RFC4062: OSPF Benchmarking Terminology and Concepts
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Related keywords:
(adjacency formation time)
(spf time)
Network Working Group V. Manral
Request for Comments: 4062 SiNett Corp.
Category: Informational R. White
Cisco Systems
A. Shaikh
AT&T Labs (Research)
April 2005
OSPF Benchmarking Terminology and Concepts
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 (2005).
Abstract
This document explains the terminology and concepts used in OSPF
benchmarking. Although some of these terms may be defined elsewhere
(and we will refer the reader to those definitions in some cases) we
include discussions concerning these terms, as they relate
specifically to the tasks involved in benchmarking the OSPF protocol.
1. Introduction
This document is a companion to [BENCHMARK], which describes basic
Open Shortest Path First [OSPF] testing methods. This document
explains terminology and concepts used in OSPF Testing Framework
Documents, such as [BENCHMARK].
2. Specification of Requirements
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].
[RFC2119] key words in this document are used to ensure
methodological control, which is very important in the specification
of benchmarks. This document does not specify a network-related
protocol.
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3. Common Definitions
Definitions in this section are well-known industry and benchmarking
terms that may be defined elsewhere.
o White Box (Internal) Measurements
- Definition
White box measurements are those reported and collected on
the Device Under Test (DUT) itself.
- Discussion
These measurements rely on output and event recording,
along with the clocking and time stamping available on the
DUT itself. Taking measurements on the DUT may impact the
actual outcome of the test, since it can increase processor
loading, memory utilization, and timing factors. Some
devices may not have the required output readily available
for taking internal measurements.
Note: White box measurements can be influenced by the
vendor's implementation of various timers and processing
models. Whenever possible, internal measurements should be
compared to external measurements to verify and validate
them.
Because of the potential for variations in collection and
presentation methods across different DUTs, white box
measurements MUST NOT be used as a basis for comparison in
benchmarks. This has been a guiding principle of the
Benchmarking Methodology Working Group.
o Black Box (External) Measurements
- Definition
Black box measurements infer the performance of the DUT
through observation of its communications with other
devices.
- Discussion
One example of a black box measurement is when a downstream
device receives complete routing information from the DUT,
it can be inferred that the DUT has transmitted all the
routing information available. External measurements of
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internal operations may suffer in that they include not
just the protocol action times, but also propagation
delays, queuing delays, and other such factors.
For the purposes of [BENCHMARK], external techniques are
more readily applicable.
o Multi-device Measurements
- Measurements assessing communications (usually in
combination with internal operations) between two or more
DUTs. Multi-device measurements may be internal or
external.
4. Terms Defined Elsewhere
Terms in this section are defined elsewhere and are included only as
they apply to [BENCHMARK].
o Point-to-Point Links
- Definition
See [OSPF], Section 1.2.
- Discussion
A point-to-point link can take less time to converge than a
broadcast link of the same speed because it does not have
the overhead of DR election. Point-to-point links can be
either numbered or unnumbered. However, in the context of
[BENCHMARK] and [OSPF], the two can be regarded as the
same.
o Broadcast Link
- Definition
See [OSPF], Section 1.2.
- Discussion
The adjacency formation time on a broadcast link can be
greater than that on a point-to-point link of the same
speed because DR election has to take place. All routers
on a broadcast network form adjacency with the DR and BDR.
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Asynchronous flooding also takes place through the DR. In
the context of convergence, it may take more time for an
LSA to be flooded from one DR-other router to another
because the LSA first has to be processed at the DR.
o Shortest Path First Execution Time
- Definition
The time taken by a router to complete the SPF process, as
described in [OSPF].
- Discussion
This does not include the time taken by the router to
install routes in the forwarding engine.
Some implementations may force two intervals, the SPF hold
time and the SPF delay, between successive SPF
calculations. If an SPF hold time exists, it should be
subtracted from the total SPF execution time. If an SPF
delay exists, it should be noted in the test results.
- Measurement Units
The SPF time is generally measured in milliseconds.
o Hello Interval
- Definition
See [OSPF], Section 7.1.
- Discussion
The hello interval must be the same for all routers on a
network.
Decreasing the hello interval can allow the router dead
interval (below) to be reduced, thus reducing convergence
times in those situations where the router dead interval's
timing out causes an OSPF process to notice an adjacency
failure. Further discussion of small hello intervals is
given in [OSPF-SCALING].
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o Router Dead Interval
- Definition
See [OSPF], Section 7.1.
- Discussion
This is advertised in the router's Hello Packets in the
Router-DeadInterval field. The router dead interval should
be some multiple of the HelloInterval (perhaps 4 times the
hello interval) and must be the same for all routers
attached to a common network.
5. Concepts
5.1. The Meaning of Single Router Control Plane Convergence
A network is termed as converged when all the devices within the
network have a loop-free path to each possible destination. However,
because we are not testing network convergence but testing
performance for a particular device within a network, this definition
needs to be streamlined to fit within a single device view.
In this case, convergence will mean the point in time when the DUT
has performed all actions needed in order to react to the change in
the topology represented by the test condition. For instance, an
OSPF device must flood any new information it has received, rebuild
its shortest path first (SPF) tree, and install any new paths or
destinations in the local routing information base (RIB, or routing
table).
Note that the word "convergence" has two distinct meanings: the
process of a group of individuals meeting at the same place, and the
process of an individual coming to the same place as an existing
group. This work focuses on the second meaning of the word, so we
consider the time required for a single device to adapt to a network
change to be Single Router Convergence.
This concept does not include the time required for the control plane
of the device to transfer the information required to forward packets
to the data plane. It also does not include the amount of time
between when the data plane receives that information and when it is
able to forward traffic.
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5.2. Measuring Convergence
Obviously, there are several elements to convergence, even under the
definition given above for a single device, including (but not
limited to) the following:
o The time it takes for the DUT to pass the information about a
network event on to its neighbors.
o The time it takes for the DUT to process information about a
network event and to calculate a new Shortest Path Tree (SPT).
o The time it takes for the DUT to make changes in its local RIB
reflecting the new shortest path tree.
5.3. Types of Network Events
A network event is an event that causes a change in the network
topology.
o Link or Neighbor Device Up
The time needed for an OSPF implementation to recognize a new
link coming up on the device, to build any necessary
adjacencies, to synchronize its database, and to perform all
other actions necessary to converge.
o Initialization
The time needed for an OSPF implementation to be initialized, to
recognize any links across which OSPF must run, to build any
needed adjacencies, to synchronize its database, and to perform
other actions necessary to converge.
o Adjacency Down
The time needed for an OSPF implementation to recognize a link
down/adjacency loss based on hello timers alone, to propagate
any information as necessary to its remaining adjacencies, and
to perform other actions necessary to converge.
o Link Down
The time needed for an OSPF implementation to recognize a link
down based on layer 2-provided information, to propagate any
information as needed to its remaining adjacencies, and to
perform other actions necessary to converge.
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6. Security Considerations
This document does not modify the underlying security considerations
in [OSPF].
7. Acknowledgements
The authors would like to thank Howard Berkowitz (hcb@clark.net),
Kevin Dubray (kdubray@juniper.net), Scott Poretsky
(sporetsky@avici.com), and Randy Bush (randy@psg.com) for their
discussion, ideas, and support.
8. Normative References
[BENCHMARK] Manral, V., White, R., and A. Shaikh, "Benchmarking
Basic OSPF Single Router Control Plane Convergence",
RFC 4061, April 2005.
[OSPF] Moy, J., "OSPF Version 2", STD 54, RFC 2328, April
1998.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
9. Informative References
[OSPF-SCALING] Choudhury, Gagan L., Editor, "Prioritized Treatment of
Specific OSPF Packets and Congestion Avoidance", Work
in Progress, August 2003.
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Authors' Addresses
Vishwas Manral,
SiNett Corp,
Ground Floor,
Embassy Icon Annexe,
2/1, Infantry Road,
Bangalore, India
EMail: vishwas@sinett.com
Russ White
Cisco Systems, Inc.
7025 Kit Creek Rd.
Research Triangle Park, NC 27709
EMail: riw@cisco.com
Aman Shaikh
AT&T Labs (Research)
180 Park Av, PO Box 971
Florham Park, NJ 07932
EMail: ashaikh@research.att.com
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