Virtual Terminal Protocol Research Articles

The ISO virtual terminal protocol: a semi-automatic implementation

While there have been reports on the automatic implementation of mid-level and lower layer protocols, there are, however, very very few such reports for application layer protocols. The main reason is that automatic implementation of application protocols is more difficult, as its protocol data units need to be specified in ASN.1 and encoding and decoding of such data is required. This paper presents a semi-automatic implementation of the ISO Virtual Terminal Protocol using a number of tools. It also discusses the experience gained and lessons learnt from this work, including the strengths and weaknesses of using such an approach, and the technical difficulties in bridging the gap between protocol specification, design decisions, implementation decisions and final implementation.

UbcVTP: An implementation of ISO VTP

Virtual terminal protocols provide remote terminal-oriented communications between network users. They are designed to allow terminals and hosts on a heterogeneous network to communicate without requiring one side to know the terminal characteristics handled by the other side. With the diversification of different types of terminals, there is a tremendous interest in research and design of such protocols. In this paper, we describe the design and implementation of a virtual terminal protocol, that is, the Basic Class VTP defined by International Standard Organization (ISO). Focus of the paper will be the description of the VT model, in particular the one that is defined by the local VTP implementation, and the mapping between terminal type specific I/O and the VT abstract operations. Furthermore, we compare the ISO VTP with other terminal protocols, such as ARPANET TELNET and CCITT PAD Facility, pointing out their differences and short-comings.

Data communications system for the state of Nordrhein-Westfalen

The common data communications system for the German federal state of Nordrhein-Westfalen has been established to provide communications between different vendors' computers within the state administration. The system uses manufacturer-independent protocols such as X.25 (Layer 3), EHKP4 as the transport protocol (Layer 4), and protocols for file transfer and dialogue (line-mode). Network management facilities, and a centre for testing services, protocols and interfaces are in operation. The virtual terminal protocol is described in some detail, as the service has only recently been developed.

A virtual terminal protocol with windowing capability

Various kinds of terminals are used in a network environment. There arises the so-called m × n problem between one terminal and another or between one application and another or application and terminal. To solve this problem various virtual terminal protocols were proposed and are now being used in many networks. In this paper, we present a virtual terminal protocol with windowing capability based on the concept of the virtual terminal protocol proposed by ISO. The virtual terminal with windowing capability refers to a virtual terminal which has graphics primitives and image processing capabilities as well as character operations used in basic class terminals and which have windowing capability. In this paper, the model for the virtual terminal, its protocol architecture, model, data structure, the service facilities, and protocol messages are presented.

Estelle formal specification of ISO virtual terminal

The ISO Virtual Terminal Protocol (VTP) is specified in Estelle, an ISO International Standard (IS) formal description technique. VTP allows heterogeneous terminals to communicate without each having to know the details of the other. This is accomplished by having two virtual terminal users negotiate a common subset of capabilities. The Estelle specification herein consists of an architecture model, a behavior model, and channel descriptions. The architecture model includes modules and their appropriate interconnections (channels). The behavior model specifies each module's underlying extended finite state machine, i.e. sequencing of events and activities. Channel descriptions declare the message elements (PDUs and service primitives) that are communicated between modules. Inconsistencies in an earlier draft ISO VTP document were discovered while specifying it in Estelle; these inconsistencies and the decisions how to formally specify them are discussed. The result is a formal specification that is precise, easy to follow, and as close to the ISO VTP IS as possible. Suggestions are given for making Estelle a more powerful and convenient formal description technique.

Building an open communication application

The implementation of a series of protocol handlers is discussed in terms of the approach taken to design and implementation methods. The target was a communication sub-system insupport of terminal handling applications. The design of an OSI transport layer handler using the Jackson system design method is described, as are the reasons for opting to use a more conventional, finite-state machine approach to session layer and ECMA virtual terminal protocol handlers. The software engineering tools used to carry out these implementations are also described. In conclusion the paper considers the lessons learned both from the application of design methods and the efficiency of protocol implementation. The paper shows that the approach taken by ISO in defining its OSI protocols is effective and leads to viable implementations.

The current status of MAP

MAP is a standard computer networking discipline and a unified set of communications protocols which make it possible to implement a vendor independent network of computers. MAP provides a complete service to the user or application program. The major facilities, file transfer, process to process messaging, inter-machine messaging, and directory service are supported. Virtual terminal protocol support is planned for the near future. MAP is a subset of the internationally agreed Open Systems Interconnection computer networking architecture, protocols and supporting standards. The MAP subset of OSI Protocols was originally selected by General Motors and has been agreed by a MAP Task Force comprised of nearly 100 user and vendor corporations. MAP specifications are also agreed by the NBS/OSI Implementor's Workshop, indicating that the MAP specifications are OSI compatible. The MAP System and Protocols are described here in sufficient detail to permit the reader to gain a general understanding of MAP against a background of OSI fundamentals. The future growth trends for MAP conclude this paper.

OSI virtual terminal service

The OSI Virtual Terminal Service (VTS) is one of several application-oriented communication services being provided by protocols operating in the Application Layer of the Open System Interconnection Environment. There will be several Virtual Terminal services and protocols, each catering to a different class of distributed terminal service. Division of the VTS into classes of service allows more timely, economical, and efficient support of distributed terminal service, both now and for the future. Current work is concentrating on the Generic Virtual Terminal Protocol and the Basic Class Virtual Terminal Protocol. The Generic Virtual Terminal Service and Protocol establishes the general approach to providing all classes of VTS and sets the style for generating Virtual Terminal Protocols (VTP's) for Specific classes of terminal use. The Basic Class VTP is the first OSI VTP being developed by the International Organization for Standardization, and is intended to provide distributed terminal access for a wide range of nonsophisticated character-oriented terminals.

DCNA Higher Level Protocols

This paper explains a philosophy for modeling the higher level communication functions into a network architecture for heterogeneous computer networks called Data Communication Network Architecture (DCNA), the logical structure of the architecture, and several protocols based on it. To specify higher level protocols among computers of different types, DCNA defines a logical model of a computer network consisting of three submodels: the basic model, the logical network model, and the virtual network model. The basic model represents a logical view of the network resources, e.g., processing power, files, data bases, I/O devices, and a layered structure for the basic mechanisms for accessing such resources, which incorporate the concept of sublevels as well as levels. The logical network model describes the mechanisms for unified management of network resources. The virtual network model describes the mechanisms for using network resources. The. common use of network resources, by several sets of interrelated applications is made easier by treating the logical network and the virtual network separately. These models form the basis for the stipulation of higher level protocols, such as network management protocols, message transfer protocols, virtual terminal protocols, and virtual file system protocols.

Terminal Protocols

Terminal protocols provide basic services for the users of computer networks. This paper presents a survey of the architecture and mechanisms used in current terminal protocols. The paper disusses both parametric terminal protocols such as the CCITT X.3, X.28, and X.29 and virtual terminal protocols, such as the ARPANET TELNET protocol. Many of the problems encountered in terminal protocols recur in more complex forms in the more sophisticated protocols.

A survey of terminal protocols

The Virtual Terminal approach has been used by first by the ARPANET in the U.S. In the last few years, the problem of defining Virtual Terminal Protocols has been approached by several international organizations such as EIN, Euronet, IFIP and by national organizations such as the Belgian University Networks, the German PIX Network, the French Cyclades Network and INFORREP (french' user association'). Asociated manufacturers have also presented a proposal. Till now, an internationally accepted Virtual Terminal Protocol has not yet emerged. The aim of this paper is to compare different protocols. It is divided into four main parts: 1. Presentation of the relationships between Virtual Terminal Protocol and network architecture. 2. Analysis of the different virtual terminal model. 3. Analysis of the set of primitives intended to data structure definition and management. 4. Introduction to the synchronization mechanisms of the protocol.

Remarks on negotiation mechanism and attention handling

Preliminary studies are at present being made for building a computer network linking the computer centres of the different Belgian Universities. This network would include:1. a packet switching network with the X25 service on top of it;2. a transport protocol;3. user level protocols (a virtual terminal protocol and a file transfer protocol).Cooperation between terminals and applications will be organized using a virtual terminal approach.Assuming a transport function offering:1. block transmission with error control on contents, error control on arrival and sequencing;2. telegram transmission with error control on contents and error control on arrival,we present the attention handling scheme proposed for the Virtual Terminal Protocol of the Belgian University Network.The proposed scheme includes a renegotiation of the data flow mode (free running or alternate).A detailed algorithmic description of the proposed attention handling scheme is given using a state diagram representation.

An informal comparison of three protocols

Three pairs of higher level computer communication protocols are compared to reveal design choices. The comparisons are between the host level protocols NCP and TCP used in the ARPANET, between the virtual terminal protocols Telnet and SMVT, and between the file transfer protocols FTP and BTF. The Telnet and FTP protocols are used in the ARPANET, while the SMVT and BTF are proposed for use in the EIN.

A virtual terminal protocol based upon the “communication variable” concept

The ideas presented here have emerged from discussions within the PIX working groups “Higher level protocols” and “Virtual Terminal protocol” and present the agreement of the PIX virtual terminal protocol. PIX = A cooperation between several network activities in the technical/scientific area in the FRG, sponsored by the Ministry of Research and Technology. The proposal is based on a general model of interprocess communication and describes how this model is used to fit the requirements of terminal control. The resulting class of terminal protocols uses one simple command set whereas the protocol for one specific terminal is derived by defining the appropriate data structure for that terminal. The complexity of a given terminal protocol therefore solely depends upon the complexity of its data structure. As an example, a protocol definition for a simple “line” and “page” mode terminal is given.

The virtual terminal approach in the Belgian University network

Preliminary studies are at present being made for building a computer network linking the computer centres of the different Belgian Universities. This network would include: 1. a packet switching network with the x25 interface; 2. a transport protocol at the user level; 3. application oriented protocols (e.g. virtual terminal protocol, file transfer protocol). Cooperation between terminals and applications will be organized using a virtual terminal approach. The aim of this paper is twofold: 1. to present the general concept of a virtual terminal and discuss its definition. Three classes of virtual terminal are introduced which exhibit increasing levels of terminal complexity (i.e. scroll mode, page mode and data entry); 2. to analyze fundamental protocol mechanisms (i.e. negotiation, data exchange synchronization and attention handling) with particular emphasis on attention handling. In this paper are mainly underlined the options which have been chosen for the Belgian University Network and the motivations which lead to make those choices.

A File Transfer Protocol (FTP)

This paper presents a File Transfer Protocol (FTP) which can be used to transfer files between heterogeneous computer systems connected by a communication network. The basic mechanisms as well as the set of protocol commands and responses have been defined in the context of a general architecture. A simplified architecture, to help first implementations is also proposed. This can easily be extended to the more general one. The proposed protocol is completely compatible (even identical in some areas) with the currently proposed Virtual Terminal Protocol (VTP).

A virtual terminal protocol based on the use of zones

Companion papers have considered the concept of the Virtual Terminal in the context of a network (ref 1) and as an interface between users and computers (ref 2). These papers give a background for the present paper which proposes a Virtual Terminal protocol for use between an applications program and a remote terminal handler, based on the concept of the dynamic assignment of zones, introduced by Luca (ref 3). The paper by Luca contains an excellent discussion of the concept and use of zones, which seem well suited for a Virtual Terminal protocol although this was not considered in his paper. Some simplification of Luca's work is appropriate because he is concerned both with the interface between an application and the terminal handler, which he calls the programmable communications controller (PCC), and with the mapping between the PCC and a real terminal. The first aspect only will be considered here as the basis of a simple applications level protocol for a Virtual Terminal. However, Luca's paper is also a useful guide for the designers of the terminal handler and users who are concerned with mapping the Virtual Terminal onto particular real terminals. This paper draws on the ideas contained in the earlier papers reviewed in Appendix 1 of ref 2, and uses the notation introduced in Appendix 2 of ref 2. The zones concept is similar to the idea of the Communication Unit introduced by Schulze (ref 4) and this has also influenced the proposals set out below.

An RJE protocol for a resource sharing network

For many users of networks like the ARPANET, an RJE protocol is probably as important in terms of utility as the TELNET (or Virtual Terminal) protocol. In fact, the facilities provided by a TELNET and an RJE protocol are probably of most interest to the majority of users of computer networks. For these users, the network provides a fast, cheap telephone surrogate to a variety of computers for RJE and terminal access. The collection (and layers) of protocols that are used to provide these services must be organized to efficiently support a wide variety of applications and user needs. They should not pose an undue software burden on the user. The "official" NETRJE protocol for the ARPANET is built on top of the Telnet and File Transfer Protocols (FTP). A user of NETRJE establishes a Telnet connection using the User NETRJE program on his local host to a Server NETRJE process at the host where he wishes to submit a job (see Figure 1). The user then sends to the Server process appropriate commands to log in, to describe where the job is that is to be submitted, to describe how the job is to be transferred, and to describe what is co be done with output when the job is completed. The Server NETRJE process on the foreign host then uses the FTP to transfer the input file to the foreign host, and then may use FTP again later to send the output back to the user. This protocol is very general and allows a user to submit his input from one host, process it at a second, and send the output to a third.

Proposal for a scroll mode virtual terminal

The scope of this document is to present the concepts of a Virtual Terminal and to define in detail that portion of the Virtual Terminal Protocol which is adequate to support a basic Scroll Mode Virtual Terminal.