Mario Tokoro

An Object Calculus for Asynchronous Communication

This paper presents a formal system based on the notion of objects and asynchronous communication. Built on Milners work on π-calculus, the communication primitive of the formal system is purely based on asynchronous communication, which makes it unique among various concurrency formalisms. Computationally this results in a consistent reduction of Milners calculus, while retaining the same expressive power. Seen semantically asynchronous communication induces a surprisingly different framework where bisimulation is strictly more general than its synchronous counterpart. This paper shows basic construction of the formal system along with several illustrative examples.

A network architecture providing host migration transparency

The continued expansion of computer networks and the miniaturization of computers increase the desire to use ones computer in a consistent computational environment, regardless of location and time. In this situation, host migration transparency is very important. This paper introduces a new network architecture which provides host migration transparency in large interconnected networks and proposes a protocol based on the propagating cache method. We introduce the concept virtual network and divide the conventional network layer into two sublayers to realize host migration transparency. Virtual Internet Protocol, or VIP for short, is derived from DARPA-IP as an example of the architecture. We estimate the overhead of VIP. Host migration transparency can be realized without signi cant processing and tra c overhead. This architecture also preserves host migration transparency across virtual circuits in radio networks.

On Asynchronous Communication Semantics

This paper presents some results concerning equational theories for an elementary calculus based on a fragment of Milners π-calculus. The system is interesting because it realises asynchronous message passing not by extending but reducing the original fragment, while preserving the computational power. The bisimulation based on a novel asynchronous transition system is introduced and studied. Presented results include congruence of the bisimilarity for the calculus, its relationship with two other asynchronous theories based on traces and failures, strict inclusion of its synchronous counterpart in the asynchronous theory, and the method called the Icompletion that transforms two asynchronously bisimilar terms into synchronously bisimilar ones.

Host migration transparency in IP networks: the VIP approach

Increasing portability of computers will require users in the future to access the network regardless of location. Host migration transparency will be an essential feature of wide area network environments. We proposed the concept of virtual network and the propagating cache method to achieve host migration transparency [12]. We also established the feasibility of Virtual Internet Protocol (VIP), derived from Internet Protocol (IP), as an example of a virtual network protocol. In this paper, we define VIP in detail and describe how VIP achieves host migration transparency in IP networks. We also show two VIP implementation approaches: 1) VIP as a sublayer; and 2) VIP as an IP option. By modifying an operating systems kernel in each case, we implement VIP using both approaches. We then compare both implementations in terms of overhead, backward compatibility, and other metrics. The measured performance indicates that VIP can provide host migration transparency in IP networks with negligible overhead.

The muse object architecture: a new operating system structuring concept

A next generation operating system should accommodate an ultra large-scale, open, self-advancing, and distributed environment. This environment is dynamic and versatile in nature. In it, an unlimited number of objects, ranging from fine to coarse-grained, are emerging, vanishing, evolving, and being replaced; computers of various processing capacities are dynamically connected and disconnected to networks; systems can optimize object execution by automatically detecting the users and/or programmers requirements. In this paper, we investigate several structuring concepts in existing operating systems. These structuring concepts include layered structuring, hierarchical structuring, policy/mechanism separation, collective kernel structuring, object-based structuring, open operating system structuring, virtual machine structuring, and proxy structuring.We adjudge that these structuring concepts are not sufficient to support the environment described above because they lack the abilities to handle dynamic system behavior and transparency and to control dependency. Thus, we propose a new operating system structuring concept which we call the Muse object architecture. In this architecture, an object is a single abstraction of a computing resource in the system. Each object has a group of meta-objects which provide an execution environment. These meta-objects constitute a meta-space which is represented within the meta-hierarchy. An object is causally connected with its meta-objects: the internal structure of an object is represented by meta-objects; an object can make a request of meta-computing; a meta-object can reflect the results of meta-computing to its object. We discuss object/meta-object separation, the meta-hierarchy, and reflective computing of the architecture. We then compare the Muse object architecture with the existing structuring concepts.We also demonstrate that the Muse object architecture is suitable for structuring future operating systems by presenting several system services of the Muse operating system such as class systems, a real-time scheduler with hierarchical policies, and free-grained objects management. Class systems facilitate programming by several classes of programming languages. A real-time scheduler with hierarchical policies can meet various types of real-time constraints presented by applications. Free-grained objects management can suit the object granularity to the application, so that an object is efficiently managed according to its granularity. Finally, we present the implementation of the Muse operating system which is designed based on the Muse object architecture. Version 0.3 of the Muse kernel is running on the MC68030 based Sony NEWS workstations.

Experience and evolution of concurrent Smalltalk

ConcurrentSmalltalk is an object-oriented concurrent programming language/system which has been running since late 1985. ConcurrentSmalltalk has the following features: Upper-compatibility with Smalltalk-80. Asynchronous method calls and CBox objects yield concurrency. Atomic objects have the property of running one at a time so that it can serialize the many requests sent to it. Through experience in writing programs, some disadvantages have become apparent related to concurrency control and the behavior of a block context. In this paper, these issues are re-examined in detail, and then the evolution of the solutions for overcoming these shortcomings is described along with the model of computation in ConcurrentSmalltalk. New features are explained with an example program. The implementation of the ConcurrentSmalltalk virtual machine is also presented along with the evaluation of it.

A Typing System for a Calculus of Objects

The present paper introduces an implicitly typed object calculus intended to capture intrinsic aspects of concurrent objects communicating via asynchronous message passing, together with a typing system assigning typings to terms in the calculus. Types meant to describe the kind of messages an object may receive are assigned to the free names in a program, resulting in a scenario where a program is assigned multiple name-type pairs, constituting a typing for the process. Programs that comply to the typing discipline are shown not to suffer from runtime errors. Furthermore the calculus possesses a notion of principal typings, from which all typings that make a program well-typed can be extracted. We present an efficient algorithm to extract the principal typing of a process.

A concurrent object-oriented knowledge representation language Orient84/K: its features and implementation

Orient84/K is an object oriented concurrent programming language for describing knowledge systems. In Orient84/K, an object is composed of the behavior part , the knowledge-base part , and the monitor part , in order to provide object-oriented, logic-based, demon-oriented , and concurrent-programming paradigms in the object framework. Every object is capable of concurrent execution in Orient84/K. In this paper, after describing an overview of Orient84/K, we will describe implementation issues in a concurrent object oriented language. Then, a new method for an efficient implementation of concurrent objects is proposed and formally described. A new virtual machine for Orient84/K is designed using this method, and some preliminary results of evaluation are presented.

The design and implementation of Concurrent Smalltalk

ConcurrentSmalltalk is a programming language/system which incorporates the facilities of concurrent programming in Smalltalk-80 1 . Such facilities are realized by providing concurrent constructs and atomic objects . This paper first gives an outline of ConcurrentSmalltalk. Then, the design of ConcurrentSmalltalk is described. The implementation of ConcurrentSmalltalk is presented in detail.

Design, implementation, and evaluation of Virtual Internet Protocol

The design and implementation of the Virtual Internet Protocol (VIP) are described. The VIP was implemented by modifying an operating system kernel based on 4.3BSD. The overhead of VIP is compared to that of IP. Measured results indicate that VIP can achieve host migration transparency in the Internet with negligible overhead.<<ETX>>