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Network Working Group V. Raisanen Request for Comments: 3432 Nokia Category: Standards Track G. Grotefeld

                                                             A. Morton
                                                             AT&T Labs
                                                         November 2002
       Network performance measurement with periodic streams

Status 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.


 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 in RFC 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

Raisanen, et al. Standards Track [Page 1] RFC 3432 Network performance measurement November 2002

Table of Contents

 1.  Conventions used in this document...........................  2
 2.  Introduction................................................  3
     2.1 Motivation..............................................  3
 3.  Periodic Sampling Methodology...............................  4
 4.  Sample metrics for periodic streams.........................  5
     4.1 Metric name.............................................  5
     4.2 Metric parameters.......................................  5
     4.3 High level description of the procedure to collect a
         sample..................................................  7
     4.4 Discussion..............................................  8
     4.5 Additional Methodology Aspects..........................  9
     4.6 Errors and uncertainties................................  9
     4.7 Reporting............................................... 13
 5.  Additional discussion on periodic sampling.................. 14
     5.1 Measurement applications................................ 15
     5.2 Statistics calculable from one sample................... 18
     5.3 Statistics calculable from multiple samples............. 18
     5.4 Background conditions................................... 19
     5.5 Considerations related to delay......................... 19
 6.  Security Considerations..................................... 19
     6.1 Denial of Service Attacks............................... 19
     6.2 User data confidentiality............................... 20
     6.3 Interference with the metric............................ 20
 7.  IANA Considerations......................................... 20
 8.  Normative References........................................ 20
 9.  Informative References...................................... 21
 10. Acknowledgments............................................. 21
 11. Author's Addresses.......................................... 22
 12. Full Copyright Statement.................................... 23

1. Conventions used in this document

 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
 document are to be interpreted as described in BCP 14, RFC 2119 [2].
 Although RFC 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 2002

2. 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
 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
 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 framework RFC 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


  • 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
 +  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 in RFC 2679[4].
 Singletons defined in [3] and [4] are applicable here.

4.1 Metric name


4.2 Metric parameters

4.2.1 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

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
 +  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 (see section 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 2002

4.4 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
 +  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.}

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4.5 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,

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)
 +  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),

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)| <=
 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 2002

4.6.3. 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
   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 by

Raisanen, 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 the

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 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

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 the

Raisanen, 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.
  1. ————–

| application |

  1. ————–

| transport | ←-

  1. ————–

| network | ←-

  1. ————–

| link | ←-

  1. ————–

| physical |

  1. ————–
 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
 +  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.
  1. —————< IP >——————–

| | | |

  1. —— ——- ——– ——–

| Src | | MP | | MP | | Dst |

  1. —— |(Src)| |(Dst) | ——–
    1. —— ——–
                  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 2002

5.1.1 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 the

Raisanen, 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 =

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, different

Raisanen, 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

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 Considerations

6.1 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 this

Raisanen, 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
 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", BCP
      9, 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

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 2002

11. Author's Addresses

 Vilho Raisanen
 Nokia Networks
 P.O. Box 300
 FIN-00045 Nokia Group
 Phone: +358 7180 8000
 Fax:   +358 9 4376 6852
 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
 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

Raisanen, et al. Standards Track [Page 22] RFC 3432 Network performance measurement November 2002

12. 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
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 copyrights defined in the Internet Standards process must be
 followed, or as required to translate it into languages other than
 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


 Funding for the RFC Editor function is currently provided by the
 Internet Society.

Raisanen, et al. Standards Track [Page 23]

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