Content Processing C. Edward Chow Department of Computer Scien

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Reachability Analysis

• Starting from initial global state, where
  channels are empty and machines at their initial
  state, explore all possible reachable state by
  firing the possible transitions (and generating
  global states) from any given reachable state.
• All deadlock and unspecified reception errors
  will be captured/marked as individual global
  state.
• By examining the number of msgs in the channels
  we can design the buffer size for the protocol.
• Can detect non-executable states and transitions
  by marking those state are touched and transition
  that are fired during the reachability analysis.

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Reability analysis
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Reachability Analysis Exercise

• Perform the reachability analysis on the Network
  (M, N).
• What sizes of buffers are needed for the two FIFO
  channels?
• Are there non-executable states or transitions?

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Solution

• One unspecified reception.
• Both channels needbuffer size of 2.(see gs8 and
  gs9)
• No non-executablestates and transitions.
• Since both machinessend same typesof msgs. We
  useMachine to specifywhich machine firesthe
  transition.

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Pros and Cons of Reachability Analysis

• Advantages
• Easily automated.
• Many logical errors can be detected by only
  examining individual global states in the
  reachability graph.
• Disadvantages
• State space explosion problem.(The state
  space of a system can be very large, or even
  infinite. Thus, at the outset, it is impossible
  to explore the entire state space with limited
  resources of time and memory. )
• Does not work on unbounded protocols.
• Many relationships among the protocol state
  variables, expressing the desirable logical
  correctness properties of the protocol, are not
  apparent from simply traversing the reachability
  graph.

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