Emulators that translate algorithms from the shared-memory model to two different message-passing models are presented. Both are achieved by implementing a wait-free, atomic, single-writer multi-reader register in unreliable, asynchronous networks. The two message-passing models considered are a complete network with processor failures and an arbitrary network with dynamic link failures.These results make it possible to view the shared-memory model as a higher-level language for designing algorithms in asynchronous distributed systems. Any wait-free algorithm based on atomic, single-writer multi-reader registers can be automatically emulated in message-passing systems, provided that at least a majority of the processors are not faulty and remain connected. The overhead introduced by these emulations is polynomial in the number of processors in the system.Immediate new results are obtained by applying the emulators to known shared-memory algorithms. These include, among others, protocols to solve the following problems in the message-passing model in the presence of processor or link failures: multi-writer multi-reader registers, concurrent time-stamp systems, l-exclusion, atomic snapshots, randomized consensus, and implementation of data structures.

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Sharing memory robustly in message-passing systems

A unified approach to the formal description and validation of several distributed protocols is presented. After introducing two basic protocols, a series of known and new protocols for connectivity test, shortest path, and path updating are described and validated. All protocols are extended to networks with changing topology.

Distributed network protocols

The first self-stabilizing end-to-end communication protocol and the most efficient known self-stabilizing network reset protocol are introduced. A simple method of local checking and correction, by which distributed protocols can be made self-stabilizing without the use of unbounded counters, is used. The self-stabilization model distinguishes between catastrophic faults that abstract arbitrary corruption of global state, and other restricted kinds of anticipated faults. It is assumed that after the execution starts there are no further catastrophic faults, but the anticipated faults may continue to occur. >

Self-stabilization by local checking and correction

A method of providing loop free and shortest path routing of data packets in a network having a plurality of switches, routing messages for communicating network topology information between the switches, a plurality of links connecting the switches and a plurality of channels connecting the switches to the links. The loop free routing of data packets is achieved through modifications to known link state packet (LSP) routing protocols and permits each switch to inform adjacent switches in the network of the information in the switch's database used to compute forwarding tables. A switch uses a received LSP to compute a forwarding table and informs neighboring switches on attached links of the routing change. The switch discards any subsequent data packets whose path would be affected by the changed routing information. The discarding of data packets continues until the switch receives notification from each adjacent switch affected by the changed routing information that all affected routing paths have been recalculated and the forwarding table of each affected switch has been updated. Thus, while adjacent switches temporarily contain inconsistent LSP databases and possibly inconsistent forwarding tables, the looping of data packets is prevented. Shortest path routing for data packets from a source endnode to a destination endnode is achieved by assuring that the first switch to forward the packet is on the shortest path to the packet's destination endnode. A source endnode transmits a data packet with an appropriate destination header and the determination of the actual routing path is performed transparently to endnodes. A data packet reaches its destination endnode by following the shortest path possible based on the network topology as represented in the database of the first switch that forwards it.

Synchronization mechanism for link state packet routing

In this paper a new class of network synchronization procedures, called Resynch Procedures, is described. A resynch procedure is a mechanism for effectively bringing all nodes of a distributed network to a known state simultaneously, despite arbitrary finite delays between nodes. The procedures presented have the interesting property that no time-outs are required. One use of a resynch procedure is to implement a network protocol that can guarantee that no packets will be lost and no duplicate packets will be inadvertently received, despite arbitrary node and link failures. This appears to be the first demonstration that such fail-safe protocols exist.

S. Finn

Papers

Resynch Procedures and a Fail-Safe Network Protocol