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Network Working Group                                        V. RaisanenRequest for Comments: 3432                                         NokiaCategory: Standards Track                                   G. Grotefeld                                                                Motorola                                                               A. Morton                                                               AT&T Labs                                                           November 2002Network performance measurement with periodic streamsStatus 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 (2002).  All Rights Reserved.Abstract   This memo describes a periodic sampling method and relevant metrics   for assessing the performance of IP networks.  First, the memo   motivates periodic sampling and addresses the question of its value   as an alternative to the Poisson sampling described inRFC 2330.  The   benefits include applicability to active and passive measurements,   simulation of constant bit rate (CBR) traffic (typical of multimedia   communication, or nearly CBR, as found with voice activity   detection), and several instances in which analysis can be   simplified.  The sampling method avoids predictability by mandating   random start times and finite length tests.  Following descriptions   of the sampling method and sample metric parameters, measurement   methods and errors are discussed.  Finally, we give additional   information on periodic measurements, including security   considerations.Raisanen, et al.            Standards Track                     [Page 1]

RFC 3432            Network performance measurement        November 2002Table of Contents1.  Conventions used in this document...........................22.  Introduction................................................32.1 Motivation..............................................33.  Periodic Sampling Methodology...............................44.  Sample metrics for periodic streams.........................54.1 Metric name.............................................54.2 Metric parameters.......................................5       4.3 High level description of the procedure to collect a           sample..................................................74.4 Discussion..............................................84.5 Additional Methodology Aspects..........................94.6 Errors and uncertainties................................94.7 Reporting...............................................135.  Additional discussion on periodic sampling..................145.1 Measurement applications................................155.2 Statistics calculable from one sample...................185.3 Statistics calculable from multiple samples.............185.4 Background conditions...................................195.5 Considerations related to delay.........................196.  Security Considerations.....................................196.1 Denial of Service Attacks...............................196.2 User data confidentiality...............................206.3 Interference with the metric............................207.  IANA Considerations.........................................208.  Normative References........................................209.  Informative References......................................2110. Acknowledgments.............................................2111. Author's Addresses..........................................2212. Full Copyright Statement....................................231. Conventions used in this document   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 inBCP 14,RFC 2119 [2].   AlthoughRFC 2119 was written with protocols in mind, the key words   are used in this document for similar reasons.  They are used to   ensure that the results of measurements from two different   implementations are comparable, and to note instances in which an   implementation could perturb the network.Raisanen, et al.            Standards Track                     [Page 2]

RFC 3432            Network performance measurement        November 20022. Introduction   This memo describes a sampling method and performance metrics   relevant to certain applications of IP networks.  The original driver   for this work was Quality of Service of interactive periodic streams,   such as multimedia conferencing over IP, but the idea of periodic   sampling and measurement has wider applicability.  Interactive   multimedia traffic is used as an example below to illustrate the   concept.   Transmitting equally sized packets (or mostly same-size packets)   through a network at regular intervals simulates a constant bit-rate   (CBR), or a nearly CBR multimedia bit stream.  Hereafter, these   packets are called periodic streams.  Cases of "mostly same-size   packets" may be found in applications that have multiple coding   methods (e.g.  digitally coded comfort noise during silence gaps in   speech).   In the following sections, a sampling methodology and metrics are   presented for periodic streams.  The measurement results may be used   in derivative metrics such as average and maximum delays.  The memo   seeks to formalize periodic stream measurements to achieve comparable   results between independent implementations.2.1 Motivation   As noted in the IPPM frameworkRFC 2330 [3], a sample metric using   regularly spaced singleton tests has some limitations when considered   from a general measurement point of view: only part of the network   performance spectrum is sampled.  However, some applications also   sample this limited performance spectrum and their performance may be   of critical interest.   Periodic sampling is useful for the following reasons:   * It is applicable to passive measurement, as well as active     measurement.   * An active measurement can be configured to match the     characteristics of media flows, and simplifies the estimation of     application performance.   * Measurements of many network impairments (e.g., delay variation,     consecutive loss, reordering) are sensitive to the sampling     frequency.  When the impairments themselves are time-varying (and     the variations are somewhat rare, yet important), a constant     sampling frequency simplifies analysis.Raisanen, et al.            Standards Track                     [Page 3]

RFC 3432            Network performance measurement        November 2002   * Frequency Domain analysis is simplified when the samples are     equally spaced.   Simulation of CBR flows with periodic streams encourages dense   sampling of network performance, since typical multimedia flows have   10 to 100 packets in each second.  Dense sampling permits the   characterization of network phenomena with short duration.3. Periodic Sampling Methodology   The Framework RFC [3] points out the following potential problems   with Periodic Sampling:   1. The performance sampled may be synchronized with some other      periodic behavior, or the samples may be anticipated and the      results manipulated.  Unpredictable sampling is preferred.   2. Active measurements can cause congestion, and periodic sampling      might drive congestion-aware senders into a synchronized state,      producing atypical results.   Poisson sampling produces an unbiased sample for the various IP   performance metrics, yet there are situations where alternative   sampling methods are advantageous (as discussed under Motivation).   We can prescribe periodic sampling methods that address the problems   listed above.  Predictability and some forms of synchronization can   be mitigated through the use of random start times and limited stream   duration over a test interval.  The periodic sampling parameters   produce bias, and judicious selection can produce a known bias of   interest.  The total traffic generated by this or any sampling method   should be limited to avoid adverse affects on non-test traffic   (packet size, packet rate, and sample duration and frequency should   all be considered).   The configuration parameters of periodic sampling are:   +  T, the beginning of a time interval where a periodic sample is      desired.   +  dT, the duration of the interval for allowed sample start times.   +  T0, a time that MUST be selected at random from the interval      [T, T+dT] to start generating packets and taking measurements.   +  Tf, a time, greater than T0, for stopping generation of packets      for a sample (Tf may be relative to T0 if desired).   +  incT, the nominal duration of inter-packet interval, first bit to      first bit.Raisanen, et al.            Standards Track                     [Page 4]

RFC 3432            Network performance measurement        November 2002   T0 may be drawn from a uniform distribution, or T0 = T + Unif(0,dT).   Other distributions may also be appropriate.  Start times in   successive time intervals MUST use an independent value drawn from   the distribution.  In passive measurement, the arrival of user media   flows may have sufficient randomness, or a randomized start time of   the measurement during a flow may be needed to meet this requirement.   When a mix of packet sizes is desired, passive measurements usually   possess the sequence and statistics of sizes in actual use, while   active measurements would need to reproduce the intended distribution   of sizes.4. Sample metrics for periodic streams   The sample metric presented here is similar to the sample metric   Type-P-One-way-Delay-Poisson-Stream presented inRFC 2679[4].   Singletons defined in [3] and [4] are applicable here.4.1 Metric name   Type-P-One-way-Delay-Periodic-Stream4.2 Metric parameters4.2.1 Global metric parameters   These parameters apply in the following sub-sections (4.2.2, 4.2.3,   and 4.2.4).   Parameters that each Singleton usually includes:     +  Src, the IP address of a host     +  Dst, the IP address of a host     +  IPV, the IP version (IPv4/IPv6) used in the measurement     +  dTloss, a time interval, the maximum waiting time for a packet        before declaring it lost.     +  packet size p(j), the desired number of bytes in the Type-P        packet, where j is the size index.   Optional parameters:     +  PktType, any additional qualifiers (transport address)     +  Tcons, a time interval for consolidating parameters collected at        the measurement points.   While a number of applications will use one packet size (j = 1),   other applications may use packets of different sizes (j > 1).   Especially in cases of congestion, it may be useful to use packets   smaller than the maximum or predominant size of packets in the   periodic stream.Raisanen, et al.            Standards Track                     [Page 5]

RFC 3432            Network performance measurement        November 2002   A topology where Src and Dst are separate from the measurement points   is assumed.4.2.2 Parameters collected at the measurement point MP(Src)   Parameters that each Singleton usually includes:   +  Tstamp(Src)[i], for each packet [i], the time of the packet as      measured at MP(Src)   Additional parameters:   +  PktID(Src) [i], for each packet [i], a unique identification or      sequence number.   +  PktSi(Src) [i], for each packet [i], the actual packet size.   Some applications may use packets of different sizes, either because   of application requirements or in response to IP performance   experienced.4.2.3 Parameters collected at the measurement point MP(Dst)   +  Tstamp(Dst)[i], for each packet [i], the time of the packet as      measured at MP(Dst)   +  PktID(Dst) [i], for each packet [i], a unique identification or      sequence number.   +  PktSi(Dst) [i], for each packet [i], the actual packet size.   Optional parameters:   +  dTstop, a time interval, used to add to time Tf to determine when      to stop collecting metrics for a sample   +  PktStatus [i], for each packet [i], the status of the packet      received.  Possible status includes OK, packet header corrupt,      packet payload corrupt, duplicate, fragment. The criteria to      determine the status MUST be specified, if used.4.2.4 Sample Metrics resulting from combining parameters at MP(Src)      and MP(Dst)   Using the parameters above, a delay singleton would be calculated as   follows:   +  Delay [i], for each packet [i], the time interval                   Delay[i] = Tstamp(Dst)[i] - Tstamp(Src)[i]Raisanen, et al.            Standards Track                     [Page 6]

RFC 3432            Network performance measurement        November 2002   For the following conditions, it will not be possible to compute   delay singletons:   Spurious: There will be no Tstamp(Src)[i] time   Not received: There will be no Tstamp (Dst) [i]   Corrupt packet header: There will be no Tstamp (Dst) [i]   Duplicate:  Only the first non-corrupt copy of the packet   received at  Dst should have Delay [i] computed.   A sample metric for average delay is as follows           AveDelay = (1/N)Sum(from i=1 to N, Delay[i])   assuming all packets i= 1 through N have valid singletons.   A delay variation [5] singleton can also be computed:   + IPDV[i], for each packet [i] except the first one, delay variation     between successive packets would be calculated as                     IPDV[i] = Delay[i] - Delay [i-1]   IPDV[i] may be negative, zero, or positive. Delay singletons for   packets i and i-1 must be calculable or IPDV[i] is undefined.   An example metric for the IPDV sample is the range:                   RangeIPDV = max(IPDV[]) - min(IPDV[])4.3 High level description of the procedure to collect a sample   Beginning on or after time T0, Type-P packets are generated by Src   and sent to Dst until time Tf is reached with a nominal interval   between the first bit of successive packets of incT, as measured at   MP(Src).  incT may be nominal due to a number of reasons: variation   in packet generation at Src, clock issues (seesection 4.6), etc.   MP(Src) records the parameters above only for packets with timestamps   between and including T0 and Tf having the required Src, Dst, and any   other qualifiers.  MP (Dst) also records for packets with time stamps   between T0 and (Tf + dTstop).   Optionally at a time Tf + Tcons (but eventually in all cases), the   data from MP(Src) and MP(Dst) are consolidated to derive the sample   metric results.  To prevent stopping data collection too soon, dTcons   should be greater than or equal to dTstop.  Conversely, to keep data   collection reasonably efficient, dTstop should be some reasonable   time interval  (seconds/minutes/hours), even if dTloss is infinite or   extremely long.Raisanen, et al.            Standards Track                     [Page 7]

RFC 3432            Network performance measurement        November 20024.4 Discussion   This sampling methodology is intended to quantify the delays and the   delay variation as experienced by multimedia streams of an   application.  Due to the definitions of these metrics, packet loss   status is also recorded.  The nominal interval between packets   assesses network performance variations on a specific time scale.   There are a number of factors that should be taken into account when   collecting a sample metric of Type-P-One-way-Delay-Periodic-Stream.   +  The interval T0 to Tf should be specified to cover a long enough      time interval to represent a reasonable use of the application      under test, yet not excessively long in the same context (e.g.      phone calls last longer than 100ms, but less than one week).   +  The nominal interval between packets (incT) and the packet size(s)      (p(j)) should not define an equivalent bit rate that exceeds the      capacity of the egress port of Src, the ingress port of Dst, or      the capacity of the intervening network(s), if known.  There may      be exceptional cases to test the response of the application to      overload conditions in the transport networks, but these cases      should be strictly controlled.   +  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 the delay      errors.   +  Depending on measurement topology, delay values may be as low as      100 usec to 10 msec, whereby it may be important for Src and Dst      to synchronize very closely.  GPS systems afford one way to      achieve synchronization to within several 10s of usec.  Ordinary      application of NTP may allow synchronization to within several      msec, but this depends on the stability and symmetry of delay      properties among the NTP agents used, and this delay is what we      are trying to measure.   +  A given methodology will have to include a way to determine      whether a packet was lost or whether delay is merely very large      (and  the packet is yet to arrive at Dst).  The global metric      parameter dTloss defines a time interval such that delays larger      than dTloss are interpreted as losses.  {Comment: For many      applications, the treatment of a large delay as infinite/loss will      be inconsequential.  A TCP data packet, for example, that arrives      only after several multiples of the usual RTT may as well have      been lost.}Raisanen, et al.            Standards Track                     [Page 8]

RFC 3432            Network performance measurement        November 20024.5 Additional Methodology Aspects   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).4.6 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 RFC [3] provides general guidance on this point, but we   note here the following specifics related to periodic streams and   delay metrics:   +  Error due to variation of incT.  The reasons for this can be      uneven process scheduling, possibly due to CPU load.   +  Errors or uncertainties due to uncertainties in the clocks of the      MP(Src) and MP(Dst) measurement points.   +  Errors or uncertainties due to the difference between 'wire time'      and 'host time'.4.6.1. 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 MP(Src) and MP(Dst).  In the   following, we refer to the clock used to measure when the packet was   measured at MP(Src) as the MP(Src) clock and we refer to the clock   used to measure when the packet was received at MP(Dst) as the   MP(Dst) clock.  Alluding to the notions of synchronization, accuracy,   resolution, and skew, we note the following:   +  Any error in the synchronization between the MP(Src) clock and the      MP(Dst) clock will contribute to error in the delay measurement.      We say that the MP(Src) clock and the MP(Dst) clock have a      synchronization error of Tsynch if the MP(Src) clock is Tsynch      ahead of the MP(Dst) clock.  Thus, if we know the value of Tsynch      exactly, we could correct for clock synchronization by adding      Tsynch to the uncorrected value of Tstamp(Dst)[i] - Tstamp(Src)      [i].   +  The resolution of a clock adds to uncertainty about any time      measured with it.  Thus, if the MP(Src) 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 MP(Dst) clock as ResMP(Src) and ResMP(Dst),      respectively.Raisanen, et al.            Standards Track                     [Page 9]

RFC 3432            Network performance measurement        November 2002   +  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      measurement or calculation must be repeated 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   Tstamp(Dst)[i] - Tstamp(Src) [i] will be off by Tsynch(t) +/-   (ResMP(SRc) + ResMP(Dst)).  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.4.6.2. Errors or uncertainties related to wire time vs host time   We would like to measure the time between when a packet is measured   and time-stamped at MP(Src) and when it arrives and is time-stamped   at MP(Dst); we refer to these as "wire times."  However, if   timestamps are applied by software on Src and Dst, then this software   can only directly measure the time between when Src generates the   packet just prior to sending the test packet and when Dst has started   to process the packet after having received the test packet; we refer   to these two points as "host times".   To the extent that the difference between wire time and host time is   accurately known, this knowledge can be used to correct for wire time   measurements.  The corrected value more accurately estimates the   desired (host time) metric, and visa-versa.   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 of   MP(Src) and host time on the Src host, and similarly define Hdest for   the difference between the host time on the Dst host and the wire   time of MP(Dst).  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.Raisanen, et al.            Standards Track                    [Page 10]

RFC 3432            Network performance measurement        November 20024.6.3. Calibration   Generally, the measured values can be decomposed as follows:     measured value = true value + systematic error + random error   If the systematic error (the constant bias in measured values) can be   determined, it can be compensated for in the reported results.     reported value = measured value - systematic error   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 due to reasons discussed in [4], briefly   summarized as (1) some confidence level is desirable to be able to   remove outliers, which will be found in measuring any physical   property; (2) a particular confidence level should be specified so   that the results of independent implementations can be compared.}   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) + ResMP(Src) + ResMP(Dst) + 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 instruments 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) +   ResMP(Src) + ResMP(Dst) 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 byRaisanen, et al.            Standards Track                    [Page 11]

RFC 3432            Network performance measurement        November 2002   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 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   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.4.6.4 Errors in incT   The nominal interval between packets, incT, can vary during either   active or passive measurements.  In passive measurement, packet   headers may include a timestamp applied prior to most of the protocol   stack, and the actual sending time may vary due to processor   scheduling.  For example, H.323 systems are required to have packets   ready for the network stack within 5 ms of their ideal time.  There   may be additional variation from the network between the Src and theRaisanen, et al.            Standards Track                    [Page 12]

RFC 3432            Network performance measurement        November 2002   MP(Src).  Active measurement systems may encounter similar errors,   but to a lesser extent.  These errors must be accounted for in some   types of analysis.4.7 Reporting   The calibration and context in which the method is used MUST be   carefully considered, and SHOULD always be reported along with metric   results.  We next present five items to consider: the Type-P of test   packets, the threshold of delay equivalent to loss, error   calibration, the path traversed by the test packets, and background   conditions at Src, Dst, and the intervening networks during a sample.   This list is not exhaustive; any additional information that could be   useful in interpreting applications of the metrics should also be   reported.4.7.1. Type-P   As noted in the Framework document [3], the value of a metric may   depend on the type of IP packets used to make the measurement, or   "type-P".  The value of Type-P-One-way-Periodic-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 reported.4.7.2. Threshold for delay equivalent to loss   In addition, the threshold for delay equivalent to loss (or   methodology to determine this threshold) MUST be reported.4.7.3. 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.4.7.4. Path   The path traversed by the packets 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 packets on short or stable paths.  If Type-P includes the   record route (or loose-source route) option in the IP header, and theRaisanen, et al.            Standards Track                    [Page 13]

RFC 3432            Network performance measurement        November 2002   path is short enough, and all routers on the path support record (or   loose-source) route, then the path will be precisely recorded.   This may be 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 one commercial setup, a Src on one NAP   can reach a Dst on another NAP by either of several different   backbone networks.}5. Additional discussion on periodic sampling   Fig.1 illustrates measurements on multiple protocol levels that are   relevant to this memo.  The user's focus is on transport quality   evaluation from the application point of view.  However, to properly   separate the quality contribution of the operating system and codec   on packet voice, for example, it is beneficial to be able to measure   quality at the IP level [6].  Link layer monitoring provides a way of   accounting for link layer characteristics such as bit error rates.        ---------------        | application |        ---------------        |  transport  | <--        ---------------        |   network   | <--        ---------------        |    link     | <--        ---------------        |   physical  |        ---------------   Fig. 1: Different possibilities for performing measurements: a   protocol view.  Above, "application" refers to all layers above L4   and is not used in the OSI sense.   In general, the results of measurements may be influenced by   individual application requirements/responses related to the   following issues:   +  Lost packets: Applications may have varying tolerance to lost      packets.  Another consideration is the distribution of lost      packets (i.e. random or bursty).Raisanen, et al.            Standards Track                    [Page 14]

RFC 3432            Network performance measurement        November 2002   +  Long delays: Many applications will consider packets delayed      longer than a certain value to be equivalent to lost packets (i.e.      real time applications).   +  Duplicate packets: Some applications may be perturbed if duplicate      packets are received.   +  Reordering: Some applications may be perturbed if packets arrive      out of sequence.  This may be in addition to the possibility of      exceeding the "long" delay threshold as a result of being out of      sequence.   +  Corrupt packet header: Most applications will probably treat a      packet with a corrupt header as equivalent to a lost packet.   +  Corrupt packet payload: Some applications (e.g. digital voice      codecs) may accept corrupt packet payload.  In some cases, the      packet payload may contain application specific forward error      correction (FEC) that can compensate for some level of corruption.   +  Spurious packet: Dst may receive spurious packets (i.e. packets      that are not sent by the Src as part of the metric).  Many      applications may be perturbed by spurious packets.   Depending, e.g., on the observed protocol level, some issues listed   above may be indistinguishable from others by the application, it may   be important to preserve the distinction for the operators of Src,   Dst, and/or the intermediate network(s).5.1 Measurement applications   This sampling method provides a way to perform measurements   irrespective of the possible QoS mechanisms utilized in the IP   network. As an example, for a QoS mechanism without hard guarantees,   measurements may be used to ascertain that the "best" class gets the   service that has been promised for the traffic class in question.   Moreover, an operator could study the quality of a cheap, low-   guarantee service implemented using possible slack bandwidth in other   classes. Such measurements could be made either in studying the   feasibility of a new service, or on a regular basis.   IP delivery service measurements have been discussed within the   International Telecommunications Union (ITU).  A framework for IP   service level measurements (with references to the framework for IP   performance [3]) that is intended to be suitable for service planning   has been approved as I.380 [7].  ITU-T Recommendation I.380 covers   abstract definitions of performance metrics.  This memo describes a   method that is useful, both for service planning and end-user testing   purposes, in both active and passive measurements.Raisanen, et al.            Standards Track                    [Page 15]

RFC 3432            Network performance measurement        November 2002   Delay measurements can be one-way [3,4], paired one-way, or round-   trip [8]. Accordingly, the measurements may be performed either with   synchronized or unsynchronized Src/Dst host clocks.  Different   possibilities are listed below.   The reference measurement setup for all measurement types is shown in   Fig. 2.        ----------------< IP >--------------------        |          |                  |          |      -------   -------           --------    --------      | Src |   | MP  |           | MP   |    | Dst  |      -------   |(Src)|           |(Dst) |    --------                -------           --------                    Fig. 2: Example measurement setup.   An example of the use of the method is a setup with a source host   (Src), a destination host (Dst), and corresponding measurement points   (MP(Src) and MP(Dst)) as shown in Figure 2.  Separate equipment for   measurement points may be used if having Src and/or Dst conduct the   measurement may significantly affect the delay performance to be   measured.  MP(Src) should be placed/measured close to the egress   point  of packets from Src.  MP(Dst) should be placed/measure close   to the ingress point of packets for Dst.  "Close" is defined as a   distance sufficiently small so that application-level performance   characteristics measured (such as delay) can be expected to follow   the corresponding performance characteristic between Src and Dst to   an adequate accuracy. The basic principle here is that measurement   results between MP(Src) and MP(Dst) should be the same as for a   measurement between Src and Dst, within the general error margin   target of the measurement (e.g., < 1 ms; number of lost packets is   the same).  If this is not possible, the difference between MP-MP   measurement and Src-Dst measurement should preferably be systematic.   The test setup just described fulfills two important criteria:   1) The test is made with realistic stream metrics, emulating - for      example - a full-duplex Voice over IP (VoIP) call.   2) Either one-way or round-trip characteristics may be obtained.   It is also possible to have intermediate measurement points between   MP(Src) and MP(Dst), but that is beyond the scope of this document.Raisanen, et al.            Standards Track                    [Page 16]

RFC 3432            Network performance measurement        November 20025.1.1 One way measurement   In the interests of specifying metrics that are as generally   applicable as possible, application-level measurements based on one-   way delays are used in the example metrics.  The implication of   application-level measurement for bi-directional applications, such   as interactive multimedia conferencing, is discussed below.   Performing a single one-way measurement only yields information on   network behavior in one direction.  Moreover, the stream at the   network transport level does not emulate accurately a full-duplex   multimedia connection.5.1.2 Paired one way measurement   Paired one way delay refers to two multimedia streams: Src to Dst and   Dst to Src for the same Src and Dst.  By way of example, for some   applications, the delay performance of each one way path is more   important than the round trip delay.  This is the case for delay-   limited signals such as VoIP.  Possible reasons for the difference   between one-way delays is different routing of streams from Src to   Dst vs. Dst to Src.   For example, a paired one way measurement may show that Src to Dst   has an average delay of 30ms, while Dst to Src has an average delay   of 120ms.  To a round trip delay measurement, this example would look   like an average of 150ms delay.  Without the knowledge of the   asymmetry, we might miss a problem that the application at either end   may have with delays averaging more than 100ms.   Moreover, paired one way delay measurement emulates a full-duplex   VoIP call more accurately than a single one-way measurement only.5.1.3 Round trip measurement   From the point of view of periodic multimedia streams, round-trip   measurements have two advantages: they avoid the need of host clock   synchronization and they allow for a simulation of full-duplex   communication.  The former aspect means that a measurement is easily   performed, since no special equipment or NTP setup is needed.  The   latter property means that measurement streams are transmitted in   both directions.  Thus, the measurement provides information on   quality of service as experienced by two-way applications.   The downsides of round-trip measurement are the need for more   bandwidth than a one-way test and more complex accounting of packet   loss.  Moreover, the stream that is returning towards the original   sender may be more bursty than the one on the first "leg" of theRaisanen, et al.            Standards Track                    [Page 17]

RFC 3432            Network performance measurement        November 2002   round-trip journey.  The last issue, however, means in practice that   the returning stream may experience worse QoS than the out-going one,   and the performance estimates thus obtained are pessimistic ones.   The possibility of asymmetric routing and queuing must be taken into   account during an analysis of the results.   Note that with suitable arrangements, round-trip measurements may be   performed using paired one way measurements.5.2 Statistics calculable from one sample   Some statistics may be particularly relevant to applications   simulated by periodic streams, such as the range of delay values   recorded during the sample.   For example, a sample metric generates 100 packets at MP(Src) with   the following measurements at MP(Dst):   +  80 packets received with delay [i] <= 20 ms   +   8 packets received with delay [i] > 20 ms   +   5 packets received with corrupt packet headers   +   4 packets from MP(Src) with no matching packet recorded at      MP(Dst) (effectively lost)   +   3 packets received with corrupt packet payload and delay      [i] <= 20 ms   +   2 packets that duplicate one of the 80 packets received correctly      as indicated in the first item   For this example, packets are considered acceptable if they are   received with less than or equal to 20ms delays and without corrupt   packet headers or packet payload.  In this case, the percentage of   acceptable packets is 80/100 = 80%.   For a different application that will accept packets with corrupt   packet payload and no delay bounds (so long as the packet is   received), the percentage of acceptable packets is (80+8+3)/100 =   91%.5.3 Statistics calculable from multiple samples   There may be value in running multiple tests using this method to   collect a "sample of samples".  For example, it may be more   appropriate to simulate 1,000 two-minute VoIP calls rather than a   single 2,000 minute call.  When considering a collection of multiple   samples, issues like the interval between samples (e.g. minutes,   hours), composition of samples (e.g. equal Tf-T0 duration, differentRaisanen, et al.            Standards Track                    [Page 18]

RFC 3432            Network performance measurement        November 2002   packet sizes), and network considerations (e.g. run different samples   over different intervening link-host combinations) should be taken   into account.  For items like the interval between samples, the usage   pattern for the application of interest should be considered.   When computing statistics for multiple samples, more general   statistics (e.g. median, percentile, etc.) may have relevance with a   larger number of packets.5.4 Background conditions   In many cases, the results may be influenced by conditions at Src,   Dst, and/or any intervening networks.  Factors that may affect the   results include: traffic levels and/or bursts during the sample, link   and/or host failures, etc.  Information about the background   conditions may only be available by external means (e.g. phone calls,   television) and may only become available days after samples are   taken.5.5 Considerations related to delay   For interactive multimedia sessions, end-to-end delay is an important   factor.  Too large a delay reduces the quality of the multimedia   session as perceived by the participants.  One approach for managing   end-to-end delays on an Internet path involving heterogeneous link   layer technologies is to use per-domain delay quotas (e.g. 50 ms for   a particular IP domain).  However, this scheme has clear   inefficiencies, and can over-constrain the problem of achieving some   end-to-end delay objective.  A more flexible implementation ought to   address issues like the possibility of asymmetric delays on paths,   and sensitivity of an application to delay variations in a given   domain. There are several alternatives as to the delay statistic one   ought to use in managing end-to-end QoS.  This question, although   very interesting, is not within the scope of this memo and is not   discussed further here.6. Security Considerations6.1 Denial of Service Attacks   This method generates a periodic stream of packets from one host   (Src) to another host (Dst) through intervening networks.  This   method could be abused for denial of service attacks directed at Dst   and/or the intervening network(s).   Administrators of Src, Dst, and the intervening network(s) should   establish bilateral or multi-lateral agreements regarding the timing,   size, and frequency of collection of sample metrics.  Use of thisRaisanen, et al.            Standards Track                    [Page 19]

RFC 3432            Network performance measurement        November 2002   method in excess of the terms agreed between the participants may be   cause for immediate rejection, discard of packets, or other   escalation procedures defined between the affected parties.6.2 User data confidentiality   Active use of this method generates packets for a sample, rather than   taking samples based on user data, and does not threaten user data   confidentiality.  Passive measurement must restrict attention to the   headers of interest.  Since user payloads may be temporarily stored   for length analysis, suitable precautions MUST be taken to keep this   information safe and confidential.6.3 Interference with the metric   It may be possible to identify that a certain packet or stream of   packets is part of a sample.  With that knowledge at Dst and/or the   intervening networks, it is possible to change the processing of the   packets (e.g. increasing or decreasing delay) that may distort the   measured performance.  It may also be possible to generate additional   packets that appear to be part of the sample metric.  These   additional packets are likely to perturb the results of the sample   measurement.   To discourage the kind of interference mentioned above, packet   interference checks, such as cryptographic hash, MAY be used.7. IANA Considerations   Since this method and metric do not define a protocol or well-known   values, there are no IANA considerations in this memo.8. Normative References   [1]  Bradner, S., "The Internet Standards Process -- Revision 3",BCP9,RFC 2026, October 1996.   [2]  Bradner, S., "Key words for use in RFCs to Indicate Requirement        Levels",BCP 14,RFC 2119, March 1997.   [3]  Paxson, V., Almes, G., Mahdavi, J. and M. Mathis, "Framework for        IP Performance Metrics",RFC 2330, May 1998.   [4]  Almes, G., Kalidindi, S. and M. Zekauskas, "A one-way delay        metric for IPPM",RFC 2679, September 1999.Raisanen, et al.            Standards Track                    [Page 20]

RFC 3432            Network performance measurement        November 2002   [5]  Demichelis, C. and P. Chimento, "IP Packet Delay Variation        Metric for IP Performance Metrics (IPPM)",RFC 3393, November        2002.9. Informative References   [6] "End-to-end Quality of Service in TIPHON systems; Part 5: Quality        of Service (QoS) measurement methodologies", ETSI TS 101 329-5        V1.1.2, January 2002.   [7]  International Telecommunications Union, "Internet protocol data        communication service _ IP packet transfer and availability        performance parameters", Telecommunications Sector        Recommendation I.380 (re-numbered Y.1540), February 1999.   [8]  Almes, G., Kalidindi, S. and M. Zekauskas, "A round-trip delay        metric for IPPM",RFC 2681, September 1999.10. Acknowledgments   The authors wish to thank the chairs of the IPPM WG (Matt Zekauskas   and Merike Kaeo) for comments that have made the present document   more clear and focused.  Howard Stanislevic and Will Leland have also   presented useful comments and questions.  We also gratefully   acknowledge Henk Uijterwaal's continued challenge to develop the   motivation for this method.  The authors have built on the   substantial foundation laid by the authors of the framework for IP   performance [3].Raisanen, et al.            Standards Track                    [Page 21]

RFC 3432            Network performance measurement        November 200211. Author's Addresses   Vilho Raisanen   Nokia Networks   P.O. Box 300   FIN-00045 Nokia Group   Finland   Phone: +358 7180 8000   Fax:   +358 9 4376 6852   EMail: Vilho.Raisanen@nokia.com   Glenn Grotefeld   Motorola, Inc.   1501 W. Shure Drive, MS 2F1   Arlington Heights, IL 60004 USA   Phone:  +1 847 435-0730   Fax:    +1 847 632-6800   EMail: g.grotefeld@motorola.com   Al Morton   AT&T Labs   Room D3 - 3C06   200 Laurel Ave. South   Middletown, NJ 07748 USA   Phone:  +1 732 420 1571   Fax:    +1 732 368 1192   EMail: acmorton@att.comRaisanen, et al.            Standards Track                    [Page 22]

RFC 3432            Network performance measurement        November 200212.  Full Copyright Statement   Copyright (C) The Internet Society (2002).  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.Raisanen, et al.            Standards Track                    [Page 23]