Protocol Converter Synthesis
Using Timed Petri Nets
Kevin Camera
EE249 Fall 2000
10/24/2000
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
Services and Protocols

Layer N services provide functions for
layer N+1 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
Converter Design Approaches

Top-down:
service-level
conversion


Easy to implement,
tends to be “passive”
Bottom-up:
protocol-level
conversion

Very powerful, very
complex
Converter Properties

Safety


Liveness


Free from deadlock or livelock, and is
complete
Performs the required functionality
Timeliness

Satisfies the timing of both protocols
Design Criteria

Modeling formalism


Design approach


CFSM, TPN, etc.
Service level, protocol level, or hybrid
Design methodology


Analytic: trial-and-error iterations
Synthetic: systematic, safe generation
Design Criteria (con’t)

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
Design Criteria (con’t)

Timeliness



Dynamicity



Internal timing and protocol requirements
Data loss and recovery
Self-induced, active communication
Concurrency
Complexity
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
Example: Alternating Bit
Example: Poll-End
Synthesis Technique




Greatest common service definition
Trace generation and collection
Trace synchronization
Synthesis of Petri Net model
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
GCSD  P{upper _ N ,upper _ M } ( S N || SIC || S M )
Example: GCSD
?
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 S’N
Find us_N, composition of lower services
and communication channel
TN = N’ || us_N
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)*}
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

(N+m,N-c)  (M-m) = (N+m,M-m,N-c)
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
Result (after PN synthesis)
Summary

Hybrid approach


Timed Petri net model



Starts with service specification, but performs
all synthesis on protocols
Can incorporate timing in specification
Models concurrency and comes with well-known
analysis algorithms
Resulting converter is safe and functional