Network Working Group Vishwas Manral
Internet Draft Netplane Systems
Russ White
Cisco Systems
Aman Shaikh
Expiration Date: March 2003 University of California
File Name: draft-ietf-bmwg-ospfconv-term-01.txt October 2002
OSPF Benchmarking Terminology and Concepts
draft-ietf-bmwg-ospfconv-term-01.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.
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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. Taking measurements on the DUT may impact the
actual outcome of the test, since it can increase proces-
sor loading, memory utilization, and timing factors. Some
devices may not have the required output readily available
for taking internal measurements, as well.
Note: Internal measurements can be influenced by the
vendor's implementation of the various timers and process-
ing models. Whenever possible, internal measurements
should be compared to external measurements to verify and
validate them.
o External Measurements
- Definition
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External measurements infer the performance of the DUT
through observation of its communications with other dev-
ices.
- Discussion
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. External measure-
ments 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 this paper, external techniques are
more readily applicable.
o Multi-device Measurements
- Definition
Multi-device measurements require the measurement of
events occurring 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
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- 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.
- Discussion
A point-to-point link will take lesser 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 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 processed at the
DR.
o Shortest Path First Time
- Definition
The time taken by a router to complete the SPF process.
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- Discussion
This does not include the time taken by the router to give
routes to the forwarding engine.
o Measurement Units
The SPF time is generally measured in milliseconds.
o Hello Interval
- Definition
The length of time, in seconds, between the Hello Packets
that the router sends hello packets on the interface. The
typical hello interval is 10 seconds on broadcast net-
works, and 30 seconds for point-to-multipoint and point-
to-point networks. On multicast capable media, hellos are
sent to a multicast address; on non-multicast capable
media, they are sent unicast to each neighbor.
- 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.
The default dead interval is four times the hello inter-
val; 40 seconds on broadcast networks, and 120 seconds on
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non-broadcast networks.
- 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.
o Incremental SPF
- Definition
The ability to recalculate a small portion of the SPF
tree, rather than the entire SPF tree, on receiving notif-
ication of a change in the network topology. At worst,
incremental SPF should perform no worse than a full SPF.
In better situations, an incremental SPF run will rebuild
the SPF tree in much shorter time than a full SPF run.
6. Concepts
6.1. The Meaning of Convergence
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).
Note that the word convergence has two distinct meanings; the process
of a group of individuals meeting the same place, and the process of
a single individual meeting in 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
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be SR-Convergence, or Single Router Convergence.
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
A network event is an event which causes a change in the network
topology.
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
The time needed for an OSPF implementation 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,
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propogate any information as necessary to its remaining adja-
cencies, 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 as needed to its remaining adjacencies, 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.
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
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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 ABR's 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. 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.
6.5. Tree Shape and the SPF Algorithm
The complexity of Dijkstra's algo 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 reach-
able from the source. 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. More-
over, after the vertex with min cost is found, the algo 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. 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
|
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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 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).
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 possible, 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 level within the
original tree.
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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:
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.
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.
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8. Acknowedgements
The authors would like to thank Howard Berkowitz (hcb@clark.net),
Kevin Dubray, (kdubray@juniper.net), and Randy Bush (randy@psg.com)
for their discussion, ideas, and support.
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-bmwg-ospfconv-intraarea-00, May 2002
[3] Moy, J., "OSPF Version 2", RFC 2328, April 1998.
[4] Ash, J., "Proposed Mechanisms for Congestion Control/Failure
Recovery in OSPF & ISIS Networks", October, 2001
[5] draft-ietf-ospf-scalability-00.txt Choudhury, G., et al, "Explicit
Marking and Prioritized Treatment of Specific IGP Packets for Fas-
ter IGP Convergence and Improved Network Scalability and Stabil-
ity", April 2002
10. Authors' Addresses
Vishwas Manral,
Netplane Systems,
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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Aman Shaikh
University of California
School of Engineering
1156 High Street
Santa Cruz, CA 95064
aman@soe.ucsc.edu
Manral, et. all [Page 13]
