💾 Archived View for gemini.bortzmeyer.org › rfc-mirror › rfc6390.txt captured on 2023-07-22 at 18:54:41.
⬅️ Previous capture (2021-11-30)
-=-=-=-=-=-=-
Internet Engineering Task Force (IETF) A. Clark
Request for Comments: 6390 Telchemy Incorporated
BCP: 170 B. Claise
Category: Best Current Practice Cisco Systems, Inc.
ISSN: 2070-1721 October 2011
Guidelines for Considering New Performance Metric Development
Abstract
This document describes a framework and a process for developing
Performance Metrics of protocols and applications transported over
IETF-specified protocols. These metrics can be used to characterize
traffic on live networks and services.
Status of This Memo
This memo documents an Internet Best Current Practice.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
BCPs is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc6390.
Copyright Notice
Copyright (c) 2011 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Clark & Claise Best Current Practice [Page 1]
RFC 6390 Guidelines Perf. Metric Devel. October 2011
This document may contain material from IETF Documents or IETF
Contributions published or made publicly available before November
10, 2008. The person(s) controlling the copyright in some of this
material may not have granted the IETF Trust the right to allow
modifications of such material outside the IETF Standards Process.
Without obtaining an adequate license from the person(s) controlling
the copyright in such materials, this document may not be modified
outside the IETF Standards Process, and derivative works of it may
not be created outside the IETF Standards Process, except to format
it for publication as an RFC or to translate it into languages other
than English.
Clark & Claise Best Current Practice [Page 2]
RFC 6390 Guidelines Perf. Metric Devel. October 2011
Table of Contents
1. Introduction ....................................................4
1.1. Background and Motivation ..................................4
1.2. Organization of This Document ..............................5
2. Terminology .....................................................5
2.1. Requirements Language ......................................5
2.2. Performance Metrics Directorate ............................5
2.3. Quality of Service .........................................5
2.4. Quality of Experience ......................................6
2.5. Performance Metric .........................................6
3. Purpose and Scope ...............................................6
4. Relationship between QoS, QoE, and Application-Specific
Performance Metrics .............................................7
5. Performance Metrics Development .................................7
5.1. Identifying and Categorizing the Audience ..................7
5.2. Definitions of a Performance Metric ........................8
5.3. Computed Performance Metrics ...............................9
5.3.1. Composed Performance Metrics ........................9
5.3.2. Index ..............................................10
5.4. Performance Metric Specification ..........................10
5.4.1. Outline ............................................10
5.4.2. Normative Parts of Performance Metric Definition ...11
5.4.3. Informative Parts of Performance Metric
Definition .........................................13
5.4.4. Performance Metric Definition Template .............14
5.4.5. Example: Loss Rate .................................15
5.5. Dependencies ..............................................16
5.5.1. Timing Accuracy ....................................16
5.5.2. Dependencies of Performance Metric Definitions on
Related Events or Metrics ..........................16
5.5.3. Relationship between Performance Metric and
Lower-Layer Performance Metrics ....................17
5.5.4. Middlebox Presence .................................17
5.6. Organization of Results ...................................17
5.7. Parameters: The Variables of a Performance Metric .........18
6. Performance Metric Development Process .........................18
6.1. New Proposals for Performance Metrics .....................18
6.2. Reviewing Metrics .........................................19
6.3. Performance Metrics Directorate Interaction with
Other WGs .................................................19
6.4. Standards Track Performance Metrics .......................20
7. Security Considerations ........................................20
8. Acknowledgements ...............................................20
9. References .....................................................21
9.1. Normative References ......................................21
9.2. Informative References ....................................21
Clark & Claise Best Current Practice [Page 3]
RFC 6390 Guidelines Perf. Metric Devel. October 2011
1. Introduction
Many networking technologies, applications, or services are
distributed in nature, and their performance may be impacted by IP
impairments, server capacity, congestion, and other factors. It is
important to measure the performance of applications and services to
ensure that quality objectives are being met and to support problem
diagnosis. Standardized metrics help ensure that performance
measurement is implemented consistently, and they facilitate
interpretation and comparison.
There are at least three phases in the development of performance
standards. They are as follows:
1. Definition of a Performance Metric and its units of measure
2. Specification of a method of measurement
3. Specification of the reporting format
During the development of metrics, it is often useful to define
performance objectives and expected value ranges. This additional
information is typically not part of the formal specification of the
metric but does provide useful background for implementers and users
of the metric.
The intended audience for this document includes, but is not limited
to, IETF participants who write Performance Metrics documents in the
IETF, reviewers of such documents, and members of the Performance
Metrics Directorate.
1.1. Background and Motivation
Previous IETF work related to the reporting of application
Performance Metrics includes "Real-time Application Quality-of-
Service Monitoring (RAQMON) Framework" [RFC4710]. This framework
extends the remote network monitoring (RMON) family of specifications
to allow real-time quality-of-service (QoS) monitoring of various
applications that run on devices such as IP phones, pagers, Instant
Messaging clients, mobile phones, and various other handheld
computing devices. Furthermore, "RTP Control Protocol Extended
Reports (RTCP XR)" [RFC3611] and "Session Initiation Protocol Event
Package for Voice Quality Reporting" [RFC6035] define protocols that
support real-time Quality of Experience (QoE) reporting for Voice
over IP (VoIP) and other applications running on devices such as IP
phones and mobile handsets.
Clark & Claise Best Current Practice [Page 4]
RFC 6390 Guidelines Perf. Metric Devel. October 2011
The IETF is also actively involved in the development of reliable
transport protocols, such as TCP [RFC0793] or the Stream Control
Transmission Protocol (SCTP) [RFC4960], which would affect the
relationship between IP performance and application performance.
Thus, there is a gap in the currently chartered coverage of IETF
Working Groups (WGs): development of Performance Metrics for
protocols above and below the IP layer that can be used to
characterize performance on live networks.
Similar to "Guidelines for Considering Operations and Management of
New Protocols and Protocol Extensions" [RFC5706], which is the
reference document for the IETF Operations Directorate, this document
should be consulted as part of the new Performance Metric review by
the members of the Performance Metrics Directorate.
1.2. Organization of This Document
This document is divided into two major sections beyond the "Purpose
and Scope" section. The first is a definition and description of a
Performance Metric and its key aspects. The second defines a process
to develop these metrics that is applicable to the IETF environment.
2. Terminology
2.1. 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].
2.2. Performance Metrics Directorate
The Performance Metrics Directorate is a directorate that provides
guidance for Performance Metrics development in the IETF.
The Performance Metrics Directorate should be composed of experts in
the performance community, potentially selected from the IP
Performance Metrics (IPPM), Benchmarking Methodology (BMWG), and
Performance Metrics for Other Layers (PMOL) WGs.
2.3. Quality of Service
Quality of Service (QoS) is defined in a way similar to the ITU
"Quality of Service (QoS)" section of [E.800], i.e., "Totality of
characteristics of a telecommunications service that bear on its
ability to satisfy stated and implied needs of the user of the
service".
Clark & Claise Best Current Practice [Page 5]
RFC 6390 Guidelines Perf. Metric Devel. October 2011
2.4. Quality of Experience
Quality of Experience (QoE) is defined in a way similar to the ITU
"QoS experienced/perceived by customer/user (QoSE)" section of
[E.800], i.e., "a statement expressing the level of quality that
customers/users believe they have experienced".
NOTE 1 - The level of QoS experienced and/or perceived by the
customer/user may be expressed by an opinion rating.
NOTE 2 - QoE has two main components: quantitative and
qualitative. The quantitative component can be influenced by the
complete end-to-end system effects (including user devices and
network infrastructure).
NOTE 3 - The qualitative component can be influenced by user
expectations, ambient conditions, psychological factors,
application context, etc.
NOTE 4 - QoE may also be considered as QoS delivered, received,
and interpreted by a user with the pertinent qualitative factors
influencing his/her perception of the service.
2.5. Performance Metric
A Performance Metric is a quantitative measure of performance,
specific to an IETF-specified protocol or specific to an application
transported over an IETF-specified protocol. Examples of Performance
Metrics are the FTP response time for a complete file download, the
DNS response time to resolve the IP address, a database logging time,
etc.
3. Purpose and Scope
The purpose of this document is to define a framework and a process
for developing Performance Metrics for protocols above and below the
IP layer (such as IP-based applications that operate over reliable or
datagram transport protocols). These metrics can be used to
characterize traffic on live networks and services. As such, this
document does not define any Performance Metrics.
The scope of this document covers guidelines for the Performance
Metrics Directorate members for considering new Performance Metrics
and suggests how the Performance Metrics Directorate will interact
with the rest of the IETF. However, this document is not intended to
supersede existing working methods within WGs that have existing
chartered work in this area.
Clark & Claise Best Current Practice [Page 6]
RFC 6390 Guidelines Perf. Metric Devel. October 2011
This process is not intended to govern Performance Metric development
in existing IETF WGs that are focused on metrics development, such as
the IPPM and BMWG WGs. However, this guidelines document may be
useful in these activities and MAY be applied where appropriate. A
typical example is the development of Performance Metrics to be
exported with the IP Flow Information eXport (IPFIX) protocol
[RFC5101], with specific IPFIX information elements [RFC5102], which
would benefit from the framework in this document.
The framework in this document applies to Performance Metrics derived
from both active and passive measurements.
4. Relationship between QoS, QoE, and Application-Specific Performance
Metrics
Network QoS deals with network and network protocol performance,
while QoE deals with the assessment of a user's experience in the
context of a task or a service. The topic of application-specific
Performance Metrics includes the measurement of performance at layers
between IP and the user. For example, network QoS metrics (packet
loss, delay, and delay variation [RFC5481]) can be used to estimate
application-specific Performance Metrics (de-jitter buffer size and
RTP-layer packet loss), and then combined with other known aspects of
a VoIP application (such as codec type) using an algorithm compliant
with ITU-T P.564 [P.564] to estimate a Mean Opinion Score (MOS)
[P.800]. However, the QoE for a particular VoIP user depends on the
specific context, such as a casual conversation, a business
conference call, or an emergency call. Finally, QoS and application-
specific Performance Metrics are quantitative, while QoE is
qualitative. Also, network QoS and application-specific Performance
Metrics can be directly or indirectly evident to the user, while the
QoE is directly evident.
5. Performance Metrics Development
This section provides key definitions and qualifications of
Performance Metrics.
5.1. Identifying and Categorizing the Audience
Many of the aspects of metric definition and reporting, even the
selection or determination of the essential metrics, depend on who
will use the results, and for what purpose. For example, the metric
description SHOULD include use cases and example reports that
illustrate service quality monitoring and maintenance or
identification and quantification of problems.
Clark & Claise Best Current Practice [Page 7]
RFC 6390 Guidelines Perf. Metric Devel. October 2011
All documents defining Performance Metrics SHOULD identify the
primary audience and its associated requirements. The audience can
influence both the definition of metrics and the methods of
measurement.
The key areas of variation between different metric users include:
o Suitability of passive measurements of live traffic or active
measurements using dedicated traffic
o Measurement in laboratory environment or on a network of deployed
devices
o Accuracy of the results
o Access to measurement points and configuration information
o Measurement topology (point-to-point, point-to-multipoint)
o Scale of the measurement system
o Measurements conducted on-demand or continuously
o Required reporting formats and periods
o Sampling criteria [RFC5474], such as systematic or probabilistic
o Period (and duration) of measurement, as the live traffic can have
patterns
5.2. Definitions of a Performance Metric
A Performance Metric is a measure of an observable behavior of a
networking technology, an application, or a service. Most of the
time, the Performance Metric can be directly measured; however,
sometimes, the Performance Metric value is computed. The process for
determining the value of a metric may assume an implicit or explicit
underlying statistical process; in this case, the Performance Metric
is an estimate of a parameter of this process, assuming that the
statistical process closely models the behavior of the system.
A Performance Metric should serve some defined purposes. This may
include the measurement of capacity, quantifying how bad some
problems are, measurement of service level, problem diagnosis or
location, and other such uses. A Performance Metric may also be an
input to some other processes, for example, the computation of a
composite Performance Metric or a model or simulation of a system.
Tests of the "usefulness" of a Performance Metric include:
Clark & Claise Best Current Practice [Page 8]
RFC 6390 Guidelines Perf. Metric Devel. October 2011
(i) the degree to which its absence would cause significant loss
of information on the behavior or performance of the application
or system being measured
(ii) the correlation between the Performance Metric, the QoS, and
the QoE delivered to the user (person or other application)
(iii) the degree to which the Performance Metric is able to
support the identification and location of problems affecting
service quality
(iv) the requirement to develop policies (Service Level Agreement,
and potentially Service Level Contract) based on the Performance
Metric
For example, consider a distributed application operating over a
network connection that is subject to packet loss. A Packet Loss
Rate (PLR) Performance Metric is defined as the mean packet loss
ratio over some time period. If the application performs poorly over
network connections with a high packet loss ratio and always performs
well when the packet loss ratio is zero, then the PLR Performance
Metric is useful to some degree. Some applications are sensitive to
short periods of high loss (bursty loss) and are relatively
insensitive to isolated packet loss events; for this type of
application, there would be very weak correlation between PLR and
application performance. A "better" Performance Metric would
consider both the packet loss ratio and the distribution of loss
events. If application performance is degraded when the PLR exceeds
some rate, then a useful Performance Metric may be a measure of the
duration and frequency of periods during which the PLR exceeds that
rate (as, for example, in RFC 3611).
5.3. Computed Performance Metrics
5.3.1. Composed Performance Metrics
Some Performance Metrics may not be measured directly, but can be
composed from base metrics that have been measured. A composed
Performance Metric is derived from other metrics by applying a
deterministic process or function (e.g., a composition function).
The process may use metrics that are identical to the metric being
composed, or metrics that are dissimilar, or some combination of both
types. Usually, the base metrics have a limited scope in time or
space, and they can be combined to estimate the performance of some
larger entities.
Clark & Claise Best Current Practice [Page 9]
RFC 6390 Guidelines Perf. Metric Devel. October 2011
Some examples of composed Performance Metrics and composed
Performance Metric definitions are as follows:
Spatial composition is defined as the composition of metrics of
the same type with differing spatial domains [RFC5835] [RFC6049].
Ideally, for spatially composed metrics to be meaningful, the
spatial domains should be non-overlapping and contiguous, and the
composition operation should be mathematically appropriate for the
type of metric.
Temporal composition is defined as the composition of sets of
metrics of the same type with differing time spans [RFC5835]. For
temporally composed metrics to be meaningful, the time spans
should be non-overlapping and contiguous, and the composition
operation should be mathematically appropriate for the type of
metric.
Temporal aggregation is a summarization of metrics into a smaller
number of metrics that relate to the total time span covered by
the original metrics. An example would be to compute the minimum,
maximum, and average values of a series of time-sampled values of
a metric.
In the context of flow records in IP Flow Information eXport (IPFIX),
the IPFIX Mediation Framework [RFC6183], based on "IP Flow
Information Export (IPFIX) Mediation: Problem Statement" [RFC5982],
also discusses some aspects of the temporal and spatial composition.
5.3.2. Index
An index is a metric for which the output value range has been
selected for convenience or clarity, and the behavior of which is
selected to support ease of understanding, for example, the R Factor
[G.107]. The deterministic function for an index is often developed
after the index range and behavior have been determined.
5.4. Performance Metric Specification
5.4.1. Outline
A Performance Metric definition MUST have a normative part that
defines what the metric is and how it is measured or computed, and it
SHOULD have an informative part that describes the Performance Metric
and its application.
Clark & Claise Best Current Practice [Page 10]
RFC 6390 Guidelines Perf. Metric Devel. October 2011
5.4.2. Normative Parts of Performance Metric Definition
The normative part of a Performance Metric definition MUST define at
least the following:
(i) Metric Name
Performance Metric names are RECOMMENDED to be unique within the
set of metrics being defined for the protocol layer and context.
While strict uniqueness may not be attainable (see the IPPM
registry [RFC6248] for an example of an IANA metric registry
failing to provide sufficient specificity), broad review must be
sought to avoid naming overlap. Note that the Performance Metrics
Directorate can help with suggestions for IANA metric registration
for unique naming. The Performance Metric name MAY be
descriptive.
(ii) Metric Description
The Performance Metric description MUST explain what the metric
is, what is being measured, and how this relates to the
performance of the system being measured.
(iii) Method of Measurement or Calculation
The method of measurement or calculation MUST define what is being
measured or computed and the specific algorithm to be used. Does
the measurement involve active or only passive measurements?
Terms such as "average" should be qualified (e.g., running average
or average over some interval). Exception cases SHOULD also be
defined with the appropriate handling method. For example, there
are a number of commonly used metrics related to packet loss;
these often don't define the criteria by which a packet is
determined to be lost (versus very delayed) or how duplicate
packets are handled. For example, if the average PLR during a
time interval is reported, and a packet's arrival is delayed from
one interval to the next, then was it "lost" during the interval
during which it should have arrived or should it be counted as
received?
Some methods of calculation might require discarding some data
collected (due to outliers) so as to make the measurement
parameters meaningful. One example is burstable billing that
sorts the 5-min samples and discards the top 5 percentile.
Clark & Claise Best Current Practice [Page 11]
RFC 6390 Guidelines Perf. Metric Devel. October 2011
Some parameters linked to the method MAY also be reported, in
order to fully interpret the Performance Metric, for example, the
time interval, the load, the minimum packet loss, the potential
measurement errors and their sources, the attainable accuracy of
the metric (e.g., +/- 0.1), the method of calculation, etc.
(iv) Units of Measurement
The units of measurement MUST be clearly stated.
(v) Measurement Point(s) with Potential Measurement Domain
If the measurement is specific to a measurement point, this SHOULD
be defined. The measurement domain MAY also be defined.
Specifically, if measurement points are spread across domains, the
measurement domain (intra-, inter-) is another factor to consider.
The Performance Metric definition should discuss how the
Performance Metric value might vary, depending on which
measurement point is chosen. For example, the time between a SIP
request [RFC3261] and the final response can be significantly
different at the User Agent Client (UAC) or User Agent Server
(UAS).
In some cases, the measurement requires multiple measurement
points: all measurement points SHOULD be defined, including the
measurement domain(s).
(vi) Measurement Timing
The acceptable range of timing intervals or sampling intervals for
a measurement, and the timing accuracy required for such
intervals, MUST be specified. Short sampling intervals or
frequent samples provide a rich source of information that can
help assess application performance but may lead to excessive
measurement data. Long measurement or sampling intervals reduce
the amount of reported and collected data such that it may be
insufficient to understand application performance or service
quality, insofar as the measured quantity may vary significantly
with time.
In the case of multiple measurement points, the potential
requirement for synchronized clocks must be clearly specified. In
the specific example of the IP delay variation application metric,
the different aspects of synchronized clocks are discussed in
[RFC5481].
Clark & Claise Best Current Practice [Page 12]
RFC 6390 Guidelines Perf. Metric Devel. October 2011
5.4.3. Informative Parts of Performance Metric Definition
The informative part of a Performance Metric specification is
intended to support the implementation and use of the metric. This
part SHOULD provide the following data:
(i) Implementation
The implementation description MAY be in the form of text, an
algorithm, or example software. The objective of this part of the
metric definition is to help implementers achieve consistent
results.
(ii) Verification
The Performance Metric definition SHOULD provide guidance on
verification testing. This may be in the form of test vectors, a
formal verification test method, or informal advice.
(iii) Use and Applications
The use and applications description is intended to help the
"user" understand how, when, and where the metric can be applied,
and what significance the value range for the metric may have.
This MAY include a definition of the "typical" and "abnormal"
range of the Performance Metric, if this was not apparent from the
nature of the metric. The description MAY include information
about the influence of extreme measurement values, i.e., if the
Performance Metric is sensitive to outliers. The Use and
Application section SHOULD also include the security implications
in the description.
For example:
(a) it is fairly intuitive that a lower packet loss ratio would
equate to better performance. However, the user may not know
the significance of some given packet loss ratio.
Clark & Claise Best Current Practice [Page 13]
RFC 6390 Guidelines Perf. Metric Devel. October 2011
(b) the speech level of a telephone signal is commonly expressed
in dBm0. If the user is presented with:
Speech level = -7 dBm0
this is not intuitively understandable, unless the user
is a telephony expert. If the metric definition explains
that the typical range is -18 to -28 dBm0, a value higher
than -18 means the signal may be too high (loud), and
less than -28 means that the signal may be too low
(quiet), it is much easier to interpret the metric.
(iv) Reporting Model
The reporting model definition is intended to make any
relationship between the metric and the reporting model clear.
There are often implied relationships between the method of
reporting metrics and the metric itself; however, these are often
not made apparent to the implementor. For example, if the metric
is a short-term running average packet delay variation (e.g., the
inter-arrival jitter in [RFC3550]) and this value is reported at
intervals of 6-10 seconds, the resulting measurement may have
limited accuracy when packet delay variation is non-stationary.
5.4.4. Performance Metric Definition Template
Normative
o Metric Name
o Metric Description
o Method of Measurement or Calculation
o Units of Measurement
o Measurement Point(s) with Potential Measurement Domain
o Measurement Timing
Clark & Claise Best Current Practice [Page 14]
RFC 6390 Guidelines Perf. Metric Devel. October 2011
Informative
o Implementation
o Verification
o Use and Applications
o Reporting Model
5.4.5. Example: Loss Rate
The example used is the loss rate metric as specified in RFC 3611
[RFC3611].
Metric Name: LossRate
Metric Description: The fraction of RTP data packets from the source
lost since the beginning of reception.
Method of Measurement or Calculation: This value is calculated by
dividing the total number of packets lost (after the effects of
applying any error protection, such as Forward Error Correction
(FEC)) by the total number of packets expected, multiplying the
result of the division by 256, limiting the maximum value to 255
(to avoid overflow), and taking the integer part.
Units of Measurement: This metric is expressed as a fixed-point
number with the binary point at the left edge of the field. For
example, a metric value of 12 means a loss rate of
approximately 5%.
Measurement Point(s) with Potential Measurement Domain: This metric
is made at the receiving end of the RTP stream sent during a Voice
over IP call.
Measurement Timing: This metric can be used over a wide range of
time intervals. Using time intervals of longer than one hour may
prevent the detection of variations in the value of this metric
due to time-of-day changes in network load. Timing intervals
should not vary in duration by more than +/- 2%.
Implementation: The numbers of duplicated packets and discarded
packets do not enter into this calculation. Since receivers
cannot be required to maintain unlimited buffers, a receiver MAY
categorize late-arriving packets as lost. The degree of lateness
that triggers a loss SHOULD be significantly greater than that
which triggers a discard.
Clark & Claise Best Current Practice [Page 15]
RFC 6390 Guidelines Perf. Metric Devel. October 2011
Verification: The metric value ranges between 0 and 255.
Use and Applications: This metric is useful for monitoring VoIP
calls, more precisely, to detect the VoIP loss rate in the
network. This loss rate, along with the rate of packets discarded
due to jitter, has some effect on the quality of the voice stream.
Reporting Model: This metric needs to be associated with a defined
time interval, which could be defined by fixed intervals or by a
sliding window. In the context of RFC 3611, the metric is
measured continuously from the start of the RTP stream, and the
value of the metric is sampled and reported in RTCP XR VoIP
Metrics reports.
5.5. Dependencies
This section introduces several Performance Metrics dependencies,
which the Performance Metric designer should keep in mind during
Performance Metric development. These dependencies, and any others
not listed here, SHOULD be documented in the Performance Metric
specifications.
5.5.1. Timing Accuracy
The accuracy of the timing of a measurement may affect the accuracy
of the Performance Metric. This may not materially affect a sampled-
value metric; however, it would affect an interval-based metric.
Some metrics -- for example, the number of events per time interval
-- would be directly affected; for example, a 10% variation in time
interval would lead directly to a 10% variation in the measured
value. Other metrics, such as the average packet loss ratio during
some time interval, would be affected to a lesser extent.
If it is necessary to correlate sampled values or intervals, then it
is essential that the accuracy of sampling time and interval start/
stop times is sufficient for the application (for example, +/- 2%).
5.5.2. Dependencies of Performance Metric Definitions on Related Events
or Metrics
Performance Metric definitions may explicitly or implicitly rely on
factors that may not be obvious. For example, the recognition of a
packet as being "lost" relies on having some method of knowing the
packet was actually lost (e.g., RTP sequence number), and some time
threshold after which a non-received packet is declared lost. It is
important that any such dependencies are recognized and incorporated
into the metric definition.
Clark & Claise Best Current Practice [Page 16]
RFC 6390 Guidelines Perf. Metric Devel. October 2011
5.5.3. Relationship between Performance Metric and Lower-Layer
Performance Metrics
Lower-layer Performance Metrics may be used to compute or infer the
performance of higher-layer applications, potentially using an
application performance model. The accuracy of this will depend on
many factors, including:
(i) The completeness of the set of metrics (i.e., are there
metrics for all the input values to the application performance
model?)
(ii) Correlation between input variables (being measured) and
application performance
(iii) Variability in the measured metrics and how this variability
affects application performance
5.5.4. Middlebox Presence
Presence of a middlebox [RFC3303], e.g., proxy, network address
translation (NAT), redirect server, session border controller (SBC)
[RFC5853], and application layer gateway (ALG), may add variability
to or restrict the scope of measurements of a metric. For example,
an SBC that does not process RTP loopback packets may block or
locally terminate this traffic rather than pass it through to its
target.
5.6. Organization of Results
The IPPM Framework [RFC2330] organizes the results of metrics into
three related notions:
o singleton: an elementary instance, or "atomic" value.
o sample: a set of singletons with some common properties and some
varying properties.
o statistic: a value derived from a sample through deterministic
calculation, such as the mean.
Performance Metrics MAY use this organization for the results, with
or without the term names used by the IPPM WG. Section 11 of
RFC 2330 [RFC2330] should be consulted for further details.
Clark & Claise Best Current Practice [Page 17]
RFC 6390 Guidelines Perf. Metric Devel. October 2011
5.7. Parameters: the Variables of a Performance Metric
Metrics are completely defined when all options and input variables
have been identified and considered. These variables are sometimes
left unspecified in a metric definition, and their general name
indicates that the user must set and report them with the results.
Such variables are called "parameters" in the IPPM metric template.
The scope of the metric, the time at which it was conducted, the
length interval of the sliding-window measurement, the settings for
timers, and the thresholds for counters are all examples of
parameters.
All documents defining Performance Metrics SHOULD identify all key
parameters for each Performance Metric.
6. Performance Metric Development Process
6.1. New Proposals for Performance Metrics
This process is intended to add more considerations to the processes
for adopting new work as described in RFC 2026 [RFC2026] and RFC 2418
[RFC2418]. Note that new Performance Metrics work item proposals
SHALL be approved using the existing IETF process. The following
entry criteria will be considered for each proposal.
Proposals SHOULD be prepared as Internet-Drafts, describing the
Performance Metric and conforming to the qualifications above as much
as possible. Proposals SHOULD be deliverables of the corresponding
protocol development WG charters. As such, the proposals SHOULD be
vetted by that WG prior to discussion by the Performance Metrics
Directorate. This aspect of the process includes an assessment of
the need for the Performance Metric proposed and assessment of the
support for its development in the IETF.
Proposals SHOULD include an assessment of interaction and/or overlap
with work in other Standards Development Organizations (SDOs).
Proposals SHOULD identify additional expertise that might be
consulted.
Proposals SHOULD specify the intended audience and users of the
Performance Metrics. The development process encourages
participation by members of the intended audience.
Proposals SHOULD identify any security and IANA requirements.
Security issues could potentially involve revealing data identifying
a user, or the potential misuse of active test tools. IANA
considerations may involve the need for a Performance Metrics
registry.
Clark & Claise Best Current Practice [Page 18]
RFC 6390 Guidelines Perf. Metric Devel. October 2011
6.2. Reviewing Metrics
Each Performance Metric SHOULD be assessed according to the following
list of qualifications:
o Are the performance metrics unambiguously defined?
o Are the units of measure specified?
o Does the metric clearly define the measurement interval where
applicable?
o Are significant sources of measurement errors identified and
discussed?
o Does the method of measurement ensure that results are repeatable?
o Does the metric or method of measurement appear to be
implementable (or offer evidence of a working implementation)?
o Are there any undocumented assumptions concerning the underlying
process that would affect an implementation or interpretation of
the metric?
o Can the metric results be related to application performance or
user experience, when such a relationship is of value?
o Is there an existing relationship to metrics defined elsewhere
within the IETF or within other SDOs?
o Do the security considerations adequately address denial-of-
service attacks, unwanted interference with the metric/
measurement, and user data confidentiality (when measuring live
traffic)?
6.3. Performance Metrics Directorate Interaction with Other WGs
The Performance Metrics Directorate SHALL provide guidance to the
related protocol development WG when considering an Internet-Draft
that specifies Performance Metrics for a protocol. A sufficient
number of individuals with expertise must be willing to consult on
the draft. If the related WG has concluded, comments on the proposal
should still be sought from key RFC authors and former chairs.
As with expert reviews performed by other directorates, a formal
review is recommended by the time the document is reviewed by the
Area Directors or an IETF Last Call is being conducted.
Clark & Claise Best Current Practice [Page 19]
RFC 6390 Guidelines Perf. Metric Devel. October 2011
Existing mailing lists SHOULD be used; however, a dedicated mailing
list MAY be initiated if necessary to facilitate work on a draft.
In some cases, it will be appropriate to have the IETF session
discussion during the related protocol WG session, to maximize
visibility of the effort to that WG and expand the review.
6.4. Standards Track Performance Metrics
The Performance Metrics Directorate will assist with the progression
of RFCs along the Standards Track. See [IPPM-STANDARD-ADV-TESTING].
This may include the preparation of test plans to examine different
implementations of the metrics to ensure that the metric definitions
are clear and unambiguous (depending on the final form of the draft
mentioned above).
7. Security Considerations
In general, the existence of a framework for Performance Metric
development does not constitute a security issue for the Internet.
Performance Metric definitions, however, may introduce security
issues, and this framework recommends that persons defining
Performance Metrics should identify any such risk factors.
The security considerations that apply to any active measurement of
live networks are relevant here. See [RFC4656].
The security considerations that apply to any passive measurement of
specific packets in live networks are relevant here as well. See the
security considerations in [RFC5475].
8. Acknowledgements
The authors would like to thank Al Morton, Dan Romascanu, Daryl
Malas, and Loki Jorgenson for their comments and contributions, and
Aamer Akhter, Yaakov Stein, Carsten Schmoll, and Jan Novak for their
reviews.
Clark & Claise Best Current Practice [Page 20]
RFC 6390 Guidelines Perf. Metric Devel. October 2011
9. References
9.1. Normative References
[RFC2026] Bradner, S., "The Internet Standards Process --
Revision 3", BCP 9, RFC 2026, October 1996.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2418] Bradner, S., "IETF Working Group Guidelines and
Procedures", BCP 25, RFC 2418, September 1998.
[RFC4656] Shalunov, S., Teitelbaum, B., Karp, A., Boote, J., and M.
Zekauskas, "A One-way Active Measurement Protocol
(OWAMP)", RFC 4656, September 2006.
9.2. Informative References
[E.800] "ITU-T Recommendation E.800. E SERIES: OVERALL NETWORK
OPERATION, TELEPHONE SERVICE, SERVICE OPERATION AND HUMAN
FACTORS", September 2008.
[G.107] "ITU-T Recommendation G.107. The E-model: a computational
model for use in transmission planning", April 2009.
[IPPM-STANDARD-ADV-TESTING]
Geib, R., Ed., Morton, A., Fardid, R., and A. Steinmitz,
"IPPM standard advancement testing", Work in Progress,
June 2011.
[P.564] "ITU-T Recommendation P.564. Conformance Testing for
Voice over IP Transmission Quality Assessment Models",
November 2007.
[P.800] "ITU-T Recommendation P.800. Methods for subjective
determination of transmission quality", August 1996.
[RFC0793] Postel, J., "Transmission Control Protocol", STD 7,
RFC 793, September 1981.
[RFC2330] Paxson, V., Almes, G., Mahdavi, J., and M. Mathis,
"Framework for IP Performance Metrics", RFC 2330,
May 1998.
Clark & Claise Best Current Practice [Page 21]
RFC 6390 Guidelines Perf. Metric Devel. October 2011
[RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
A., Peterson, J., Sparks, R., Handley, M., and E.
Schooler, "SIP: Session Initiation Protocol", RFC 3261,
June 2002.
[RFC3303] Srisuresh, P., Kuthan, J., Rosenberg, J., Molitor, A., and
A. Rayhan, "Middlebox communication architecture and
framework", RFC 3303, August 2002.
[RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V.
Jacobson, "RTP: A Transport Protocol for Real-Time
Applications", STD 64, RFC 3550, July 2003.
[RFC3611] Friedman, T., Ed., Caceres, R., Ed., and A. Clark, Ed.,
"RTP Control Protocol Extended Reports (RTCP XR)",
RFC 3611, November 2003.
[RFC4710] Siddiqui, A., Romascanu, D., and E. Golovinsky, "Real-time
Application Quality-of-Service Monitoring (RAQMON)
Framework", RFC 4710, October 2006.
[RFC4960] Stewart, R., Ed., "Stream Control Transmission Protocol",
RFC 4960, September 2007.
[RFC5101] Claise, B., Ed., "Specification of the IP Flow Information
Export (IPFIX) Protocol for the Exchange of IP Traffic
Flow Information", RFC 5101, January 2008.
[RFC5102] Quittek, J., Bryant, S., Claise, B., Aitken, P., and J.
Meyer, "Information Model for IP Flow Information Export",
RFC 5102, January 2008.
[RFC5474] Duffield, N., Ed., Chiou, D., Claise, B., Greenberg, A.,
Grossglauser, M., and J. Rexford, "A Framework for Packet
Selection and Reporting", RFC 5474, March 2009.
[RFC5475] Zseby, T., Molina, M., Duffield, N., Niccolini, S., and F.
Raspall, "Sampling and Filtering Techniques for IP Packet
Selection", RFC 5475, March 2009.
[RFC5481] Morton, A. and B. Claise, "Packet Delay Variation
Applicability Statement", RFC 5481, March 2009.
[RFC5706] Harrington, D., "Guidelines for Considering Operations and
Management of New Protocols and Protocol Extensions",
RFC 5706, November 2009.
Clark & Claise Best Current Practice [Page 22]
RFC 6390 Guidelines Perf. Metric Devel. October 2011
[RFC5835] Morton, A., Ed., and S. Van den Berghe, Ed., "Framework
for Metric Composition", RFC 5835, April 2010.
[RFC5853] Hautakorpi, J., Ed., Camarillo, G., Penfield, R.,
Hawrylyshen, A., and M. Bhatia, "Requirements from Session
Initiation Protocol (SIP) Session Border Control (SBC)
Deployments", RFC 5853, April 2010.
[RFC5982] Kobayashi, A., Ed., and B. Claise, Ed., "IP Flow
Information Export (IPFIX) Mediation: Problem Statement",
RFC 5982, August 2010.
[RFC6035] Pendleton, A., Clark, A., Johnston, A., and H. Sinnreich,
"Session Initiation Protocol Event Package for Voice
Quality Reporting", RFC 6035, November 2010.
[RFC6049] Morton, A. and E. Stephan, "Spatial Composition of
Metrics", RFC 6049, January 2011.
[RFC6183] Kobayashi, A., Claise, B., Muenz, G., and K. Ishibashi,
"IP Flow Information Export (IPFIX) Mediation: Framework",
RFC 6183, April 2011.
[RFC6248] Morton, A., "RFC 4148 and the IP Performance Metrics
(IPPM) Registry of Metrics Are Obsolete", RFC 6248,
April 2011.
Authors' Addresses
Alan Clark
Telchemy Incorporated
2905 Premiere Parkway, Suite 280
Duluth, Georgia 30097
USA
EMail: alan.d.clark@telchemy.com
Benoit Claise
Cisco Systems, Inc.
De Kleetlaan 6a b1
Diegem 1831
Belgium
Phone: +32 2 704 5622
EMail: bclaise@cisco.com
Clark & Claise Best Current Practice [Page 23]