draft-ietf-ippm-ipdv-00.txt | draft-ietf-ippm-ipdv-01.txt | |||
---|---|---|---|---|

Network Working Group C.Demichelis CSELT | Network Working Group C.Demichelis CSELT | |||

expires 16 August 1998 | expires 16 January 1999 | |||

Instantaneous Packet Delay Variation Metric for IPPM | Instantaneous Packet Delay Variation Metric for IPPM | |||

<draft-ietf-ippm-ipdv-01.txt> | ||||

<draft-ietf-ippm-ipdv-00.txt> | ||||

1. Status of this Memo | 1. Status of this Memo | |||

This document is an Internet Draft. Internet Drafts are working doc- | This document is an Internet Draft. Internet Drafts are working doc- | |||

uments of the Internet Engineering Task Force (IETF), its areas, and | uments of the Internet Engineering Task Force (IETF), its areas, and | |||

its working groups. Note that other groups may also distribute work- | its working groups. Note that other groups may also distribute work- | |||

ing documents as Internet Drafts. | ing documents as Internet Drafts. | |||

Internet Drafts are draft documents valid for a maximum of six | Internet Drafts are draft documents valid for a maximum of six | |||

months, and may be updated, replaced, or obsoleted by other documents | months, and may be updated, replaced, or obsoleted by other documents | |||

at any time. It is inappropriate to use Internet Drafts as reference | at any time. It is inappropriate to use Internet Drafts as reference | |||

material or to cite them other than as ``work in progress''. | material or to cite them other than as ``work in progress''. | |||

To learn the current status of any Internet Draft, please check the | To learn the current status of any Internet Draft, please check the | |||

``1id-abstracts.txt'' listing contained in the Internet Drafts shadow | ``1id-abstracts.txt'' listing contained in the Internet Drafts shadow | |||

directories on ftp.is.co.za (Africa), nic.nordu.net (Europe), | directories on ftp.is.co.za (Africa), nic.nordu.net (Europe), | |||

munnari.oz.au (Pacific Rim), ftp.ietf.org (US East Coast), or | ||||

munnari.oz.au (Pacific Rim), ds.internic.net (US East Coast), or | ||||

ftp.isi.edu (US West Coast). | ftp.isi.edu (US West Coast). | |||

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 | |||

relative skew. Measurements performed on both directions (Two-ways | relative skew. Measurements performed on both directions (Two-ways | |||

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. | |||

This memo is intended to have, as much as possible, the structure of | This memo is intended to have, as much as possible, the structure of | |||

the ippm draft on one-way delay metric. | the ippm draft on one-way delay metric. | |||

I-D Ipdv Metric March 1998 | I-D Ipdv Metric July 1998 | |||

3. Introduction | 3. Introduction | |||

This memo defines a metric for variation in delay of packets that go | This memo refers to the Draft-ietf "One-way-delay metric for IPPM" that | |||

supposes as known. Part of the text in this memo is directly taken from | ||||

that Draft. | ||||

This memo defines a metric for variation in delay of packets that flow | ||||

from one host to another one through an IP path. Since the metric is | from one host to another one through an IP path. Since the metric is | |||

related to a variation, different definitions are possible according | related to a variation, different definitions are possible according | |||

to what the variation is measured against. | 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 | 3.1. Definition | |||

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 measu- | 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 packet | ||||

rement point MP2, is the difference of the One-Way Delay of that pac- | and the One-Way Delay of preceding packet in the stream. | |||

ket and the One-Way Delay of preceding packet in the 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 va- | regular delivery of packets and can be disturbed by instantaneous va- | |||

riations in delay, while they are not disturbed by slow variations, | riations in delay, while they are not disturbed by slow variations, | |||

that can last a relatively long time. A specific metric for quick va- | that can last a relatively long time. A specific metric for quick va- | |||

riations is therefore desirable. Metrics that can be derived from the | ||||

riations is therefore desirable. | analysis of statistics of ipdv can also be used for buffer | |||

dimensioning, but this memo is not intended in that sense. The scope | ||||

is to provide a way for measurement of the quality delivered by a | ||||

path. | ||||

In addition, this type of metric is particularly robust with respect | In addition, this type of metric is particularly robust with respect | |||

differences and variations of the clocks of the two hosts. This allow | 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- | the use of the metric even if the two hosts that support the measure- | |||

ment points are not synchronized. The related precision is comparable | ment points are not synchronized. The related precision is comparable | |||

with the one that can be achieved with synchronized clocks. This will | with the one that can be achieved with synchronized clocks. This will | |||

be discussed below. | be discussed below. | |||

3.3. General Issues Regarding Time | 3.3. General Issues Regarding Time | |||

All what is contained in the paragraph 2.2. of the Draft ippm on one- | 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 | way delay metric (2.2. General Issues Regarding Time) applies also in | |||

this case. | this case. | |||

In addition, it is here considered that the relative skew of the two | In addition, it is here considered that the relative skew of the two | |||

clocks can be decomposed into two parts: | clocks can be decomposed into two parts: | |||

* A fixed one, called in this context "skew", given, for example, by | * A fixed one, called in this context "skew", given, for example, by | |||

tolerances in physical dimension of crystals. | tolerances in physical dimension of crystals. | |||

I-D Ipdv Metric March 1998 | I-D Ipdv Metric July 1998 | |||

* A variable one, called in this context "drift", given, for example, | * A variable one, called in this context "drift", given, for example, | |||

by changes in temperature or other conditions of operation. | by changes in temperature or other conditions of operation. | |||

Both of this components are part of the term "skew" as defined in the | Both of this components are part of the term "skew" as defined in the | |||

referenced Draft and in the Framework document. | referenced Draft and in the Framework document. | |||

4. Structure of this memo | 4. Structure of this memo | |||

The metric will be defined as applicable to a stream of packets that | 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 | flow from a source host to a destination host (one-way ipdv). The ini | |||

-tial assumption is that source and destination hosts have synchronized | ||||

-tial assumption is that source and destination hosts have synchroni- | clocks. | |||

zed clocks. | ||||

The definition of a singleton of one-way ipdv metric is first consi- | 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. | dered, and then a definition of samples for ipdv will be given. | |||

Then the case of application to not synchronized hosts will be dis- | Then the case of application to not synchronized hosts will be dis- | |||

cussed, and the precision will be compared with the one of the previous | ||||

cussed, and the precision will be compared with the one of the pre- | case. | |||

vious case. | ||||

A bidirectional ipdv metric will be defined, and the methodology for | A bidirectional ipdv metric will be defined, and the methodology for | |||

error corrections. This will not be a two-ways metric, but a "paired" | error corrections. This will not be a two-ways metric, but a "paired" | |||

one-way in opposite directions. Some statistics describing the IP | one-way in opposite directions. Some statistics describing the IP | |||

path's behavior will be proposed. | ||||

path behavior will be proposed. | ||||

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, that is supposed to be known. This section makes use | 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 | 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. | One-way-ipdv 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 | |||

+ Scr, the IP address of a host | + Scr, the IP address of a host | |||

+ Dst, the IP address of a host | + Dst, the IP address of a host | |||

+ T1, a time | + T1, a time | |||

+ T2, a time. It is explicitly noted that also the difference T2-T1 | ||||

+ T2, a time | is a parameter of the measurement though this is already implicit, | |||

since the times T1 and T2 exactly define the time conditions in which | ||||

TTTTTTT | the measurement takes place. | |||

+ Path, the path from Src to Dst; in cases where there is only one | + 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. | I-D Ipdv Metric July 1998 | |||

path from Src to Dst, this optional parameter can be omitted. | ||||

{Comment: the presence of path is motivated by cases such as with | {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 | Merit's NetNow setup, in which a Src on one NAP can reach a Dst on | |||

I-D Ipdv Metric March 1998 | ||||

another NAP by either of several different backbone networks. Gener- | another NAP by either of several different backbone networks. Gener- | |||

ally, this optional parameter is useful only when several different | ally, this optional parameter is useful only when several different | |||

routes are possible from Src to Dst. Using the loose source route IP | routes are possible from Src to Dst. Using the loose source route IP | |||

option is avoided since it would often artificially worsen the per- | option is avoided since it would often artificially worsen the per- | |||

formance observed, and since it might not be supported along some | formance observed, and since it might not be supported along some | |||

paths.} | paths.} | |||

EEEEEEE | ||||

5.2. Metric unit | 5.2. 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. | ||||

or an undefined number of seconds | ||||

5.3. Definition | 5.3. 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 [via path] and the value of the type-P- | 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 | 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 | 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. This | ||||

metric is therefore ideally derived from the One-Way-Delay metric. | ||||

wire-time at which Src sent the first bit of the second packet. | NOTE: The requirement of "consecutive" packets is not essential. The | |||

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 | ||||

T2 are such to describe the investigated characteristics. These times | ||||

will be better defined later. | ||||

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 [via path] is ddT" means that Src sent two consecutive | 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 | 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 | wire-time T2 (first bit) and the packets were received by Dst at wire | |||

-time dT1+T1 (last bit of the first packet), and, respectively, at | ||||

-time dT1+T1 (last bit of the first packet), and at wire-time dT2+T2 | wire-time dT2+T2 (last bit of the second packet), and that dT2-dT1=ddT. | |||

(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- | "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 | 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 | first bit of a second packet at T2 and that Dst did not receive one | |||

or both packets. | or both packets. | |||

5.4. Discussion | 5.4. Discussion | |||

Type-P-One-way-ipdv is a metric that makes use of the same measure- | Type-P-One-way-ipdv is a metric that makes use of the same measurement | |||

methods provided for delay metrics. | ||||

ment methods provided for delay metrics. | I-D Ipdv Metric July 1998 | |||

The following practical issues have to be considered: | The following practical issues have to be considered: | |||

+ Being a differential measurement, this metric is less sensitive | + Being a differential measurement, this metric is less sensitive | |||

to clock synchronization problems. This issue will be more care- | to clock synchronization problems. This issue will be more care- | |||

fully examined in section 6. of this memo. It is pointed out | fully examined in section 6. of this memo. It is pointed out | |||

that, if the reciprocal clock conditions change in time, the ac- | that, if the reciprocal clock conditions change in time, the ac- | |||

curacy of the measurement will depend on the time interval T2-T1 | curacy of the measurement will depend on the time interval T2-T1 | |||

and the amount of possible errors will be discussed below. | and the amount of possible errors will be discussed below. | |||

I-D Ipdv Metric March 1998 | ||||

TTTTTTT | ||||

+ A given methodology will have to include a way to deter- | + A given methodology will have to include a way to deter- | |||

mine whether a delay value is infinite or whether it is mere- | mine whether a delay value is infinite or whether it is mere- | |||

ly very large (and the packet is yet to arrive at Dst). | ly very large (and the packet is yet to arrive at Dst). | |||

As noted by Mahdavi and Paxson, simple upper bounds (such as the | As noted by Mahdavi and Paxson, simple upper bounds (such as the | |||

255 seconds theoretical upper bound on the lifetimes of IP | 255 seconds theoretical upper bound on the lifetimes of IP | |||

packets [Postel: RFC 791]) could be used, but good engineering, | packets [Postel: RFC 791]) could be used, but good engineering, | |||

including an understanding of packet lifetimes, will be nee- | including an understanding of packet lifetimes, will be nee- | |||

ded in practice. {Comment: Note that, for many applications of | ded in practice. {Comment: Note that, for many applications of | |||

these metrics, the harm in treating a large delay as infinite | these metrics, the harm in treating a large delay as infinite | |||

might be zero or very small. A TCP data packet, for example, | might be zero or very small. A TCP data packet, for example, | |||

that arrives only after several multiples of the RTT may as well | that arrives only after several multiples of the RTT may as well | |||

have been lost.} | have been lost.} | |||

+ Usually a path is such that if the first packet is largely delayed, | ||||

it can "stop" the second packet of the pair and vary its delay. | ||||

This is not a problem for the definition since is, in any case, | ||||

part of the description of the path's behavior. | ||||

+ As with other 'type-P' metrics, the value of the metric may de- | + 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) | pend on such properties of the packet as protocol,(UDP or TCP) | |||

port number, size, and arrangement for special treatment (as | port number, size, and arrangement for special treatment (as | |||

with IP precedence or with RSVP). | with IP 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, reas- | + If the packet is fragmented and if, for whatever reason, reas- | |||

sembly does not occur, then the packet will be deemed lost. | sembly does not occur, then the packet will be deemed lost. | |||

EEEEEEE | ||||

5.5. Methodologies | 5.5. Methodologies | |||

TTTTTTT | ||||

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). | precedence). | |||

Generally, for a given Type-P, the methodology would proceed as fol- | Generally, for a given Type-P, the methodology would proceed as fol- | |||

lows: | lows: | |||

EEEEEEE | ||||

+ The need of synchronized clocks for Src and Dst will be discus- | + The need of synchronized clocks for Src and Dst will be discus- | |||

sed later. Here a methodology is supposed that is based on | ||||

sed later. Here a methodology is supposed that is bases on | ||||

synchronized clocks. | synchronized clocks. | |||

TTTTTTT | I-D Ipdv Metric July 1998 | |||

+ At the Src host, select Src and Dst IP addresses, and form a | ||||

test packet of Type-P with these addresses. Any 'padding' por- | ||||

+ At the Src host, select Src and Dst IP addresses, and form two | ||||

test packets of Type-P with these addresses. Any 'padding' por- | ||||

tion of the packet needed only to make the test packet a given | tion of the packet needed only to make the test packet a given | |||

size should be filled with randomized bits to avoid a situation | size should be filled with randomized bits to avoid a situation | |||

in which the measured delay is lower than it would otherwise | in which the measured delay is lower than it would otherwise | |||

be due to compression techniques along the path. | be due to compression techniques along the path. | |||

I-D Ipdv Metric March 1998 | ||||

+ Optionally, select a specific path and arrange for Src to send | + Optionally, select a specific path and arrange for Src to send | |||

the packets to that path. {Comment: This could be done, for | ||||

the packet to that path. {Comment: This could be done, for | ||||

example, by installing a temporary host-route for Dst in Src's | example, by installing a temporary host-route for Dst in Src's | |||

routing table.} | routing table.} | |||

+ At the Dst host, arrange to receive the packets. | ||||

+ At the Dst host, arrange to receive the packet. | + At the Src host, place a timestamp in the prepared first | |||

Type-P packet, and send it towards Dst [via path]. | ||||

+ 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 | + 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. | ||||

EEEEEEE | + At the Src host, place a timestamp in the prepared second | |||

Type-P packet, and send it towards Dst [via path]. | ||||

Record this first delay value. | + If the packet arrives within a reasonable period of time, take a | |||

timestamp as soon as possible upon the receipt of the packet. By | ||||

+ Repeat the procedure with the same parameters and record the se- | subtracting the two timestamps, an estimate of one-way delay can | |||

be computed. | ||||

cond delay value. By subtracting the second value 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. | ||||

the ipdv value 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. | ||||

ot time, the ipdv is taken to be undefined. | ||||

5.6. Errors and Uncertainties | 5.6. 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. | |||

TTTTTTT | ||||

The Framework document provides general guidance on this point, but | The Framework document provides general guidance on this point, but | |||

we note here the following specifics related to delay metrics: | we note here the following specifics related to delay metrics: | |||

+ Errors/uncertainties due to uncertainties in the clocks of the | + Errors/uncertainties due to uncertainties in the clocks of the | |||

Src and Dst hosts. | Src and Dst hosts. | |||

+ Errors/uncertainties due to the difference between 'wire time' | + Errors/uncertainties due to the difference between 'wire time' | |||

and 'host time'. | and 'host time'. | |||

Each of these are discussed in more detail below. | Each of these are discussed in more detail below. | |||

EEEEEEE | ||||

5.6.1. Errors/Uncertainties related to Clocks | 5.6.1. Errors/Uncertainties related to Clocks | |||

If, as a first approximation, the error that affects the first measu- | 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 | rement of one-way delay were the same of the one affecting the second | |||

measurement, they will cancel each other when calculating ipdv. The | I-D Ipdv Metric July 1998 | |||

measurement, they will cancel each other when calculating ipdv. The | ||||

residual error related to clocks is the difference of the said errors | 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 | 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- | measurement is performed, to the time T2 at which the second measure- | |||

ment is performed. Synchronization, skew, accuracy and resolution are | ment is performed. Synchronization, skew, accuracy and resolution are | |||

here considered with the following notes: | 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. | |||

I-D Ipdv Metric March 1998 | ||||

+ If the synchronization error is Tsync, and it is a linear func- | + 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 | 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- | be Tsync1 and at time T2 the error will be Tsync2. The ipdv mea- | |||

surement will be affected by the error Tsync2-Tsync1, depending | surement will be affected by the error Tsync2-Tsync1, depending | |||

from skew and T2-T1. To minimize this error it is possible to | 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- | reduce the time interval T2-T1, but this could limit the genera- | |||

lity of the metric. Methods for evaluating the synchronization | lity of the metric. Methods for evaluating the synchronization | |||

error will be discussed below, since they come from a statistic | error will be discussed below, since they come from a statistic | |||

over a significant sample. | ||||

of a significant sample. | ||||

+ As far as accuracy and resolution are concerned, what is noted | + 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, | in the above referenced Draft on one-way delay at section 3.7.1, | |||

applies also in this case, with the further consideration, about | applies also in this case, with the further consideration, about | |||

resolution, that in this case the uncertainty introduced is two | resolution, that in this case the uncertainty introduced is two | |||

times the one of a single delay measurement. | times the one of a single delay measurement. | |||

5.6.2. Errors/uncertainties related to Wire-time vs Host-time | 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 | The content of sec. 3.7.2 of the above referenced Draft applies also | |||

in this case, with the following further consideration: | in this case, with the following further consideration: | |||

The difference between Host-time and Wire-time can be in general de- | 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 | composed into two components, whose one is constant and the other is | |||

variable around zero. Only the variable components will produce measu- | ||||

variable aroud zero. Only the variable components will produce measu- | rement errors, while the constant one will be canceled while calcu- | |||

rement errors, while the constant one will be cancelled while calcu- | ||||

lating ipdv. | lating ipdv. | |||

6. Definitions for Samples of One-way ipdv | 6. Definitions for Samples of One-way ipdv | |||

Starting from the definition of the singleton metric 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 | some ways of building a sample of such singletons are here described | |||

that have to be further analyzed in order to find the best way of con- | ||||

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 | sidering all the related problems. In the following, the two packets | |||

needed for a singleton measurement will be called a "pair". | needed for a singleton measurement will be called a "pair". | |||

I-D Ipdv Metric July 1998 | ||||

6.1. A "discontinuous" definition | 6.1. A "discontinuous" definition | |||

A general definition can be the following: | A general definition can be the following: | |||

Given particular binding of the parameters Src, Dst, path, and | Given particular binding of the parameters Src, Dst, path, and | |||

Type-P, a sample of values of parameters T1 and T2 is defined. | 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 | 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 | time T0, a final time Tf, and an average rate lambda, then | |||

define a pseudo-random Poisson arrival process of rate lambda, | define a pseudo-random Poisson arrival process of rate lambda, | |||

whose values fall between T0 and Tf. The time interval between | whose values fall between T0 and Tf. The time interval between | |||

successive values of T1 will then average 1/lambda. Another si- | successive values of T1 will then average 1/lambda. Another si- | |||

milar, but independent, pseudo-random Poisson arrival process | milar, but independent, pseudo-random Poisson arrival process | |||

based on T0', Tf' and lambda', will produce a series of t' | ||||

values. The time interval between successive t' values will then | ||||

average 1/lambda'. For each T1 value that has been obtained | ||||

by the first process, it is then possible to calculate the | ||||

successive T2 values as the successive T1 values plus the | ||||

successive intervals of t'. | ||||

based on T0', Tf' and lambda', for each T1 value that has been | The result is shown in figure 1. | |||

obtained by the first process, is used for obtaining the inter- | ||||

I-D Ipdv Metric March 1998 | ||||

val T2-T1, that falls between T0' and Tf' with an average of | ||||

1/lambda' | |<- average interval 1/lambda ->| | |||

| | | ||||

|<- av.int. | |<- av.int. | | ||||

|1/lambda'->| | 1/lambda'->| | ||||

_____|___________|___________________|_____________|________ | ||||

pair i pair i+1 | ||||

Figure 1 | ||||

This general definition is likely go give problems, if no limits are | This general definition is likely go give problems, if no limits are | |||

considered for the obtained values. For example, the emission | ||||

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 first packet of a pair, could fall before the emission | |||

time of the second packet of the preceding pair. Probably this could | time of the second packet of the preceding pair. Probably this could | |||

be acceptable (provided that there are means to recognize pairs -e.g. | 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 | use of sequence numbers-), but the concept itself of ipdv would be,at | |||

least, slightly changed. A way for avoiding this type of philosophical | ||||

least, slightly changed. A way for avoiding problems can be to give | problems can be to give some rules on the values T0, Tf, lambda, | |||

T0', Tf', lambda', without changing the meaning of the metric. | ||||

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 | 6.2. A "continuous" definition | |||

A way to naturally avoid the previous problem is to adopt the following | ||||

definition. | ||||

A continuous stream of test packets can be supposed, where the second | 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 | packet of a pair is, at the same time, the first packet of the next | |||

pair. Therefore the preceding definition becomes: | pair. Therefore the preceding definition becomes: | |||

+ Given particular binding of the parameters Src, Dst, path, and | I-D Ipdv Metric July 1998 | |||

+ Given particular binding of the parameters Src, Dst, path, and | ||||

Type-P, a sample of values of parameter T1 is defined. | Type-P, a sample of values of parameter T1 is defined. | |||

The means for defining the values of T1 is to select a beginning | 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 | time T0, a final time Tf, and an average rate lambda, then | |||

define a pseudo-random Poisson arrival process of rate lambda, | define a pseudo-random Poisson arrival process of rate lambda, | |||

whose values fall between T0 and Tf. The time interval between | whose values fall between T0 and Tf. The time interval between | |||

successive values of T1 will then average 1/lambda. From the | 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 | 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- | pair n-1, and the first packet of pair n coincides with the se- | |||

cond packet of the pair n-1. | cond packet of the pair n-1. | |||

For the moment, in the following, this second definition will be con- | For the moment, in the following, this second definition will be con- | |||

sidered. Further refinement is required and is for further discussion. | ||||

sidered. Further refinement is required and is for further discussion | ||||

6.3. Metric name | 6.3. Metric name | |||

Type-P-One-way-ipdv-stream | Type-P-One-way-ipdv-stream | |||

6.4. Parameters | 6.4. Parameters | |||

+ Src, the IP address of a host | + Src, the IP address of a host | |||

+ Dst, 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 | + 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 | one path from Src to Dst, this optional parameter can be omitted | |||

+ T0, a time | + T0, a time | |||

+ Tf, a time | + Tf, a time | |||

+ lambda, a rate in reciprocal seconds | + lambda, a rate in reciprocal seconds | |||

I-D Ipdv Metric March 1998 | ||||

6.5. Metric Units: | 6.5. Metric Units: | |||

A sequence of triads whose elements are: | A sequence of triads whose elements are: | |||

+ T, a time | + T, a time | |||

+ Ti, a time interval. | + Ti, a time interval. | |||

+ dT a real number or an undefined number of seconds | + dT a real number or an undefined number of seconds | |||

6.6. Definition | 6.6. Definition | |||

A pseudo-random Poisson process is defined such that ir begins at or | A pseudo-random Poisson process is defined such that it begins at or | |||

before T0, with average arrival rate lambda, and ends at or after Tf. | 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 | 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 pair of | ||||

times T(i), T(i+1)of this process a value of Type-P-One-way-ipdv is | ||||

obtained. The value of the sample is the sequence made up of the | ||||

resulting <time, time interval, ipdv> triad, where the time interval | ||||

is given by T(i+1)-T(i). Each obtained time T(i), excluding the first | ||||

and the last, is therefore at the same time the the second time of | ||||

pair i and the first time of pair i+1. The result is shown in figure 2 | ||||

equal to Tf are then selected. Starting from time T, at each time Ti | |T(i-2) |T(i-1) |T(i) |T(i+1) | |||

_____|__________|___________________|__________|________ | ||||

of this process a value of Type-P-One-way-ipdv is obtained, and the | pair i-1 pair i pair i+1 | |||

Figure 2 | ||||

time T becomes T = T+Ti. The value of the sample is the sequence made | ||||

up of the resulting <time, time interval, ipdv> triad. | I-D Ipdv Metric July 1998 | |||

6.7. Discussion | 6.7. Discussion | |||

TTTTTTT | ||||

Note first that, since a pseudo-random number sequence is employed, | Note first that, since a pseudo-random number sequence is employed, | |||

the sequence of times, and hence the value of the sample, is not | the sequence of times, and hence the value of the sample, is not | |||

fully specified. Pseudo-random number generators of good quality | fully specified. Pseudo-random number generators of good quality | |||

will be needed to achieve the desired qualities. | will be needed to achieve the desired qualities. | |||

The sample is defined in terms of a Poisson process both to avoid the | 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 | effects of self-synchronization and also capture a sample that is | |||

statistically as unbiased as possible. {Comment: there is, of | statistically as unbiased as possible. {Comment: there is, of | |||

course, no claim that real Internet traffic arrives according to a | course, no claim that real Internet traffic arrives according to a | |||

Poisson arrival process.} | Poisson arrival process.} | |||

EEEEEEE | ||||

6.8. Methodology | 6.8. Methodology | |||

Since packets can be lost or duplicated or can arrive in a different | 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 | order with respect the one of emission, in order to recognize the | |||

pairs of test packets, they should be marked with a Sequence Number | 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 | 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- | 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, | ket is lost, two values of ipdv will be undefined, since each packet, | |||

in the supposed "continuous" definition, is common to two pairs. | in the supposed "continuous" definition, is common to two pairs. | |||

Steps for measurement can be the following: | Steps for measurement can be the following: | |||

+ Starting from a given time T, Src generates a test packet as for | + Starting from a given time T, Src generates a test packet as for | |||

a singleton metrics, inserts in the packet a Sequence Number | ||||

a singleton metrics, adding also a Sequence Number, and sorts a | and the transmission Time Stamp Tx,then sorts the time Ti at | |||

which the next packet has to be sent. | ||||

Ti interval to determine the time at wich the next packet has to | + At time Ti, Src repeats the previous step, unless T(i) > Tf. | |||

+ On reception of the first packet, or the first packet after a SN | ||||

be sent. | error, Dst records SN and Tx timestamp that are contained in | |||

the packet and the reception time Rx as "old values". | ||||

I-D Ipdv Metric March 1998 | + On reception of the other packets Dst verifies the SN and if it is | |||

correct, by using the "old values" and the newly received ones, | ||||

+ On reception of the packet, Dst verifies the Sequence Number SN, | a value of ipdv is computed. Then Dst records the new SN, Tx | |||

and Rx timestamps as "old values". | ||||

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 | 6.9. Errors and uncertainties | |||

The same considerations apply that have been made about the single- | The same considerations apply that have been made about the single- | |||

ton metric. An additional error can be introduced by the pseudo-ran- | ton metric. An additional error can be introduced by the pseudo-ran- | |||

dom Poisson process as focused in the above referenced Draft. | ||||

dom Poisson process as focushed in the above referenced Draft. | ||||

Further considerations will be made in section 7. | Further considerations will be made in section 7. | |||

I-D Ipdv Metric July 1998 | ||||

6.10 Some statistics for One-way-ipdv | 6.10 Some statistics for One-way-ipdv | |||

Some statistics are here considered, that can provide useful informa- | Some statistics are here considered, that can provide useful informa- | |||

tion in analyzing the behavior of the packets flowing from Src to Dst | 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. The | ||||

These statistics are given having in mind a practical use of them. | focus is on the instantaneous behavior of the connection, while buffer | |||

dimensioning is not in the scope of this document. | ||||

Other statistics can be defined if needed. | Other statistics can be defined if needed. | |||

6.10.1. Type-P-One-way-ipdv-inverse-percentile | 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 | Given a Type-P-One-way-ipdv-Stream and a time threshold, that can be | |||

either positive or negative, the fraction of all the ipdv values in | ||||

either positive or negative, the fraction of all the dT values in the | the Stream less than or equal to the threshold, if the threshold is | |||

positive, or greater or equal to the threshold if the threshold is ne- | ||||

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. | gative. | |||

For many real-time services that require a regular delivery of the | For many real-time services that require a regular delivery of the | |||

packets, this statistics can give the amount of packets received | ||||

packets, this statistics can give the amount of packets beyond accep- | beyond acceptable limits. | |||

table limits. | ||||

6.10.2 Type-P-One-way-ipdv-standard-deviation | 6.10.2 Type-P-One-way-ipdv-standard-deviation | |||

Given a Type-P-One-way-ipdv-Stream, the distribution of ipdv values | Given a Type-P-One-way-ipdv-Stream, the distribution of ipdv values | |||

is considered and the Standard Deviation can be calculated as an | is considered and the Standard Deviation can be calculated as an | |||

indication of regularity of delivery. For practical purposes it can | indication of regularity of delivery. For practical purposes it can | |||

useful to define a total standard deviation, computed over the com- | useful to define a total standard deviation, computed over the com- | |||

plete set of value, and a standard deviation computed over the sub- | plete set of value, and a standard deviation computed over the sub- | |||

set of those values that do not exceed given positive and negative | ||||

set of those values that do not exceede given positive and negative | ||||

thresholds. This allows a more accurate description of the performan- | thresholds. This allows a more accurate description of the performan- | |||

ce experienced by packets. | ce experienced by packets. | |||

6.10.3 Type-P-One-way-ipdv-average | 6.10.3 Type-P-One-way-ipdv-average | |||

This statistic should tend to a value of ZERO for a number of ipdv | This statistic should tend to a value of ZERO for a number of ipdv | |||

I-D Ipdv Metric March 1998 | ||||

values that tend to infinite. The behavior of Type-P-One-way-ipdv- | values that tend to infinite. The behavior of Type-P-One-way-ipdv- | |||

average, and its meaning, are issues for the next section 7. | ||||

average is an issue for the next section 7. | ||||

7. Discussion on clock synchronization | 7. Discussion on clock synchronization | |||

This section gives some considerations about the need of having syn- | This section gives some considerations about the need of having syn- | |||

chronized clocks at Src and Dst. These considerations are given as a | chronized clocks at Src and Dst. These considerations are given as a | |||

basis for discussion, they require further investigation. We start | basis for discussion, they require further investigation. We start | |||

from the analysis of the mean value of the ipdv distribution related | from the analysis of the mean value of the ipdv distribution related | |||

to a "continuous" sample. | to a "continuous" sample. | |||

I-D Ipdv Metric July 1998 | ||||

7.1. Mean value of ipdv distribution. | 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 | 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 | ipdv in the distribution of a sample of "n" values, collected with | |||

the described methodology, we can write: | the described methodology, we can write: | |||

ipdv(1) = D1 - D0 | ipdv(1) = D1 - D0 | |||

.......... | .......... | |||

ipdv(i) = D(i) - D(i-1) | ipdv(i) = D(i) - D(i-1) | |||

.......... | .......... | |||

ipdv(n) = D(n) - D(n-1) | ipdv(n) = D(n) - D(n-1) | |||

The mean value of ipdv distribution will result in | The mean value of ipdv distribution will result in | |||

E(ipdv) = (D(n) - D(0))/n | E(ipdv) = (D(n) - D(0))/n | |||

If an actual measurement is performed, that lasts a period of time | If an actual measurement is performed, that lasts a period of time | |||

long enough to contain a number "n" sufficiently large and, supposing | long enough to contain a number "n" sufficiently large and, supposing | |||

synchronized clocks, such that the network conditions (traffic) allow | synchronized clocks, such that the network conditions (traffic) allow | |||

to find a D(n) not too different from D(0), e.g. a time of | ||||

to find a D(n) not too diferent from D(0), e.g. a time of 24 hours, | n x 24 hours, E(ipdv) will tend to zero, since the difference | |||

D(n) - D(0) will remain finite. | ||||

E(ipdv) will tend to zero. | ||||

7.2. Effects of a varying traffic | 7.2. Effects of a varying traffic | |||

If the mean values of delay D are changing inside a given period of | 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, | time, for example they are increasing due to an increment of traffic, | |||

we can consider, as a first approximation, the ipdv values as decom- | we can consider, as a first approximation, the ipdv values as decom- | |||

posed into two components, one being instantaneous and another one | posed into two components, one being instantaneous and another one | |||

as having a constant rate dD and corresponding to the increment "per | 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 | 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 | will be shifted of the value dD corresponding to the mean value of | |||

the interval between test packets. When the conditions will come back | the interval between test packets. When the conditions will come back | |||

to the initial ones, the distribution will resume a mean value around | to the initial ones, the distribution will resume a mean value around | |||

zero. At any time the distribution will correctly describe the | zero. At any time the distribution will correctly describe the | |||

effects of the path on the packet flow. | ||||

behavior of the packet flow. | 7.3. Effects of synchronization errors | |||

I-D Ipdv Metric March 1998 | ||||

7.3. Effects of syncronization errors | ||||

We refer here to the two components that can generate this type of | 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 | 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" | clocks. It is first of all noted that the variable component "drift" | |||

is physically limited and its effects can be interpreted by saying | 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 | 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 | to a max value in the time. This type of variation takes place very | |||

slowly being mostly connected to variations in temperature. | ||||

slowly being most connected to variations in temperature. | I-D Ipdv Metric July 1998 | |||

We suppose to perform a measurement between a Src and a Dst that have | 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, | 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 | 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 | 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 | the same time T0. In order to analyze the effects produced by this | |||

situation we suppose that packets are transferred, from Src to Dst, | situation we suppose that packets are transferred, from Src to Dst, | |||

with a constant delay D. In this conditions the measured ipdv should | ||||

with a consatant delay D. In this conditions the measured ipdv should | ||||

always be zero, and what is actually measured is the error. | 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 | 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 | 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). | of T with two packets having a time interval Ti(2). | |||

On our purposes other errors (like wire-time vs host-time) are not | On our purposes other errors (like wire-time vs host-time) are not | |||

considered since they are not relevant in this analysis. | considered since they are not relevant in this analysis. | |||

It is then possible to calculate the values of the Tx and Rx time- | 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 | stamps as they are seen by the two clocks, and the related values of | |||

the two ipdv values. | the two ipdv values. | |||

The first ipdv value will be: ipdv1 = ts1*Ti(1) + ((ts2-ts1)/T)*Ti(1) | 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 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- | 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 order | ||||

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. | term due to the variation of that variation in the same interval. | |||

If, as in practical cases, the drift can be considered zero, then | ||||

ts1 = ts2, and the error is not depending on the time at which the | ||||

measurement is done. | ||||

7.4. Related precision | 7.4. Related precision | |||

This means that: | This means that: | |||

1) + If the skew is constant and is = ts all the ipdv(i) values are | 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. | 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 | 2) + Considering the total skew as subdivided into a fixed part and a | |||

variable part (skew and drift),respectively, ts and + or - td, and | 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 | 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 | -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 | Ti(i)*ts plus or minus, as a maximum, the quantity 2*td*Ti(i)/T | |||

I-D Ipdv Metric March 1998 | I-D Ipdv Metric July 1998 | |||

3) + If the duration of the measurement is such that it is possible | 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, | to consider that the effect of the items at points 7.1 and 7.2, | |||

and the effect of the drift are negligible (related average ten- | ||||

and the effect of the drift are negligeables (related average ten- | ||||

ding to zero), the mean value of the ipdv distribution will have | 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 | the value of the skew multiplied by the mean value of the emission | |||

interval. | interval. | |||

4) + We observe that the displacement due to the skew does not change | ||||

4) + We observe that the displacemet due to the skew does not change | ||||

the shape of the distribution, and, for example the Standard Devi- | the shape of the distribution, and, for example the Standard Devi- | |||

ation remains the same. What introduces a distortion is the effect | 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 | of the drift, even if the mean value of this effect is zero at the | |||

end of the measurement. The value of this distortion is limited to | ||||

end of the measurement. This is, anyway, a "second order" effect. | the effect of the total skew variation on the emission interval. | |||

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 | 8. Definition for a bidirectional ipdv metric | |||

We now consider that the action of the skew on one direction is the | 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 | same, with opposite sign, of the action on the other direction. The | |||

idea of performing at the same time two independent measurements in | idea of performing at the same time two independent measurements in | |||

the two directions is suggested by this fact. | the two directions is suggested by this fact. | |||

If, after a long measurement, the variable conditions of the system | If, after a long measurement, the variable conditions of the system | |||

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 fixed action of the skew has modified the measured mean value. It | 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 | is therefore expected that on one direction that value is equal and | |||

opposite to the one measured in the other direction. | opposite to the one measured in the other direction. | |||

A bidirectional measurement can be defined not only as twin one-way | This fact offers the possibility of defining a theoretical reference | |||

measurement duration in the following way: | ||||

independent metrics that take place (nearly) at the same time, but | 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 > 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. | ||||

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 | 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 | end. This metric, that can be object of further study/Draft, would be | |||

able to measure also the Round Trip Delay and its variations. | able to measure also the Round Trip Delay and its variations. | |||

I-D Ipdv Metric July 1998 | ||||

9. References | 9. References | |||

V.Paxon, G.Almes, J.Mahdavi, M.Mathis - "Framework for IP Performance | V.Paxon, G.Almes, J.Mahdavi, M.Mathis - "Framework for IP Performance | |||

Metrics", Internet Draft <draft-ietf-ippm-framework-01.txt> Feb. 1998 | Metrics", Internet Draft <draft-ietf-ippm-framework-01.txt> Feb. 1998 | |||

G.Almes, S.Kalidindi - "A One-way Delay Metric for IPPM", Internet | G.Almes, S.Kalidindi - "A One-way Delay Metric for IPPM", Internet | |||

Draft <draft-ietf-ippm-delay-01.txt> Nov. 1997 | Draft <draft-ietf-ippm-delay-01.txt> Nov. 1997 | |||

I-D Ipdv Metric March 1998 | ||||

10. Author's Address | 10. Author's Address | |||

Carlo Demichelis <carlo.demichelis@cselt.it> | Carlo Demichelis <carlo.demichelis@cselt.it> | |||

CSELT - Centro Studi E Laboratori Telecomunicazioni S.p.A | ||||

CSELT - Centro Studi E Laburatori Telecomunicazioni S.p.A | ||||

Via G. Reiss Romoli 274 | Via G. Reiss Romoli 274 | |||

10148 - TORINO | 10148 - TORINO | |||

Italy | Italy | |||

Phone +39 11 228 5057 | Phone +39 11 228 5057 | |||

Fax. +39 11 228 5069 | Fax. +39 11 228 5069 | |||

End of changes. | ||||

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