Network Working Group Jerry Perser
INTERNET-DRAFT Spirent
Expires in: December 2001 David Newman
Network Test
Sumit Khurana
Telcordia
Shobha Erramilli
Telcordia
Scott Poretsky
Avici Systems
June 2001
Terminology for Benchmarking Network-layer
Traffic Control Mechanisms
<draft-ietf-bmwg-dsmterm-01.txt>
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
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Table of Contents
1. Introduction ............................................... 2
2. Existing definitions ....................................... 3
3. Term definitions.............................................3
3.1 Channel Capacity..........................................3
3.2 Classification............................................4
3.3 Codepoint Set.............................................4
3.4 Conforming................................................5
3.5 Congestion................................................5
3.6 Congestion Management.....................................6
3.7 Delay.....................................................7
3.8 Expected Vector...........................................7
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3.9 Flow......................................................8
3.10 Forwarding Vector .......................................9
3.11 Jitter...................................................9
3.12 Nonconforming...........................................10
3.13 Offered Vector..........................................11
3.14 Stream..................................................11
3.15 Tail dropping...........................................12
3.16 Test Sequence number....................................12
3.17 Unburdened Response.....................................13
4. Security Considerations.....................................13
5. References..................................................14
6. Author's Address............................................14
7. Full Copyright Statement....................................15
1. Introduction
Driven by Internet economics, service providers and enterprises
alike have shown strong interest in adding traffic-control
capabilities to network devices. These capabilities would enable
network operators to define and deliver minimum or maximum levels of
bandwidth, delay, and jitter for multiple classes of traffic.
Perhaps more importantly, network operators would be able to set
pricing according to the level of service delivered.
Networking device manufacturers have responded with a wide variety
of approaches for controlling network traffic. While there are
numerous ôpolicy managementö and ôquality of serviceö frameworks,
many of them rely on one of two network-layer mechanisms for the
actual control of forwarding rate, delay, and jitter. These two
mechanisms are the IP precedence setting in the IP header and the
diff-serv code point (DSCP) defined in [3].
This document describes the various terms to be used in benchmarking
devices that implement traffic control based on IP precedence or
DSCP criteria. This document is narrowly focused, in that it
describes only terms for measuring behavior of a device or a group
of devices using one of these two mechanisms. End-to-end and
service-level measurements are beyond the scope of this document.
This document introduces several new terms that will allow
measurements to be taken during periods of congestion. New
terminology is needed because most existing benchmarking terms
assume the absence of congestion. For example, throughput is one of
the most widely used measurements û- yet RFC 1242 defines throughput
as a rate in the absence of loss. As a result, throughput is not a
meaningful measurement where congestion exists.
Another key difference from existing benchmarking terminology is the
definition of measurements as observed on egress as well as ingress
of a device/system under test. Again, the existence of congestion
requires the addition of egress measurements as well as those taken
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on ingress; without observing traffic leaving a device it is not
possible to say whether traffic-control mechanisms effectively dealt
with congestion.
The principal measurements introduced in this document are rate
vectors, delay, and jitter, all of which can be observed with or
without congestion of the DUT/SUT.
2. Existing definitions
RFC 1242 "Benchmarking Terminology for Network Interconnect Devices"
and RFC 2285 "Benchmarking Terminology for LAN Switching Devices"
should be consulted before attempting to make use of this document.
RFC 2474 "Definition of the Differentiated Services Field (DS Field)
in the IPv4 and IPv6 Headers" section 2, contains discussions of a
number of terms relevant to network-layer traffic control mechanisms
and should also be consulted.
For the sake of clarity and continuity this RFC adopts the template
for definitions set out in Section 2 of RFC 1242. Definitions are
indexed and grouped together in sections for ease of reference.
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.
3. Term definitions
3.1 Channel Capacity
Definition:
The maximum forwarding rate of a link or set of aggregated
links at which none of the offered packets are dropped by the
DUT/SUT.
Discussion:
Channel capacity measures the data rate at the egress
interface(s) of the DUT/SUT. In contrast, throughput as defined
in RFC 1242 measures the data rate is based on the ingress
interface(s) of the DUT/SUT.
Ingress-based measurements do not account for congestion of the
DUT/SUT. Channel capacity, as an egress measurement, does take
congestion into account.
Understanding channel capacity is a necessary precursor to any
measurement involving congestion. Throughput numbers can be
higher than channel capacity because of queueing.
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Measurement units:
N-octet packets per second
Issues:
See Also:
Throughput [1]
3.2 Classification
Definition:
Selection of packets based on the contents of packet header
according to defined rules.
Discussion:
Packets can be selected based on the DS field or IP Precedence
in the packet header. Classification can also be based on
Multi-Field (MF) criteria such as IP Source and destination
addresses, protocol and port number.
Classification determines the per-hop behaviors and traffic
conditioning functions such as shaping and dropping that are to
be applied to the packet.
Measurement units:
n/a
Issues:
See Also:
Rules
3.3 Codepoint Set
Definition:
The set of all DS Code-points or IP precedence values used
during the test duration.
Discussion:
Describes all the code-point markings associated with packets
that are input to the DUT/SUT. For each entry in the codepoint
set, there are associated vectors describing the rate of
traffic containing that particular DSCP or IP precedence value.
The treatment that a packet belonging to a particular code-
point gets is subject to the DUT classifying packets to map to
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the correct PHB. Moreover, the forwarding treatment in general
is also dependent on the complete set of offered vectors.
Measurement Units:
n/a
See Also:
3.4 Conforming
Definition:
Packets that lie within the bounds specified by a traffic
profile.
Discussion:
Rules may be configured that allow a given traffic class to
consume only X bit/s of channel capacity and no more. All
additional packets are dropped. All packets that constitute
the first X bits/s measured over a period of time specified by
the traffic profile, are then said to be conforming whereas
those exceeding the bound are non conforming.
In particular in a congestion scenario, some individual packets
will be conforming and others will not.
Measurement units:
n/a
Issues:
See Also:
Expected Vector
Forwarding Vector
Offered Vector
3.5 Congestion
Definition:
A condition in which one or more egress interfaces are offered
more packets than can be forwarded at any given instant.
Discussion:
This condition is a superset of the overload definition [2].
That definition assumes the overload is introduced strictly by
the tester on ingress of a DUT/SUT. That may or may not be the
case here.
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Another difference is that with multiple-DUT measurements,
congestion may occur at multiple points. For example, multiple
edge devices collectively may congest a core device. In
contrast, overload [1] deals only with overload on ingress.
Ingress observations alone are not sufficient to cover all
cases in which congestion may occur. A device with an infinite
amount of memory could buffer an infinite amount of packets,
and eventually forward all of them. However, these packets may
or may not be forwarded during the test duration. Even though
ingress interfaces accept all packets without loss, this
hypothetical device may still be congested.
The definition presented here explicitly defines congestion as
an event observable on egress interfaces. Regardless of
internal architecture, any device that cannot forward packets
on one or more egress interfaces is congested.
Measurement units:
n/a
Issues:
See Also:
3.6 Congestion Management
Definition:
An implementation of one or more per-hop-behaviors to avoid or
minimize the condition of congestion.
Discussion:
Congestion management may seek either to control congestion or
avoid it altogether. Such mechanisms classify packets based
upon IP Precedence or DSCP settings in a packetÆs IP header.
Congestion avoidance mechanisms seek to prevent congestion
before it actually occurs.
Congestion control mechanisms gives one or more service classes
preferential treatment over other classes during periods of
congestion.
Measurement units:
n/a
Issues:
See Also:
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3.7 Delay
Definition:
The time interval starting when the last bit of the input IP
packets reaches the input port of the DUT/SUT and ending when
the last bit of the output IP packets is seen on the output
port of the DUT/SUT.
Discussion:
Delay is measured the same regardless of the type of DUT/SUT.
Latency [1] require some knowledge of whether the DUT/SUT is a
"store and forward" or a "bit forwarding" device. The fact
that a DUT/SUT's technology has a lower delay than another
technology should be visible.
The measurement point at the end is more like the way an
internet datagram is processed. An internet datagram is not
passed up or down the stack unless it is complete. Completion
occurs once the last bit of the IP packet has been received.
Delay can be run at any offered load. Recommend at or below
the channel capacity for non-congested delay. For congested
delay, run the offered load above the channel capacity.
Measurement units:
Seconds.
Issues:
See Also:
3.8 Expected Vector
Definition:
A vector describing the expected output rate of packets having
a specific code-point. The value is dependent on the set of
offered vectors and configuration of the DUT.
Discussion:
The DUT is configured in a certain way in order that service
discrimination happens for behavior aggregates when a specific
traffic mix consisting of multiple behavior aggregates is
applied. This term attempts to capture the expected behavior,
for which the device is configured, when subjected to a certain
offered load.
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The actual algorithms or mechanism, that the DUT uses to
achieve service discrimination, is not important in describing
the expected vector.
Measurement units:
N-octets packets per second
See Also:
Forwarding Vector
Offered Vector
Codepoint Set
3.9 Flow
Definition:
A flow is a one or more of packets sharing a common intended
pair of source and destination interfaces.
Discussion:
Packets are groups by the ingress and egress interfaces they
use on a given DUT/SUT.
A flow can contain multiple source IP addresses and/or
destination IP addresses. All packets in a flow must enter on
the same ingress interface and exit on the same egress
interface, and have some common network layer content.
Microflows [3] are a subset of flows. As defined in [3],
microflows require application-to-application measurement. In
contrast, flows use lower-layer classification criteria. Since
this document focuses on network-layer classification criteria,
we concentrate here on the use of network-layer identifiers in
describing a flow. Flow identifiers also may reside at the
data-link, transport, or application layers of the ISO model.
However, identifiers other than those at the network layer are
out of scope for this document.
A flow may contain a single code point/IP precedence value or
may contain multiple values destined for a single egress
interface. This is determined by the test methodology.
Measurement units:
n/a
Issues:
See Also:
Microflow [3]
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Streams
3.10 Forwarding Vector
Definition:
The number of packets per second for all packets containing a
single DSCP (or IP precedence) that a device can be observed to
successfully transmit to the correct destination interface in
response to an offered vector.
Discussion:
Forwarding Vector is expressed as pair of numbers. Both the
codepoint (or IP precedence) value AND the packets per second
value combine to make a vector.
The forwarding vector represents packet rate based on their
codepoint or IP precedence value. It is not necessary based on
stream or flow. The forwarding vector may be expresses as ôper
portö or ôof the DUT/SUTö.
Forwarding Vector is a per-hop measurement. The DUT/SUT may
change the codepoint or IP precedence value for a multiple-hop
measurement.
Measurement units:
N-octet packets per second
Issues:
See Also:
Codepoint Set
Expected Vector
Offered Vector.
3.11 Jitter
Definition:
Variation in a stream's delay.
Discussion:
Jitter is the absolute value of the difference between the
delay measurement of two packets belonging to the same stream.
The jitter between two consecutive packets in a stream is
reported as the "instantaneous jitter". Instantaneous jitter
can be expressed as |D(i) û D(i+1)| where D equals the delay
and i is the test sequence number. Packets lost are not
counted in the jitter measurement.
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Average Jitter is the average of the instantaneous jitter
measured during the test duration.
Peak-to-peak jitter is the maximum delay minus the minimum
delay of the packets forwarded by the DUT/SUT.
Measurement units:
Seconds (instantaneous)
Seconds P-P (peak to peak)
Seconds avg (average)
Issues:
Mean
Standard Deviation
Median
90th percentile
Inter Quartile Range
See Also:
Stream
3.12 Nonconforming
Definition:
Packets that lie outside the parameter bounds of a given
traffic profile.
Discussion:
Rules may be configured for a given traffic class based on
parameters, such as an upper bound on the rate of packet
arrivals. All packets that lie outside the bounds specified by
the traffic profile, measured over a period of time specified
in the traffic profile, are said to be nonconforming.
Measurement units:
n/a
Issues:
See Also:
Conforming
3.13 Offered Vector
Definition:
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A vector describing the rate at which packets having a specific
code-point are offered to the DUT/SUT.
Discussion:
Offered loads across the different code-point classes,
constituting a code-point set, determine the metrics associated
with a specific code-point traffic class.
Measurement Units:
N-octets packets per second
Issues:
Packet size.
See Also:
Expected Vector
Forwarding Vector
Codepoint Set
3.14 Stream
Definition:
A group of packets tracked as a single entity by the traffic
receiver. A stream shares a common content such as type (IP,
UDP), frame size, or payload.
Discussion:
Streams are tracked by "sequence number" or "unique signature
field" (RFC 2889). Streams define how individual packet's
statistics are grouped together to form an intelligible
summary.
Common stream groupings would be by egress interface,
destination address, source address, DSCP, or IP precedence. A
stream using sequence numbers can track the ordering of packets
as they transverse the DUT/SUT.
Streams are not restricted to a pair of source and destination
interfaces as long as all packets are tracked as a single
entity. A mulitcast stream can be forward to multiple
destination interfaces.
Measurement units:
n/a
Issues:
See Also:
Flow
MicroFlow [3]
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Test sequence number
3.15 Tail dropping
Definition:
The condition in which a congested DUT/SUT discards newly
arriving packets.
Discussion:
Every DUT/SUT has a finite amount of traffic it can forward,
beyond which congestion occurs. Once the offered load crosses
the congestion threshold, the device may discard any additional
traffic that arrives until congestion clears.
Tail dropping is typically a function of offered load exceeding
a DUT/SUTÆs buffer capacity, but other factors internal to the
DUT/SUT may also come into play. In terms of what is externally
observable, tail dropping can be said to occur only when
offered load exceeds channel capacity. Since a DUT/SUT may
buffer traffic on ingress, the actual threshold for tail
dropping may be higher than channel capacity.
Measurement units:
n/a
Issues:
Some congestion management mechanisms seek to avoid tail
dropping by discarding packets before offered load exceeds
channel capacity. In the presence of such mechanisms, neither
congestion nor tail dropping should occur.
See Also:
Channel capacity
Congestion
3.16 Test Sequence number
Definition:
A field in the IP payload portion of the packet that is used to
verify the order of the packets on the egress of the DUT/SUT.
Discussion:
The traffic generator sets the sequence number value and the
traffic receiver checks the value upon receipt of the packet.
The traffic generator changes the value on each packet
transmitted based on an algorithm agreed to by the traffic
receiver.
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The traffic receiver keeps track of the sequence numbers on a
per stream basis. In addition to number of received packets,
the traffic receiver may also report number of in-sequence
packets, number of out-sequence packets and number of duplicate
packets.
The recommended algorithm to use to change the sequence number
on sequential packets is an incrementing value.
Measurement units:
n/a
Issues:
See Also:
Stream
3.17 Unburdened Response
Definition:
A performance measure obtained when mechanisms used to support
IP precedence and DiffServ are disabled.
Discussion:
Enabling Diffserv mechanisms such as scheduling algorithms may
impose an additional processing overhead for packets, which may
cause the aggregate response to suffer even when traffic
belonging to only one class, the best effort class, is offered
to the device. Comparisons with "unburdened performance" may
thus be in order when obtaining metrics to ensure that enabling
Diffserv mechanisms doesn't impose an excessive performance
penalty.
Measurement units:
n/a
4. Security Considerations
Documents of this type do not directly effect the security of
the Internet or of corporate networks as long as benchmarking
is not performed on devices or systems connected to operating
networks.
5. References
[1] Bradner, S., Editor, "Benchmarking Terminology for Network
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Interconnection Devices", RFC 1242, July 1991.
[2] Mandeville, R., "Benchmarking Terminology for LAN Switching
Devices", RFC 2285, February 1998.
[3] K. Nichols, S. Blake, F. Baker, D. Black,"Definition of the
Differentiated Services Field (DS Field) in the IPv4 and
IPv6 Headers", RFC 2474, December 1998.
[4] S. Blake, D. Black, M. Carlson, E. Davies, Z. Wang, W.
Weiss, "An Architecture for Differentiated Services", RFC
2475, December 1998.
6. Author's Address
Jerry Perser
Spirent Communications
26750 Agoura Road
Calabasas, CA 91302
USA
Phone: + 1 818 676 2300
EMail: jerry.perser@spirentcom.com
David Newman
Network Test
31324 Via Colinas, Suite 113
Westlake Village, CA 91362
USA
Phone: + 1 818 889 0011, x10
EMail: dnewman@networktest.com
Sumit Khurana
Telcordia Technologies
445 South Street
Morristown, NJ 07960
USA
Phone: + 1 973 829 3170
EMail: sumit@research.telcordia.com
Shobha Erramilli
Telcordia Technologies
331 Newman Springs Road,
Navesink, NJ 07701
USA
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Phone: + 1 732 758 5508
EMail: shobha@research.telcordia.com
Scott Poretsky
Avici Systems
101 Billerica AveùBuilding #6
N. Billerica, MA 01862
USA
Phone: + 1 978 964 2287
EMail: sporetsky@avici.com
7. Full Copyright Statement
Copyright (C) The Internet Society (1998). All Rights
Reserved.
This document and translations of it may be copied and
furnished to others, and derivative works that comment on or
otherwise explain it or assist in its implementation may be
prepared, copied, published and distributed, in whole or in
part, without restriction of any kind, provided that the above
copyright notice and this paragraph are included on all such
copies and derivative works. However, this document itself may
not be modified in any way, such as by removing the copyright
notice or references to the Internet Society or other Internet
organizations, except as needed for the purpose of developing
Internet standards in which case the procedures for copyrights
defined in the Internet Standards process must be followed, or
as required to translate it into languages other than English.
The limited permissions granted above are perpetual and will
not be revoked by the Internet Society or its successors or
assigns. This document and the information contained herein is
provided on an "AS IS" basis and THE INTERNET SOCIETY AND THE
INTERNET ENGINEERING TASK FORCE DISCLAIMS ALL WARRANTIES,
EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY
THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY
RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS
FOR A PARTICULAR PURPOSE.
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