Transcript SIF8072 Distributed Artificial Intelligence and

Lecture 8:
Mobile Agents
SIF8072
Distributed Artificial Intelligence
and
Intelligent Agents
http://www.idi.ntnu.no/~agent/
6 March 2003
Lecturer: Sobah Abbas Petersen
Email: [email protected]
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Lecture Outline
1.
What are mobile agents?
2.
Remote Procedure Calls vs. mobile agents
3.
Security
•
Threats, security of host, security of agent
4.
Typology of mobile agents
5.
Implementation of mobile agents
•
Telescript
•
TCL/TK
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References - Curriculum
•
Wooldridge: ”Introduction to MAS”,
–
•
Chapter 10, Section 10.4
Not in curriculum:
–
White, J. E., Mobile Agents, in Bradshaw, J. (ed.), Software
Agents, MIT Press, Cambridge, MA, 1997, p. 437-472.
–
D. M. Chess, C. G. Harrison, A. Kershenbaum. Mobile Agents: Are
they a good idea? Research Report, IBM Research Division, T. J.
Watson Research Center, 1995, 21 pages.
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What are Mobile Agents
• Agents that are capable of transmitting themselves – their programs
and their state – across a computer network, and recommencing
execution at a remote site.
• The program chooses when and where to migrate.
• It can suspend its execution at an arbitrary point, transport itself to
another machine and resume execution.
 Moving programs while they run!
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Current Approach Remote Procedure Calls
Client
Network
PC
Service
Server
•Enables one computer to call procedures in another.
•The two computers agree in advance upon a protocol:
•The effects of each remotely accessible procedure and the types of its arguments
and results.
•Each interaction entails two acts of communication - request & acknowledge
ongoing interaction requires ongoing communication!
e.g.: From a file server, delete all files that are atleast two months old.
RPC: n files => 2(n+1) messages
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New Approach Remote Programming
Local procedure Call
Client
Network
Agent
Client
PC
•
Service
Server
One computer not only calls procedures on another computer, but also provides the
procedures.
•
Each message contains the procedure + its arguments.
•
The two computers agree in advance upon a language:
– instructions and the types of data that are allowed.
•
A user computer and a server can interact without using the network once the agent
is transported
 ongoing interaction does not require ongoing communication!
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Advantages of RP over RPC
• Tactical
– Performance - due to less message passing over the network.
– Less connection time - need network connection to transport the agent.
– Computer delegates a task to the agent.
• Strategic
– Customisation - agents let manufacturers of user s/w extend the functionalities of
the server s/w.
– In a RPC application, the server component needs to be statically installed by the
user. In RP, they are dynamically installed by the application itself - each is an
agent.
– New RPC-based applications - business decisions by the provider. New RP-based
applications - buying decision by the user.
– A public network becomes like a platform.
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Why Mobile Agents
 Mobile agent = procedure + its state.
• Efficient use of network resources.
• Low-bandwidth networks.
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Basic Mobile Agent Environment
• A mobile agent environment is a software system which
is distributed over a network of heterogeneous computers.
• Its primary task is to provide an environment in which
mobile agents can execute.
• It implements the majority of models which appear in the
mobile agent definition.
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What is security
”If I take a letter, lock it in a safe, hide the safe somewhere in New York,
then tell you to read the letter, that’s not security. That’s obscurity. On
the other hand, if I take the letter and lock it in a safe and then give
you the safe along with the design specifications of the safe and a
hundred identical safes with their combinations so that you and the
world’s best safecrackers can study the locking mechanisms – and
you still can’t open the safe and read the letter – that’s security.”
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Security Model
• General security requirements:
• Secrecy
• requires that the information in a computer system be
accessible for reading by authorised parties only.
• Integrity
• requires that the computer system assets can be modified by
authorised parties only.
• Availability
• requires that the computer system assets be available to
authorised parties.
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Types of Threats 1
• Interruption: an asset of
the system is destroyed
(threat of availability).
Information
source
Information
source
• Interception:
unauthorised party gains
access to an asset (threat
of secrecy)
Information
source
Information
destination
Information
destination
Information
destination
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Types of Threats 2
• Modification: unauthorised
party tampers with an asset
(threat of integrity)
• Fabrication: unauthorised
party inserts counterfeit
object into the system (threat
of integrity)
Information
source
Information
source
Information
destination
Information
destination
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Assets in a Computer System
Software
Data
Communication
lines and
networks
Security
Aspect
Security violations
Availability
a key threat, easy to delete software
Integrity
computer virus and related attacks
Secrecy
unauthorised copying
Availability
destruction of data files
Integrity
files are modified
Secrecy
unauthorised reading
Availability
Integrity &
active threats such as message stream
modification, denial of service.
Secrecy
passive threats such as release of
message contents, traffic analysis.
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Design Principles for Security
• Least privilege – (need-to-know) only the smallest set of privileges to
complete the job; the access rights should be acquired by explicit
permission only.
• Economy of mechanism- security mechanisms should be as small as
possible; an integral part of the design.
• Acceptability - security mechanisms should not interfere unduly with
the work of the users.
• Complete mediation – every access must be checked against the
access control information.
• Open design – mechanisms can be reviewed by many experts, and
users must have high confidence in them.
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Main Intruders
• Hackers
• Viruses
• Fraudulent users
• Incompetent users
• Technology by itself cannot solve the security problem.
• Technology for security must be complemented by an
awareness of security issues and disciplined application
of techniques.
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What does security do? 1
• Security systems attempt to get you reliable answers to the
following types of questions:
• Who is accessing our information system? (Identification)
• Can I be sure who made the request? (Verification)
• Can I be sure that this message has not been modified since it was
sent? (Integrity)
• Is this user entitled to make use of this resource? (Access control)
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What does security do? 2
• Will this program cause damage or disruption? (Viruses)
• Can I safely send this message on a public network?
(Privacy)
• Can I be sure that this user will actually pay for the service
s/he is requesting? (Nonrepudiation)
• Can I be sure that this server will do what I want?
(reputation)
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Security in Using Agents 1
• Some general issues in using agents:
• Delegation: you are delegating to the agent some of
your authority. This means that agents are doing things
that you cannot always see.
• Mobility: they may be doing it on the other side of the
planet. Or, an agent from the other side of the planet
may be doing it on your server.
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Security in Using Agents 2
• Some general issues in using agents, contd.:
• Viruses: agents share many characteristics with
viruses. In creating an environment for agents, there is
the additional risk that we expose weaknesses that may
enable viruses to breed.
• Trust: humans have classified their co-workers into
those who are reliable and those who are not.
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Mobile Agents and Viruses
• It’s impossible, in principle, to verify with complete certainty that an
arbitrary program is not a virus.
• In practice, the problem of writing a program that can verify the
correct behaviour of another program is unsolved.
• It’s difficult to define the necessary and sufficient tests that an agent
must pass in order to determine its intentions.
• Some precautions:
• Restriction of access to critical resources.
• Restriction on altering other programs.
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Delegation
• The purpose of an agent is to perform some tasks that would otherwise
be performed by its user.
• The agent may need many, if not all, of the access rights of the user.
• In a security environment, this can be readily achieved by passing the
copy of the user’s certificate to the agent.
• In this regard, the agent is indistinguishable from any other applications
employed by the user.
• However, the certificates are valid for a finite period, defined by the
security administrators.
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Security for Hosts 1
• Limiting delegation:
• Give the agent and the user separate identities
• Secure co-processors: have a physically separate processor on which the
agent is run, execute the agent in a ”padded cell”
• Allow the agent to interact with the system environment only in a
language with limited expressiveness (e.g. ACL)
• Limiting resource consumption:
• Limit the amount of each resource that an agent is permitted to consume
• Limit the amount of e.g. money and processor time an agent can access
(e.g. Telescript)
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Security for Hosts 2
• Unix-like access rights on hosts.
• Safe libraries for access to filestore, process space, etc.
• Some agent languages allow security properties of an
agent to be verified on receipt.
• Host must handle crashed programs cleanly – what do you
tell an owner when their agent crashes?
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Security for Agents 1
• We need to protect mobile agents from malicious hosts
because:
• Agents have a right to privacy!
• We often do not want to send our programs, as to do so might
enable the recepient to determine its purpose, and hence our intent.
• The agent might be modified (sabotaged) in some way, without the
owner’s knowledge or approval.
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Security for Agents 2
• Some possibilities for protection:
• Data integrity - an agent can be protected in transit by using
conventional encryption techniques, (e.g. PGP).
• In order to ensure that an agent is not tampered with, it is possible
to use digital watermarks.
• Origin authentication – certification.
• Access itenary control – restriction on visiting some environments.
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Proposed Advantages of Mobile
Agents
• Efficiency – mobile agents consume less network resources
• Asynchronous autonomous interaction
• Interaction with real-time entities
• Support for heterogeneous environment
• Support for electronic commerce
• Convenient development paradigm
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General Requirements to Mobile
Agent Environments
• Expressiveness as a programming language
• Ability to execute remotely or to transport state
• Support for agent communication language
• Security support
• Management support
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Typology of Mobile Agents 1
•
Mobile agents can be divided into atleat 3 types:
1. Autonomous
2. On-demand
3. ”Active-mail” type
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Typology 2
Autonomous Mobile Agents
•
Autonomous mobile means that the agents are able to
decide for themselves where to go, when and what to
do when they get there (subject to certain resource
constraints, e.g. how much emoney they can spend).
•
Such agents are generally programmed in a special
language that provides a go instruction (e.g. Telescript).
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Typology 3
On-demand Mobility
•
The idea here is that a host is only required to execute an agent when it
explicitly demands the agent.
•
The best known example of such functionality is that provided by the
JAVA language, as embedded within HTML.
•
A user with a JAVA-compatible browser can request HTML pages that
contain applets.
•
These applets are downloaded along with all other images, text, forms,
etc. and once downloaded, are executed on the user’s machine.
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Typology 4
Active-mail Agents
•
The idea here is to ”piggy-back” agent programs onto
mail.
•
The best known example of this work is the mime
extension to email.
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Mobile Agent Implementation
•
Java
•
Contains no mobile agents. It’s simply an enabling technology for
mobile agents. Most important are Remote Method Invocation and
Object Serialisation.
•
•
Telescript
•
•
e.g. Aglets
Language-based environment for constructing MAS.
Tcl/TK and other scripting languages
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Telescript 1
•
Telescript was a language-based environment for constructing mobile
agent systems.
•
2 key concepts: agents and places.
•
Telescript Technology is focussed towards an electronic marketplace,
where agents are the providers and consumers of goods in the electronic
market place.
•
Agents are mobile – they are able to move from one place to another, in
which case their program and state are encided and transmitted across
the network to another place, where execution recommences.
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Telescript 2 - Concepts
• Implements the concepts places, agents, travel, go,
meeting, connections, authorities and permits.
• The network is a collection of places. The
application consists of agents, each occupying a
place.
• Agents can travel from one place to another to
have a meeting with another agents.
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Telescript 3 - Concepts
PC
Shopping
agent
Network
Directory
Tickets
Florist
Shopping Centre
• Places: Shopping centre, with a Directory service, a ticket office and a
florist.
• Agents: Agents reside in Places and can move from one place to
another. e.g. The shopping agent can go to the florist.
• Travel: Lets an agent obtain a service offered remotely and return to
its starting place.
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Telescript 4 - Concepts
meeting
go
PC
Shopping
agent
Network
Directory
Tickets
Florist
Shopping Centre
• Travel is achieved by executing go.
• Go requires a ticket - specifies the agent’s destination and the terms of
the trip.
• Meetings let agents in the same computer call one another’s procedures.
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Telescript 5 - Concepts
connection
PC
Shopping
agent
Network
Directory
Tickets
Florist
Shopping Centre
• Connections let two agents on different computers communicate.
• A connection requires a target - specifies the distance agent and the
terms of the connection.
• One agent or place can discern the authority of another.
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Telescript 6 - Concepts
authority
PC
Shopping
agent
Network
Directory
Tickets
Florist
Shopping Centre
• Authority is associated with regions - verifies the authority when the
agent travels from one region to another.
• Permits grant capabilities. They let authorities limit what agents and
places can do. 2 kinds:
– Grant the right to execute a certain instruction. e.g. create another agent.
– Grant the right to use a certain resource in a certain amount - allowance.
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Tcl/TK and Scripting Languages 1
•
TCL was primarily intended as a standard command language. But
every time a new application is developed, a new command language
must be as well. TCL provides the facilities to easily implement your
own command language.
•
It provides facilities for making GUI features such as buttons, labels,
text and graphic windows.
•
It provides powerful facilities for interprocess communication, via the
exchange of Tcl scripts.
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Tcl/TK 2
•
TCL is an interpreted language.
•
It is extendable: it provides a core set of primitives
implemented in C/C++ and allows the user to build on
these, as required.
•
It can be embedded – the interpreter itself is available as
C++ code, which can be embedded in an application, and
can itself be extended.
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Tcl/TK 2
•
So, where does the idea of an agent come in?
•
It’s easy to build operations where Tcl scripts are exchanged across a
network, and executed on remote machines.
•
A key issue is safety. You don’t want to provide someone elses script
with the full access rights to your computer that an ordinary scripting
language provides.
•
•
Safe TCL: provides mechanisms for limiting the access provided to a script.
It was not intended as an agent programming environment.
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Summary
•
Efficient use of bandwidth!
•
Main issue is that of security, both for the host and the
mobile agent.
•
Mobile agents can be implemented in Java, agent
programming environment (e.g. Telescript) and some
scripting languages (e.g. Tcl/TK).
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Next Lecture:
Agent Architectures
•
Wooldridge: ”Introduction to MAS”,
–
Chapters 4 and 5
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