draft-morton-ippm-delay-var-as-00.txt   draft-morton-ippm-delay-var-as-01.txt 
Network Working Group A. Morton Network Working Group A. Morton
Internet-Draft AT&T Labs Internet-Draft AT&T Labs
Intended status: Informational October 15, 2006 Intended status: Informational November 27, 2006
Expires: April 18, 2007 Expires: May 31, 2007
Packet Delay Variation Applicability Statement Packet Delay Variation Applicability Statement
draft-morton-ippm-delay-var-as-00 draft-morton-ippm-delay-var-as-01
Status of this Memo Status of this Memo
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Copyright Notice Copyright Notice
Copyright (C) The Internet Society (2006). Copyright (C) The Internet Society (2006).
Abstract Abstract
Many definitions of packet delay variation exist, and two different Packet delay variation metrics appear in many different standards
formulations have come into wide use in the context of active documents. The metric definition in RFC 3393 has considerable
measurements. This memo examines a range of circumstances for active flexibility, and it allows multiple formulations of delay variation
measurements and their uses, and recommends which of these two forms through the specification of different packet selection functions.
is best matched to the conditions and task.
Although flexibility provides wide coverage and room for new ideas,
it can be a challenge when attempting to make comparisons. Two
different formulations of delay variation have come into wide use in
the context of active measurements. This memo examines a range of
circumstances for active measurements of delay variation and their
uses, and recommends which of the two forms is best matched to
particular conditions and tasks.
Requirements Language Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119]. document are to be interpreted as described in RFC 2119 [RFC2119].
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Purpose and Scope . . . . . . . . . . . . . . . . . . . . . . 4 2 Purpose and Scope . . . . . . . . . . . . . . . . . . . . . . 5
3. Uses of Delay Variation Metrics . . . . . . . . . . . . . . . 4 3 Uses and Circumstances for Delay Variation Metrics . . . . . . 5
3.1. Determining De-jitter Buffer Size . . . . . . . . . . . . 4 3.1 Uses . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
3.2. Inferring Queue Occupation on a Path . . . . . . . . . . . 5 3.1.1 Determining De-jitter Buffer Size . . . . . . . . . . . 6
3.3. Spatial Composition . . . . . . . . . . . . . . . . . . . 5 3.1.2 Inferring Queue Occupation on a Path . . . . . . . . . 6
3.4. Challenging Circumstances . . . . . . . . . . . . . . . . 5 3.1.3 Spatial Composition . . . . . . . . . . . . . . . . . . 6
3.5. <your favorite here> . . . . . . . . . . . . . . . . . . . 6 3.1.4 Service Level Comparison . . . . . . . . . . . . . . . 7
4. Formulations of IPDV and PDV . . . . . . . . . . . . . . . . . 6 3.1.5 <your favorite here> . . . . . . . . . . . . . . . . . 7
4.1. IPDV: Inter-Packet Delay Variation . . . . . . . . . . . . 6 3.2 Challenging Circumstances . . . . . . . . . . . . . . . . . 7
4.2. PDV: Packet Delay Variation . . . . . . . . . . . . . . . 6 4 Formulations of IPDV and PDV . . . . . . . . . . . . . . . . . 7
4.3. Examples and Initial Comparisons . . . . . . . . . . . . . 6 4.1 IPDV: Inter-Packet Delay Variation . . . . . . . . . . . . 8
5. Earlier Comparisons . . . . . . . . . . . . . . . . . . . . . 6 4.2 PDV: Packet Delay Variation . . . . . . . . . . . . . . . . 8
5.1. Demichelis' Comparison . . . . . . . . . . . . . . . . . . 7 4.3 Examples and Initial Comparisons . . . . . . . . . . . . . 8
5.2. Ciavattone et al. . . . . . . . . . . . . . . . . . . . . 8 5 Earlier Comparisons . . . . . . . . . . . . . . . . . . . . . 8
5.3. IPPM List Discussion from 2001 . . . . . . . . . . . . . . 8 5.1 Demichelis' Comparison . . . . . . . . . . . . . . . . . . 9
5.4. Y.1540 Appendix II . . . . . . . . . . . . . . . . . . . . 9 5.2 Ciavattone et al. . . . . . . . . . . . . . . . . . . . . . 10
6. Additional Properties and Comparisons . . . . . . . . . . . . 9 5.3 IPPM List Discussion from 2001 . . . . . . . . . . . . . . 10
6.1. Jitter in RTCP Reports . . . . . . . . . . . . . . . . . . 9 5.4 Y.1540 Appendix II . . . . . . . . . . . . . . . . . . . . 11
6.2. Path Changes . . . . . . . . . . . . . . . . . . . . . . . 9 6 Additional Properties and Comparisons . . . . . . . . . . . . 11
6.2.1. Lossless Path Change . . . . . . . . . . . . . . . . . 10 6.1 Jitter in RTCP Reports . . . . . . . . . . . . . . . . . . 11
6.2.2. Path Change with Loss . . . . . . . . . . . . . . . . 11 6.2 Path Changes . . . . . . . . . . . . . . . . . . . . . . . 11
6.3. Measurement Clock Issues . . . . . . . . . . . . . . . . . 11 6.2.1 Lossless Path Change . . . . . . . . . . . . . . . . . 12
6.4. Reporting a Single Number . . . . . . . . . . . . . . . . 12 6.2.2 Path Change with Loss . . . . . . . . . . . . . . . . . 13
6.5. MAPDV2 . . . . . . . . . . . . . . . . . . . . . . . . . . 12 6.3 Measurement Clock Issues . . . . . . . . . . . . . . . . . 13
7. Applicability of the Delay Variation Forms with Tasks . . . . 13 6.4 Reporting a Single Number . . . . . . . . . . . . . . . . . 14
7.1. Challenging Circumstances . . . . . . . . . . . . . . . . 13 6.5 MAPDV2 . . . . . . . . . . . . . . . . . . . . . . . . . . 14
7.2. Spatial Composition . . . . . . . . . . . . . . . . . . . 13 7 Applicability of the Delay Variation Forms . . . . . . . . . . 15
7.3. Inferring Queue Occupancy . . . . . . . . . . . . . . . . 14 7.1 Challenging Circumstances . . . . . . . . . . . . . . . . . 15
7.4. Determining De-jitter Buffer Size . . . . . . . . . . . . 14 7.2 Spatial Composition . . . . . . . . . . . . . . . . . . . . 15
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 15 7.3 Inferring Queue Occupancy . . . . . . . . . . . . . . . . . 16
9. Security Considerations . . . . . . . . . . . . . . . . . . . 15 7.4 Determining De-jitter Buffer Size . . . . . . . . . . . . . 16
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 15 8 IANA Considerations . . . . . . . . . . . . . . . . . . . . . 17
11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 15 9 Security Considerations . . . . . . . . . . . . . . . . . . . 17
11.1. Normative References . . . . . . . . . . . . . . . . . . . 15 10 Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 17
11.2. Informative References . . . . . . . . . . . . . . . . . . 16 11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 17
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 17 11.1 Normative References . . . . . . . . . . . . . . . . . . . 17
Intellectual Property and Copyright Statements . . . . . . . . . . 18 11.2 Informative References . . . . . . . . . . . . . . . . . . 18
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 19
Intellectual Property and Copyright Statements . . . . . . . . . . 20
1. Introduction 1 Introduction
There are many ways to formulate delay variation metrics for packet There are many ways to formulate packet delay variation metrics for
networks. The IETF itself has several specifications for delay the Internet and other IP-based networks. The IETF itself has
variation, sometimes called jitter, and these have achieved wide several specifications for delay variation [RFC3393], sometimes
adoption. The International Telecommunication Union - called jitter [RFC3550], and these have achieved wide adoption. The
Telecommunication Standardization Sector has also recommended several International Telecommunication Union - Telecommunication
delay variation metrics (called parameters in their terminology), and Standardization Sector (ITU-T) has also recommended several delay
some of these are widely cited and used. 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 standards 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 "packet 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 Most (if not all) delay variation metrics are derived metrics, in
that their definitions rely on another fundamental metric. In this that their definitions rely on another fundamental metric. In this
case, the fundamental metric is one-way delay, and variation is case, the fundamental metric is one-way delay, and variation is
assessed by computing the difference between two individual one-way assessed by computing the difference between two individual one-way
delay measurements, or a pair of singletons. One of the delay delay measurements, or a pair of singletons. One of the delay
singletons is taken as a reference value, and the result is the singletons is taken as a reference, and the result is the variation
variation with respect to the reference. The variation is usually with respect to the reference. The variation is usually summarized
summarized for all packets in a stream (or sample) using statistics. for all packets in a stream using statistics.
Two main formulations of delay variation are preferred (according to Two main formulations of delay variation are preferred (according to
[Krzanowski]): [Krzanowski]):
1. Inter-Packet Delay Variation, IPDV, where the reference is the 1. Inter-Packet Delay Variation, IPDV, where the reference is the
previous packet in the stream (according to sending sequence), previous packet in the stream (according to sending sequence),
and the reference changes for each packet in the stream. and the reference changes for each packet in the stream.
Properties of variation and packet sequence are captured in this Properties of variation and packet sequence are captured in this
formulation. formulation.
2. Packet Delay Variation, PDV, where a single reference is chosen 2. Packet Delay Variation, PDV, where a single reference is chosen
from the stream based on specific criteria, and the reference is from the stream based on specific criteria, and the reference is
fixed once selected. The most common criterion for the reference fixed once selected. The most common criterion for the reference
is the packet with the minimum delay in the sample. 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 It is important to note that the authors of relevant standards for
delay variation recognized there are many different users with delay variation recognized there are many different users with
varying needs, and allowed sufficient flexibility to formulate varying needs, and allowed sufficient flexibility to formulate
several metrics with different properties. Therefore, the comparison several metrics with different properties. Therefore, the comparison
is not so much between standards bodies or their specifications as it is not so much between standards bodies or their specifications as it
is between specific formulations of delay variation. For instance, is between specific formulations of delay variation. For instance,
both Inter-Packet Delay Variation and Packet Delay Variation can be both Inter-Packet Delay Variation and Packet Delay Variation can be
assessed using options of [RFC3393], especially the packet selection assessed using options of [RFC3393], especially the packet selection
function. function.
The IPPM framework [RFC2330] and other RFCs describing IPPM metrics The IPPM framework [RFC2330] and other RFCs describing IPPM metrics
provide a background for this memo, especially for terms such as provide a background for this memo, especially for terms such as
singleton, sample, and statistic. singleton, sample, and statistic.
2. Purpose and Scope 2 Purpose and Scope
The purpose of this memo is to compare two forms of delay variation, The IPDV and PDV formulations have certain features that make them
so that it will be evident which of the two is better suited for each more suitable for one circumstance and less so for another. The
of many possible uses and their related circumstances. 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 The scope of this memo is limited to the two forms of delay variation
briefly described above (Inter-Packet Delay Variation and Packet briefly described above (Inter-Packet Delay Variation and Packet
Delay Variation), circumstances related to active measurement, and Delay Variation), circumstances related to active measurement, and
uses that are deemed relevant and worthy of inclusion here through uses that are deemed relevant and worthy of inclusion here through
IPPM Working Group consensus. IPPM Working Group consensus.
The scope excludes assessment of delay variation for packets with The scope excludes assessment of delay variation for packets with
undefined delay. The is accomplished by conditioning the delay undefined delay. This is accomplished by conditioning the delay
distribution on arrival within a reasonable time based on an distribution on arrival within a reasonable waiting time based on an
understanding of the path under test and packet lifetimes. This is understanding of the path under test and packet lifetimes. The
consistent with [RFC3393], where the Type-P-One-way-ipdv is undefined waiting time is sometimes called the loss threshold [RFC2680]: if a
when the destination fails to receive one or both packets in the packet arrives beyond this threshold, it may as well have been lost
selected pair. Furthermore, it is consistent with application because it is no longer useful. This is consistent with [RFC3393],
performance analysis to consider only arriving packets, because a where the Type-P-One-way-ipdv is undefined when the destination fails
finite waiting time-out is a feature of many protocols. 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 3 Uses and Circumstances for Delay Variation Metrics
This section presents a set of tasks that call for delay variation This section presents a set of tasks that call for delay variation
measurements and their possible circumstances. It answers the measurements and their possible circumstances.
question, "How will the results be used?" for the delay variation
metric.
3.1. Determining De-jitter Buffer Size 3.1 Uses
Here, the memo provides several answers to the question, "How will
the results be used?" for the delay variation metric.
3.1.1 Determining De-jitter Buffer Size
Most Isochronous applications (a.k.a. real-time applications) employ Most Isochronous applications (a.k.a. real-time applications) employ
a buffer to smooth out delay variation encountered on the path from a buffer to smooth out delay variation encountered on the path from
source to destination. The buffer must be big enough to accommodate source to destination. The buffer must be big enough to accommodate
(most of) the expected variation, or packet loss will result. (most of) the expected variation, or packet loss will result.
However, if the buffer is too large, then some of the desired However, if the buffer is too large, then some of the desired
spontaneity of communication will be lost and conversational dynamics spontaneity of communication will be lost and conversational dynamics
will be affected. Therefore, application designers need to know the will be affected. Therefore, application designers need to know the
extent of delay variation they must accommodate, whether they are extent of delay variation they must accommodate, whether they are
designing fixed or adaptive buffer systems. designing fixed or adaptive buffer systems.
Network service providers also attempt to constrain delay variation Network service providers also attempt to constrain delay variation
to ensure the quality of real-time applications, and monitor this to ensure the quality of real-time applications, and monitor this
metric (possibly to compare with a numerical objective or Service metric (possibly to compare with a numerical objective or Service
Level Agreement). Level Agreement).
3.2. Inferring Queue Occupation on a Path 3.1.2 Inferring Queue Occupation on a Path
As packets travel along the path from source to destination, they As packets travel along the path from source to destination, they
pass through a series of router queues. Many of the sources of delay pass through many network elements, including a series of router
along the path are constant, but the latency encountered in each queues. Some types of the delay sources along the path are constant,
queue varies, depending on the number of packets in the queue when a such as links between two locations. But the latency encountered in
particular packet arrives. If one assumes that at least one of the each queue varies, depending on the number of packets in the queue
packets in a test stream encounters virtually empty queues all along when a particular packet arrives. If one assumes that at least one
the path (and the path is stable), then the additional delay observed of the packets in a test stream encounters virtually empty queues all
on other packets can be attributed to the time spent in one or more along the path (and the path is stable), then the additional delay
queues. Otherwise, the delay variation observed is the variation in observed on other packets can be attributed to the time spent in one
queue time experienced by the test stream. or more queues. Otherwise, the delay variation observed is the
variation in queue time experienced by the test stream.
3.3. Spatial Composition 3.1.3 Spatial Composition
In Spatial Composition, the tasks are similar to those described In Spatial Composition, the tasks are similar to those described
above, but with the additional complexity of a multiple network path above, but with the additional complexity of a multiple network path
where several sub-paths are measured separately, and no source to where several sub-paths are measured separately, and no source to
destination measurements are available. In this case, the source to destination measurements are available. In this case, the source to
destination performance must be estimated, using Composed Metrics as destination performance must be estimated, using Composed Metrics as
described in [I-D.ietf-ippm-framework-compagg] described in [I-D.ietf-ippm-framework-compagg] and [Y.1541]. Note
that determining the composite delay variation is not trivial: simply
summing the sub-path variations is not accurate.
3.4. Challenging Circumstances 3.1.4 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.
3.1.5 <your favorite here>
3.2 Challenging Circumstances
Any of the tasks above are made more "interesting" when certain Any of the tasks above are made more "interesting" when certain
circumstances are present. Among these are: circumstances are present. Among these are:
1. Low cost or low complexity measurement systems. These systems 1. Low cost or low complexity measurement systems. These systems
may be embedded in communication devices that do not have access may be embedded in communication devices that do not have access
to high stability clocks, and time errors will almost certainly to high stability clocks, and time errors will almost certainly
be present. These devices may not have sufficient memory to be present. However, larger time-related errors may offer an
store all singletons for later processing. acceptable trade-off for monitoring performance over a large
population (the accuracy needed to detect problems may be much
than required for a scientific study). 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 2. Extremely dynamic network conditions. When there is little or no
stability in the network under test, then the devices that stability in the network under test, then the devices that
attempt to characterize the network are equally stressed, attempt to characterize the network are equally stressed,
especially if the results displayed are used to make inferences especially if the results displayed are used to make inferences
which may not be valid. Frequent path changes and prolonged which may not be valid. Frequent path changes and prolonged
congestion with substantial packet loss clearly make delay congestion with substantial packet loss clearly make delay
variation measurements challenging. variation measurements challenging.
3.5. <your favorite here> 4 Formulations of IPDV and PDV
4. Formulations of IPDV and PDV
This section presents the formulations of IPDV and PDV, and provides This section presents the formulations of IPDV and PDV, and provides
some illustrative examples. We use the basic singleton definition in some illustrative examples. We use the basic singleton definition in
[RFC3393] (which itself is based on [RFC2679]): [RFC3393] (which itself is based on [RFC2679]):
"Type-P-One-way-ipdv is defined for two packets from Src to Dst "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 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 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." of the Type-P-One-Way-Delay from Src to Dst at T1."
4.1. IPDV: Inter-Packet Delay Variation 4.1 IPDV: Inter-Packet Delay Variation
An example selection function given in [RFC3393] is "Consecutive An example selection function given in [RFC3393] is "Consecutive
Type-P packets within the specified interval." This is exactly the Type-P packets within the specified interval." This is exactly the
function needed for IPDV. The reference packet in the pair is always function needed for IPDV. The reference packet in the pair is always
the previous packet in the sending sequence. the previous packet in the sending sequence.
If we have packets in a stream consecutively numbered i = 1,2,3,... 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 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. D(i) denotes the one-way-delay of the ith packet of a stream.
4.2. PDV: Packet Delay Variation Note that IPDV can take on positive and negative values (and zero),
although one of the useful ways to analyze the results is to
concentrate on the positive excursions. This is discussed in more
detail below.
4.2 PDV: Packet Delay Variation
(The name Packet Delay Variation is from [Y.1540], and refers to a
performance parameter equivalent to the metric described below.)
The Selection Function for PDV requires two specific roles for the The Selection Function for PDV requires two specific roles for the
packets in the pair. The first packet is any Type-P packet within packets in the pair. The first packet is any Type-P packet within
the specified interval. The second, or reference packet is the the specified interval. The second, or reference packet is the
Type-P packet within the specified interval with the minimum one-way- Type-P packet within the specified interval with the minimum one-way-
delay. delay.
Therefore, PDV(i) = D(i)-D(min) (using the nomenclature introduced in Therefore, PDV(i) = D(i)-D(min) (using the nomenclature introduced in
the IPDV section). the IPDV section). D(min) is the delay of the packet with the lowest
value for delay (minimum) over the current test interval. Values of
PDV may be zero or positive, and quantiles of the PDV distribution
are direct indications of delay variation.
4.3. Examples and Initial Comparisons 4.3 Examples and Initial Comparisons
This section will discuss the examples in slides 2 and 3 of This section will discuss the examples in slides 2 and 3 of
http://www3.ietf.org/proceedings/06mar/slides/ippm-4.pdf http://www3.ietf.org/proceedings/06mar/slides/ippm-4.pdf
5. Earlier Comparisons 5 Earlier Comparisons
This section summarizes previous work to compare these two forms of This section summarizes previous work to compare these two forms of
delay variation. delay variation.
5.1. Demichelis' Comparison 5.1 Demichelis' Comparison
In [Demichelis], Demichelis compared the early draft versions of the In [Demichelis], Demichelis compared the early draft versions of the
two forms we consider here. Although the IPDV form would eventually two forms we consider here. Although the IPDV form would eventually
be standardized under the IETF IPPM effort, the ITU-T work cited here be standardized under the IETF IPPM effort, the ITU-T work cited here
was significantly modified based on further study and analysis. was significantly modified based on further study and analysis.
Demichelis considered the possibilities of using the delay of the Demichelis considered the possibilities of using the delay of the
first packet in the stream and the mean delay of the stream as the first packet in the stream and the mean delay of the stream as the
PDV reference packet. Neither of these alternative references were PDV reference packet. Neither of these alternative references were
used in practice, and they are now depreciated in favor of the used in practice, and they are now depreciated in favor of the
minimum delay of the stream [Y.1540] . minimum delay of the stream [Y.1540] .
skipping to change at page 8, line 13 skipping to change at page 10, line 13
support shorter interval sessions. support shorter interval sessions.
5. PDV characterizes the range of queue occupancies along the 5. PDV characterizes the range of queue occupancies along the
measurement path (assuming the path is fixed), but the range says measurement path (assuming the path is fixed), but the range says
nothing about how the variation took place. nothing about how the variation took place.
The summary metrics used in this comparison were the number of values The summary metrics used in this comparison were the number of values
exceeding a +/-50ms range around the mean, the Inverse Percentiles, exceeding a +/-50ms range around the mean, the Inverse Percentiles,
and the Inter-Quartile Range. and the Inter-Quartile Range.
5.2. Ciavattone et al. 5.2 Ciavattone et al.
In [Cia03], the authors compared IPDV and PDV (referred to as delta) In [Cia03], the authors compared IPDV and PDV (referred to as delta)
using a periodic packet stream conforming to [RFC3432] with inter- using a periodic packet stream conforming to [RFC3432] with inter-
packet interval of 20 ms. packet interval of 20 ms.
One of the comparisons between IPDV and PDV involves a laboratory One of the comparisons between IPDV and PDV involves a laboratory
set-up where a queue was temporarily congested by a competing packet set-up where a queue was temporarily congested by a competing packet
burst. The additional queuing delay was 85ms to 95ms, much larger burst. The additional queuing delay was 85ms to 95ms, much larger
than the inter-packet interval. The first packet in the stream that than the inter-packet interval. The first packet in the stream that
follows the competing burst spends the longest time enqueued, and follows the competing burst spends the longest time enqueued, and
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interval. This preference for information from the positive IPDV interval. This preference for information from the positive IPDV
values has prompted some to ignore the negative values, or to take values has prompted some to ignore the negative values, or to take
the absolute value of each IPDV measurement (sacrificing key the absolute value of each IPDV measurement (sacrificing key
properties of IPDV in the process, such as its ability to distinguish properties of IPDV in the process, such as its ability to distinguish
delay trends). delay trends).
Elsewhere, the authors considered the range as a summary statistic Elsewhere, the authors considered the range as a summary statistic
for IPDV, and the 99.9%-ile minus the minimum delay as a summary for IPDV, and the 99.9%-ile minus the minimum delay as a summary
statistic for delay variation, or PDV. statistic for delay variation, or PDV.
5.3. IPPM List Discussion from 2001 5.3 IPPM List Discussion from 2001
Summary To Be Provided. But to indicate one of the key points: Summary To Be Provided. But to indicate one of the key points:
IPDV values can be viewed as the adjustments that an adaptive de- IPDV values can be viewed as the adjustments that an adaptive de-
jitter buffer would make, IF it could make adjustments on a packet- jitter buffer would make, IF it could make adjustments on a packet-
by-packet basis. However, adaptive de-jitter buffers don't make by-packet basis. However, adaptive de-jitter buffers don't make
adjustments so frequently, so in this respect IPDV provides "too much adjustments so frequently, so in this respect IPDV provides "too much
information". information".
5.4. Y.1540 Appendix II 5.4 Y.1540 Appendix II
This Appendix compares IPDV, PDV (referred to as 2-point PDV), and This Appendix compares IPDV, PDV (referred to as 2-point PDV), and
1-point packet delay variation (which assume a periodic stream and 1-point packet delay variation (which assume a periodic stream and
assesses variation against an ideal arrival schedule constructed at assesses variation against an ideal arrival schedule constructed at
the single measurement point). the single measurement point).
6. Additional Properties and Comparisons 6 Additional Properties and Comparisons
This section treats some of the earlier comparison areas in more This section treats some of the earlier comparison areas in more
detail, and introduces new areas for comparison. detail, and introduces new areas for comparison.
6.1. Jitter in RTCP Reports 6.1 Jitter in RTCP Reports
[RFC3550] gives the calculation of the inter-arrival Jitter field for [RFC3550] gives the calculation of the inter-arrival Jitter field for
the RTCP report, with a sample implementation in an Appendix. the RTCP report, with a sample implementation in an Appendix.
The RTCP Jitter value can be calculated using IPDV singletons. If The RTCP Jitter value can be calculated using IPDV singletons. If
there is packet reordering, as defined in [I-D.ietf-ippm-reordering], there is packet reordering, as defined in [I-D.ietf-ippm-reordering],
then estimates of Jitter based on IPDV may vary slightly, because then estimates of Jitter based on IPDV may vary slightly, because
[RFC3550] specifies the use of receive packet order. [RFC3550] specifies the use of receive packet order.
Just as there is no simple way to convert PDV singletons to IPDV Just as there is no simple way to convert PDV singletons to IPDV
singletons without returning to the original sample of delay singletons without returning to the original sample of delay
singletons, there is no clear relationship between PDV and [RFC3550] singletons, there is no clear relationship between PDV and [RFC3550]
Jitter. Jitter.
6.2. Path Changes 6.2 Path Changes
Sometimes the path characteristics change during a measurement Sometimes the path characteristics change during a measurement
interval. The change may be due to link or router failure, interval. The change may be due to link or router failure,
administrative changes prior to maintenance (e.g., link cost change), administrative changes prior to maintenance (e.g., link cost change),
or re-optimization of routing using new information. All these or re-optimization of routing using new information. All these
causes are usually infrequent, and network providers take appropriate causes are usually infrequent, and network providers take appropriate
measures to ensure this. Automatic restoration to a back-up path is measures to ensure this. Automatic restoration to a back-up path is
seen as a desirable feature of IP networks. seen as a desirable feature of IP networks.
Path changes are usually accompanied by a persistent increase or Path changes are usually accompanied by a persistent increase or
decrease in one-way-delay. [Cia03] gives one such example. We decrease in one-way-delay. [Cia03] gives one such example. We
assume that a restoration path either accepts a stream of packets, or assume that a restoration path either accepts a stream of packets, or
is not used for that particular stream (e.g., no multipath for is not used for that particular stream (e.g., no multi-path for
flows). flows).
In any case, a change in the TTL (or Hop Limit) of the received 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 packets indicates that the path is no longer the same. Transient
packet reordering may also be observed with path changes, due to use packet reordering may also be observed with path changes, due to use
of non-optimal routing while updates propagate through the network of non-optimal routing while updates propagate through the network
(see [Casner] and [Cia03] ) (see [Casner] and [Cia03] )
Many, if not all, packet streams experience packet loss in Many, if not all, packet streams experience packet loss in
conjunction with a path change. However, it is certainly possible conjunction with a path change. However, it is certainly possible
that the active measurement stream does not experience loss. This that the active measurement stream does not experience loss. This
may be due to use of a long inter-packet sending interval with may be due to use of a long inter-packet sending interval with
respect to the restoration time, and this becomes more likely as respect to the restoration time, and this becomes more likely as
"fast restoration" techniques see wider deployment. "fast restoration" techniques see wider deployment.
Thus, there are two main cases to consider, path changes accompanied 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 by loss, and those that are lossless from the point of view of the
active measurement stream. active measurement stream.
6.2.1. Lossless Path Change 6.2.1 Lossless Path Change
In the lossless case, a path change will typically affect only two In the lossless case, a path change will typically affect only two
IPDV singletons. However, if the change in delay is negative and IPDV singletons. However, if the change in delay is negative and
larger than the inter-packet sending interval, then more than two larger than the inter-packet sending interval, then more than two
IPDV singletons may be affected because packet reordering is also IPDV singletons may be affected because packet reordering is also
likely to occur. likely to occur.
The use of the new path and its delay variation can be quantified by The use of the new path and its delay variation can be quantified by
treating the PDV distribution as bi-modal, and characterizing each treating the PDV distribution as bi-modal, and characterizing each
mode separately. This would involve declaring a new path within the mode separately. This would involve declaring a new path within the
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in an automated decision. in an automated decision.
The effect of path changes may also be reduced by making PDV The effect of path changes may also be reduced by making PDV
measurements over short intervals (minutes, as opposed to hours). measurements over short intervals (minutes, as opposed to hours).
This way, a path change will affect one sample and its PDV values. 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 Assuming that the mean or median one-way-delay changes appreciably on
the new path, then subsequent measurements can confirm a path change, the new path, then subsequent measurements can confirm a path change,
and trigger special processing on the interval containing a path and trigger special processing on the interval containing a path
change and the affected PDV result. change and the affected PDV result.
6.2.2. Path Change with Loss 6.2.2 Path Change with Loss
If the path change is accompanied by loss, such that the are no If the path change is accompanied by loss, such that the are no
consecutive packet pairs that span the change, then no IPDV consecutive packet pairs that span the change, then no IPDV
singletons will reflect the change. This may or may not be singletons will reflect the change. This may or may not be
desirable, depending on the ultimate use of the delay variation desirable, depending on the ultimate use of the delay variation
measurement. measurement.
PDV will again produce a bimodal distribution. But here, the PDV will again produce a bimodal distribution. But here, the
decision process to define sub-intervals associated with each path is decision process to define sub-intervals associated with each path is
further assisted by the presence of loss, in addition to TTL, further assisted by the presence of loss, in addition to TTL,
reordering information, and use of short measurement intervals reordering information, and use of short measurement intervals
consistent with the duration of user sessions. It is reasonable to consistent with the duration of user sessions. It is reasonable to
assume that at least loss and delay will be measured simultaneously assume that at least loss and delay will be measured simultaneously
with PDV or IPDV. with PDV or IPDV.
6.3. Measurement Clock Issues 6.3 Measurement Clock Issues
As mentioned above, [Demichelis] observed that PDV places greater As mentioned above, [Demichelis] observed that PDV places greater
demands on clock synchronization than for IPDV. This observation demands on clock synchronization than for IPDV. This observation
deserves more discussion. Synchronization errors have two deserves more discussion. Synchronization errors have two
components: time of day errors and clock frequency errors (resulting components: time of day errors and clock frequency errors (resulting
in skew). in skew).
Both IPDV and PDV are sensitive to time-of-day errors when attempting Both IPDV and PDV are sensitive to time-of-day errors when attempting
to align measurement intervals at the source and destination. Gross to align measurement intervals at the source and destination. Gross
mis-alignment of the measurement intervals can lead to lost packets, mis-alignment of the measurement intervals can lead to lost packets,
skipping to change at page 12, line 7 skipping to change at page 14, line 7
provides some relief from the effects of skew error. provides some relief from the effects of skew error.
If skew is present in a sample of one-way-delays, its symptom is 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 typically a linear growth or decline over all the one-way-delay
values. As a practical matter, if the same slope appears values. As a practical matter, if the same slope appears
consistently in the measurements, then it may be possible to fit the consistently in the measurements, then it may be possible to fit the
slope and compensate for the skew in the one-way-delay measurements, slope and compensate for the skew in the one-way-delay measurements,
thereby avoiding the issue in the PDV calculations that follow. See thereby avoiding the issue in the PDV calculations that follow. See
[RFC3393] for additional information on compensating for skew. [RFC3393] for additional information on compensating for skew.
6.4. Reporting a Single Number 6.4 Reporting a Single Number
Despite the risk of over-summarization, measurements must often be Despite the risk of over-summarization, measurements must often be
displayed for easy consumption. If the right summary report is displayed for easy consumption. If the right summary report is
prepared, then the "dashboard" view correctly indicates whether there prepared, then the "dashboard" view correctly indicates whether there
is something different and worth investigating further, or that the is something different and worth investigating further, or that the
status has not changed. The dashboard model restricts every status has not changed. The dashboard model restricts every
instrument display to a single number. The packet network dashboard instrument display to a single number. The packet network dashboard
could have different instruments for loss, delay, delay variation, could have different instruments for loss, delay, delay variation,
reordering, etc., and each must be summarized as a single number for reordering, etc., and each must be summarized as a single number for
each measurement interval. each measurement interval.
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IPDV does not lend itself to summarization so easily. The mean IPDV IPDV does not lend itself to summarization so easily. The mean IPDV
is typically zero. As the IPDV distribution may have two tails is typically zero. As the IPDV distribution may have two tails
(positive and negative) the range or pseudo-range would not match the (positive and negative) the range or pseudo-range would not match the
needed de-jitter buffer size. Additional complexity may be needed de-jitter buffer size. Additional complexity may be
introduced when the variation exceeds the inter-packet sending introduced when the variation exceeds the inter-packet sending
interval, as discussed above. Should the Inter-Quartile Range be interval, as discussed above. Should the Inter-Quartile Range be
used? Should the singletons beyond some threshold be counted (e.g., used? Should the singletons beyond some threshold be counted (e.g.,
mean +/- 50ms)? A strong rationale for one of these summary mean +/- 50ms)? A strong rationale for one of these summary
statistics has yet to emerge. statistics has yet to emerge.
6.5. MAPDV2 6.5 MAPDV2
MAPDV2 stands for Mean Absolute Packet Delay Variation (version) 2, MAPDV2 stands for Mean Absolute Packet Delay Variation (version) 2,
and is specified in [G.1020]. The MAPDV2 algorithm computes a and is specified in [G.1020]. The MAPDV2 algorithm computes a
smoothed running estimate of the mean delay using the one-way delays 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 of 16 previous packets. It compares the current one-way-delay to the
estimated mean, separately computes the means of positive and estimated mean, separately computes the means of positive and
negative deviations, and sums these deviation means to produce negative deviations, and sums these deviation means to produce
MAPVDV2. In effect, there is a MAPDV2 singleton for every arriving MAPVDV2. In effect, there is a MAPDV2 singleton for every arriving
packet, so further summarization is usually warranted. packet, so further summarization is usually warranted.
Neither IPDV or PDV assists in the computation of MAPDV2. Neither IPDV or PDV assists in the computation of MAPDV2.
7. Applicability of the Delay Variation Forms with Tasks 7 Applicability of the Delay Variation Forms
Based on the comparisons of IPDV and PDV presented above, this Based on the comparisons of IPDV and PDV presented above, this
section matches the attributes of each form with the tasks described section matches the attributes of each form with the tasks described
in section 3. We discuss the more general circumstances first. in section 3. We discuss the more general circumstances first.
Note: the conclusions of this section should be regarded as Note: the conclusions of this section should be regarded as
preliminary, pending discussion and further development by the IPPM preliminary, pending discussion and further development by the IPPM
WG. WG.
7.1. Challenging Circumstances 7.1 Challenging Circumstances
When appreciable skew is present between measurement system clocks, When appreciable skew is present between measurement system clocks,
then IPDV has a clear advantage, since that PDV would require then IPDV has a clear advantage, since that PDV would require
processing over the entire sample to remove the skew error. Neither 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 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 summarization without memory, and this is one of the reasons that
[RFC3550] RTCP Jitter and MAPDV2 in [G.1020] have attained deployment [RFC3550] RTCP Jitter and MAPDV2 in [G.1020] have attained deployment
in low-cost systems. in low-cost systems.
If the network under test exhibits frequent path changes, on the If the network under test exhibits frequent path changes, on the
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If the network under test exhibits frequent loss, then PDV may If the network under test exhibits frequent loss, then PDV may
produce a larger set of singletons for the sample than IPDV. This is produce a larger set of singletons for the sample than IPDV. This is
due to IPDV requiring consecutive packet arrivals to assess delay due to IPDV requiring consecutive packet arrivals to assess delay
variation, compared to PDV where any packet arrival is useful. The variation, compared to PDV where any packet arrival is useful. The
worst case is when no consecutive packets arrive, and the entire IPDV worst case is when no consecutive packets arrive, and the entire IPDV
sample would be undefined. PDV would successfully produce a sample sample would be undefined. PDV would successfully produce a sample
based on the arriving packets. based on the arriving packets.
Note that delay variation may not be the top concern under these Note that delay variation may not be the top concern under these
unstable and un-reliable circumstances, as this author has pointed- unstable and unreliable circumstances, as this author has pointed-out
out many times in discussion. many times in discussion.
7.2. Spatial Composition 7.2 Spatial Composition
ITU-T Recommendation [Y.1541] gives a provisional method to compose a 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 metric using PDV measurement results from two or more sub-paths.
PDV has a clear advantage at this time, since there is no known PDV has a clear advantage at this time, since there is no known
method to compose an IPDV metric. In addition, IPDV results depend method to compose an IPDV metric. In addition, IPDV results depend
greatly on the exact sequence of packets and may not lend themselves greatly on the exact sequence of packets and may not lend themselves
easily to the composition problem. easily to the composition problem.
7.3. Inferring Queue Occupancy 7.3 Inferring Queue Occupancy
The PDV distribution is anchored at the minimum delay observed in the The PDV distribution is anchored at the minimum delay observed in the
measurement interval. When the sample minimum coincides with the measurement interval. When the sample minimum coincides with the
true minimum delay of the path, then the PDV distribution is true minimum delay of the path, then the PDV distribution is
equivalent to the queuing time distribution experienced by the test equivalent to the queuing time distribution experienced by the test
stream. If the minimum delay is not the true minimum, then the PDV stream. If the minimum delay is not the true minimum, then the PDV
distribution captures the variation in queuing time and some distribution captures the variation in queuing time and some
additional amount of queuing time is experienced, but unknown. One additional amount of queuing time is experienced, but unknown. One
can summarize the PDV distribution with the mean, median, and other can summarize the PDV distribution with the mean, median, and other
statistics. statistics.
IPDV can capture the difference in queuing time from one packet to IPDV can capture the difference in queuing time from one packet to
the next, but this is a different distribution from the queue the next, but this is a different distribution from the queue
occupancy revealed by PDV. occupancy revealed by PDV.
7.4. Determining De-jitter Buffer Size 7.4 Determining De-jitter Buffer Size
This task is complimentary to the problem of inferring queue This task is complimentary to the problem of inferring queue
occupancy through measurement. Again, use of the sample minimum as occupancy through measurement. Again, use of the sample minimum as
the reference delay for PDV yields a distribution that is very the reference delay for PDV yields a distribution that is very
relevant to de-jitter buffer size. This is because the minimum delay relevant to de-jitter buffer size. This is because the minimum delay
is an alignment point for the smoothing operation of de-jitter is an alignment point for the smoothing operation of de-jitter
buffers. A de-jitter buffer that is ideally aligned with the delay buffers. A de-jitter buffer that is ideally aligned with the delay
variation adds zero buffer time to packets with the longest variation adds zero buffer time to packets with the longest
accommodated network delay (any packets with longer delays are accommodated network delay (any packets with longer delays are
discarded). Thus, a packet experiencing minimum network delay should discarded). Thus, a packet experiencing minimum network delay should
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The PDV distribution is also useful for this task, but different The PDV distribution is also useful for this task, but different
statistics are preferred. The range (max-min) or the 99.9%-ile of 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 PDV (pseudo-range) are closely related to the buffer size needed to
accommodate the observed network delay variation. accommodate the observed network delay variation.
In some cases, the positive excursions of IPDV may help to In some cases, the positive excursions of IPDV may help to
approximate the de-jitter buffer size, but there is no guarantee that approximate the de-jitter buffer size, but there is no guarantee that
a good buffer estimate will emerge, especially when the delay varies a good buffer estimate will emerge, especially when the delay varies
as a positive trend over several test packets. as a positive trend over several test packets.
8. IANA Considerations 8 IANA Considerations
This document makes no request of IANA. This document makes no request of IANA.
Note to RFC Editor: this section may be removed on publication as an Note to RFC Editor: this section may be removed on publication as an
RFC. RFC.
9. Security Considerations 9 Security Considerations
The security considerations that apply to any active measurement of The security considerations that apply to any active measurement of
live networks are relevant here as well. See [RFC4656] live networks are relevant here as well. See [RFC4656]
10. Acknowledgements 10 Acknowledgements
The author would like to thank Phil Chimento for his suggestion to The author would like to thank Phil Chimento for his suggestion to
employ the convention of conditional distributions for Delay to deal employ the convention of conditional distributions for Delay to deal
with packet loss, and his encouragement to "write the memo" after with packet loss, and his encouragement to "write the memo" after
hearing the talk. hearing the talk. Also, thanks to Benoit Claise for many suggestions
to broaden the memo's applicability and other comments.
11. References 11. References
11.1. Normative References 11.1. Normative References
[I-D.ietf-ippm-reordering] [I-D.ietf-ippm-reordering]
Morton, A., "Packet Reordering Metric for IPPM", Morton, A., "Packet Reordering Metric for IPPM",
draft-ietf-ippm-reordering-13 (work in progress), draft-ietf-ippm-reordering-13 (work in progress),
May 2006. May 2006.
skipping to change at page 16, line 9 skipping to change at page 18, line 9
Packet Loss Metric for IPPM", RFC 2680, September 1999. Packet Loss Metric for IPPM", RFC 2680, September 1999.
[RFC3393] Demichelis, C. and P. Chimento, "IP Packet Delay Variation [RFC3393] Demichelis, C. and P. Chimento, "IP Packet Delay Variation
Metric for IP Performance Metrics (IPPM)", RFC 3393, Metric for IP Performance Metrics (IPPM)", RFC 3393,
November 2002. November 2002.
[RFC3432] Raisanen, V., Grotefeld, G., and A. Morton, "Network [RFC3432] Raisanen, V., Grotefeld, G., and A. Morton, "Network
performance measurement with periodic streams", RFC 3432, performance measurement with periodic streams", RFC 3432,
November 2002. November 2002.
[RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V.
Jacobson, "RTP: A Transport Protocol for Real-Time
Applications", STD 64, RFC 3550, July 2003.
[RFC4656] Shalunov, S., Teitelbaum, B., Karp, A., Boote, J., and M. [RFC4656] Shalunov, S., Teitelbaum, B., Karp, A., Boote, J., and M.
Zekauskas, "A One-way Active Measurement Protocol Zekauskas, "A One-way Active Measurement Protocol
(OWAMP)", RFC 4656, September 2006. (OWAMP)", RFC 4656, September 2006.
11.2. Informative References 11.2. Informative References
[Casner] "A Fine-Grained View of High Performance Networking, NANOG [Casner] "A Fine-Grained View of High Performance Networking, NANOG
22 Conf.; http://www.nanog.org/mtg-0105/agenda.html", May 22 Conf.; http://www.nanog.org/mtg-0105/agenda.html", May
20-22 2001. 20-22 2001.
skipping to change at page 16, line 33 skipping to change at page 18, line 37
http://www.advanced.org/ippm/archive.3/att-0075/ http://www.advanced.org/ippm/archive.3/att-0075/
01-pap02.doc, "Packet Delay Variation Comparison between 01-pap02.doc, "Packet Delay Variation Comparison between
ITU-T and IETF Draft Definitions", November 2000. ITU-T and IETF Draft Definitions", November 2000.
[G.1020] ITU-T Recommendation G.1020, ""Performance parameter [G.1020] ITU-T Recommendation G.1020, ""Performance parameter
definitions for the quality of speech and other voiceband definitions for the quality of speech and other voiceband
applications utilizing IP networks"", 2006. applications utilizing IP networks"", 2006.
[I-D.ietf-ippm-framework-compagg] [I-D.ietf-ippm-framework-compagg]
Morton, A. and S. Berghe, "Framework for Metric Morton, A. and S. Berghe, "Framework for Metric
Composition", draft-ietf-ippm-framework-compagg-01 (work Composition", draft-ietf-ippm-framework-compagg-02 (work
in progress), June 2006. in progress), October 2006.
[Krzanowski] [Krzanowski]
Presentation at IPPM, IETF-64, "Jitter Definitions: What Presentation at IPPM, IETF-64, "Jitter Definitions: What
is What?", November 2005. 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 [Y.1540] ITU-T Recommendation Y.1540, "Internet protocol data
communication service - IP packet transfer and communication service - IP packet transfer and
availability performance parameters", December 2002. availability performance parameters", December 2002.
[Y.1541] ITU-T Recommendation Y.1540, "Network Performance [Y.1541] ITU-T Recommendation Y.1540, "Network Performance
Objectives for IP-Based Services", February 2006. Objectives for IP-Based Services", February 2006.
Author's Address Author's Address
Al Morton Al Morton
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