MobileAgents

8/7/2019 MobileAgents

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Mobile agents

EEL 5937 Multi Agent Systems

Lotzi Blni


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Mobile agents

Mobile agents are autonomous programs which movethough a network and maintain their identitythrough this move.

This is a stronger concept than code mobilitysuch as Java applets, or client-side Javascript.

Many agent systems were implemented with supportfor mobility. And for many researchers, agents == mobile agents


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Motivation for mobility(contd)

Mobile agents can provide better support formobile clients. Reduction of network traffic Asynchronous interaction (good in case of intermittent

connection) Remote searching and filtering

Mobile agents facilitate semantic informationretrieval. Move one step above simple keyword based search.

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

Mobile agent based transactions avoid the need topreserve process state in clients and servers Instead, the process state is carried in an agent


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Motivations for mobility(contd)

Agent based transactions scale better than RPC-based transactions

Secure agent-based transactions have loweroverhead than secure RPC.

Mobile agents allow users to personalize serverbehaviour.

Agents enable semantic routing.

Not all these arguments are valid.


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Counter arguments andanswers

Most counter arguments are based on the factthat 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 insome (ad-hoc) manner without mobile agents, amobile agent framework addresses them all ofthem 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


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Mobile agents without codemobility

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 atany 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 Magicaround 1994-95 Highly influential, without being highly successful

Interpreted language, which runs on a Telescriptengine. The company implemented engines running on PDAs, PCs 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 (contd)

The basic network configuration is to run aTelescript Engine on each node of the network.

A network of Telescript Engines provides ahomogenous environment on which to builddistributed systems.

Basic class: Process. Telescript supportspreemptive, prioritized multi-tasking of Processobjects. A Process instance can be thought of asan object with a life of its own.

A Place object represents a virtual space in whichother objects can interwork (through localcommunication). Each Telescript Engine can supporta number of places.


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Telescript (contd)

An Agent object is a Process object which can migratebetween Places. An agent may move between Places on thesame Engine, or between Places which exist on differentEngines. 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 andrecover object state information.

The Telescript world is divided into "regions". Each Engineuses a "regions" database to route migrating Agents. Placesand 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 theAgent and the entity responsible for it. Theseattributes may be used for authentication.Telescript objects also have a "permit" attributewhich may be used to limit the amount of resourceswhich they may consume (e.g. a Place may ask anAgent to pay it 30 "Teleclicks" before granting itaccess to some resource).

A secure "permits" feature is crucial to stopAgents from creating a crash-limited number of

clones of themselves, exhausting resources, orother such anti-social behaviour. Apparently you can't define a legal Telescript Place which holds

visiting Agents to ransom unless you can circumvent securityfeatures and hack the Interpreter code!)


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Specialized JVM:NOMADS/Aroma

NOMADS/Aroma is a Java based agent systemwith strong mobility support, developed at Boeingand University of West Florida.

The standard Java JVM does not allow explicitexecution 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 executionenvironment (called Oasis) and the AromaVM. Thisprovides two key enhancements:

Strong mobility: the ability to capture and transferthe full execution state.

Safe execution: the ability to control the resourcesconsumed by the agents thereby facilitatingguarantees of quality of service and protectingagainst denial of service attacks.

These features, however come with a performancepenalty.

Documents

MobileAgents