Protocol Converter Synthesis Using Timed Petri Nets

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Protocol Converter Synthesis Using Timed Petri
Nets

• Kevin CameraEE249 Fall 200010/24/2000

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Motivation

• Evolution of heterogeneous, distributed networks
• Protocol converters act as mediators between
  otherwise incompatible protocols
• This work improves on previous converter modeling
  and synthesis methodologies

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Services and Protocols

• Layer N services provide functions for layer N1
  via service access points
• User-based functional specification
• Protocol entities (PE) exchange protocol data
  units (PDU) via lower and upper SAPs
• Low-level behavioral specification

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Converter Design Approaches

• Top-downservice-level conversion
• Easy to implement, tends to be passive
• Bottom-upprotocol-level conversion
• Very powerful, very complex

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Converter Properties

• Safety
• Free from deadlock or livelock, and is complete
• Liveness
• Performs the required functionality
• Timeliness
• Satisfies the timing of both protocols

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Design Criteria

• Modeling formalism
• CFSM, TPN, etc.
• Design approach
• Service level, protocol level, or hybrid
• Design methodology
• Analytic trial-and-error iterations
• Synthetic systematic, safe generation

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Design Criteria (cont)

• Information transfer issues
• Direct no buffers, messages transmitted
  immediately to each protocol
• Indirect messages stored in non-FIFO buffer,
  re-ordered, and transmitted
• Synchronization issues
• Mapping of messages (traces) to ensure
  compatbility

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Design Criteria (cont)

• Timeliness
• Internal timing and protocol requirements
• Data loss and recovery
• Dynamicity
• Self-induced, active communication
• Concurrency
• Complexity

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Timed Petri Net Model

• Standard Petri net with predicated and timed
  transitions
• New notations
• Input/Output actions marked with /-
• Parallel composition PN1 PN2
• Trace schuffling t1 ? t2
• Complement of a trace t

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Example Alternating Bit
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Example Poll-End
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Synthesis Technique

• Greatest common service definition
• Trace generation and collection
• Trace synchronization
• Synthesis of Petri Net model

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Greatest Common Service

• Start with both service descriptions
• I/O operations are service primitives
• Map equivalent primitives into a service
  interface converter (SIC)
• Remove primitives not mapped in SIC

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Example GCSD
?
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Trace Generation

• Interested in traces of each separate network
  which contribute to the GCSD
• Can be found with following analysis
• Let TN be set of traces at lower,upper_N
• Find N, a pruning with contributions to SN
• Find us_N, composition of lower services and
  communication channel
• TN N us_N

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Example Trace Generation

• TABP ACCEPT DATA(bit) (ACK(bit)
  DATA(bit)) ACK(bit),DATA(bit) (-ACK(bit)
  DATA(bit)) DELIVER -ACK(bit),DATA(bit)
  DELIVER (-ACK(bit) DATA(bit))
  -ACK(bit),DATA(bit) (DATA(bit)) DELIVER
  -ACK(bit),DATA(bit) DELIVER (DATA(bit))
  -ACK(bit)

• TPE SEND poll (-data SEND) (-data SEND)
  -end,poll SEND (-data SEND) (-data SEND)
  -end,(poll),-poll (data RECEIVE) (data
  RECEIVE) end,(-poll)

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Trace Synchronization

• For trace sets TN and TM found above
• Prune the protocol components TRN, TRM
• Take complements to get TCN
• Schuffle the complements (TCN ? TCM)
• 14 rules for ordering data, confirmation, ack,
  and nack messages safely
• (Nm,N-c) ? (M-m) (Nm,M-m,N-c)

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Example Trace Synchronization

• TCABP lower_ABP TCABP -DATA(bit)
  (ACK(bit) -DATA(bit)) ACK(bit),DATA(bit)
  (-ACK(bit) DATA(bit)) -ACK(bit),-DATA(bit)
  (-DATA(bit)) ACK(bit)

• TCPE lower_ABP TCABP -poll (data SEND)
  (data SEND) end,(-poll),poll (-data SEND)
  (-data SEND) -end, (poll)

TC TCABP ? TCPE
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Result (after PN synthesis)
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Summary

• Hybrid approach
• Starts with service specification, but performs
  all synthesis on protocols
• Timed Petri net model
• Can incorporate timing in specification
• Models concurrency and comes with well-known
  analysis algorithms
• Resulting converter is safe and functional