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Internet Engineering Task Force (IETF)                          G. AlmesRequest for Comments: 7679                                     Texas A&MSTD: 81                                                     S. KalidindiObsoletes:2679                                                     IxiaCategory: Standards Track                                   M. ZekauskasISSN: 2070-1721                                                Internet2                                                          A. Morton, Ed.                                                               AT&T Labs                                                            January 2016A 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 makesRFC 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 inSection 2 of RFC 5741.   Information about the current status of this document, any errata,   and how to provide feedback on it may be obtained athttp://www.rfc-editor.org/info/rfc7679.Almes, et al.                Standards Track                    [Page 1]

RFC 7679             A One-Way Delay Metric for IPPM        January 2016Copyright Notice   Copyright (c) 2016 IETF Trust and the persons identified as the   document authors.  All rights reserved.   This document is subject toBCP 78 and the IETF Trust's Legal   Provisions Relating to IETF Documents   (http://trustee.ietf.org/license-info) in effect on the date of   publication of this document.  Please review these documents   carefully, as they describe your rights and restrictions with respect   to this document.  Code Components extracted from this document must   include Simplified BSD License text as described in Section 4.e of   the Trust Legal Provisions and are provided without warranty as   described in the Simplified BSD License.Almes, et al.                Standards Track                    [Page 2]

RFC 7679             A One-Way Delay Metric for IPPM        January 2016Table of Contents1. Introduction ....................................................41.1. Motivation .................................................42. General Issues regarding Time ...................................63. A Singleton Definition for One-Way Delay ........................73.1. Metric Name ................................................73.2. Metric Parameters ..........................................73.3. Metric Units ...............................................73.4. Definition .................................................73.5. Discussion .................................................83.6. Methodologies ..............................................93.7. Errors and Uncertainties ..................................103.7.1. Errors or Uncertainties Related to Clocks ..........10           3.7.2. Errors or Uncertainties Related to Wire                  Time vs. Host Time .................................113.7.3. Calibration of Errors and Uncertainties ............123.8. Reporting the Metric ......................................143.8.1. Type-P .............................................143.8.2. Loss Threshold .....................................153.8.3. Calibration Results ................................153.8.4. Path ...............................................154. A Definition for Samples of One-Way Delay ......................154.1. Metric Name ...............................................164.2. Metric Parameters .........................................164.3. Metric Units ..............................................164.4. Definition ................................................174.5. Discussion ................................................174.6. Methodologies .............................................184.7. Errors and Uncertainties ..................................184.8. Reporting the Metric ......................................185. Some Statistics Definitions for One-Way Delay ..................185.1. Type-P-One-way-Delay-Percentile ...........................195.2. Type-P-One-way-Delay-Median ...............................195.3. Type-P-One-way-Delay-Minimum ..............................205.4. Type-P-One-way-Delay-Inverse-Percentile ...................206. Security Considerations ........................................217. Changes fromRFC 2679 ..........................................228. References .....................................................248.1. Normative References ......................................248.2. Informative References ....................................25   Acknowledgements ..................................................26   Authors' Addresses ................................................27Almes, et al.                Standards Track                    [Page 3]

RFC 7679             A One-Way Delay Metric for IPPM        January 20161.  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 inSection 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.Almes, et al.                Standards Track                    [Page 4]

RFC 7679             A One-Way Delay Metric for IPPM        January 2016   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.Almes, et al.                Standards Track                    [Page 5]

RFC 7679             A One-Way Delay Metric for IPPM        January 20162.  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 anAlmes, et al.                Standards Track                    [Page 6]

RFC 7679             A One-Way Delay Metric for IPPM        January 2016   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 Delay3.1.  Metric Name   Type-P-One-way-Delay3.2.  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 time3.3.  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 inSection 3.8 after a discussion of the metric, methodologies for   measuring the metric, and error analysis.Almes, et al.                Standards Track                    [Page 7]

RFC 7679             A One-Way Delay Metric for IPPM        January 20163.5.  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.  SeeSection 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.Almes, et al.                Standards Track                    [Page 8]

RFC 7679             A One-Way Delay Metric for IPPM        January 2016   o  A given methodology will include a way to determine whether the      packet is standard-formed, the default criteria for all metric      definitions defined inSection 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, seeSection 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.Almes, et al.                Standards Track                    [Page 9]

RFC 7679             A One-Way Delay Metric for IPPM        January 2016   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:Almes, et al.                Standards Track                   [Page 10]

RFC 7679             A One-Way Delay Metric for IPPM        January 2016   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 canAlmes, et al.                Standards Track                   [Page 11]

RFC 7679             A One-Way Delay Metric for IPPM        January 2016   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 removeAlmes, et al.                Standards Track                   [Page 12]

RFC 7679             A One-Way Delay Metric for IPPM        January 2016   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 aAlmes, et al.                Standards Track                   [Page 13]

RFC 7679             A One-Way Delay Metric for IPPM        January 2016   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 inSection 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.Almes, et al.                Standards Track                   [Page 14]

RFC 7679             A One-Way Delay Metric for IPPM        January 2016   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 forAlmes, et al.                Standards Track                   [Page 15]

RFC 7679             A One-Way Delay Metric for IPPM        January 2016   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, seeSection 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-Stream4.2.  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.Almes, et al.                Standards Track                   [Page 16]

RFC 7679             A One-Way Delay Metric for IPPM        January 20164.4.  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.Almes, et al.                Standards Track                   [Page 17]

RFC 7679             A One-Way Delay Metric for IPPM        January 20164.6.  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 inSection 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.Almes, et al.                Standards Track                   [Page 18]

RFC 7679             A One-Way Delay Metric for IPPM        January 20165.1.  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.  SeeSection 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.Almes, et al.                Standards Track                   [Page 19]

RFC 7679             A One-Way Delay Metric for IPPM        January 2016   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%.Almes, et al.                Standards Track                   [Page 20]

RFC 7679             A One-Way Delay Metric for IPPM        January 20166.  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.  SeeAlmes, et al.                Standards Track                   [Page 21]

RFC 7679             A One-Way Delay Metric for IPPM        January 2016Section 7 of [RFC7594] (Security Considerations of the LMAP   Framework) for system requirements that help to avoid measurement   system compromise.7.  Changes fromRFC 2679   The text above constitutes a revision toRFC 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 ofSection 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 inSection 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 ofSection 3.6,Section 3.8, and       inSection 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 inSection 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 ofSection 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.Almes, et al.                Standards Track                   [Page 22]

RFC 7679             A One-Way Delay Metric for IPPM        January 2016   2.   Explicit inclusion of the maximum waiting time input parameter        in Sections3.2 and4.2, reflecting recognition of this        parameter in more recent RFCs and ITU-T Recommendation Y.1540.   3.   Addition of a reference toRFC 6703 in the discussion of packet        lifetime and application timeouts inSection 3.5.   4.   Addition of a reference to the default requirement (that packets        be standard-formed) fromRFC 2330 as a new list item inSection 3.5.   5.   GPS-based NTP experience replaces "to be tested" inSection 3.5.   6.   Replaced "precedence" with updated terminology (DS Field) in        Sections3.6 and3.8.1(with reference).   7.   Added parenthetical guidance on minimizing the interval between        timestamp placement to send time inSection 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" inSection 3.7.3 andSection 3.8.3.   10.  Added reference toRFC 3432 regarding periodic sampling        alongside Poisson sampling inSection 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 toRFC 4737 regarding reordering metrics in        the related discussion of "Methodologies (Section 4.6).   12.  Modified the formatting of the example inSection 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 newAlmes, et al.                Standards Track                   [Page 23]

RFC 7679             A One-Way Delay Metric for IPPM        January 2016   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.  References8.1.  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>.Almes, et al.                Standards Track                   [Page 24]

RFC 7679             A One-Way Delay Metric for IPPM        January 2016   [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 ofRFC 2679 on the              Standards Track",RFC 6808, DOI 10.17487/RFC6808, December              2012, <http://www.rfc-editor.org/info/rfc6808>.Almes, et al.                Standards Track                   [Page 25]

RFC 7679             A One-Way Delay Metric for IPPM        January 2016   [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.Almes, et al.                Standards Track                   [Page 26]

RFC 7679             A One-Way Delay Metric for IPPM        January 2016Authors' 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/Almes, et al.                Standards Track                   [Page 27]