One document matched: draft-morton-ippm-active-passive-01.xml
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<front>
<title abbrev="Active and Passive">Active and Passive Metrics and Methods
(and everything in-between)</title>
<author fullname="Al Morton" initials="A." surname="Morton">
<organization abbrev="AT&T Labs">AT&T Labs</organization>
<address>
<postal>
<street>200 Laurel Avenue South</street>
<city>Middletown, NJ</city>
<country>USA</country>
</postal>
<email>acmorton@att.com</email>
</address>
</author>
<date day="23" month="February" year="2015"/>
<abstract>
<t>This memo provides clear definitions for Active and Passive
performance assessment. The construction of Metrics and Methods can be
described as Active or Passive. Methods can take on some of the
attributes of both, and we refer to these as Hybrid Methods.</t>
</abstract>
</front>
<middle>
<section title="Introduction">
<t>The adjectives "active" and "passive" have been used for many years
to distinguish two different classes of Internet performance assessment.
The first Passive and Active Measurement (PAM) Conference was held in
2000, but the earliest proceedings available on-line are from the second
PAM conference in 2001
[https://www.ripe.net/ripe/meetings/pam-2001].</t>
<t>The notions of "active" and "passive" are well-established. In
general:<list style="empty">
<t>An Active metric or method depends on a dedicated measurement
packet stream.</t>
<t>A Passive metric or method depends solely on observation of one
or more packet streams. The streams only serve measurement when they
are observed for that purpose, and are present whether measurements
take place or not.</t>
</list></t>
<t>As new techniques for assessment emerge it is helpful to have clear
definitions of these notions. This memo provides more detailed
definitions and discusses means to evaluate new techniques as they
emerge.</t>
<t>This memo provides definitions for Active and Passive Metrics and
Methods based on long usage in the Internet measurement community, and
especially the Internet Engineering Task Force.</t>
<section title="Requirements Language">
<t>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 <xref
target="RFC2119">RFC 2119</xref>.</t>
</section>
</section>
<section title="Purpose and Scope">
<t>The scope of this memo is to define and describe Active and Passive
versions of metrics and methods which are consistent with the long-time
usage of these adjectives in the Internet measurement community and
especially the Internet Engineering Task Force.</t>
<t>Further, this memo's purpose includes describing multiple dimensions
in which to evaluate methods as they emerge.</t>
</section>
<section title="Terms and Definitions">
<t>This section defines the key terms of the memo.</t>
<section title="Performance Metric">
<t>The standard definition of a quantity, produced in an assessment of
performance and/or reliability of the network, which has an intended
utility and is carefully specified to convey the exact meaning of a
measured value. (This definition is consistent with that of
Performance Metric in RFC 2330 and RFC 6390).</t>
</section>
<section title="Method of Measurement">
<t>The procedure or set of operations having the object of determining
a Measured Value or Measurement Result.</t>
</section>
<section title="Observation Point">
<t>See section 2 of <xref target="RFC7011"/> for this definition (a
location in the network where packets can be observed), and related
definitions. The comparable term defined in IETF literature on Active
measurement is Measurement Point, see section 4.1 of <xref
target="RFC5835"/>. Two terms have come into use describing somewhat
actions at the identified point in the network path.</t>
</section>
<section title="Active Methods">
<t>Active measurement methods have the following attributes:</t>
<t><list style="numbers">
<t>Commonly, the packet stream of interest is generated as the
basis of measurement. A packet stream may be generated to increase
traffic load, but the loading stream itself may not be
measured.</t>
<t>The packets in the stream of interest have fields (or are
augmented or modified to include fields) which are dedicated to
measurement. Since measurement usually requires determining the
corresponding packets at multiple measurement points, a sequence
number is the most common information dedicated to
measurement.</t>
<t>The Source and Destination of the packet stream are usually
known a' priori.</t>
<t>Packet stream characteristics are known at the Source at least,
and may be communicated to Destination as part of the method.</t>
</list>When adding traffic to the network for measurement, Active
Methods influence the quantities measured to some degree, and those
performing tests should take steps to quantify the effect(s) and/or
minimize such effects.</t>
</section>
<section title="Active Metric">
<t>An Active Metric incorporates one or more of the aspects of Active
Methods in the metric definition.</t>
<t>For example, IETF metrics for IP performance (developed according
to the <xref target="RFC2330"/> framework) include the Source packet
stream characteristics as metric input parameters, and also specify
the packet characteristics (Type-P) and Source and Destination IP
addresses (with their implications on both stream treatment and
interfaces associated with measurement points).</t>
</section>
<section title="Passive Methods">
<t>Passive measurement methods are based on observations of
undisturbed packet traffic. Some passive methods simply observe and
collect information on all packets that pass Observation Point(s),
while others filter the packets as a first step and only collect
information on packets that match the filter criteria.</t>
<t>It is common that passive methods are conducted at one or more
Observation Points. Passive methods to assess Performance Metrics
often require multiple observation points, e.g., to assess latency of
packet transfer across a network path between two Observation Points.
In this case, the observed packets must include enough information to
determine the corresponding packets at different Observation
Points.</t>
<t>Communication of the observations (in some form) to a collector is
an essential aspect of Passive Methods. In some configurations, the
traffic load associated with results export to a collector may
influence the network performance. However, the collection of results
is not unique to Passive Methods, and the load from management and
operations of measurement systems must always be considered for
potential effects on the measured values.</t>
</section>
<section title="Passive Metric">
<t>Passive Metrics apply to observations of packet traffic (traffic
flows in <xref target="RFC7011"/>).</t>
<t>Passive performance metrics are assessed independent of the packets
or traffic flows, and solely through observation. Some refer to such
assessments as "out-of-band".</t>
<t>One example of passive performance metrics for IP packet transfer
can be found in ITU-T Recommendation Y.1540, where the metrics are
defined on the basis of reference events as packet pass reference
points, so the metrics are agnostic to the distinction between active
and passive when packet correspondence can be derived from the
observed stream when required.</t>
</section>
<section title="Hybrid Methods and Metrics">
<t>Methods of Measurement which use a combination of Active Methods
and Passive Methods, to assess Active Metrics, Passive Metrics, or a
new metrics derived from the observations. ITU-T Recommendation Y.1540
defines metrics are applicable to the hybrid category, since packet
correspondence at different observation/reference points could be
derived from "fields which are dedicated to measurement", but
otherwise the methods are passive.</t>
</section>
</section>
<section title="Discussion">
<t>If we compare the Active and Passive Methods, there are at least two
dimensions on which methods can be evaluated. This evaluation space may
be useful when a method is a combination of the two alternative
methods.</t>
<t>The two dimensions (initially chosen) are:<list style="numbers">
<t>The degree to which the measured stream effects overall network
conditions. There is also the notion of time averages - a
measurement stream may have significant effect while it is present,
but the stream is only generated 0.1% of the time. On the other
hand, observations alone have no effect on network performance. To
keep things simple, we consider the stream effect only when it is
present.</t>
<t>The methodological advantages of knowing the source stream
characteristics, and having complete control of the stream
characteristics. For example, knowing the number of packets in a
stream allows more efficient operation of the measurement receiver,
and so is an asset for active measurement methods. Passive methods
(with no sample filter) have few clues available to anticipate what
the protocol first packet observed will use or how many packets will
comprise the flow, but once the standard protocol of a flow is known
the possibilities narrow (for some compliant flows).</t>
</list></t>
<t>There are a few examples we can plot on a two-dimensional space. We
can anchor the dimensions with reference point descriptions.</t>
<t><figure>
<artwork><![CDATA[Effect of the measured stream on network conditions
^ Max
|* Active using max capacity stream
|
|
|
|
|* Active using stream with load of typical user
|
|
|
|* Active using extremely sparse, randomized stream
| * PDM Passive
| Min *
+----------------------------------------------------------------|
| |
Stream None
Characteristics
completely
known
]]></artwork>
</figure></t>
<t>We recognize that method categorization could be based on additional
dimensions, but this would require a different graphical approach.</t>
<t>For example, "effect of measured stream on network conditions" could
easily be further qualified into:</t>
<t><list style="numbers">
<t>effect on the performance of the measured stream itself: for
example, choosing a packet marking or DSCP resulting in domain
treatment as a real-time stream (as opposed to default/best-effort
marking.</t>
<t>effect on unmeasured flows that share the path and/or
bottlenecks: for example, an extremely sparse measured stream of
minimal size packets typically has little effect on other flows (and
itself), while a stream designed to characterize path capacity may
effect all other flows passing through the capacity bottleneck
(including itself).</t>
<t>effect on network conditions, resulting in network adaptation:
for example, a network monitoring load and congestion conditions
might change routing, placing some flows to alternate paths to
mitigate the congestion.</t>
</list>As suggestions emerge we will examine the possibilities.</t>
<section title="Discussion of PDM">
<t>In <xref target="I-D.elkins-ippm-pdm-option"/>, an IPv6 Option
Header is described which (when added to the stream at strategic
interfaces) supports performance measurements. This method processes a
user traffic stream and adds "fields which are dedicated to
measurement". Thus:</t>
<t><list style="symbols">
<t>The method may have a small effect on the measured stream and
other streams in the network.</t>
<t>The measured stream has unknown characteristics until it is
processed to add the PDM Option header.</t>
</list>We conclude that this is a Hybrid method, having at least one
characteristic of both active and passive methods.</t>
</section>
<section title="Discussion of "Coloring" Method">
<t>Draft <xref target="I-D.tempia-opsawg-p3m"/>, proposed to color
packets by re-writing a field of the stream at strategic interfaces to
support performance measurements. This method processes a user traffic
stream and inserts "fields which are dedicated to measurement".
Thus:</t>
<t><list style="symbols">
<t>The method may have a small effect on the measured stream and
other streams in the network (smaller than PDM above).</t>
<t>The measured stream has unknown characteristics until it is
processed to add the coloring in the header, and the stream could
be measured and time-stamped during that process.</t>
</list>We note that <xref
target="I-D.chen-ippm-coloring-based-ipfpm-framework"/> proposes a
method similar to <xref target="I-D.tempia-opsawg-p3m"/>, and
ippm-list discussion indicates <xref
target="I-D.chen-ippm-coloring-based-ipfpm-framework"/> may be covered
by the same IPR as <xref target="I-D.tempia-opsawg-p3m"/>.</t>
<t>We conclude that this is a Hybrid method, having at least one
characteristic of both active and passive methods.</t>
</section>
</section>
<section title="Security considerations">
<t>When considering privacy of those involved in measurement or those
whose traffic is measured, there is sensitive information communicated
and observed at observation and measurement points described above. We
refer the reader to the privacy considerations described in the Large
Scale Measurement of Broadband Performance (LMAP) Framework <xref
target="I-D.ietf-lmap-framework"/>, which covers active and passive
measurement techniques and supporting material on measurement
context.</t>
</section>
<section title="IANA Considerations">
<t>This memo makes no requests for IANA consideration.</t>
</section>
<section title="Acknowledgements">
<t>Thanks to Mike Ackermann for asking the right question, and for
several suggestions on terminology. Brian Trammell provided key terms
and references for the passive category. Tiziano Ionta reviewed the
draft and suggested the classification for the "coloring" method of
measurement. Nalini Elkins identified several areas for clarification
following her review. Bill Jouris reviewed 01 editorially and suggested
several improvements. </t>
</section>
</middle>
<back>
<references title="Normative References">
<?rfc include='reference.RFC.2330'?>
<?rfc include='reference.RFC.2119'?>
<?rfc include='reference.RFC.3432'?>
<?rfc include='reference.RFC.5835'?>
<?rfc include='reference.RFC.7011'?>
</references>
<references title="Informative References">
<?rfc include='reference.I-D.ietf-lmap-framework'?>
<?rfc include='reference.I-D.elkins-ippm-pdm-option'?>
<?rfc include='reference.I-D.tempia-opsawg-p3m'?>
<?rfc include='reference.I-D.chen-ippm-coloring-based-ipfpm-framework'?>
</references>
</back>
</rfc>
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