Computer Networks (1976)最新文献

The aim of this paper is to analyse the relation between inhouse and public videotex systems and show that these systems are and should not be independent of each other and that they can and should complement each other rather than complete.

This paper outlines the principles on which the U.S. Department of Defense packet internet architecture is based and characterizes some of the protocols which implement the architecture. Major factors which influenced the development of this architectural model include experimental and operational experience with a large number of interconnected packet networks, assessments and evaluations of military requirements for national and international interoperability and multiple jurisdiction operation, and specific concerns regarding security, survivability and operation under crisis conditions.

This paper outlines the nature of the military requirements for packet-switched data communications and contrasts these requirements with those of commercial, industrial and private users served by common carrier public packet-switched data networks. Current US Department of Defense policy on the use of commercial standards in defense systems is reviewed. Deficiencies in existing standards for military applications are identified and implications for the development of a suitable military data communications architecture are drawn. A strategy of military standardization is outlined which adopts commercial standards where they apply and supplements these with special military standards as needed to satisfy unique military requirements. The thesis of this paper is illustrated by a discussion of the applicability of the CCITT X.25 protocol standard to military communications.

The US Department of Defense (DoD) has, for some years now, been pursuing the objective of implementing a new data communications system responsive to its requirements. In consonance with this goal, it has also been heavily involved in developing data communications protocol standards. It has participated in related research, test, and development of a Transmission Control Protocol and an Internet Protocol and has plans for developing other protocols in layers 4–7 of the OSI model.

This paper is essentially structured into two parts. The first part will provide background information for the DoD protocol standardization program, which describes policy issues, status, and problems inherent in the development of such a program. This background description is then followed by an outline of current DoD planning for future work in this area. The point being made here is that the DoD by virtue of its needs, investigations, and accomplishments to date, is eminently qualified to contribute significantly to the development of data protocol standards. The second part of the paper describes the actual current DoD activities for developing data protocol standards, as well as its efforts to coordinate with NATO, the National Bureau of Standards (NBS), and commercial standards groups.

The potential for cost-effective distribution and coordinated sharing of information processing systems is substantially enhanced by the recent emergence of comparatively cheap, very reliable, high speed local area networks (LANs). The lack of common interface specifications is however a major impediment to progress. The benefits to be gained by ensuring that the user accessible interface to a local area network provides identical functionality to that offered over a long haul network argue for a subset of features common to both types. This need for commonality argues against exploitation of some of the more novel features intrinsic to certain styles of technologically advanced systems. By this means, the enormous investment in existing (wide area) networked applications can be applied within a local context and, perhaps even more importantly, applications mounted in a LAN environment will also be accessible from, and be able to communicate with, other more remote systems.

In a world increasingly populated with Ethernets and Ethernet-like nets there nevertheless continues to be a strong interest in rings of active repeaters for local data communication. This paper explores some of the engineering problems involved in designing a ring that has no central control. It then compares one ring design with the Ethernet on several different operational and subtle technical points of design, maintainability, and future prospects. On each of these points the ring possesses important or interesting advantages. At the same time, the most commonly cited advantage of a ring, “deterministic access time,” is shown to be illusory. The paper concludes that the data communication ring is a sound idea that will prove its value on operational rather than theoretical issues.

This paper examines alternative shared channel access procedures for mobile packet radio data/voice systems. Natural pauses in speech allow the possibility of bandwidth savings through interleaving of talkspurts from many sources, as is done in packet data systems. The acceptability of such schemes lies in the resulting performance level; inevitable degradation through delay and blocking should be small. Access procedures are outlined for a two hop virtual circuit in which a fixed base station acts as a relay transmitter. Analysis shows that a reduction of 30–35% in bandwidth requirements can be obtained without serious performance loss.

In any computer system it is valuable to have methods for enforcing expected behaviour at runtime. (Capabilities and object-oriented architectures are mechanisms commonly used to address the problem.) In a message-switched system, all inter-process communication is done by the transmission of messages, and the problem of enforcing expected behaviour can be reduced to the problem of enforcing expected patterns of message transmission.

In this paper we give a method, called the Task Graph Language, for expressing expected patterns (policy) of message transmission. Such an expression is quite independent of the actual programs which pass messages, and can be used at runtime to check the validity of observed behaviour. The mechanism we provide to perform the runtime checking of observed vs. expected behaviour is called Token Lists.

This mechanism requires no esoteric hardware support and it can be used in both distributed and centralized systems. From another perspective, the user is provided with two independent, hence mutually redundant, ways to express his intent (the programs which pass messages and the Task Graph Language description), and the two expressions of intent are compared or matched at runtime as a checking mechanism. From still another perspective, the Task Graph Language and Token Lists permit a centralized representation of distributed control.