1. A Formal Treatment of an Abstract Channel Implementation Using Java Sockets and TCP
Chryssis Georgiou, University of Cyprus
Peter Musial, VeroModo, Inc.
Alexander Shvartsman, University of Connecticut
Elaine Sonderegger, University of Connecticut

2. Motivation
Abstract models and specifications of distributed systems allow formal reasoning about their safety properties
Mapping the functionality of abstract specifications to executable code for target distributed platforms is a challenging and error-prone process
Formal specifications and faithful implementations of asynchronous communication channels are particularly challenging

3. Related Work Traditional communication channel models
Fixed, pre-initialized channels
Examples
Reliable FIFO channel
Lossy reordering channel
Josh Tauber’s IOA compiler used Java/MPI to implement pre-initialized channels

4. Our Work
First formal specification of an asynchronous communication channel with:
Explicit initialization
Dynamic interconnections with graceful comings and goings
Implementation of the specification using Java’s interface to TCP sockets
Proof by forward simulation that the implementation preserves the safety properties of the specification

5. Initialization Sender Receiver senderOpen receiverListening send
respReceiverListening
send
receive
send
receive

6. Sender Closing emptying closed Sender Receiver senderClose receive

7. Receiver Closing
closed
Sender
Receiver
receiverClose
Bit Bucket

8. Abstract Channel Input/Output Automata formalism
Transitions (where m is a message, i & j are nodes)
input send (m, i, j)
output receive (m, i, j)
input receiverListening (j)
input receiverStopListening (j)
input senderOpen (i, j)
output respReceiverListening (i, j)
input senderClose (i, j)
internal senderClosing (i, j)
input receiverClose (i, j)
internal lose (m)

9. Implementation Distributed Abstract Channel functionality among nodes
Developed a Composite Channel with three types of component automata
JVM-TCP Channel
Sender Mediator
Receiver Mediator
Based on Josh Tauber’s IOA compiler for a Java/MPI interface

10. Node Automaton Node i TCP Send Sockets Mediator JVM- TCP Channel
Application
Automaton
TCP
Sockets
Receive
Mediator

11. Main Result Theorem: Composite Channel implements Abstract Channel
The set of traces of Composite Channel is a subset of the set of traces of Abstract Channel
Proved using forward simulation
Established a simulation relation mapping the states of Composite Channel to the states of Abstract Channel
Showed the mapping holds for the initial states of each automaton and is maintained by every transition of Composite Channel

12. Summary
First formal specification and implementation of an abstract asynchronous communication channel with explicit support for dynamic creation and teardown of communication links
Provides a building block for modeling dynamic distributed applications and systems
Serves as an aid to automated code generation
Future Work (supported by an NSF grant)
Bi-directional channels
Multiple concurrent channels between node pairs

13. Thank You