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Research and Advances May 1 1982

On an improved algorithm for decentralized extrema finding in circular configurations of processors

This note presents a more efficient algorithm for finding the largest element in a circular list of processors when messages can be passed in either direction. It passes 2N*floor(lg N) + 3N messages in the worst case, compared to Chang and Roberts' N(N + 1)/2 and Hirschberg and Sinclair's 8N + 8*ceiling(N lg N) messages. The technique is a selective elimination of possible processes, which then merely relay future messages between the remaining contenders.
Research and Advances Apr 1 1982

Grapevine: an exercise in distributed computing

Grapevine is a multicomputer system on the Xerox research internet. It provides facilities for the delivery of digital messages such as computer mail; for naming people, machines, and services; for authenticating people and machines; and for locating services on the internet. This paper has two goals: to describe the system itself and to serve as a case study of a real application of distributed computing. Part I describes the set of services provided by Grapevine and how its data and function are divided among computers on the internet. Part II presents in more detail selected aspects of Grapevine that illustrate novel facilities or implementation techniques, or that provide insight into the structure of a distributed system. Part III summarizes the current state of the system and the lesson learned from it so far.
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Research and Advances Apr 1 1982

Performing remote operations efficiently on a local computer network

A communication model is described that can serve as a basis for a highly efficient communication subsystem for local networks. The model contains a taxonomy of communication instructions that can be implemented efficiently and can be a good basis for interprocessor communication. These communication instructions, called remote references, cause an operation to be performed by a remote process and, optionally, cause a value to be returned. This paper also presents implementation considerations for a communication system based upon the model and describes an experimental communication subsystem that provides one class of remote references. These remote references take about 150 microseconds or 50 average instruction times to perform on Xerox Alto computers connected by a 2.94 megabit Ethernet.
Research and Advances Apr 1 1982

A technique for testing command and control software

A technique for testing embedded-microprocessor command and control programs is described. The continuity inherent in functions computed by programs which monitor natural phenomena is exploited by a simple difference equation-based algorithm to predict a program's next output from its preceding ones. The predicted output is compared with the actual output while indexing through the program's domain. Outputs which cannot be predicted are flagged as potential errors. Data are presented which show that this technique can be a sensitive measure of a program's correctness.
Research and Advances Mar 1 1982

The “worm” programs—early experience with a distributed computation

The “worm” programs were an experiment in the development of distributed computations: programs that span machine boundaries and also replicate themselves in idle machines. A “worm” is composed of multiple “segments,” each running on a different machine. The underlying worm maintenance mechanisms are responsible for maintaining the worm—finding free machines when needed and replicating the program for each additional segment. These techniques were successfully used to support several real applications, ranging from a simple multimachine test program to a more sophisticated real-time animation system harnessing multiple machines.
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Research and Advances Nov 1 1981

Authentication of signatures using public key encryption

One of Needham and Schroeder's proposed signature authentication protocols is shown to fail when there is a possibility of compromised keys: this invalidates one of the applications of their technique. A more elaborate mechanism is proposed which does not require a network clock, but does require a third party to the transaction. The latter approach is shown to be reliable in a fairly strong sense.
Research and Advances Nov 1 1981

The evolution of user behavior in a computerized conferencing system

Data from 18-month operational trials of the EIES system indicate that the range of features considered valuable in a computer-based communication system increases with the amount of experience gained by using this medium of communication. Simple message systems alone are not likely to satisfy the communications needs of long term, regular users of computerized communications systems. Among the capabilities which long term, regular users find valuable are group conferences, notebooks for text composition, and self-defined commands.
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Research and Advances Sep 1 1981

Comparison of synonym handling and bucket organization methods

A theoretical description of the access times required in open addressing and external chaining is given. Values are calculated for different record and bucket sizes and load factors, and the corresponding values for the two methods are compared. Practical guidelines for determining bucket sizes and load factors are presented. It is proved that open addressing is almost always superior to external chaining and the optimal bucket size is between 1 and 4.
Research and Advances May 1 1981

The cube-connected cycles: a versatile network for parallel computation

An interconnection pattern of processing elements, the cube-connected cycles (CCC), is introduced which can be used as a general purpose parallel processor. Because its design complies with present technological constraints, the CCC can also be used in the layout of many specialized large scale integrated circuits (VLSI). By combining the principles of parallelism and pipelining, the CCC can emulate the cube-connected machine and the shuffle-exchange network with no significant degradation of performance but with a more compact structure. We describe in detail how to program the CCC for efficiently solving a large class of problems that include Fast Fourier transform, sorting, permutations, and derived algorithms.
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Research and Advances May 1 1981

Strip trees: a hierarchical representation for curves

The use of curves to represent two-dimensional structures is an important part of many scientific investigations. For example, geographers use curves extensively to represent map features such as contour lines, roads, and rivers. Circuit layout designers use curves to specify the wiring between circuits. Because of the very large amount of data involved and the need to perform operations on this data efficiently, the representation of such curves is a crucial issue. A hierarchical representation consisting of binary trees with a special datum at each node is described. This datum is called a strip and the tree that contains such data is called a strip tree. Lower levels in the tree correspond to finer resolution representations of the curve. The strip tree structure is a direct consequence of using a special method for digitizing lines and retaining all intermediate steps. This gives several desirable properties. For curves that are well-behaved, intersection and point-membership (for closed curves) calculations can be solved in 0(log n) where n is the number of points describing the curve. The curves can be efficiently encoded and displayed at various resolutions. The representation is closed under intersection and union and these operations can be carried out at different resolutions. All these properties depend on the hierarchical tree structure which allows primitive operations to be performed at the lowest possible resolution with great computational time savings. Strip trees is a linear interpolation scheme which realizes an important space savings by not representing all the points explicitly. This means that even when the overhead of the tree indexing is added, the storage requirement is comparable to raster representations which do represent most of the points explicitly.
Research and Advances Dec 1 1978

Using encryption for authentication in large networks of computers

Use of encryption to achieve authenticated communication in computer networks is discussed. Example protocols are presented for the establishment of authenticated connections, for the management of authenticated mail, and for signature verification and document integrity guarantee. Both conventional and public-key encryption algorithms are considered as the basis for protocols.
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Research and Advances Dec 1 1978

Reverse path forwarding of broadcast packets

A broadcast packet is for delivery to all nodes of a network. Algorithms for accomplishing this delivery through a store-and-forward packet switching computer network include (1) transmission of separately addressed packets, (2) multidestination addressing, (3) hot potato forwarding, (4) spanning tree forwarding, and (5) source based forwarding. To this list of algorithms we add (6) reverse path forwarding, a broadcast routing method which exploits routing procedures and data structures already available for packet switching. Reverse path forwarding is a practical algorithm for broadcast routing in store-and-forward packet switching computer networks. The algorithm is described as being practical because it is not optimal according to metrics developed for its analysis in this paper, and also because it can be implemented in existing networks with less complexity than that required for the known alternatives.
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Research and Advances Nov 1 1978

Performance evaluation of highly concurrent computers by deterministic simulation

Simulation is presented as a practical technique for performance evaluation of alternative configurations of highly concurrent computers. A technique is described for constructing a detailed deterministic simulation model of a system. In the model a control stream replaces the instruction and data streams of the real system. Simulation of the system model yields the timing and resource usage statistics needed for performance evaluation, without the necessity of emulating the system. As a case study, the implementation of a simulator of a model of the CPU-memory subsystem of the IBM 360/91 is described. The results of evaluating some alternative system designs are discussed. The experiments reveal that, for the case study, the major bottlenecks in the system are the memory unit and the fixed point unit. Further, it appears that many of the sophisticated pipelining and buffering techniques implemented in the architecture of the IBM 360/91 are of little value when high-speed (cache) memory is used, as in the IBM 360/195.
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Research and Advances Oct 1 1978

Counting large numbers of events in small registers

It is possible to use a small counter to keep approximate counts of large numbers. The resulting expected error can be rather precisely controlled. An example is given in which 8-bit counters (bytes) are used to keep track of as many as 130,000 events with a relative error which is substantially independent of the number n of events. This relative error can be expected to be 24 percent or less 95 percent of the time (i.e. &sgr; = n/8). The techniques could be used to advantage in multichannel counting hardware or software used for the monitoring of experiments or processes.
Research and Advances Sep 1 1978

Hybrid simulation models of computer systems

This paper describes the structure and operation of a hybrid simulation model in which both discrete-event simulation and analytic techniques are combined to produce efficient yet accurate system models. In an example based on a simple hypothetical computer system, discrete-event simulation is used to model the arrival and activation of jobs, and a central-server queueing network models the use of system processors. The accuracy and efficiency of the hybrid technique are demonstrated by comparing the result and computational costs of the hybrid model of the example with those of an equivalent simulation-only model.
Research and Advances Jul 1 1978

Time, clocks, and the ordering of events in a distributed system

The concept of one event happening before another in a distributed system is examined, and is shown to define a partial ordering of the events. A distributed algorithm is given for synchronizing a system of logical clocks which can be used to totally order the events. The use of the total ordering is illustrated with a method for solving synchronization problems. The algorithm is then specialized for synchronizing physical clocks, and a bound is derived on how far out of synchrony the clocks can become.
Research and Advances May 1 1978

Optimal shift strategy for a block-transfer CCD memory

For the purposes of this paper, a block-transfer CCD memory is composed of serial shift registers whose shift rate can vary, but which have a definite minimum shift rate (the refresh rate) and a definite maximum shift rate. The bits in the shift registers are numbered 0 to N - 1, and blocks of N bits are always transferred, always starting at bit 0. What is the best shift strategy so that a block transfer request occurring at a random time will have to wait the minimal amount of time before bit 0 can be reached? The minimum shift rate requirement does not allow one to simply “park” at bit 0 and wait for a transfer request. The optimal strategy involves shifting as slowly as possible until bit 0 is passed, then shifting as quickly as possible until a critical boundary is reached, shortly before bit 0 comes around again. This is called the “hurry up and wait” strategy and is well known outside the computer field. The block-transfer CCD memory can also be viewed as a paging drum with a variable (bounded) rotation speed.
Research and Advances Apr 1 1978

List processing in real time on a serial computer

A real-time list processing system is one in which the time required by the elementary list operations (e.g. CONS, CAR, CDR, RPLACA, RPLACD, EQ, and ATOM in LISP) is bounded by a (small) constant. Classical implementations of list processing systems lack this property because allocating a list cell from the heap may cause a garbage collection, which process requires time proportional to the heap size to finish. A real-time list processing system is presented which continuously reclaims garbage, including directed cycles, while linearizing and compacting the accessible cells into contiguous locations to avoid fragmenting the free storage pool. The program is small and requires no time-sharing interrupts, making it suitable for microcode. Finally, the system requires the same average time, and not more than twice the space, of a classical implementation, and those space requirements can be reduced to approximately classical proportions by compact list representation. Arrays of different sizes, a program stack, and hash linking are simple extensions to our system, and reference counting is found to be inferior for many applications.
Research and Advances Apr 1 1978

Secure communications over insecure channels

According to traditional conceptions of cryptographic security, it is necessary to transmit a key, by secret means, before encrypted massages can be sent securely. This paper shows that it is possible to select a key over open communications channels in such a fashion that communications security can be maintained. A method is described which forces any enemy to expend an amount of work which increases as the square of the work required of the two communicants to select the key. The method provides a logically new kind of protection against the passive eavesdropper. It suggests that further research on this topic will be highly rewarding, both in a theoretical and a practical sense.

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