Mobile Software Agents Tomasz Müldner

Mobile Software AgentsTomasz Müldner

October 2, 1998

Introduction: History

• research on agents was originated by J. McCarthy in the mid-1950’s

• the term agent was coined by O.G. Selfridge

Introduction: According to some...

Agent

is an intelligent robot, or “humanoid”, that has emotions, feelings and perceptions, and therefore is concerned with cognitive science, speech acts, etc.

“...asking the question of what an agent is to a DAI researcher is as embarrassing as the question of what intelligence is for an AI researcher...”

Carl Hewitt

Introduction:DAI

Introduction: contents of this talk

• concentrate on mobile agents• discuss:

– communication

– security

– MALs

Agents - what are they?

a tool to perform client-server computing by transmitting running programs between clients and servers; (White 1994)

Agents - what are they?

server

find and filter information

customize views of information (e.g. email)

automate work (respond to events, such as a new version)

Agents - what can they do?

make recommendations and perform corporate tasks; e.g. scheduling of meetings

• execute diagnostics, e.g. in networks

• rapid (re)deployment of applications

• active network load balancing

Agents - what can they do?

distributed OO adaptive learning systems AI, expert systems, genetic algorithms electronic commerce

• collaborative environment

• mobile (nomadic) computing

Agents - where are they used?

Agents - main characteristics

autonomous execution (life); have control over their own actions and may operate without the direct intervention of humans

intelligent (perform domain oriented reasoning)


perceive their environment

adaptive (they learn)

mobile (they move)

persistent (they have their own idea as to how to accomplish a task)


goal oriented (they realize a set of goals)

reactive (or reflexive); they perceive environment and timely and accurately respond to changes that occur in it

• active, or proactive: act to accomplish goals (take initiative not only respond to the environment)


• An intelligent agent has some artificial intelligence; for example based on a set of facts and inference rules

• learning agents are adaptive; they can learn themselves about a subject in question by statistically matching subjects of interest with particular people

Intelligent Agents

Basic Definitions

• An agent system can create, interpret, execute, transfer and terminate agents

• A host can contain several agent systems; each is uniquely identified by its name and address.

Basic Definitions

• Both, an agent and an agent system have an authority; a person or organization for whom they act

• An agent executes in a context, called the place; there may be one or more places within an agent system

Basic Definitions

• agents have locations (names of their current places)

• agents have names (the agent’s authority and identity; a unique value within the scope of the authority)

Basic Definitions

Basic Definitions

There are two kinds of agents:

• stationary agent is permanently attached to a place; often resource managers, server programs or search engines

• mobile agent can move from one place to another

Mobility is not new:

• submitting batch jobs on mainframes

• perform distributed, real-time processing by executing scripts on networks of mini-computers

• databases have been using stored procedures.

Basic Definitions

Agents and DOOP

L. Lamport:

A distributed system is one in which I cannot get something done because a machine I have never heard of is down.

Agents and DOOP: DOOP

•distribute applications and use a number of network nodes, rather than a single node

•communicate; for example by •messaging (asynchronous)•RPC (synchronous)

•CORBA supports language and system independence

Agents and DOOP: Efficiency

Which is better

•to send data to the program

• to send a program to the source of data (move the code closer to data)

Agents and DOOP: Robustness

Agents are better in their ability to recover from server breakdowns or unavailability

(disconnected operations)

Agents and DOOP: Flexibility

•Mobile agents are particularly useful for rapid deployment of applications and dynamic updates of software; a code server can provide required code (this is also called code on demand).

•agents can be used for dynamic extensions of server services.

Agents and DOOP: Design

Designing a client/server architecture requires making all decisions about the communication between the server(s) and the client(s). These decisions are tightly coupled with the underlying problem of specific design and are very difficult or even impossible to change

• supports rapid deployment of applications, by using a “thin” bootstrap program and code-on-demand approach– objects serialization– local dynamic linking – remote linking that would fetch the code to be

linked from a remote site– reflection

Agents and DOOP: Java

• code servers

• applets - no need for installation


Java does not support mobility


Mobile Agent Languages

• An executing unit, EU is a single process (or thread) image of execution. EU is always considered in the context of a place, which contains components; either EUs, or resources, such as files.

• A MAL is a language designed for distributed systems, which supports EUs migrating between various places.

Mobile Agent Languages: Definitions

• An EU consists of – a static code segment – a program state:

• data space containing accessible resources

• execution state containing system information such as program counter and return address.

• The EU has a distributed state if its data space exists in more than one place.


• For an agent to be moved, first it has to be suspended. An entry point is a point where execution of a suspended agent is resumed

• Two kinds of resumption:– standard resumption (after “go”)– itinerary; one or more entry points may be

explicitly specified.


• strong mobility means that the entire code and execution state of EUs can be moved

• Strong mobility combined with standard resumption == strong MAL (implies that it must be possible to save the state of the execution, and later, to restore this state)


• Strong MALs:– Java with a modified JVM– Tcl with modified interpreter– Scheme (no modifications)

• Weak MALs:– aglets


Mobile Agent Languages: Dynamic Linking

• Name resolution must be able to bind names to both local and remote entities

• remote code dynamic linking: the code downloaded from a remote site and linked with an EU

• local resource dynamic linking: arriving EU links with resources available in this place; e.g. link with libraries.

Mobile Agent Languages: Dynamic Linking

• the standard class loader provides local resource linking and the user defined class loader can support remote code linking

• The programmer has a choice of– fully resolving the class, i.e. load the code

closure – partially resolving the class, i.e. postpone

resolving dependent classes until later time.

Mobile Agent Languages: Dynamic Linking in Java

Mobile Agent Languages: Dynamic Linking in MALs

• library site provides precompiled code that can be picked up by an agent to use at other sites

• we consider four kinds of dynamic linking. For each kind, we consider an example of a search on a list of sites, using a search procedure.

Mobile Agent Languages: Local only Dynamic Linking

All the bindings are voided when a program moves.

For our example, search is rebound at every site.

Mobile Agent Languages: Code-with-a-reference-is-sticky

Dynamic Linking

A binding is retained as long as there is at least one reference; otherwise it is garbage-collected.

In our example, the first site provides binding which will be retained for remaining sites.

The user specifies the initial binding which is then retained (as long as there is a reference).

In our example, the user can specify which version of search should be used for the traversal.

Mobile Agent Languages: User-specified-sticky links

Dynamic Linking

Gives the programmer complete control over linking.

In our example, the user can specify which version of search should be used for the traversal.

Mobile Agent Languages: User-specified Dynamic Linking

Mobile Agent Languages: Communication

• agent to service agent; this is typically a client/ server type of interaction (e.g. based on RPC)

• agent to agent; this a peer-to-peer type of interaction and could be supported by messaging

• agent to group; this is a group communication (e.g. based on an observer pattern)

• user to agent interaction; this a standard human-computer interaction, HCI.

Mobile Agent Languages: Types of Communication

• agent to service agent and agent to agent are session oriented

Mobile Agent Languages: Types of Communication

Security

• a security policy is a set of guidelines describing whether various actions are allowed or not(may also include royalties)

• a security policy is static if its guidelines do not depend on external conditions; otherwise it is dynamic.

• a security policy may include credentials, or level of trust.

Security:Definitions

Java Security Manager implements a static security policy, which controls access to resources such as file I/O, network access, and others. There is no provision to limit access to other resources such as CPU cycles.

Security:Definitions

In general, we need to:• protect agent hosts from agents• agents from agent hosts• one agent from another• one agent host from another• a group of hosts• the communication between agent hosts.

Security: Protection

• Doable, but two agents operating on the same host share the same interpreter and they share memory (no hardware faults)

• Software Fault Isolation inserts a checking code before each unsafe instruction

• Sandboxing before each unsafe instruction inserts a code which sets the high order bits of the target address to the correct segment identifier

Security: Protecting one agent from another

• cryptography can be used to authenticate credentials

• a credential means that the receiver will trust the sender

• access-level monitoring and control uses a security manager which maintains a list of allowed activities (perhaps associated with fees)- can be misleading

Security: Protecting agent hosts from

agents

• code verification by the host may try to examine the code of the agent (does not help if the agent is self-modifiable)

• the host can apply various limitation techniques, e.g. limit the time, or record all agent activities.


agents

• A static byte code verifier checks for the right magic number in the class file, and performs data flow analysis on each method to test for things such as branches must be within the bounds of the code, or there is no attempt to access variables which are not in the scope

• JVM performs various run-time checks.


agents

• Trail obscuring means that the agent constantly modifies its image so it can avoid tracing by hosts

• Code obfuscation means that the agent is sent together with a kind of interpreter, so the host doesn’t really see the code of this agent.


agents

• a host can steal information (normal routing)

• a host can mutate an agent (e.g. rerouting)

• since the host agent has access to an agent, it is not possible to protect an agent from the agent host (unless a specialized hardware is used)

Security: Protecting agent from agents

hosts

• visit only trusted sites and from there use safer mechanisms, such as RPC or stateless agents

• maintain reliable data that can be used (after the fact) to determine whether the agent has been tampered with

• divide an agent into components, encrypted each component (when traveling through untrusted sites); update only while on trusted sites


hosts

• maintain a safe migration history and use it against rerouting attacks; or keep audit logs

• there is new research on encrypting programs which are equivalent to the original programs, are directly executable and produce encrypted output.


hosts

Network Awareness

Network Awareness:Requirements

• awareness - an ability to monitor resources

• agility - an ability to react to changes in resources

• authority - an ability to control the way resources are used on their behalf by support code.


Resource awareness can be represented by:

• on-demand monitoring

• continuous monitoring (specific filters should be used to avoid jitters).


Authority can

• give a complete control over resources, with explicit authorization for every use (like in applets)

• consider a resource violation as an asynchronous event and associate a handler with every restriction on resource.

Criticism

Criticism

Sheinderman:

• “There is a growing danger that agents will be a deception and an empty promise.”

• “… machines are not people, nor can they ever become so ...”

• “… For me, computers have no more intelligence that a wooden pencil ...”

Mobile Software Agents Tomasz Müldner

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