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Network Working Group                                  R. Mandeville
INTERNET-DRAFT                         European Network Laboratories
Expiration Date: September 1997                           March 1997


          Benchmarking Terminology for LAN Switching Devices
                < draft-ietf-bmwg-lanswitch-04.txt >



Status of this Document

This document is an Internet-Draft. Internet-Drafts are working
documents of the Internet Engineering Task Force (IETF), its areas,
and its working groups. Note that other groups may also distribute
working documents as Internet-Drafts.

Internet-Drafts are draft documents valid for a maximum of six
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Distribution of this document is unlimited. Please send comments to
bmwg@harvard.edu or to the editor.


Abstract

The purpose of this draft is to define and discuss benchmarking
terminology for local area switching devices. It is meant to extend the
terminology already defined for network interconnect devices in
RFCs 1242 and 1944 by the Benchmarking Methodology Working
Group (BMWG) of the Internet Engineering Task Force (IETF) and
prepare the way for a discussion on benchmarking methodology for
local area switches.

LAN switches are one of the principal sources of new bandwidth in
the local area. The multiplicity of products brought to market makes
it desirable to define a set of terms to be used when evaluating the
performance characteristics of local area switching devices. Well-
defined terminology will help in providing the user community with
complete, reliable and comparable data on LAN switches.

1. Introduction

The purpose of this draft is to discuss and define terminology for the
benchmarking of local area network switches. Although it might be
found useful to apply some of the terms defined here to a broader
range of network interconnect devices, this draft primarily deals with
devices which switch frames at the Medium Access Control (MAC)
layer. It defines terms in relation to forwarding performance, latency,
address handling and filtering.

2. Term definitions

RFC 1242 "Benchmarking Terminology for Network Interconnect
Devices" defines many of the terms that are used in this document.
The terminology document should be consulted before attempting to
make use of this document. RFC 1944 "Benchmarking Methodology
for Network Interconnect Devices" defines a number of test
procedures which are directly applicable to switches. Since it
discusses a number of terms relevant to benchmarking switches it
should also be consulted.
A number of new terms applicable to benchmarking switches are
defined below using the format for definitions set out in Section 2 of
RFC 1242. RFCs 1242 and 1944 already contain discussions of
some of these terms. The new terms are defined in groups.

2. 1. Reminder of RFC 1242 definition format

Term to be defined. (e.g., Latency)

Definition:
The specific definition for the term.

Discussion:
A brief discussion about the term, its application
and any restrictions on measurement procedures.

Measurement units:
The units used to report measurements of this
term, if applicable.

Issues:
List of issues or conditions that effect this term.

See Also:
List of other terms that are relevant to the discussion
of this term.

3. Index of terms defined

3.1 Devices
3.1.1 Device under test (DUT)
3.1.2 System under test (SUT)

3.2 Traffic patterns
3.2.1 Unidirectional traffic
3.2.2 Bidirectional traffic
3.2.3 Partially meshed traffic
3.2.4 Fully meshed traffic

3.3 Bursts
3.3.1 Burst
3.3.2 Burst size
3.3.3 Inter-burst gap (IBG)

3.4 Loads
3.4.1 Intended load
3.4.2 Offered load
3.4.3 Maximum offered load
3.4.4 Overloading

3.5 Forwarding rates
3.5.1 Forwarding rate at maximum load
3.5.2 Maximum forwarding rate

3.6 Congestion control
3.6.1 Backpressure
3.6.2 Forward pressure
3.6.3 Head of line blocking

3.7 Address handling
3.7.1 Address caching capacity
3.7.2 Address learning rate
3.7.3 Flood count

3.8 Errored frame filtering
3.8.1 Errored frames

3.9 Broadcasts
3.9.1 Broadcast forwarding rate at maximum load
3.9.2 Broadcast latency

3.1 Devices

This group applies to all types of networking devcies.

3.1.1 Device under test (DUT)

Definition:
The network forwarding device to which stimuli is offered and
response measured.

Discussion:
A single stand-alone or modular unit generally equipped with its own
power supply.

Measurement units:
n/a

Issues:

See Also:
System under test (SUT) (3.1.2)

3.1.2 System Under Test (SUT).

Definition:
The collective set of network devices to which stimuli is offered as a
single entity and response measured.

Discussion:
A system under test may be comprised of a variety of networking
devices.  Some devices may be active in the forwarding decision-
making process, such as routers or switches; other devices may be
passive such as CSU/DSUs.  Regardless of constituent components,
the system is treated as a singular entity to which stimuli is offered
and response measured.

Measurement units:
n/a

Issues:

See Also:
Device under test (DUT) (3.1.1)

3.2 Traffic patterns
This group applies to the distribution of frames forwarded by any
DUT/SUT.

3.2.1 Unidirectional traffic

Definition:

Single or multiple streams of frames forwarded in one direction only
from a set of input ports to a set of output ports.

Discussion:
This definition conforms to the discussion in section 16 of RFC
1944 on multi-port testing which describes how unidirectional traffic
can be offered to ports of a device to measure throughput.
Unidirectional traffic is also appropriate for:
- the measurement of the minimum inter-frame gap
- the creation of many-to-one or one-to-many port overload
- the detection of head of line blocking
- the measurement of throughput when congestion control
mechanisms are active

Unidirectional traffic can be used to load the ports of a DUT/SUT in
different ways. For example unidirectional traffic can be sent to two
or more input ports from an external source and forwarded by the
DUT/SUT to a single output port (n-to-1) or such traffic can be sent
to a single input port and forwarded by the DUT/SUT to two or more
output ports (1-to-n). Such patterns can be combined to test for head
of line blocking or to measure throughput when congestion control
mechanisms are active.
When a DUT/SUT is equipped with ports running at different media
rates the number of input streams required to load or overload an
output port or ports will vary.
It should be noted that measurement of the minimum inter-frame gap
serves to detect violations of the IEEE 802.3 standard.

Issues:
half duplex / full duplex

Measurement units:
n/a

See Also:
bidirectional traffic (3.2.2)
partially meshed traffic (3.2.3)
fully meshed traffic (3.2.4)

3.2.2 Bidirectional traffic

Definition:
Two or more streams of frames forwarded in opposite directions
between two or more ports of a DUT/SUT.

Discussion:
This definition conforms to the discussions in sections 14 and 16 of
RFC 1944 on bidirectional traffic and multi-port testing.
Bidirectional traffic MUST be offered when measuring throughput
on full duplex ports of a switching device.

Issues:
truncated binary exponential back-off algorithm

Measurement units:
n/a

See Also:
unidirectional traffic (3.2.1)
partially meshed traffic (3.2.3)
fully meshed traffic (3.2.4)

3.2.3 Partially meshed traffic

Definition:
Streams of frames forwarded between a set of input ports and a set of
output ports of a DUT/SUT with a one to many, many to one or many
to many mapping of input ports to output ports.

Discussion:
This definition follows from the discussions in sections 14 and 16 of
RFC 1944 on bidirectional traffic and multi-port testing and readily
extends to configurations with multiple switching devices linked
together over backbone connections. Meshed traffic can be
unidirectional or bidirectional.

Measurement units:
n/a

Issues:
half duplex / full duplex

See Also:
unidrectional traffic (3.2.1)
bidirectional traffic (3.2.2)
fully meshed traffic (3.2.4)
bursts (3.3)

3.2.4 Fully meshed traffic

Definition:
Streams of frames switched simultaneously between all of a
designated number of ports of a device such that each of the ports
under test will both send frames to and receive frames from all of the
other ports under test.

Discussion:
As with bidirectional multi-port traffic, meshed traffic exercises both
the transmission and reception sides of the ports of a switching
device. Since ports are not divided into two groups every port
forwards frames to and receives frames from every other port. The
total number of individual streams when traffic is meshed over n
switched ports equals n x (n - 1). This compares with n x (n / 2) such
streams in a bidirectional multi-port test. It should be noted that
bidirectional multiport traffic can load backbone connections linking
together two switching devices more than meshed traffic.
It should be noted that bidirectional meshed traffic on half duplex
ports is inherently bursty since ports must interrupt transmission
whenever they receive frames. This kind of bursty meshed traffic is
characteristic of real network traffic and can be advantageously used
to diagnose a DUT/SUT by exercising many of its component parts
simultaneously. Additional inspection may be warranted to correlate
the frame forwarding capacity of a DUT/SUT when offered meshed
traffic and the behavior of individual elements such as input and
output buffers, buffer allocation mechanisms, aggregate switching
capacity, processing speed or medium access control.
When offering bursty meshed traffic to a DUT/SUT a number of
variables have to be considered. These include frame size, the number
of frames within bursts, the interval between bursts as well as the
distribution of load between incoming and outgoing traffic. Terms
related to bursts are defined in section 3.3 below.

Measurement units:
n/a

Issues:
half duplex / full duplex

See Also:
unidrectional traffic (3.2.1)
bidirectional traffic (3.2.2)
partially meshed traffic (3.2.4)
bursts (3.3)

3.3 Bursts

This group applies to the intervals defining traffic forwarded by
DUT/SUT.

3.3.1 Burst

Definition:
A sequence of frames transmitted with the minimum inter-frame gap
allowed by the medium.

Discussion:
This definition follows from discussions in section 3.16 of RFC
1242 and section 21 of RFC 1944 which describes cases where it is
useful to consider isolated frames as single frame bursts.

Measurement units:
n/a

Issues:

See Also:
burst size (3.3.2)
inter-burst gap (IBG) (3.2.3)
3.2.2 Burst size

Definition:
The number of frames in a burst.

Discussion:
Burst size can range from one to infinity. In unidirectional streams
there is no theoretical limit to burst length. When traffic is
bidirectional or meshed bursts on half duplex media are finite since
ports interrupt transmission intermittently to receive frames.
On real networks burst size will normally increase with window size.
This makes it desirable to test devices with small as well as large
burst sizes.

Measurement units:
number of N-octet frames

Issues:

See Also:
burst (3.3.1)
inter-burst gap (IBG) (3.2.3)

3.2.3 Inter-burst gap (IBG)

Definition:
The interval between two bursts.

Discussion:
This definition conforms to the discussion in section 20 of RFC
1944 on bursty traffic.
Bidirectional and meshed streams of traffic are inherently bursty
since ports share their time between receiving and transmitting
frames. External sources offering bursty traffic for a given frame size
and burst size must adjust the inter-burst gap to achieve a specified
rate of transmission.

Measurement units:
nanoseconds
microseconds
milliseconds
seconds

Issues:

See Also:
burst (3.3.1)
burst size (3.2.2)

3.4 Loads

This group applies to the rates that traffic is offered to any
DUT/SUT.

3.4.1 Intended load

Definition:
The number of frames per second that an external source attempts to
transmit to a DUT/SUT for forwarding to a specified output port or
ports.

Discussion:
Collisions on CSMA/CD links or the action of congestion control
mechanisms can effect the rate at which an external source of traffic
transmits frames to a DUT/SUT. This makes it useful to distinguish
the load that an external source attempts to apply to a DUT/SUT and
the load it is observed or measured to apply.
In the case of Ethernet an external source of traffic must implement
the truncated binary exponential back-off algorithm to ensure that it
is accessing the medium legally.

Measurement units:
bits per second
N-octets per second
(N-octets per second / media_maximum-octets per second) x 100

Issues:

See Also:
offered load (3.4.2)

3.4.2 Offered load

Definition:
The number of frames per second that an external source can be
observed or measured to transmit to a DUT/SUT for forwarding to a
specified output port or ports.

Discussion:
The load which an external device can be observed to apply to a
DUT/SUT may be less than the load the external device attempts to
apply due to collisions or the action of congestion control
mechanisms.
Frames which are not successfully transmitted by an external source
of traffic to a DUT/SUT MUST NOT be counted as transmitted
frames when measuring the forwarding rate of a DUT/SUT.
The frame count on a port of a DUT/SUT may exceed the rate at
which an external device offers frames due to the presence of
spanning tree BPDUs (Bridge Protocol Data Units) on 802.1D-
compliant switches or SNMP frames. Such frames should be treated
as modifiers as described in section 11 of RFC 1944.

Measurement units:
bits per second
N-octets per second
(N-octets per second / media_maximum-octets per second) x 100

Issues:
token ring

See also:
intended load (3.4.1)

3.4.3 Maximum offered load

Definition:
The highest number of frames per second that an external source can
transmit to a DUT/SUT for forwarding to a specified output port or
ports.

Discussion:
The maximum load that an external device can apply to a DUT/SUT
may not equal the maximum load allowed by the medium. This will
be the case when an external source lacks the resources to transmit
frames at the minimum legal inter-frame gap or when it has sufficient
resources to transmit frames below the minimum legal inter-frame
gap. Moreover, maximum load may vary with respect to parameters
other than a medium's maximum theoretical utilization. For example,
on those media employing tokens, maximum load may vary as a
function of Token Rotation Time, Token Holding Time, or the ability
to chain multiple frames to a single token.
The maximum load that an external device applies to a DUT/SUT
MUST be specified when measuring forwarding rates.

Measurement units:
bits per second
N-octets per second
(N-octets per second / media_maximum-octets per second) x 100


Issues:

See Also:
offered load (3.4.2)

3.4.4 Overloading

Definition:
Attempting to load a DUT/SUT in excess of the maximum rate of
transmission allowed by the medium.

Discussion:
Overloading can serve to exercise buffers and buffer allocation
algorithms as well as congestion control mechanisms.
The number of input port or ports required to overload one or more
output ports of a DUT/SUT will vary according to the media rates of
the ports involved. An external source can also overload a port by
transmitting frames below the minimum inter-frame gap. This can
serve to determine whether a device respects the minimum inter-frame
gap. Overloading can be achieved with unidirectional, bidirectional
and meshed traffic.

Measurement units:
N-octets per second
(N-octets per second / media_maximum-octets per second) x 100
N-octet frames per second

Issues:

See Also:
offered load (3.4.2)

3.5 Forwarding rates

This group applies to the rates at which traffic is forwarded by any
DUT/SUT in response a stimulus.

3.5.1 Forwarding rate

Definition:
The number of frames per second that a device can be observed to
successfully transmit to the correct destination port in response to a
specified offered load.

Discussion:
Unlike throughput defined in section 3.17 of RFC 1242, forwarding
rate makes no explicit reference to frame loss. Forwarding rate,
which must only be sampled on the output side of the ports under
test, can be measured for unidirectional, bidirectional or meshed
traffic and should be sampled in fixed time intervals of one second. If
longer or shorter intervals are used they should be cited when
reporting a device's forwarding rate.
It should be noted that the forwarding rate of a DUT/SUT may be
sensitive to the action of congestion control mechanisms.

Measurement units:
N-octet frames per second

Issues:

See Also:
offered load (3.4.2)
forwarding rate at maximum offered load (3.5.2)
maximum forwarding rate (3.5.3)

3.5.2 Forwarding rate at maximum offered load

Definition:
The number of frames per second that a device can be observed to
successfully transmit to the correct destination port in response to the
maximum offered load.

Discussion:
Forwarding rate at maximum offered load may be less than the
maximum rate at which a device can be observed to successfully
forward traffic. This will be the case when the ability of a device to
forward frames degenerates when offered traffic at maximum load.
Maximum offered load must be cited when reporting forwarding rate
at maximum offered load.

Measurement units:
N-octet frames per second

Issues:

See Also:
maximum offered load (3.4.3)
forwarding rate (3.5.1)
maximum forwarding rate (3.5.3)

3.5.3 Maximum forwarding rate

Definition:
The highest forwarding rate of a DUT/SUT taken from an iterative set
of forwarding rate measurements.

Discussion:
The forwarding rate of a device may degenerate before maximum load
is reached. The load applied to a device must be cited when reporting
maximum forwarding rate.

Measurement units:
N-octet frames per second

Issues:

See Also:
offered load (3.4.2)
forwarding rates (3.5.1)
forwarding rate at maximum load (3.5.2)

3.6 Congestion
This group applies to the behavior of a DUT/SUT when congestion
or contention is present.

3.6.1 Backpressure

Definition:
Any technique used by a DUT/SUT to attempt to avoid frame loss by
impeding external sources of traffic from transmitting frames to
congested ports.

Discussion:
Some switches send jam signals, for example preamble bits, back to
traffic sources when their transmit amd/or receive buffers start to
overfill. Switches implementing full duplex Ethernet links may use
IEEE 802.3x Flow Control for the same purpose. Such devices may
incur no frame loss when external sources attempt to offer traffic to
congested or overloaded ports.
It shoulkd be noted that jamming and other flow control methods
may slow all traffic transmitted to congested input ports including
traffic intended for uncongested output ports.

Measurement units:
frame loss on congested port or ports
N--octet frames per second between the port applying backpressure
and an uncongested
destination port

Issues:
jamming not explicitly described in standards

See Also:
forward pressure (3.6.2)

3.6.2 Forward pressure

Definition:
Methods which depart from or otherwise violate a defined
standardized protocol in an attempt to increase the forwarding
performance of a DUT/SUT.

Discussion:
A DUT/SUT may be found to inhibit or abort backoff algorithms in
order to force access to the medium when contention occurs. It
should be noted that the backoff algorithm should be fair whether the
DUT/SUT is in a congested or an uncongested state. Transmission
below the minimum inter-frame gap or the disregard of flow control
primitives fall into this category.

Measurement units:
intervals between frames in microseconds
intervals in microseconds between transmission retries during 16
successive collisions.

Issues:
truncated binary exponential backoff algorithm

See also:
backpressure (3.6.1)

3.6.3 Head of line blocking

Definition:
Frame loss observed on an uncongested output port whenever frames
are received from an input port which is also attempting to forward
frames to a congested output port.

Discussion:
It is important to verify that a switch does not slow transmission or
drop frames on ports which are not congested whenever overloading
on one of its other ports occurs.

Measurement units:
frame loss recorded on an uncongested port when receiving frames
from a port which is also forwarding frames to a congested port.

Issues:
input buffers

See Also:
unidirectional traffic (3.2.1)

3.7 Address handling
This group applies to the process of address resolution which enables
a DUT/SUT to forward frames to the correct destination.

3.7.1 Address caching capacity

Definition:
The number of MAC addresses per n ports, per module or per device
that a DUT/SUT can cache and successfully forward frames to
without flooding or dropping frames.

Discussion:

Users building networks will want to know how many nodes they can
connect to a DUT/SUT. This makes it necessary to verify the number
of MAC addresses that can be assigned per n ports, per module and
per chassis before a DUT/SUT begins flooding frames.

Measurement units:
number of MAC addresses per n ports, per module and/or per chassis

Issues:

See Also:
Address learning rate (3.7.2)

3.7.2 Address learning rate

Definition:
The maximum rate at which a switch can learn new MAC addresses
before starting to flood or drop frames.

Discussion:
Users may want to know how long it takes a switch to build its
address tables. This information is useful to have when considering
how long it takes a network to come up when many users log on in
the morning or after a network crash.

Measurement units:
frames per second with each successive frame sent to the switch
containing a different source address.

Issues:

See Also: address caching capacity (3.7.1)

3.7.3 Flood count

Definition:
Frames forwarded to ports which do not correspond to the
destination MAC address information when traffic is offered to a
DUT/SUT for forwarding.

Discussion:
When recording throughput statistics it is important to check that
frames have been forwarded to their proper destinations. Flooded
frames MUST NOT be counted as received frames. Both known and
unknown unicast frames can be flooded.

Measurement units:
N-octet valid frames

Issues:
Spanning tree BPDUs.

See Also:
address caching capacity (3.7.1)

3.8 Errored frame filtering
This group applies to frames with errors which a DUT/SUT may
filter.

3.8.1 Errored frames

Definition:
Frames which are over-sized, under-sized, misaligned or with an
errored Frame Check Sequence.

Discussion:
Switches, unlike IEEE 802.1d compliant bridges, do not necessarily
filter all types of illegal frames. Some switches, for example, which
do not store frames before forwarding them to their destination ports
may not filter over-sized frames (jabbers) or verify the validity of the
Frame Check Sequence field. Other illegal frames are under-sized
frames (runts) and misaligned frames.

Measurement units:
n/a

Issues:

See Also:

3.9 Broadcasts
This group applies to MAC layer and network layer broadcast
frames.

3.9.1 Broadcast forwarding rate

Definition:
The number of broadcast frames per second that a DUT/SUT can be
observed to deliver to all ports located within a broadcast domain in
response to a specified offered load.

Discussion:
There is no standard forwarding mechanism used by switches to
forward broadcast frames. It is useful to determine the broadcast
forwarding rate for frames switched between ports on the same card,
ports on different cards in the same chassis and ports on different
chassis linked together over backbone connections. The terms
maximum broadcast forwarding rate and broadcast forwarding rate at
maximum load follow directly from the terms already defined for
forwarding rate measurements in section 3.5 above.

Measurement units:
N-octet frames per second

Issues:

See Also:
forwarding rate at maximum load (3.5.2)
maximum forwarding rate (3.5.3)
broadcast latency (3.9.2)

3.9.2 Broadcast latency

Definition:
The time required by a DUT/SUT to forward a broadcast frame to
each port located within a broadcast domain.

Discussion:
Since there is no standard way for switches to process broadcast
frames, broadcast latency may not be the same on all receiving ports
of a switching device. The latency measurements SHOULD be bit
oriented as described in 3.8 of RFC 1242. It is useful to determine
broadcast latency for frames forwarded between ports on the same
card, ports on different cards in the same chassis and ports on
different chassis linked together over backbone connections.

Measurement units:
nanoseconds
microseconds
milliseconds
seconds

Issues:

See Also:
broadcast forwarding rate (3.9.1)

4. References:
1. RFC 1242 "Benchmarking Terminology for Network Interconnect
Devices"
2. RFC 1944 "Benchmarking Methodology for Network Interconnect
Devices"

5. Acknowledgments

A special thanks goes to the IETF BenchMark WorkGroup for the
many suggestions it collectively made to help complete this draft.
Kevin Dubray (Bay Networks), Jean-Christophe Bestaux (ENL),
Ajay Shah ( WG), Henry Hamon (Netcom Systems), Stan Kopek
(3Com) and Doug Ruby (Prominet) all provided valuable input at
various stages of this project.

The editor
Bob Mandeville

5. Editor's Address

Robert Mandeville
ENL (European Network Laboratories)
35, rue Beaubourg
75003 Paris
France
mobile phone: +33 6 07 47 67 10
phone: +33 1 39 44 12 05
fax: + 33 1 39 44 12 06
email: bob.mandeville@eunet.fr


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