Layered communication protocols frequently implement a FIFO message facility On top of an unreliable non-FIFO service such as that provided by a packet-switching network. This paper investigates the possibility of implementing a reliable message layer on top of an underlying layer that can lose packets and deliver them out of order, with the additional restriction that the implementation uses only a fixed finite number of different packets. A new formalism is presented to specify communication layers and their properties, the notion of their implementation by I/O automata, and the properties of such implementations. An I/O automaton that implements a reliable layer over an unreliable layer is presented. In this implementation, the number of packets needed to deliver each succeeding message increases permanently as additional packetloss and reordering faults occur. A proof is given that no protocol can avoid such performance degradation.

We survey results from distributed computing that show tasks to be impossible, either outright or within given resource bounds, in various models. The parameters of the models considered include synchrony, fault-tolerance, different communication media, and randomization. The resource bounds refer to time, space and message complexity. These results are useful in understanding the inherent difficulty of individual problems and in studying the power of different models of distributed computing.

End-to-end communication is the problem of sending a sequence of messages from a sender to a receiver when the network through which they communicate is unreliable. The model considered is an asynchronous network in which intermediate nodes are assumed to have no memory. Dynamic link failures are allowed: links can lose messages, but cannot reorder or duplicate them.

The end-to-end communication problem is a protocol design problem, for sending a packet from a speci ed source-node s to a speci ed target-node t, through an unreliable asynchronous communication network G. The protocol must insure reception and termination. In this paper, we measure the complexity of the protocol in term of header size, i.e., the quantity of information that must be attached to the packets to insure their delivery. We show that headers of (log log ) bits are required in every network, where denotes the tree-width of the network.

End-to-end communication is the problem of sending a sequence of messages from one processor, the sender, to another processor, the receiver, through an unreliable communication network. This paper surveys positive and negative results concerning deterministic protocols for end-to-end communication in asynchronous networks and presents open questions. 1 Introduction Sending information from one designated processor to another designated processor is a fundamental activity in network based systems. However, networks, especially large networks, are subject to faults, for example, message loss, link failures, and processor crashes. Because of their complexity, communication networks are usually built in a sequence of layers [25]. At each layer (above the physical layer), there is a protocol that uses the services provided by the layer below to provide better or more complicated services to the layer above. This may introduce other sorts of faults, for example, message duplication or reor...

A semantic framework for analyzing safe composition of distributed programs is presented. Its applicability is illustrated by a study of program composition when communication is reliable but not necessarily FIFO. In this model, special care must be taken to ensure that messages do not accidentally overtake one another in the composed program. We show that barriers do not exist in this model. Indeed, no program that sends or receives messages can automatically be composed with arbitrary programs without jeopardizing their intended behavior. Safety of composition becomes context-sensitive and new tools are needed for ensuring it. A notion of sealing is defined, where if a program P is immediately followed by a program Q that seals P then P will be communication-closed—it will execute as if it runs in isolation. The investigation of sealing in this model reveals a novel connection between Lamport causality and safe composition. A characterization of sealable programs is given, as well as efficient algorithms for testing if Q seals P and for constructing a seal for a significant class of programs. It is shown that every sealable program that is open to interference on O(n 2) channels can be sealed using O(n) messages. 1.

The fundamental question considered in this paper is when program Q, if executed immediately after program P, is guaranteed not to interfere with P and be safe from interference by P. If a message sent by one of these programs is received by the other, it may affect and modify the other’s execution. The notion of communication-closed layers (CCLs) introduced by Elrad and Francez in 1982 is a useful tool for studying such interference. CCLs have been considered mainly in the context of reliable FIFO channels (without duplication), where one can design program layers that do not interfere with any other layer. When channels are less than perfect such programs are no longer feasible. The absence of interference between layers becomes context-dependent. In this paper we study the impact of message duplication and loss on the safety of layer composition. Using a communication phase operator, the fits after relation among programs is defined. If program Q fits after P then P and Q will not interfere with each other in executions of P ∗ Q (P immediately followed by Q). For programs P and Q in a natural class of programs we outline efficient algorithms for the following: (1) deciding whether Q fits after P; (2) deciding whether Q seals P, meaning that Q fits after P and no following program can communicate with P; and (3) constructing a separator S that both fits after P and satisfies that Q fits after P ∗ S. 1

The end-to-end communication problem is a protocol design problem, for sending a packet from a speci ed source-node s to a speci ed target-node t, through an unreliable asynchronous memoryless communication network. The protocol must insure reception and termination. In this paper, we measure the complexity of the protocol in term of header size, i.e., the quantity of information that must be attached to the packets to insure their delivery. We show that headers of 14 log ) bits are required in every network, where denotes the tree-width of the network. In planar networks, ks, ) bits are required.

The end-to-end communication problem is a protocol design problem, for sending a (sequence of) packet(s) from a specified source-node s to a specified target-node t, through an unreliable asynchronous communication network G. The protocol must insure reception and termination. In this paper