One document matched: draft-morton-ippm-delay-var-as-00.txt
Network Working Group A. Morton
Internet-Draft AT&T Labs
Intended status: Informational October 15, 2006
Expires: April 18, 2007
Packet Delay Variation Applicability Statement
draft-morton-ippm-delay-var-as-00
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Copyright Notice
Copyright (C) The Internet Society (2006).
Abstract
Many definitions of packet delay variation exist, and two different
formulations have come into wide use in the context of active
measurements. This memo examines a range of circumstances for active
measurements and their uses, and recommends which of these two forms
is best matched to the conditions and task.
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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].
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Purpose and Scope . . . . . . . . . . . . . . . . . . . . . . 4
3. Uses of Delay Variation Metrics . . . . . . . . . . . . . . . 4
3.1. Determining De-jitter Buffer Size . . . . . . . . . . . . 4
3.2. Inferring Queue Occupation on a Path . . . . . . . . . . . 5
3.3. Spatial Composition . . . . . . . . . . . . . . . . . . . 5
3.4. Challenging Circumstances . . . . . . . . . . . . . . . . 5
3.5. <your favorite here> . . . . . . . . . . . . . . . . . . . 6
4. Formulations of IPDV and PDV . . . . . . . . . . . . . . . . . 6
4.1. IPDV: Inter-Packet Delay Variation . . . . . . . . . . . . 6
4.2. PDV: Packet Delay Variation . . . . . . . . . . . . . . . 6
4.3. Examples and Initial Comparisons . . . . . . . . . . . . . 6
5. Earlier Comparisons . . . . . . . . . . . . . . . . . . . . . 6
5.1. Demichelis' Comparison . . . . . . . . . . . . . . . . . . 7
5.2. Ciavattone et al. . . . . . . . . . . . . . . . . . . . . 8
5.3. IPPM List Discussion from 2001 . . . . . . . . . . . . . . 8
5.4. Y.1540 Appendix II . . . . . . . . . . . . . . . . . . . . 9
6. Additional Properties and Comparisons . . . . . . . . . . . . 9
6.1. Jitter in RTCP Reports . . . . . . . . . . . . . . . . . . 9
6.2. Path Changes . . . . . . . . . . . . . . . . . . . . . . . 9
6.2.1. Lossless Path Change . . . . . . . . . . . . . . . . . 10
6.2.2. Path Change with Loss . . . . . . . . . . . . . . . . 11
6.3. Measurement Clock Issues . . . . . . . . . . . . . . . . . 11
6.4. Reporting a Single Number . . . . . . . . . . . . . . . . 12
6.5. MAPDV2 . . . . . . . . . . . . . . . . . . . . . . . . . . 12
7. Applicability of the Delay Variation Forms with Tasks . . . . 13
7.1. Challenging Circumstances . . . . . . . . . . . . . . . . 13
7.2. Spatial Composition . . . . . . . . . . . . . . . . . . . 13
7.3. Inferring Queue Occupancy . . . . . . . . . . . . . . . . 14
7.4. Determining De-jitter Buffer Size . . . . . . . . . . . . 14
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 15
9. Security Considerations . . . . . . . . . . . . . . . . . . . 15
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 15
11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 15
11.1. Normative References . . . . . . . . . . . . . . . . . . . 15
11.2. Informative References . . . . . . . . . . . . . . . . . . 16
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 17
Intellectual Property and Copyright Statements . . . . . . . . . . 18
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1. Introduction
There are many ways to formulate delay variation metrics for packet
networks. The IETF itself has several specifications for delay
variation, sometimes called jitter, and these have achieved wide
adoption. The International Telecommunication Union -
Telecommunication Standardization Sector has also recommended several
delay variation metrics (called parameters in their terminology), and
some of these are widely cited and used.
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 value, and the result is the
variation with respect to the reference. The variation is usually
summarized for all packets in a stream (or sample) using statistics.
Two main formulations of delay variation are preferred (according to
[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 and packet sequence are captured in this
formulation.
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.
Each of these metric formulations has certain advantages and
disadvantages that make them more suitable for one circumstance and
less so for another. This memo examines a range of circumstances for
active measurements of delay variation and their uses, and recommends
the form that is best matched to the conditions and task.
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. For instance,
both Inter-Packet Delay Variation and Packet Delay Variation can be
assessed using options of [RFC3393], especially the packet selection
function.
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The IPPM framework [RFC2330] and other RFCs describing IPPM metrics
provide a background for this memo, especially for terms such as
singleton, sample, and statistic.
2. Purpose and Scope
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. The is accomplished by conditioning the delay
distribution on arrival within a reasonable time based on an
understanding of the path under test and packet lifetimes. 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. Uses of Delay Variation Metrics
This section presents a set of tasks that call for delay variation
measurements and their possible circumstances. It answers the
question, "How will the results be used?" for the delay variation
metric.
3.1. Determining De-jitter Buffer Size
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
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to ensure the quality of real-time applications, and monitor this
metric (possibly to compare with a numerical objective or Service
Level Agreement).
3.2. Inferring Queue Occupation on a Path
As packets travel along the path from source to destination, they
pass through a series of router queues. Many of the sources of delay
along the path are constant, 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.
3.3. 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 [I-D.ietf-ippm-framework-compagg]
3.4. Challenging Circumstances
Any of the tasks above are made more "interesting" when certain
circumstances are present. Among these are:
1. Low cost or low complexity measurement systems. These systems
may be embedded in communication devices that do not have access
to high stability clocks, and time errors will almost certainly
be present. These devices may not have sufficient memory to
store all singletons for later processing.
2. Extremely dynamic network conditions. 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. Frequent path changes and prolonged
congestion with substantial packet loss clearly make delay
variation measurements challenging.
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3.5. <your favorite here>
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
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.
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.
4.2. PDV: Packet Delay Variation
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.
Therefore, PDV(i) = D(i)-D(min) (using the nomenclature introduced in
the IPDV section).
4.3. Examples and Initial Comparisons
This section will discuss the examples in slides 2 and 3 of
http://www3.ietf.org/proceedings/06mar/slides/ippm-4.pdf
5. 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 the
two forms we consider here. Although the IPDV form would eventually
be standardized under the IETF IPPM effort, the ITU-T work cited here
was significantly modified based on further study and analysis.
Demichelis considered the possibilities 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 depreciated 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 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.
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 long measurement intervals can
indicate more PDV than would be experienced by streams that
support shorter interval sessions.
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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
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
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.
5.3. IPPM List Discussion from 2001
Summary To Be Provided. But to indicate one of the key points:
IPDV values can be viewed as the adjustments that an adaptive de-
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jitter buffer would make, IF it could make adjustments on a packet-
by-packet basis. However, adaptive de-jitter buffers don't make
adjustments so frequently, so in this respect IPDV provides "too much
information".
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. 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 [I-D.ietf-ippm-reordering],
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]
Jitter.
6.2. Path Changes
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.
Path changes are usually accompanied by a 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 multipath for
flows).
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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
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.
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.
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
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 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.
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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.
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 or IPDV.
6.3. Measurement Clock Issues
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
arrives. However, both IPDV and PDV assess time differences, so the
error present in two one-way-delay singletons will cancel as long as
it is constant.
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,
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thereby avoiding the issue in the PDV calculations that follow. See
[RFC3393] for additional information on compensating for skew.
6.4. Reporting a Single Number
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 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.
6.5. 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 assists in the computation of MAPDV2.
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7. Applicability of the Delay Variation Forms with Tasks
Based on the comparisons of IPDV and PDV presented above, this
section matches the attributes of each form with the tasks described
in section 3. 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
WG.
7.1. Challenging Circumstances
When appreciable skew is present between measurement system clocks,
then IPDV has a clear advantage, since that 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 is one of the reasons that
[RFC3550] RTCP Jitter and MAPDV2 in [G.1020] have attained deployment
in low-cost systems.
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.
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.
Note that delay variation may not be the top concern under these
unstable and un-reliable circumstances, as this author has pointed-
out many times in discussion.
7.2. 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
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greatly on the exact sequence of packets and may not lend themselves
easily to the composition problem.
7.3. 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.
7.4. 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 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.
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8. IANA Considerations
This document makes no request of IANA.
Note to RFC Editor: this section may be removed on publication as an
RFC.
9. Security Considerations
The security considerations that apply to any active measurement of
live networks are relevant here as well. See [RFC4656]
10. 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.
11. References
11.1. Normative References
[I-D.ietf-ippm-reordering]
Morton, A., "Packet Reordering Metric for IPPM",
draft-ietf-ippm-reordering-13 (work in progress),
May 2006.
[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.
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[RFC3432] Raisanen, V., Grotefeld, G., and A. Morton, "Network
performance measurement with periodic streams", RFC 3432,
November 2002.
[RFC4656] Shalunov, S., Teitelbaum, B., Karp, A., Boote, J., and M.
Zekauskas, "A One-way Active Measurement Protocol
(OWAMP)", RFC 4656, September 2006.
11.2. Informative References
[Casner] "A Fine-Grained View of High Performance Networking, NANOG
22 Conf.; http://www.nanog.org/mtg-0105/agenda.html", May
20-22 2001.
[Cia03] "Standardized Active Measurements on a Tier 1 IP Backbone,
IEEE Communications Mag., pp 90-97.", June 2003.
[Demichelis]
http://www.advanced.org/ippm/archive.3/att-0075/
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.
[I-D.ietf-ippm-framework-compagg]
Morton, A. and S. Berghe, "Framework for Metric
Composition", draft-ietf-ippm-framework-compagg-01 (work
in progress), June 2006.
[Krzanowski]
Presentation at IPPM, IETF-64, "Jitter Definitions: What
is What?", November 2005.
[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 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.
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Author's Address
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/
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Full Copyright Statement
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