Network Working Group Vishwas Manral
Internet Draft Netplane Networks
Russ White
Expiration Date: June 2002 Cisco Systems
File Name: draft-manral-ospfconv-term-00.txt December 2001
OSPF Benchmarking Terminology and Concepts
draft-manral-ospfconv-term-00.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 explains the terminology and concepts used in [2] and
future OSPF benchmarking drafts.
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3. Conventions used in this document
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 [1].
4. Motivation
This draft is a companion to [2], which describes basic Open Shortest
Path First (OSPF [3]) testing methods. This draft explains
terminology and concepts used in OSPF Testing Framework Drafts, such
as [2].
5. Definitions
o Internal Measurements
- Definition
Internal measurements are measurements taken on the Device
Under Test (DUT) itself.
- Discussion
These measurement rely on output and event recording,
along with the clocking and timestamping available on the
DUT itself. Internal measurements are preferred for all
tests that can be completely contained on the DUT (which
is very rare).
o External Measurements
- Definition
External measurements infer the performance of the DUT
through observation of its communications with other dev-
ices.
- Discussion
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One example of an external measurement is when a down-
stream device receives complete routing information from
the DUT, it can be inferred that the DUT has transmitted
all the routing information available.
For the purposes of this paper, external technique are
more readily applicable. However, external measurements
have their own problems because they include the time to
advertise the new route downstream and transmission times
for the advertisement within the device under test.
o Multi-device Measurements
- Definition
Multi-device measurements require the measurement of
events occuring on multiple devices within the testbed.
- Discussion
For instance, the timestamp on a device generating an
event could be used as the marker for the beginning of a
test, while the timestamp on the DUT or some other device
might be used to determine when the DUT has finished pro-
cessing the event.
These sorts of measurements are the most problematic, and
are to be avoided where possible, since the timestamps of
the devices in the test bed must be synchronized within
milliseconds for the test results to be meaningful. Given
the state of network time protocol implementation, expect-
ing the timestamps on several devices to be within mil-
liseconds of each other is highly optimistic.
o Point-to-Point links
- Definition
A network that joins a single pair of routers is called a
point-to-point link. For OSPF [3], point-to-point links
are those on which a designated router are not elected.
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- Discussion
A point-to-point link will take lesser time to converge
than a braodcast 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 con-
text of [2], the two can be regarded the same.
o Broadcast Link
- Definition
Networks supporting many (more than two) attached routers,
together with the capability to address a single physical
message to all of the attached routers (broadcast). In the
context of [2] and [3], broadcast links are taken as those
on which a designated router is elected.
- Discussion
The adjacency formation time on a broadcast link can be
more 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.
Async flooding also takes place thru the DR. In context of
convergence, it may take more time for an LSU to be
flooded from one DR-other router to another DR-other
router, because the LSA has to be first processsed at the
DR.
o Shortest Path First Time
- Definition
The time taken by a router to complete the SPF process.
- Discussion
This does not include the time taken by the router to give
routes to the forwarding engine.
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o Measurement Units
The LSA time is generally measured in milliseconds.
o Hello Interval
- Definition
The length of time, in seconds, between the Hello Packets
that the router sends on the interface.
- Discussion
The hello interval should be the same for all routers on
the 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
timing out causes an OSPF process to notice an adjacency
failure. Very small router dead intervals accompanied by
very small hello intervals can produce more problems than
they resolve, as described in [4] & [5].
o Router Dead interval
- Definition
After ceasing to hear a router's Hello Packets, the number
of seconds before its neighbors declare the router down.
- Discussion
This is advertised in the router's Hello Packets in the
RouterDeadInterval field. The router dead interval should
be some multiple of the HelloInterval (say 4 times the
hello interval), and must be the same for all routers
attached to a common network.
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6. Concepts
6.1. A network is termed to be converged when all of the devices within
the network have a loop free path to each possible destination. Since
we are not testing network convergence, but performance for a partic-
ular device within a network, however, this definition needs to be
narrowed somewhat 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 to react to the change in 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).
6.2. Measuring Convergence
Obviously, there are several elements to convergence, even under the
definition given above for a single device. We will try to provide
tests to measure each of these:
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 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.
6.3. Types of Network Events
o Link or Neighbor Device Up
The time needed for an OSPF implementation to recoginize a
new link coming up on the device, build any necessarily adja-
cencies, synchronize its database, and perform all other
needed actions to converge.
o Initialization
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The time needed for an OSPF implemention to be initialized,
recognize any links across which OSPF must run, build any
needed adjacencies, synchronize its database, and perform
other actions needed to converge.
o Adjacency Down
The time needed for an OSPF implementation to recognize a
link down/adjacency loss based on hello timers alone, propo-
gate any information as necessary to its remaining adjacen-
cies, and perform other actions needed to converge.
o Link Down
The time needed for an OSPF implementation to recognize a
link down based on layer 2 provided information, propogate
any information information as needed to its remaining adja-
cencies, and perform other actions needed to converge.
6.4. 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.
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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 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. The ordering can be changed in various
tests to provide insight on the efficency of storage within
the DUT. Any such changes in ordering should be noted in test
results.
6.5. Tree Shape and the SPF Algorithm
The shortest path first algorithm is a simple algorithm which handles
complexity by breaking the problem of finding the shortest paths
through a network into smaller parts and recursing (calling itself)
to compute the best path within each smaller part. Because of this,
moving along a single level of the tree, along the tree's width, is
fundamentally different than moving along the depth of the tree.
root root
/ \ |
1 2 1
/ \ / \ |
3 4 5 6 2
|
3
|
4
|
5
|
6
For instance, the shortest path first algorithm would go through two
recursions when finding the shortest paths on the left topology, with
an average of two nodes processed per level. The topology on the
right would produce five recursions, with one node processed per
recursion. While this may not produce dramatically different test
results, there may be some apparent difference between the two.
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In general, those benchmarking link state protocols which use the
shortest path first algorithm to compute the best paths through the
network need to be aware that the construction of the tree may impact
the performance of the algorithm. Best practice would be to try and
make any emulated network look as much like a real network as possi-
ble, especially in the area of the tree depth, the meshiness of the
network, the number of stub links verses transit links, and the
number of connections and nodes to process at each recursion level.
7. Route 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 topologues 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:
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.
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.
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o The ability to change the rate at which OSPF LSAs are transmitted.
8. Acknowedgements
The authors would like to thank Aman Shaikh
(ashaikh@research.att.com) for his comments and help on this draft.
9. References
[1] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", RFC2119, March 1997.
[2] Manral, V., "Benchmarking Methodology for Basic OSPF Convergence",
draft-manral-ospconv-intraarea-00, November 2001
[3] Moy, J., "OSPF Version 2", RFC 2328, April 1998.
[4] draft-ash-ospf-isis-congestion-control-01.txt
[5] draft-ietf-ospf-scalability-00.txt
10. Authors' Addresses
Vishwas Manral,
Netplane Networks,
189 Prashasan Nagar,
Road number 72,
Jubilee Hills,
Hyderabad.
vmanral@netplane.com
Russ White
Cisco Systems, Inc.
7025 Kit Creek Rd.
Research Triangle Park, NC 27709
riw@cisco.com
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