Quantitative Network Engineering with Fault Tree Analysis

This is a simple paper outlining how to use some Risk Analysis Techniques with Network Engineering to obtain quantitative results. An attempt to turn network engineering from an artful guessing game into a science.

Topics

1. Background
2. Modeling
3. Intro to FTA
4. Top Event

Iterative Analysis

From this point on, we repeat the previous analsyis moving one step lower in the tree. Once we've completed the analysis of that level (and made any necessary adjustments to the model), we repeat at the next lower level. We continue iterating in this fasion until:

• The event is a basic event.
• We belive that the portion of the tree is sufficently developed
• We cannot develop the tree further

Continuing with our example, we examine the reason we may have had a control plane failure and a forwarding plane failure in more depth, focusing in on the contributing events:

• Forwarding Plane Failure
Ask yourself, what could happen that would take out the router's ability to forward packets?

• Failure of our network links.
• A system power failure.
• A hardware failure. Careful not to jump ahead here again and start blaming line cards and SFPs already.

Analysing what sequence of these three events would cause the event above, we can see that any one of thise events occuring independently would cause a forwarding plane failure. Thus we use an OR gate to link them together.

• Control Plane Failure
Again, what could cause this to happen?

• A power outate could certainly take out the control plane.
• A hardware failure of the Routing Brain subsystem.
• Misconfiguration, more than likely due to operator error.

When looking at what combinaions of these events would cause the Control Plane to fail, it's easy to see that any one of these events could cause the event above. We link the events together with an OR gate.

At this stage our fault tree diagram looks as follows:

      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |   Version 0   |       C       |            Plenty             |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |           Router ID - www.blackhole-networks.com              |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |           Area ID - FTA with Network Engineering              |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |          Checksum  OK         |         Construction          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                                                               |
      +-                                                             -+
      |                        PAGE STILL                             | 
      +-                         UNDER                               -+
      |                       CONSTRUCTION                            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Last page update 17 March 2013

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