Active and Passive Metrics and Methods (and everything in-between)
draft-morton-ippm-active-passive-00

Abstract

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.

Status of This Memo

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Table of Contents

• 1. Introduction
• 1.1. Requirements Language
• 2. Purpose and Scope
• 3. Terms and Definitions
• 3.1. Performance Metric
• 3.2. Method of Measurement
• 3.3. Observation Point
• 3.4. Active Methods
• 3.5. Active Metric
• 3.6. Passive Methods
• 3.7. Passive Metric
• 3.8. Hybrid Methods
• 4. Discussion
• 5. Security considerations
• 6. IANA Considerations
• 7. Acknowledgements
• 8. References
• 8.1. Normative References
• 8.2. Informative References
• Author's Address

1. Introduction

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].

The notions of "active" and "passive" are well-established. In general:

• An Active metric or method depends on a dedicated measurement packet stream.
• 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.

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.

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.

1.1. 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. Purpose and Scope

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.

Further, this memo's purpose includes describing multiple dimensions in which to evaluate methods as they emerge.

3. Terms and Definitions

This section defines the key terms of the memo.

3.1. Performance Metric

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).

3.2. Method of Measurement

The procedure or set of operations having the object of determining a Measured Value or Measurement Result.

3.3. Observation Point

See section 2 of [RFC7101] 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 [RFC5835]. Two terms have come into use describing somewhat actions at the identified point in the network path.

3.4. Active Methods

Active measurement methods have the following attributes:

1. 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.
2. The packets in the stream of interest have 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.
3. The Source and Destination of the packet stream are usually known a' priori.
4. Packet stream characteristics are known at the Source at least, and may be communicated to Destination as part of the method.

When adding traffic to the network for measurement, Active Methods influence the quantities measured to some degree, and should take steps to quantify the effect(s) and/or minimize such effects.

3.5. Active Metric

An Active Metric incorporates one or more of the aspects of Active Methods in the metric definition.

For example, IETF metrics for IP performance (developed according to the [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).

3.6. Passive Methods

Passive measurement methods are based on observations of un-disturbed 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.

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.

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.

3.7. Passive Metric

Passive Metrics apply to observations of packet traffic (traffic flows in [RFC7101]).

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".

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, and the metrics are therefore agnostic to the distinction between active and passive).

3.8. Hybrid Methods

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.

4. Discussion

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.

The two dimensions are:

1. The degree to which the measurement stream affects network conditions. For example, an extremely sparse stream of minimal size packets typically has little effect, while a stream designed to characterize path capacity may affect all other flows passing through the capacity bottleneck. There is also the notion of time averages - a measurement stream may have significant affect while it is present, but the stream is only generated 0.1% of the time. On the other hand, observations alone have no affect on network performance. To keep things simple, we consider the stream affect only when it is present.
2. 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 first packet observed will be, but once the standard protocol of a flow is known the possibilities narrow (for compliant flows).

There are a few examples we can plot on a two-dimensional space. We can anchor the dimensions with reference point descriptions.

Affect of the measurement 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

5. Security considerations

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 [I-D.ietf-lmap-framework], which covers active and passive measurement techniques and supporting material on measurement context.

6. IANA Considerations

This memo makes no requests for IANA consideration.

7. Acknowledgements

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.

8. References

8.1. Normative References

8.2. Informative References

Author's Address