internet Standard.  This section tracks the changes from [RFC2679].

   [RFC6808] provides the test plan and results supporting [RFC2679]
   advancement along the Standards Track, according to the process in
   [RFC6576].  The conclusions of [RFC6808] list four minor
   modifications:

   1.  Section 6.2.3 of [RFC6808] asserts that the assumption of post-
       processing to enforce a constant waiting time threshold is
       compliant and that the text of the RFC should be revised slightly
       to include this point.  The applicability of post-processing was
       added in the last list item of Section 3.6.

   2.  Section 6.5 of [RFC6808] indicates that the Type-P-One-way-Delay-
       Inverse-Percentile statistic has been ignored in both
       implementations, so it was a candidate for removal or deprecation
       in this document (this small discrepancy does not affect
       candidacy for advancement).  This statistic was deprecated in
       Section 5.4.

   3.  The IETF has reached consensus on guidance for reporting metrics
       in [RFC6703], and the memo is referenced in this document to
       incorporate recent experience where appropriate.  This reference
       was added in the last list item of Section 3.6, Section 3.8, and
       in Section 5.

   4.  There is currently one erratum with status "Held for Document
       Update" (EID 398) for [RFC2679], and this minor revision and
       additional text was incorporated in this document in Section 5.1.

   A number of updates to the [RFC2679] text have been implemented in
   the text above to reference key IPPM RFCs that were approved after
   [RFC2679] and to address comments on the IPPM mailing list describing
   current conditions and experience.

   1.   Near the end of Section 1.1, there is an update of a network
        example using ATM, a clarification of TCP's affect on queue
        occupation, and discussion of the importance of one-way delay
        measurement.





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   2.   Explicit inclusion of the maximum waiting time input parameter
        in Sections 3.2 and 4.2, reflecting recognition of this
        parameter in more recent RFCs and ITU-T Recommendation Y.1540.

   3.   Addition of a reference to RFC 6703 in the discussion of packet
        lifetime and application timeouts in Section 3.5.

   4.   Addition of a reference to the default requirement (that packets
        be standard-formed) from RFC 2330 as a new list item in
        Section 3.5.

   5.   GPS-based NTP experience replaces "to be tested" in Section 3.5.

   6.   Replaced "precedence" with updated terminology (DS Field) in
        Sections 3.6 and 3.8.1(with reference).

   7.   Added parenthetical guidance on minimizing the interval between
        timestamp placement to send time in Section 3.6.

   8.   Section 3.7.2 notes that some current systems perform host time
        stamping on the network-interface hardware.

   9.   "instrument" replaced by the defined term "host" in
        Section 3.7.3 and Section 3.8.3.

   10.  Added reference to RFC 3432 regarding periodic sampling
        alongside Poisson sampling in Section 4 and also noted that a
        truncated Poisson distribution may be needed with modern
        networks as described in the IPPM Framework update [RFC7312].

   11.  Added a reference to RFC 4737 regarding reordering metrics in
        the related discussion of "Methodologies (Section 4.6).

   12.  Modified the formatting of the example in Section 5.1 to match
        the original (issue with conversion to XML in this version).

   13.  Clarified the conclusions on two related points on harm to
        measurements (recognition of measurement traffic for unexpected
        priority treatment and attacker traffic which emulates
        measurement) in "Security Considerations (Section 6).

   14.  Expanded and updated the material on Privacy and added cautions
        on the use of measurements for reconnaissance in "Security
        Considerations" (Section 6).

   Section 5.4.4 of [RFC6390] suggests a common template for performance
   metrics partially derived from previous IPPM and Benchmarking
   Methodology Working Group (BMWG) RFCs, but it also contains some new



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   items.  All of the normative parts of [RFC6390] are covered, but not
   quite in the same section names or orientation.  Several of the
   informative parts are covered.  Maintaining the familiar outline of
   IPPM literature has both value and minimizes unnecessary differences
   between this revised RFC and current/future IPPM RFCs.

   The publication of [RFC6921] suggested an area where this memo might
   need updating.  Packet transfer on Faster-Than-Light (FTL) networks
   could result in negative delays and packet reordering; however, both
   are covered as possibilities in the current text and no revisions are
   deemed necessary (we also note that [RFC6921] is an April 1st RFC).

8.  References

8.1.  Normative References

   [RFC791]   Postel, J., "Internet Protocol", STD 5, RFC 791,
              DOI 10.17487/RFC0791, September 1981,
              <http://www.rfc-editor.org/info/rfc791>.

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

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

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

   [RFC2679]  Almes, G., Kalidindi, S., and M. Zekauskas, "A One-way
              Delay Metric for IPPM", RFC 2679, DOI 10.17487/RFC2679,
              September 1999, <http://www.rfc-editor.org/info/rfc2679>.

   [RFC2780]  Bradner, S. and V. Paxson, "IANA Allocation Guidelines For
              Values In the Internet Protocol and Related Headers",
              BCP 37, RFC 2780, DOI 10.17487/RFC2780, March 2000,
              <http://www.rfc-editor.org/info/rfc2780>.

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




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   [RFC6576]  Geib, R., Ed., Morton, A., Fardid, R., and A. Steinmitz,
              "IP Performance Metrics (IPPM) Standard Advancement
              Testing", BCP 176, RFC 6576, DOI 10.17487/RFC6576, March
              2012, <http://www.rfc-editor.org/info/rfc6576>.

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

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

8.2.  Informative References

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

   [RFC3168]  Ramakrishnan, K., Floyd, S., and D. Black, "The Addition
              of Explicit Congestion Notification (ECN) to IP",
              RFC 3168, DOI 10.17487/RFC3168, September 2001,
              <http://www.rfc-editor.org/info/rfc3168>.

   [RFC4737]  Morton, A., Ciavattone, L., Ramachandran, G., Shalunov,
              S., and J. Perser, "Packet Reordering Metrics", RFC 4737,
              DOI 10.17487/RFC4737, November 2006,
              <http://www.rfc-editor.org/info/rfc4737>.

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

   [RFC6703]  Morton, A., Ramachandran, G., and G. Maguluri, "Reporting
              IP Network Performance Metrics: Different Points of View",
              RFC 6703, DOI 10.17487/RFC6703, August 2012,
              <http://www.rfc-editor.org/info/rfc6703>.

   [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, DOI 10.17487/RFC6808, December
              2012, <http://www.rfc-editor.org/info/rfc6808>.



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   [RFC6921]  Hinden, R., "Design Considerations for Faster-Than-Light
              (FTL) Communication", RFC 6921, DOI 10.17487/RFC6921,
              April 2013, <http://www.rfc-editor.org/info/rfc6921>.

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

Acknowledgements

   For [RFC2679], special thanks are due to Vern Paxson of Lawrence
   Berkeley Labs for his helpful comments on issues of clock uncertainty
   and statistics.  Thanks also to Garry Couch, Will Leland, Andy
   Scherrer, Sean Shapira, and Roland Wittig for several useful
   suggestions.

   For this document, thanks to Joachim Fabini, Ruediger Geib, Nalini
   Elkins, and Barry Constantine for sharing their measurement
   experience as part of their careful reviews.  Brian Carpenter and
   Scott Bradner provided useful feedback at IETF Last Call.





























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Authors' Addresses

   Guy Almes
   Texas A&M

   Email: almes@acm.org


   Sunil Kalidindi
   Ixia

   Email: skalidindi@ixiacom.com


   Matt Zekauskas
   Internet2

   Email: matt@internet2.edu


   Al Morton (editor)
   AT&T Labs
   200 Laurel Avenue South
   Middletown, NJ  07748
   United States

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





















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