Network Working Group Vishwas Manral
Internet Draft Netplane Systems
Russ White
Cisco Systems
Aman Shaikh
Expiration Date: August 2004 University of California
File Name: draft-ietf-bmwg-ospfconv-applicability-04.txt February 2004
Benchmarking Applicability for Basic OSPF Convergence
draft-ietf-bmwg-ospfconv-applicability-04.txt
1. Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
Internet Drafts are working documents of the Internet Engineering
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2. Abstract
This draft describes the applicability of [BENCHMARK] and similar
work which may be done in the future. Refer to [TERM] for terminology
used in this draft and [BENCHMARK]. The draft defines the advantages
as well as limitations of using the method defined in [BENCHMARK],
besides describing the pitfalls to avoid during measurement.
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3. Motivation
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].
4. Advantages of Such Measurement
o To be able to compare the iterations of a protocol implemen-
tation. It is often useful to be able to compare the perfor-
mance of two iterations of a given implementation of a proto-
col to determine where improvements have been made and where
further improvements can be made.
o To understand, given a set of parameters (network condi-
tions), how a particular implementation on a particular dev-
ice is going to perform. For instance, if you were trying to
decide the processing power (size of device) required in a
certain location within a network, you can emulate the condi-
tions 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
network, using routes taken from the network where the equip-
ment 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
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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 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.
5. 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 testing
methods presented in [BENCHMARK] could give inaccurate measure-
ments.
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 ([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 components
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 which must be taken into considera-
tion when comparing different vendor's products, and it's diffi-
cult to align the resources available on one device to the
resources available on another device.
6. Observations on the Tests Described in [BENCHMARK]
Some observations taken while implementing the tests described in
[BENCHMARK] are noted in this section.
6.1. 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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6.2. 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 com-
pletely, 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.
6.3. 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.
6.4. 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.
7. 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 pri-
mary 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 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 characteri-
zations 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 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
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type-5 LSA's reach a router in 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.
8. 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).
Therefore, 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 construction 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.
9. 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 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 should be able 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 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 transmitted.
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.
10. 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.
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11. Normative References
[BENCHMARK]
Manral, V., "Benchmarking Basic OSPF Single Router Control Plane
Convergence", draft-bmwg-ospfconv-intraarea-07, November 2003
[TERM]Manral, V., "OSPF Convergence Testing Terminiology and Concepts",
draft-bmwg-ospfconv-term-07, January 2004
12. 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.
[GATED]
http://www.gated.org
13. 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.
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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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