Mobile Agents

Mobile agents EEL 5937 Multi Agent Systems Lotzi Bölöni

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Mobile agents • Mobile agents are autonomous programs which move though a network and maintain their identity through this move. • This is a stronger concept than “code mobility” such as Java applets, or client-side Javascript. • Many agent systems were implemented with support for mobility. – And for many researchers, agents == mobile agents

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Motivation for mobility (cont’d)

• Mobile agents can provide better support for mobile clients. – Reduction of network traffic – Asynchronous interaction (good in case of intermittent connection) – Remote searching and filtering

• Mobile agents facilitate semantic information retrieval. – Move one step above simple keyword based search.

• Mobile agents facilitate real-time interaction with a server – Eg. space probes, real time control of a machine tool

• Mobile agent based transactions avoid the need to preserve process state in clients and servers – Instead, the process state is carried in an agent

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Motivations for mobility (cont’d) • Agent based transactions scale better than RPCbased transactions • Secure agent-based transactions have lower overhead than secure RPC. • Mobile agents allow users to personalize server behaviour. • Agents enable semantic routing. • Not all these arguments are valid.

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Counter arguments and answers • Most counter arguments are based on the fact that – What can be done with mobile agents can be done with RPC or – What can be done on the server, you can do it on the client.

• The “software engineering counterargument”: whereas each individual case can be addressed in some (ad-hoc) manner without mobile agents, a mobile agent framework addresses them all of them at once.

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Mobile code • != mobile agents – But, the majority of mobile agent systems imply mobile code

• Transferring code between (heterogeneous) machines. • Implies machine independent code. – Usually, it is implemented with some kind of virtual machine – But it can be also implemented with adaptation, recompilation etc.

• Types of mobile code: – Partially Turing machine complete languages (e.g. SQL, SVG) – Interpreted programming languages (Perl, Python, Javascript) – Virtual machine based compiled languages (Java, Telescript)

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Mobile code - applications • Client-server queries (SQL) • Client side browser applets: – Java applets – Javascript – ActiveX controls

• Remote code updates: – Software updates – Plugins

• Active E-mail – Confirmations – Javascript, Visual Basic for Applications – E-mail viruses and worms

• Mobile agents EEL 5937

Mobile agents without code mobility • Seeing control handoff as mobility – No code mobility involved. – Multithreading involves problems.

• Distributed systems as mobile agent systems • In this approach, mobility is an analysis approach, not a design principle.

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Strong mobility

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Strong mobility • Strong mobility assumes that agents can move at any point during their execution • They are usually relying on: – Specially designed programming languages (eg. Telescript). – Modified virtual machines (eg. NOMADS / AromaVM)

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Custom language: Telescript • Proprietary language, created by General Magic around 1994-95 – Highly influential, without being highly successful

• Interpreted language, which runs on a Telescript engine. – The company implemented engines running on PDA’s, PC’s etc

• “High Telescript”: – Object oriented language, inspired by Smalltalk – Compiled to Low Telescript

• “Low Telescript” – Postfix syntax for stack based implementation

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Telescript (cont’d) • The basic network configuration is to run a Telescript Engine on each node of the network. • A network of Telescript Engines provides a homogenous environment on which to build distributed systems. • Basic class: Process. Telescript supports preemptive, prioritized multi-tasking of Process objects. A Process instance can be thought of as an object with a life of its own. • A Place object represents a virtual space in which other objects can interwork (through local communication). Each Telescript Engine can support a number of places. EEL 5937

Telescript (cont’d) • An Agent object is a Process object which can migrate between Places. An agent may move between Places on the same Engine, or between Places which exist on different Engines. – The Telescript notion of a distributed system is a number of distinctly located places and a number of Agents which move between these Places.

• Places provide meeting locations for Agents. At a Place, Agents can exchange information and perform computation. Places also route travelling Agents. • Persistent Objects --- Telescript Engines implicitly save and recover object state information. • The Telescript world is divided into "regions". Each Engine uses a "regions" database to route migrating Agents. Places and Agents are identified using "Telenames": –

Telename(Locally-Unique-Name, Region-Name)

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Telescript security • Agents have "attributes" such as "identify" and "owning authority" which uniquely identify the Agent and the entity responsible for it. These attributes may be used for authentication. Telescript objects also have a "permit" attribute which may be used to limit the amount of resources which they may consume (e.g. a Place may ask an Agent to pay it 30 "Teleclicks" before granting it access to some resource). • A secure "permits" feature is crucial to stop Agents from creating a crash-limited number of clones of themselves, exhausting resources, or other such anti-social behaviour. – Apparently you can't define a legal Telescript Place which holds visiting Agents to ransom unless you can circumvent security features and hack the Interpreter code!)

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Specialized JVM: NOMADS/Aroma • NOMADS/Aroma is a Java based agent system with strong mobility support, developed at Boeing and University of West Florida. • The standard Java JVM does not allow explicit execution state capture, thus we can not implement hard mobility. • There are several solutions: – Modify (patch) the Sun JVM » Difficulty because of the native thread usage. – Implement a new JVM – Use preprocessors and a standard JVM.

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NOMADS • Is composed of two parts: the agent execution environment (called Oasis) and the AromaVM. This provides two key enhancements: • Strong mobility: the ability to capture and transfer the full execution state. • Safe execution: the ability to control the resources consumed by the agents thereby facilitating guarantees of quality of service and protecting against denial of service attacks. • These features, however come with a performance penalty.

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