RFC2432 - Terminology for IP Multicast Benchmarking

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　　Network Working Group K. Dubray
Request for Comments: 2432 IronBridge Networks
Category: Informational October 1998

Terminology for IP Multicast Benchmarking

Status of this Memo

This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.

Copyright Notice

Copyright (C) The Internet Society (1998). All Rights Reserved.

Abstract

The purpose of this document is to define terminology specific to the
benchmarking of multicast IP forwarding devices. It builds upon the
tenets set forth in RFC1242, RFC2285, and other IETF Benchmarking
Methodology Working Group (BMWG) efforts. This document seeks to
extend these efforts to the multicast paradigm.

The BMWG produces two major classes of documents: Benchmarking
Terminology documents and Benchmarking Methodology documents. The
Terminology documents present the benchmarks and other related terms.
The Methodology documents define the procedures required to collect
the benchmarks cited in the corresponding Terminology documents.

1. Introduction

Network forwarding devices are being required to take a single frame
and support delivery to a number of destinations having membership to
a particular group. As such, multicast support may place a different
burden on the resources of these network forwarding devices than with
unicast or broadcast traffic types.

Such burdens may not be readily apparent at first glance - the IP
multicast packet's Class D address may be the only noticeable
difference from an IP unicast packet. However, there are many
factors that may impact the treatment of IP multicast packets.

Consider how a device's architecture may impact the handling of a
multicast frame. For example, is the multicast packet subject to the
same processing as its unicast analog? Or is the multicast packet
treated as an exeception and processed on a different data path?

Consider, too, how a shared memory architecture may demonstrate a
different performance profile than an architecture which explicitly
passes each individual packet between the processing entities.

In addition to forwarding device architecture, there are other
factors that may impact a device's or system's multicast related
performance. Protocol requirements may demand that routers and
switches consider destination and source addressing in its multicast
forwarding decisions. Capturing multicast source/destination
addressing information may impact forwarding table size and lengthen
lookups. Topological factors such as the degree of packet
replication, the number of multicast groups being supported by the
system, or the placement of multicast packets in unicast wrappers to
span non-multicast network paths may all potentially affect a
system's multicast related performance. For an overall understanding
of IP multicasting, the reader is directed to [Se98], [Hu95], and
[Mt98].

By clearly identifying IP multicast benchmarks and related
terminology in this document, it is hoped that detailed methodologies
can be generated in subsequent documents. Taken in tandem, these two
efforts endeavor to assist the clinical, empirical, and consistent
characterization of certain aspects of multicast technologies and
their individual implementations. Understanding the operational
profile of multicast forwarding devices may assist the network
designer to better deploy multicast in his or her networking
environment.

Moreover, this document focuses on one source to many destinations
profiling. Elements of this document may require extension when
considering multiple source to multiple destination IP multicast
communication.

2. Definition Format

This section cites the template suggested by RFC1242 in the
specification of a term to be defined.

Term to be defined.

Definition:
The specific definition for the term.

Discussion:
A brief discussion of the term, its application, or other
information that would build understanding.

Measurement units:
Units used to record measurements of this term, if applicable.

[Issues:]
List of issues or conditions that affect this term. This field can
present items the may impact the term's related methodology or
otherwise restrict its measurement procedures. This field is
optional in this document.

[See Also:]
List of other terms that are relevant to the discussion of this
term. This field is optional in this document.

2.1 Existing Terminology

This document draws on existing terminology defined in other BMWG
work. Examples include, but are not limited to:

Throughput [RFC1242, section 3.17]
Latency [RFC1242, section 3.8]
Constant Load [RFC1242, section 3.4]
Frame Loss Rate [RFC1242, section 3.6]
Overhead behavior [RFC1242, section 3.11]
Forwarding Rates [RFC2285, section 3.6]
Loads [RFC2285, section 3.5]
Device Under Test (DUT) [RFC2285, section 3.1.1]
System Under Test (SUT) [RFC2285, section 3.1.2]

Note: "DUT/SUT" refers to a metric that may be applicable to a DUT or
SUT.

3. Table of Defined Terms

3.1 General Nomenclature

3.1.1 Traffic Class. (TC)
3.1.2 Group Class. (GC)
3.1.3 Service Class. (SC)

3.2 Forwarding and Throughput
3.2.1 Mixed Class Throughput (MCT).
3.2.2 Scaled Group Forwarding Matrix (SGFM).
3.2.3 Aggregated Multicast Throughput (AMT)
3.2.4 Encapsulation Throughput (ET)
3.2.5 Decapsulation Throughput (DT)
3.2.6 Re-encapsulation Throughput (RET)

3.3 Forwarding Latency
3.3.1 Multicast Latency (ML)
3.3.2 Min/Max Multicast Latency (Min/Max ML)

3.4 Overhead
3.4.1 Group Join Delay. (GJD)
3.4.2 Group Leave Delay. (GLD)

3.5 Capacity
3.5.1 Multicast Group Capacity. (MGC)

3.6 Interaction
3.6.1 Burdened Response
3.6.2 Forwarding Burdened Multicast Latency (FBML)
3.6.3 Forwarding Burdened Join Delay (FBJD)

3.1 General Nomenclature

This section will present general terminology to be used in this and
other documents.

3.1.1 Traffic Class. (TC)

Definition:
An equivalence class of packets comprising one or more data
streams.

Discussion:
In the scope of this document, Traffic Class will be considered a
logical identifier used to discriminate between a set or sets of
packets offered the DUT.

For example, one Traffic Class may identify a set of unicast
packets offered to the DUT. Another Traffic Class may
differentiate the multicast packets destined to multicast group X.
Yet another Class may distinguish the set of multicast packets
destined to multicast group Y.

Unless otherwise qualified, the usage of the word "Class" in this
document will refer simply to a Traffic Class.

Measurement units:
Not applicable.

3.1.2 Group Class. (GC)

Definition:
A specific type of Traffic Class where the packets comprising the
Class are destined to a particular multicast group.

Discussion:

Measurement units:
Not applicable.

3.1.3 Service Class. (SC)

Definition:
A specific type of Traffic Class where the packets comprising the
Class require particular treatment or treatments by the network
forwarding devices along the path to the packets' destination(s).

Discussion:

Measurement units:
Not applicable.

3.2 Forwarding and Throughput.

This section presents terminology related to the characterization of
the packet forwarding ability of a DUT/SUT in a multicast
environment. Some metrics extend the concept of throughput presented
in RFC1242. The notion of Forwarding Rate is cited in RFC2285.

3.2.1 Mixed Class Throughput (MCT).

Definition:
The maximum rate at which none of the offered frames, comprised
from a unicast Class and a multicast Class, to be forwarded are
dropped by the device across a fixed number of ports.

Discussion:
Often times, throughput is collected on a homogenous traffic class
- the offered load to the DUT is either singularly unicast or
singularly multicast. In most networking environments, the
traffic mix is seldom so uniformly distributed.

Based on the RFC1242 definition for throughput, the Mixed Class
Throughput benchmark attempts to characterize the DUT's ability to
process both unicast and multicast frames in the same aggregated
traffic stream.

Measurement units:
Frames per second

Issues:
Related methodology may have to address the ratio of unicast
packets to multicast packets.

Since frame size can sometimes be a factor in frame forwarding
benchmarks, the corresponding methodology for this metric will
need to consider frame size distribution(s).

3.2.2 Scaled Group Forwarding Matrix (SGFM).

Definition:
A table that demonstrates Forwarding Rate as a function of tested
multicast groups for a fixed number of tested DUT/SUT ports.

Discussion:
A desirable attribute of many Internet mechanisms is the ability
to "scale." This benchmark seeks to demonstrate the ability of a
SUT to forward as the number of multicast groups is scaled
upwards.

Measurement units:
Packets per second, with corresponding tested multicast group and
port configurations.

Issues:
The corresponding methodology may have to reflect the impact that
the pairing (source, group) has on many multicast routing
protocols.

Since frame size can sometimes be a factor in frame forwarding
benchmarks, the corresponding methodology for this metric will
need to consider frame size distribution(s).

3.2.3 Aggregated Multicast Throughput (AMT)

Definition:
The maximum rate at which none of the offered frames to be
forwarded through N destination interfaces of the same multicast
group are dropped.

Discussion:
Another "scaling" type of exercise, designed to identify the
DUT/SUT's ability to handle traffic as a function of the multicast
destination ports it is required to support.

Measurement units:
The ordered pair (N,t) where,

N = the number of destination ports of the multicast group.
t = the throughput, in frames per second, relative to the
source stream.

Issues:
Since frame size can sometimes be a factor in frame forwarding
benchmarks, the corresponding methodology for this metric will
need to consider frame size distribution(s).

3.2.4 Encapsulation Throughput (ET)

Definition:
The maximum rate at which frames offered a DUT are encapsulated
and correctly forwarded by the DUT without loss.

Discussion:
A popular technique in presenting a frame to a device that may not
support a protocol feature is to encapsulate, or tunnel, the
packet containing the unsupported feature in a format that is
supported by that device.

More specifically, encapsulation refers to the act of taking a
frame or part of a frame and embedding it as a payload of another
frame. This benchmark attempts to characterize the overhead
behavior associated with that translational process.

Measurement units:
Frames per second.

Issues:
Consideration may need to be given with respect to the impact of
different frame formats on usable bandwidth.

Since frame size can sometimes be a factor in frame forwarding
benchmarks, the corresponding methodology for this metric will
need to consider frame size distribution(s).

3.2.5 Decapsulation Throughput (DT)

Definition:
The maximum rate at which frames offered a DUT are decapsulated
and correctly forwarded by the DUT without loss.

Discussion:
A popular technique in presenting a frame to a device that may not
support a protocol feature is to encapsulate, or tunnel, the
packet containing the unsupported feature in a format that is
supported by that device. At some point, the frame may be required
to be returned its orginal format from its encapsulation wrapper
for use by the frame's next destination.

More specifically, decapsulation refers to the act of taking a
frame or part of a frame embedded as a payload of another frame
and returning it to the payload's appropriate format. This
benchmark attempts to characterize the overhead behavior
associated with that translational process.

Measurement units:
Frames per second.

Issues:
Consideration may need to be given with respect to the impact of
different frame formats on usable bandwidth.

Since frame size can sometimes be a factor in frame forwarding
benchmarks, the corresponding methodology for this metric will
need to consider frame size distribution(s).

3.2.6 Re-encapsulation Throughput (RET)

Definition:
The maximum rate at which frames of one encapsulated format
offered a DUT are converted to another encapsulated format and
correctly forwarded by the DUT without loss.

Discussion:
A popular technique in presenting a frame to a device that may not
support a protocol feature is to encapsulate, or tunnel, the
packet containing the unsupported feature in a format that is
supported by that device. At some point, the frame may be required
to be converted from one encapsulation format to another
encapsulation format.

More specifically, re-encapsulation refers to the act of taking an
encapsulated payload of one format and replacing it with another
encapsulated format - all the while preserving the original
payload's contents. This benchmark attempts to characterize the
overhead behavior associated with that translational process.

Measurement units:
Frames per second.

Issues:
Consideration may need to be given with respect to the impact of
different frame formats on usable bandwidth.

Since frame size can sometimes be a factor in frame forwarding
benchmarks, the corresponding methodology for this metric will
need to consider frame size distribution(s).

3.3 Forwarding Latency.

This section presents terminology relating to the characterization of
the forwarding latency of a DUT/SUT in a multicast environment. It
extends the concept of latency presented in RFC1242.

3.3.1 Multicast Latency. (ML)

Definition:
The set of individual latencies from a single input port on the
DUT or SUT to all tested ports belonging to the destination
multicast group.

Discussion:
This benchmark is based on the RFC1242 definition of latency.
While it is useful to collect latency between a pair of source and
destination multicast ports, it may be insightful to collect the
same type of measurements across a range of ports supporting that
Group Class.

A variety of statistical exercises can be applied to the set of
latencies measurements.

Measurement units:
Time units with enough precision to reflect a latency measurement.

3.3.2 Min/Max Multicast Latency. (Min/Max ML)

Definition:
The difference between the maximum latency measurement and the
minimum latency measurement from the set of latencies produced by
the Multicast Latency benchmark.

Discussion:
This statistic may yield some insight into how a particular
implementation handles its multicast traffic. This may be useful
to users of multicast synchronization types of applications.

Measurement units:
Time units with enough precision to reflect latency measurement.

3.4 Overhead

This section presents terminology relating to the characterization of
the overhead delays associated with explicit operations found in
multicast environments.

3.4.1 Group Join Delay. (GJD)

Definition:
The time duration it takes a DUT to start forwarding multicast
packets from the time a successful IGMP group membership report
has been issued to the DUT.

Discussion:
Many factors can contribute to different results, such as the
number or type of multicast-related protocols configured on the
device under test. Other factors are physical topology and "tree"
configuration.

Because of the number of variables that could impact this metric,
the metric may be a better characterization tool for a device
rather than a basis for comparisons with other devices.

Issues:
A consideration for the related methodology: possible need to
differentiate a specifically-forwarded multicast frame from those
sprayed by protocols implementing a flooding tactic to solicit
prune feedback.

While this metric attempts to identify a simple delay, the
underlying and contributing delay components (e.g., propagation
delay, frame processing delay, etc.) make this a less than simple
measurement. The corresponding methodology will need to consider
this and similar factors to ensure a consistent and precise metric
result.

Measurement units:
Microseconds.

3.4.2 Group Leave Delay. (GLD)

Definition:
The time duration it takes a DUT to cease forwarding multicast
packets after a corresponding IGMP "Leave Group" message has been
successfully offered to the DUT.

Discussion:
While it is important to understand how quickly a device can
process multicast frames; it may be beneficial to understand how
quickly that same device can stop the process as well.

Because of the number of variables that could impact this metric,
the metric may be a better characterization tool for a device
rather than a basis for comparisons with other devices.

Measurement units:
Microseconds.

Issues:
The Methodology may need to consider protocol-specific timeout
values.

While this metric attempts to identify a simple delay, the
underlying and contributing delay components (e.g., propagation
delay, frame processing delay, etc.) make this a less than simple
measurement. Moreover, the cessation of traffic is a rather
unobservable event (i.e., at what point is the multicast forwarded
considered stopped on the DUT interface processing the Leave?).
The corresponding methodology will need to consider this and
similar factors to ensure a consistent and precise metric result.

3.5 Capacity

This section offers terms relating to the identification of multicast
group limits of a DUT/SUT.

3.5.1 Multicast Group Capacity. (MGC)

Definition:
The maximum number of multicast groups a SUT/DUT can support while
maintaining the ability to forward multicast frames to all
multicast groups registered to that SUT/DUT.

Discussion:

Measurement units:
Multicast groups.

Issues:
The related methodology may have to consider the impact of
multicast sources per group on the ability of a SUT/DUT to "scale
up" the number of supportable multicast groups.

3.6 Interaction

Network forwarding devices are generally required to provide more
functionality than than the forwarding of traffic. Moreover, network
forwarding devices may be asked to provide those functions in a
variety of environments. This section offers terms to assist in the
charaterization of DUT/SUT behavior in consideration of potentially
interacting factors.

3.6.1 Burdened Response.

Definition:
A measured response collected from a DUT/SUT in light of
interacting, or potentially interacting, distinct stimulii.

Discussion:
Many metrics provide a one dimensional view into an operating
characteristic of a tested system. For example, the forwarding
rate metric may yield information about the packet processing
ability of a device. Collecting that same metric in view of
another control variable can oftentimes be very insightful. Taking
that same forwarding rate measurement, for instance, while the
device's address table is injected with an additional 50,000
entries may yield a different perspective.

Measurement units:
A burdened response is a type of metric. Metrics of this this
type must follow guidelines when reporting results.

The metric's principal result MUST be reported in conjunction with
the contributing factors.

For example, in reporting a Forwarding Burdened Latency, the
latency measurement should be reported with respect to
corresponding Offered Load and Forwarding Rates.

Issues: A Burdened response may be very illuminating when trying to
characterize a single device or system. Extreme care must be
exercised when attempting to use that characterization as a basis
of comparison with other devices or systems. Test agents must
ensure that the measured response is a function of the controlled
stimulii, and not secondary factors. An example of of such an
interfering factor would be configuration mismatch of a timer
impacting a response process.

3.6.2 Forwarding Burdened Multicast Latency. (FBML)

Definition:
A multicast latency taken from a DUT/SUT in the presence of a
traffic forwarding requirement.

Discussion:
This burdened response metric builds on the Multicast Latency
definition offered in section 3.3.1. It mandates that the DUT be
subjected to an additional measure of traffic not required by the
non-burdened metric.

This metric attempts to provide a means by which to evaluate how
traffic load may or may not impact a device's or system's packet
processing delay.

Measurement units:
Time units with enough precision to reflect the latencies
measurements.

Latency measurements MUST be reported with the corresponding
sustained Forwarding Rate and associated Offered Load.

3.6.3 Forwarding Burdened Group Join Delay. (FBGJD)

Definition:
A multicast Group Join Delay taken from a DUT in the presence of a
traffic forwarding requirement.

Discussion:
This burdened response metric builds on the Group Join Delay
definition offered in section 3.4.1. It mandates that the DUT be
subjected to an additional measure of traffic not required by the
non-burdened metric.

Many factors can contribute to different results, such as the
number or type of multicast-related protocols configured on the
device under test. Other factors could be physical topology or the
logical multicast "tree" configuration.

Because of the number of variables that could impact this metric,
the metric may be a better characterization tool for a device
rather than a basis for comparisons with other devices.

Measurement units:
Time units with enough precision to reflect the delay
measurements.

Delay measurements MUST be reported with the corresponding
sustained Forwarding Rate and associated Offered Load.

Issues:
While this metric attempts to identify a simple delay, the
underlying and contributing delay components (e.g., propagation
delay, frame processing delay, etc.) make this a less than simple
measurement. The corresponding methodology will need to consider
this and similar factors to ensure a consistent and precise metric
result.

4. Security Considerations

This document addresses metrics and terminology relating to the
performance benchmarking of IP Multicast forwarding devices. The
information contained in this document does not impact the security
of the Internet.

Methodologies regarding the collection of the metrics described
within this document may need to cite security considerations. This
document does not address methodological issues.

5. Acknowledgments

The IETF BMWG participants have made several comments and suggestions
regarding this work. Particular thanks goes to Harald Alvestrand,
Scott Bradner, Brad Cain, Eric Crawley, Bob Mandeville, David Newman,
Shuching Sheih, Dave Thaler, Chuck Winter, Zhaohui Zhang, and John
Galgay for their insightful review and assistance.

6. References

[Br91] Bradner, S., "Benchmarking Terminology for Network
Interconnection Devices", RFC1242, July 1991.

[Br96] Bradner, S., and J. McQuaid, "Benchmarking Methodology for
Network Interconnect Devices", RFC1944, May 1996.

[Hu95] Huitema, C. "Routing in the Internet." Prentice-Hall, 1995.

[Se98] Semeria, C. and Maufer, T. "Introduction to IP Multicast
Routing." http://www.3com.com/nsc/501303.html 3Com Corp.,
1998.

[Ma98] Mandeville, R., "Benchmarking Terminology for LAN Switching
Devices", RFC2285, February 1998.

[Mt98] Maufer, T. "Deploying IP Multicast in the Enterprise."
Prentice-Hall, 1998.

7. Author's Address

Kevin Dubray
IronBridge Networks
55 Hayden Avenue
Lexington, MA 02421
USA

Phone: 781 372 8118
EMail: kdubray@ironbridgenetworks.com

8. Full Copyright Statement

Copyright (C) The Internet Society (1998). All Rights Reserved.

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