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

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
   Task Force (IETF), its Areas, and its Working Groups. Note that other
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   The list of current Internet-Drafts can be accessed at

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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",
   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-

         -    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

      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

      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:


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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:

              /    \
             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 (,
   Kevin Dubray, (, and Randy Bush (
   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,

      Russ White
      Cisco Systems, Inc.
      7025 Kit Creek Rd.
      Research Triangle Park, NC 27709

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      Aman Shaikh
      University of California
      School of Engineering
      1156 High Street
      Santa Cruz, CA  95064

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