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Network Working Group                                           G. AlmesRequest for Comments: 2681                                  S. KalidindiCategory: Standards Track                                   M. Zekauskas                                             Advanced Network & Services                                                          September 1999A Round-trip Delay Metric for IPPMStatus of this Memo   This document specifies an Internet standards track protocol for the   Internet community, and requests discussion and suggestions for   improvements.  Please refer to the current edition of the "Internet   Official Protocol Standards" (STD 1) for the standardization state   and status of this protocol.  Distribution of this memo is unlimited.Copyright Notice   Copyright (C) The Internet Society (1999).  All Rights Reserved.1. Introduction   This memo defines a metric for round-trip delay of packets across   Internet paths.  It builds on notions introduced and discussed in the   IPPM Framework document,RFC 2330 [1], and follows closely the   corresponding metric for One-way Delay ("A One-way Delay Metric for   IPPM") [2]; the reader is assumed to be familiar with those   documents.   The memo was largely written by copying material from the One-way   Delay metric.  The intention is that, where the two metrics are   similar, they will be described with similar or identical text, and   that where the two metrics differ, new or modified text will be used.   This memo is intended to be parallel in structure to a future   companion document for Packet Loss.   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 inRFC 2119 [6].   AlthoughRFC 2119 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.Almes, et al.               Standards Track                     [Page 1]

RFC 2681          Round-trip for Delay Metric for IPPM    September 1999   The structure of the memo is as follows:   +  A 'singleton' analytic metric, called Type-P-Round-trip-Delay,      will be introduced to measure a single observation of round-trip      delay.   +  Using this singleton metric, a 'sample', called Type-P-Round-trip-      Delay-Poisson-Stream, will be introduced to measure a sequence of      singleton delays measured 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.1.1. Motivation   Round-trip 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 interactive 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.Almes, et al.               Standards Track                     [Page 2]

RFC 2681          Round-trip for Delay Metric for IPPM    September 1999   The measurement of round-trip delay instead of one-way delay has   several weaknesses, summarized here:   +  The Internet path from a source to a destination may differ from      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.   +  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.   +  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.   On the other hand, the measurement of round-trip delay has two   specific advantages:   +  Ease of deployment: unlike in one-way measurement, it is often      possible to perform some form of round-trip delay measurement      without installing measurement-specific software at the intended      destination.  A variety of approaches are well-known, including      use of ICMP Echo or of TCP-based methodologies (similar to those      outlined in "IPPM Metrics for Measuring Connectivity" [4]).      However, some approaches may introduce greater uncertainty in the      time for the destination to produce a response (seeSection 2.7.3).   +  Ease of interpretation: in some circumstances, the round-trip time      is in fact the quantity of interest. Deducing the round-trip time      from matching one-way measurements and an assumption of the      destination processing time is less direct and potentially less      accurate.1.2. General Issues Regarding Time   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:Almes, et al.               Standards Track                     [Page 3]

RFC 2681          Round-trip for Delay Metric for IPPM    September 1999   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.   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.   resolution*        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.   skew*        measures the change of accuracy, or of synchronization, with        time.  For example, the clock on a given host might gain 1.3        msec per hour and thus be 27.1 msec behind UTC at one time and        only 25.8 msec an 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.2. A Singleton Definition for Round-trip Delay2.1. Metric Name:   Type-P-Round-trip-Delay2.2. Metric Parameters:   +  Src, the IP address of a host   +  Dst, the IP address of a host   +  T, a time2.3. Metric Units:   The value of a Type-P-Round-trip-Delay is either a real number, or an   undefined (informally, infinite) number of seconds.Almes, et al.               Standards Track                     [Page 4]

RFC 2681          Round-trip for Delay Metric for IPPM    September 19992.4. Definition:   For a real number dT, >>the *Type-P-Round-trip-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, that Dst received that packet, then immediately   sent a Type-P packet back to Src, and that Src received the last bit   of that packet at wire-time T+dT.   >>The *Type-P-Round-trip-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 (either Dst did not   receive the packet, Dst did not send a Type-P packet in response, or)   Src did not receive that response packet.   >>The *Type-P-Round-trip-Delay between Src and Dst at T<< means   either the *Type-P-Round-trip-Delay from Src to Dst at T or the   *Type-P-Round-trip-Delay from Dst to Src at T.  When this notion is   used, it is understood to be specifically ambiguous which host acts   as Src and which as Dst.  {Comment: This ambiguity will usually be a   small price to pay for being able to have one measurement, launched   from either Src or Dst, rather than having two measurements.}   Suggestions for what to report along with metric values appear inSection 3.8 after a discussion of the metric, methodologies for   measuring the metric, and error analysis.2.5. Discussion:   Type-P-Round-trip-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:   +  The timestamp values (T) for the time at which delays are measured      should be fairly accurate in order to draw meaningful conclusions      about the state of the network at a given T.  Therefore, Src      should have an accurate knowledge of time-of-day.  NTP [3] affords      one way to achieve time accuracy to within several milliseconds.      Depending on the NTP server, higher accuracy may be achieved, for      example when NTP servers make use of GPS systems as a time source.      Note that NTP will adjust the instrument's clock.  If an      adjustment is made between the time the initial timestamp is taken      and the time the final timestamp is taken the adjustment will      affect the uncertainty in the measured delay.  This uncertainty      must be accounted for in the instrument's calibration.Almes, et al.               Standards Track                     [Page 5]

RFC 2681          Round-trip for Delay Metric for IPPM    September 1999   +  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 [4], simple upper bounds (such as the 255      seconds theoretical upper bound on the lifetimes of IP      packets [5]) 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.}   +  If the packet is duplicated so that multiple non-corrupt instances      of the response arrive back at the source, then the packet is      counted as received, and the first instance to arrive back at the      source determines the packet's round-trip delay.   +  If the packet is fragmented and if, for whatever reason,      reassembly does not occur, then the packet will be deemed lost.2.6. 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,   precedence).   Generally, for a given Type-P, the methodology would proceed as   follows:   +  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.  The test packet must have      some identifying information so that the response to it can be      identified by Src when Src receives the response; one means to do      this is by placing the timestamp generated just before sending the      test packet in the packet itself.   +  At the Dst host, arrange to receive and respond to the test      packet.  At the Src host, arrange to receive the corresponding      response packet.Almes, et al.               Standards Track                     [Page 6]

RFC 2681          Round-trip for Delay Metric for IPPM    September 1999   +  At the Src host, take the initial timestamp and then send the      prepared Type-P packet towards Dst.  Note that the timestamp could      be placed inside the packet, or kept separately as long as the      packet contains a suitable identifier so the received timestamp      can be compared with the send timestamp.   +  If the packet arrives at Dst, send a corresponding response packet      back from Dst to Src as soon as possible.   +  If the response packet arrives within a reasonable period of time,      take the final timestamp as soon as possible upon the receipt of      the packet.  By subtracting the two timestamps, an estimate of      round-trip delay can be computed.  If the delay between the      initial 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 response packet and final 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      discussion.   +  If the packet fails to arrive within a reasonable period of time,      the round-trip delay is taken to be undefined (informally,      infinite).  Note that the threshold of 'reasonable' is a parameter      of the methodology.   Issues such as the packet format and the means by which Dst knows   when to expect the test packet are outside the scope of this   document.   {Comment: Note that you cannot in general add two Type-P-One-way-   Delay values (see [2]) to form a Type-P-Round-trip-Delay value.  In   order to form a Type-P-Round-trip-Delay value, the return packet must   be triggered by the reception of a packet from Src.}   {Comment: "ping" would qualify as a round-trip measure under this   definition, with a Type-P of ICMP echo request/reply with 60-byte   packets.  However, the uncertainties associated with a typical ping   program must be analyzed as in the next section, including the type   of reflecting point (a router may not handle an ICMP request in the   fast path) and effects of load on the reflecting point.}Almes, et al.               Standards Track                     [Page 7]

RFC 2681          Round-trip for Delay Metric for IPPM    September 19992.7. 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 uncertainty in the clock of the Src      host.   +  Errors or uncertainties due to the difference between 'wire time'      and 'host time'.   +  Errors or uncertainties due to time required by the Dst to receive      the packet from the Src and send the corresponding response.   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.2.7.1. Errors or Uncertainties Related to Clocks   The uncertainty in a measurement of round-trip delay is related, in   part, to uncertainty in the clock of the Src host.  In the following,   we refer to the clock used to measure when the packet was sent from   Src as the source clock, and we refer to the observed time when the   packet was sent by the source as Tinitial, and the observed time when   the packet was received by the source as Tfinal.  Alluding to the   notions of synchronization, accuracy, resolution, and skew mentioned   in the Introduction, we note the following:   +  While in one-way delay there is an issue of the synchronization of      the source clock and the destination clock, in round-trip delay      there is an (easier) issue of self-synchronization, as it were,      between the source clock at the time the test packet is sent and      the (same) source clock at the time the response packet is      received.  Theoretically a very severe case of skew could threaten      this.  In practice, the greater threat is anything that would      cause a discontinuity in the source clock during the time between      the taking of the initial and final timestamp.  This might happen,      for example, with certain implementations of NTP.   +  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.Almes, et al.               Standards Track                     [Page 8]

RFC 2681          Round-trip for Delay Metric for IPPM    September 1999   +  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 as Rsource.   Taking these items together, we note that naive computation Tfinal-   Tinitial will be off by 2*Rsource.2.7.2. Errors or Uncertainties Related to Wire-time vs Host-time   As we have defined round-trip delay, we would like to measure the   time between when the test packet leaves the network interface of Src   and when the corresponding response packet (completely) arrives at   the network interface of Src, and we refer to these as "wire times".   If the timings are themselves performed by software on Src, 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   it grabs a timestamp just after having received the response packet,   and we refer to these two points as "host times".   Another contributor to this problem is time spent at Dst between the   receipt there of the test packet and the sending of the response   packet.  Ideally, this time is zero; it is explored further in the   next section.   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 Hinitial an   upper bound on the uncertainty in the difference between wire time   and host time on the Src host in sending the test packet, and   similarly define Hfinal for the difference on the Src host in   receiving the response packet.  We then note that these problems   introduce a total uncertainty of Hinitial + Hfinal.  This estimate of   total wire-vs-host uncertainty should be included in the   error/uncertainty analysis of any measurement implementation.2.7.3. Errors or Uncertainties Related to Dst Producing a Response   Any time spent by the destination host in receiving and recognizing   the packet from Src, and then producing and sending the corresponding   response adds additional error and uncertainty to the round-trip   delay measurement.  The error equals the difference between the wireAlmes, et al.               Standards Track                     [Page 9]

RFC 2681          Round-trip for Delay Metric for IPPM    September 1999   time the first bit of the packet is received by Dst and the wire time   the first bit of the response is sent by Dst.  To the extent that   this difference is accurately known, this knowledge can be used to   correct the desired metric.  To the extent, however, that this   difference is uncertain, this uncertainty must be accounted for in   the error analysis of a measurement implementation. We denote this   uncertainty by Hrefl.  This estimate of uncertainty should be   included in the error/uncertainty analysis of any measurement   implementation.2.7.4. 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   therefore       reported value = true value + random error   The goal of calibration is to determine the systematic and random   error generated by the instruments 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 instrument 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; and (2) a particular confidence level should be   specified so that the results of independent implementations can be   compared.}   From the discussion in the previous three sections, the error in   measurements could be bounded by determining all the individual   uncertainties, and adding them together to form       2*Rsource + Hinitial + Hfinal + Hrefl.Almes, et al.               Standards Track                    [Page 10]

RFC 2681          Round-trip for Delay Metric for IPPM    September 1999   However, reasonable bounds on both the clock-related uncertainty   captured by the first term and the host-related uncertainty captured   by the last three terms should be possible by careful design   techniques and calibrating the instruments using a known, isolated,   network in a lab.   The host-related uncertainties, Hinitial + Hfinal + Hrefl, could be   bounded by connecting two instruments back-to-back with a high-speed   serial link or isolated LAN segment.  In this case, repeated   measurements are measuring the same round-trip 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   instrumentation.  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 instrument, 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   instruments will see in the field.   We wish to reiterate that this statistical treatment refers to the   calibration of the instrument; it is used to "calibrate the meter   stick" and say how well the meter stick reflects reality.Almes, et al.               Standards Track                    [Page 11]

RFC 2681          Round-trip for Delay Metric for IPPM    September 1999   In addition to calibrating the instruments for finite 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 instrument.2.8. 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.2.8.1. Type-P   As noted in the Framework document [1], 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-Round-trip-Delay could change if the   protocol (UDP or TCP), port number, size, or arrangement for special   treatment (e.g., IP precedence or RSVP) changes.  The exact Type-P   used to make the measurements MUST be accurately reported.2.8.2. Loss threshold   In addition, the threshold (or methodology to distinguish) between a   large finite delay and loss MUST be reported.2.8.3. 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 instrument SHOULD be reported.Almes, et al.               Standards Track                    [Page 12]

RFC 2681          Round-trip for Delay Metric for IPPM    September 19992.8.4. 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.  For example, 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, and the Dst host copies   the path from Src to Dst into the corresponding reply packet, 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.}3. A Definition for Samples of Round-trip Delay   Given the singleton metric Type-P-Round-trip-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/lambda.   {Comment: Note that Poisson sampling is only one way of defining a   sample.  Poisson has the advantage of limiting bias, but other   methods of sampling might be appropriate for different situations.   We encourage others who find such appropriate cases to use this   general framework and submit their sampling method for   standardization.}3.1. Metric Name:   Type-P-Round-trip-Delay-Poisson-StreamAlmes, et al.               Standards Track                    [Page 13]

RFC 2681          Round-trip for Delay Metric for IPPM    September 19993.2. Metric Parameters:   +  Src, the IP address of a host   +  Dst, the IP address of a host   +  T0, a time   +  Tf, a time   +  lambda, a rate in reciprocal seconds3.3. 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-Round-trip-Delay, and that dT   would be a valid value of Type-P-Round-trip-Delay.3.4. 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-Round-trip-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.3.5. Discussion:   The reader should be familiar with the in-depth discussion of Poisson   sampling in the Framework document [1], 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.}Almes, et al.               Standards Track                    [Page 14]

RFC 2681          Round-trip for Delay Metric for IPPM    September 1999   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, nor will response packets   arrive at Src according to a Poisson distribution, since they are   influenced by the network.   All the singleton Type-P-Round-trip-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-Round-trip-Delay-Poisson-Stream   sample.3.6. 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-Round-trip-Delay metric.   Care must, of course, be given to correctly handle out-of-order   arrival of test or response packets; it is possible that the Src   could send one test packet at TS[i], then send a second test packet   (later) at TS[i+1], and it could receive the second response packet   at TR[i+1], and then receive the first response packet (later) at   TR[i].3.7. 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 severalAlmes, et al.               Standards Track                    [Page 15]

RFC 2681          Round-trip for Delay Metric for IPPM    September 1999   things, including problems with the pseudo-random number techniques   used to generate the Poisson arrival process, or with jitter in the   value of Hinitial (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.}3.8. Reporting the Metric:   You MUST report the calibration and context for the underlying   singletons along with the stream.  (See "Reporting the metric" for   Type-P-Round-trip-Delay.)4. Some Statistics Definitions for Round-trip Delay   Given the sample metric Type-P-Round-trip-Delay-Poisson-Stream, we   now offer several statistics of that sample.  These statistics are   offered mostly to be illustrative of what could be done.4.1. Type-P-Round-trip-Delay-Percentile   Given a Type-P-Round-trip-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-   Round-trip-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 110 msec and 'undefined' are larger.   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.Almes, et al.               Standards Track                    [Page 16]

RFC 2681          Round-trip for Delay Metric for IPPM    September 19994.2. Type-P-Round-trip-Delay-Median   Given a Type-P-Round-trip-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-Round-trip-Delay-   Percentile, Type-P-Round-trip-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.   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.4.3. Type-P-Round-trip-Delay-Minimum   Given a Type-P-Round-trip-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-Round-trip-Delay-Minimum is   undefined if the sample is empty.   In the above example, the minimum would be 90 msec.4.4. Type-P-Round-trip-Delay-Inverse-Percentile   Given a Type-P-Round-trip-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-Round-   trip-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 17]

RFC 2681          Round-trip for Delay Metric for IPPM    September 19995. 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.  If an attacker   injects artificial traffic that is accepted as legitimate, the loss   rate will be artificially lowered.  Therefore, the measurement   methodologies SHOULD include appropriate techniques to reduce the   probability measurement traffic can be distinguished from "normal"   traffic.  Authentication techniques, such as digital signatures, may   be used where appropriate to guard against injected traffic attacks.   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.6. Acknowledgements   Special thanks are due to Vern Paxson and to Will Leland for several   useful suggestions.7. References   [1]  Paxson, D., Almes, G., Mahdavi, J. and M. Mathis, "Framework for        IP Performance Metrics",RFC 2330, May 1998.   [2]  Almes, G., Kalidindi,S. and M. Zekauskas, "A One-way Delay        Metric for IPPM",RFC 2679, September 1999.   [3]  Mills, D., "Network Time Protocol (v3)",RFC 1305, April 1992.Almes, et al.               Standards Track                    [Page 18]

RFC 2681          Round-trip for Delay Metric for IPPM    September 1999   [4]  Mahdavi, J. and V. Paxson, "IPPM Metrics for Measuring        Connectivity",RFC 2678, September 1999.   [5]  Postel, J., "Internet Protocol", STD 5,RFC 791, September 1981.   [6]  Bradner, S., "Key words for use in RFCs to Indicate Requirement        Levels",BCP 14,RFC 2119, March 1997.8. Authors' Addresses   Guy Almes   Advanced Network & Services, Inc.   200 Business Park Drive   Armonk, NY  10504   USA   Phone: +1 914 765 1120   EMail: almes@advanced.org   Sunil Kalidindi   Advanced Network & Services, Inc.   200 Business Park Drive   Armonk, NY  10504   USA   Phone: +1 914 765 1128   EMail: kalidindi@advanced.org   Matthew J. Zekauskas   Advanced Network & Services, Inc.   200 Business Park Drive   Armonk, NY 10504   USA   Phone: +1 914 765 1112   EMail: matt@advanced.orgAlmes, et al.               Standards Track                    [Page 19]

RFC 2681          Round-trip for Delay Metric for IPPM    September 19999.  Full Copyright Statement   Copyright (C) The Internet Society (1999).  All Rights Reserved.   This document and translations of it may be copied and furnished to   others, and derivative works that comment on or otherwise explain it   or assist in its implementation may be prepared, copied, published   and distributed, in whole or in part, without restriction of any   kind, provided that the above copyright notice and this paragraph are   included on all such copies and derivative works.  However, this   document itself may not be modified in any way, such as by removing   the copyright notice or references to the Internet Society or other   Internet organizations, except as needed for the purpose of   developing Internet standards in which case the procedures for   copyrights defined in the Internet Standards process must be   followed, or as required to translate it into languages other than   English.   The limited permissions granted above are perpetual and will not be   revoked by the Internet Society or its successors or assigns.   This document and the information contained herein is provided on an   "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING   TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING   BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF   MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.Acknowledgement   Funding for the RFC Editor function is currently provided by the   Internet Society.Almes, et al.               Standards Track                    [Page 20]