draft-ietf-ippm-rt-delay-00.txt   rfc2681.txt 
Network Working Group G. Almes Network Working Group G. Almes
Internet Draft S. Kalidindi Request for Comments: 2681 S. Kalidindi
Expiration Date: May 1999 M. Zekauskas Category: Standards Track M. Zekauskas
Advanced Network & Services Advanced Network & Services
November 1998 September 1999
A Round-trip Delay Metric for IPPM A Round-trip Delay Metric for IPPM
<draft-ietf-ippm-rt-delay-00.txt>
1. Status of this Memo
This document is an Internet-Draft. Internet-Drafts are working Status of this Memo
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and its working groups. Note that other groups may also distribute
working documents as Internet-Drafts.
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at any time. It is inappropriate to use Internet-Drafts as reference improvements. Please refer to the current edition of the "Internet
material or to cite them other than as "work in progress." Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
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This memo provides information for the Internet community. This memo Copyright (C) The Internet Society (1999). All Rights Reserved.
does not specify an Internet standard of any kind. Distribution of
this memo is unlimited.
2. Introduction 1. Introduction
This memo defines a metric for round-trip delay of packets across This memo defines a metric for round-trip delay of packets across
Internet paths. It builds on notions introduced and discussed in the Internet paths. It builds on notions introduced and discussed in the
IPPM Framework document, RFC 2330 [1], and follows closely the IPPM Framework document, RFC 2330 [1], and follows closely the
corresponding metric for One-way Delay ("A One-way Delay Metric for corresponding metric for One-way Delay ("A One-way Delay Metric for
IPPM" <draft-ietf-ippm-delay-05.txt>) [2]; the reader is assumed to IPPM") [2]; the reader is assumed to be familiar with those
be familiar with those documents. documents.
The memo was largely written by copying material from the One-way The memo was largely written by copying material from the One-way
Delay metric. The intention is that, where the two metrics are Delay metric. The intention is that, where the two metrics are
similar, they will be described with similar or identical text, and similar, they will be described with similar or identical text, and
that where the two metrics differ, new or modified text will be used. that where the two metrics differ, new or modified text will be used.
This memo is intended to be parallel in structure to a future This memo is intended to be parallel in structure to a future
companion document for Packet Loss. companion document for Packet Loss.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
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+ Using this sample, several 'statistics' of the sample will be + Using this sample, several 'statistics' of the sample will be
defined and discussed. defined and discussed.
This progression from singleton to sample to statistics, with clear This progression from singleton to sample to statistics, with clear
separation among them, is important. separation among them, is important.
Whenever a technical term from the IPPM Framework document is first Whenever a technical term from the IPPM Framework document is first
used in this memo, it will be tagged with a trailing asterisk. For used in this memo, it will be tagged with a trailing asterisk. For
example, "term*" indicates that "term" is defined in the Framework. example, "term*" indicates that "term" is defined in the Framework.
2.1. Motivation 1.1. Motivation
Round-trip delay of a Type-P* packet from a source host* to a Round-trip delay of a Type-P* packet from a source host* to a
destination host is useful for several reasons: destination host is useful for several reasons:
+ Some applications do not perform well (or at all) if end-to-end + Some applications do not perform well (or at all) if end-to-end
delay between hosts is large relative to some threshold value. delay between hosts is large relative to some threshold value.
+ Erratic variation in delay makes it difficult (or impossible) to + Erratic variation in delay makes it difficult (or impossible) to
support many real-time applications. support many interactive real-time applications.
+ The larger the value of delay, the more difficult it is for + The larger the value of delay, the more difficult it is for
transport-layer protocols to sustain high bandwidths. transport-layer protocols to sustain high bandwidths.
+ The minimum value of this metric provides an indication of the + The minimum value of this metric provides an indication of the
delay due only to propagation and transmission delay. delay due only to propagation and transmission delay.
+ The minimum value of this metric provides an indication of the + The minimum value of this metric provides an indication of the
delay that will likely be experienced when the path* traversed is delay that will likely be experienced when the path* traversed is
lightly loaded. lightly loaded.
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+ The Internet path from a source to a destination may differ from + The Internet path from a source to a destination may differ from
the path from the destination back to the source ("asymmetric the path from the destination back to the source ("asymmetric
paths"), such that different sequences of routers are used for the paths"), such that different sequences of routers are used for the
forward and reverse paths. Therefore round-trip measurements forward and reverse paths. Therefore round-trip measurements
actually measure the performance of two distinct paths together. actually measure the performance of two distinct paths together.
+ Even when the two paths are symmetric, they may have radically + Even when the two paths are symmetric, they may have radically
different performance characteristics due to asymmetric queueing. different performance characteristics due to asymmetric queueing.
+ Performance of an application may depend mostly on the performance + Performance of an application may depend mostly on the performance
in one direction. For example, a file transfer using TCP may in one direction.
depend more on the performance in the direction that data flows,
rather than the direction in which acknowledgements travel.
+ In quality-of-service (QoS) enabled networks, provisioning in one + In quality-of-service (QoS) enabled networks, provisioning in one
direction may be radically different than provisioning in the direction may be radically different than provisioning in the
reverse direction, and thus the QoS guarantees differ. reverse direction, and thus the QoS guarantees differ.
On the other hand, the measurement of round-trip delay has two On the other hand, the measurement of round-trip delay has two
specific advantages: specific advantages:
+ Ease of deployment: unlike in one-way measurement, it is often + Ease of deployment: unlike in one-way measurement, it is often
possible to perform some form of round-trip delay measurement possible to perform some form of round-trip delay measurement
without installing measurement-specific software at the intended without installing measurement-specific software at the intended
destination. A variety of approaches are well-known, including destination. A variety of approaches are well-known, including
use of ICMP Echo or of TCP-based methodologies (similar to those use of ICMP Echo or of TCP-based methodologies (similar to those
outlined in "IPPM Metrics for Measuring Connectivity" [4]). outlined in "IPPM Metrics for Measuring Connectivity" [4]).
However, some approaches may introduce greater uncertainty in the However, some approaches may introduce greater uncertainty in the
time for the destination to produce a response (see time for the destination to produce a response (see
Section 3.7.3). Section 2.7.3).
+ Ease of interpretation: in some circumstances, the round-trip time + Ease of interpretation: in some circumstances, the round-trip time
is in fact the quantity of interest; deducing it from matching is in fact the quantity of interest. Deducing the round-trip time
one-way measurements and an assumption of the destination from matching one-way measurements and an assumption of the
processing time is less direct and potentially less accurate. destination processing time is less direct and potentially less
accurate.
2.2. General Issues Regarding Time 1.2. General Issues Regarding Time
Whenever a time (i.e., a moment in history) is mentioned here, it is Whenever a time (i.e., a moment in history) is mentioned here, it is
understood to be measured in seconds (and fractions) relative to UTC. understood to be measured in seconds (and fractions) relative to UTC.
As described more fully in the Framework document, there are four As described more fully in the Framework document, there are four
distinct, but related notions of clock uncertainty: distinct, but related notions of clock uncertainty:
synchronization* synchronization*
measures the extent to which two clocks agree on what time it measures the extent to which two clocks agree on what time it
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measures the change of accuracy, or of synchronization, with measures the change of accuracy, or of synchronization, with
time. For example, the clock on a given host might gain 1.3 time. For example, the clock on a given host might gain 1.3
msec per hour and thus be 27.1 msec behind UTC at one time and msec per hour and thus be 27.1 msec behind UTC at one time and
only 25.8 msec an hour later. In this case, we say that the only 25.8 msec an hour later. In this case, we say that the
clock of the given host has a skew of 1.3 msec per hour relative clock of the given host has a skew of 1.3 msec per hour relative
to UTC, which threatens accuracy. We might also speak of the to UTC, which threatens accuracy. We might also speak of the
skew of one clock relative to another clock, which threatens skew of one clock relative to another clock, which threatens
synchronization. synchronization.
3. A Singleton Definition for Round-trip Delay 2. A Singleton Definition for Round-trip Delay
3.1. Metric Name: 2.1. Metric Name:
Type-P-Round-trip-Delay Type-P-Round-trip-Delay
3.2. Metric Parameters: 2.2. Metric 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
+ T, a time + T, a time
3.3. Metric Units: 2.3. Metric Units:
The value of a Type-P-Round-trip-Delay is either a non-negative real The value of a Type-P-Round-trip-Delay is either a real number, or an
number, or an undefined (informally, infinite) number of seconds. undefined (informally, infinite) number of seconds.
3.4. Definition: 2.4. Definition:
For a non-negative real number dT, >>the *Type-P-Round-trip-Delay* For a real number dT, >>the *Type-P-Round-trip-Delay* from Src to Dst
from Src to Dst at T is dT<< means that Src sent the first bit of a at T is dT<< means that Src sent the first bit of a Type-P packet to
Type-P packet to Dst at wire-time* T, that Dst received that packet, Dst at wire-time* T, that Dst received that packet, then immediately
then sent a Type-P packet back to Src, and that Src received the last sent a Type-P packet back to Src, and that Src received the last bit
bit of that packet at wire-time T+dT. of that packet at wire-time T+dT.
>>The *Type-P-Round-trip-Delay* from Src to Dst at T is undefined >>The *Type-P-Round-trip-Delay* from Src to Dst at T is undefined
(informally, infinite)<< means that Src sent the first bit of a Type- (informally, infinite)<< means that Src sent the first bit of a
P packet to Dst at wire-time T and that (either Dst did not receive Type-P packet to Dst at wire-time T and that (either Dst did not
the packet, Dst did not send a Type-P packet in response, or) Src did receive the packet, Dst did not send a Type-P packet in response, or)
not receive that response packet. Src did not receive that response packet.
>>The *Type-P-Round-trip-Delay between Src and Dst at T<< means >>The *Type-P-Round-trip-Delay between Src and Dst at T<< means
either the *Type-P-Round-trip-Delay from Src to Dst at T or the either the *Type-P-Round-trip-Delay from Src to Dst at T or the
*Type-P-Round-trip-Delay from Dst to Src at T. When this notion is *Type-P-Round-trip-Delay from Dst to Src at T. When this notion is
used, it is understood to be specifically ambiguous which host acts used, it is understood to be specifically ambiguous which host acts
as Src and which as Dst. {This ambiguity will usually be a small as Src and which as Dst. {Comment: This ambiguity will usually be a
price to pay for being able to have one measurement, launched from small price to pay for being able to have one measurement, launched
either Src or Dst, rather than having two measurements.} from either Src or Dst, rather than having two measurements.}
Suggestions for what to report along with metric values appear in Suggestions for what to report along with metric values appear in
Section 3.8 after a discussion of the metric, methodologies for Section 3.8 after a discussion of the metric, methodologies for
measuring the metric, and error analysis. measuring the metric, and error analysis.
3.5. Discussion: 2.5. Discussion:
Type-P-Round-trip-Delay is a relatively simple analytic metric, and Type-P-Round-trip-Delay is a relatively simple analytic metric, and
one that we believe will afford effective methods of measurement. one that we believe will afford effective methods of measurement.
The following issues are likely to come up in practice: The following issues are likely to come up in practice:
+ The timestamp values (T) for the time at which delays are measured + The timestamp values (T) for the time at which delays are measured
should be fairly accurate in order to draw meaningful conclusions should be fairly accurate in order to draw meaningful conclusions
about the state of the network at a given T. Therefore, Src about the state of the network at a given T. Therefore, Src
should have an accurate knowledge of time-of-day. NTP [3] affords should have an accurate knowledge of time-of-day. NTP [3] affords
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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.}
+ If the packet is duplicated so that multiple non-corrupt instances + If the packet is duplicated so that multiple non-corrupt instances
of the response arrive back at the source, then the packet is of the response arrive back at the source, then the packet is
counted as received, and the first instance to arrive back at the counted as received, and the first instance to arrive back at the
source determines the packet's round-trip delay. source determines the packet's round-trip 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.
3.6. Methodologies: 2.6. 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). precedence).
Generally, for a given Type-P, the methodology would proceed as Generally, for a given Type-P, the methodology would proceed as
follows: follows:
+ At the Src host, select Src and Dst IP addresses, and form a test + At the Src host, select Src and Dst IP addresses, and form a test
packet of Type-P with these addresses. Any 'padding' portion of packet of Type-P with these addresses. Any 'padding' portion of
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order to form a Type-P-Round-trip-Delay value, the return packet must order to form a Type-P-Round-trip-Delay value, the return packet must
be triggered by the reception of a packet from Src.} be triggered by the reception of a packet from Src.}
{Comment: "ping" would qualify as a round-trip measure under this {Comment: "ping" would qualify as a round-trip measure under this
definition, with a Type-P of ICMP echo request/reply with 60-byte definition, with a Type-P of ICMP echo request/reply with 60-byte
packets. However, the uncertainties associated with a typical ping packets. However, the uncertainties associated with a typical ping
program must be analyzed as in the next section, including the type program must be analyzed as in the next section, including the type
of reflecting point (a router may not handle an ICMP request in the of reflecting point (a router may not handle an ICMP request in the
fast path) and effects of load on the reflecting point.} fast path) and effects of load on the reflecting point.}
3.7. Errors and Uncertainties: 2.7. Errors and Uncertainties:
The description of any specific measurement method should include an The description of any specific measurement method should include an
accounting and analysis of various sources of error or uncertainty. accounting and analysis of various sources of error or uncertainty.
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 or uncertainties due to uncertainty in the clock of the Src + Errors or uncertainties due to uncertainty in the clock of the Src
host. host.
+ Errors or uncertainties due to the difference between 'wire time' + Errors or uncertainties due to the difference between 'wire time'
and 'host time'. and 'host time'.
+ Errors or uncertainties due to time required by the Dst to receive + Errors or uncertainties due to time required by the Dst to receive
the packet from the Src and send the corresponding response. the packet from the Src and send the corresponding response.
In addition, the loss threshold may affect the results. Each of In addition, the loss threshold may affect the results. Each of
these are discussed in more detail below, along with a section these are discussed in more detail below, along with a section
("Calibration") on accounting for these errors and uncertainties. ("Calibration") on accounting for these errors and uncertainties.
3.7.1. Errors or Uncertainties Related to Clocks 2.7.1. Errors or Uncertainties Related to Clocks
The uncertainty in a measurement of round-trip delay is related, in The uncertainty in a measurement of round-trip delay is related, in
part, to uncertainty in the clock of the Src host. In the following, part, to uncertainty in the clock of the Src host. In the following,
we refer to the clock used to measure when the packet was sent from we refer to the clock used to measure when the packet was sent from
Src as the source clock, and we refer to the observed time when the Src as the source clock, and we refer to the observed time when the
packet was sent by the source as Tinitial, and the observed time when packet was sent by the source as Tinitial, and the observed time when
the packet was received by the source as Tfinal. Alluding to the the packet was received by the source as Tfinal. Alluding to the
notions of synchronization, accuracy, resolution, and skew mentioned notions of synchronization, accuracy, resolution, and skew mentioned
in the Introduction, we note the following: in the Introduction, we note the following:
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+ The resolution of a clock adds to uncertainty about any time + The resolution of a clock adds to uncertainty about any time
measured with it. Thus, if the source clock has a resolution of measured with it. Thus, if the source clock has a resolution of
10 msec, then this adds 10 msec of uncertainty to any time value 10 msec, then this adds 10 msec of uncertainty to any time value
measured with it. We will denote the resolution of the source measured with it. We will denote the resolution of the source
clock as Rsource. clock as Rsource.
Taking these items together, we note that naive computation Tfinal- Taking these items together, we note that naive computation Tfinal-
Tinitial will be off by 2*Rsource. Tinitial will be off by 2*Rsource.
3.7.2. Errors or Uncertainties Related to Wire-time vs Host-time 2.7.2. Errors or Uncertainties Related to Wire-time vs Host-time
As we have defined round-trip delay, we would like to measure the As we have defined round-trip delay, we would like to measure the
time between when the test packet leaves the network interface of Src time between when the test packet leaves the network interface of Src
and when the corresponding response packet (completely) arrives at and when the corresponding response packet (completely) arrives at
the network interface of Src, and we refer to these as "wire times". the network interface of Src, and we refer to these as "wire times".
If the timings are themselves performed by software on Src, however, If the timings are themselves performed by software on Src, however,
then this software can only directly measure the time between when then this software can only directly measure the time between when
Src grabs a timestamp just prior to sending the test packet and when Src grabs a timestamp just prior to sending the test packet and when
it grabs a timestamp just after having received the response packet, it grabs a timestamp just after having received the response packet,
and we refer to these two points as "host times". and we refer to these two points as "host times".
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accurately known, this knowledge can be used to correct for host time accurately known, this knowledge can be used to correct for host time
measurements and the corrected value more accurately estimates the measurements and the corrected value more accurately estimates the
desired (wire time) metric. desired (wire time) metric.
To the extent, however, that the difference between wire time and To the extent, however, that the difference between wire time and
host time is uncertain, this uncertainty must be accounted for in an host time is uncertain, this uncertainty must be accounted for in an
analysis of a given measurement method. We denote by Hinitial an analysis of a given measurement method. We denote by Hinitial an
upper bound on the uncertainty in the difference between wire time upper bound on the uncertainty in the difference between wire time
and host time on the Src host in sending the test packet, and and host time on the Src host in sending the test packet, and
similarly define Hfinal for the difference on the Src host in similarly define Hfinal for the difference on the Src host in
receiving the reponse packet. We then note that these problems receiving the response packet. We then note that these problems
introduce a total uncertainty of Hinitial + Hfinal. This estimate of introduce a total uncertainty of Hinitial + Hfinal. This estimate of
total wire-vs-host uncertainty should be included in the total wire-vs-host uncertainty should be included in the
error/uncertainty analysis of any measurement implementation. error/uncertainty analysis of any measurement implementation.
3.7.3. Errors or Uncertainties Related to Dst Producing a Response 2.7.3. Errors or Uncertainties Related to Dst Producing a Response
Any time spent by the destination host in receiving and recognizing Any time spent by the destination host in receiving and recognizing
the packet from Src, and then producing and sending the corresponding the packet from Src, and then producing and sending the corresponding
response adds additional error and uncertainty to the round-trip response adds additional error and uncertainty to the round-trip
delay measurement. The error equals the difference between the wire- delay measurement. The error equals the difference between the wire
time the first bit of the packet is received by Dst and the wire-time time the first bit of the packet is received by Dst and the wire time
the first bit of the response is sent by Dst. To the extent that the first bit of the response is sent by Dst. To the extent that
this difference is accurately known, this knowledge can be used to this difference is accurately known, this knowledge can be used to
correct the desired metric. To the extent, however, that this correct the desired metric. To the extent, however, that this
difference is uncertain, this uncertainty must be accounted for in difference is uncertain, this uncertainty must be accounted for in
the error analysis of a measurement implementation. We denote by the error analysis of a measurement implementation. We denote this
Hrefl the difference between the two wire-times. uncertainty by Hrefl. This estimate of uncertainty should be
included in the error/uncertainty analysis of any measurement
implementation.
3.7.4. Calibration 2.7.4. Calibration
Generally, the measured values can be decomposed as follows: Generally, the measured values can be decomposed as follows:
measured value = true value + systematic error + random error measured value = true value + systematic error + random error
If the systematic error (the constant bias in measured values) can be If the systematic error (the constant bias in measured values) can be
determined, it can be compensated for in the reported results. determined, it can be compensated for in the reported results.
reported value = measured value - systematic error reported value = measured value - systematic error
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The goal of calibration is to determine the systematic and random The goal of calibration is to determine the systematic and random
error generated by the instruments themselves in as much detail as error generated by the instruments themselves in as much detail as
possible. At a minimum, a bound ("e") should be found such that the possible. At a minimum, a bound ("e") should be found such that the
reported value is in the range (true value - e) to (true value + e) reported value is in the range (true value - e) to (true value + e)
at least 95 percent of the time. We call "e" the calibration error at least 95 percent of the time. We call "e" the calibration error
for the measurements. It represents the degree to which the values for the measurements. It represents the degree to which the values
produced by the measurement instrument are repeatable; that is, how produced by the measurement instrument are repeatable; that is, how
closely an actual delay of 30 ms is reported as 30 ms. {Comment: 95 closely an actual delay of 30 ms is reported as 30 ms. {Comment: 95
percent was chosen because (1) some confidence level is desirable to percent was chosen because (1) some confidence level is desirable to
be able to remove outliers which will be found in measuring any be able to remove outliers, which will be found in measuring any
physical property; and (2) a particular confidence level should be physical property; and (2) a particular confidence level should be
specified so that the results of independent implementations can be specified so that the results of independent implementations can be
compared.} compared.}
From the discussion in the previous three sections, the error in From the discussion in the previous three sections, the error in
measurements could be bounded by determining all the individual measurements could be bounded by determining all the individual
uncertainties, and adding them together to form uncertainties, and adding them together to form
2*Rsource + Hinitial + Hfinal + Hrefl. 2*Rsource + Hinitial + Hfinal + Hrefl.
However, reasonable bounds on both the clock-related uncertainty However, reasonable bounds on both the clock-related uncertainty
captured by the first term and the host-related uncertainty captured captured by the first term and the host-related uncertainty captured
by the last three terms should be possible by careful design by the last three terms should be possible by careful design
techniques and calibrating the instruments using a known, isolated, techniques and calibrating the instruments using a known, isolated,
network in a lab. network in a lab.
The host-related uncertainties, Hinitial + Hfinal + Hrefl, could be The host-related uncertainties, Hinitial + Hfinal + Hrefl, could be
bounded by connecting two instruments back-to-back with a high-speed bounded by connecting two instruments back-to-back with a high-speed
serial link or isolated LAN segment. In this case, repeated serial link or isolated LAN segment. In this case, repeated
measurements are measuring the same round-trip delay. measurements are measuring the same round-trip delay.
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checks should be made to ensure that packets reported as losses were checks should be made to ensure that packets reported as losses were
really lost. First, the threshold for loss should be verified. In really lost. First, the threshold for loss should be verified. In
particular, ensure the "reasonable" threshold is reasonable: that it particular, ensure the "reasonable" threshold is reasonable: that it
is very unlikely a packet will arrive after the threshold value, and is very unlikely a packet will arrive after the threshold value, and
therefore the number of packets lost over an interval is not therefore the number of packets lost over an interval is not
sensitive to the error bound on measurements. Second, consider the sensitive to the error bound on measurements. Second, consider the
possibility that a packet arrives at the network interface, but is possibility that a packet arrives at the network interface, but is
lost due to congestion on that interface or to other resource lost due to congestion on that interface or to other resource
exhaustion (e.g. buffers) in the instrument. exhaustion (e.g. buffers) in the instrument.
3.8. Reporting the Metric: 2.8. Reporting the Metric:
The calibration and context in which the metric is measured MUST be The calibration and context in which the metric is measured MUST be
carefully considered, and SHOULD always be reported along with metric carefully considered, and SHOULD always be reported along with metric
results. We now present four items to consider: the Type-P of test results. We now present four items to consider: the Type-P of test
packets, the threshold of infinite delay (if any), error calibration, packets, the threshold of infinite delay (if any), error calibration,
and the path traversed by the test packets. This list is not and the path traversed by the test packets. This list is not
exhaustive; any additional information that could be useful in exhaustive; any additional information that could be useful in
interpreting applications of the metrics should also be reported. interpreting applications of the metrics should also be reported.
3.8.1. Type-P 2.8.1. Type-P
As noted in the Framework document [1], the value of the metric may As noted in the Framework document [1], the value of the metric may
depend on the type of IP packets used to make the measurement, or depend on the type of IP packets used to make the measurement, or
"type-P". The value of Type-P-Round-trip-Delay could change if the "type-P". The value of Type-P-Round-trip-Delay could change if the
protocol (UDP or TCP), port number, size, or arrangement for special protocol (UDP or TCP), port number, size, or arrangement for special
treatment (e.g., IP precedence or RSVP) changes. The exact Type-P treatment (e.g., IP precedence or RSVP) changes. The exact Type-P
used to make the measurements MUST be accurately reported. used to make the measurements MUST be accurately reported.
3.8.2. Loss threshold 2.8.2. Loss threshold
In addition, the threshold (or methodology to distinguish) between a In addition, the threshold (or methodology to distinguish) between a
large finite delay and loss MUST be reported. large finite delay and loss MUST be reported.
3.8.3. Calibration Results 2.8.3. Calibration Results
+ If the systematic error can be determined, it SHOULD be removed + If the systematic error can be determined, it SHOULD be removed
from the measured values. from the measured values.
+ You SHOULD also report the calibration error, e, such that the + You SHOULD also report the calibration error, e, such that the
true value is the reported value plus or minus e, with 95% true value is the reported value plus or minus e, with 95%
confidence (see the last section.) confidence (see the last section.)
+ If possible, the conditions under which a test packet with finite + If possible, the conditions under which a test packet with finite
delay is reported as lost due to resource exhaustion on the delay is reported as lost due to resource exhaustion on the
measurement instrument SHOULD be reported. measurement instrument SHOULD be reported.
3.8.4. Path 2.8.4. Path
Finally, the path traversed by the packet SHOULD be reported, if Finally, the path traversed by the packet SHOULD be reported, if
possible. In general it is impractical to know the precise path a possible. In general it is impractical to know the precise path a
given packet takes through the network. The precise path may be given packet takes through the network. The precise path may be
known for certain Type-P on short or stable paths. For example, if known for certain Type-P on short or stable paths. For example, if
Type-P includes the record route (or loose-source route) option in Type-P includes the record route (or loose-source route) option in
the IP header, and the path is short enough, and all routers* on the the IP header, and the path is short enough, and all routers* on the
path support record (or loose-source) route, and the Dst host copies path support record (or loose-source) route, and the Dst host copies
the path from Src to Dst into the corresponding reply packet, then the path from Src to Dst into the corresponding reply packet, then
the path will be precisely recorded. This is impractical because the the path will be precisely recorded. This is impractical because the
route must be short enough, many routers do not support (or are not route must be short enough, many routers do not support (or are not
configured for) record route, and use of this feature would often configured for) record route, and use of this feature would often
artificially worsen the performance observed by removing the packet artificially worsen the performance observed by removing the packet
from common-case processing. However, partial information is still from common-case processing. However, partial information is still
valuable context. For example, if a host can choose between two valuable context. For example, if a host can choose between two
links* (and hence two separate routes from Src to Dst), then the links* (and hence two separate routes from Src to Dst), then the
initial link used is valuable context. {Comment: For example, with initial link used is valuable context. {Comment: For example, with
Merit's NetNow setup, a Src on one NAP can reach a Dst on another NAP Merit's NetNow setup, a Src on one NAP can reach a Dst on another NAP
by either of several different backbone networks.} by either of several different backbone networks.}
4. A Definition for Samples of Round-trip Delay 3. A Definition for Samples of Round-trip Delay
Given the singleton metric Type-P-Round-trip-Delay, we now define one Given the singleton metric Type-P-Round-trip-Delay, we now define one
particular sample of such singletons. The idea of the sample is to particular sample of such singletons. The idea of the sample is to
select a particular binding of the parameters Src, Dst, and Type-P, select a particular binding of the parameters Src, Dst, and Type-P,
then define a sample of values of parameter T. The means for then define a sample of values of parameter T. The means for
defining the values of T is to select a beginning time T0, a final defining the values of T is to select a beginning time T0, a final
time Tf, and an average rate lambda, then define a pseudo-random time Tf, and an average rate lambda, then define a pseudo-random
Poisson process of rate lambda, whose values fall between T0 and Tf. Poisson process of rate lambda, whose values fall between T0 and Tf.
The time interval between successive values of T will then average The time interval between successive values of T will then average
1/lambda. 1/lambda.
{Comment: Note that Poisson sampling is only one way of defining a {Comment: Note that Poisson sampling is only one way of defining a
sample. Poisson has the advantage of limiting bias, but other sample. Poisson has the advantage of limiting bias, but other
methods of sampling might be appropriate for different situations. methods of sampling might be appropriate for different situations.
We encourage others who find such appropriate cases to use this We encourage others who find such appropriate cases to use this
general framework and submit their sampling method for general framework and submit their sampling method for
standardization.} standardization.}
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The time interval between successive values of T will then average The time interval between successive values of T will then average
1/lambda. 1/lambda.
{Comment: Note that Poisson sampling is only one way of defining a {Comment: Note that Poisson sampling is only one way of defining a
sample. Poisson has the advantage of limiting bias, but other sample. Poisson has the advantage of limiting bias, but other
methods of sampling might be appropriate for different situations. methods of sampling might be appropriate for different situations.
We encourage others who find such appropriate cases to use this We encourage others who find such appropriate cases to use this
general framework and submit their sampling method for general framework and submit their sampling method for
standardization.} standardization.}
4.1. Metric Name: 3.1. Metric Name:
Type-P-Round-trip-Delay-Poisson-Stream Type-P-Round-trip-Delay-Poisson-Stream
4.2. Metric Parameters: 3.2. Metric 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
+ T0, a time + T0, a time
+ Tf, a time + Tf, a time
+ lambda, a rate in reciprocal seconds + lambda, a rate in reciprocal seconds
4.3. Metric Units: 3.3. Metric Units:
A sequence of pairs; the elements of each pair are: A sequence of pairs; the elements of each pair are:
+ T, a time, and + T, a time, and
+ dT, either a non-negative real number or an undefined number of + dT, either a real number or an undefined number of seconds.
seconds.
The values of T in the sequence are monotonic increasing. Note that The values of T in the sequence are monotonic increasing. Note that
T would be a valid parameter to Type-P-Round-trip-Delay, and that dT T would be a valid parameter to Type-P-Round-trip-Delay, and that dT
would be a valid value of Type-P-Round-trip-Delay. would be a valid value of Type-P-Round-trip-Delay.
4.4. Definition: 3.4. Definition:
Given T0, Tf, and lambda, we compute a pseudo-random Poisson process Given T0, Tf, and lambda, we compute a pseudo-random Poisson process
beginning at or before T0, with average arrival rate lambda, and beginning at or before T0, with average arrival rate lambda, and
ending at or after Tf. Those time values greater than or equal to T0 ending at or after Tf. Those time values greater than or equal to T0
and less than or equal to Tf are then selected. At each of the times and less than or equal to Tf are then selected. At each of the times
in this process, we obtain the value of Type-P-Round-trip-Delay at in this process, we obtain the value of Type-P-Round-trip-Delay at
this time. The value of the sample is the sequence made up of the this time. The value of the sample is the sequence made up of the
resulting <time, delay> pairs. If there are no such pairs, the resulting <time, delay> pairs. If there are no such pairs, the
sequence is of length zero and the sample is said to be empty. sequence is of length zero and the sample is said to be empty.
4.5. Discussion: 3.5. Discussion:
The reader should be familiar with the in-depth discussion of Poisson The reader should be familiar with the in-depth discussion of Poisson
sampling in the Framework document [1], which includes methods to sampling in the Framework document [1], which includes methods to
compute and verify the pseudo-random Poisson process. compute and verify the pseudo-random Poisson process.
We specifically do not constrain the value of lambda, except to note We specifically do not constrain the value of lambda, except to note
the extremes. If the rate is too large, then the measurement traffic the extremes. If the rate is too large, then the measurement traffic
will perturb the network, and itself cause congestion. If the rate will perturb the network, and itself cause congestion. If the rate
is too small, then you might not capture interesting network is too small, then you might not capture interesting network
behavior. {Comment: We expect to document our experiences with, and behavior. {Comment: We expect to document our experiences with, and
skipping to change at page 16, line 6 skipping to change at page 15, line 28
All the singleton Type-P-Round-trip-Delay metrics in the sequence All the singleton Type-P-Round-trip-Delay metrics in the sequence
will have the same values of Src, Dst, and Type-P. will have the same values of Src, Dst, and Type-P.
Note also that, given one sample that runs from T0 to Tf, and given Note also that, given one sample that runs from T0 to Tf, and given
new time values T0' and Tf' such that T0 <= T0' <= Tf' <= Tf, the new time values T0' and Tf' such that T0 <= T0' <= Tf' <= Tf, the
subsequence of the given sample whose time values fall between T0' subsequence of the given sample whose time values fall between T0'
and Tf' are also a valid Type-P-Round-trip-Delay-Poisson-Stream and Tf' are also a valid Type-P-Round-trip-Delay-Poisson-Stream
sample. sample.
4.6. Methodologies: 3.6. Methodologies:
The methodologies follow directly from: The methodologies follow directly from:
+ the selection of specific times, using the specified Poisson + the selection of specific times, using the specified Poisson
arrival process, and arrival process, and
+ the methodologies discussion already given for the singleton Type- + the methodologies discussion already given for the singleton Type-
P-Round-trip-Delay metric. P-Round-trip-Delay metric.
Care must, of course, be given to correctly handle out-of-order Care must, of course, be given to correctly handle out-of-order
arrival of test or response packets; it is possible that the Src arrival of test or response packets; it is possible that the Src
could send one test packet at TS[i], then send a second test packet could send one test packet at TS[i], then send a second test packet
(later) at TS[i+1], and it could receive the second response packet (later) at TS[i+1], and it could receive the second response packet
at TR[i+1], and then receive the first response packet (later) at at TR[i+1], and then receive the first response packet (later) at
TR[i]. TR[i].
4.7. Errors and Uncertainties: 3.7. Errors and Uncertainties:
In addition to sources of errors and uncertainties associated with In addition to sources of errors and uncertainties associated with
methods employed to measure the singleton values that make up the methods employed to measure the singleton values that make up the
sample, care must be given to analyze the accuracy of the Poisson sample, care must be given to analyze the accuracy of the Poisson
process with respect to the wire-times of the sending of the test process with respect to the wire-times of the sending of the test
packets. Problems with this process could be caused by several packets. Problems with this process could be caused by several
things, including problems with the pseudo-random number techniques things, including problems with the pseudo-random number techniques
used to generate the Poisson arrival process, or with jitter in the used to generate the Poisson arrival process, or with jitter in the
value of Hinitial (mentioned above as uncertainty in the singleton value of Hinitial (mentioned above as uncertainty in the singleton
delay metric). The Framework document shows how to use the Anderson- delay metric). The Framework document shows how to use the
Darling test to verify the accuracy of a Poisson process over small Anderson-Darling test to verify the accuracy of a Poisson process
time frames. {Comment: The goal is to ensure that test packets are over small time frames. {Comment: The goal is to ensure that test
sent "close enough" to a Poisson schedule, and avoid periodic packets are sent "close enough" to a Poisson schedule, and avoid
behavior.} periodic behavior.}
4.8. Reporting the Metric: 3.8. Reporting the Metric:
You MUST report the calibration and context for the underlying You MUST report the calibration and context for the underlying
singletons along with the stream. (See "Reporting the metric" for singletons along with the stream. (See "Reporting the metric" for
Type-P-Round-trip-Delay.) Type-P-Round-trip-Delay.)
5. Some Statistics Definitions for Round-trip Delay 4. Some Statistics Definitions for Round-trip Delay
Given the sample metric Type-P-Round-trip-Delay-Poisson-Stream, we Given the sample metric Type-P-Round-trip-Delay-Poisson-Stream, we
now offer several statistics of that sample. These statistics are now offer several statistics of that sample. These statistics are
offered mostly to be illustrative of what could be done. offered mostly to be illustrative of what could be done.
5.1. Type-P-Round-trip-Delay-Percentile 4.1. Type-P-Round-trip-Delay-Percentile
Given a Type-P-Round-trip-Delay-Poisson-Stream and a percent X Given a Type-P-Round-trip-Delay-Poisson-Stream and a percent X
between 0% and 100%, the Xth percentile of all the dT values in the between 0% and 100%, the Xth percentile of all the dT values in the
Stream. In computing this percentile, undefined values are treated Stream. In computing this percentile, undefined values are treated
as infinitely large. Note that this means that the percentile could as infinitely large. Note that this means that the percentile could
thus be undefined (informally, infinite). In addition, the Type-P- thus be undefined (informally, infinite). In addition, the Type-P-
Round-trip-Delay-Percentile is undefined if the sample is empty. Round-trip-Delay-Percentile is undefined if the sample is empty.
Example: suppose we take a sample and the results are: Example: suppose we take a sample and the results are:
Stream1 = < Stream1 = <
<T1, 100 msec> <T1, 100 msec>
<T2, 110 msec> <T2, 110 msec>
<T3, undefined> <T3, undefined>
<T4, 90 msec> <T4, 90 msec>
<T5, 500 msec> <T5, 500 msec>
> >
Then the 50th percentile would be 110 msec, since 90 msec and 100 Then the 50th percentile would be 110 msec, since 90 msec and 100
msec are smaller and 110 msec and 'undefined' are larger. msec are smaller and 110 msec and 'undefined' are larger.
Note that if the possibility that a packet with finite delay is Note that if the possibility that a packet with finite delay is
reported as lost is significant, then a high percentile (90th or reported as lost is significant, then a high percentile (90th or
95th) might be reported as infinite instead of finite. 95th) might be reported as infinite instead of finite.
5.2. Type-P-Round-trip-Delay-Median 4.2. Type-P-Round-trip-Delay-Median
Given a Type-P-Round-trip-Delay-Poisson-Stream, the median of all the Given a Type-P-Round-trip-Delay-Poisson-Stream, the median of all the
dT values in the Stream. In computing the median, undefined values dT values in the Stream. In computing the median, undefined values
are treated as infinitely large. As with Type-P-Round-trip-Delay- are treated as infinitely large. As with Type-P-Round-trip-Delay-
Percentile, Type-P-Round-trip-Delay-Median is undefined if the sample Percentile, Type-P-Round-trip-Delay-Median is undefined if the sample
is empty. is empty.
As noted in the Framework document, the median differs from the 50th As noted in the Framework document, the median differs from the 50th
percentile only when the sample contains an even number of values, in percentile only when the sample contains an even number of values, in
which case the mean of the two central values is used. which case the mean of the two central values is used.
skipping to change at page 17, line 48 skipping to change at page 17, line 18
dT values in the Stream. In computing the median, undefined values dT values in the Stream. In computing the median, undefined values
are treated as infinitely large. As with Type-P-Round-trip-Delay- are treated as infinitely large. As with Type-P-Round-trip-Delay-
Percentile, Type-P-Round-trip-Delay-Median is undefined if the sample Percentile, Type-P-Round-trip-Delay-Median is undefined if the sample
is empty. is empty.
As noted in the Framework document, the median differs from the 50th As noted in the Framework document, the median differs from the 50th
percentile only when the sample contains an even number of values, in percentile only when the sample contains an even number of values, in
which case the mean of the two central values is used. which case the mean of the two central values is used.
Example: suppose we take a sample and the results are: Example: suppose we take a sample and the results are:
Stream2 = < Stream2 = <
<T1, 100 msec> <T1, 100 msec>
<T2, 110 msec> <T2, 110 msec>
<T3, undefined> <T3, undefined>
<T4, 90 msec> <T4, 90 msec>
> >
Then the median would be 105 msec, the mean of 100 msec and 110 msec, Then the median would be 105 msec, the mean of 100 msec and 110 msec,
the two central values. the two central values.
5.3. Type-P-Round-trip-Delay-Minimum 4.3. Type-P-Round-trip-Delay-Minimum
Given a Type-P-Round-trip-Delay-Poisson-Stream, the minimum of all Given a Type-P-Round-trip-Delay-Poisson-Stream, the minimum of all
the dT values in the Stream. In computing this, undefined values are the dT values in the Stream. In computing this, undefined values are
treated as infinitely large. Note that this means that the minimum treated as infinitely large. Note that this means that the minimum
could thus be undefined (informally, infinite) if all the dT values could thus be undefined (informally, infinite) if all the dT values
are undefined. In addition, the Type-P-Round-trip-Delay-Minimum is are undefined. In addition, the Type-P-Round-trip-Delay-Minimum is
undefined if the sample is empty. undefined if the sample is empty.
In the above example, the minimum would be 90 msec. In the above example, the minimum would be 90 msec.
5.4. Type-P-Round-trip-Delay-Inverse-Percentile 4.4. Type-P-Round-trip-Delay-Inverse-Percentile
Given a Type-P-Round-trip-Delay-Poisson-Stream and a non-negative Given a Type-P-Round-trip-Delay-Poisson-Stream and a time duration
time duration threshold, the fraction of all the dT values in the threshold, the fraction of all the dT values in the Stream less than
Stream less than or equal to the threshold. The result could be as or equal to the threshold. The result could be as low as 0% (if all
low as 0% (if all the dT values exceed threshold) or as high as 100%. the dT values exceed threshold) or as high as 100%. Type-P-Round-
Type-P-Round-trip-Delay-Inverse-Percentile is undefined if the sample trip-Delay-Inverse-Percentile is undefined if the sample is empty.
is empty.
In the above example, the Inverse-Percentile of 103 msec would be In the above example, the Inverse-Percentile of 103 msec would be
50%. 50%.
6. Security Considerations 5. Security Considerations
Conducting Internet measurements raises both security and privacy Conducting Internet measurements raises both security and privacy
concerns. This memo does not specify an implementation of the concerns. This memo does not specify an implementation of the
metrics, so it does not directly affect the security of the Internet metrics, so it does not directly affect the security of the Internet
nor of applications which run on the Internet. However, nor of applications which run on the Internet. However,
implementations of these metrics must be mindful of security and implementations of these metrics must be mindful of security and
privacy concerns. privacy concerns.
There are two types of security concerns: potential harm caused by There are two types of security concerns: potential harm caused by
the measurements, and potential harm to the measurements. The the measurements, and potential harm to the measurements. The
skipping to change at page 19, line 24 skipping to change at page 18, line 39
rate will be artificially lowered. Therefore, the measurement rate will be artificially lowered. Therefore, the measurement
methodologies SHOULD include appropriate techniques to reduce the methodologies SHOULD include appropriate techniques to reduce the
probability measurement traffic can be distinguished from "normal" probability measurement traffic can be distinguished from "normal"
traffic. Authentication techniques, such as digital signatures, may traffic. Authentication techniques, such as digital signatures, may
be used where appropriate to guard against injected traffic attacks. be used where appropriate to guard against injected traffic attacks.
The privacy concerns of network measurement are limited by the active The privacy concerns of network measurement are limited by the active
measurements described in this memo. Unlike passive measurements, measurements described in this memo. Unlike passive measurements,
there can be no release of existing user data. there can be no release of existing user data.
7. Acknowledgements 6. Acknowledgements
Special thanks are due to Vern Paxson and to Will Leland for several Special thanks are due to Vern Paxson and to Will Leland for several
useful suggestions. useful suggestions.
8. References 7. References
[1] V. Paxson, G. Almes, J. Mahdavi, and M. Mathis, "Framework for [1] Paxson, D., Almes, G., Mahdavi, J. and M. Mathis, "Framework for
IP Performance Metrics", RFC 2330, May 1998. IP Performance Metrics", RFC 2330, May 1998.
[2] G. Almes, S. Kalidindi, and M. Zekauskas, "A One-way Delay [2] Almes, G., Kalidindi,S. and M. Zekauskas, "A One-way Delay
Metric for IPPM", Internet Draft <draft-ietf-ippm-delay-05.txt>, Metric for IPPM", RFC 2679, September 1999.
November 1998.
[3] D. Mills, "Network Time Protocol (v3)", RFC 1305, April 1992. [3] Mills, D., "Network Time Protocol (v3)", RFC 1305, April 1992.
[4] J. Mahdavi and V. Paxson, "IPPM Metrics for Measuring [4] Mahdavi, J. and V. Paxson, "IPPM Metrics for Measuring
Connectivity", Internet-Draft <draft-ietf-ippm- Connectivity", RFC 2678, September 1999.
connectivity-03.txt>, October 1998.
[5] J. Postel, "Internet Protocol", RFC 791, September 1981. [5] Postel, J., "Internet Protocol", STD 5, RFC 791, September 1981.
[6] S. Bradner, "Key words for use in RFCs to Indicate Requirement [6] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", RFC 2119, March 1997. Levels", BCP 14, RFC 2119, March 1997.
9. Authors' Addresses 8. Authors' Addresses
Guy Almes Guy Almes
Advanced Network & Services, Inc. Advanced Network & Services, Inc.
200 Business Park Drive 200 Business Park Drive
Armonk, NY 10504 Armonk, NY 10504
USA USA
Phone: +1 914 765 1120 Phone: +1 914 765 1120
EMail: almes@advanced.org EMail: almes@advanced.org
skipping to change at page 20, line 27 skipping to change at page 19, line 35
Advanced Network & Services, Inc. Advanced Network & Services, Inc.
200 Business Park Drive 200 Business Park Drive
Armonk, NY 10504 Armonk, NY 10504
USA USA
Phone: +1 914 765 1128 Phone: +1 914 765 1128
EMail: kalidindi@advanced.org EMail: kalidindi@advanced.org
Matthew J. Zekauskas Matthew J. Zekauskas
Advanced Network & Services, Inc. Advanced Network & Services, Inc.
200 Buisiness Park Drive 200 Business Park Drive
Armonk, NY 10504 Armonk, NY 10504
USA USA
Phone: +1 914 765 1112 Phone: +1 914 765 1112
EMail: matt@advanced.org EMail: matt@advanced.org
Expiration date: May, 1999 9. Full Copyright Statement
Copyright (C) The Internet Society (1999). All Rights Reserved.
This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any
kind, provided that the above copyright notice and this paragraph are
included on all such copies and derivative works. However, this
document itself may not be modified in any way, such as by removing
the copyright notice or references to the Internet Society or other
Internet organizations, except as needed for the purpose of
developing Internet standards in which case the procedures for
copyrights defined in the Internet Standards process must be
followed, or as required to translate it into languages other than
English.
The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assigns.
This document and the information contained herein is provided on an
"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Acknowledgement
Funding for the RFC Editor function is currently provided by the
Internet Society.
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