Network Working Group                                          A. Morton
Internet-Draft                                                 AT&T Labs
Intended status: Informational                                 B. Claise
Expires: January 8, 2008                             Cisco Systems, Inc.
                                                            July 7, 2007

             Packet Delay Variation Applicability Statement

Status of this Memo

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Copyright Notice

   Copyright (C) The IETF Trust (2007).


   Packet delay variation metrics appear in many different standards
   documents.  The metric definition in RFC 3393 has considerable
   flexibility, and it allows multiple formulations of delay variation
   through the specification of different packet selection functions.

   Although flexibility provides wide coverage and room for new ideas,
   it can make comparisons of independent implementations more

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   difficult.  Two different formulations of delay variation have come
   into wide use in the context of active measurements.  This memo
   examines a range of circumstances for active measurements of delay
   variation and their uses, and recommends which of the two forms is
   best matched to particular conditions and tasks.

Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [RFC2119].

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
     1.1.  Background Literature in IPPM and Elsewhere  . . . . . . .  5
     1.2.  Organization of the Memo . . . . . . . . . . . . . . . . .  6
   2.  Purpose and Scope  . . . . . . . . . . . . . . . . . . . . . .  6
   3.  Brief Descriptions of Delay Variation Uses . . . . . . . . . .  7
     3.1.  Inferring Queue Occupation on a Path . . . . . . . . . . .  7
     3.2.  Determining De-jitter Buffer Size  . . . . . . . . . . . .  7
     3.3.  Spatial Composition  . . . . . . . . . . . . . . . . . . .  7
     3.4.  Service Level Comparison . . . . . . . . . . . . . . . . .  8
     3.5.  <your favorite here> . . . . . . . . . . . . . . . . . . .  8
   4.  Formulations of IPDV and PDV . . . . . . . . . . . . . . . . .  8
     4.1.  IPDV: Inter-Packet Delay Variation . . . . . . . . . . . .  8
     4.2.  PDV: Packet Delay Variation  . . . . . . . . . . . . . . .  9
     4.3.  Examples and Initial Comparisons . . . . . . . . . . . . .  9
   5.  Survey of Earlier Comparisons  . . . . . . . . . . . . . . . . 10
     5.1.  Demichelis' Comparison . . . . . . . . . . . . . . . . . . 11
     5.2.  Ciavattone et al.  . . . . . . . . . . . . . . . . . . . . 12
     5.3.  IPPM List Discussion from 2000 . . . . . . . . . . . . . . 13
     5.4.  Y.1540 Appendix II . . . . . . . . . . . . . . . . . . . . 14
   6.  Additional Properties and Comparisons  . . . . . . . . . . . . 14
     6.1.  Packet Loss  . . . . . . . . . . . . . . . . . . . . . . . 14
     6.2.  Path Changes . . . . . . . . . . . . . . . . . . . . . . . 15
       6.2.1.  Lossless Path Change . . . . . . . . . . . . . . . . . 16
       6.2.2.  Path Change with Loss  . . . . . . . . . . . . . . . . 17
     6.3.  Clock Stability and Error  . . . . . . . . . . . . . . . . 17
     6.4.  Spatial Composition  . . . . . . . . . . . . . . . . . . . 18
     6.5.  Reporting a Single Number  . . . . . . . . . . . . . . . . 18
     6.6.  Jitter in RTCP Reports . . . . . . . . . . . . . . . . . . 19
     6.7.  MAPDV2 . . . . . . . . . . . . . . . . . . . . . . . . . . 19
     6.8.  Load Balancing . . . . . . . . . . . . . . . . . . . . . . 20
   7.  Applicability of the Delay Variation Forms and
       Recommendations  . . . . . . . . . . . . . . . . . . . . . . . 20
     7.1.  Uses . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

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       7.1.1.  Inferring Queue Occupancy  . . . . . . . . . . . . . . 20
       7.1.2.  Determining De-jitter Buffer Size  . . . . . . . . . . 21
       7.1.3.  Spatial Composition  . . . . . . . . . . . . . . . . . 21
     7.2.  Challenging Circumstances  . . . . . . . . . . . . . . . . 21
       7.2.1.  Clock Issues . . . . . . . . . . . . . . . . . . . . . 21
       7.2.2.  Frequent Path Changes  . . . . . . . . . . . . . . . . 22
       7.2.3.  Frequent Loss  . . . . . . . . . . . . . . . . . . . . 22
       7.2.4.  Load Balancing . . . . . . . . . . . . . . . . . . . . 22
   8.  Measurement Considerations for Vendors, Testers, and Users . . 22
     8.1.  Measurement Stream Characteristics . . . . . . . . . . . . 22
     8.2.  Measurement Units  . . . . . . . . . . . . . . . . . . . . 22
     8.3.  Test Duration  . . . . . . . . . . . . . . . . . . . . . . 23
     8.4.  Clock Sync Options . . . . . . . . . . . . . . . . . . . . 23
     8.5.  Distinguishing Long Delay from Loss  . . . . . . . . . . . 23
     8.6.  Accounting for Packet Reordering . . . . . . . . . . . . . 23
     8.7.  Results Representation and Reporting . . . . . . . . . . . 23
   9.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 23
   10. Security Considerations  . . . . . . . . . . . . . . . . . . . 23
   11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 23
   12. Appendix on Reducing Delay Variation in Networks . . . . . . . 23
   13. Appendix on Calculating the D(min) in PDV  . . . . . . . . . . 24
   14. References . . . . . . . . . . . . . . . . . . . . . . . . . . 24
     14.1. Normative References . . . . . . . . . . . . . . . . . . . 24
     14.2. Informative References . . . . . . . . . . . . . . . . . . 25
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 26
   Intellectual Property and Copyright Statements . . . . . . . . . . 28

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1.  Introduction

   There are many ways to formulate packet delay variation metrics for
   the Internet and other packet-based networks.  The IETF itself has
   several specifications for delay variation [RFC3393], sometimes
   called jitter [RFC3550] or even interarrival jitter [RFC3550], and
   these have achieved wide adoption.  The International
   Telecommunication Union - Telecommunication Standardization Sector
   (ITU-T) has also recommended several delay variation metrics (called
   parameters in their terminology) [Y.1540] [G.1020], and some of these
   are widely cited and used.  Most of the standards above specify more
   than one way to quantify delay variation, so one can conclude that
   standardization efforts have tended to be inclusive rather than

   This memo uses the term "delay variation" for metrics that quantify a
   path's ability to transfer packets with consistent delay.  [RFC3393]
   and [Y.1540] both prefer this term.  Some refer to this phenomenon as
   "jitter" (and the buffers that attempt to smooth the variations as
   de-jitter buffers).  Applications of the term "jitter" are much
   broader than packet transfer performance, with "unwanted signal
   variation" as a general definition.  "Jitter" has been used to
   describe frequency or phase variations, such as data stream rate
   variations or carrier signal phase noise.  The phrase "delay
   variation" is almost self-defining and more precise, so it is
   preferred in this memo.

   Most (if not all) delay variation metrics are derived metrics, in
   that their definitions rely on another fundamental metric.  In this
   case, the fundamental metric is one-way delay, and variation is
   assessed by computing the difference between two individual one-way
   delay measurements, or a pair of singletons.  One of the delay
   singletons is taken as a reference, and the result is the variation
   with respect to the reference.  The variation is usually summarized
   for all packets in a stream using statistics.

   The industry has predominantly implemented two specific formulations
   of delay variation (for one survey of the situation,

   1.  Inter-Packet Delay Variation, IPDV, where the reference is the
       previous packet in the stream (according to sending sequence),
       and the reference changes for each packet in the stream.
       Properties of variation are coupled with packet sequence in this
       formulation.  This form was called Instantaneous Packet Delay
       Variation in early contributions.

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   2.  Packet Delay Variation, PDV, where a single reference is chosen
       from the stream based on specific criteria, and the reference is
       fixed once selected.  The most common criterion for the reference
       is the packet with the minimum delay in the sample.  This term
       derives its name from a similar definition for Cell Delay
       Variation, an ATM performance metric.

   It is important to note that the authors of relevant standards for
   delay variation recognized there are many different users with
   varying needs, and allowed sufficient flexibility to formulate
   several metrics with different properties.  Therefore, the comparison
   is not so much between standards bodies or their specifications as it
   is between specific formulations of delay variation.  Both Inter-
   Packet Delay Variation and Packet Delay Variation are compliant with
   [RFC3393], because different packet selection functions will produce
   either form.

1.1.  Background Literature in IPPM and Elsewhere

   With more people joining the measurement community every day, it is
   possible this memo is the first from the IP Performance Metrics
   (IPPM) Working Group that the reader has consulted.  This section
   provides a brief roadmap and background on the IPPM literature, and
   the published specifications of other relevant standards

   The IPPM framework [RFC2330] provides a background for this memo and
   other IPPM RFCs.  Key terms such as singleton, sample, and statistic
   are defined there, along with methods of collecting samples (Poisson
   streams), time related issues, and the "packet of Type-P" convention.

   There are two fundamental and related metrics that can be applied to
   every packet transfer attempt: one-way loss [RFC2680] and one-way
   delay [RFC2679].  Lost and delayed packets are separated by a waiting
   time threshold.  Packets that arrive at the measurement destination
   within their waiting time have finite delay and are not lost.
   Otherwise, packets are designated lost and their delay is undefined.
   Guidance on setting the waiting time threshold may be found in
   [RFC2680] and [I-D.morton-ippm-reporting-metrics].

   Another fundamental metric is packet reordering as specified in
   [RFC4737].  The reordering metric was defined to be "orthogonal" to
   packet loss.  In other words, the gap in a packet sequence caused by
   loss does not result in reordered packets, but a re-arrangement of
   packet arrivals from their sending order constitutes reordering.

   Derived metrics are based on the fundamental metrics.  The derived
   metric of primary interest here is delay variation [RFC3393], a

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   metric which is derived from one-way delay [RFC2680].  Another
   derived metric is the loss patterns metric [RFC3357], which is
   derived from loss.

   In the ITU-T, the framework, fundamental metrics and derived metrics
   for IP performance are all specified in Recommendation Y.1540

1.2.  Organization of the Memo

   The Purpose and Scope follows in Section 2.  We then give a summary
   of the main tasks for delay variation metrics in section 3.  Section
   4 defines the two primary forms of delay variation, and section 5
   presents summaries of four earlier comparisons.  Section 6 adds new
   comparisons to the analysis, and section 7 reviews the applicability
   and recommendations for each form of delay variation.  Section 8 then
   looks at many important delay variation measurement considerations.
   Following IANA and Security Considerations, there two Appendices.
   One presents guidance on reducing delay variation in networks, and
   the other calculation of the minimum delay for the PDV form.

2.  Purpose and Scope

   The IPDV and PDV formulations have certain features that make them
   more suitable for one circumstance and less so for another.  The
   purpose of this memo is to compare two forms of delay variation, so
   that it will be evident which of the two is better suited for each of
   many possible uses and their related circumstances.

   The scope of this memo is limited to the two forms of delay variation
   briefly described above (Inter-Packet Delay Variation and Packet
   Delay Variation), circumstances related to active measurement, and
   uses that are deemed relevant and worthy of inclusion here through
   IPPM Working Group consensus.

   The scope excludes assessment of delay variation for packets with
   undefined delay.  This is accomplished by conditioning the delay
   distribution on arrival within a reasonable waiting time based on an
   understanding of the path under test and packet lifetimes.  The
   waiting time is sometimes called the loss threshold [RFC2680]: if a
   packet arrives beyond this threshold, it may as well have been lost
   because it is no longer useful.  This is consistent with [RFC3393],
   where the Type-P-One-way-ipdv is undefined when the destination fails
   to receive one or both packets in the selected pair.  Furthermore, it
   is consistent with application performance analysis to consider only
   arriving packets, because a finite waiting time-out is a feature of
   many protocols.

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3.  Brief Descriptions of Delay Variation Uses

   This section presents a set of tasks that call for delay variation
   measurements.  Here, the memo provides several answers to the
   question, "How will the results be used?" for the delay variation

3.1.  Inferring Queue Occupation on a Path

   As packets travel along the path from source to destination, they
   pass through many network elements, including a series of router
   queues.  Some types of the delay sources along the path are constant,
   such as links between two locations.  But the latency encountered in
   each queue varies, depending on the number of packets in the queue
   when a particular packet arrives.  If one assumes that at least one
   of the packets in a test stream encounters virtually empty queues all
   along the path (and the path is stable), then the additional delay
   observed on other packets can be attributed to the time spent in one
   or more queues.  Otherwise, the delay variation observed is the
   variation in queue time experienced by the test stream.

3.2.  Determining De-jitter Buffer Size

   Note - while this memo and other IPPM literature prefer the term
   delay variation, the terms "jitter buffer" and the more accurate "de-
   jitter buffer" are widely adopted names for a component of packet
   communication systems, and they will be used here to designate that
   system component.

   Most Isochronous applications (a.k.a. real-time applications) employ
   a buffer to smooth out delay variation encountered on the path from
   source to destination.  The buffer must be big enough to accommodate
   (most of) the expected variation, or packet loss will result.
   However, if the buffer is too large, then some of the desired
   spontaneity of communication will be lost and conversational dynamics
   will be affected.  Therefore, application designers need to know the
   extent of delay variation they must accommodate, whether they are
   designing fixed or adaptive buffer systems.

   Network service providers also attempt to constrain delay variation
   to ensure the quality of real-time applications, and monitor this
   metric (possibly to compare with a numerical objective or Service
   Level Agreement).

3.3.  Spatial Composition

   In Spatial Composition, the tasks are similar to those described
   above, but with the additional complexity of a multiple network path

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   where several sub-paths are measured separately, and no source to
   destination measurements are available.  In this case, the source to
   destination performance must be estimated, using Composed Metrics as
   described in [I-D.ietf-ippm-framework-compagg] and [Y.1541].  Note
   that determining the composite delay variation is not trivial: simply
   summing the sub-path variations is not accurate.

3.4.  Service Level Comparison

   IP performance measurements are often used as the basis for
   agreements (or contracts) between service providers and their
   customers.  The measurement results must compare favorably with the
   performance levels specified in the agreement.

   Packet delay variation is usually one of the metrics specified in
   these agreements.  In principle, any formulation could be specified
   in the Service Level Agreement (SLA).  However, the SLA is most
   useful when the measured quantities can be related to ways in which
   the communication service will be utilized by the customer, and this
   can usually be derived from one of the tasks described above.

3.5.  <your favorite here>

   Note: At the IETF-68 IPPM session, Alan Clark suggested another
   possible task for DV measurements, that of detecting and somehow
   removing the delay variation associated with a smoothing buffer used
   with a video codec.  Further work is needed to define the problem and
   to investigate the applicability of IPDV and PDV.

4.  Formulations of IPDV and PDV

   This section presents the formulations of IPDV and PDV, and provides
   some illustrative examples.  We use the basic singleton definition in
   [RFC3393] (which itself is based on [RFC2679]):

   "Type-P-One-way-ipdv is defined for two packets from Src to Dst
   selected by the selection function F, as the difference between the
   value of the Type-P-One-way-delay from Src to Dst at T2 and the value
   of the Type-P-One-Way-Delay from Src to Dst at T1."

4.1.  IPDV: Inter-Packet Delay Variation

   If we have packets in a stream consecutively numbered i = 1,2,3,...
   falling within the test interval, then IPDV(i) = D(i)-D(i-1) where
   D(i) denotes the one-way-delay of the ith packet of a stream.

   0ne-way delays are the difference between timestamps applied at the

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   ends of the path, or the receiver time minus the transmission time.
   So D(2) = R2-T2.  With this timestamp notation, it can be shown that
   IPDV also represents the change in inter-packet spacing between
   transmission and reception:

   IPDV(2) = D(2) - D(1) = (R2-T2) - (R1-T1) = (R2-R1) - (T2-T1)

   An example selection function given in [RFC3393] is "Consecutive
   Type-P packets within the specified interval."  This is exactly the
   function needed for IPDV.  The reference packet in the pair is always
   the previous packet in the sending sequence.

   Note that IPDV can take on positive and negative values (and zero),
   although one of the useful ways to analyze the results is to
   concentrate on the positive excursions.  This is discussed in more
   detail below.

4.2.  PDV: Packet Delay Variation

   The name Packet Delay Variation is used in [Y.1540] and its
   predecessors, and refers to a performance parameter equivalent to the
   metric described below.

   The Selection Function for PDV requires two specific roles for the
   packets in the pair.  The first packet is any Type-P packet within
   the specified interval.  The second, or reference packet is the
   Type-P packet within the specified interval with the minimum one-way-

   Therefore, PDV(i) = D(i)-D(min) (using the nomenclature introduced in
   the IPDV section).  D(min) is the delay of the packet with the lowest
   value for delay (minimum) over the current test interval.  Values of
   PDV may be zero or positive, and quantiles of the PDV distribution
   are direct indications of delay variation.

4.3.  Examples and Initial Comparisons

   Note: This material originally presented in slides 2 and 3 of

   The Figure below gives a sample of packet delays and calculates IPDV
   and PDV values and depicts a histogram for each one.

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                       Packet #     1   2   3   4   5
                       Delay, ms   20  10  20  25  20

                       IPDV         U -10  10   5  -5

                       PDV         10   0  10  15  10

                          |                 |
                         4|                4|
                          |                 |
                         3|                3|         H
                          |                 |         H
                         2|                2|         H
                          |                 |         H
                  H   H  1|   H   H        1|H        H   H
                  H   H   |   H   H         |H        H   H
                 ---------+--------         +---------------
                -10  -5   0   5  10          0   5   10  15

                   IPDV Histogram             PDV Histogram

                     Figure 1: IPDV and PDV Comparison

   The sample of packets contains three packets with "typical" delays of
   20ms, one packet with a low delay of 10ms (the minimum of the sample)
   and one packet with 25ms delay.

   As noted above, this example illustrates that IPDV may take on
   positive and negative values, while the PDV values are greater than
   or equal to zero.  The Histograms of IPDV and PDV are quite different
   in general shape, and the ranges are different, too (IPDV range =
   20ms, PDV range = 15 ms).  Note that the IPDV histogram will change
   if the sequence of delays is modified, but the PDV histogram will
   stay the same.  PDV normalizes the one-way delay distribution to the
   minimum delay and emphasizes the variation independent from the
   sequence of delays.

5.  Survey of Earlier Comparisons

   This section summarizes previous work to compare these two forms of
   delay variation.

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5.1.  Demichelis' Comparison

   In [Demichelis], Demichelis compared the early draft versions of two
   forms of delay variation.  Although the IPDV form would eventually
   see widespread use, the ITU-T work-in-progress he cited did not
   utilize the same reference packets as PDV.  Demichelis compared IPDV
   with the alternatives of using the delay of the first packet in the
   stream and the mean delay of the stream as the PDV reference packet.
   Neither of these alternative references were used in practice, and
   they are now deprecated in favor of the minimum delay of the stream

   Active measurements of a transcontinental path (Torino to Tokyo)
   provided the data for the comparison.  The Poisson test stream had
   0.764 second average inter-packet interval, with more than 58
   thousand packets over 13.5 hours.  Among Demichelis' observations
   about IPDV are the following:

   1.  IPDV is a measure of the network's ability to preserve the
       spacing between packets.

   2.  The distribution of IPDV is usually symmetrical about the origin,
       having a balance of negative and positive values (for the most
       part).  The mean is usually zero, unless some long-term delay
       trend is present.

   3.  IPDV distinguishes quick delay variations (on the order of the
       interval between packets) from longer term variations.

   4.  IPDV places reduced demands on the stability and skew of
       measurement clocks.

   He also notes these features of PDV:

   1.  PDV does not distinguish quick variation from variation over the
       complete test interval.

   2.  The location of the distribution is very sensitive to the delay
       of the first packet, IF this packet is used as the reference.
       This would be a new formulation that differs from the PDV
       definition in this memo (PDV references the packet with minimum
       delay, so it does not have this drawback).

   3.  The shape of the PDV distribution is identical to the delay
       distribution, but shifted by the reference delay.

   4.  Use of a common reference over measurement intervals that are
       longer than a typical session length may indicate more PDV than

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       would be experienced by streams that support such sessions.

   5.  PDV characterizes the range of queue occupancies along the
       measurement path (assuming the path is fixed), but the range says
       nothing about how the variation took place.

   The summary metrics used in this comparison were the number of values
   exceeding a +/-50ms range around the mean, the Inverse Percentiles,
   and the Inter-Quartile Range.

5.2.  Ciavattone et al.

   In [Cia03], the authors compared IPDV and PDV (referred to as delta)
   using a periodic packet stream conforming to [RFC3432] with inter-
   packet interval of 20 ms.

   One of the comparisons between IPDV and PDV involves a laboratory
   set-up where a queue was temporarily congested by a competing packet
   burst.  The additional queuing delay was 85ms to 95ms, much larger
   than the inter-packet interval.  The first packet in the stream that
   follows the competing burst spends the longest time enqueued, and
   others experience less and less queuing time until the queue is

   The authors observed that PDV reflects the additional queuing time of
   the packets affected by the burst, with values of 85, 65, 45, 25, and
   5ms.  Also, it is easy to determine (by looking at the PDV range)
   that a de-jitter buffer of 90 ms would have been sufficient to
   accommodate the delay variation.

   The distribution of IPDV values in the congested queue example are
   very different: 85, -20, -20, -20, -20, -5ms.  Only the positive
   excursion of IPDV gives an indication of the de-jitter buffer size
   needed.  Although the variation exceeds the inter-packet interval,
   the extent of negative IPDV values is limited by that sending
   interval.  This preference for information from the positive IPDV
   values has prompted some to ignore the negative values, or to take
   the absolute value of each IPDV measurement (sacrificing key
   properties of IPDV in the process, such as its ability to distinguish
   delay trends).

   Elsewhere, the authors considered the range as a summary statistic
   for IPDV, and the 99.9%-ile minus the minimum delay as a summary
   statistic for delay variation, or PDV.

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5.3.  IPPM List Discussion from 2000

   Mike Pierce made many comments in the context of the 05 version of
   draft-ietf-ippm-ipdv.  One of his main points was that a delay
   histogram is a useful approach to quantifying variation.  Another was
   the that the time duration of evaluation is a critical aspect.

   Carlo Demichelis then mailed his comparison paper to the IPPM list,
   [Demichelis] as discussed in more detail above.

   Ruediger Geib observed that both IPDV and the delay histogram (PDV)
   are useful, and suggested that they might be applied to different
   variation time scales.  He pointed out that loss has a significant
   effect on IPDV, and encouraged that the loss information be retained
   in the arrival sequence.

   Several example delay variation scenarios were discussed, including:

          Packet #     1   2   3   4   5   6   7   8   9  10  11
          Ex. A

          Delay, ms  100 110 120 130 140 150 140 130 120 110 100

          IPDV        U   10  10  10  10  10 -10 -10 -10 -10 -10

          PDV         0   10  20  30  40  50  40  30  20  10   0

          Ex. B
          Lost                     L

          Delay, ms  100 110 150   U 120 100 110 150 130 120 100

          IPDV        U   10  40   U   U -10  10  40 -20 -10 -20

          PDV         0   10  50   U  20   0  10  50  30  20   0

                         Figure 2: Delay Examples

   Clearly, the range of PDV values is 50 ms in both cases above, while
   the IPDV range tends to minimize the smooth variation in Example A
   (20 ms), and responds to the faster variations in Example B (60 ms).

   IPDV values can be viewed as the adjustments that an adaptive de-
   jitter buffer would make, IF it could make adjustments on a packet-
   by-packet basis.  However, adaptive de-jitter buffers don't make

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   adjustments this frequently, so the value of this information is

5.4.  Y.1540 Appendix II

   This Appendix compares IPDV, PDV (referred to as 2-point PDV), and
   1-point packet delay variation (which assume a periodic stream and
   assesses variation against an ideal arrival schedule constructed at
   the single measurement point).

6.  Additional Properties and Comparisons

   This section treats some of the earlier comparison areas in more
   detail, and introduces new areas for comparison.

6.1.  Packet Loss

   The measurement packet loss is of great influence for the delay
   variation results, as displayed in the figure 2 and 3 (L means Lost
   and U means undefined).  Figure 3 shows that in the extreme case of
   every other packet loss, the IPDV doesn't produce any results, while
   the PDV produces results for all arriving packets.

                  Packet #   1  2  3  4  5  6  7  8  9 10
                  Lost          L     L     L     L     L
                  Delay, ms  3  U  5  U  4  U  3  U  4  U

                  IPDV       U  U  U  U  U  U  U  U  U  U

                  PDV        0  U  2  U  1  U  0  U  1  U

                  Figure 3: Path Loss Every Other Packet

   In case of a burst of packet loss, as displayed in figure 3, both the
   IPDV and PDV produces some results.  Note that the PDV.

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                  Packet #   1  2  3  4  5  6  7  8  9 10
                  Lost             L  L  L  L  L
                  Delay, ms  3  4  U  U  U  U  U  5  4  3

                  IPDV       U  1  U  U  U  U  U  U -1 -1

                  PDV        0  1  U  U  U  U  U  2  1  0

                      Figure 4: Burst of Packet Loss

   In conclusion, the PDV results are affected by the packet loss ratio.
   While the IPDV results are affected by the packet loss ratio, they
   are also affected by the packet loss distribution.  Indeed, in the
   extreme case of every other packet loss, the IPDV doesn't provide any

6.2.  Path Changes

   When there is little or no stability in the network under test, then
   the devices that attempt to characterize the network are equally
   stressed, especially if the results displayed are used to make
   inferences which may not be valid.

   Sometimes the path characteristics change during a measurement
   interval.  The change may be due to link or router failure,
   administrative changes prior to maintenance (e.g., link cost change),
   or re-optimization of routing using new information.  All these
   causes are usually infrequent, and network providers take appropriate
   measures to ensure this.  Automatic restoration to a back-up path is
   seen as a desirable feature of IP networks.

   Frequent path changes and prolonged congestion with substantial
   packet loss clearly make delay variation measurements challenging.
   Path changes are usually accompanied by a sudden, persistent increase
   or decrease in one-way-delay.  [Cia03] gives one such example.  We
   assume that a restoration path either accepts a stream of packets, or
   is not used for that particular stream (e.g., no multi-path for

   In any case, a change in the TTL (or Hop Limit) of the received
   packets indicates that the path is no longer the same.  Transient
   packet reordering may also be observed with path changes, due to use
   of non-optimal routing while updates propagate through the network
   (see [Casner] and [Cia03] )

   Many, if not all, packet streams experience packet loss in
   conjunction with a path change.  However, it is certainly possible

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   that the active measurement stream does not experience loss.  This
   may be due to use of a long inter-packet sending interval with
   respect to the restoration time, and this becomes more likely as
   "fast restoration" techniques see wider deployment (e.g., [RFC4090].

   Thus, there are two main cases to consider, path changes accompanied
   by loss, and those that are lossless from the point of view of the
   active measurement stream.  The subsections below examine each of
   these cases.

6.2.1.  Lossless Path Change

   In the lossless case, a path change will typically affect only two
   IPDV singletons.  However, if the change in delay is negative and
   larger than the inter-packet sending interval, then more than two
   IPDV singletons may be affected because packet reordering is also
   likely to occur.

   The use of the new path and its delay variation can be quantified by
   treating the PDV distribution as bi-modal, and characterizing each
   mode separately.  This would involve declaring a new path within the
   sample, and using a new local minimum delay as the PDV reference
   delay for the sub-sample (or time interval) where the new path is

   The process of detecting a bi-modal delay distribution is made
   difficult if the typical delay variation is larger than the delay
   change associated with the new path.  However, information on TTL (or
   Hop Limit) change or the presence of transient reordering can assist
   in an automated decision.

   The effect of path changes may also be reduced by making PDV
   measurements over short intervals (minutes, as opposed to hours).
   This way, a path change will affect one sample and its PDV values.
   Assuming that the mean or median one-way-delay changes appreciably on
   the new path, then subsequent measurements can confirm a path change,
   and trigger special processing on the interval containing a path
   change and the affected PDV result.

   Alternatively, if the path change is detected, by monitoring the test
   packets TTL or Hop Limit, or monitoring the change in the IGP link-
   state database, the results of measurement before and after the path
   change could be kept separated, presenting two different
   distributions.  This avoids the difficult task of determining the
   different modes of a multi-modal distribution.

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6.2.2.  Path Change with Loss

   If the path change is accompanied by loss, such that the are no
   consecutive packet pairs that span the change, then no IPDV
   singletons will reflect the change.  This may or may not be
   desirable, depending on the ultimate use of the delay variation
   measurement.  The Figure 3, in which L means Lost and U means
   undefined, illustrates this case.

                    Packet #   1  2  3  4  5  6  7  8  9
                    Lost                   L  L
                    Delay, ms  3  4  3  3  U  U  8  9  8

                    IPDV       U  1 -1  0  U  U  U  1 -1

                    PDV        0  1  0  0  U  U  5  6  5

                      Figure 5: Path Change with Loss

   PDV will again produce a bimodal distribution.  But here, the
   decision process to define sub-intervals associated with each path is
   further assisted by the presence of loss, in addition to TTL,
   reordering information, and use of short measurement intervals
   consistent with the duration of user sessions.  It is reasonable to
   assume that at least loss and delay will be measured simultaneously
   with PDV and/or IPDV.

6.3.  Clock Stability and Error

   Low cost or low complexity measurement systems may be embedded in
   communication devices that do not have access to high stability
   clocks, and time errors will almost certainly be present.  However,
   larger time-related errors may offer an acceptable trade-off for
   monitoring performance over a large population (the accuracy needed
   to detect problems may be much less than required for a scientific

   As mentioned above, [Demichelis] observed that PDV places greater
   demands on clock synchronization than for IPDV.  This observation
   deserves more discussion.  Synchronization errors have two
   components: time of day errors and clock frequency errors (resulting
   in skew).

   Both IPDV and PDV are sensitive to time-of-day errors when attempting
   to align measurement intervals at the source and destination.  Gross
   mis-alignment of the measurement intervals can lead to lost packets,
   for example if the receiver is not ready when the first test packet

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   arrives.  However, both IPDV and PDV assess delay differences, so the
   error present in two one-way-delay singletons will cancel as long as
   it is constant.  So, NTP or GPS synchronization is not required to
   correct the time-of-day error in case the delay variation
   measurement, while it is required for the one-way delay measurement.

   Skew is a measure of the change in clock time over an interval w.r.t.
   a reference clock.  Both IPDV and PDV are affected by skew, but the
   error sensitivity in IPDV singletons is less because the intervals
   between consecutive packets are rather small, especially when
   compared to the overall measurement interval.  Since PDV computes the
   difference between a single reference delay (the sample minimum) and
   all other delays in the measurement interval, the constraint on skew
   error is greater to attain the same accuracy as IPDV.  Again, use of
   short PDV measurement intervals (on the order of minutes, not hours)
   provides some relief from the effects of skew error.

   If skew is present in a sample of one-way-delays, its symptom is
   typically a linear growth or decline over all the one-way-delay
   values.  As a practical matter, if the same slope appears
   consistently in the measurements, then it may be possible to fit the
   slope and compensate for the skew in the one-way-delay measurements,
   thereby avoiding the issue in the PDV calculations that follow.  See
   [RFC3393] for additional information on compensating for skew.

   There is a third factor related to clock error and stability: this is
   the presence of a clock synchronization protocol (e.g., NTP) and the
   time adjustment operations that result.  When a small time error is
   detected (typically on the order of a few milliseconds), the host
   clock frequency is continuously adjusted to reduce the time error.
   If these adjustments take place during a measurement interval, they
   may appear as delay variation when none was present, and therefore
   are a source of error.

6.4.  Spatial Composition

   ITU-T Recommendation [Y.1541] gives a provisional method to compose a
   PDV metric using PDV measurement results from two or more sub-paths.

   PDV has a clear advantage at this time, since there is no known
   method to compose an IPDV metric.  In addition, IPDV results depend
   greatly on the exact sequence of packets and may not lend themselves
   easily to the composition problem.

6.5.  Reporting a Single Number

   Despite the risk of over-summarization, measurements must often be
   displayed for easy consumption.  If the right summary report is

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   prepared, then the "dashboard" view correctly indicates whether there
   is something different and worth investigating further, or that the
   status has not changed.  The dashboard model restricts every
   instrument display to a single number.  The packet network dashboard
   could have different instruments for loss, delay, delay variation,
   reordering, etc., and each must be summarized as a single number for
   each measurement interval.

   The simplicity of the PDV distribution lends itself to this
   summarization process (including use of the median or mean).
   [Y.1541] introduced the notion of a pseudo-range when setting an
   objective for the 99.9%-ile of PDV.  The conventional range (max-min)
   was avoided for several reasons, including stability of the maximum
   delay.  The 99.9%-ile of PDV is helpful to performance planners
   (seeking to meet some user-to-user objective for delay) and in design
   of de-jitter buffer sizes, even those with adaptive capabilities.

   IPDV does not lend itself to summarization so easily.  The mean IPDV
   is typically zero.  As the IPDV distribution may have two tails
   (positive and negative) the range or pseudo-range would not match the
   needed de-jitter buffer size.  Additional complexity may be
   introduced when the variation exceeds the inter-packet sending
   interval, as discussed above.  Should the Inter-Quartile Range be
   used?  Should the singletons beyond some threshold be counted (e.g.,
   mean +/- 50ms)?  A strong rationale for one of these summary
   statistics has yet to emerge.

6.6.  Jitter in RTCP Reports

   [RFC3550] gives the calculation of the inter-arrival Jitter field for
   the RTCP report, with a sample implementation in an Appendix.

   The RTCP Jitter value can be calculated using IPDV singletons.  If
   there is packet reordering, as defined in [RFC4737], then estimates
   of Jitter based on IPDV may vary slightly, because [RFC3550]
   specifies the use of receive packet order.

   Just as there is no simple way to convert PDV singletons to IPDV
   singletons without returning to the original sample of delay
   singletons, there is no clear relationship between PDV and [RFC3550]

6.7.  MAPDV2

   MAPDV2 stands for Mean Absolute Packet Delay Variation (version) 2,
   and is specified in [G.1020].  The MAPDV2 algorithm computes a
   smoothed running estimate of the mean delay using the one-way delays
   of 16 previous packets.  It compares the current one-way-delay to the

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   estimated mean, separately computes the means of positive and
   negative deviations, and sums these deviation means to produce
   MAPVDV2.  In effect, there is a MAPDV2 singleton for every arriving
   packet, so further summarization is usually warranted.

   Neither IPDV or PDV forms assist in the computation of MAPDV2.

6.8.  Load Balancing

   TO DO: What if there is load-balancing in an ISP network?  Load-
   balancing is based on the IGP metrics, while the delay depends on the
   path.  So, we could have a multi-modal distribution, if we send test
   packets with different characteristics such as IP addresses/ports.

   Should the delay singletons using multiple addresses/ports be
   combined in the same sample?

   The PDV form makes the identification of multiple modes possible.
   Should we characterize each mode separately?  This question also
   applies to the Path Change case.

7.  Applicability of the Delay Variation Forms and Recommendations

   Based on the comparisons of IPDV and PDV presented above, this
   section matches the attributes of each form with the tasks described
   earlier.  We discuss the more general circumstances first.

   Note: the conclusions of this section should be regarded as
   preliminary, pending discussion and further development by the IPPM

7.1.  Uses

7.1.1.  Inferring Queue Occupancy

   The PDV distribution is anchored at the minimum delay observed in the
   measurement interval.  When the sample minimum coincides with the
   true minimum delay of the path, then the PDV distribution is
   equivalent to the queuing time distribution experienced by the test
   stream.  If the minimum delay is not the true minimum, then the PDV
   distribution captures the variation in queuing time and some
   additional amount of queuing time is experienced, but unknown.  One
   can summarize the PDV distribution with the mean, median, and other

   IPDV can capture the difference in queuing time from one packet to
   the next, but this is a different distribution from the queue

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   occupancy revealed by PDV.

7.1.2.  Determining De-jitter Buffer Size

   This task is complimentary to the problem of inferring queue
   occupancy through measurement.  Again, use of the sample minimum as
   the reference delay for PDV yields a distribution that is very
   relevant to de-jitter buffer size.  This is because the minimum delay
   is an alignment point for the smoothing operation of de-jitter
   buffers.  A de-jitter buffer that is ideally aligned with the delay
   variation adds zero buffer time to packets with the longest
   accommodated network delay (any packets with longer delays are
   discarded).  Thus, a packet experiencing minimum network delay should
   be aligned to wait the maximum length of the de-jitter buffer.  With
   this alignment, the stream is smoothed with no unnecessary delay
   added.  [G.1020] illustrates the ideal relationship between network
   delay variation and buffer time.

   The PDV distribution is also useful for this task, but different
   statistics are preferred.  The range (max-min) or the 99.9%-ile of
   PDV (pseudo-range) are closely related to the buffer size needed to
   accommodate the observed network delay variation.

   In some cases, the positive excursions (or series of positive
   excursions) of IPDV may help to approximate the de-jitter buffer
   size, but there is no guarantee that a good buffer estimate will
   emerge, especially when the delay varies as a positive trend over
   several test packets.

7.1.3.  Spatial Composition

   PDV has a clear advantage at this time, since there is no known
   method to compose an IPDV metric.

7.2.  Challenging Circumstances

   Note that measurement of delay variation may not be the primary
   concern under unstable and unreliable circumstances.

7.2.1.  Clock Issues

   When appreciable skew is present between measurement system clocks,
   then IPDV has a clear advantage because PDV would require processing
   over the entire sample to remove the skew error.  Neither form of
   delay variation is more suited than the other to on-the-fly
   summarization without memory, and this may be one of the reasons that
   [RFC3550] RTCP Jitter and MAPDV2 in [G.1020] have attained deployment
   in low-cost systems.

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7.2.2.  Frequent Path Changes

   If the network under test exhibits frequent path changes, on the
   order of several new routes per minute, then IPDV appears to isolate
   the delay variation on each path from the transient effect of path
   change (especially if there is packet loss at the time of path
   change).  It is possible to make meaningful PDV measurements when
   paths are unstable, but great importance would be placed on the
   algorithms that infer path change and attempt to divide the sample on
   path change boundaries.

7.2.3.  Frequent Loss

   If the network under test exhibits frequent loss, then PDV may
   produce a larger set of singletons for the sample than IPDV.  This is
   due to IPDV requiring consecutive packet arrivals to assess delay
   variation, compared to PDV where any packet arrival is useful.  The
   worst case is when no consecutive packets arrive, and the entire IPDV
   sample would be undefined.  PDV would successfully produce a sample
   based on the arriving packets.

7.2.4.  Load Balancing

   TO DO: this section will give the brief conclusions of the discussion
   and analysis in section 6.

8.  Measurement Considerations for Vendors, Testers, and Users

   TO DO:

8.1.  Measurement Stream Characteristics

8.2.  Measurement Units

   TO DO: Al, you mentioned somewhere above: "These devices may not have
   sufficient memory to store all singletons for later processing."  And
   also an important conclusion: "Just as there is no simple way to
   convert PDV singletons to IPDV singletons without returning to the
   original sample of delay singletons" I'm thinking we should develop
   around that:

   - Do we want to get IPDV and DV?

   - Do we want to reconstruct the IPDV and DV later on, with a
   different interval?

   + a reference to the appendix where we would describe:

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   - how is this D(min) calculated?  Is it DV(99%) as mentioned by Roman

   - do we need to keep all the values from the interval, then take the
   minimum?  Or do we keep the minimum from previous intervals?

8.3.  Test Duration

8.4.  Clock Sync Options

8.5.  Distinguishing Long Delay from Loss

   Setting the max waiting time threshold...

8.6.  Accounting for Packet Reordering

8.7.  Results Representation and Reporting

9.  IANA Considerations

   This document makes no request of IANA.

   Note to RFC Editor: this section may be removed on publication as an

10.  Security Considerations

   The security considerations that apply to any active measurement of
   live networks are relevant here as well.  See [RFC4656]

11.  Acknowledgements

   The author would like to thank Phil Chimento for his suggestion to
   employ the convention of conditional distributions for Delay to deal
   with packet loss, and his encouragement to "write the memo" after
   hearing the talk on this topic at IETF-65.

12.  Appendix on Reducing Delay Variation in Networks

   This text is both preliminary and generic but we want to explain the
   basic troubleshooting.

   If there is a DV problem, it may be because:

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   1. there is congestion.  Find where the bottleneck is, and increase
   the buffer Alternatively, increase the bandwidth Alternatively,
   remove some applications from that class of service

   2. there is a variability of the traffic Discover that traffic, then
   change/apply QoS (for example, rate-limiting)

13.  Appendix on Calculating the D(min) in PDV

   Note: These are the questions this section intends to answer:

   - how is this D(min) calculated?  Is it DV(99%) as mentioned by Roman

   - do we need to keep all the values from the interval, then take the
   minimum?  Or do we keep the minimum from previous intervals?

   The value of D(min) used as the reference delay for PDV calculations
   is simply the minimum delay of all packets in the current sample.
   The usual single value summary of the PDV distribution is D(99.9%-
   ile) minus D(min).

   It may be appropriate to segregate sub-sets and revise the minimum
   value during a sample.  For example, if it can be determined with
   certainty that the path has changed by monitoring the Time to Live or
   Hop Count of arriving packets, this may be sufficient justification
   to reset the minimum for packets on the new path.

   It is not necessary to store all delay values in a sample when
   storage is a major concern.  D(min) can be found by comparing each
   new singleton value with the current value and replacing it when
   required.  In a sample with 5000 packets, evaluation of the 99.9%-ile
   can also be achieved with limited storage.  One method calls for
   storing the top 50 delay singletons and revising the top value list
   each time 50 more packets arrive.

14.  References

14.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2330]  Paxson, V., Almes, G., Mahdavi, J., and M. Mathis,
              "Framework for IP Performance Metrics", RFC 2330,
              May 1998.

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

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

   [RFC3393]  Demichelis, C. and P. Chimento, "IP Packet Delay Variation
              Metric for IP Performance Metrics (IPPM)", RFC 3393,
              November 2002.

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

   [RFC3550]  Schulzrinne, H., Casner, S., Frederick, R., and V.
              Jacobson, "RTP: A Transport Protocol for Real-Time
              Applications", STD 64, RFC 3550, July 2003.

   [RFC4090]  Pan, P., Swallow, G., and A. Atlas, "Fast Reroute
              Extensions to RSVP-TE for LSP Tunnels", RFC 4090,
              May 2005.

   [RFC4656]  Shalunov, S., Teitelbaum, B., Karp, A., Boote, J., and M.
              Zekauskas, "A One-way Active Measurement Protocol
              (OWAMP)", RFC 4656, September 2006.

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

14.2.  Informative References

   [Casner]   "A Fine-Grained View of High Performance Networking, NANOG
              22 Conf.;", May
              20-22 2001.

   [Cia03]    "Standardized Active Measurements on a Tier 1 IP Backbone,
              IEEE Communications Mag., pp 90-97.", June 2003.

              01-pap02.doc, "Packet Delay Variation Comparison between
              ITU-T and IETF Draft Definitions", November 2000.

   [G.1020]   ITU-T Recommendation G.1020, ""Performance parameter
              definitions for the quality of speech and other voiceband
              applications utilizing IP networks"", 2006.

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              Morton, A. and S. Berghe, "Framework for Metric
              Composition", draft-ietf-ippm-framework-compagg-03 (work
              in progress), March 2007.

              Morton, A., "Reporting Metrics: Different Points of View",
              draft-morton-ippm-reporting-metrics-02 (work in progress),
              April 2007.

              Presentation at IPPM, IETF-64, "Jitter Definitions: What
              is What?", November 2005.

   [RFC3357]  Koodli, R. and R. Ravikanth, "One-way Loss Pattern Sample
              Metrics", RFC 3357, August 2002.

   [Y.1540]   ITU-T Recommendation Y.1540, "Internet protocol data
              communication service - IP packet transfer and
              availability performance parameters", December  2002.

   [Y.1541]   ITU-T Recommendation Y.1540, "Network Performance
              Objectives for IP-Based Services", February  2006.

Authors' Addresses

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

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

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   Benoit Claise
   Cisco Systems, Inc.
   De Kleetlaan 6a b1
   Diegem,   1831

   Phone: +32 2 704 5622

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Full Copyright Statement

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