draft-ietf-ippm-ipdv-04.txt   draft-ietf-ippm-ipdv-05.txt 
Network Working Group C. Demichelis Network Working Group C. Demichelis
INTERNET-DRAFT CSELT INTERNET-DRAFT CSELT
Expiration Date: December 1999 P. Chimento Expiration Date: December 2000 P. Chimento
CTIT CTIT
October 1999 July 2000
Instantaneous Packet Delay Variation Metric for IPPM Instantaneous Packet Delay Variation Metric for IPPM
<draft-ietf-ippm-ipdv-04.txt> <draft-ietf-ippm-ipdv-05.txt>
1. Status of this Memo 1. Status of this Memo
This document is an Internet-Draft and is in full conformance with | This document is an Internet-Draft and is in full conformance with |
all provisions of Section 10 of RFC2026. all provisions of Section 10 of RFC2026.
Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as Internet- other groups may also distribute working documents as Internet-
Drafts. Drafts.
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http://www.ietf.org/shadow.html http://www.ietf.org/shadow.html
This memo provides information for the Internet community. This memo This memo provides information for the Internet community. This memo
does not specify an Internet standard of any kind. Distribution of does not specify an Internet standard of any kind. Distribution of
this memo is unlimited. this memo is unlimited.
2. Abstract 2. Abstract
This memo refers to a metric for variation in delay of packets across This memo refers to a metric for variation in delay of packets across
Internet paths. The metric is based on statistics of the difference Internet paths. The metric is based on statistics of the difference
in One-Way-Delay of consecutive packets. This particular definition in One-way-Delay of consecutive packets. This particular definition
of variation is called "Instantaneous Packet Delay Variation (ipdv)". of variation is called "Instantaneous Packet Delay Variation (ipdv)".
The metric is valid for measurements between two hosts both in the The metric is valid for measurements between two hosts both in the
case that they have synchronized clocks and in the case that they are case that they have synchronized clocks and in the case that they are
not synchronized. In the second case it allows an evaluation of the not synchronized. In the second case it allows an evaluation of the
reciprocal skew. Measurements performed on both directions (Two-way reciprocal skew. Measurements performed on both directions (Two-way
measurements) allow a better estimation of clock differences. The measurements) allow a better estimation of clock differences. The
precision that can be obtained is evaluated. precision that can be obtained is evaluated.
3. Introduction 3. Introduction
This memo is based on "A One-Way-Delay metric for IPPM", RFC 2679 | This memo is based on "A One-way-Delay metric for IPPM", RFC 2679 |
[2]. Part of the text in this memo is taken directly from that [2]. Part of the text in this memo is taken directly from that
document. document.
This memo defines a metric for variation in delay of packets that > This memo defines a metric for variation in delay of packets that >
flow from one host to another one through an IP path. This quantity > flow from one host to another one through an IP path. This quantity >
is sometimes called "jitter". This term, however, causes confusion > is sometimes called "jitter". This term, however, causes confusion >
because it is used in different ways by different groups of people. | because it is used in different ways by different groups of people. |
"Jitter" commonly has two meanings: The first meaning is the | "Jitter" commonly has two meanings: The first meaning is the |
variation of a signal with respect to some clock signal, where the | variation of a signal with respect to some clock signal, where the |
arrival time of the signal is expected to coincide with the arrival | arrival time of the signal is expected to coincide with the arrival |
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3.1. Definition 3.1. Definition
A definition of the Instantaneous Packet Delay Variation (ipdv) can A definition of the Instantaneous Packet Delay Variation (ipdv) can
be given for a pair of packets or for a packet inside a stream of be given for a pair of packets or for a packet inside a stream of
packets. packets.
For a pair of packets: For a pair of packets:
+ The ipdv of a pair of IP packets, that are transmitted from the + The ipdv of a pair of IP packets, that are transmitted from the
measurement point MP1 to the measurement point MP2, is the measurement point MP1 to the measurement point MP2, is the
difference between the One-Way-Delay measured for the second difference between the One-way-Delay measured for the second
packet and the One-Way-Delay measured for the first packet of the packet and the One-way-Delay measured for the first packet of the
pair. pair.
For a stream of packets: For a stream of packets:
+ The Instantaneous Packet Delay Variation of an IP packet, inside a + The Instantaneous Packet Delay Variation of an IP packet, inside a
stream of packets, going from the measurement point MP1 to the stream of packets, going from the measurement point MP1 to the
measurement point MP2, is the difference of the One-Way-Delay of measurement point MP2, is the difference of the One-way-Delay of
that packet and the One-Way-Delay of the preceding packet in the that packet and the One-way-Delay of the preceding packet in the
stream. stream.
3.2. Motivation 3.2. Motivation
A number of services that can be supported by IP are sensitive to the A number of services that can be supported by IP are sensitive to the
regular delivery of packets and can be disturbed by instantaneous regular delivery of packets and can be disturbed by instantaneous
variations in delay, while they are not disturbed by slow variations, variations in delay, while they are not disturbed by slow variations,
that can last a relatively long time. A specific metric for quick that can last a relatively long time. A specific metric for quick
variations is therefore desirable. Metrics that can be derived from variations is therefore desirable. Metrics that can be derived from
the analysis of statistics of ipdv can also be used, for example, for | the analysis of statistics of ipdv can also be used, for example, for |
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discussed, and the precision will be compared with the one of discussed, and the precision will be compared with the one of
synchronized clocks. synchronized clocks.
A bidirectional ipdv metric will be defined, as well as the > A bidirectional ipdv metric will be defined, as well as the >
methodology for error corrections. This will not be a two-way metric, > methodology for error corrections. This will not be a two-way metric, >
but a "paired" one-way in opposite directions. but a "paired" one-way in opposite directions.
5. A singleton definition of a One-way ipdv metric | 5. A singleton definition of a One-way ipdv metric |
This definition makes use of the corresponding definition of type-P- This definition makes use of the corresponding definition of type-P-
One-Way-Delay metric [2]. This section makes use of those parts of One-way-Delay metric [2]. This section makes use of those parts of
the One-Way-Delay Draft that directly apply to the One-Way-ipdv the One-way Delay Draft that directly apply to the One-way-ipdv
metric, or makes direct references to that Draft. metric, or makes direct references to that Draft.
5.1. Metric name 5.1. Metric name
Type-P-One-way-ipdv Type-P-One-way-ipdv
5.2. Metric parameters 5.2. Metric parameters
+ Src, the IP address of a host + Src, the IP address of a host
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packets sent for ipdv measurements are all of the same length. packets sent for ipdv measurements are all of the same length.
5.3. Metric unit 5.3. Metric unit
The value of a Type-P-One-way-ipdv is either a real number of seconds The value of a Type-P-One-way-ipdv is either a real number of seconds
(positive, zero or negative) or an undefined number of seconds. (positive, zero or negative) or an undefined number of seconds.
5.4. Definition 5.4. Definition
Type-P-One-way-ipdv is defined for two (consecutive) packets from Src Type-P-One-way-ipdv is defined for two (consecutive) packets from Src
to Dst, as the difference between the value of the type-P-One-way- to Dst, 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 delay from Src to Dst at T2 and the value of the Type-P-One-way-Delay
from Src to Dst at T1. T1 is the wire-time at which Scr sent the from Src to Dst at T1. T1 is the wire-time at which Scr sent the
first bit of the first packet, and T2 is the wire-time at which Src first bit of the first packet, and T2 is the wire-time at which Src
sent the first bit of the second packet. This metric is therefore sent the first bit of the second packet. This metric is therefore
ideally derived from the One-Way-Delay metric. ideally derived from the One-way-Delay metric.
NOTE: The requirement of "consecutive" packets is not essential. The NOTE: The requirement of "consecutive" packets is not essential. The
measured value is anyway the difference in One-Way-Delay at the times measured value is anyway the difference in One-way-Delay at the times
T1 and T2, which is meaningful by itself, as long as the times T1 and | T1 and T2, which is meaningful by itself, as long as the times T1 and |
T2 denote the wire times of the packets sent from Src to Dst. T2 denote the wire times of the packets sent from Src to Dst.
Therefore, for a real number ddT "The type-P-one-way-ipdv from Src to Therefore, for a real number ddT "The Type-P-one-way-ipdv from Src to
Dst at T1, T2 is ddT" means that Src sent two consecutive packets, Dst at T1, T2 is ddT" means that Src sent two consecutive packets,
the first at wire-time T1 (first bit), and the second at wire-time T2 the first at wire-time T1 (first bit), and the second at wire-time T2
(first bit) and the packets were received by Dst at wire-time dT1+T1 (first bit) and the packets were received by Dst at wire-time dT1+T1
(last bit of the first packet), and at wire-time dT2+T2 (last bit of (last bit of the first packet), and at wire-time dT2+T2 (last bit of
the second packet), and that dT2-dT1=ddT. the second packet), and that dT2-dT1=ddT.
"The type-P-one-way-ipdv from Src to Dst at T1,T2 is undefined" means "The Type-P-one-way-ipdv from Src to Dst at T1,T2 is undefined" means
that Src sent the first bit of a packet at T1 and the first bit of a that Src sent the first bit of a packet at T1 and the first bit of a
second packet at T2 and that Dst did not receive one or both packets. second packet at T2 and that Dst did not receive one or both packets.
5.5. Discussion 5.5. Discussion
Type-P-One-way-ipdv is a metric that makes use of the same Type-P-One-way-ipdv is a metric that makes use of the same
measurement methods provided for delay metrics. measurement methods provided for delay metrics.
The following practical issues have to be considered: The following practical issues have to be considered:
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several multiples of the RTT may as well have been lost.} several multiples of the RTT may as well have been lost.}
+ As with other 'type-P' metrics, the value of the metric may depend + As with other 'type-P' metrics, the value of the metric may depend
on such properties of the packet as protocol,(UDP or TCP) port on such properties of the packet as protocol,(UDP or TCP) port
number, size, and arrangement for special treatment (as with IP number, size, and arrangement for special treatment (as with IP
precedence or with RSVP). precedence or with RSVP).
+ If the packet is duplicated along the path (or paths!) so that + If the packet is duplicated along the path (or paths!) so that
multiple non-corrupt copies arrive at the destination, then the multiple non-corrupt copies arrive at the destination, then the
packet is counted as received, and the first copy to arrive packet is counted as received, and the first copy to arrive
determines the packet's One-Way-Delay. determines the packet's One-way-Delay.
+ If the packet is fragmented and if, for whatever reason, + If the packet is fragmented and if, for whatever reason,
reassembly does not occur, then the packet will be deemed lost. reassembly does not occur, then the packet will be deemed lost.
5.6. Methodologies 5.6. Methodologies
As with other Type-P-* metrics, the detailed methodology will depend As with other Type-P-* metrics, the detailed methodology will depend
on the Type-P (e.g., protocol number, UDP/TCP port number, size, on the Type-P (e.g., protocol number, UDP/TCP port number, size,
precedence). Generally, for a given Type-P, the methodology would precedence). Generally, for a given Type-P, the methodology would
proceed as follows: proceed as follows:
+ The need of synchronized clocks for Src and Dst will be discussed + The need of synchronized clocks for Src and Dst will be discussed
later. Here a methodology is supposed that is based on later. Here a methodology is presented that is based on
synchronized clocks. synchronized clocks.
+ At the Src host, select Src and Dst IP addresses, and form two + At the Src host, select Src and Dst IP addresses, and form two
test packets of Type-P with these addresses. Any 'padding' portion test packets of Type-P with these addresses. Any 'padding' portion
of the packet needed only to make the test packet a given size of the packet needed only to make the test packet a given size
should be filled with randomized bits to avoid a situation in should be filled with randomized bits to avoid a situation in
which the measured delay is lower than it would otherwise be due which the measured delay is lower than it would otherwise be due
to compression techniques along the path. to compression techniques along the path.
+ At the Dst host, arrange to receive the packets. + At the Dst host, arrange to receive the packets.
+ At the Src host, place a timestamp in the first Type-P packet, + At the Src host, place a timestamp in the first Type-P packet,
and send it towards Dst. and send it towards Dst.
+ If the packet arrives within a reasonable period of time, take a + If the packet arrives within a reasonable period of time, take a
timestamp as soon as possible upon the receipt of the packet. By timestamp as soon as possible upon the receipt of the packet. By
subtracting the two timestamps, an estimate of One-Way-Delay can subtracting the two timestamps, an estimate of One-way-Delay can
be computed. be computed.
+ Record this first delay value. + Record this first delay value.
+ At the Src host, place a timestamp in the second Type-P packet, + At the Src host, place a timestamp in the second Type-P packet,
and send it towards Dst. and send it towards Dst.
+ If the packet arrives within a reasonable period of time, take a + If the packet arrives within a reasonable period of time, take a
timestamp as soon as possible upon the receipt of the packet. By timestamp as soon as possible upon the receipt of the packet. By
subtracting the two timestamps, an estimate of One-Way-Delay can subtracting the two timestamps, an estimate of One-way-Delay can
be computed. be computed.
+ By subtracting the second value of One-Way-Delay from the first + By subtracting the second value of One-way-Delay from the first
value the ipdv value of the pair of packets is obtained. value the ipdv value of the pair of packets is obtained.
+ If one or both packets fail to arrive within a reasonable period + If one or both packets fail to arrive within a reasonable period
of time, the ipdv is taken to be undefined. of time, the ipdv is taken to be undefined.
5.7. Errors and Uncertainties 5.7. Errors and Uncertainties
In the singleton metric of ipdv, factors that affect the measurement In the singleton metric of ipdv, factors that affect the measurement
are the same that can affect the One-Way-Delay measurement, even if, are the same that can affect the One-way-Delay measurement, even if,
in this case, the influence is different. in this case, the influence is different.
The Framework document [1] provides general guidance on this point, The Framework document [1] provides general guidance on this point,
but we note here the following specifics related to delay metrics: but we note here the following specifics related to delay metrics:
+ Errors/uncertainties due to uncertainties in the clocks of the Src + Errors/uncertainties due to uncertainties in the clocks of the Src
and Dst hosts. and Dst hosts.
+ Errors/uncertainties due to the difference between 'wire time' and + Errors/uncertainties due to the difference between 'wire time' and
'host time'. 'host time'.
Each of these errors is discussed in more detail in the next Each of these errors is discussed in more detail in the next
paragraphs. paragraphs.
5.7.1. Errors/Uncertainties related to Clocks 5.7.1. Errors/Uncertainties related to Clocks
If, as a first approximation, the error that affects the first If, as a first approximation, the error that affects the first
measurement of One-Way-Delay were the same of the one affecting the measurement of One-way-Delay were the same of the one affecting the
second measurement, they will cancel each other when calculating second measurement, they will cancel each other when calculating
ipdv. The residual error related to clocks is the difference of the ipdv. The residual error related to clocks is the difference of the
errors that are supposed to change from the time T1, at which the errors that are supposed to change from the time T1, at which the
first measurement is performed, to the time T2 at which the second first measurement is performed, to the time T2 at which the second
measure ment is performed. Synchronization, skew, accuracy and measure ment is performed. Synchronization, skew, accuracy and
resolution are here considered with the following notes: resolution are here considered with the following notes:
+ Errors in synchronization between source and destination clocks + Errors in synchronization between source and destination clocks
contribute to errors in both of the delay measurements required contribute to errors in both of the delay measurements required
for calculating ipdv. for calculating ipdv.
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under test have reached the situation of a contribution close to zero under test have reached the situation of a contribution close to zero
to the mean value of the ipdv distribution, it is expected that only to the mean value of the ipdv distribution, it is expected that only
the action of the average skew has modified the measured mean value. the action of the average skew has modified the measured mean value.
It is therefore expected that in one direction that value is equal It is therefore expected that in one direction that value is equal
and opposite to the one measured in the other direction. and opposite to the one measured in the other direction.
This fact offers the possibility of defining a theoretical reference This fact offers the possibility of defining a theoretical reference
measurement duration in the following way: measurement duration in the following way:
The reference duration of a bidirectional ipdv measurement between an The reference duration of a bidirectional ipdv measurement between an
host E and an host W is reached at time Tf such that for each time T host E and an host W is reached at time Tf such that for each time
> Tf the expression ABS(E(ipdv E-W) - E(ipdv W-E))< epsilon, where T > Tf the expression ABS(E(ipdv E-W) - E(ipdv W-E))< epsilon, where
epsilon is what we can consider as zero, is always verified. This is epsilon is what we can consider as zero, is always verified. This is
one, but not the only method for verifying that the mean ipdv value one, but not the only method for verifying that the mean ipdv value
has reached the value of the average relative skew. has reached the value of the average relative skew.
At this point it is possible to evaluate the relative skew. This At this point it is possible to evaluate the relative skew. This
will require the knowledge of the mean value of the intervals between will require the knowledge of the mean value of the intervals between
consecutive packets, that can be calculated over the transmitted consecutive packets, that can be calculated over the transmitted
stream, by using the collected time stamps. stream, by using the collected time stamps.
A bidirectional measurement can be defined not only as twin one-way A bidirectional measurement can be defined not only as twin one-way
independent metrics that take place (nearly) at the same time, but independent metrics that take place (nearly) at the same time, but
also as a two-way metric making use of packets looped back at one also as a two-way metric making use of packets looped back at one
end. This metric, that can be object of further study/Draft, would be end. This metric, that can be object of further study, would be
able to measure also the Round Trip Delay and its variations. able to measure also the Round Trip Delay and its variations.
Problems will anyway arise on the characterization of emission Problems will anyway arise on the characterization of emission
intervals in the backward direction. They would be produced by the intervals in the backward direction. They would be produced by the
combination of the original Poisson arrival process and the effect of combination of the original Poisson arrival process and the effect of
ipdv on the forward direction. It has to be studied if this sequence ipdv on the forward direction. It has to be studied if this sequence
of intervals is still suitable for the measurement. also other of intervals is still suitable for the measurement. also other
possibilities can be envisaged for obtaining a proper backward possibilities can be envisaged for obtaining a proper backward
sequence and still maintain the loopback concept. sequence and still maintain the loopback concept.
9. Relationship to other standards | 9. Relationship to other standards |
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Thanks to Matt Zekauskas from Advanced and Ruediger Geib from | Thanks to Matt Zekauskas from Advanced and Ruediger Geib from |
Deutsche Telekom for discussions relating to the contents of this | Deutsche Telekom for discussions relating to the contents of this |
revised draft. For this additional revision, discussions by e-mail > revised draft. For this additional revision, discussions by e-mail >
with Andy Scherrer were very helpful. with Andy Scherrer were very helpful.
12. References 12. References
[1] V.Paxon, G.Almes, J.Mahdavi, M.Mathis - "Framework for IP [1] V.Paxon, G.Almes, J.Mahdavi, M.Mathis - "Framework for IP
Performance Metrics", RFC 2330 Feb. 1998 Performance Metrics", RFC 2330 Feb. 1998
[2] G.Almes, S.Kalidindi - "A One-Way-Delay Metric for IPPM", RFC [2] G.Almes, S.Kalidindi - "A One-way-Delay Metric for IPPM", RFC
2679, September 1999 2679, September 1999
[3] ITU-T Recommendation I.380 "Internet Protocol Data [3] ITU-T Recommendation I.380 "Internet Protocol Data
Communication Service - IP Packet Transfer and Availability Communication Service - IP Packet Transfer and Availability
Performance Parameters", February 1999 Performance Parameters", February 1999
[4] Demichelis, Carlo - "Packet Delay Variation Comparison between [4] Demichelis, Carlo - "Packet Delay Variation Comparison between
ITU-T and IETF Draft Definitions" March 1999 ITU-T and IETF Draft Definitions" March 1999
[5] ITU-T Recommendation I.356 "B-ISDN ATM Layer Cell Transfer [5] ITU-T Recommendation I.356 "B-ISDN ATM Layer Cell Transfer
skipping to change at page 20, line 13 skipping to change at page 20, line 13
Fax. +39 11 228 5069 Fax. +39 11 228 5069
Philip Chimento <chimento@ctit.utwente.nl> Philip Chimento <chimento@ctit.utwente.nl>
CTIT - Centre for Telematics and Information Technology CTIT - Centre for Telematics and Information Technology
University of Twente University of Twente
Postbox 217 Postbox 217
7500 AE Enschede 7500 AE Enschede
The Netherlands The Netherlands
Phone +31 53 489 4331 Phone +31 53 489 4331
FAX +31 53 489 4524 FAX +31 53 489 4524
Expiration date: April 2000 Expiration date: December 2000
 End of changes. 

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