M.P. Singh - Agent Communication Languages: Rethinking the Principles

Alessandro Giusti

March, 28 2006

Agent Communication Languages

• Allow agents to communicate• Interoperability (key feature)

• Other key agent features• Autonomy• Heterogeneity

Sony

Philips

Microsoft

Reality check (1998)

Verbatim:“Theoretically, an ACL should let

heterogeneous agents communicate.

However, none currently do.”• No interoperability

Who to blame?

Philips

Microsoft

Thesis

• Blame current ACLs• Knowledge Query Management Language

(KQML):

based on wrong principles• France Telecom’s Arcol:

based on wrong principles• FIPA ACL:

based on wrong principles

A paradigm shift is needed

What principles?

Analysis of communication dimensions:

• Perspective

• Type of meaning

• Semantic / Pragmatic focus

• Context

• Coverage of communicative acts

1 - Perspective

• Private• Sender’s perspective• Receiver’s perspective

• Public• Multiagent system’s perspective

Private perspectives are approximations of the public perspective

1 - Perspective

• Public perspective is needed:• ACLs must be normative• Agents must be tested for compliance• The ACL must have a public perspective (or

compliance testing is not possible)

• KQML and Arcol: private perspective

2 - Type of meaning

• Personal• Meaning: intent or interpretation of receiver

or sender

• Conventional• Meaning: usage conventions

Language is a system of conventions

Different conventions need different communicative acts

2 - Type of meaning

• Conventional meaning is needed

• KQML and Arcol: personal meaning• Different communicative acts do not capture

different conventions

Dialects

• KQML failed because many dialects arose;

• Blame private perspective and personal meaning:• Idiolects

"When I use a word," Humpty Dumpty said, in rather a scornful tone, "it means just what I choose it to mean -- neither more nor less.“

Lewis Carroll, “Through the Looking-Glass, and What Alice Found There” (1871)

3 - Semantics versus pragmatics

Meaning =Semantics + Pragmatics

• Semantics• what symbols denote

• Pragmatics• how syntactic symbols are interpreted and

used• involves mental states and the environment• constrain how agents interact

3 - Semantics versus pragmatics

• Semantics-focused language is needed

• Pragmatics require fully-cooperative agents

• Pragmatics fail where sincerity cannot be taken for granted

• KQML and Arcol: Pragmatics-focused languages

4 - Context

Communication context: needed for understanding.

• Fixed context

• Flexible context

Goal: flexible context

5 - Coverage of communicative acts

• Seven categories:• Assertives• Directives• Commissives• Permissives• Prohibitives• Declaratives• Expressives

• Limited coverage vs Full coverage• Full coverage is needed

• KQML and Arcol have limited coverage

Opposing paradigms

• Mental Agency• Focus on mental state (e.g. BDI)• Assumes intentional stance• How to determine the mental state of

agents?• Introspection: unsatisfactory or impossible• “Mental state” is an abstract concept: only

the agent designer warrants compliance.

• Social Agency• Focus on agent behavior (external)• “Social creatures” (sic)• Compliance : obey conventions in society

(self-evident)

Autonomy

Design autonomy: agent designer’s freedom:• Promotes heterogeneity and

applications• KQML and Arcol require that agents

have BDI-based mental states

Execution autonomy: agent’s freedom• Arcol assumes sincere, cooperative,

benevolent agents• KQML is less strict

Proposed solution

• Social agency• Different from traditional ACLs• Goals:

• Public perspective• Conventional meaning• Semantics over pragmatics• Flexible context• Full communicative acts coverage

Protocols

• Agents play different roles

• Roles• Define commitments/obligations

• Restrictions on behavior and communication

• Agents can manipulate/cancel commitments Metacommitments (avoid chaos)

• Protocol• Set of commitments

• Testability without introspection; closed-source friendly.

• Autonomy• Everything is allowed as soon as commitments are met

• Context is society (“Social context”)• Context is better known and agreed on better communication

Dialects in societies

• Agent societies are free from idiolects• No private perspective nor personal

meaning

• Dialects good• Allow “context sensitivity” and real-

world applications• Do not involve introspection• No risk of Humpty Dumptyism

Instantiation

• How is this translated into practice?• No clear answer• A purely behavior-based approach is not viable –

too limiting.• The purely-mentalist approach has been criticized so

far

• Combine both solutions:• Define when a communicative act is satisfied

• Assertive: if the world matches what is described• Directive: the receiver acts to ensure success• Commissive: the sender acts to ensure success

• Coarse canonical set of objective definitions• Do not ascribe beliefs and intentions to agents

Comments / critique

• Rewrite:• BDI-based languages have drawbacks:

• Too strict• Require introspection for compliance testing• Limits autonomy• Requires full cooperation... but many of the critiques are not adequately justified.

• Behavior-Commitments based agencies sound good• Upon closer inspection, they have their limits as well: not

powerful enough.

• Proposed solution is a not-better defined mix between the two

Conclusion

• FIPA ACL is based on wrong principles...• every possible communication dimension is

wrong

• ... but after 8 years FIPA ACL is the standard.

• Some of the proposed concepts are intriguing, but they can not be easily translated into practice.