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Network Working Group                                     Vishwas Manral
Internet Draft                                          Netplane Systems
                                                              Russ White
                                                           Cisco Systems
                                                             Aman Shaikh
Expiration Date: December 2004                  University of California
File Name: draft-ietf-bmwg-ospfconv-applicability-07.txt       June 2004

      Considerations When Using Basic OSPF Convergence Benchmarks
             draft-ietf-bmwg-ospfconv-applicability-07.txt

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Copyright Notice

   Copyright (C) The Internet Society (2004).  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 performance 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 performance
        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 up to 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 at least as small as desired from
        the test results.
















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8. IANA Considerations

   This document requires no IANA considerations.


9. Security Considerations

   This document does not modify the underlying security considerations
   in [OSPF].


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.


11. Normative References


   [BENCHMARK]
        Manral, V., "Benchmarking Basic OSPF Single Router Control Plane
        Convergence", draft-bmwg-ospfconv-intraarea-10, June 2004


   [TERM]Manral, V., "OSPF Convergence Testing Terminology and Con-
        cepts", draft-bmwg-ospfconv-term-10, May 2004


   [RFC2119]
        Bradner, S., "Key words for use in RFCs to Indicate Requirement
        Levels", BCP 14, RFC 2119, March 1997


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.





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   [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.
   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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