Network Working Group                                  R. Mandeville
INTERNET-DRAFT                         European Network Laboratories
Expiration Date:  May 1997                                  Nov 1996

        Benchmarking Terminology for LAN Switching Devices

                < draft-ietf-bmwg-lanswitch-01.txt >

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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).

LAN switches are one of the principal sources of new bandwidth in the local
area and are handling a significantly increasing proportion of network
traffic. 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 LAN switching devices. This draft covers local area devices
which switch frames at the Media Access Control (MAC) layer. It discusses
throughput, latency, address handling and filtering.

2. Term definitions

A previous document, "Benchmarking Terminology for Network Interconnect
Devices" (RFC 1242), defined many of the terms that are used in this
document.  The terminology document should be consulted before attempting to
make use of this document. A more recent document, "Benchmarking Methodology
for Network Interconnect Devices" (RFC 1944), defined 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.

2. 1. Reminder of RFC 1242 definition format

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

The specific definition for the term.

A brief discussion about the term, it's application
and any restrictions on measurement procedures.

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

List of issues or conditions that effect this term.

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

2.2. Unidirectional traffic


Unidirectional traffic is made up of a single or multiple streams of frames
forwarded in one direction only from one or more ports of a switching device
designated as input ports to one or more other ports of the device
designated as output ports.

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 maximum rate of throughput.
Unidirectional traffic SHOULD be offered to devices 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 streams of traffic can be used to create different patterns
of traffic. For example unidirectional streams can be offered to two input
ports so as to overload a single output port (2-to-1) or they can be offered
to a single input port and switched by the device under test to two or more
output ports (1-to-2). Such patterns can be combined to test for head of
line blocking or to measure throughput when congestion control mechanisms
are active.
When devices are 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.

half duplex / full duplex

Measurement units:

See Also:
bidirectional traffic (2.3)
multidirectional traffic (2.4)

2.3. Bidirectional traffic

Bidirectional traffic is made up of two or more streams of frames forwarded
in opposite directions between at two or more ports of a switching device.

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 the maximum rate of
throughput on full duplex ports of a switching device.

truncated binary exponential back-off algorithm

Measurement units:

See Also:
unidirectional traffic (2.2)
multidirectional traffic (2.4)

2.4. Multidirectional traffic

Multidirectional traffic is made up of streams of frames that are switched
simultaneously between multiple ports of a switching device. When such
streams are fully meshed each of the ports under test will both send frames
to and receive frames from all of the other ports under test.

This definition extends the discussions in sections 14 and 16 of RFC 1944 on
bidirectional traffic and multi-port testing.
As with bidirectional multi-port tests, multidirectional 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 unidirectional streams offered in a multidirectional test for 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 however that
bidirectional multiport tests create a greater load than multidirectional
tests on backbone connections linking together two switching devices because
none of the transmitted frames are forwarded locally. Backbone tests SHOULD
use bidirectional multi-port traffic.
Multidirectional traffic on half duplex ports is inherently bursty since
ports must interrupt transmission intermittently to receive frames. When
offering such bursty traffic to a device under test a number of variables
have to be considered. They include frame size, the number of frames within
bursts as well as the interval between bursts. The terms burst, burst size
and inter-burst gap are defined in sections 2.5, 2.6 and 2.7 below.
Bursty multidirectional traffic is characteristic of real network traffic.
It simultaneously exercises many of the component parts of a switching
device such as input and output buffers, buffer allocation mechanisms,
aggregate switching capacity, processing speed and behavior of the media
access controller.

Measurement units:

half duplex / full duplex

See Also:
unidirectional traffic (2.2)
bidirectional traffic (2.3)

2.5 Burst

A group of frames transmitted with the minimum inter-frame gap allowed by
the media. This definition allows for single frame bursts and infinite bursts.

This definition follows from the discussion in section 21 of RFC 1944. It is
useful to consider isolated frames as single frame bursts.

Measurement units:


See Also:
burst size (2.6)

2.6 Burst size

The number of frames in a burst.

Burst size can range from one to infinity. In unidirectional streams there
is no theoretical limit to the burst length. Bursts in bidirectional and
multidirectional streams of traffic on half duplex media are finite since
ports interrupt transmission intermittently to receive frames.
On real networks burst size can 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


See Also: burst (2.5)

2.7 Inter-burst gap (IBG)

The interval between two bursts.

This definition conforms to the discussion in section 20 of RFC 1944 on
bursty traffic.
Bidirectional and multidirectional streams of traffic are inherently bursty
since ports share their time between receiving and transmitting frames. The
rate of transmission of an external source of traffic is a function of the
number of frames per burst, frame length and the inter-burst gap. External
sources offering bursty multidirectional traffic for a given frame size and
burst size MUST adjust the inter-burst gap to achieve a specified rate of
When a burst contains a single frame inter-burst gap and inter-frame gap are
When a burst is infinite the interburst gap equals the minimum inter-frame gap.

Measurement units:


See Also: burst size (2.6), load (2.8)

2.8 Load, nominal and real

The amount of traffic per second that a port transmits and receives.

Load can be expressed in a number of ways: bits per second, frames per
second with the frame size specified or as a percentage of the maximum frame
rate allowed by the media for a given frame size. A unidirectional stream of
7440 64-byte Ethernet frames per second is equivalent to a 50% load given
that the maximum rate of transmission on an Ethernet is 14880 64-byte frames
per second.
In the case of bidirectional or multidirectional traffic port load is the
sum of the frames transmitted and received on a port per second.
There is room for varying the balance between incoming and outgoing traffic
when loading ports with bidirectional and multidirectional traffic. In the
case of bidirectional traffic a 100% load can be created by offering a n%
load on one port and a (100 - n)% load on the opposite port.
Multidirectional traffic will be equally distributed over all ports under
test when all ports are offered 50% of the target load.
It has to kept in mind that an external source may not deliver frames to a
device under test at the desired rate due to collisions on CSMA/CD links or
the action of congestion control mechanisms. Because of this it is often
necessary to distinguish between the desired or target load (nominal load)
and the actual load (real load) offered to the device under test.
External sources of Ethernet traffic MUST implement the truncated binary
exponential back-off algorithm when executing bidirectional and
multidirectional performance tests to ensure that the external source of
traffic is accessing the medium legally.
Frames which are not successfully transmitted by the external source of
traffic to the device under test MUST NOT be not counted as transmitted
frames in performance benchmarks.

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

token ring

2.9 Overload

Loading a port or ports in excess of the maximum rate of transmission
allowed by the media.

Overloading can serve to exercise input and/or output buffers, buffer
allocation algorithms and congestion control mechanisms. Unidirectional
overloads require a minimum of two input and one output ports when all ports
run at the same nominal speed.
Bidirectional and multidirectional overloading occurs when the sum of the
traffic transmitted and received on each port exceeds the maximum media
rate. The external source of traffic MUST transmit at the same rate situated
between more than 50% and a 100% of the maximum media rate to each of the
ports under test in order to equally distribute an overload over all ports
under test.

Measurement units:
N-octet frames per second

Issues: nominal/real load

See Also:

2.10 Speed

The number of frames that a device is capable of delivering to the correct
destination port in a given time interval. The maximum speed of a switching
device is the highest number of frames it can deliver during a one second
interval to the correct destination port.

Switching devices which exhibit no frame loss may be found to deliver frames
to their proper destination ports at differing rates. This may be due to the
action of congestion control mechanisms at high loads or the relative
aggressiveness of the truncated binary back-off algorithm. Speed MUST only
be sampled on the output side of the ports under test. This is because an
input port may receive frames at higher rates when the device under test
drops frames.

Measurement units:
N-octet frames per second


See Also:

2.11 Flooded frame

A unicast frame which is received on ports which do not correspond to the
frame's destination MAC address information.

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.

Measurement units:
N-octet valid frames per second

Spanning tree BPDUs.

See Also:

2.11 Backpressure

A jamming technique used by some switching devices to avoid frame loss when
one or more of its ports are saturated.

Some switches are designed to send jamming signals back to traffic sources
when ports begin to saturate. Such devices may incur no frame loss when
ports are offered target loads in excess of 100% by external traffic
sources. Jamming however affects traffic destined to congested as well as
uncongested ports so it is important to measure the maximum speed at which a
device can forward frames to both congested and uncongested ports when
backpressure mechanisms are active.

Measurement units:
N--octet frames per second between the jamming port and an uncongested
destination port

not explicitly described in standards

See Also:
forward pressure (2.12)

2.12 Forward pressure

A technique which modifies the truncated binary exponential backoff
algorithm to avoid frame loss when congestion on one or more of the ports
under test occurs.

Some switches avoid buffer overload by retransmitting buffered frames
without waiting for the interval calculated by the normal operation of the
backoff algorithm. It is useful to measure how aggressive a switch's backoff
algorithm is in both congested and uncongested states. Forward pressure
reduces the number of collisions when congestion on a port builds up.

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

not explicitly described in standards

See also:
backpressure (2.11)

2.13 Head of line blocking

A pathological state whereby a switch drops frames forwarded to an
uncongested port whenever frames are forwarded from the same source port to
a congested port.

It is important to verify that a switch does not propagate frame loss to
ports which are not congested whenever overloading on one of its 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 destination port.

Input buffers

See Also:

2.14 Address handling

The number of MAC addresses per port, per module or per device which a
switch can cache and successfully forward frames to without flooding or
dropping frames.


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

Measurement units:
number of MAC addresses


See Also:

2.15 Address learning rate

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

Users may want to know how long it takes a switch to build up 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.


See Also: address handling (2.14)

2.16 Filtering illegal frames

Switches do not necessarily filter all types of illegal frames. Some
switches, for example, do not store frames before forwarding them to their
destination ports. These so-called cut-through switches forward frames after
reading the destination and source address fields. They do not normally
filter over-sized frames (jabbers) or verify the validity of the Frame Check
Sequence field. Other examples of illegal frame types are under-sized frames
(runts), misaligned frames and frames followed by dribble bits.

Measurement units:
N-octet frames filtered or not filtered


See Also:

2.17 Broadcast rate

The number of broadcast frames forwarded by the device under test per
second. The maximum broadcast rate corresponds to highest number of
broadcast frames a switch can forward either locally or over a backbone

There is no standard forwarding mechanism used by switches to forward
broadcast frames. It is useful to determine the broadcast forwarding rate
both locally and over backbone connections.

Measurement units:
N-octet frames per second


See Also:
broadcast latency

2.18 Broadcast latency

The time it takes a broadcast frame to go through a switching device and be
forwarded to each destination port.

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. Broadcast latency SHOULD be determined on all receiving ports both
locally and, if applicable,  over backbone connections.

Measurement units:
The latency measurements SHOULD be bit oriented as described in 3.8 of RFC
1242 and reported for all connected receive ports.


See Also:
broadcast rate

3. Acknowledgments

In order of appearance Ajay Shah of Wandel & Goltermann, Jean-Christophe
Bestaux of European Network Laboratories, Stan Kopek of Digital Equipment
Corporation, Henry Hamon of Netcom Systems and Kevin Dubray of Bay Networks
were all instrumental in getting this draft done.
A special thanks goes to the IETF BenchMark WorkGroup for the many
suggestions it collectively made to help shape this draft.
The editor
Bob Mandeville

4. Editor's Address

Robert Mandeville
ENL  (European Network Laboratories)
35, rue Beaubourg
75003 Paris
phone: +33 6 07 47 67 10
fax: + 33 1 42 78 36 71


Robert Mandeville, ENL
European Network Laboratories
6 Parc Ariane, le Mercure
Blvd des Chenes
78284 Guyancourt

NEW LAB PHONE, FAX, VOICE MAIL: +33 1 39 44 12 05
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