Self-stabilization of dynamic systems assuming only read/write atomicity

Summary Three self-stabilizing protocols for distributed systems in the shared memory model are presented. The first protocol is a mutual-exclusion prootocol for tree structured systems. The second protocol is a spanning tree protocol for systems with any connected communication graph. The thrid protocol is obtianed by use offair protoco combination, a simple technique which enables the combination of two self-stabilizing dynamic protocols. The result protocol is a self-stabilizing, mutualexclusion protocol for dynamic systems with a general (connected) communication graph. The presented protocols improve upon previous protocols in two ways: First, it is assumed that the only atomic operations are either read or write to the shared memory. Second, our protocols work for any connected network and even for dynamic network, in which the topology of the network may change during the excution. Shlomi Dolev received his B.Sc. in Civil Engineering and B.A. in Computer Science in 1984 and 1985, and his M.Sc. and Ph.D. in computer Sciene in 1989 and 1992 from the Technion Israel Institute of Technology. He is currently a post-dotoral fellow in the Department of Computer Science at Texas A & M Univeristy. His current research interests include the theoretical aspects of distributed computing and communcation networks. Amos Israeli received his B.Sc. in Mathematics and Physics from Hebrew University in 1976, and his M.Sc. and D.Sc. in Computer Science from the Weizmann Institute in 1980 and the Technion in 1985, respectively. Currently he is a sensior lecturer at the Electrical Engineering Department at the Technion. Prior tot his he was a postdoctoral fellow at the Aiken Computation Laboratory at harvard. His research interests are in Parellel and Distributed Computing and in Robotics. In particular he has worked on the design and analysis of Wait-Free and Self-Stabilizing distributed protocols. Shlomo Moran received his B.Sc. and D.Sc. degrees in matheamtics from Technion, Israel Institute of Technology, Haifa, in 1975 and 1979, respectively. From 1979 to 1981 he was assistant professors and a visiting research specialist at the University of Minnesota, Minneapolis. From 1981 to 1985 he was a senior lecturer at the Department of Computer Science. Technion, and from 1985 to 1986 he visted at IBM Thoas J. Watson Research Center, Yorktown Heights. From 1986 to 1993 he was an associated professor at the Department of Computer Science, Technin. in 1992–3 he visited at AT & T Bell Labs at Murray Hill and at Centrum voor Wiskunde en Informatica, Amsterdam. From 1993 he is a full professor at the Department of Computer Science, Technion. His researchinterests include distributed algorithm, computational complexity, combinatorics and grapth theory.Part of this research was supported in part by Technion V.P.R. Funds — Wellner Research Fund, and by the Foundation for Research in Electronics, Computers and Communictions, administrated by the Israel Academy of Sciences and Humanities.     

Efficient emulation of single-hop radio network with collision detection on multi-hop radio network with no collision detection

Summary This paper presents an efficient randomized emulation ofsingle-hop radio networkwith collision detection onmulti-hop radio networkwithout collision detection. Each step of the single-hop network is emulated by rounds of the multi-hop network and succeeds with probability 1–. (n is the number of processors,D the diameter and the maximum degree). It is shown how to emulate any polynomial algorithm such that the probability of failure remains . A consequence of the emulation is an efficient randomized algorithm for choosing a leader in a multi-hop network. Reuven Bar-Yehuda was born in Iran, on July 17th 1951. Received B.A., M.Sc., and D.Sc. in Computer Science from the Technion — Israel Institute of Technology, Haifa, Israel, in 1978, 1980, and 1983, respectively. He is currently a Senior Lecturer of Computer Science at the Technion. From 1984 to 1986, he was a visiting assistant professor in the Computer Science Dept. at the Duke Univesity His research interests include computational geometry, VLSI, graph algorithms and distributed algorithms. Oded Goldreich was born in Tel-Aviv, Israel, on February 4th 1957. Received B.A., M.Sc., and D.Sc. in Computer Science from the Technion — Israel Institute of Technology, Haifa, Israel, in 1980, 1982, and 1983, respectively. He is currently an Associate Professor of Computer Science at the Technion. From 1983 to 1986, he was a postdoctoral fellow at MIT's Laboratory for Computer Science. His research interests include cryptography and related areas, relation between randomness and algorithms, and distributed computation. Alon Itai was born in Scotland, on December 12th 1946. Received B.Sc. in Mathematics from the Hebrew University in Jerusalem in 1969. M.Sc., and Ph.D. in Computer Science from the Weizmann Institute of Science, Rehovot, Israel in 1971 and 1976. He is currently an Associate Professor of Computer Science at the Technion. His research interests include randomized and distributed algorithms, computational learning theory and performance evaluation.The second author was partially supported by grant No. 86-00301 from the United States—Israel Bi-national Science Foundation BSF), Jerusalem, Israel.     

Counting networks with arbitrary fan-out

Summary It is shown that an acyclic smoothing network (and hence counting network) with fan-outn cannot be constructed from balancers of fan-outb ₁,...,b _k , if there exists a prime factorp ofn, such thatp does not divideb _i , for alli, 1ik. This holds regardless of the depth, fan-in or size of the network, as long as they are finite. On the positive side, a simple construction ofcyclic counting networks with fan-outn, for arbitraryn, is presented. An acyclic counting network with fan-in and fan-outp2 ^k , for any integerk0, is constructed out of 2-balancers andp-balancers. Eran Aharonson received the B.A. and M.Sc. degrees in Computer Science from the Technion, Israel Institute of Technology (Haifa, Israel) in 1989 and 1992, respectively. He is currently vice president for research and development at ART-Advanced Recognition Technolgies Ltd., a company dedicated to handwriting and voice recognition. His general research interests are distributed computation, theoretical computer science and pattern recognition. Hagit Attiya received the B.Sc. degree in Mathematics and Computer Science from the Hebrew University of Jerusalem, in 1981, the M.Sc. and Ph.D. degrees in Computer Science from the Hebrew University of Jerusalem, in 1983 and 1987, respectively. She is presently a senior lecturer at the department of Computer Science at the Technion, Israel Institute of Technology. Prior to this, she has been a post-doctoral research associate at the Laboratory for Computer Science at M.I.T. Her general research interests are distributed computation and theoretical computer science. More specific interests include fault-tolerance, timing-based and asynchronous algorithms.A preliminary version of this paper appears in proceedings of the3rd Annual ACM-SIAM Symposium on Discrete Algorithms, January 1992, pp. 104–113. This research was supported by Technion V.P.R.-B. and G. Greenberg Research Fund (Ottawa)Supported by Rashi Enterprise graduate fellowship     

Atomicm-register operations

Summary We investigate systems where it is possible to access several shared registers in one atomic step. We characterize those systems in which the consensus problem can be solved in the presence of faults and give bounds on the space required. We also describe a fast solution to the mutual exclusion problem using atomicm-register operations. Michael Merritt received a B.S. degree in Philosophy and in Computer Science from Yale College in 1978, the M.S. and Ph. D. degrees in Information and Computer Science in 1980 and 1983, respectively, from the Georgia Institute of Technology. Since 1983 he has been a member of technical staff at AT&T Bell Laboratories, and has taught as an adjunct or visiting lecturer at Stevens Institute of Technology and Columbia University. In 1989 he was program chair for the ACM Symposium on Principles of Distributed Computing. His research interests include distributed and concurrent computation, both algorithms and formal methods for verifying their correctness, cryptography, and security. He is an editor for Distributed Computing and for Information and Computation, recently coauthored a book on database concurrency control algorithms, and is a member of the ACM and of Computer Professionals for Social Responsibility. Gadi Taubenfeld received the B.A., M.Sc. and Ph.D. degrees in Computer Science from the Technion (Israel Institute of Technology), in 1982, 1984 and 1988, respectively. From 1988 to 1990 he was a research scientist at Yale University. Since 1991 he has been a member of technical staff at AT&T Bell Laboratories. His primary research interests are in concurrent and distributed computing.A preliminary version of this workappeared in theProceedings of the Fifth International Workshop on Distributed Algorithms, Delphi, Greece, October 1991, pp 289–294     

Composite registers

Summary We introduce a shared data object, called acomposite register, that generalizes the notion of an atomic register. A composite register is an array-like shared data object that is partitioned into a number of components. An operation of a composite register either writes a value to a single component or reads the values of all components. A composite register reduces to an ordinary atomic register when there is only one component. In this paper, we show that multi-reader, singlewriter atomic registers can be used to implement a composite register in which there is only one writer per component. In a related paper, we show how to use the composite register construction of this paper to implement a composite register with multiple writers per component. These two constructions show that it is possible to implement a shared memory that can be read in its entirety in a single snapshot operation, without using mutual exclusion. James H. Anderson received the B.S. degree in Computer Science from Michigan State University in 1982, the M.S. degree in Computer Science from Purdue University in 1983, and the Ph.D. degree in Computer Sciences from the University of Texas at Austin in 1990. Since August 1990, he has been with the University of Maryland at College Park, where he is now an Assistant Professor of Computer Science. Since January 1992, he has been a staff scientist at NASA's Center of Excellence in Space Data and Information Sciences, located at the Goddard Space Flight Center in Greenbelt, Maryland. Professor Anderson's primary research interests are within the area of concurrent and distributed computing.Preliminary version presented at theNinth Annual ACM Symposium on Principles of Distributed Computing [2]Work supported, in part, at the University of Texas at Austin by Office of Naval Research Contract N00014-89-J-1913, and at the University of Maryland by an award from the University of Maryland General Research Board     

Bounded time-stamps

Summary Time-stamps are labels which a system adds to its data items. These labels enable the system to keep track of the temporal precedence relations among its data elements. Many distributed protocols and some applications use the natural numbers as time-stamps. The natural numbers however are not useful for bounded protocols. In this paper we develop a theory ofbounded time-stamps. Time-stamp schemes are defined and the complexity of their implementation is analyzed. This indicates a direction for developing a general tool for converting time-stamp based protocols to bounded protocols. Amos Israeli received his B.Sc. in Mathematics and Physics from Hebrew University in 1976, and his M.Sc. and D.Sc. in Computer Science from the Weizmann Institute in 1980 and the Technion in 1985, respectively. Currently he is a senior lecturer at the Tlectrical Engineering Department at the Technion. Prior to this he was a postdoctoral fellow at the Aiken Computation Laboratory at Harvard. His research interests are in Parallel and Distributed Computing and in Robotics. In particular he has worked on the design and analysis of Wait-Free and Self-Stabilizing distributed protocols. Ming Li received his M.S. and Ph.D. in Computer Science from Wayne State University in 1980 and Cornell University 1985, respectively. Currently he is an associate professor at the Computer Science Department at the University of Waterloo. His research interests are in Theory of Computing, Kolmogorov Complexity, and Machine Learning.Supported in part by the Weizmann fellowship and NSF Grant DCR-86-00379Supported in part by ONR Grant N00014-85-k-0445 and Army Research Office Grant DAAL03-86-K-0171 at Harvard University, by NSF Grant kDCR-86-06366 at Ohio State University, and by NSERC Operating Grant OGP0036747. Most of this work was done when the authors were at Aiken Computation Laboratory at Harvard University. The authors also acknowledge the hospitality of the computer science department at York University, Canada     

One-bit algorithms

Many algorithms in distributed systems assume that the size of a single message depends on the number of processors. In this paper, we assume in contrast that messages consist of a single bit. Our main goal is to explore how the one-bit translation of unbounded message algorithms can be sped up by pipelining. We consider two problems. The first is routing between two processors in an arbitrary network and in some special networks (ring, grid, hypercube). The second problem is coloring a synchronous ring with three colors. The routing problem is a very basic subroutine in many distributed algorithms; the three coloring problem demonstrates that pipelining is not always useful. Amotz Bar-Noy received his B.Sc. degree in Mathematics and Computer Science in 1981, and his Ph.D. degree in Computer Science in 1987, both from the Hebrew University of Jerusalem, Israel. Between 1987 and 1989 he was a post-doctoral fellow in the Department of Computer Science at Stanford University. He is currently a visiting scientist at the IBM Thomas J. Watson Research Center. His current research interests include the theoretical aspects of distributed and parallel computing, computational complexity and combinatorial optimization. Joseph (Seffi) Naor received his B.A. degree in Computer Science in 1981 from the Technion, Israel Institute of Technology. He received his M.Sc. in 1983 and Ph.D. in 1987 in Computer Science, both from the Hebrew University of Jerusalem, Israel. Between 1987 and 1988 he was a post-doctoral fellow at the University of Southern California, Los Angeles, CA. Since 1988 he has been a post-doctoral fellow in the Department of Computer Science at Stanford University. His research interests include combinatorial optimization, randomized algorithms, computational complexity and the theoretical aspects of parallel and distributed computing. Moni Naor received his B.A. in Computer Science from the Technion, Israel Institute of Technology, in 1985, and his Ph.D. in Computer Science from the University of California at Berkeley in 1989. He is currently a visiting scientist at the IBM Almaden Research Center. His research interests include computational complexity, data structures, cryptography, and parallel and distributed computation.Supported in part by a Weizmann fellowship and by contract ONR N00014-85-C-0731Supported by contract ONR N00014-88-K-0166 and by a grant from Stanford's Center for Integrated Systems. This work was done while the author was a post-doctoral fellow at the University of Southern California, Los Angeles, CAThis work was done while the author was with the Computer Science Division, University of California at Berkeley, and Supported by NSF grant DCR 85-13926     

Verification of distributed programs using representative interleaving sequences

Summary We present a formal proof method for distributed programs. The semantics used to justify the proof method explicitly identifies equivalence classes of execution sequences which are equivalent up to permuting commutative operations. Each equivalence class is called an interleaving set or a run. The proof rules allow concluding the correctness of certain classes of properties for all execution sequences, even though such properties are demonstrated directly only for a subset of the sequences. The subset used must include a representative sequence from each interleaving set, and the proof rules, when applicable, guarantee that this is the case. By choosing a subset with appropriate sequences, simpler intermediate assertions can be used than in previous formal approaches. The method employs proof lattices, and is expressed using the temporal logic ISTL. Shmuel Katz received his B.A. in Mathematics and English Literature from U.C.L.A., and his M.Sc. and Ph.D. in Computer Science (1976) from the Weizmann Institute in Rechovot, Israel. From 1976 to 1981 he was at the IBM Israel Scientific Center. Presently, he is on the faculty of the Computer Science Department at the Technion in Haifa, Israel. In 1977–1978 he visited for a year at the University of California, Berkeley, and in 1984–1985 was at the University of Texas at Austin. He has been a consultant and visitor at the MCC Software Technology Program, and in 1988–1989 was a visiting scientist at the I.B.M. Watson Research Center. His research interests include the methodology of programming, specification methods, program verification and semantics, distributed programming, data structures, and programming languages. Doron Peled was born in 1962 in Haifa. He received his B.Sc. and M.Sc. in Computer Science from the Technion, Israel in 1984 and 1987, respectively. Between 1987 and 1991 he did his military service. He also completed his D.Sc. degree in the Technion during these years. Dr. Peled was with the Computer Science department at Warwick University in 1991–1992. He is currently a member of the technical staff with AT & T Bell Laboratories. His main research interests are specification and verification of programs, especially as related to partial order models, fault-tolerance and real-time. He is also interested in semantics and topology.This research was carried out while the second author was at the Department of Computer Science, The Technion, Haifa 32000, Israel     

TEG—a hybrid approach to information extraction

This paper describes a hybrid statistical and knowledge-based information extraction model, able to extract entities and relations at the sentence level. The model attempts to retain and improve the high accuracy levels of knowledge-based systems while drastically reducing the amount of manual labour by relying on statistics drawn from a training corpus. The implementation of the model, called TEG (trainable extraction grammar), can be adapted to any IE domain by writing a suitable set of rules in a SCFG (stochastic context-free grammar)-based extraction language and training them using an annotated corpus. The system does not contain any purely linguistic components, such as PoS tagger or shallow parser, but allows to using external linguistic components if necessary. We demonstrate the performance of the system on several named entity extraction and relation extraction tasks. The experiments show that our hybrid approach outperforms both purely statistical and purely knowledge-based systems, while requiring orders of magnitude less manual rule writing and smaller amounts of training data. We also demonstrate the robustness of our system under conditions of poor training-data quality. Ronen Feldman is a senior lecturer at the Mathematics and Computer Science Department of Bar-Ilan University in Israel, and the Director of the Data Mining Laboratory. He received his B.Sc. in Math, Physics and Computer Science from the Hebrew University, M.Sc. in Computer Science from Bar-Ilan University, and his Ph.D. in Computer Science from Cornell University in NY. He was an Adjunct Professor at NYU Stern Business School. He is the founder of ClearForest Corporation, a Boston based company specializing in development of text mining tools and applications. He has given more than 30 tutorials on next mining and information extraction and authored numerous papers on these topics. He is currently finishing his book “The Text Mining Handbook” to the published by Cambridge University Press. Benjamin Rosenfeld is a research scientist at ClearForest Corporation. He received his B.Sc. in Mathematics and Computer Science from Bar-Ilan University. He is the co-inventor of the DIAL information extraction language. Moshe Fresko is finalizing his Ph.D. in Computer Science Department at Bar-Ilan University in Israel. He received his B.Sc. in Computer Engineering from Bogazici University, Istanbul/Turkey on 1991, and M.Sc. on 1994. He is also an adjunct lecturer at the Computer Science Department of Bar-Ilan University and functions as the Information-Extraction Group Leader in the Data Mining Laboratory.     

Multi-writer composite registers

Summary Acomposite register is an array-like shared data object that is partitioned into a number of components. An operation of such a register either writes a value to a single component, or reads the values of all components. A composite register reduces to an ordinary atomic register when there is only one component. In this paper, we show that a composite register with multiple writers per component can be implemented in a wait-free manner from a composite register with a single writer per component. It has been previously shown that registers of the latter kind can be implemented from atomic registers without waiting. Thus, our results establish that any composite register can be implemented in a wait-free manner from atomic registers. We show that our construction has comparable space compexity and better time complexity than other constructions that have been presented in the literature. James H. Anderson received the B.S. degree in Computer Science from Michigan State University in 1982, the M.S. degree in Computer Science from Purdue University in 1983, and the Ph.D. degree in Computer Sciences from the University of Texas at Austin in 1990. He recently joined the Computer Science Department at the University of North Carolina at Chapel Hill, where he is now an Assistant Professor. Prior to joining the University of North Carolina, he was an Assistant Professor of Computer Science for three years at the University of Maryland at College Park. Professor Anderson's main research interests are within the area of concurrent and distributed computing. His current interests primarily involve the implementation of resilient and scalable synchronization mechanisms.Preliminary version was presented at the Ninth Annual ACM Symposium on Principles of Distributed Computing [2]Much of the work described herein was completed while the author was with the University of Texas at Austin and the University of Maryland at College Park. This work was supported at the University of Texas by ONR Contract N00014-89-J-1913, and at the University of Maryland by NSF Contract CCR 9109497 and by an award from the University of Maryland General Research Board     

Greedy hot-potato routing on the two-dimensional mesh

Summary We propose hot-potato (or, deflection) packet routing algorithms on the two-dimensional mesh. The algorithms are strongly greedy in the sense that they attempt to send packets in good directions whenever possible. Furthermore, the routing operations are simple and independent of the time that has elapsed. The first algorithm gives the best evacuation time known for delivering all the packets to their destinations. A batch ofk packets with maximal source-to-destination distanced _max is delivered in 2(k-1)+d _max. The second algorithm improves this bound tok+d _max when all packets are destined to the same node. This also implies a new bound for the multitarget case, which is the first to take into account the number of in-edges of a node. The third algorithm is designed for routing permutations with source-to-destination distance at most three, in which case the algorithm terminates in at most seven steps. We also show a lower bound of five steps for this problem. Ishai Ben-Aroya received the B.A. and M.Sc. in computer science from the Technion (Israel Institute of Technology). He is currently working with Microsoft Israel R&D group. His main interests include Routing Algorithms, Cryptography and Computer Security. Tamar Eilam received the B.A. degree in Computer Science from the Technion IIL in 1995, and is currently studying towards her M.A. degree. Assaf Schuster received his B.A., M.A. and Ph.D. degrees in Computer Science from the Hebrew University of Jerusalem (the last one in 1991). He is currently a lecturer at the Technion IIL. His main interests include Networks and Routing Algorithms, Parallel and Distributed Computation, Optical Computation and Communication, Dynamically Reconfiguring Networks, and Greedy Hot Potato Routing.This work was supported in part by the French-Israeli grant for cooperation in Computer Science, and by a grant from the Israeli Ministry of Science. An extended abstract appeared in proc. 2nd European Symposium on Algorithms, September 1994     

Using mappings to prove timing properties

Summary A new technique for proving timing properties for timing-based algorithms is described; it is an extension of the mapping techniques previously used in proofs of safety properties for asynchronous concurrent systems. The key to the method is a way of representing a system with timing constraints as an automaton whose state includes predictive timing information. Timing assumptions and timing requirements for the system are both represented in this way. A multi-valued mapping from the assumptions automaton to the requirements automaton is then used to show that the given system satisfies the requirements. One type of mapping is based on a collection of progress functions providing measures of progress toward timing goals. The technique is illustrated with two examples, a simple resource manager and a two-process race system. Nancy A. Lynch received the B.S. degree in mathematics from Brooklyn College, Brooklyn, NY, in 1968, and the Ph.D. degree in mathematics from the Massachusetts Institute of Technology, Cambridge, MA, in 1972. She is presently a professor of computer science and electrical engineering at Massachusetts Institute of Technology. She has also been on the computer science faculty at Georgia Institute of Technology and on the mathematics faculty at Tufts University and the University of Southern California. Her research interests are in distributed and real-time computing and theoretical computer science. In particular, she has worked on formal models and verification methods, on algorithm design and analysis, and on impossibility results. She also likes to hike and ski. Hagit Attiya received the B.Sc. degree in Mathematics and Computer Science from the Hebrew University of Jerusalem, in 1981, the M.Sc. and Ph.D. degrees in Computer Science from the Hebrew University of Jerusalem, in 1983 and 1987, respectively. She is presently a senior lecturer at the department of Computer Science at the Technion, Israel Institute of Technology. Prior to this, she has been a post-doctoral research associate at the Laboratory for Computer Science at M.I.T. Her general research interests are distributed computation and theoretical computer science. More specific interests include fault-tolerance, timing-based and asynchronous algorithms.This work was supported by ONR contracts N00014-85-K-0168 and N00014-91-J-1046, by NSF grants CCR-8611442 and CCR-8915206, and by DARPA contracts N00014-87-K-0825 and N00014-89-J-1988     

On the complexity of computation in the presence of link failures: the case of a ring

Summary We investigate the message complexity of distributed computations on rings of asynchronous processors. In such computations, each processor has an initial local value and the task is to compute some predetermined function of all local values. Our work deviates from previous works concerning the complexity of ring computations in that we consider the effect oflink failures. A link is said to fail if some message sent through it never reaches its destination. We show that the message complexity of any function, which is sensitive to all its inputs, is (n logn) whenn, the number of processors, is a-priori known; and is (n ² ) whenn is not known. Interestingly, these tight bounds do not depend on whether the identity of a leader is a-priori known before the computation starts. These results stand in sharp contrast to the situation in asynchronous rings with no link failures, where the message complexity is affected by the a-priori knowledge of a leader but is not affected by the knowledge ofn. Oded Goldreich was born in Tel-Aviv, Israel, on February 4th 1957. Received B.A., M.Sc., and D.Sc. in Computer Science from the Technion — Israel Institute of Technology, Haifa, Israel, in 1980, 1982, and 1983, respectively. He is currently an Associate Professor of Computer Science in the Technion. From 1983 to 1986, he was a postdoctoral fellow in MIT's Laboratory for Computer Science. His research interests include cryptography and related areas, relation between randomness and algorithms, and distributed computation. Luiba Shrira was born in Vilnius, Lithuania. Received B.A., M.Sc., and D.Sc. in Computer Science from the Technion—Israel Institute of Technology, Haifa, Israel in 1977, 1980, and 1985, respectively. from 1986 to 1989 she was a postdoctoral fellow at Laboratory for Computer science at MIT, where she is currently a Research Associate. Her research interests include highly-available and reliable distributed algorithms and systems, persistent object systems, and programming methodology.Part of the work has been done while the first author was in the Laboratory for Computer Science of MIT and the second author was in the Computer Science Department of the Technion. First author was partially supported by a Weizmann Postdoctoral Fellowship, an IBM Postdoctoral Fellowship, and Albert Einstein Research Fund (through Technion's V.P.R. Fund)     

Self-stabilization over unreliable communication media

Summary A self-stabilizing system has the property that it will converge to a desirable state when started from any state. Most previous researchers assumed that processes in self-stabilizing systems may communicate through shared variables while those that studied meassage passing systems allowed messages with unbounded size. This paper discusses the development of self-stabilizing systems which communicate through message passing, and in which messages may be lost in transit. The systems presented all use fixed size message headers. First, a selfstabilizing version of theAlternating Bit Protocol, a fundamental communication protocol for transmitting data across an unreliable communication medium, is presented. Secondly, the alternating-bit protocol is used to construct a self-stabilizing token ring. Yehuda Afek received a B.Sc. in Electrical Engineering from the Technion and an M.S. and Ph.D. in Computer Science from the University of California, Los Angeles. In 1985 he joined the Distributed Systems research Department in AT&T Bell Laboratories and in 1988 he joined the Department of Computer Science in Tel-Aviv University. His interests include communication protocols, distributed systems, and asynchronous shared memories. Geoffrey M. Brown received the BS degree in Engineering from Swarthmore College in 1982, the MS degree in Electrical Engineering from Stanford University in 1983, and the Ph.D. degree in Electrical Engineering from the University of Texas at Austin in 1987. From 1983 to 1984 he worked for Motorola in Austin, TX. Currently he is an Assistant Professor in the School of Electrical Engineering at Cornell University. In 1990, Brown was named a Presidential Young Investigator by the National Science Foundation.This work supported in part by NSF grant CCR-9058180     

Adaptive solutions to the mutual exclusion problem

Summary Algorithms for mutual exclusion that adapt to the current degree of contention are developed. Afilter and a leader election algorithm form the basic building blocks. The algorithms achieve system response times that are independent of the total number of processes and governed instead by the current degree of contention. The final algorithm achieves a constant amortized system response time. Manhoi Choy was born in 1967 in Hong Kong. He received his B.Sc. in Electrical and Electronic Engineerings from the University of Hong Kong in 1989, and his M.Sc. in Computer Science from the University of California at Santa Barbara in 1991. Currently, he is working on his Ph.D. in Computer Science at the University of California at Santa Barbara. His research interests are in the areas of parallel and distributed systems, and distributed algorithms. Ambuj K. Singh is an Assistant Professor in the Department of Computer Science at the University of California, Santa Barbara. He received a Ph.D. in Computer Science from the University of Texas at Austin in 1989, an M.S. in Computer Science from Iowa State University in 1984, and a B.Tech. from the Indian Institute of Technology at Kharagpur in 1982. His research interests are in the areas of adaptive resource allocation, concurrent program development, and distributed shared memory.A preliminary version of the paper appeared in the 12th Annual ACM Symposium on Principles of Distributed ComputingWork supported in part by NSF grants CCR-9008628 and CCR-9223094     

Distributed algorithms for constructing a minimum-weight spanning tree in a broadcast network

Two distributed algorithms are presented for a network using a common communication channel (e.g. radio) in which all nodes are within signal range and in line of sight of each other: (a) an algorithm to compute all internode distances (in terms of propagation delays) in the network. the algorithm requires only 2 messages per node, and provides each node with the distances to all other nodes. (b) An algorithm for constructing a minimum-weight spanning tree (MST) in such a network. This algorithm starts out with the information provided by (a) and ends with each node possessing the explicit knowledge of the full MST. The algorithm requires at most log₂ N messages per node. The internal processing in each node needsO(N logN) time andO(N) space. All messages required by (a) and (b) contain at most one edge weight plus 2 log₂ N bits. Some possible applications of the algorithms are: position-location, tuning acknowledgement time-out mechanisms, tuning the scheduling functions of access protocols that are sensitive to individual internode propagation delays, and selecting performance effective transmission sequences for round robin access protocols.Yaron I. Gold received the B.Sc. (Cum Laude, 1970) and M.Sc. (1975), both in Electrical Engineering, from the Technion, Israel institute of Technology, and the Ph.D. (1981) in Computer Science, from the University of Minnesota, Minneapolis.From 1970 to 1975 Yaron Gold served as Research and Development Officer in the Israeli Defense Forces, leading a group of several scientists, engineers and technicians. From 1982 to 1984 he was on the faculty of the Department of Electrical Engineering the Department of Electrical Engineering and Computer Science at the University of Connecticut. During that period he also served as consultant for United Technologies Corporation and for Battelle Laboratories. Presently, Dr. Gold is on the faculty of the Computer Science Department at the Technion.His research interests include Computer Networks and Communications, Simulation and Intelligent Systems.Shlomo Moran received the B.Sc. and D.Sc. degrees in mathematics from Technion, Israel Institute of Technology, Haifa, in 1975 and 1979, respectively.From 1979 to 1981 he was assistant professor and a visiting research specialist at the University of Minnesota, Minneapolis. From 1981 to 1985 he was a senior lecturer at the Department of Computer Science, Technion, and from 1985 to 1986 he was a World Trade visitor at IBM Thomas J. Watson Research Center, Yorktown Heights. Currently he is associate professor at the Department of Computer Science, Technion.His research interests include distributed algorithms, computational complexity, combinatorics, graph theory and geometric aspects of communication networks.This work was supported in part by NSF grant ECS-8307186Part of this research was done while this author was with the University of ConnecticutPart of this research was done while this author was with IBM, Thomas J. Watson Research Center     

Fairness and hyperfairness in multi-party interactions

Summary In this paper, a new fairness notion is proposed for languages withmulti-party interactions as the sole interprocess synchronization and communication primitive. The main advantage of this fairness notion is the elimination of starvation occurring solely due to race conditions (i.e., ordering of independent actions). Also, this is the first fairness notion for such languages which is fully adequate with respect to the criteria presented in [2]. The paper defines the notion, proves its properties, and presents examples of its usefulness. Orna Grumberg received her B.Sc. degree, M.Sc. and Ph.D. in the Computer Science Department at the Technion—Israel Institute of Technology. Since 1984 she is a faculty member in the Computer Science Department at the Technion. Her research interests include verification of distributed systems, computer-aided verification, model checking, temporal logics and automata. Paul Attie received a B.A. degree in engineering science from the University of Oxford, and an M.Sc. degree in computer science from the University of London. Since 1986, Paul has been with the Microelectronics and Computer Technology Corporation, where he is currently a member of technical staff. He is also a candidate for the Ph.D. in computer science degree at the University of Texas at Austin. His research interests include temporal logic, fairness, algebraic process theory, formal semantics, and concurrent program verification.The photograph and autobiography of Dr. Nissim Francez were published in Volume 2, Issue No. 4, 1988 on page 226     

Closed schedulers: a novel technique for analyzing asynchronous protocols

Summary Analyzing distributed protocols in various models often involves a careful analysis of the set ofadmissible runs, for which the protocols should behave correctly. In particular, the admissible runs assumed by at-resilient protocol are runs which are fair for all but at mostt processors. In this paper we defineclosed sets of runs, and suggest a technique to prove impossibility results fort-resilient protocols, by restricting the corresponding sets of admissible runs to smaller sets, which are closed, as follows: For each protocolPR and for each initial configurationc, the set of admissible runs ofPR which start fromc defines a tree in a natural way: the root of the tree is the empty run, and each vertex in it denotes a finite prefix of an admissible run; a vertexu in the tree has a sonv iffv is also a prefix of an admissible run, which extendsu by one atomic step.The tree of admissible runs described above may contain infinite paths which are not admissible runs. A set of admissible runs isclosed if for every possible initial configurationc, each path in the tree of admissible runs starting fromc is also an admissible run. Closed sets of runs have the simple combinatorial structure of the set of paths of an infinite tree, which makes them easier to analyze. We introduce a unified method for constructing closed sets of admissible runs by using a model-independent construction of closedschedulers, and then mapping these schedulers to closed sets of runs. We use this construction to provide a unified proof of impossibility of consensus protocols. Ronit Lubitch received her B.Sc. degree in Mathematics and Computer Science from Tel Aviv University in 1989, and her Master degree in Computer Science from the Technion, in 1993. From 1992 she is working in Graffiti Software Industries, which expertise in the design and development of advanced photo realistic rendering, and animation software systems. Shlomo Moran received his B.Sc. and Ph.D. degrees in Mathematics from the Technion in 1975 and 1979 resp. In 1979–1981 he was at the University of Minnesota as a visiting research specialist. In 1981 he joined the Computer Science Department at the Technion, where he is now a full professor. In 1985–1986 he visited at IBM T.J. Watson Research Center. In 1992–1993 he visited at AT&T Bell Laboratories and in Centrum voor Wiskunde en Informatica, Amsterdam. His research interests include distributed computing, Combinatorics and Graph Theory, and Complexity Theory.A preliminary extended version of this paper appeared in the Proceedings of 6-th International Workshop on Distributed Algorithms, Haifa, November 1992This work was supported in part by the Technion V.P.R. fund. Part of this research was conducted while this author was visiting at AT&T Bell Labs at Murray Hill and at CWI, Amsterdam     

Self-stabilizing extensions for meassage-passing systems

Summary A self-stabilizing program eventually resumes normal behavior even if excution begins in, an abnormal initial state. In this paper, we explore the possibility of extending an arbitrary program into a self-stabilizing one. Our contributions are: (1) a formal definition of the concept of one program being aself-stabilizing extension of another; (2) a characterization of what properties may hold in such extensions; (3) a demonstration of the possibility of mechanically creating such extensions. The computtional model used is that of an asynchronous distributed message-passing system whose communication topology is an arbitrary graph. We contrast the difficulties of self-stabilization in thismodel with those of themore common shared-memory models. Shmuel Katz received his B.A. in Mathematics and Englisch Literature from U.C.L.A., and his M.Sc. and Ph.D. in Computer Science (1976) from the Weizmann Institute in Rechovot, Israel. From 1976 to 1981 he was a research at the IBM Israel Scientific Center. Presently, he is an Associate Professor in the Computer Science Department at the Technion in Haifa, Israel. In 1977–78 he visited for a year at the University of California, Berkeley, and in 1984–85 was at the University of Texas at Austin. He has been a consultant and vistor at the MCC Software Technology Program, and in 1988–89 was a visiting scientist at the IBM Watson Research Center. His research interests include the methodology of programming, specification methods, program verification and semantics, distributed programming, data structure, and programming languages. Kenneth J. Pery has performed research in the area of distributed computing since obtaining Masters and Doctorate degrees in Computer Science from Cornell Univesity. His current interest is in studying problems of a partical nature in a formal context. He was graduated from Princeton University in 1979 with a B.S.E. degree in Electrical Engineering and Computer Science.The Research of this author was partially supported by Research Grant 120-749 and the Argentinian Research Fund at the Technion     

Knowledge in shared memory systems

Summary We study the relation between knowledge and space. That is, we analyze how much shared memory space is needed in order to learn certain kinds of facts. Such results are useful tools for reasoning about shared memory systems. In addition we generalize a known impossibility result, and show that results about how knowledge can be gained and lost in message passing systems also hold for shared memory systems. Michael Merritt received a B.S. degree in Philosophy and in Computer Science from Yale College in 1978, the M.S. and Ph.D. degrees in Information and Computer Science in 1980 and 1983, respectively, from the Georgia Institute of Technology. Since 1983 he has been a member of technical staff at AT & T Bell Laboratories, and has taught as an adjunct or visiting lecturer at Stevens Institute of Technology, Massachusetts Institute of Technology, and Columbia University. In 1989 he was program chair for the ACM Symposium on Principles of Distributed Computing. His research interests include distributed and concurrent computation, both algorithms and formal methods for verifying their correctness, cryptography, and security. He is an editor for Distributed Computing and for Information and Computation, recently co-authored a book on database concurrency control algorithms, and is a member of the ACM and of Computer Professionals for Social Responsibility. Gadi Taubenfeld received the B.A., M.Sc. and Ph.D. degrees in Computer Science from the Technion (Israel Institute of Technology), in 1982, 1984 and 1988, respectively. From 1988 to 1990 he was a research scientist at Yale University. Since 1991 he has been a member of technical staff at AT & T Bell Laboratories. His primary research interests are in concurrent and distributed computing.A preliminary version of this work appeared in the Proceedings of the Tenth Annual ACM Symposium on Principles of Distributed Computing, pages 189–200, Montreal, Canada, August 1991