Network Working Group                                           G. Almes
Internet-Draft                                                 Texas A&M
Obsoletes: 2679 (if approved)                               S. Kalidindi
Intended status: Standards Track                                    Ixia
Expires: July 30, December 22, 2015                                  M. Zekauskas
                                                          A. Morton, Ed.
                                                               AT&T Labs
                                                        January 26,
                                                           June 20, 2015

                    A One-Way Delay Metric for IPPM


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

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on July 30, December 22, 2015.

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Table of Contents

   1.  RFC 2679 bis  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   5
     2.1.  Motivation  . . . . . . . . . . . . . . . . . . . . . . .   6
     2.2.  General Issues Regarding Time . . . . . . . . . . . . . .   7
   3.  A Singleton Definition for One-way Delay  . . . . . . . . . .   8
     3.1.  Metric Name:  . . . . . . . . . . . . . . . . . . . . . .   8
     3.2.  Metric Parameters:  . . . . . . . . . . . . . . . . . . .   8
     3.3.  Metric Units: . . . . . . . . . . . . . . . . . . . . . .   8
     3.4.  Definition: . . . . . . . . . . . . . . . . . . . . . . .   8
     3.5.  Discussion: . . . . . . . . . . . . . . . . . . . . . . .   9
     3.6.  Methodologies:  . . . . . . . . . . . . . . . . . . . . .  10
     3.7.  Errors and Uncertainties: . . . . . . . . . . . . . . . .  11
       3.7.1.  Errors or uncertainties related to Clocks . . . . . .  11
       3.7.2.  Errors or uncertainties related to Wire-time vs Host-
               time  . . . . . . . . . . . . . . . . . . . . . . . .  12
       3.7.3.  Calibration . . . . . . . . . . . . . . . . . . . . .  13
     3.8.  Reporting the metric: . . . . . . . . . . . . . . . . . .  15
       3.8.1.  Type-P  . . . . . . . . . . . . . . . . . . . . . . .  15
       3.8.2.  Loss Threshold  . . . . . . . . . . . . . . . . . . .  16
       3.8.3.  Calibration Results . . . . . . . . . . . . . . . . .  16
       3.8.4.  Path  . . . . . . . . . . . . . . . . . . . . . . . .  16
   4.  A Definition for Samples of One-way Delay . . . . . . . . . .  16
     4.1.  Metric Name:  . . . . . . . . . . . . . . . . . . . . . .  17
     4.2.  Metric Parameters:  . . . . . . . . . . . . . . . . . . .  17
     4.3.  Metric Units: . . . . . . . . . . . . . . . . . . . . . .  17
     4.4.  Definition: . . . . . . . . . . . . . . . . . . . . . . .  17
     4.5.  Discussion: . . . . . . . . . . . . . . . . . . . . . . .  18
     4.6.  Methodologies:  . . . . . . . . . . . . . . . . . . . . .  18
     4.7.  Errors and Uncertainties: . . . . . . . . . . . . . . . .  19
     4.8.  Reporting the metric: . . . . . . . . . . . . . . . . . .  19
   5.  Some Statistics Definitions for One-way Delay . . . . . . . .  19
     5.1.  Type-P-One-way-Delay-Percentile . . . . . . . . . . . . .  19
     5.2.  Type-P-One-way-Delay-Median . . . . . . . . . . . . . . .  20
     5.3.  Type-P-One-way-Delay-Minimum  . . . . . . . . . . . . . .  21
     5.4.  Type-P-One-way-Delay-Inverse-Percentile . . . . . . . . .  21
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  21
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  22
   8.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  22
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  22
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  22
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  23
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  24  23

1.  RFC 2679 bis

   The following text constitutes RFC 2769 bis proposed for advancement
   on the IETF Standards Track.  This section tracks the changes from

   [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

   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 (see the last list item of section
       3.6, below).

   2.  Section 6.5 of [RFC6808] indicates that Type-P-One-way-Delay-
       Inverse-Percentile statistic has been ignored in both
       implementations, so it is a candidate for removal or deprecation
       in RFC2679bis (this small discrepancy does not affect candidacy
       for advancement) (see section 5.4, below).

   3.  The IETF has reached consensus on guidance for reporting metrics
       in [RFC6703], and this memo should be referenced in RFC2679bis to
       incorporate recent experience where appropriate (see the last
       list item of section 3.6, section 3.8, and section 5 below).

   4.  There is currently one erratum with status "Held for document
       update" for [RFC2679], and it appears this minor revision and
       additional text should be incorporated in RFC2679bis (see section

   A number of updates to the [RFC2679] text have been implemented in
   the text below, 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.   Add that RFC2330 was updated by RFC7312 in the Introduction
        (section 2).

   2.   Near the end of section 2.1, update of a network example using
        ATM and clarification of TCP's affect on queue occupation and
        importance of one-way delay measurement.

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

   4.   Addition of reference to RFC6703 in the discussion of packet
        life time and application timeouts in section 3.5.

   5.   Addition of reference to the default requirement (that packets
        be standard-formed) from RFC2330 as a new list item in section

   6.   GPS-based NTP experience replaces "to be tested" in section 3.5.

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

   8.   Added parenthetical guidance on minimizing interval between
        timestamp placement to send time in section 3.6.

   9.   Added text recognizing the impending deployment of transport
        layer encryption in section 3.6.

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

   11.  "instrument" replaced by the defined term "host" in sections
        3.7.3 and 3.8.3.

   12.  Added reference to RFC 3432 Periodic sampling alongside Poisson
        sampling in section 4, and also noting that a truncated Poisson
        distribution may be needed with modern networks as described in
        the IPPM Framework update, RFC7312.

   13.  Add reference to RFC 4737 Reordering metric in the related
        discussion of section 4.6, Methodologies.

   14.  Formatting of Example in section 5.1 modified to match the
        original (issue with conversion to XML in bis version).

   15.  Clarifying 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 section 6, Security Considerations.

   Section 5.4.4 of [RFC6390] suggests a common template for performance
   metrics partially derived from previous IPPM and BMWG RFCs, but also
   contains some new items.  All of the [RFC6390] Normative points are
   covered, but not quite in the same section names or orientation.

   Several of the Informative points are covered.  Maintaining the
   familiar outline of IPPM literature has both value and minimizes
   unnecessary differences between this revised RFC and current/future

   The publication of RFC 6921 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 this is an April 1st RFC).

2.  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")

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

   The structure of the memo is as follows:

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

   + Using this singleton metric, a 'sample', called Type-P-One-way-
   Delay-Poisson-Stream, will be introduced to measure a sequence of
   singleton delays sent at times taken from a Poisson process.

   + 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.

   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.

2.1.  Motivation

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

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

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

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

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

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

   + 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:

   + 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 which 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 vs. wireline

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

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

   + In quality-of-service (QoS) enabled networks, 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.

2.2.  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) 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:


   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


   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".}


   measures the precision of a given clock.  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".}


   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
   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:


3.2.  Metric Parameters:

   + Src, the IP address of a host

   + Dst, the IP address of a host

   + T, a time

   + 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 along with metric values appear in
   Section 3.8 after a discussion of the metric, methodologies for
   measuring the metric, and error analysis.

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:

   + 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

   + 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.  GPS systems afford one way to achieve synchronization to
   within several 10s 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.

   + 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 [RFC0791]) 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.}

   + 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.

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

   + 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.

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

   + 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.

   + 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.  Note that use of transport layer
   encryption will counteract the deployment of network-based analysis
   and may reduce the adoption of payload optimizations like

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

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

   + 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 the
   next section, "Errors and Uncertainties", for a more detailed

   + 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 elsewhere our own work in describing such more
   detailed implementation techniques and we encourage others to as

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:

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

   + 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
   ("Calibration") 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 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:

   + 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.

   + 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.

   + 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.

   + 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)| <=

   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, and we
   refer to these as "wire times."  If the timings are themselves
   performed by software on Src and Dst, however, then this software can
   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, and 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

   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


   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 percent 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 percent
   was chosen because (1) some confidence level is desirable to be able
   to remove 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
   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 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] or RSVP) changes.  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

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

   + 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.)

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

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

   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

4.1.  Metric Name:


4.2.  Metric Parameters:

   + Src, the IP address of a host

   + Dst, the IP address of a host

   + T0, a time

   + Tf, a time

   + Tmax, a loss threshold waiting time

   + lambda, a rate in reciprocal seconds (or parameters for another

4.3.  Metric Units:

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

   + T, a time, and

   + 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.

4.4.  Definition:

   Given T0, Tf, and lambda, we compute a pseudo-random 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 times
   in this process, we obtain the value of Type-P-One-way-Delay at this
   time.  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 pseudo-random 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
   practices" document.}

   Since a pseudo-random number sequence is employed, the sequence of
   times, and hence the value of the sample, is not fully specified.
   Pseudo-random 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

   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.

4.6.  Methodologies:

   The methodologies follow directly from:

   + the selection of specific times, using the specified Poisson
   arrival process, and
   + the methodologies discussion already given for the singleton Type-
   P-One-way-Delay metric.

   Care must, of course, 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 pseudo-random 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

4.8.  Reporting the metric:

   You MUST report the calibration and context for the underlying
   singletons along with the stream.  (See "Reporting the metric" for

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 be illustrative of illustrate what could be done.  See [RFC6703] for
   additional discussion of statistics that are relevant to different

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.

   Example: suppose we take a sample and the results are:

   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

   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.

   Example: suppose we take a sample and the results are:

   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 version of the memo
   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

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 which 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 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

   The privacy concerns of network measurement are limited by the active
   measurements described in this memo.  Unlike passive measurements,
   there can be no release of existing user data.

7.  IANA Considerations

   This memo makes no requests of IANA.

8.  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

   For RFC 2679 bis, thanks to Joachim Fabini, Ruediger Geib, Nalini
   Elkins, and Barry Constantine for sharing their measurement
   experience as part of their careful reviews.

9.  References

9.1.  Normative References

   [RFC0791]  Postel, J., "Internet Protocol", STD 5, RFC 791, September

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

   [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

   [RFC2678]  Mahdavi, J. and V. Paxson, "IPPM Metrics for Measuring
              Connectivity", RFC 2678, September 1999.

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

   [RFC2680]  Almes, G., Kalidindi, S., and M. Zekauskas, "A One-way
              Packet Loss Metric for IPPM", RFC 2680, September 1999.

   [RFC2780]  Bradner, S. and V. Paxson, "IANA Allocation Guidelines For
              Values In the Internet Protocol and Related Headers", BCP
              37, RFC 2780, March 2000.

   [RFC3432]  Raisanen, V., Grotefeld, G., and A. Morton, "Network
              performance measurement with periodic streams", RFC 3432,
              November 2002.

   [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.

   [RFC7312]  Fabini, J. and A. Morton, "Advanced Stream and Sampling
              Framework for IP Performance Metrics (IPPM)", RFC 7312,
              August 2014.

9.2.  Informative References

   [RFC4737]  Morton, A., Ciavattone, L., Ramachandran, G., Shalunov,
              S., and J. Perser, "Packet Reordering Metrics", RFC 4737,
              November 2006.

   [RFC6390]  Clark, A. and B. Claise, "Guidelines for Considering New
              Performance Metric Development", BCP 170, RFC 6390,
              October 2011.

   [RFC6703]  Morton, A., Ramachandran, G., and G. Maguluri, "Reporting
              IP Network Performance Metrics: Different Points of View",
              RFC 6703, August 2012.

   [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.

Authors' Addresses
   Guy Almes
   Texas A&M


   Sunil Kalidindi


   Matt Zekauskas


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

   Phone: +1 732 420 1571
   Fax:   +1 732 368 1192