[bmwg] OSPF convergence terminology

["Howard C. Berkowitz" <[e-mail]>]

I'm delighted that someone is taking on this work. Quite frankly, we 
considered BGP convergence (without extensive policy) to be a simpler 
problem in many respects.  Most comments here deal with lessons 
learned as our documents evolved.


Comments inline, with snippage for space.  Apologies for the mess 
made by Mac and Eudora reformatting...will do it differently next 
time.

--------------------
>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



>2. Abstract    This draft explains the terminology and concepts used 
>in [2] and    future OSPF benchmarking drafts. INTERNET DRAFT 
>OSPF Benchmarking Terminology         December 2001



>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

Internal and external measurements, also called, respectively, white 
box and black box, are a basic issue in convergence testing. In the 
BGP convergence work, we elected to emphasize external measurements 
for several reasons:

     -- collecting the data inside the router MAY increase loading on it
        and give the effect of reduced performance
     -- a given implementation may not have the interfaces needed for
        internal testing.
     -- time synchronization among the sources, sinks, and DUT may be
        more difficult, especially if NTP proves inadequate.

There are other factors, but the general topic of internal versus 
external measurement is probably well worth discussing as a WG-wide 
subject.  I don't think it's desirable to have some protocols 
measured internally and others measured externally.

>    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               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.

In the BGP convergence work, we are attempting to define measurement 
points at which these additional times can be factored out. It is not 
a trivial task, especially when you are dealing with >1 load source.

>       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.          - 
>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.

For completeness, should you not deal with NBMA and demand links, and 
possibly any special cases associated with virtual links and stub 
routes?

>       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.



>o    Measurement Units               The LSA time is generally 
>measured in milliseconds.

We should not preclude the experimental work going on that puts these 
times into the microsecond range.

>       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].

It's probably worth discussing the reality that there can be 
different hello/dead intervals depending on link type (e.g., NBMA).

>       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.



>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.

Is there an assumption here that the entire new SPT tree is presented 
to the RIB installation process as a single unit, or could the 
presentation be incremental?




>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            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

This material is probably more appropriate for the methodology 
document. We also started with it in terminology, but found it worked 
better in methodology.

>    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. 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

Again, we found this to be a methodology issue.

>    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.    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

Methodology again.

>    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.



>o    The ability to change the rate at which OSPF LSAs are transmitted.



>8. Acknowedgements    The authors would like to thank Aman Shaikh 
>([e-mail]) 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. 
>[e-mail]       Russ White       Cisco Systems, Inc. 
>7025 Kit Creek Rd.       Research Triangle Park, NC 27709 
>[e-mail]

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