Philip M. Merlin

A Failsafe Distributed Routing Protocol

An algorithm for constructing and adaptively maintaining routing tables in communication networks is presented. The algorithm can be employed in message as well as circuit switching networks, uses distributed computation, provides routing tables that are loop-free for each destination at all times, adapts to changes in network flows, and is completely failsafe. The latter means that after arbitrary failures and additions, the network recovers in finite time in the sense of providing routing paths between all physically connected nodes. For each destination, the routes are independently updated by an update cycle triggered by the destination.

Deadlock Avoidance in Store-and-Forward Networks--I: Store-and-Forward Deadlock

Store-and-forward deadlock in store-and-forward networks may be avoided by forwarding messages from buffer to buffer in accordance with a loop-free directed buffer graph which accommodates all the desired message routes. Schemes for designing such buffer graphs are presented, together with methods for using them to forward the messages in an efficient and deadlock-free manner. These methods can be implemented by a set of counters at each node. Such an implementation increases the efficiency of buffer use, and simplifies jumping between normal lowoverhead operation when deadlock is far and more careful operation when deadlock is near. The proposed deadlock avoidance mechanism works for any network topology and any finite routing algorithm.

On the Construction of Submodule Specifications and Communication Protocols

The problem of elaborating the specification for the submodules of a system is considered. A new method for the construction of submodule specifications is described. If the system is to consist of n submodules and the system as well as (n 1) submodules are specified, then the method described determines the specification of the additional n t h submodule. A formula is given which defines the specification of the additional submodule in the general case where module specifications are given in terms of sets of possible execution sequences, and interaction occurs when several modules participate in the execution of an atomic interaction. For the restricted context of finite-state machines, a constructive algorithm for the evaluation of the formula is given. The use of this design method is demonstrated by examples, including a simple communication protocol involving error detection and retransmission. Possible applications in other areas, as well as remaining problems, are indicated.

Deadlock Avoidance in Store-and-Forward Networks--II: Other Deadlock Types

This paper describes the construction of loop-free buffer graphs which avoid four types of buffer deadlocks in store-and-forward networks. 1) Progeny deadlock, where original messages spawnother ones, and buffer contention occurs between the original and progeny messages. This occurs when positive or negative acknowledgments are created, e.g., if messages reverse direction after encountering a path failure. 2) Copy-release deadlock, where a message copy is stored at the source node and the buffer is not released until an acknowledgment is received from the destination node. Buffer contention may arise among the original messages, stored copies, and acknowledgments. 3) Pacing deadlock, where a local flow control protocol is used between a network node and attached terminals. Buffer contention may arise between the message flows into and out of the terminal, preventing the transmission of go-ahead commands. 4) Reassembly deadlock, whereby reassembly of packetized messages at the destination node cannot be completed. The solution presented here has the novel features of not requiring preallocation of reassembly buffers before transmission of multiple packets of a multipacket message, and not requiring dedication of buffer space at intermediate nodes for individual messages. These schemes are believed to have modest buffer requirements at each node, and if adequate buffer pools are provided, will incur negligible performance degradations under normal conditions, with overhead increasing under heavy buffer usage when deadlock is near.

Some theoretical results in multiple path definition in networks

This paper is concerned with the problem of defining K multiple paths between pairs of nodes in a network. The treatment presented here is on a theoretical level, although the problem is motivated by a practical scheme for message routing in store-and-forward packet switched computer networks. Theorems are presented which offer readily testable sufficient conditions on the topology of a network which guarantee the existence of at least one set of paths under consideration. The proof of this theorem provides a generalized systematic method for constructing these paths if the sufficient conditions are met.