Transcript on mobile agents

Mobile Software Agents
Tomasz 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.
Introduction:DAI
“...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: contents of this talk
• concentrate on mobile agents
• discuss:
– communication
– security
– MALs
Agents - what are they?
Agents - what are they?
a tool to perform client-server computing by
transmitting running programs between
clients and servers; (White 1994)
server
Agents - what can they do?
 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 - where are they used?




•
•
distributed OO
adaptive learning systems
AI, expert systems, genetic algorithms
electronic commerce
collaborative environment
mobile (nomadic) computing
Agents - main characteristics
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)
Agents - main characteristics
 perceive their environment
 adaptive (they learn)
 mobile (they move)
 persistent (they have their own idea as to
how to accomplish a task)
Agents - main characteristics
 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)
Intelligent Agents
• 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
Basic Definitions
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
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
Basic Definitions
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.
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
Agents and DOOP: Java
• 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
Agents and DOOP: Java
Java does not support mobility
Mobile Agent Languages
Mobile Agent Languages:
Definitions
• 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.
Mobile Agent Languages:
Definitions
• 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.
Mobile Agent Languages:
Definitions
• 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)
Mobile Agent Languages:
Definitions
• Strong MALs:
– Java with a modified JVM
– Tcl with modified interpreter
– Scheme (no modifications)
• Weak MALs:
– aglets
Mobile Agent Languages:
Dynamic Linking
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 in Java
• 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 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 garbagecollected.
In our example, the first site provides
binding which will be retained for
remaining sites.
Mobile Agent Languages:
User-specified-sticky links
Dynamic Linking
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 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:
Communication
Mobile Agent Languages:
Types of 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 humancomputer interaction, HCI.
Mobile Agent Languages:
Types of Communication
• agent to service agent and agent to agent are
session oriented
Security
Security:
Definitions
• 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:
Protection
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:
Protecting one agent from another
• 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 agent hosts from agents
• 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.
Security:
Protecting agent hosts from 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.
Security:
Protecting agent hosts from 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.
Security:
Protecting agent from agents hosts
• 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
Security:
Protecting agent from agents 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.
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.
Network Awareness:
Requirements
Resource awareness can be represented by:
• on-demand monitoring
• continuous monitoring (specific filters should
be used to avoid jitters).
Network Awareness:
Requirements
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 ...”