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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.
The findings of a study designed to address the pressing problems associated with the strategic planning of the computing effort in higher education are presented here. A planning methodology was developed and tested through implementation at a university. Two years after the methodology was implemented, the effectiveness of the planning methodology was assessed in terms of the improvement of the delivery of computing services to the major institutional roles of instruction, research, and administration. Two control institutions were employed to contrast the improvements at the test institution. The results of the research indicate the planning methodology significantly enhanced the delivery of computing services.
Detection of logical errors in decision table programs
In this paper an algorithm to detect logical errors in a limited-entry decision table and in loop-free programs with embedded decision tables is developed. All the conditions in the decision tables are assumed to be inequalities or equalities relating linear expressions. It is also assumed that actions in a decision table are linear in variables which occur in the condition stub of the decision table (or tables) to which control is transferred from the table. The algorithm is based on determining whether a set of linear inequalities has or does not have a solution. The algorithm described in the paper is implemented in Fortran IV.
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.
Abstract data types and software validation
A data abstraction can be naturally specified using algebraic axioms. The virtue of these axioms is that they permit a representation-independent formal specification of a data type. An example is given which shows how to employ algebraic axioms at successive levels of implementation. The major thrust of the paper is twofold. First, it is shown how the use of algebraic axiomatizations can simplify the process of proving the correctness of an implementation of an abstract data type. Second, semi-automatic tools are described which can be used both to automate such proofs of correctness and to derive an immediate implementation from the axioms. This implementation allows for limited testing of programs at design time, before a conventional implementation is accomplished.
An example of hierarchical design and proof
Hierarchical programming is being increasingly recognized as helpful in the construction of large programs. Users of hierarchical techniques claim or predict substantial increases in productivity and in the reliability of the programs produced. In this paper we describe a formal method for hierarchical program specification, implementation, and proof. We apply this method to a significant list processing problem and also discuss a number of extensions to current programming languages that ease hierarchical program design and proof.
A linear sieve algorithm for finding prime numbers
A new algorithm is presented for finding all primes between 2 and n. The algorithm executes in time proportional to n (assuming that multiplication of integers not larger than n can be performed in unit time). The method has the same arithmetic complexity as the algorithm presented by Mairson [6]; however, our version is perhaps simpler and more elegant. It is also easily extended to find the prime factorization of all integers between 2 and n in time proportional to n.
Distributed processes: a concurrent programming concept
A language concept for concurrent processes without common variables is introduced. These processes communicate and synchronize by means of procedure calls and guarded regions. This concept is proposed for real-time applications controlled by microcomputer networks with distributed storage. The paper gives several examples of distributed processes and shows that they include procedures, coroutines, classes, monitors, processes, semaphores, buffers, path expressions, and input/output as special cases.
A simple recovery-only procedure for simple precedence parsers
A simple method is described enabling simple precedence parsers to recover from syntax errors. No attempt to repair errors is made, yet parsing and most semantic processing can continue. The result is a good “first approximation” to syntax error handling with negligible increase in parsing time, space, and complexity of both the parser and its table generator.
On-the-fly garbage collection: an exercise in cooperation
As an example of cooperation between sequential processes with very little mutual interference despite frequent manipulations of a large shared data space, a technique is developed which allows nearly all of the activity needed for garbage detection and collection to be performed by an additional processor operating concurrently with the processor devoted to the computation proper. Exclusion and synchronization constraints have been kept as weak as could be achieved; the severe complexities engendered by doing so are illustrated.
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.
Systems design education: a gaming approach
One of the problems facing managers of computer installations is the problem of configuring the computer system to meet the demands made by the mix of jobs that the computer center must service, This paper presents a management game that allows the player to configure a computer system to meet a hypothetical job mix. The job mix is under the control of a game administrator and can be varied to simulate a variety of real-world situations (I/0 bound jobs, compute bound jobs, etc.). The player of the game receives a set of detailed reports on the cost of his choices and a simulated run of the center operating under his choices.
A note on conditional expressions
Evaluation of a conditional expression may succeed even when the “deciding predicate” diverges and the alternatives are records (or nodes) whose fields have different content.
Cold-start vs. warm-start miss ratios
In a two-level computer storage hierarchy, miss ratio measurements are often made from a “cold start”, that is, made with the first-level store initially empty. For large capacities the effect on the measured miss ratio of the misses incurred while filling the first-level store can be significant, even for long reference strings. Use of “warm-start” rather than “cold-start” miss ratios cast doubt on the widespread belief that the observed “S-shape” of lifetime (reciprocal of miss ratio) versus capacity curve indicates a property of behavior of programs that maintain a constant number of pages in main storage. On the other hand, if cold-start miss ratios are measured as a function of capacity and measurement length, then they are useful in studying systems in which operation of a program is periodically interrupted by task switches. It is shown how to obtain, under simple assumptions, the cache miss ratio for multiprogramming from cold-start miss ratio values and how to obtain approximate cold-start miss ratios from warm-start miss ratios.
Scatter tables for open addressing benefit from recursive entry displacements, cutoffs for unsuccessful searches, and auxiliary cost functions. Compared with conventional methods, the new techniques provide substantially improved tables that resemble exact-solution optimal packings. The displacements are depth-limited approximations to an enumerative (exhaustive) optimization, although packing costs remain linear—O(n)—with table size n. The techniques are primarily suited for important fixed (but possibly quite large) tables for which reference frequencies may be known: op-code tables, spelling dictionaries, access arrays. Introduction of frequency weights further improves retrievals, but the enhancement may degrade cutoffs.
Jump searching: a fast sequential search technique
When sequential file structures must be used and binary searching is not feasible, jump searching becomes an appealing alternative. This paper explores variants of the classic jump searching scheme where the optimum jump size is the square root of the number of records. Multiple level and variable size jump strategies are explored, appropriate applications are discussed and performance is evaluated.
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