One document matched: draft-ietf-ippm-ipdv-00.txt
Network Working Group C.Demichelis CSELT
Internet Draft March 1998
expires 16 August 1998
Instantaneous Packet Delay Variation Metric for IPPM
<draft-ietf-ippm-ipdv-00.txt>
1. Status of this Memo
This document is an Internet Draft. Internet Drafts are working doc-
uments of the Internet Engineering Task Force (IETF), its areas, and
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ftp.isi.edu (US West Coast).
This memo provides information for the Internet community. This memo
does not specify an Internet standard of any kind. Distribution of
this memo is unlimited.
2. Abstract
This memo refers to a metric for variation in delay of packets across
Internet paths. The metric is based on statistics of the difference
in One-way Delay of consecutive packets. This particular definition
of variation is called ''Instantaneous Packet Delay Variation (ipdv)''.
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
not synchronized. In the second case it allows an evaluation of the
relative skew. Measurements performed on both directions (Two-ways
measurements) allow a better estimation of clock differences. The
precision that can be obtained is evaluated.
This memo is intended to have, as much as possible, the structure of
the ippm draft on one-way delay metric.
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3. Introduction
This memo defines a metric for variation in delay of packets that go
from one host to another one through an IP path. Since the metric is
related to a variation, different definitions are possible according
to what the variation is measured against.
[Editor's Note: This memo refers to the Draft-ietf "One-way-delay metric
for IPPM" that supposes as known. For sake of readability, some text is
directly taken from that Draft. Text taken without modification is mark-
ed with trailing "TTTTTTT" and ending "EEEEEEE". These marks will be re-
moved in next versions]
3.1. Definition
The Instantaneous Packet Delay Variation of an IP packet, inside a
stream of packets, going from the measurement point MP1 to the measu-
rement point MP2, is the difference of the One-Way Delay of that pac-
ket and the One-Way Delay of preceding packet in the stream.
3.2. Motivation
A number of services that can be supported by IP are sensitive to the
regular delivery of packets and can be disturbed by instantaneous va-
riations in delay, while they are not disturbed by slow variations,
that can last a relatively long time. A specific metric for quick va-
riations is therefore desirable.
In addition, this type of metric is particularly robust with respect
differences and variations of the clocks of the two hosts. This allow
the use of the metric even if the two hosts that support the measure-
ment points are not synchronized. The related precision is comparable
with the one that can be achieved with synchronized clocks. This will
be discussed below.
3.3. General Issues Regarding Time
All what is contained in the paragraph 2.2. of the Draft ippm on one-
way delay metric (2.2. General Issues Regarding Time) applies also in
this case.
In addition, it is here considered that the relative skew of the two
clocks can be decomposed into two parts:
* A fixed one, called in this context "skew", given, for example, by
tolerances in physical dimension of crystals.
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* A variable one, called in this context "drift", given, for example,
by changes in temperature or other conditions of operation.
Both of this components are part of the term "skew" as defined in the
referenced Draft and in the Framework document.
4. Structure of this memo
The metric will be defined as applicable to a stream of packets that
flow from a source host to a destination host (one-way ipdv). The ini
-tial assumption is that source and destination hosts have synchroni-
zed clocks.
The definition of a singleton of one-way ipdv metric is first consi-
dered, and then a definition of samples for ipdv will be given.
Then the case of application to not synchronized hosts will be dis-
cussed, and the precision will be compared with the one of the pre-
vious case.
A bidirectional ipdv metric will be defined, and the methodology for
error corrections. This will not be a two-ways metric, but a "paired"
one-way in opposite directions. Some statistics describing the IP
path behavior will be proposed.
5. A singleton definition of a One-way ipdv metric
This definition makes use of the corresponding definition of type-P-
One-way-delay, that is supposed to be known. This section makes use
of those parts of the One-way-delay Draft that directly apply to the
One-way-ipdv metric, or makes direct references to that Draft.
5.1. Metric name
Type-P-One-way-ipdv
5.2. Metric parameters
+ Scr, the IP address of a host
+ Dst, the IP address of a host
+ T1, a time
+ T2, a time
TTTTTTT
+ Path, the path from Src to Dst; in cases where there is only one
path from Src to Dst, this optional parameter can be omitted.
{Comment: the presence of path is motivated by cases such as with
Merit's NetNow setup, in which a Src on one NAP can reach a Dst on
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another NAP by either of several different backbone networks. Gener-
ally, this optional parameter is useful only when several different
routes are possible from Src to Dst. Using the loose source route IP
option is avoided since it would often artificially worsen the per-
formance observed, and since it might not be supported along some
paths.}
EEEEEEE
5.2. Metric unit
The value of a Type-P-One-way-ipdv is either a real number of seconds
or an undefined number of seconds
5.3. Definition
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-
delay from Src to Dst at T2 [via path] and the value of the type-P-
One-way-delay from Src to Dst at T1 [via path]. 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 sent the first bit of the second packet.
Therefore, for a real number ddT "The type-P-one-way-ipdv from Src to
Dst at T1, T2 [via path] is ddT" means that Src sent two consecutive
packets whose the first at wire-time T1 (first bit), and the second
wire-time T2 (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 the second packet), and that dT2-dT1=ddT.
"The type-P-one-way-ipdv from Src to Dst at T1,T2 [via path] is unde-
fined" means 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.
5.4. Discussion
Type-P-One-way-ipdv is a metric that makes use of the same measure-
ment methods provided for delay metrics.
The following practical issues have to be considered:
+ Being a differential measurement, this metric is less sensitive
to clock synchronization problems. This issue will be more care-
fully examined in section 6. of this memo. It is pointed out
that, if the reciprocal clock conditions change in time, the ac-
curacy of the measurement will depend on the time interval T2-T1
and the amount of possible errors will be discussed below.
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TTTTTTT
+ A given methodology will have to include a way to deter-
mine whether a delay value is infinite or whether it is mere-
ly very large (and the packet is yet to arrive at Dst).
As noted by Mahdavi and Paxson, simple upper bounds (such as the
255 seconds theoretical upper bound on the lifetimes of IP
packets [Postel: RFC 791]) could be used, but good engineering,
including an understanding of packet lifetimes, will be nee-
ded in practice. {Comment: Note that, for many applications of
these metrics, the harm in treating a large delay as infinite
might be zero or very small. A TCP data packet, for example,
that arrives only after several multiples of the RTT may as well
have been lost.}
+ As with other 'type-P' metrics, the value of the metric may de-
pend on such properties of the packet as protocol,(UDP or TCP)
port number, size, and arrangement for special treatment (as
with IP precedence or with RSVP).
+ If the packet is duplicated along the path (or paths!) so that
multiple non-corrupt copies arrive at the destination, then the
packet is counted as received, and the first copy to arrive
determines the packet's one-way delay.
+ If the packet is fragmented and if, for whatever reason, reas-
sembly does not occur, then the packet will be deemed lost.
EEEEEEE
5.5. Methodologies
TTTTTTT
As with other Type-P-* metrics, the detailed methodology will depend
on the Type-P (e.g., protocol number, UDP/TCP port number, size,
precedence).
Generally, for a given Type-P, the methodology would proceed as fol-
lows:
EEEEEEE
+ The need of synchronized clocks for Src and Dst will be discus-
sed later. Here a methodology is supposed that is bases on
synchronized clocks.
TTTTTTT
+ At the Src host, select Src and Dst IP addresses, and form a
test packet of Type-P with these addresses. Any 'padding' por-
tion of the packet needed only to make the test packet a given
size should be filled with randomized bits to avoid a situation
in which the measured delay is lower than it would otherwise
be due to compression techniques along the path.
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+ Optionally, select a specific path and arrange for Src to send
the packet to that path. {Comment: This could be done, for
example, by installing a temporary host-route for Dst in Src's
routing table.}
+ At the Dst host, arrange to receive the packet.
+ At the Src host, place a timestamp in the prepared Type-P pac-
ket, and send it towards Dst [via path].
+ If the packet arrives within a reasonable period of time, take a
timestamp as soon as possible upon the receipt of the packet. By
subtracting the two timestamps, an estimate of one-way delay can
be computed.
EEEEEEE
Record this first delay value.
+ Repeat the procedure with the same parameters and record the se-
cond delay value. By subtracting the second value from the first
the ipdv value is obtained.
+ If one or both packets fail to arrive within a reasonable period
ot time, the ipdv is taken to be undefined.
5.6. Errors and Uncertainties
In the singleton metric of ipdv, factors that affect the measurement
are the same that can affect the one-way delay measurement, even if,
in this case, the influence is different.
TTTTTTT
The Framework document provides general guidance on this point, but
we note here the following specifics related to delay metrics:
+ Errors/uncertainties due to uncertainties in the clocks of the
Src and Dst hosts.
+ Errors/uncertainties due to the difference between 'wire time'
and 'host time'.
Each of these are discussed in more detail below.
EEEEEEE
5.6.1. Errors/Uncertainties related to Clocks
If, as a first approximation, the error that affects the first measu-
rement of one-way delay were the same of the one affecting the second
measurement, they will cancel each other when calculating ipdv. The
residual error related to clocks is the difference of the said 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 measure-
ment is performed. Synchronization, skew, accuracy and resolution are
here considered with the following notes:
+ Errors in synchronization between source and destination clocks
contribute to errors in both of the delay measurements required
for calculating ipdv.
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+ If the synchronization error is Tsync, and it is a linear func-
tion of time, through the skew value, at time T1 the error will
be Tsync1 and at time T2 the error will be Tsync2. The ipdv mea-
surement will be affected by the error Tsync2-Tsync1, depending
from skew and T2-T1. To minimize this error it is possible to
reduce the time interval T2-T1, but this could limit the genera-
lity of the metric. Methods for evaluating the synchronization
error will be discussed below, since they come from a statistic
of a significant sample.
+ As far as accuracy and resolution are concerned, what is noted
in the above referenced Draft on one-way delay at section 3.7.1,
applies also in this case, with the further consideration, about
resolution, that in this case the uncertainty introduced is two
times the one of a single delay measurement.
5.6.2. Errors/uncertainties related to Wire-time vs Host-time
The content of sec. 3.7.2 of the above referenced Draft applies also
in this case, with the following further consideration:
The difference between Host-time and Wire-time can be in general de-
composed into two components, whose one is constant and the other is
variable aroud zero. Only the variable components will produce measu-
rement errors, while the constant one will be cancelled while calcu-
lating ipdv.
6. Definitions for Samples of One-way ipdv
Starting from the definition of the singleton metric of one-way ipdv,
some ways of building a sample of such singletons are here described
that have to be further anlyzed in order to find the best way of con-
sidering all the related problems. In the following, the two packets
needed for a singleton measurement will be called a "pair".
6.1. A "discontinuous" definition
A general definition can be the following:
Given particular binding of the parameters Src, Dst, path, and
Type-P, a sample of values of parameters T1 and T2 is defined.
The means for defining the values of T1 is to select a beginning
time T0, a final time Tf, and an average rate lambda, then
define a pseudo-random Poisson arrival process of rate lambda,
whose values fall between T0 and Tf. The time interval between
successive values of T1 will then average 1/lambda. Another si-
milar, but independent, pseudo-random Poisson arrival process
based on T0', Tf' and lambda', for each T1 value that has been
obtained by the first process, is used for obtaining the inter-
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val T2-T1, that falls between T0' and Tf' with an average of
1/lambda'
This general definition is likely go give problems, if no limits are
considered for the values T0, Tf, T0', Tf'. For example, the emission
time of the first packet of a pair, could fall before the emission
time of the second packet of the preceding pair. Probably this could
be acceptable (provided that there are means to recognize pairs -e.g.
use of sequence numbers-), but the concept itself of ipdv would be,at
least, slightly changed. A way for avoiding problems can be to give
some rules on the values T0, Tf, lambda, T0', Tf', lambda', without
changing the meaning of the metric. For example it can be required
that Tf'<T0 in order to assure that pairs of packets consist of two
consecutive packets.
6.2. A "continuous" definition
A continuous stream of test packets can be supposed, where the second
packet of a pair is, at the same time, the first packet of the next
pair. Therefore the preceding definition becomes:
+ Given particular binding of the parameters Src, Dst, path, and
Type-P, a sample of values of parameter T1 is defined.
The means for defining the values of T1 is to select a beginning
time T0, a final time Tf, and an average rate lambda, then
define a pseudo-random Poisson arrival process of rate lambda,
whose values fall between T0 and Tf. The time interval between
successive values of T1 will then average 1/lambda. From the
second value on, T1 value of the pair n coincides with T2 of the
pair n-1, and the first packet of pair n coincides with the se-
cond packet of the pair n-1.
For the moment, in the following, this second definition will be con-
sidered. Further refinement is required and is for further discussion
6.3. Metric name
Type-P-One-way-ipdv-stream
6.4. Parameters
+ Src, the IP address of a host
+ Dst, the IP address of a host
+ Path, the path* from Src to Dst; in cases where there is only
one path from Src to Dst, this optional parameter can be omitted
+ T0, a time
+ Tf, a time
+ lambda, a rate in reciprocal seconds
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6.5. Metric Units:
A sequence of triads whose elements are:
+ T, a time
+ Ti, a time interval.
+ dT a real number or an undefined number of seconds
6.6. Definition
A pseudo-random Poisson process is defined such that ir begins at or
before T0, with average arrival rate lambda, and ends at or after Tf.
Those time values Ti greater than or equal to T0 and less than or
equal to Tf are then selected. Starting from time T, at each time Ti
of this process a value of Type-P-One-way-ipdv is obtained, and the
time T becomes T = T+Ti. The value of the sample is the sequence made
up of the resulting <time, time interval, ipdv> triad.
6.7. Discussion
TTTTTTT
Note first that, since a pseudo-random number sequence is employed,
the sequence of times, and hence the value of the sample, is not
fully specified. Pseudo-random number generators of good quality
will be needed to achieve the desired qualities.
The sample is defined in terms of a Poisson process both to avoid the
effects of self-synchronization and also capture a sample that is
statistically as unbiased as possible. {Comment: there is, of
course, no claim that real Internet traffic arrives according to a
Poisson arrival process.}
EEEEEEE
6.8. Methodology
Since packets can be lost or duplicated or can arrive in a different
order with respect the one of emission, in order to recognize the
pairs of test packets, they should be marked with a Sequence Number
or make use of any other tool suitable to the scope. For duplicated
packets only the first received copy should be considered. If a pac-
ket is lost, two values of ipdv will be undefined, since each packet,
in the supposed "continuous" definition, is common to two pairs.
Steps for measurement can be the following:
+ Starting from a given time T, Src generates a test packet as for
a singleton metrics, adding also a Sequence Number, and sorts a
Ti interval to determine the time at wich the next packet has to
be sent.
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+ On reception of the packet, Dst verifies the Sequence Number SN,
and records SN and Tx timestamp that are contained in the packet
and the Rx timestamp.
+ if the packet is not the first received and the SN is correct,
ipdv is computed and Ti is recorded. Then Dst records SN, Tx
and Rx timestamps as "old" values.
6.9. Errors and uncertainties
The same considerations apply that have been made about the single-
ton metric. An additional error can be introduced by the pseudo-ran-
dom Poisson process as focushed in the above referenced Draft.
Further considerations will be made in section 7.
6.10 Some statistics for One-way-ipdv
Some statistics are here considered, that can provide useful informa-
tion in analyzing the behavior of the packets flowing from Src to Dst
These statistics are given having in mind a practical use of them.
Other statistics can be defined if needed.
6.10.1. Type-P-One-way-ipdv-inverse-percentile
Given a Type-P-One-way-ipdv-Stream and a time threshold, that can be
either positive or negative, the fraction of all the dT values in the
Stream less than or equal to the threshold, if the threshold is po-
sitive, or greater or equal to the threshold if the threshold is ne-
gative.
For many real-time services that require a regular delivery of the
packets, this statistics can give the amount of packets beyond accep-
table limits.
6.10.2 Type-P-One-way-ipdv-standard-deviation
Given a Type-P-One-way-ipdv-Stream, the distribution of ipdv values
is considered and the Standard Deviation can be calculated as an
indication of regularity of delivery. For practical purposes it can
useful to define a total standard deviation, computed over the com-
plete set of value, and a standard deviation computed over the sub-
set of those values that do not exceede given positive and negative
thresholds. This allows a more accurate description of the performan-
ce experienced by packets.
6.10.3 Type-P-One-way-ipdv-average
This statistic should tend to a value of ZERO for a number of ipdv
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values that tend to infinite. The behavior of Type-P-One-way-ipdv-
average is an issue for the next section 7.
7. Discussion on clock synchronization
This section gives some considerations about the need of having syn-
chronized clocks at Src and Dst. These considerations are given as a
basis for discussion, they require further investigation. We start
from the analysis of the mean value of the ipdv distribution related
to a "continuous" sample.
7.1. Mean value of ipdv distribution.
If D(i) is the delay of packet "i", and ipdv(i) is the i-th value of
ipdv in the distribution of a sample of "n" values, collected with
the described methodology, we can write:
ipdv(1) = D1 - D0
..........
ipdv(i) = D(i) - D(i-1)
..........
ipdv(n) = D(n) - D(n-1)
The mean value of ipdv distribution will result in
E(ipdv) = (D(n) - D(0))/n
If an actual measurement is performed, that lasts a period of time
long enough to contain a number "n" sufficiently large and, supposing
synchronized clocks, such that the network conditions (traffic) allow
to find a D(n) not too diferent from D(0), e.g. a time of 24 hours,
E(ipdv) will tend to zero.
7.2. Effects of a varying traffic
If the mean values of delay D are changing inside a given period of
time, for example they are increasing due to an increment of traffic,
we can consider, as a first approximation, the ipdv values as decom-
posed into two components, one being instantaneous and another one
as having a constant rate dD and corresponding to the increment "per
interval" of the mean value of D. The mean value of the distribution
will be shifted of the value dD corresponding to the mean value of
the interval between test packets. When the conditions will come back
to the initial ones, the distribution will resume a mean value around
zero. At any time the distribution will correctly describe the
behavior of the packet flow.
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7.3. Effects of syncronization errors
We refer here to the two components that can generate this type of
errors that are the relative "skew" and "drift" of the Src and Dst
clocks. It is first of all noted that the variable component "drift"
is physically limited and its effects can be interpreted by saying
that the total skew of the two clocks can vary, ranging from a min
to a max value in the time. This type of variation takes place very
slowly being most connected to variations in temperature.
We suppose to perform a measurement between a Src and a Dst that have
a reciprocal, initial skew of "ts1" and a reciprocal drift such that,
after the time T the total skew is "ts2". It is not here a limitation
to consider that at the beginning of time T the two clocks indicate
the same time T0. In order to analyze the effects produced by this
situation we suppose that packets are transferred, from Src to Dst,
with a consatant delay D. In this conditions the measured ipdv should
always be zero, and what is actually measured is the error.
An ipdv value is measured at the beginning of time T with two packets
having an interval of Ti(1).Another ipdv value is measured at the end
of T with two packets having a time interval Ti(2).
On our purposes other errors (like wire-time vs host-time) are not
considered since they are not relevant in this analysis.
It is then possible to calculate the values of the Tx and Rx time-
stamps as they are seen by the two clocks, and the related values of
the two ipdv values.
The first ipdv value will be: ipdv1 = ts1*Ti(1) + ((ts2-ts1)/T)*Ti(1)
The second ipdv value will be: ipdv2 = ts2*Ti(2) +((ts2-ts1)/T)*Ti(2)
The error is given by the amount of variation during the time inter-
val Ti(i) between the two packets of the pair, and a second orded
term due to the variation of that variation in the same interval.
7.4. Related precision
This means that:
1) + If the skew is constant and is = ts all the ipdv(i) values are
increased by the quantity Ti(i)*ts with respect the actual value.
2) + Considering the total skew as subdivided into a fixed part and a
variable part (skew and drift),respectively, ts and + or - td, and
a minimum time T in which the drift can go from -td to +td or vice
-versa, each ipdv(i) value will be increased of the fixed quantity
Ti(i)*ts plus or minus, as a maximum, the quantity 2*td*Ti(i)/T
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3) + If the duration of the measurement is such that it is possible
to consider that the effect of the items at points 7.1 and 7.2,
and the effect of the drift are negligeables (related average ten-
ding to zero), the mean value of the ipdv distribution will have
the value of the skew multiplied by the mean value of the emission
interval.
4) + We observe that the displacemet due to the skew does not change
the shape of the distribution, and, for example the Standard Devi-
ation remains the same. What introduces a distortion is the effect
of the drift, even if the mean value of this effect is zero at the
end of the measurement. This is, anyway, a "second order" effect.
If, for example, a drift of 30 parts per million (ppm) takes place
along a time of 4 hours, and the used Ti(i) interval ranges from
200 ms to 1200 ms, with an average of 700 ms, the maximum error on
ipdv(i) values will be in the order of:
Ti(i)*td/(4*3600)= 2.25 E -9 seconds
8. Definition for a bidirectional ipdv metric
We now consider that the action of the skew on one direction is the
same, with opposite sign, of the action on the other direction. The
idea of performing at the same time two independent measurements in
the two directions is suggested by this fact.
If, after a long measurement, the variable conditions of the system
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
the fixed action of the skew has modified the measured mean value. It
is therefore expected that on one direction that value is equal and
opposite to the one measured in the other direction.
A bidirectional measurement can be defined not only as twin one-way
independent metrics that take place (nearly) at the same time, but
also as a two-ways metric making use of packets looped back at one
end. This metric, that can be object of further study/Draft, would be
able to measure also the Round Trip Delay and its variations.
9. References
V.Paxon, G.Almes, J.Mahdavi, M.Mathis - "Framework for IP Performance
Metrics", Internet Draft <draft-ietf-ippm-framework-01.txt> Feb. 1998
G.Almes, S.Kalidindi - "A One-way Delay Metric for IPPM", Internet
Draft <draft-ietf-ippm-delay-01.txt> Nov. 1997
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10. Author's Address
Carlo Demichelis <carlo.demichelis@cselt.it>
CSELT - Centro Studi E Laburatori Telecomunicazioni S.p.A
Via G. Reiss Romoli 274
10148 - TORINO
Italy
Phone +39 11 228 5057
Fax. +39 11 228 5069
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