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rfc:rfc5481

Network Working Group A. Morton Request for Comments: 5481 AT&T Labs Category: Informational B. Claise

                                                   Cisco Systems, Inc.
                                                            March 2009
           Packet Delay Variation Applicability Statement

Status of This Memo

 This memo provides information for the Internet community.  It does
 not specify an Internet standard of any kind.  Distribution of this
 memo is unlimited.

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Morton & Claise Informational [Page 1] RFC 5481 Delay Variation AS March 2009

Abstract

 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
 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.

Table of Contents

 1. Introduction ....................................................4
    1.1. Requirements Language ......................................5
    1.2. Background Literature in IPPM and Elsewhere ................5
    1.3. Organization of the Memo ...................................6
 2. Purpose and Scope ...............................................7
 3. Brief Descriptions of Delay Variation Uses ......................7
    3.1. Inferring Queue Occupation on a Path .......................7
    3.2. Determining De-Jitter Buffer Size ..........................8
    3.3. Spatial Composition .......................................10
    3.4. Service-Level Comparison ..................................10
    3.5. Application-Layer FEC Design ..............................10
 4. Formulations of IPDV and PDV ...................................10
    4.1. IPDV: Inter-Packet Delay Variation ........................11
    4.2. PDV: Packet Delay Variation ...............................11
    4.3. A "Point" about Measurement Points ........................12
    4.4. Examples and Initial Comparisons ..........................12
 5. Survey of Earlier Comparisons ..................................13
    5.1. Demichelis' Comparison ....................................13
    5.2. Ciavattone et al. .........................................15
    5.3. IPPM List Discussion from 2000 ............................16
    5.4. Y.1540 Appendix II ........................................18
    5.5. Clark's ITU-T SG 12 Contribution ..........................18
 6. Additional Properties and Comparisons ..........................18
    6.1. Packet Loss ...............................................18
    6.2. Path Changes ..............................................19
         6.2.1. Lossless Path Change ...............................20
         6.2.2. Path Change with Loss ..............................21
    6.3. Clock Stability and Error .................................22
    6.4. Spatial Composition .......................................24
    6.5. Reporting a Single Number (SLA) ...........................24
    6.6. Jitter in RTCP Reports ....................................25

Morton & Claise Informational [Page 2] RFC 5481 Delay Variation AS March 2009

    6.7. MAPDV2 ....................................................25
    6.8. Load Balancing ............................................26
 7. Applicability of the Delay Variation Forms and
    Recommendations ................................................27
    7.1. Uses ......................................................27
         7.1.1. Inferring Queue Occupancy ..........................27
         7.1.2. Determining De-Jitter Buffer Size (and FEC
                Design) ............................................27
         7.1.3. Spatial Composition ................................28
         7.1.4. Service-Level Specification: Reporting a
                Single Number ......................................28
    7.2. Challenging Circumstances .................................28
         7.2.1. Clock and Storage Issues ...........................28
         7.2.2. Frequent Path Changes ..............................29
         7.2.3. Frequent Loss ......................................29
         7.2.4. Load Balancing .....................................29
    7.3. Summary ...................................................30
 8. Measurement Considerations .....................................31
    8.1. Measurement Stream Characteristics ........................31
    8.2. Measurement Devices .......................................32
    8.3. Units of Measurement ......................................33
    8.4. Test Duration .............................................33
    8.5. Clock Sync Options ........................................33
    8.6. Distinguishing Long Delay from Loss .......................34
    8.7. Accounting for Packet Reordering ..........................34
    8.8. Results Representation and Reporting ......................35
 9. Security Considerations ........................................35
 10. Acknowledgments ...............................................35
 11. Appendix on Calculating the D(min) in PDV .....................35
 12. References ....................................................36
    12.1. Normative References .....................................36
    12.2. Informative References ...................................37

Morton & Claise Informational [Page 3] RFC 5481 Delay Variation AS March 2009

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 inter-arrival 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
 selective.
 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, see
 [Krzanowski]):
 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 IETF contributions, and is similar to the
     packet spacing difference metric used for interarrival jitter
     calculations in [RFC3550].

Morton & Claise Informational [Page 4] RFC 5481 Delay Variation AS March 2009

 2.  Packet Delay Variation, PDV, where a single reference is chosen
     from the stream based on specific criteria.  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
     [I.356].
 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. Requirements Language

 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
 document are to be interpreted as described in RFC 2119 [RFC2119].

1.2. 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 road map and background on the IPPM literature, and
 the published specifications of other relevant standards
 organizations.
 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].  The metrics use a waiting time threshold to
 distinguish between lost and delayed packets.  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 [IPPM-Reporting].

Morton & Claise Informational [Page 5] RFC 5481 Delay Variation AS March 2009

 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 rearrangement of
 packet arrivals from their sending order constitutes reordering.
 Derived metrics are based on the fundamental metrics.  The metric of
 primary interest here is delay variation [RFC3393], a metric that is
 derived from one-way delay [RFC2680].  Another derived metric is the
 loss patterns metric [RFC3357], which is derived from loss.
 The measured values of all metrics (both fundamental and derived)
 depend to great extent on the stream characteristics used to collect
 them.  Both Poisson streams [RFC3393] and Periodic streams [RFC3432]
 have been used with the IPDV and PDV metrics.  The choice of stream
 specification for active measurement will depend on the purpose of
 the characterization and the constraints of the testing environment.
 Periodic streams are frequently chosen for use with IPDV and PDV,
 because the application streams that are most sensitive to delay
 variation exhibit periodicity.  Additional details that are method-
 specific are discussed in Section 8 on "Measurement Considerations".
 In the ITU-T, the framework, fundamental metrics, and derived metrics
 for IP performance are specified in Recommendation Y.1540 [Y.1540].
 [G.1020] defines additional delay variation metrics, analyzes the
 operation of fixed and adaptive de-jitter buffers, and describes an
 example adaptive de-jitter buffer emulator.  Appendix II of [G.1050]
 describes the models for network impairments (including delay
 variation) that are part of standardized IP network emulator that may
 be useful when evaluating measurement techniques.

1.3. 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 the Security Considerations, there is an
 appendix on the calculation of the minimum delay for the PDV form.

Morton & Claise Informational [Page 6] RFC 5481 Delay Variation AS March 2009

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.
 It is entirely possible that the analysis and conclusions drawn here
 are applicable beyond the intended scope, but the reader is cautioned
 to fully appreciate the circumstances of active measurement on IP
 networks before doing so.
 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.

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
 metric.

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

Morton & Claise Informational [Page 7] RFC 5481 Delay Variation AS March 2009

 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.
 It is worth noting that delay variation can occur beyond IP router
 queues, in other communication components.  Examples include media
 contention: DOCSIS, IEEE 802.11, and some mobile radio technologies.
 However, delay variation from all sources at the IP layer and below
 will be quantified using the two formulations discussed here.

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
 the expected variation of delay, 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
 range 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).
 De-jitter buffer size can be expressed in units of octets of storage
 space for the packet stream, or in units of time that the packets are
 stored.  It is relatively simple to convert between octets and time
 when the buffer read rate (in octets per second) is constant:
 read_rate * storage_time = storage_octets
 Units of time are used in the discussion below.
 The objective of a de-jitter buffer is to compensate for all prior
 sources of delay variation and produce a packet stream with constant
 delay.  Thus, a packet experiencing the minimum transit delay from
 source to destination, D_min, should spend the maximum time in a

Morton & Claise Informational [Page 8] RFC 5481 Delay Variation AS March 2009

 de-jitter buffer, B_max.  The sum of D_min and B_max should equal the
 sum of the maximum transit delay (D_max) and the minimum buffer time
 (B_min).  We have
 Constant = D_min + B_max = D_max + B_min,
 after rearranging terms,
 B_max - B_min = D_max - D_min = range(B) = range(D)
 where range(B) is the range of packet buffering times, and range(D)
 is the range of packet transit delays from source to destination.
 Packets with transit delay between the max and min spend a
 complementary time in the buffer and also see the constant delay.
 In practice, the minimum buffer time, B_min, may not be zero, and the
 maximum transit delay, D_max, may be a high percentile (99.9th
 percentile) instead of the maximum.
 Note that B_max - B_min = range(B) is the range of buffering times
 needed to compensate for delay variation.  The actual size of the
 buffer may be larger (where B_min > 0) or smaller than range(B).
 There must be a process to align the de-jitter buffer time with
 packet transit delay.  This is a process to identify the packets with
 minimum delay and schedule their play-out time so that they spend the
 maximum time in the buffer.  The error in the alignment process can
 be accounted for by a variable, A.  In the equation below, the range
 of buffering times *available* to the packet stream, range(b),
 depends on buffer alignment with the actual arrival times of D_min
 and D_max.
 range(b) = b_max - b_min = D_max - D_min + A
 where variable b represents the *available* buffer in a system with a
 specific alignment, A, and b_max and b_min represent the limits of
 the available buffer.
 When A is positive, the de-jitter buffer applies more delay than
 necessary (where Constant = D_max + b_min + A represents one possible
 alignment).  When A is negative, there is insufficient buffer time
 available to compensate for range(D) because of misalignment.
 Packets with D_min may be arriving too early and encountering a full
 buffer, or packets with D_max may be arriving too late, and in either
 case, the packets would be discarded.

Morton & Claise Informational [Page 9] RFC 5481 Delay Variation AS March 2009

 In summary, the range of transit delay variation is a critical factor
 in the determination of de-jitter buffer size.

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
 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 [IPPM-Framework] 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. Application-Layer FEC Design

 The design of application-layer Forward Error Correction (FEC)
 components is closely related to the design of a de-jitter buffer in
 several ways.  The FEC designer must choose a protection interval
 (time to send/receive a block of packets in a constant packet rate
 system) consistent with the packet-loss characteristics, but also
 mindful of the extent of delay variation expected.  Further, the
 system designer must decide how long to wait for "late" packets to
 arrive.  Again, the range of delay variation is the relevant
 expression delay variation for these tasks.

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]):

Morton & Claise Informational [Page 10] RFC 5481 Delay Variation AS March 2009

 "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.
 One-way delays are the difference between timestamps applied at the
 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 the
 previous packet in the sending sequence.
 Note that IPDV can take on positive and negative values (and zero).
 One way to analyze the IPDV results is to concentrate on the positive
 excursions.  However, this approach has limitations that are
 discussed in more detail below (see Section 5.3).
 The mean of all IPDV(i) for a stream is usually zero.  However, a
 slow delay change over the life of the stream, or a frequency error
 between the measurement system clocks, can result in a non-zero mean.

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
 delay.

Morton & Claise Informational [Page 11] RFC 5481 Delay Variation AS March 2009

 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.
 PDV is a version of the one-way-delay distribution, shifted to the
 origin by normalizing to the minimum delay.

4.3. A "Point" about Measurement Points

 Both IPDV and PDV are derived from the one-way-delay metric.  One-way
 delay requires knowledge of time at two points, e.g., the source and
 destination of an IP network path in end-to-end measurement.
 Therefore, both IPDV and PDV can be categorized as 2-point metrics
 because they are derived from one-way delay.  Specific methods of
 measurement may make assumptions or have a priori knowledge about one
 of the measurement points, but the metric definitions themselves are
 based on information collected at two measurement points.

4.4. Examples and Initial Comparisons

 Note: This material originally presented in Slides 2 and 3 of
 [Morton06].
 The Figure below gives a sample of packet delays, calculates IPDV and
 PDV values, and depicts a histogram for each one.

Morton & Claise Informational [Page 12] RFC 5481 Delay Variation AS March 2009

                     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
 20 ms, one packet with a low delay of 10 ms (the minimum of the
 sample) and one packet with 25 ms 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.

5.1. Demichelis' Comparison

 In [Demichelis], Demichelis compared the early 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

Morton & Claise Informational [Page 13] RFC 5481 Delay Variation AS March 2009

 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 [Y.1540].
 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 singletons distinguish quick-delay variations (short-term,
     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.  The PDV distribution does not distinguish short-term variation
     from variation over the complete test interval.  (Comment: PDV
     can be determined over any sub-intervals when the singletons are
     stored.)
 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
     would be experienced by streams that support such sessions.

Morton & Claise Informational [Page 14] RFC 5481 Delay Variation AS March 2009

     (Ideally, the measurement interval should be aligned with the
     session length of interest, and this influences determination of
     the reference delay, D(min).)
 5.  The PDV distribution 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
 setup where a queue was temporarily congested by a competing packet
 burst.  The additional queuing delay was 85 ms to 95 ms, much larger
 than the inter-packet interval.  The first packet in the stream that
 follows the competing burst spends the longest time queued, and
 others experience less and less queuing time until the queue is
 drained.
 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
 5 ms.  Also, it is easy to determine (by looking at the PDV range)
 that a de-jitter buffer of >85 ms would have been sufficient to
 accommodate the delay variation.  Again, the measurement interval is
 a key factor in the validity of such observations (it should have
 similar length to the session interval of interest).
 The IPDV values in the congested queue example are very different:
 85, -20, -20, -20, -20, -5 ms.  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).

Morton & Claise Informational [Page 15] RFC 5481 Delay Variation AS March 2009

 Note that this example illustrates a case where the IPDV distribution
 is asymmetrical, because the delay variation range (85 ms) exceeds
 the inter-packet spacing (20 ms).  We see that the IPDV values 85,
 -20, -20, -20, -20, -5 ms have zero mean, but the left side of the
 distribution is truncated at -20 ms.
 Elsewhere in the article, the authors considered the range as a
 summary statistic for IPDV, and the 99.9th percentile minus the
 minimum delay as a summary statistic for delay variation, or PDV.

5.3. IPPM List Discussion from 2000

 Mike Pierce made many comments in the context of a working version of
 [RFC3393].  One of his main points was that a delay histogram is a
 useful approach to quantifying variation.  Another point was that the
 time duration of evaluation is a critical aspect.
 Carlo Demichelis then mailed his comparison paper [Demichelis] to the
 IPPM list, 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:

Morton & Claise Informational [Page 16] RFC 5481 Delay Variation AS March 2009

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

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, and
 this is the statistic that determines the size of a de-jitter buffer.
 The IPDV range is minimal in response to the smooth variation in
 Example A (20 ms).  However, IPDV responds to the faster variations
 in Example B (60 ms range from 40 to -20).  Here the IPDV range is
 larger than the PDV range, and overestimates the buffer size
 requirements.
 A heuristic method to estimate buffer size using IPDV is to sum the
 consecutive positive or zero values as an estimate of PDV range.
 However, this is more complicated to assess than the PDV range, and
 has strong dependence on the actual sequence of IPDV values (any
 negative IPDV value stops the summation, and again causes an
 underestimate).
 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
 adjustments this frequently, so the value of this information is
 unknown.  The short-term variations may be useful to know in some
 other cases.

Morton & Claise Informational [Page 17] RFC 5481 Delay Variation AS March 2009

5.4. Y.1540 Appendix II

 Appendix II of [Y.1540] describes a secondary terminology for delay
 variation.  It compares IPDV, PDV (referred to as 2-point PDV), and
 1-point packet delay variation (which assumes a periodic stream and
 assesses variation against an ideal arrival schedule constructed at a
 single measurement point).  This early comparison discusses some of
 the same considerations raised in Section 6 below.

5.5. Clark's ITU-T SG 12 Contribution

 Alan Clark's contribution to ITU-T Study Group 12 in January 2003
 provided an analysis of the root causes of delay variation and
 investigated different techniques for measurement and modeling of
 "jitter" [COM12.D98].  Clark compared a metric closely related to
 IPDV, Mean Packet-to-Packet Delay Variation, MPPDV = mean(abs(D(i)-
 D(i-1))) to the newly proposed Mean Absolute Packet Delay Variation
 (MAPDV2, see [G.1020]).  One of the tasks for this study was to
 estimate the number of packet discards in a de-jitter buffer.  Clark
 concluded that MPPDV did not track the ramp delay variation he
 associated access link congestion (similar to Figure 2, Example A
 above), but MAPDV2 did.
 Clark also briefly looked at PDV (as described in the 2002 version of
 [Y.1541]).  He concluded that if PDV was applied to a series of very
 short measurement intervals (e.g., 200 ms), it could be used to
 determine the fraction of intervals with high packet discard rates.

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 of packet loss is of great influence for the delay
 variation results, as displayed in the Figures 3 and 4 (L means Lost
 and U means Undefined).  Figure 3 shows that in the extreme case of
 every other packet loss, the IPDV metric doesn't produce any results,
 while the PDV produces results for all arriving packets.

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                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 4, both the
 IPDV and PDV metrics produce some results.  Note that PDV still
 produces more values than IPDV.
                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.
 The IPDV results are affected by both the packet-loss ratio and the
 packet-loss distribution.  In the extreme case of loss of every other
 packet, IPDV doesn't provide any results.

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 that 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.

Morton & Claise Informational [Page 19] RFC 5481 Delay Variation AS March 2009

 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
 flows).
 In any case, a change in the Time to Live (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
 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 it 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 one
 IPDV singleton.  For example, the delay sequence in the Figure below
 always produces IPDV=0 except in the one case where the value is 5
 (U, 0, 0, 0, 5, 0, 0, 0, 0).
                  Packet #   1  2  3  4  5  6  7  8  9
                  Lost
                  ------------------------------------
                  Delay, ms  4  4  4  4  9  9  9  9  9
                  IPDV       U  0  0  0  5  0  0  0  0
                  PDV        0  0  0  0  5  5  5  5  5
                    Figure 5: Lossless Path Change
 However, if the change in delay is negative and larger than the
 inter-packet sending interval, then more than one IPDV singleton may
 be affected because packet reordering is also likely to occur.

Morton & Claise Informational [Page 20] RFC 5481 Delay Variation AS March 2009

 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
 present.
 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 a 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 to revise the 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.

6.2.2. Path Change with Loss

 If the path change is accompanied by loss, such that there 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.  Figure 6, 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 6: Path Change with Loss

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 PDV will again produce a bi-modal 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.
 IPDV does not help to detect path changes when accompanied by loss,
 and this is a disadvantage for those who rely solely on IPDV
 measurements.

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 (~1 ms) 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 study, ~0.01 ms for example).
 Maintaining time accuracy <<1 ms has typically required access to
 dedicated time receivers at all measurement points.  Global
 positioning system (GPS) receivers have often been installed to
 support measurements.  The GPS installation conditions are fairly
 restrictive, and many prospective measurement efforts have found the
 deployment complexity and system maintenance too difficult.
 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
 misalignment of the measurement intervals can lead to lost packets,
 for example, if the receiver is not ready when the first test packet
 arrives.  However, both IPDV and PDV assess delay differences, so the
 error present in any two one-way-delay singletons will cancel as long
 as the error is constant.  So, the demand for NTP or GPS
 synchronization comes primarily from one-way-delay measurement time-
 of-day accuracy requirements.  Delay variation and measurement
 interval alignment are relatively less demanding.

Morton & Claise Informational [Page 22] RFC 5481 Delay Variation AS March 2009

 Skew is a measure of the change in clock time over an interval with
 respect to 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.  Thus, the additional accuracy demand of PDV can be expressed
 as a ratio of the measurement interval to the inter-packet spacing.
 A practical example is a measurement between two hosts, one with a
 synchronized clock and the other with a free-running clock having 50
 parts per million (ppm) long term accuracy.
 o  If IPDV measurements are made on packets with a 1 second spacing,
    the maximum singleton error will be 1 x 5 x 10^-5 seconds, or 0.05
    ms.
 o  If PDV measurements are made on the same packets over a 60 second
    measurement interval, then the delay variation due to the max
    free-running clock error will be 60 x 5 x 10-5 seconds, or 3 ms
    delay variation error from the first packet to the last.
 Therefore, the additional accuracy required for equivalent PDV error
 under these conditions is a factor of 60 more than for IPDV.  This is
 a rather extreme scenario, because time-of-day error of 1 second
 would accumulate in ~5.5 hours, potentially causing the measurement
 interval alignment issue described above.
 If skew is present in a sample of one-way delays, its symptom is
 typically a nearly 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.
 Values for IPDV may have non-zero mean over a sample when clock skew
 is present.  This tends to complicate IPDV analysis when using the
 assumptions of a zero mean and a symmetric distribution.
 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 time error is
 detected (typically on the order of a few milliseconds), the host

Morton & Claise Informational [Page 23] RFC 5481 Delay Variation AS March 2009

 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 (regardless of the form of delay variation
 considered).

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.
 Additional methods are considered in [IPPM-Spatial].
 PDV has a clear advantage at this time, since there is no validated
 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, where segments must be assumed to
 have independent delay distributions.

6.5. Reporting a Single Number (SLA)

 Despite the risk of over-summarization, measurements must often be
 displayed for easy consumption.  If the right summary report is
 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 single number summary statistic is a
 key component of SLAs, where a threshold on that number must be met
 x% of the time.
 The simplicity of the PDV distribution lends itself to this
 summarization process (including use of the percentiles, median or
 mean).  An SLA of the form "no more than x% of packets in a
 measurement interval shall have PDV >= y ms, for no less than z% of
 time" is relatively straightforward to specify and implement.
 [Y.1541] introduced the notion of a pseudo-range when setting an
 objective for the 99.9th percentile of PDV.  The conventional range
 (max-min) was avoided for several reasons, including stability of the
 maximum delay.  The 99.9th percentile 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 will have two tails
 (positive and negative), the range or pseudo-range would not match

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 the needed de-jitter buffer size.  Additional complexity may be
 introduced when the variation exceeds the inter-packet sending
 interval, as discussed above (in Sections 5.2 and 6.2.1).  Should the
 Inter-Quartile Range be used?  Should the singletons beyond some
 threshold be counted (e.g., mean +/- 50 ms)?  A strong rationale for
 one of these summary statistics has yet to emerge.
 When summarizing IPDV, some prefer the simplicity of the single-sided
 distribution created by taking the absolute value of each singleton
 result, abs(D(i)-D(i-1)).  This approach sacrifices the two-sided
 inter-arrival spread information in the distribution.  It also makes
 the evaluation using percentiles more confusing, because a single
 late packet that exceeds the variation threshold will cause two pairs
 of singletons to fail the criteria (one positive, the other negative
 converted to positive).  The single-sided PDV distribution is an
 advantage in this category.

6.6. Jitter in RTCP Reports

 Section 6.4.1 of [RFC3550] gives the calculation of the "inter-
 arrival jitter" field for the RTP Control Protocol (RTCP) report,
 with a sample implementation in an Appendix.
 The RTCP "interarrival 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]
 "interarrival jitter".

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
 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.

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6.8. Load Balancing

 Network traffic load balancing is a process to divide packet traffic
 in order to provide a more even distribution over two or more equally
 viable paths.  The paths chosen are based on the IGP cost metrics,
 while the delay depends on the path's physical layout.  Usually, the
 balancing process is performed on a per-flow basis to avoid delay
 variation experienced when packets traverse different physical paths.
 If the sample includes test packets with different characteristics
 such as IP addresses/ports, there could be multi-modal delay
 distributions present.  The PDV form makes the identification of
 multiple modes possible.  IPDV may also reveal that multiple paths
 are in use with a mixed-flow sample, but the different delay modes
 are not easily divided and analyzed separately.
 Should the delay singletons using multiple addresses/ports be
 combined in the same sample?  Should we characterize each mode
 separately?  (This question also applies to the Path Change case.)
 It depends on the task to be addressed by the measurement.
 For the task of de-jitter buffer sizing or assessing queue
 occupation, the modes should be characterized separately because
 flows will experience only one mode on a stable path.  Use of a
 single flow description (address/port combination) in each sample
 simplifies this analysis.  Multiple modes may be identified by
 collecting samples with different flow attributes, and
 characterization of multiple paths can proceed with comparison of the
 delay distributions from each sample.
 For the task of capacity planning and routing optimization,
 characterizing the modes separately could offer an advantage.
 Network-wide capacity planning (as opposed to link capacity planning)
 takes as input the core traffic matrix, which corresponds to a matrix
 of traffic transferred from every source to every destination in the
 network.  Applying the core traffic matrix along with the routing
 information (typically the link state database of a routing protocol)
 in a capacity planning tool offers the possibility to visualize the
 paths where the traffic flows and to optimize the routing based on
 the link utilization.  In the case where equal cost multiple paths
 (ECMPs) are used, the traffic will be load balanced onto multiple
 paths.  If each mode of the IP delay multi-modal distribution can be
 associated with a specific path, the delay performance offers an
 extra optimization parameter, i.e., the routing optimization based on
 the IP delay variation metric.  As an example, the load balancing
 across ECMPs could be suppressed so that the Voice over IP (VoIP)
 calls would only be routed via the path with the lower IP delay

Morton & Claise Informational [Page 26] RFC 5481 Delay Variation AS March 2009

 variation.  Clearly, any modifications can result in new delay
 performance measurements, so there must be a verification step to
 ensure the desired outcome.

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.

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
 statistics.
 IPDV can capture the difference in queuing time from one packet to
 the next, but this is a different distribution from the queue
 occupancy revealed by PDV.

7.1.2. Determining De-Jitter Buffer Size (and FEC Design)

 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.  Figure 5 of [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 99.9th
 percentile of PDV (pseudo-range) are closely related to the buffer
 size needed to accommodate the observed network delay variation.

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 The PDV distribution directly addresses the FEC waiting time
 question.  When the PDV distribution has a 99th percentile of 10 ms,
 then waiting 10 ms longer than the FEC protection interval will allow
 99% of late packets to arrive and be used in the FEC block.
 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 validated
 method to compose an IPDV metric.

7.1.4. Service-Level Specification: Reporting a Single Number

 The one-sided PDV distribution can be constrained with a single
 statistic, such as an upper percentile, so it is preferred.  The IPDV
 distribution is two-sided, usually has zero mean, and no universal
 summary statistic that relates to a physical quantity has emerged in
 years of experience.

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 and Storage Issues

 When appreciable skew is present between measurement system clocks,
 IPDV has an advantage because PDV would require processing over the
 entire sample to remove the skew error.  However, significant skew
 can invalidate IPDV analysis assumptions, such as the zero-mean and
 symmetric-distribution characteristics.  Small skew may well be
 within the error tolerance, and both PDV and IPDV results will be
 usable.  There may be a portion of the skew, measurement interval,
 and required accuracy 3-D space where IPDV has an advantage,
 depending on the specific measurement specifications.
 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.

Morton & Claise Informational [Page 28] RFC 5481 Delay Variation AS March 2009

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).  However, if one intends to use IPDV to indicate path
 changes, it cannot do this when the change is accompanied by loss.
 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.
 When path changes are frequent and cause packet loss, delay variation
 is probably less important than the loss episodes and attention
 should be turned to the loss metric instead.

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, yet PDV would successfully produce a
 sample based on the arriving packets.

7.2.4. Load Balancing

 PDV distributions offer the most straightforward way to identify that
 a sample of packets have traversed multiple paths.  The tasks of
 de-jitter buffer sizing or assessing queue occupation with PDV should
 be use a sample with a single flow because flows will experience only
 one mode on a stable path, and it simplifies the analysis.

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7.3. Summary

 +---------------+----------------------+----------------------------+
 | Comparison    | PDV = D(i)-D(min)    | IPDV = D(i)-D(i-1)         |
 | Area          |                      |                            |
 +---------------+----------------------+----------------------------+
 | Challenging   | Less sensitive to    | Preferred when path        |
 | Circumstances | packet loss, and     | changes are frequent or    |
 |               | simplifies analysis  | when measurement clocks    |
 |               | when load balancing  | exhibit some skew          |
 |               | or multiple paths    |                            |
 |               | are present          |                            |
 |---------------|----------------------|----------------------------|
 | Spatial       | All validated        | Has sensitivity to         |
 | Composition   | methods use this     | sequence and spacing       |
 | of DV metric  | form                 | changes, which tends to    |
 |               |                      | break the requirement for  |
 |               |                      | independent distributions  |
 |               |                      | between path segments      |
 |---------------|----------------------|----------------------------|
 | Determine     | "Pseudo-range"       | No reliable relationship,  |
 | De-Jitter     | reveals this         | but some heuristics        |
 | Buffer Size   | property by          |                            |
 | Required      | anchoring the        |                            |
 |               | distribution at the  |                            |
 |               | minimum delay        |                            |
 |---------------|----------------------|----------------------------|
 | Estimate of   | Distribution has     | No reliable relationship   |
 | Queuing Time  | one-to-one           |                            |
 | and Variation | relationship on a    |                            |
 |               | stable path,         |                            |
 |               | especially when      |                            |
 |               | sample min = true    |                            |
 |               | min                  |                            |
 |---------------|----------------------|----------------------------|
 | Specification | One constraint       | Distribution is two-sided, |
 | Simplicity:   | needed for           | usually has zero mean, and |
 | Single Number | single-sided         | no universal summary       |
 | SLA           | distribution, and    | statistic that relates to  |
 |               | easily related to    | a physical quantity        |
 |               | quantities above     |                            |
 +---------------+----------------------+----------------------------+
                        Summary of Comparisons

Morton & Claise Informational [Page 30] RFC 5481 Delay Variation AS March 2009

8. Measurement Considerations

 This section discusses the practical aspects of delay variation
 measurement, with special attention to the two formulations compared
 in this memo.

8.1. Measurement Stream Characteristics

 As stated in Section 1.2, there is a strong dependency between the
 active measurement stream characteristics and the results.  The IPPM
 literature includes two primary methods for collecting samples:
 Poisson sampling described in [RFC2330], and Periodic sampling in
 [RFC3432].  The Poisson method was intended to collect an unbiased
 sample of performance, while the Periodic method addresses a "known
 bias of interest".  Periodic streams are required to have random
 start times and limited stream duration, in order to avoid unwanted
 synchronization with some other periodic process, or cause
 congestion-aware senders to synchronize with the stream and produce
 atypical results.  The random start time should be different for each
 new stream.
 It is worth noting that [RFC3393] was developed in parallel with
 [RFC3432].  As a result, all the stream metrics defined in [RFC3393]
 specify the Poisson sampling method.
 Periodic sampling is frequently used in measurements of delay
 variation.  Several factors foster this choice:
 1.  Many application streams that are sensitive to delay variation
     also exhibit periodicity, and so exemplify the bias of interest.
     If the application has a constant packet spacing, this constant
     spacing can be the inter-packet gap for the test stream.  VoIP
     streams often use 20 ms spacing, so this is an obvious choice for
     an Active stream.  This applies to both IPDV and PDV forms.
 2.  The spacing between packets in the stream will influence whether
     the stream experiences short-range dependency, or only long-range
     dependency, as investigated in [Li.Mills].  The packet spacing
     also influences the IPDV distribution and the stream's
     sensitivity to reordering.  For example, with a 20 ms spacing the
     IPDV distribution cannot go below -20 ms without packet
     reordering.
 3.  The measurement process may make several simplifying assumptions
     when the send spacing and send rate are constant.  For example,
     the inter-arrival times at the destination can be compared with
     an ideal sending schedule, and allowing a one-point measurement

Morton & Claise Informational [Page 31] RFC 5481 Delay Variation AS March 2009

     of delay variation (described in [Y.1540]) that approximates the
     IPDV form.  Simplified methods that approximate PDV are possible
     as well (some are discussed in Appendix II of [Y.1541]).
 4.  Analysis of truncated, or non-symmetrical IPDV distributions is
     simplified.  Delay variations in excess of the periodic sending
     interval can cause multiple singleton values at the negative
     limit of the packet spacing (see Section 5.2 and [Cia03]).  Only
     packet reordering can cause the negative spacing limit to be
     exceeded.
 Despite the emphasis on inter-packet delay differences with IPDV,
 both Poisson [Demichelis] and Periodic [Li.Mills] streams have been
 used, and these references illustrate the different analyses that are
 possible.
 The advantages of using a Poisson distribution are discussed in
 [RFC2330].  The main properties are to avoid predicting the sample
 times, avoid synchronization with periodic events that are present in
 networks, and avoid inducing synchronization with congestion-aware
 senders.  When a Poisson stream is used with IPDV, the distribution
 will reflect inter-packet delay variation on many different time
 scales (or packet spacings).  The unbiased Poisson sampling brings a
 new layer of complexity in the analysis of IPDV distributions.

8.2. Measurement Devices

 One key aspect of measurement devices is their ability to store
 singletons (or individual measurements).  This feature usually is
 closely related to local calculation capabilities.  For example, an
 embedded measurement device with limited storage will like provide
 only a few statistics on the delay variation distribution, while
 dedicated measurement systems store all the singletons and allow
 detailed analysis (later calculation of either form of delay
 variation is possible with the original singletons).
 Therefore, systems with limited storage must choose their metrics and
 summary statistics in advance.  If both IPDV and PDV statistics are
 desired, the supporting information must be collected as packets
 arrive.  For example, the PDV range and high percentiles can be
 determined later if the minimum and several of the largest delays are
 stored while the measurement is in-progress.

Morton & Claise Informational [Page 32] RFC 5481 Delay Variation AS March 2009

8.3. Units of Measurement

 Both IPDV and PDV can be summarized as a range in milliseconds.
 With IPDV, it is interesting to report on a positive percentile, and
 an inter-quantile range is appropriate to reflect both positive and
 negative tails (e.g., 5% to 95%).  If the IPDV distribution is
 symmetric around a mean of zero, then it is sufficient to report on
 the positive side of the distribution.
 With PDV, it is sufficient to specify the upper percentile (e.g.,
 99.9%).

8.4. Test Duration

 At several points in this memo, we have recommended use of test
 intervals on the order of minutes.  In their paper examining the
 stability of Internet path properties [Zhang.Duff], Zhang et al.
 concluded that consistency was present on the order of minutes for
 the performance metrics considered (loss, delay, and throughput) for
 the paths they measured.
 The topic of temporal aggregation of performance measured in small
 intervals to estimate some larger interval is described in the Metric
 Composition Framework [IPPM-Framework].
 The primary recommendation here is to test using durations that are
 similar in length to the session time of interest.  This applies to
 both IPDV and PDV, but is possibly more relevant for PDV since the
 duration determines how often the D_min will be determined, and the
 size of the associated sample.

8.5. Clock Sync Options

 As with one-way-delay measurements, local clock synchronization is an
 important matter for delay variation measurements.
 There are several options available:
 1.  Global Positioning System receivers
 2.  In some parts of the world, Cellular Code Division Multiple
     Access (CDMA) systems distribute timing signals that are derived
     from GPS and traceable to UTC.
 3.  Network Time Protocol [RFC1305] is a convenient choice in many
     cases, but usually offers lower accuracy than the options above.

Morton & Claise Informational [Page 33] RFC 5481 Delay Variation AS March 2009

 When clock synchronization is inconvenient or subject to appreciable
 errors, then round-trip measurements may give a cumulative indication
 of the delay variation present on both directions of the path.
 However, delay distributions are rarely symmetrical, so it is
 difficult to infer much about the one-way-delay variation from round-
 trip measurements.  Also, measurements on asymmetrical paths add
 complications for the one-way-delay metric.

8.6. Distinguishing Long Delay from Loss

 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
 [IPPM-Reporting].
 In essence, [IPPM-Reporting] suggests to use a long waiting time to
 serve network characterization and revise results for specific
 application delay thresholds as needed.

8.7. Accounting for Packet Reordering

 Packet reordering, defined in [RFC4737], is essentially an extreme
 form of delay variation where the packet stream arrival order differs
 from the sending order.
 PDV results are not sensitive to packet arrival order, and are not
 affected by reordering other than to reflect the more extreme
 variation.
 IPDV results will change if reordering is present because they are
 sensitive to the sequence of delays of arriving packets.  The main
 example of this sensitivity is in the truncation of the negative tail
 of the distribution.
 o  When there is no reordering, the negative tail is limited by the
    sending time spacing between packets.
 o  If reordering occurs (and the reordered packets are not
    discarded), the negative tail can take on any value (in
    principal).
 In general, measurement systems should have the capability to detect
 when sequence has changed.  If IPDV measurements are made without
 regard to packet arrival order, the IPDV will be under-reported when
 reordering occurs.

Morton & Claise Informational [Page 34] RFC 5481 Delay Variation AS March 2009

8.8. Results Representation and Reporting

 All of the references that discuss or define delay variation suggest
 ways to represent or report the results, and interested readers
 should review the various possibilities.
 For example, [IPPM-Reporting] suggests reporting a pseudo-range of
 delay variation based on calculating the difference between a high
 percentile of delay and the minimum delay.  The 99.9th percentile
 minus the minimum will give a value that can be compared with
 objectives in [Y.1541].

9. Security Considerations

 The security considerations that apply to any active measurement of
 live networks are relevant here as well.  See the "Security
 Considerations" sections in [RFC2330], [RFC2679], [RFC3393],
 [RFC3432], and [RFC4656].
 Security considerations do not contribute to the selection of PDV or
 IPDV forms of delay variation, because measurements using these
 metrics involve exactly the same security issues.

10. Acknowledgments

 The authors would like to thank Phil Chimento for his suggestion to
 employ the convention of conditional distributions of delay to deal
 with packet loss, and his encouragement to "write the memo" after
 hearing "the talk" on this topic at IETF 65.  We also acknowledge
 constructive comments from Alan Clark, Loki Jorgenson, Carsten
 Schmoll, and Robert Holley.

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

 Practitioners have raised several questions that this section intends
 to answer:
  1. How is this D_min calculated? Is it DV(99%) as mentioned in

[Krzanowski]?

  1. 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_(99.9th
 percentile) minus D_min.

Morton & Claise Informational [Page 35] RFC 5481 Delay Variation AS March 2009

 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.  There is also a
 simpler approach to solving this problem: use samples collected over
 short evaluation intervals (on the order of minutes).  Intervals with
 path changes may be more interesting from the loss or one-way-delay
 perspective (possibly failing to meet one or more SLAs), and it may
 not be necessary to conduct delay variation analysis.  Short
 evaluation intervals are preferred for measurements that serve as a
 basis for troubleshooting, since the results are available to report
 soon after collection.
 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 99.9th
 percentile 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.

12. References

12.1. 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.
 [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.

Morton & Claise Informational [Page 36] RFC 5481 Delay Variation AS March 2009

 [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.

12.2. Informative References

 [COM12.D98]       Clark, A., "Analysis, measurement and modelling of
                   Jitter", ITU-T Delayed Contribution COM 12 - D98,
                   January 2003.
 [Casner]          Casner, S., Alaettinoglu, C., and C. Kuan, "A Fine-
                   Grained View of High Performance Networking",
                   NANOG 22, May 20-22, 2001,
                   <http://www.nanog.org/mtg-0105/agenda.html>.
 [Cia03]           Ciavattone, L., Morton, A., and G. Ramachandran,
                   "Standardized Active Measurements on a Tier 1 IP
                   Backbone", IEEE Communications Magazine, p. 90-97,
                   June 2003.
 [Demichelis]      Demichelis, C., "Packet Delay Variation Comparison
                   between ITU-T and IETF Draft Definitions",
                   November 2000, <http://www.advanced.org/ippm/
                   archive.3/att-0075/01-pap02.doc>.
 [G.1020]          ITU-T, "Performance parameter definitions for the
                   quality of speech and other voiceband applications
                   utilizing IP networks", ITU-T
                   Recommendation G.1020, 2006.
 [G.1050]          ITU-T, "Network model for evaluating multimedia
                   transmission performance over Internet Protocol",
                   ITU-T Recommendation G.1050, November 2005.
 [I.356]           ITU-T, "B-ISDN ATM Layer Cell Transfer
                   Performance", ITU-T Recommendation I.356,
                   March 2000.
 [IPPM-Framework]  Morton, A., "Framework for Metric Composition",
                   Work in Progress, October 2008.

Morton & Claise Informational [Page 37] RFC 5481 Delay Variation AS March 2009

 [IPPM-Reporting]  Morton, A., Ramachandran, G., and G. Maguluri,
                   "Reporting Metrics: Different Points of View", Work
                   in Progress, January 2009.
 [IPPM-Spatial]    Morton, A. and E. Stephan, "Spatial Composition of
                   Metrics", Work in Progress, July 2008.
 [Krzanowski]      Presentation at IPPM, IETF-64, "Jitter Definitions:
                   What is What?", November 2005.
 [Li.Mills]        Li, Q. and D. Mills, "The Implications of Short-
                   Range Dependency on Delay Variation Measurement",
                   Second IEEE Symposium on Network Computing
                   and Applications, 2003.
 [Morton06]        Morton, A., "A Brief Jitter Metrics Comparison, and
                   not the last word, by any means...", slide
                   presentation at IETF 65, IPPM Session, March 2006.
 [RFC1305]         Mills, D., "Network Time Protocol (Version 3)
                   Specification, Implementation", RFC 1305,
                   March 1992.
 [RFC3357]         Koodli, R. and R. Ravikanth, "One-way Loss Pattern
                   Sample Metrics", RFC 3357, August 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.
 [Y.1540]          ITU-T, "Internet protocol data communication
                   service - IP packet transfer and availability
                   performance parameters", ITU-T Recommendation
                   Y.1540, November 2007.
 [Y.1541]          ITU-T, "Network Performance Objectives for IP-Based
                   Services", ITU-T Recommendation Y.1541,
                   February 2006.
 [Zhang.Duff]      Zhang, Y., Duffield, N., Paxson, V., and S.
                   Shenker, "On the Constancy of Internet Path
                   Properties", Proceedings of ACM SIGCOMM Internet
                   Measurement Workshop, November 2001.

Morton & Claise Informational [Page 38] RFC 5481 Delay Variation AS March 2009

Authors' Addresses

 Al Morton
 AT&T Labs
 200 Laurel Avenue South
 Middletown, NJ  07748
 USA
 Phone: +1 732 420 1571
 Fax:   +1 732 368 1192
 EMail: acmorton@att.com
 URI:   http://home.comcast.net/~acmacm/
 Benoit Claise
 Cisco Systems, Inc.
 De Kleetlaan 6a b1
 Diegem,   1831
 Belgium
 Phone: +32 2 704 5622
 EMail: bclaise@cisco.com

Morton & Claise Informational [Page 39]

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