draft-ietf-ippm-reporting-metrics-08.txt   draft-ietf-ippm-reporting-metrics-09.txt 
Network Working Group A. Morton Network Working Group A. Morton
Internet-Draft G. Ramachandran Internet-Draft G. Ramachandran
Intended status: Informational G. Maguluri Intended status: Informational G. Maguluri
Expires: September 12, 2012 AT&T Labs Expires: November 11, 2012 AT&T Labs
March 11, 2012 May 10, 2012
Reporting Metrics: Different Points of View Reporting IP Network Performance Metrics: Different Points of View
draft-ietf-ippm-reporting-metrics-08 draft-ietf-ippm-reporting-metrics-09
Abstract Abstract
Consumers of IP network performance metrics have many different uses Consumers of IP network performance metrics have many different uses
in mind. The memo provides "long-term" reporting considerations in mind. The memo provides "long-term" reporting considerations
(e.g, days, weeks or months, as opposed to 10 seconds), based on (e.g., hours, days, weeks or months, as opposed to 10 seconds), based
analysis of the two key audience points-of-view. It describes how on analysis of the two key audience points-of-view. It describes how
the audience categories affect the selection of metric parameters and the audience categories affect the selection of metric parameters and
options when seeking info that serves their needs. options when seeking info that serves their needs.
Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
Status of this Memo Status of this Memo
This Internet-Draft is submitted in full conformance with the This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79. provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet- working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/. Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference 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."
This Internet-Draft will expire on September 12, 2012. This Internet-Draft will expire on November 11, 2012.
Copyright Notice Copyright Notice
Copyright (c) 2012 IETF Trust and the persons identified as the Copyright (c) 2012 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of (http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents publication of this document. Please review these documents
skipping to change at page 3, line 15 skipping to change at page 3, line 15
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Purpose and Scope . . . . . . . . . . . . . . . . . . . . . . 4 2. Purpose and Scope . . . . . . . . . . . . . . . . . . . . . . 4
3. Reporting Results . . . . . . . . . . . . . . . . . . . . . . 5 3. Reporting Results . . . . . . . . . . . . . . . . . . . . . . 5
3.1. Overview of Metric Statistics . . . . . . . . . . . . . . 5 3.1. Overview of Metric Statistics . . . . . . . . . . . . . . 5
3.2. Long-Term Reporting Considerations . . . . . . . . . . . . 6 3.2. Long-Term Reporting Considerations . . . . . . . . . . . . 6
4. Effect of POV on the Loss Metric . . . . . . . . . . . . . . . 8 4. Effect of POV on the Loss Metric . . . . . . . . . . . . . . . 8
4.1. Loss Threshold . . . . . . . . . . . . . . . . . . . . . . 8 4.1. Loss Threshold . . . . . . . . . . . . . . . . . . . . . . 8
4.1.1. Network Characterization . . . . . . . . . . . . . . . 8 4.1.1. Network Characterization . . . . . . . . . . . . . . . 8
4.1.2. Application Performance . . . . . . . . . . . . . . . 10 4.1.2. Application Performance . . . . . . . . . . . . . . . 11
4.2. Errored Packet Designation . . . . . . . . . . . . . . . . 10 4.2. Errored Packet Designation . . . . . . . . . . . . . . . . 11
4.3. Causes of Lost Packets . . . . . . . . . . . . . . . . . . 10 4.3. Causes of Lost Packets . . . . . . . . . . . . . . . . . . 11
4.4. Summary for Loss . . . . . . . . . . . . . . . . . . . . . 11 4.4. Summary for Loss . . . . . . . . . . . . . . . . . . . . . 12
5. Effect of POV on the Delay Metric . . . . . . . . . . . . . . 11 5. Effect of POV on the Delay Metric . . . . . . . . . . . . . . 12
5.1. Treatment of Lost Packets . . . . . . . . . . . . . . . . 11 5.1. Treatment of Lost Packets . . . . . . . . . . . . . . . . 12
5.1.1. Application Performance . . . . . . . . . . . . . . . 12 5.1.1. Application Performance . . . . . . . . . . . . . . . 13
5.1.2. Network Characterization . . . . . . . . . . . . . . . 12 5.1.2. Network Characterization . . . . . . . . . . . . . . . 13
5.1.3. Delay Variation . . . . . . . . . . . . . . . . . . . 13 5.1.3. Delay Variation . . . . . . . . . . . . . . . . . . . 14
5.1.4. Reordering . . . . . . . . . . . . . . . . . . . . . . 14 5.1.4. Reordering . . . . . . . . . . . . . . . . . . . . . . 15
5.2. Preferred Statistics . . . . . . . . . . . . . . . . . . . 14 5.2. Preferred Statistics . . . . . . . . . . . . . . . . . . . 15
5.3. Summary for Delay . . . . . . . . . . . . . . . . . . . . 15 5.3. Summary for Delay . . . . . . . . . . . . . . . . . . . . 16
6. Reporting Raw Capacity Metrics . . . . . . . . . . . . . . . . 15 6. Reporting Raw Capacity Metrics . . . . . . . . . . . . . . . . 16
6.1. Type-P Parameter . . . . . . . . . . . . . . . . . . . . . 16 6.1. Type-P Parameter . . . . . . . . . . . . . . . . . . . . . 17
6.2. A priori Factors . . . . . . . . . . . . . . . . . . . . . 16 6.2. A priori Factors . . . . . . . . . . . . . . . . . . . . . 17
6.3. IP-layer Capacity . . . . . . . . . . . . . . . . . . . . 16 6.3. IP-layer Capacity . . . . . . . . . . . . . . . . . . . . 17
6.4. IP-layer Utilization . . . . . . . . . . . . . . . . . . . 17 6.4. IP-layer Utilization . . . . . . . . . . . . . . . . . . . 18
6.5. IP-layer Available Capacity . . . . . . . . . . . . . . . 17 6.5. IP-layer Available Capacity . . . . . . . . . . . . . . . 18
6.6. Variability in Utilization and Avail. Capacity . . . . . . 18 6.6. Variability in Utilization and Available Capacity . . . . 19
6.6.1. General Summary of Variability . . . . . . . . . . . . 18 6.6.1. General Summary of Variability . . . . . . . . . . . . 19
7. Reporting Restricted Capacity Metrics . . . . . . . . . . . . 19 7. Reporting Restricted Capacity Metrics . . . . . . . . . . . . 20
7.1. Type-P Parameter and Type-C Parameter . . . . . . . . . . 20 7.1. Type-P Parameter and Type-C Parameter . . . . . . . . . . 21
7.2. A priori Factors . . . . . . . . . . . . . . . . . . . . . 20 7.2. A priori Factors . . . . . . . . . . . . . . . . . . . . . 21
7.3. Measurement Interval . . . . . . . . . . . . . . . . . . . 20 7.3. Measurement Interval . . . . . . . . . . . . . . . . . . . 21
7.4. Bulk Transfer Capacity Reporting . . . . . . . . . . . . . 21 7.4. Bulk Transfer Capacity Reporting . . . . . . . . . . . . . 22
7.5. Variability in Bulk Transfer Capacity . . . . . . . . . . 22 7.5. Variability in Bulk Transfer Capacity . . . . . . . . . . 23
8. Reporting on Test Streams and Sample Size . . . . . . . . . . 22 8. Reporting on Test Streams and Sample Size . . . . . . . . . . 23
8.1. Test Stream Characteristics . . . . . . . . . . . . . . . 22 8.1. Test Stream Characteristics . . . . . . . . . . . . . . . 23
8.2. Sample Size . . . . . . . . . . . . . . . . . . . . . . . 23 8.2. Sample Size . . . . . . . . . . . . . . . . . . . . . . . 24
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 23 9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 24
10. Security Considerations . . . . . . . . . . . . . . . . . . . 23 10. Security Considerations . . . . . . . . . . . . . . . . . . . 24
11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 23 11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 25
12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 24 12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 25
12.1. Normative References . . . . . . . . . . . . . . . . . . . 24 12.1. Normative References . . . . . . . . . . . . . . . . . . . 25
12.2. Informative References . . . . . . . . . . . . . . . . . . 25 12.2. Informative References . . . . . . . . . . . . . . . . . . 26
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 25 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 26
1. Introduction 1. Introduction
When designing measurements of IP networks and presenting the When designing measurements of IP networks and presenting a result,
results, knowledge of the audience is a key consideration. To knowledge of the audience is a key consideration. To present a
present a useful and relevant portrait of network conditions, one useful and relevant portrait of network conditions, one must answer
must answer the following question: the following question:
"How will the results be used?" "How will the results be used?"
There are two main audience categories: There are two main audience categories for the report of results:
1. Network Characterization - describes conditions in an IP network 1. Network Characterization - describes conditions in an IP network
for quality assurance, troubleshooting, modeling, Service Level for quality assurance, troubleshooting, modeling, Service Level
Agreements (SLA), etc. The point-of-view looks inward, toward Agreements (SLA), etc. The point-of-view looks inward toward the
the network, and the consumer intends their actions there. network where the report consumer intends their actions.
2. Application Performance Estimation - describes the network 2. Application Performance Estimation - describes the network
conditions in a way that facilitates determining affects on user conditions in a way that facilitates determining effects on user
applications, and ultimately the users themselves. This point- applications, and ultimately the users themselves. This point-
of-view looks outward, toward the user(s), accepting the network of-view looks outward, toward the user(s), accepting the network
as-is. This consumer intends to estimate a network-dependent as-is. This report consumer intends to estimate a network-
aspect of performance, or design some aspect of an application's dependent aspect of performance, or design some aspect of an
accommodation of the network. (These are *not* application application's accommodation of the network. (These are *not*
metrics, they are defined at the IP layer.) application metrics, they are defined at the IP layer.)
This memo considers how these different points-of-view affect both This memo considers how these different points-of-view affect both
the measurement design (parameters and options of the metrics) and the measurement design (parameters and options of the metrics) and
statistics reported when serving their needs. statistics reported when serving their needs.
The IPPM framework [RFC2330] and other RFCs describing IPPM metrics The IPPM framework [RFC2330] and other RFCs describing IPPM metrics
provide a background for this memo. provide a background for this memo.
2. Purpose and Scope 2. Purpose and Scope
skipping to change at page 5, line 7 skipping to change at page 5, line 7
the loss and delay metrics [RFC2680] [RFC2679]. It will also discuss the loss and delay metrics [RFC2680] [RFC2679]. It will also discuss
the delay variation [RFC3393] and reordering metrics [RFC4737] where the delay variation [RFC3393] and reordering metrics [RFC4737] where
applicable. applicable.
With capacity metrics growing in relevance to the industry, the memo With capacity metrics growing in relevance to the industry, the memo
also covers POV and reporting considerations for metrics resulting also covers POV and reporting considerations for metrics resulting
from the Bulk Transfer Capacity Framework [RFC3148] and Network from the Bulk Transfer Capacity Framework [RFC3148] and Network
Capacity Definitions [RFC5136]. These memos effectively describe two Capacity Definitions [RFC5136]. These memos effectively describe two
different categories of metrics, different categories of metrics,
o [RFC3148] includes restrictions of congestion control and the o Restricted: [RFC3148] includes restrictions of congestion control
notion of unique data bits delivered, and and the notion of unique data bits delivered, and
o [RFC5136] using a definition of raw capacity without the o Raw: [RFC5136] using a definition of raw capacity without the
restrictions of data uniqueness or congestion-awareness. restrictions of data uniqueness or congestion awareness.
It might seem at first glance that each of these metrics has an It might seem at first glance that each of these metrics has an
obvious audience (Raw = Network Characterization, Restricted = obvious audience (Raw = Network Characterization, Restricted =
Application Performance), but reality is more complex and consistent Application Performance), but reality is more complex and consistent
with the overall topic of capacity measurement and reporting. For with the overall topic of capacity measurement and reporting. For
example, TCP is usually used in Restricted capacity measurement example, TCP is usually used in Restricted capacity measurement
methods, while UDP appears in Raw capacity measurement. The Raw and methods, while UDP appears in Raw capacity measurement. The Raw and
Restricted capacity metrics will be treated in separate sections, Restricted capacity metrics will be treated in separate sections,
although they share one common reporting issue: representing although they share one common reporting issue: representing
variability in capacity metric results as part of a long-term report. variability in capacity metric results as part of a long-term report.
Sampling, or the design of the active packet stream that is the basis Sampling, or the design of the active packet stream that is the basis
for the measurements, is also discussed. for the measurements, is also discussed.
3. Reporting Results 3. Reporting Results
This section gives an overview of recommendations, followed by This section gives an overview of recommendations, followed by
additional considerations for reporting results in the "long-term", additional considerations for reporting results in the "long term",
based on the discussion and conclusions of the major sections that based on the discussion and conclusions of the major sections that
follow. follow.
3.1. Overview of Metric Statistics 3.1. Overview of Metric Statistics
This section gives an overview of reporting recommendations for the This section gives an overview of reporting recommendations for the
loss, delay, and delay variation metrics. loss, delay, and delay variation metrics.
The minimal report on measurements MUST include both Loss and Delay The minimal report on measurements must include both Loss and Delay
Metrics. Metrics.
For Packet Loss, the loss ratio defined in [RFC2680] is a sufficient For Packet Loss, the loss ratio defined in [RFC2680] is a sufficient
starting point, especially the existing guidance for setting the loss starting point, especially the existing guidance for setting the loss
threshold waiting time. We have calculated a waiting time above that threshold waiting time. We have calculated a waiting time in section
should be sufficient to differentiate between packets that are truly 4.1.1 that should be sufficient to differentiate between packets that
lost or have long finite delays under general measurement are truly lost or have long finite delays under general measurement
circumstances, 51 seconds. Knowledge of specific conditions can help circumstances, 51 seconds. Knowledge of specific conditions can help
to reduce this threshold, but 51 seconds is considered to be to reduce this threshold, and a waiting time of approximately 50
manageable in practice. seconds is considered to be manageable in practice.
We note that a loss ratio calculated according to [Y.1540] would We note that a loss ratio calculated according to [Y.1540] would
exclude errored packets from the numerator. In practice, the exclude errored packets from the numerator. In practice, the
difference between these two loss metrics is small if any, depending difference between these two loss metrics is small if any, depending
on whether the last link prior to the destination contributes errored on whether the last link prior to the destination contributes errored
packets. packets.
For Packet Delay, we recommend providing both the mean delay and the For Packet Delay, we recommend providing both the mean delay and the
median delay with lost packets designated undefined (as permitted by median delay with lost packets designated undefined (as permitted by
[RFC2679]). Both statistics are based on a conditional distribution, [RFC2679]). Both statistics are based on a conditional distribution,
and the condition is packet arrival prior to a waiting time dT, where and the condition is packet arrival prior to a waiting time dT, where
dT has been set to take maximum packet lifetimes into account, as dT has been set to take maximum packet lifetimes into account, as
discussed above for loss. Using a long dT helps to ensure that delay discussed above for loss. Using a long dT helps to ensure that delay
distributions are not truncated. distributions are not truncated.
For Packet Delay Variation (PDV), the minimum delay of the For Packet Delay Variation (PDV), the minimum delay of the
conditional distribution should be used as the reference delay for conditional distribution should be used as the reference delay for
computing PDV according to [Y.1540] or [RFC5481] and [RFC3393]. A computing PDV according to [Y.1540] or [RFC5481] and [RFC3393]. A
useful value to report is a pseudo range of delay variation based on useful value to report is a pseudo range of delay variation based on
calculating the difference between a high percentile of delay and the calculating the difference between a high percentile of delay and the
minimum delay. For example, the 99.9%-ile minus the minimum will minimum delay. For example, the 99.9 percentile minus the minimum
give a value that can be compared with objectives in [Y.1541]. will give a value that can be compared with objectives in [Y.1541].
For Capacity, both Raw and Restricted, reporting the variability in a For Capacity, both Raw and Restricted, reporting the variability in a
useful way is identified as the main challenge. The Min, Max, and useful way is identified as the main challenge. The Min, Max, and
Range statistics are suggested along with a ratio of Max to Min and Range statistics are suggested along with a ratio of Max to Min and
moving averages. In the end, a simple plot of the singleton results moving averages. In the end, a simple plot of the singleton results
over time may succeed where summary metrics fail, or serve to confirm over time may succeed where summary metrics fail, or serve to confirm
that the summaries are valid. that the summaries are valid.
3.2. Long-Term Reporting Considerations 3.2. Long-Term Reporting Considerations
[I-D.ietf-ippm-reporting] describes methods to conduct measurements [I-D.ietf-ippm-reporting] describes methods to conduct measurements
and report the results on a near-immediate time scale (10 seconds, and report the results on a near-immediate time scale (10 seconds,
which we consider to be "short-term"). which we consider to be "short-term").
Measurement intervals and reporting intervals need not be the same Measurement intervals and reporting intervals need not be the same
length. Sometimes, the user is only concerned with the performance length. Sometimes, the user is only concerned with the performance
levels achieved over a relatively long interval of time (e.g, days, levels achieved over a relatively long interval of time (e.g., days,
weeks, or months, as opposed to 10 seconds). However, there can be weeks, or months, as opposed to 10 seconds). However, there can be
risks involved with running a measurement continuously over a long risks involved with running a measurement continuously over a long
period without recording intermediate results: period without recording intermediate results:
o Temporary power failure may cause loss of all the results to date. o Temporary power failure may cause loss of all the results to date.
o Measurement system timing synchronization signals may experience a o Measurement system timing synchronization signals may experience a
temporary outage, causing sub-sets of measurements to be in error temporary outage, causing sub-sets of measurements to be in error
or invalid. or invalid.
o Maintenance may be necessary on the measurement system, or its o Maintenance may be necessary on the measurement system, or its
connectivity to the network under test. connectivity to the network under test.
For these and other reasons, such as For these and other reasons, such as
o the constraint to collect measurements on intervals similar to o the constraint to collect measurements on intervals similar to
user session length, or user session length,
o the dual-use of measurements in monitoring activities where o the dual-use of measurements in monitoring activities where
results are needed on a period of a few minutes, results are needed on a period of a few minutes, or
o the ability to inspect results of a single measurement interval
for deeper analysis,
there is value in conducting measurements on intervals that are much there is value in conducting measurements on intervals that are much
shorter than the reporting interval. shorter than the reporting interval.
There are several approaches for aggregating a series of measurement There are several approaches for aggregating a series of measurement
results over time in order to make a statement about the longer results over time in order to make a statement about the longer
reporting interval. One approach requires the storage of all metric reporting interval. One approach requires the storage of all metric
singletons collected throughout the reporting interval, even though singletons collected throughout the reporting interval, even though
the measurement interval stops and starts many times. the measurement interval stops and starts many times.
Another approach is described in [RFC5835] as "temporal aggregation". Another approach is described in [RFC5835] as "temporal aggregation".
This approach would estimate the results for the reporting interval This approach would estimate the results for the reporting interval
based on many individual measurement interval statistics (results) based on many individual measurement interval statistics (results)
alone. The result would ideally appear in the same form as though a alone. The result would ideally appear in the same form as though a
continuous measurement was conducted. A memo to address the details continuous measurement had been conducted. A memo addressing the
of temporal aggregation is yet to be prepared. details of temporal aggregation is yet to be prepared.
Yet another approach requires a numerical objective for the metric, Yet another approach requires a numerical objective for the metric,
and the results of each measurement interval are compared with the and the results of each measurement interval are compared with the
objective. Every measurement interval where the results meet the objective. Every measurement interval where the results meet the
objective contribute to the fraction of time with performance as objective contribute to the fraction of time with performance as
specified. When the reporting interval contains many measurement specified. When the reporting interval contains many measurement
intervals it is possible to present the results as "metric A was less intervals it is possible to present the results as "metric A was less
than or equal to objective X during Y% of time. than or equal to objective X during Y% of time".
NOTE that numerical thresholds of acceptability are not set in IETF NOTE that numerical thresholds of acceptability are not set in IETF
performance work and are explicitly excluded from the IPPM charter. performance work and are therefore excluded from the scope of this
memo.
In all measurement, it is important to avoid unintended In all measurement, it is important to avoid unintended
synchronization with network events. This topic is treated in synchronization with network events. This topic is treated in
[RFC2330] for Poisson-distributed inter-packet time streams, and [RFC2330] for Poisson-distributed inter-packet time streams, and
[RFC3432] for Periodic streams. Both avoid synchronization through [RFC3432] for Periodic streams. Both avoid synchronization through
use of random start times. use of random start times.
There are network conditions where it is simply more useful to report There are network conditions where it is simply more useful to report
the connectivity status of the Source-Destination path, and to the connectivity status of the Source-Destination path, and to
distinguish time intervals where connectivity can be demonstrated distinguish time intervals where connectivity can be demonstrated
from other time intervals (where connectivity does not appear to from other time intervals (where connectivity does not appear to
exist). [RFC2678] specifies a number of one-way and two connectivity exist). [RFC2678] specifies a number of one-way and two-way
metrics of increasing complexity. In this memo, we RECOMMEND that connectivity metrics of increasing complexity. In this memo, we
long term reporting of loss, delay, and other metrics be limited to RECOMMEND that long term reporting of loss, delay, and other metrics
time intervals where connectivity can be demonstrated, and other be limited to time intervals where connectivity can be demonstrated,
intervals be summarized as percent of time where connectivity does and other intervals be summarized as percent of time where
not appear to exist. We note that this same approach has been connectivity does not appear to exist. We note that this same
adopted in ITU-T Recommendation [Y.1540] where performance parameters approach has been adopted in ITU-T Recommendation [Y.1540] where
are only valid during periods of service "availability" (evaluated performance parameters are only valid during periods of service
according to a function based on packet loss, and sustained periods "availability" (evaluated according to a function based on packet
of loss ratio greater than a threshold are declared "unavailable"). loss, and sustained periods of loss ratio greater than a threshold
are declared "unavailable").
4. Effect of POV on the Loss Metric 4. Effect of POV on the Loss Metric
This section describes the ways in which the Loss metric can be tuned This section describes the ways in which the Loss metric can be tuned
to reflect the preferences of the two audience categories, or to reflect the preferences of the two audience categories, or
different POV. The waiting time to declare a packet lost, or loss different POV. The waiting time to declare a packet lost, or loss
threshold is one area where there would appear to be a difference, threshold is one area where there would appear to be a difference,
but the ability to post-process the results may resolve it. but the ability to post-process the results may resolve it.
4.1. Loss Threshold 4.1. Loss Threshold
RFC 2680 [RFC2680] defines the concept of a waiting time for packets RFC 2680 [RFC2680] defines the concept of a waiting time for packets
to arrive, beyond which they are declared lost. The text of the RFC to arrive, beyond which they are declared lost. The text of the RFC
declines to recommend a value, instead saying that "good engineering, declines to recommend a value, instead saying that "good engineering,
including an understanding of packet lifetimes, will be needed in including an understanding of packet lifetimes, will be needed in
practice." Later, in the methodology, they give reasons for waiting practice." Later, in the methodology, they give reasons for waiting
"a reasonable period of time", and leaving the definition of "a reasonable period of time", and leaving the definition of
"reasonable" intentionally vague. "reasonable" intentionally vague. We estimate a practical bound on
waiting time below.
4.1.1. Network Characterization 4.1.1. Network Characterization
Practical measurement experience has shown that unusual network Practical measurement experience has shown that unusual network
circumstances can cause long delays. One such circumstance is when circumstances can cause long delays. One such circumstance is when
routing loops form during IGP re-convergence following a failure or routing loops form during IGP re-convergence following a failure or
drastic link cost change. Packets will loop between two routers drastic link cost change. Packets will loop between two routers
until new routes are installed, or until the IPv4 Time-to-Live (TTL) until new routes are installed, or until the IPv4 Time-to-Live (TTL)
field (or the IPv6 Hop Limit) decrements to zero. Very long delays field (or the IPv6 Hop Limit) decrements to zero. Very long delays
on the order of several seconds have been measured [Casner] [Cia03]. on the order of several seconds have been measured [Casner] [Cia03].
skipping to change at page 9, line 6 skipping to change at page 9, line 12
Therefore, network characterization activities prefer a long waiting Therefore, network characterization activities prefer a long waiting
time in order to distinguish these events from other causes of loss time in order to distinguish these events from other causes of loss
(such as packet discard at a full queue, or tail drop). This way, (such as packet discard at a full queue, or tail drop). This way,
the metric design helps to distinguish more reliably between packets the metric design helps to distinguish more reliably between packets
that might yet arrive, and those that are no longer traversing the that might yet arrive, and those that are no longer traversing the
network. network.
It is possible to calculate a worst-case waiting time, assuming that It is possible to calculate a worst-case waiting time, assuming that
a routing loop is the cause. We model the path between Source and a routing loop is the cause. We model the path between Source and
Destination as a series of delays in links (t) and queues (q), as Destination as a series of delays in links (t) and queues (q), as
these two are the dominant contributors to delay. The normal path these are the dominant contributors to delay (in active measurement,
delay across n hops without encountering a loop, D, is the Source and Destination hosts contribute minimal delay). The
normal path delay, D, across n queues (where TTL is decremented at a
node with a queue) and n+1 links without encountering a loop, is
n Path model with n=5
--- Source --- q1 --- q2 --- q3 --- q4 --- q5 --- Destination
\ t0 t1 t2 t3 t4 t5
D = t + > (t + q )
0 / i i n
--- ---
i = 1 \
D = t + > (t + q )
0 / i i
---
i = 1
Figure 1: Normal Path Delay Figure 1: Normal Path Delay
and the time spent in the loop with L hops, is and the time spent in the loop with L queues, is
Path model with n=5 and L=3
Time in one Loop = (qx+tx + qy+ty + qz+tz)
j + L-1 qy -- qz
--- | ?/exit?
\ (TTL - n) qx--/\
R = C > (t + q ) where C = --------- Src --- q1 --- q2 ---/ q3 --- q4 --- q5 --- Dst
/ i i max L t0 t1 t2 t3 t4 t5
---
i=j where j is the hop number where the loop begins j + L-1
---
\ (TTL - n)
R = C > (t + q ) where C = ---------
/ i i max L
---
i=j
Figure 2: Delay due to Rotations in a Loop Figure 2: Delay due to Rotations in a Loop
where C is the number of times a packet circles the loop, and where where n is the total number of queues in the non-loop path (with n+1
TTL is the packet's initial Time-to-Live value at the source (or Hop links), j is the queue number where the loop begins, C is the number
Count in IPv6). of times a packet circles the loop, and TTL is the packet's initial
Time-to-Live value at the source (or Hop Count in IPv6).
If we take the delays of all links and queues as 100ms each, the If we take the delays of all links and queues as 100ms each, the
TTL=255, the number of hops n=5 and the hops in the loop L=4, then TTL=255, the number of queues n=5 and the queues in the loop L=4,
then using C_max:
D = 1.1 sec and R ~= 50 sec, and D + R ~= 51.1 seconds D = 1.1 sec and R ~= 50 sec, and D + R ~= 51.1 seconds
We note that the link delays of 100ms would span most continents, and We note that the link delays of 100ms would span most continents, and
a constant queue length of 100ms is also very generous. When a loop a constant queue length of 100ms is also very generous. When a loop
occurs, it is almost certain to be resolved in 10 seconds or less. occurs, it is almost certain to be resolved in 10 seconds or less.
The value calculated above is an upper limit for almost any real- The value calculated above is an upper limit for almost any real-
world circumstance. world circumstance.
A waiting time threshold parameter, dT, set consistent with this A waiting time threshold parameter, dT, set consistent with this
skipping to change at page 10, line 13 skipping to change at page 11, line 8
network. network.
It is worth noting that packets that are stored and deliberately It is worth noting that packets that are stored and deliberately
forwarded at a much later time constitute a replay attack on the forwarded at a much later time constitute a replay attack on the
measurement system, and are beyond the scope of normal performance measurement system, and are beyond the scope of normal performance
reporting. reporting.
4.1.2. Application Performance 4.1.2. Application Performance
Fortunately, application performance estimation activities are not Fortunately, application performance estimation activities are not
adversely affected by the estimated worst-case transfer time. adversely affected by the long estimated limit on waiting time,
Although the designer's tendency might be to set the Loss Threshold because most applications will use shorter time thresholds. Although
at a value equivalent to a particular application's threshold, this the designer's tendency might be to set the Loss Threshold at a value
specific threshold can be applied when post-processing the equivalent to a particular application's threshold, this specific
measurements. A shorter waiting time can be enforced by locating threshold can be applied when post-processing the measurements. A
packets with delays longer than the application's threshold, and re- shorter waiting time can be enforced by locating packets with delays
designating such packets as lost. Thus, the measurement system can longer than the application's threshold, and re-designating such
use a single loss waiting time and support both application and packets as lost. Thus, the measurement system can use a single loss
network performance POVs simultaneously. waiting time and support both application and network performance
POVs simultaneously.
4.2. Errored Packet Designation 4.2. Errored Packet Designation
RFC 2680 designates packets that arrive containing errors as lost RFC 2680 designates packets that arrive containing errors as lost
packets. Many packets that are corrupted by bit errors are discarded packets. Many packets that are corrupted by bit errors are discarded
within the network and do not reach their intended destination. within the network and do not reach their intended destination.
This is consistent with applications that would check the payload This is consistent with applications that would check the payload
integrity at higher layers, and discard the packet. However, some integrity at higher layers, and discard the packet. However, some
applications prefer to deal with errored payloads on their own, and applications prefer to deal with errored payloads on their own, and
skipping to change at page 11, line 4 skipping to change at page 11, line 47
Although many measurement systems use a waiting time to determine if Although many measurement systems use a waiting time to determine if
a packet is lost or not, most of the waiting is in vain. The packets a packet is lost or not, most of the waiting is in vain. The packets
are no-longer traversing the network, and have not reached their are no-longer traversing the network, and have not reached their
destination. destination.
There are many causes of packet loss, including: There are many causes of packet loss, including:
1. Queue drop, or discard 1. Queue drop, or discard
2. Corruption of the IP header, or other essential header info 2. Corruption of the IP header, or other essential header info
3. TTL expiration (or use of a TTL value that is too small) 3. TTL expiration (or use of a TTL value that is too small)
4. Link or router failure 4. Link or router failure
5. Layers below the source-to-destination IP layer can discard 5. Layers below the source-to-destination IP layer can discard
packets that fail error checking and link-layer checksums often packets that fail error checking, and link-layer checksums often
cover the entire packet cover the entire packet
After waiting sufficient time, packet loss can probably be attributed It is reasonable to consider a packet that has not arrived after a
to one of these causes. large amount of time to be lost (due to one of the causes above)
because packets do not "live forever" in the network, or have
infinite delay.
4.4. Summary for Loss 4.4. Summary for Loss
Given that measurement post-processing is possible (even encouraged Given that measurement post-processing is possible (even encouraged
in the definitions of IPPM metrics), measurements of loss can easily in the definitions of IPPM metrics), measurements of loss can easily
serve both points of view: serve both points of view:
o Use a long waiting time to serve network characterization and o Use a long waiting time to serve network characterization and
revise results for specific application delay thresholds as revise results for specific application delay thresholds as
needed. needed.
skipping to change at page 12, line 9 skipping to change at page 13, line 9
lost packets. We next look at how these two different treatments lost packets. We next look at how these two different treatments
align with the needs of measurement consumers who wish to align with the needs of measurement consumers who wish to
characterize networks or estimate application performance. Also, we characterize networks or estimate application performance. Also, we
look at the way that lost packets have been treated in other metrics: look at the way that lost packets have been treated in other metrics:
delay variation and reordering. delay variation and reordering.
5.1.1. Application Performance 5.1.1. Application Performance
Applications need to perform different functions, dependent on Applications need to perform different functions, dependent on
whether or not each packet arrives within some finite tolerance. In whether or not each packet arrives within some finite tolerance. In
other words, a receivers' packet processing takes one of two other words, a receiver's packet processing takes only one of two
directions (or "forks" in the road): alternative directions (a "fork" in the road):
o Packets that arrive within expected tolerance are handled by o Packets that arrive within expected tolerance are handled by
processes that remove headers, restore smooth delivery timing (as removing headers, restoring smooth delivery timing (as in a de-
in a de-jitter buffer), restore sending order, check for errors in jitter buffer), restoring sending order, checking for errors in
payloads, and many other operations. payloads, and many other operations.
o Packets that do not arrive when expected spawn other processes o Packets that do not arrive when expected lead to attempted
that attempt recovery from the apparent loss, such as recovery from the apparent loss, such as retransmission requests,
retransmission requests, loss concealment, or forward error loss concealment, or forward error correction to replace the
correction to replace the missing packet. missing packet.
So, it is important to maintain a distinction between packets that So, it is important to maintain a distinction between packets that
actually arrive, and those that do not. Therefore, it is preferable actually arrive, and those that do not. Therefore, it is preferable
to leave the delay of lost packets undefined, and to characterize the to leave the delay of lost packets undefined, and to characterize the
delay distribution as a conditional distribution (conditioned on delay distribution as a conditional distribution (conditioned on
arrival). arrival).
5.1.2. Network Characterization 5.1.2. Network Characterization
In this discussion, we assume that both loss and delay metrics will In this discussion, we assume that both loss and delay metrics will
be reported for network characterization (at least). be reported for network characterization (at least).
Assume packets that do not arrive are reported as Lost, usually as a Assume packets that do not arrive are reported as Lost, usually as a
fraction of all sent packets. If these lost packets are assigned fraction of all sent packets. If these lost packets are assigned
undefined delay, then the network's inability to deliver them (in a undefined delay, then the network's inability to deliver them (in a
timely way) is relegated only in the Loss metric when we report timely way) is relegated only in the Loss metric when we report
statistics on the Delay distribution conditioned on the event of statistics on the Delay distribution conditioned on the event of
packet arrival (within the Loss waiting time threshold). We can say packet arrival (within the Loss waiting time threshold). We can say
that the Delay and Loss metrics are Orthogonal, in that they convey that the Delay and Loss metrics are orthogonal, in that they convey
non-overlapping information about the network under test. This is a non-overlapping information about the network under test. This is a
valuable property, whose absence is discussed below. valuable property, whose absence is discussed below.
However, if we assign infinite delay to all lost packets, then: However, if we assign infinite delay to all lost packets, then:
o The delay metric results are influenced both by packets that o The delay metric results are influenced both by packets that
arrive and those that do not. arrive and those that do not.
o The delay singleton and the loss singleton do not appear to be o The delay singleton and the loss singleton do not appear to be
orthogonal (Delay is finite when Loss=0, Delay is infinite when orthogonal (Delay is finite when Loss=0, Delay is infinite when
Loss=1). Loss=1).
o The network is penalized in both the loss and delay metrics, o The network is penalized in both the loss and delay metrics,
effectively double-counting the lost packets. effectively double-counting the lost packets.
As further evidence of overlap, consider the Cumulative Distribution As further evidence of overlap, consider the Cumulative Distribution
Function (CDF) of Delay when the value positive infinity is assigned Function (CDF) of Delay when the value "positive infinity" is
to all lost packets. Figure 3 shows a CDF where a small fraction of assigned to all lost packets. Figure 3 shows a CDF where a small
packets are lost. fraction of packets are lost.
1 | - - - - - - - - - - - - - - - - - -+ 1 | - - - - - - - - - - - - - - - - - -+
| | | |
| _..----'''''''''''''''''''' | _..----''''''''''''''''''''
| ,-'' | ,-''
| ,' | ,'
| / Mass at | / Mass at
| / +infinity | / +infinity
| / = fraction | / = fraction
|| lost || lost
skipping to change at page 13, line 34 skipping to change at page 14, line 34
0 Delay +o0 0 Delay +o0
Figure 3: Cumulative Distribution Function for Delay when Loss = Figure 3: Cumulative Distribution Function for Delay when Loss =
+Infinity +Infinity
We note that a Delay CDF that is conditioned on packet arrival would We note that a Delay CDF that is conditioned on packet arrival would
not exhibit this apparent overlap with loss. not exhibit this apparent overlap with loss.
Although infinity is a familiar mathematical concept, it is somewhat Although infinity is a familiar mathematical concept, it is somewhat
disconcerting to see any time-related metric reported as infinity, in disconcerting to see any time-related metric reported as infinity.
the opinion of the authors. Questions are bound to arise, and tend Questions are bound to arise, and tend to detract from the goal of
to detract from the goal of informing the consumer with a performance informing the consumer with a performance report.
report.
5.1.3. Delay Variation 5.1.3. Delay Variation
[RFC3393] excludes lost packets from samples, effectively assigning [RFC3393] excludes lost packets from samples, effectively assigning
an undefined delay to packets that do not arrive in a reasonable an undefined delay to packets that do not arrive in a reasonable
time. Section 4.1 of [RFC3393] describes this specification and its time. Section 4.1 of [RFC3393] describes this specification and its
rationale (ipdv = inter-packet delay variation in the quote below). rationale (ipdv = inter-packet delay variation in the quote below).
"The treatment of lost packets as having "infinite" or "undefined" "The treatment of lost packets as having "infinite" or "undefined"
delay complicates the derivation of statistics for ipdv. delay complicates the derivation of statistics for ipdv.
skipping to change at page 14, line 18 skipping to change at page 15, line 17
at the same time avoiding events with undefined outcomes." at the same time avoiding events with undefined outcomes."
We note that the argument above applies to all forms of packet delay We note that the argument above applies to all forms of packet delay
variation that can be constructed using the "selection function" variation that can be constructed using the "selection function"
concept of [RFC3393]. In recent work the two main forms of delay concept of [RFC3393]. In recent work the two main forms of delay
variation metrics have been compared and the results are summarized variation metrics have been compared and the results are summarized
in [RFC5481]. in [RFC5481].
5.1.4. Reordering 5.1.4. Reordering
[RFC4737]defines metrics that are based on evaluation of packet [RFC4737] defines metrics that are based on evaluation of packet
arrival order, and include a waiting time to declare a packet lost arrival order, and include a waiting time to declare a packet lost
(to exclude them from further processing). (to exclude them from further processing).
If packets are assigned a delay value, then the reordering metric If packets are assigned a delay value, then the reordering metric
would declare any packets with infinite delay to be reordered, would declare any packets with infinite delay to be reordered,
because their sequence numbers will surely be less than the "Next because their sequence numbers will surely be less than the "Next
Expected" threshold when (or if) they arrive. But this practice Expected" threshold when (or if) they arrive. But this practice
would fail to maintain orthogonality between the reordering metric would fail to maintain orthogonality between the reordering metric
and the loss metric. Confusion can be avoided by designating the and the loss metric. Confusion can be avoided by designating the
delay of non-arriving packets as undefined, and reserving delay delay of non-arriving packets as undefined, and reserving delay
skipping to change at page 14, line 50 skipping to change at page 15, line 49
The median is another statistic that summarizes a distribution, The median is another statistic that summarizes a distribution,
having somewhat different properties from the sample mean. The having somewhat different properties from the sample mean. The
median is stable in distributions with a few outliers or without median is stable in distributions with a few outliers or without
them. However, the median's stability prevents it from indicating them. However, the median's stability prevents it from indicating
when a large fraction of the distribution changes value. 50% or more when a large fraction of the distribution changes value. 50% or more
values would need to change for the median to capture the change. values would need to change for the median to capture the change.
Both the median and sample mean have difficulty with bimodal Both the median and sample mean have difficulty with bimodal
distributions. The median will reside in only one of the modes, and distributions. The median will reside in only one of the modes, and
the mean may not lie in either mode range. For this and other the mean may not lie in either mode range. For this and other
reasons, additional statistics such as the minimum, maximum, and 95%- reasons, additional statistics such as the minimum, maximum, and 95
ile have value when summarizing a distribution. percentile have value when summarizing a distribution.
When both the sample mean and median are available, a comparison will When both the sample mean and median are available, a comparison will
sometimes be informative, because these two statistics are equal only sometimes be informative, because these two statistics are equal only
when the delay distribution is perfectly symmetrical. under unusual circumstances, such as when the delay distribution is
perfectly symmetrical.
Also, these statistics are generally useful from the Application Also, these statistics are generally useful from the Application
Performance POV, so there is a common set that should satisfy Performance POV, so there is a common set that should satisfy
audiences. audiences.
Plots of the delay distribution may also be useful when single-value Plots of the delay distribution may also be useful when single-value
statistics indicate that new conditions are present. An empirically- statistics indicate that new conditions are present. An empirically-
derived probability distribution function will usually describe derived probability distribution function will usually describe
multiple modes more efficiently than any other form of result. multiple modes more efficiently than any other form of result.
skipping to change at page 15, line 31 skipping to change at page 16, line 32
1. application/receiver analysis, where subsequent processing 1. application/receiver analysis, where subsequent processing
depends on whether the packet arrives or times-out, depends on whether the packet arrives or times-out,
2. straightforward network characterization without double-counting 2. straightforward network characterization without double-counting
defects, and defects, and
3. consistency with Delay variation and Reordering metric 3. consistency with Delay variation and Reordering metric
definitions, definitions,
the most efficient practice is to distinguish between truly lost and the most efficient practice is to distinguish between packets that
delayed packets with a sufficiently long waiting time, and to are truly lost and those that are delayed packets with a sufficiently
designate the delay of non-arriving packets as undefined. long waiting time, and to designate the delay of non-arriving packets
as undefined.
6. Reporting Raw Capacity Metrics 6. Reporting Raw Capacity Metrics
Raw capacity refers to the metrics defined in [RFC5136] which do not Raw capacity refers to the metrics defined in [RFC5136] which do not
include restrictions such as data uniqueness or flow-control response include restrictions such as data uniqueness or flow-control response
to congestion. to congestion.
The metrics considered are IP-layer Capacity, Utilization (or used The metrics considered are IP-layer Capacity, Utilization (or used
capacity), and Available Capacity, for individual links and complete capacity), and Available Capacity, for individual links and complete
paths. These three metrics form a triad: knowing one metric paths. These three metrics form a triad: knowing one metric
constrains the other two (within their allowed range), and knowing constrains the other two (within their allowed range), and knowing
two determines the third. The link metrics have another key aspect two determines the third. The link metrics have another key aspect
in common: they are single-measurement-point metrics at the egress of in common: they are single-measurement-point metrics at the egress of
a link. The path Capacity and Available Capacity are derived by a link. The path Capacity and Available Capacity are derived by
examining the set of single-point link measurements and taking the examining the set of single-point link measurements and taking the
minimum value. minimum value.
6.1. Type-P Parameter 6.1. Type-P Parameter
The concept of "packets of type-P" is defined in [RFC2330]. The The concept of "packets of Type-P" is defined in [RFC2330]. The
type-P categorization has critical relevance in all forms of capacity Type-P categorization has critical relevance in all forms of capacity
measurement and reporting. The ability to categorize packets based measurement and reporting. The ability to categorize packets based
on header fields for assignment to different queues and scheduling on header fields for assignment to different queues and scheduling
mechanisms is now common place. When un-used resources are shared mechanisms is now common place. When un-used resources are shared
across queues, the conditions in all packet categories will affect across queues, the conditions in all packet categories will affect
capacity and related measurements. This is one source of variability capacity and related measurements. This is one source of variability
in the results that all audiences would prefer to see reported in a in the results that all audiences would prefer to see reported in a
useful and easily understood way. useful and easily understood way.
Type-P in OWAMP and TWAMP is essentially confined to the Diffserv Communication of Type-P within the One-way Active Measurement
Codepoint [RFC4656]. DSCP is the most common qualifier for type-P. Protocol (OWAMP) and the Two-way Active Measurement Protocol (TWAMP)
is essentially confined to the Diffserv Codepoint [RFC4656]. DSCP is
the most common qualifier for Type-P.
Each audience will have a set of type-P qualifications and value Each audience will have a set of Type-P qualifications and value
combinations that are of interest. Measurements and reports SHOULD combinations that are of interest. Measurements and reports should
have the flexibility to report per-type and aggregate performance. have the flexibility to report per-type and aggregate performance.
6.2. A priori Factors 6.2. A priori Factors
The audience for Network Characterization may have detailed The audience for Network Characterization may have detailed
information about each link that comprises a complete path (due to information about each link that comprises a complete path (due to
ownership, for example), or some of the links in the path but not ownership, for example), or some of the links in the path but not
others, or none of the links. others, or none of the links.
There are cases where the measurement audience only has information There are cases where the measurement audience only has information
skipping to change at page 17, line 40 skipping to change at page 18, line 42
Transmission Unit (MTU) packet will observe on/off behavior and Transmission Unit (MTU) packet will observe on/off behavior and
report 100% or 0%. The smallest interval needs to be some multiple report 100% or 0%. The smallest interval needs to be some multiple
of MTU serialization time for averaging to be effective. of MTU serialization time for averaging to be effective.
6.5. IP-layer Available Capacity 6.5. IP-layer Available Capacity
The Available Capacity of a link can be calculated using the Capacity The Available Capacity of a link can be calculated using the Capacity
and Utilization metrics. and Utilization metrics.
When Available capacity of a link or path is estimated through some When Available capacity of a link or path is estimated through some
measurement technique, the following parameters SHOULD be reported: measurement technique, the following parameters should be reported:
o Name and reference to the exact method of measurement o Name and reference to the exact method of measurement
o IP packet length, octets (including IP header) o IP packet length, octets (including IP header)
o Maximum Capacity that can be assessed in the measurement o Maximum Capacity that can be assessed in the measurement
configuration configuration
o The time duration of the measurement o The time duration of the measurement
skipping to change at page 18, line 16 skipping to change at page 19, line 21
actual Available capacity of the link or path. Therefore, it is actual Available capacity of the link or path. Therefore, it is
important to know the capacity value beyond which there will be no important to know the capacity value beyond which there will be no
measured improvement. measured improvement.
The Application Design audience may have a desired target capacity The Application Design audience may have a desired target capacity
value and simply wish to assess whether there is sufficient Available value and simply wish to assess whether there is sufficient Available
Capacity. This case simplifies measurement of link and path capacity Capacity. This case simplifies measurement of link and path capacity
to some degree, as long as the measurable maximum exceeds the target to some degree, as long as the measurable maximum exceeds the target
capacity. capacity.
6.6. Variability in Utilization and Avail. Capacity 6.6. Variability in Utilization and Available Capacity
As with most metrics and measurements, assessing the consistency or As with most metrics and measurements, assessing the consistency or
variability in the results gives the user an intuitive feel for the variability in the results gives the user an intuitive feel for the
degree (or confidence) that any one value is representative of other degree (or confidence) that any one value is representative of other
results, or the spread of the underlying distribution of the results, or the spread of the underlying distribution of the
singleton measurements. singleton measurements.
How can Utilization be measured and summarized to describe the How can Utilization be measured and summarized to describe the
potential variability in a useful way? potential variability in a useful way?
skipping to change at page 19, line 32 skipping to change at page 20, line 36
Restricted capacity refers to the metrics defined in [RFC3148] which Restricted capacity refers to the metrics defined in [RFC3148] which
include criteria of data uniqueness or flow-control response to include criteria of data uniqueness or flow-control response to
congestion. congestion.
In primary metric considered is Bulk Transfer Capacity (BTC) for In primary metric considered is Bulk Transfer Capacity (BTC) for
complete paths. [RFC3148] defines complete paths. [RFC3148] defines
BTC = data_sent / elapsed_time BTC = data_sent / elapsed_time
for a connection with congestion-aware flow control, where data_sent for a connection with congestion aware flow control, where data_sent
is the total of unique payload bits (no headers). is the total of unique payload bits (no headers).
We note that this definition *differs* from the raw capacity We note that this definition *differs* from the raw capacity
definition in Section 2.3.1 of [RFC5136], where IP-layer Capacity definition in Section 2.3.1 of [RFC5136], where IP-layer Capacity
*includes* all bits in the IP header and payload. This means that *includes* all bits in the IP header and payload. This means that
Restricted Capacity BTC is already operating at a disadvantage when Restricted Capacity BTC is already operating at a disadvantage when
compared to the raw capacity at layers below TCP. Further, there are compared to the raw capacity at layers below TCP. Further, there are
cases where one IP-layer is encapsulated in another IP-layer or other cases where one IP-layer is encapsulated in another IP-layer or other
form of tunneling protocol, designating more and more of the form of tunneling protocol, designating more and more of the
fundamental transport capacity as header bits that are pure overhead fundamental transport capacity as header bits that are pure overhead
skipping to change at page 20, line 12 skipping to change at page 21, line 15
overlapping, but they reveal two different and important aspects of overlapping, but they reveal two different and important aspects of
performance. performance.
When thinking about the triad of raw capacity metrics, BTC is most When thinking about the triad of raw capacity metrics, BTC is most
akin to the "IP-Type-P Available Path Capacity", at least in the eyes akin to the "IP-Type-P Available Path Capacity", at least in the eyes
of a network user who seeks to know what transmission performance a of a network user who seeks to know what transmission performance a
path might support. path might support.
7.1. Type-P Parameter and Type-C Parameter 7.1. Type-P Parameter and Type-C Parameter
The concept of "packets of type-P" is defined in [RFC2330]. The The concept of "packets of Type-P" is defined in [RFC2330]. The
considerations for Restricted Capacity are identical to the raw considerations for Restricted Capacity are identical to the raw
capacity section on this topic, with the addition that the various capacity section on this topic, with the addition that the various
fields and options in the TCP header MUST be included in the fields and options in the TCP header must be included in the
description. description.
The vast array of TCP flow control options are not well-captured by The vast array of TCP flow control options are not well-captured by
Type-P, because they do not exist in the TCP header bits. Therefore, Type-P, because they do not exist in the TCP header bits. Therefore,
we introduce a new notion here: TCP Configuration of "Type-C". The we introduce a new notion here: TCP Configuration of "Type-C". The
elements of Type-C describe all of the settings for TCP options and elements of Type-C describe all of the settings for TCP options and
congestion control algorithm variables, including the main form of congestion control algorithm variables, including the main form of
congestion control in use. congestion control in use. Readers should consider the parameters
and variables of [RFC3148] and [RFC6349] when constructing Type-C.
7.2. A priori Factors 7.2. A priori Factors
The audience for Network Characterization may have detailed The audience for Network Characterization may have detailed
information about each link that comprises a complete path (due to information about each link that comprises a complete path (due to
ownership, for example), or some of the links in the path but not ownership, for example), or some of the links in the path but not
others, or none of the links. others, or none of the links.
There are cases where the measurement audience only has information There are cases where the measurement audience only has information
on one of the links (the local access link), and wishes to measure on one of the links (the local access link), and wishes to measure
skipping to change at page 21, line 27 skipping to change at page 22, line 33
well as the BTC). well as the BTC).
Individual measurement intervals may be short or long, but there is a Individual measurement intervals may be short or long, but there is a
need to report the results on a long-term basis that captures the BTC need to report the results on a long-term basis that captures the BTC
variability experienced between each interval. Consistent BTC is a variability experienced between each interval. Consistent BTC is a
valuable commodity along with the value attained. valuable commodity along with the value attained.
7.4. Bulk Transfer Capacity Reporting 7.4. Bulk Transfer Capacity Reporting
When BTC of a link or path is estimated through some measurement When BTC of a link or path is estimated through some measurement
technique, the following parameters SHOULD be reported: technique, the following parameters should be reported:
o Name and reference to the exact method of measurement o Name and reference to the exact method of measurement
o Maximum Transmission Unit (MTU) o Maximum Transmission Unit (MTU)
o Maximum BTC that can be assessed in the measurement configuration o Maximum BTC that can be assessed in the measurement configuration
o The time and duration of the measurement o The time and duration of the measurement
o The number of BTC connections used simultaneously o The number of BTC connections used simultaneously
skipping to change at page 22, line 23 skipping to change at page 23, line 29
measurements have come. measurements have come.
With two questions looming: With two questions looming:
1. What ways can BTC be measured and summarized to describe the 1. What ways can BTC be measured and summarized to describe the
potential variability in a useful way? potential variability in a useful way?
2. How can the variability in BTC estimates be reported, so that the 2. How can the variability in BTC estimates be reported, so that the
confidence in the results is also conveyed? confidence in the results is also conveyed?
we suggest the methods of Section 6.6.1 above, and the additional We suggest the methods of Section 6.6.1 above, and the additional
results presentations given in [RFC6349]. results presentations given in [RFC6349].
8. Reporting on Test Streams and Sample Size 8. Reporting on Test Streams and Sample Size
This section discusses two key aspects of measurement that are This section discusses two key aspects of measurement that are
sometimes omitted from the report: the description of the test stream sometimes omitted from the report: the description of the test stream
on which the measurements are based, and the sample size. on which the measurements are based, and the sample size.
8.1. Test Stream Characteristics 8.1. Test Stream Characteristics
skipping to change at page 23, line 31 skipping to change at page 24, line 34
3. The effects of active measurement traffic on user traffic. 3. The effects of active measurement traffic on user traffic.
A sample size may sometimes be referred to as "large". This is a A sample size may sometimes be referred to as "large". This is a
relative, and qualitative term. It is preferable to describe what relative, and qualitative term. It is preferable to describe what
one is attempting to achieve with their sample. For example, stating one is attempting to achieve with their sample. For example, stating
an implication may be helpful: this sample is large enough such that an implication may be helpful: this sample is large enough such that
a single outlying value at ten times the "typical" sample mean (the a single outlying value at ten times the "typical" sample mean (the
mean without the outlying value) would influence the mean by no more mean without the outlying value) would influence the mean by no more
than X. than X.
The Appendix of [RFC2330] indicates that sample size of 128
singletons worked well for goodness-of-fit testing, while a much
larger size almost always failed (8192 singletons).
9. IANA Considerations 9. IANA Considerations
This document makes no request of IANA. This document makes no request of IANA.
Note to RFC Editor: this section may be removed on publication as an Note to RFC Editor: this section may be removed on publication as an
RFC. RFC.
10. Security Considerations 10. Security Considerations
The security considerations that apply to any active measurement of The security considerations that apply to any active measurement of
live networks are relevant here as well. See [RFC4656]. live networks are relevant here as well. See [RFC4656] for mandatory
to implement security features that intend to mitigate attacks
described in the corresponding security considerations section.
Measurement systems conducting long-term measurements are more
exposed to threats as a by-product of ports open longer to perform
their task, and more easily detected measurement activity on those
ports. Further, use of long packet waiting times affords an attacker
a better opportunity to prepare and launch a replay attack.
11. Acknowledgements 11. Acknowledgements
The authors thank: Phil Chimento for his suggestion to employ The authors thank: Phil Chimento for his suggestion to employ
conditional distributions for Delay, Steve Konish Jr. for his careful conditional distributions for Delay, Steve Konish Jr. for his careful
review and suggestions, Dave McDysan and Don McLachlan for useful review and suggestions, Dave McDysan and Don McLachlan for useful
comments based on their long experience with measurement and comments based on their long experience with measurement and
reporting, Daniel Genin for his observation of non-orthogonality reporting, Daniel Genin for his observation of non-orthogonality
between Raw and Restricted Capacity metrics (and our omission of this between Raw and Restricted Capacity metrics (and our omission of this
fact), and Matt Zekauskas for suggestions on organizing the memo for fact), and Matt Zekauskas for suggestions on organizing the memo for
easier consumption. easier consumption.
12. References 12. References
12.1. Normative References 12.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2330] Paxson, V., Almes, G., Mahdavi, J., and M. Mathis, [RFC2330] Paxson, V., Almes, G., Mahdavi, J., and M. Mathis,
"Framework for IP Performance Metrics", RFC 2330, "Framework for IP Performance Metrics", RFC 2330,
May 1998. May 1998.
[RFC2678] Mahdavi, J. and V. Paxson, "IPPM Metrics for Measuring [RFC2678] Mahdavi, J. and V. Paxson, "IPPM Metrics for Measuring
Connectivity", RFC 2678, September 1999. Connectivity", RFC 2678, September 1999.
[RFC2679] Almes, G., Kalidindi, S., and M. Zekauskas, "A One-way [RFC2679] Almes, G., Kalidindi, S., and M. Zekauskas, "A One-way
Delay Metric for IPPM", RFC 2679, September 1999. Delay Metric for IPPM", RFC 2679, September 1999.
 End of changes. 61 change blocks. 
167 lines changed or deleted 199 lines changed or added

This html diff was produced by rfcdiff 1.41. The latest version is available from http://tools.ietf.org/tools/rfcdiff/