Network Working Group Vishwas Manral
Internet Draft Netplane Systems
Russ White
Cisco Systems
Aman Shaikh
Expiration Date: November 2004 University of California
File Name: draft-ietf-bmwg-ospfconv-applicability-05.txt May 2004
Benchmarking Applicability for Basic OSPF Convergence
draft-ietf-bmwg-ospfconv-applicability-05.txt
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Copyright Notice
Copyright (C) The Internet Society (2002). All Rights Reserved.
Abstract
This document discusses the applicability of various tests for
measuring single router control plane convergence, specifically in
regards to the Open Shortest First (OSPF) protocol. There are two
general sections in this document, the first discussing specific
advantages and limitations of specific OSPF convergence tests, and
the second discussing more general pitfalls to be considered when
testing routing protocols convergence testing.
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1. Introduction
There is a growing interest in testing single router control plane
convergence for routing protocols, with many people looking at
testing methodologies which 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 attempting 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 what apparent
performance routing protocols provide.
This document describes in detail the 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: The word convergence within this document refers to single
router control plane convergence [TERM].
2. Advantages of Such Measurement
o To be able to compare the iterations of a protocol implementa-
tion. It is often useful to be able to compare the performance
of two iterations of a given implementation of a protocol 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 is going
to perform. For instance, if you were trying to decide the pro-
cessing power (size of device) required in a certain location
within a network, you can emulate the conditions which are going
to exist at that point in the network and use the test described
to measure the perfomance 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 net-
work, using routes taken from the network where the equipment is
to be deployed rather than some generated topology in these
tests will give results which are closer to the real preformance
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
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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 some times 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 some
times be helpful for operators to know the delay required for
individual tasks so as to optimize the resource usage in the
network i.e. if it is found that the processing time is x
seconds on an router, it would be helpful to determine the rate
at which to flood LSA's to that router so as to not overload the
network.
3. Assumptions Made and Limitations of such measurements
o The interactions of convergence and forwarding; testing is res-
tricted 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 some implementation does
not acknowledge duplicate LSAs immediately on receipt, the test-
ing methods presented in [BENCHMARK] could give inaccurate meas-
urements.
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 pro-
cess is not running ([BENCHMARK] measures the total time taken
for SPF to run, not the amount of time that SPF actually spends
on the device's processor), thus giving inaccurate measurements.
o Some implementations may be multithreaded or use a
multiprocess/multirouter model of OSPF. If because of this any
of the assumptions taken in measurement are violated in such a
model, it could lead to inaccurate measurements.
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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 com-
ponents of an OSPF implementation's performance, and it may be
difficult to combine the measurements in a way which 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 con-
sideration when comparing different vendor's products, and it's
difficult to align the resources available on one device to the
resources available on another device.
4. Observations on the Tests Described in [BENCHMARK]
Some observations taken 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, since the amount of time
between the first LSA which 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 under test actually begins run-
ning SPF on the network change information. If the delay between the
two LSAs is too long, the device under test may finish SPF processing
before receiving the duplicate LSA. It is important to closely inves-
tigate any delays between the receipt of an LSA and the beginning of
an SPF calculation in the device under test; multiple tests with
various delays might be required to determine what delay needs to be
used to accurately measure the SPF calculation time.
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.
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4.2. Noise in the Measurement Device
The device on which measurements are taken (not the device under
test) 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 information about the packet reaching 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 remove it
from the results.
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.
o Having an estimate of noise can also be useful.
The DUT also adds noise to the measurement. Points (a) and (c)
apply to the DUT as well.
4.3. Gaining an Understanding of the Implementation Improves Measure-
ments
While the tester will (generally) not have access to internal infor-
mation 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 which can be used to become familiar with the tests
described in [BENCHMARK] is to perform the tests on an OSPF implemen-
tation for which all the internal details are available, such as
[GATED]. While there is no assurance that any two implementations
will be similar, this will provide a better understanding of the
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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 destina-
tions 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 which would be acceptable under
different situations; within an area, for instance, type 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.
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 algorithms 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
under test. For instance, AS-External LSA's have AS wide flood-
ing 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
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the adjacent area. So routes to the external destinations aren't
immediately added to the routers in the other areas. When the
routers which already have the type 5's 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 5's originated by that border router.
The ordering can be changed in various tests to provide insight
on 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, with the simplest structure being a list of vertices
currently reachable from the source. In a simple list of vertices,
finding the minimum cost vertex then would 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 min cost is found, the algorithm iterates thru all the edges of
the vertex and updates 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,
overall running time is:
O(sum(i:1, n)(size(list at level i) + E).
So, everything boils down to the size (list at level i).
If the graph is linear:
root
|
1
|
2
|
3
|
4
|
5
|
6
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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 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 upto a certain number, and then goes down by 1.
If we assumed that tree is a complete tree (like the one in the
draft) 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 graph is a complete graph (fully-connected mesh), then
size(list at level i) = n - i. Number of edges E = O(n^2). There-
fore, run-time is O(n^2).
So, the performance of the shortest path first algorithm used to
compute the best paths through the network is dependant o the con-
struction of the tree The best practice would be to try and 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
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 which can be adver-
tised to a device with varying conditions. Route generators either
tend to be 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:
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o The ability to connect to the 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 in 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 which appear to be a logi-
cal, 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 should be able to inject vary-
ing numbers of externally reachable destinations.
o The ability to withdraw and add routing information into and
from the emulated topology to emulate links flapping.
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 transmit-
ted.
o The generator and the collector should be fast enough so that
they are not bottle necks. The devices should also have a degree
of granularity of measurement atleast as small as desired from
the test results.
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8. Security Considerations
This doecument 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 of this draft.
10. Normative References
[BENCHMARK]
Manral, V., "Benchmarking Basic OSPF Single Router Control Plane
Convergence", draft-bmwg-ospfconv-intraarea-08, May 2004
[TERM]Manral, V., "OSPF Convergence Testing Terminiology and Con-
cepts", draft-bmwg-ospfconv-term-08, May 2004
[RFC2119]
Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997
11. Informative References
[INTERCONNECT]
Bradner, S., McQuaid, J., "Benchmarking Methodology for Network
Interconnect Devices", RFC2544, March 1999.
[FIB-TERM]
Trotter, G., "Terminology for Forwarding Information Base (FIB)
based Router Performance", RFC3222, October 2001.
[BROADCAST-P2P]
Shen, Naiming, et al., "Point-to-point operation over LAN in
link-state routing protocols", draft-ietf-isis-igp-p2p-over-
lan-03.txt, August, 2003.
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[GATED]
http://www.gated.org
12. Authors' Addresses
Vishwas Manral
Netplane Systems
189 Prashasan Nagar
Road number 72
Jubilee Hills
Hyderabad, India
vmanral@netplane.com
Russ White
Cisco Systems, Inc.
7025 Kit Creek Rd.
Research Triangle Park, NC 27709
riw@cisco.com
Aman Shaikh
University of California
School of Engineering
1156 High Street
Santa Cruz, CA 95064
aman@soe.ucsc.edu
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