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Keywords: spf time, adjacency formation time
Network Working Group V. Manral
Request for Comments: 4063 SiNett Corp.
Category: Informational R. White
Cisco Systems
A. Shaikh
AT&T Labs (Research)
April 2005
Considerations When Using Basic OSPF Convergence Benchmarks
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 discusses the applicability of various tests for
measuring single router control plane convergence, specifically in
regard to the Open Shortest First (OSPF) protocol. There are two
general sections in this document, the first discusses advantages and
limitations of specific OSPF convergence tests, and the second
discusses more general pitfalls to be considered when routing
protocol convergence is tested.
1. Introduction
There is a growing interest in testing single router control plane
convergence for routing protocols, and many people are looking at
testing methodologies that can provide information on how long it
takes for a network to converge after various network events occur.
It is important to consider the framework within which any given
convergence test is executed when one attempts to apply the results
of the testing, since the framework can have a major impact on the
results. For instance, determining when a network is converged, what
parts of the router's operation are considered within the testing,
and other such things will have a major impact on the apparent
performance that routing protocols provide.
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This document describes in detail various benefits and pitfalls of
tests described in [BENCHMARK]. It also explains how such
measurements can be useful for providers and the research community.
NOTE: In this document, the word "convergence" refers to single
router control plane convergence [TERM].
2. Advantages of Such Measurement
o To be able to compare the iterations of a protocol
implementation. It is often useful to be able to compare the
performance of two iterations of a given implementation of a
protocol in order to determine where improvements have been made
and where further improvements can be made.
o To understand, given a set of parameters (network conditions),
how a particular implementation on a particular device will
perform. For instance, if you were trying to decide the
processing power (size of device) required in a certain location
within a network, you could emulate the conditions that will
exist at that point in the network and use the test described to
measure the performance of several different routers. The
results of these tests can provide one possible data point for
an intelligent decision.
If the device being tested is to be deployed in a running
network, using routes taken from the network where the equipment
is to be deployed rather than some generated topology in these
tests will yield results that are closer to the real performance
of the device. Care should be taken to emulate or take routes
from the actual location in the network where the device will be
(or would be) deployed. For instance, one set of routes may be
taken from an ABR, one set from an area 0 only router, various
sets from stub area, another set from various normal areas, etc.
o To measure the performance of an OSPF implementation in a wide
variety of scenarios.
o To be used as parameters in OSPF simulations by researchers. It
may sometimes be required for certain kinds of research to
measure the individual delays of each parameter within an OSPF
implementation. These delays can be measured using the methods
defined in [BENCHMARK].
o To help optimize certain configurable parameters. It may
sometimes be helpful for operators to know the delay required
for individual tasks in order to optimize the resource usage in
the network. For example, if the processing time on a router is
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found to be x seconds, determining the rate at which to flood
LSAs to that router would be helpful so as not to overload the
network.
3. Assumptions Made and Limitations of Such Measurements
o The interactions of convergence and forwarding; testing is
restricted to events occurring within the control plane.
Forwarding performance is the primary focus in [INTERCONNECT],
and it is expected to be dealt with in work that ensues from
[FIB-TERM].
o Duplicate LSAs are Acknowledged Immediately. A few tests rely
on the property that duplicate LSA Acknowledgements are not
delayed but are done immediately. However, if an implementation
does not acknowledge duplicate LSAs immediately on receipt, the
testing methods presented in [BENCHMARK] could give inaccurate
measurements.
o It is assumed that SPF is non-preemptive. If SPF is implemented
so that it can (and will be) preempted, the SPF measurements
taken in [BENCHMARK] would include the times that the SPF
process is not running, thus giving inaccurate measurements.
([BENCHMARK] measures the total time taken for SPF to run, not
the amount of time that SPF actually spends on the device's
processor.)
o Some implementations may be multithreaded or use a
multiprocess/multirouter model of OSPF. If because of this any
of the assumptions made during measurement are violated in such
a model, measurements could be inaccurate.
o The measurements resulting from the tests in [BENCHMARK] may not
provide the information required to deploy a device in a large-
scale network. The tests described focus on individual
components of an OSPF implementation's performance, and it may
be difficult to combine the measurements in a way that
accurately depicts a device's performance in a large-scale
network. Further research is required in this area.
o The measurements described in [BENCHMARK] should be used with
great care when comparing two different implementations of OSPF
from two different vendors. For instance, there are many other
factors than convergence speed that need to be taken into
consideration when comparing different vendors' products. One
difficulty is aligning the resources available on one device to
the resources available on another.
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4. Observations on the Tests Described in [BENCHMARK]
Some observations recorded while implementing the tests described in
[BENCHMARK] are noted in this section.
4.1. Measuring the SPF Processing Time Externally
The most difficult test to perform is the external measurement of the
time required to perform an SPF calculation because the amount of
time between the first LSA that indicates a topology change and the
duplicate LSA is critical. If the duplicate LSA is sent too quickly,
it may be received before the device being tested actually begins
running SPF on the network change information. If the delay between
the two LSAs is too long, the device may finish SPF processing before
receiving the duplicate LSA. It is important to closely investigate
any delays between the receipt of an LSA and the beginning of an SPF
calculation in the tested device; multiple tests with various delays
might be required to determine what delay needs to be used to measure
the SPF calculation time accurately.
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.
4.2. Noise in the Measurement Device
The device on which measurements are taken (not the device being
tested) also adds noise to the test results, primarily in the form of
delay in packet processing and measurement output. The largest
source of noise is generally the delay between the receipt of packets
by the measuring device and the receipt of information about the
packet by the device's output, where the event can be measured. The
following steps may be taken to reduce this sampling noise:
o Increasing the number of samples taken will generally improve
the tester's ability to determine what is noise, and to remove
it from the results. This applies to the DUT as well.
o Try to take time-stamp for a packet as early as possible.
Depending on the operating system being used on the box, one can
instrument the kernel to take the time-stamp when the interrupt
is processed. This does not eliminate the noise completely, but
at least reduces it.
o Keep the measurement box as lightly loaded as possible. This
applies to the DUT as well.
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o Having an estimate of noise can also be useful.
The DUT also adds noise to the measurement.
4.3. Gaining an Understanding of the Implementation Improves
Measurements
Although the tester will (generally) not have access to internal
information about the OSPF implementation being tested using
[BENCHMARK], the more thorough the tester's knowledge of the
implementation is, the more accurate the results of the tests will
be. For instance, in some implementations, the installation of
routes in local routing tables may occur while the SPF is being
calculated, dramatically impacting the time required to calculate the
SPF.
4.4. Gaining an Understanding of the Tests Improves Measurements
One method that can be used to become familiar with the tests
described in [BENCHMARK] is to perform the tests on an OSPF
implementation for which all the internal details are available.
Although there is no assurance that any two implementations will be
similar, this will provide a better understanding of the tests
themselves.
5. LSA and Destination Mix
In many OSPF benchmark tests, a generator injecting a number of LSAs
is called for. There are several areas in which injected LSAs can be
varied in testing:
o The number of destinations represented by the injected LSAs
Each destination represents a single reachable IP network; these
will be leaf nodes on the shortest path tree. The primary
impact to performance should be the time required to insert
destinations in the local routing table and handling the memory
required to store the data.
o The types of LSAs injected
There are several types of LSAs that would be acceptable under
different situations; within an area, for instance, types 1, 2,
3, 4, and 5 are likely to be received by a router. Within a
not-so-stubby area, however, type-7 LSAs would replace the
type-5 LSAs received. These sorts of characterizations are
important to note in any test results.
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o The number of LSAs injected
Within any injected set of information, the number of each type
of LSA injected is also important. This will impact the
shortest path algorithm's ability to handle large numbers of
nodes, large shortest path first trees, etc.
o The order of LSA injection
The order in which LSAs are injected should not favor any given
data structure used for storing the LSA database on the device
being tested. For instance, AS-External LSAs have AS wide
flooding scope; any type-5 LSA originated is immediately flooded
to all neighbors. However, the type-4 LSA, which announces the
ASBR as a border router, is originated in an area at SPF time
(by ABRs on the edge of the area in which the ASBR is). If SPF
isn't scheduled immediately on the ABRs originating the type-4
LSA, the type-4 LSA is sent after the type-5 LSA's reach a
router in the adjacent area. Therefore, routes to the external
destinations aren't immediately added to the routers in the
other areas. When the routers that already have the type 5s
receive the type-4 LSA, all the external routes are added to the
tree at the same time. This timing could produce different
results than a router receiving a type 4 indicating the presence
of a border router, followed by the type 5s originated by that
border router.
The ordering can be changed in various tests to provide insight
into the efficiency of storage within the DUT. Any such changes
in ordering should be noted in test results.
6. Tree Shape and the SPF Algorithm
The complexity of Dijkstra's algorithm depends on the data structure
used for storing vertices with their current minimum distances from
the source; the simplest structure is a list of vertices currently
reachable from the source. In a simple list of vertices, finding the
minimum cost vertex would then take O(size of the list). There will
be O(n) such operations if we assume that all the vertices are
ultimately reachable from the source. Moreover, after the vertex
with minimum cost is found, the algorithm iterates through all the
edges of the vertex and updates the cost of other vertices. With an
adjacency list representation, this step, when iterated over all the
vertices, would take O(E) time, with E being the number of edges in
the graph. Thus, the overall running time is:
O(sum(i:1, n)(size(list at level i) + E).
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So everything boils down to the size(list at level i).
If the graph is linear,
root
|
1
|
2
|
3
|
4
|
5
|
6
and source is a vertex on the end, then size(list at level i) = 1 for
all i. Moreover, E = n - 1. Therefore, running time is O(n).
If the graph is a balanced binary tree,
root
/ \
1 2
/ \ / \
3 4 5 6
size(list at level i) is a little complicated. First, it increases
by 1 at each level up to a certain number, and then it goes down by
1. If we assume that the tree is a complete tree (as shown above)
with k levels (1 to k), then size(list) goes on like this: 1, 2, 3,
Then the number of edges E is still n - 1. It then turns out that
the run-time is O(n^2) for such a tree.
If the graph is a complete graph (fully-connected mesh), then
size(list at level i) = n - i. Number of edges E = O(n^2).
Therefore, run-time is O(n^2).
Therefore, the performance of the shortest path first algorithm used
to compute the best paths through the network is dependent on the
construction of the tree. The best practice would be to try to make
any emulated network look as much like a real network as possible,
especially in the area of the tree depth, the meshiness of the
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network, the number of stub links versus transit links, and the
number of connections and nodes to process at each level within the
original tree.
7. Topology Generation
As the size of networks grows, it becomes more and more difficult to
actually create a large-scale network on which to test the properties
of routing protocols and their implementations. In general, network
emulators are used to provide emulated topologies that can be
advertised to a device with varying conditions. Route generators
tend to be either a specialized device, a piece of software which
runs on a router, or a process that runs on another operating system,
such as Linux or another variant of Unix.
Some of the characteristics of this device should be as follows:
o The ability to connect to several devices using both point-to-
point and broadcast high-speed media. Point-to-point links can
be emulated with high-speed Ethernet as long as there is no hub
or other device between the DUT and the route generator, and the
link is configured as a point-to-point link within OSPF
[BROADCAST-P2P].
o The ability to create a set of LSAs that appear to be a logical,
realistic topology. For instance, the generator should be able
to mix the number of point-to-point and broadcast links within
the emulated topology and to inject varying numbers of
externally reachable destinations.
o The ability to withdraw and add routing information into and
from the emulated topology to emulate flapping links.
o The ability to randomly order the LSAs representing the emulated
topology as they are advertised.
o The ability to log or otherwise measure the time between packets
transmitted and received.
o The ability to change the rate at which OSPF LSAs are
transmitted.
o The generator and the collector should be fast enough that they
are not bottlenecks. The devices should also have a degree of
granularity of measurement at least as small as is desired from
the test results.
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8. Security Considerations
This document does not modify the underlying security considerations
in [OSPF].
9. Acknowledgements
Thanks to Howard Berkowitz (hcb@clark.net) and the rest of the BGP
benchmarking team for their support and to Kevin Dubray
(kdubray@juniper.net), who realized the need for this document.
10. Normative References
[BENCHMARK] Manral, V., White, R., and A. Shaikh, "Benchmarking
Basic OSPF Single Router Control Plane Convergence",
RFC 4061, April 2005.
[TERM] Manral, V., White, R., and A. Shaikh, "OSPF
Benchmarking Terminology and Concepts", RFC 4062,
April 2005.
[OSPF] Moy, J., "OSPF Version 2", STD 54, RFC 2328, April
1998.
11. Informative References
[INTERCONNECT] Bradner, S. and J. McQuaid, "Benchmarking Methodology
for Network Interconnect Devices", RFC 2544, March
1999.
[FIB-TERM] Trotter, G., "Terminology for Forwarding Information
Base (FIB) based Router Performance", RFC 3222,
December 2001.
[BROADCAST-P2P] Shen, Naiming, et al., "Point-to-point operation over
LAN in link-state routing protocols", 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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