FIPA ACL (Agent Communication Language)

A FIPA ACL message is a structured object with a set of communicative acts and parameters. The core structure includes:

• Performative: The type of communicative act (e.g., inform, request, cfp).
• Sender & Receiver: Identifiers for the participating agents.
• Content: The propositional content of the message.
• Language & Ontology: Specifies the format (e.g., SL, FIPA-SL) and shared vocabulary for interpreting the content.
• Protocol & Conversation-id: Links the message to a specific interaction protocol and conversation thread.
• Reply-with & In-reply-to: Manages message correlation for asynchronous dialogues.

These are the core performatives defining the intention behind a message. They are based on speech act theory. Key acts include:

• Inform: Assert a proposition believed to be true.
• Request: Ask another agent to perform an action.
• Call for Proposal (cfp): Initiate a contract-net negotiation.
• Propose: Submit a bid in response to a cfp.
• Accept/Reject Proposal: Respond to a propose act.
• Query-ref: Ask for the value of a referential expression.
• Subscribe: Request ongoing notifications about a condition.
• Cancel: Retract a previous request before it is completed.

The Semantic Language (SL) is FIPA's recommended content language for expressing the logical propositions within a message. It is a modal logic that can represent:

• Beliefs: B(i, p) means agent i believes proposition p.
• Desires/Goals: D(i, p) means agent i desires p.
• Uncertainty: U(i, p) means agent i is uncertain about p.
• Actions: done(a) or agent(i, a) to represent performed actions.
• Temporal Operators: feasible, optional. This formal semantics allows agents to reason about each other's mental states, enabling sophisticated interactions like persuasion or conditional requests.

These are pre-defined patterns of message exchange that structure common agent dialogues. They provide a deterministic framework for negotiations and coordination. Standard FIPA protocols include:

• FIPA-Request: A simple request/agreement/refusal sequence.
• FIPA-Query: For asking questions and receiving answers.
• FIPA-Contract-Net: A canonical protocol for decentralized task allocation (manager, cfp, bids, award).
• FIPA-Iterated-Contract-Net: An extension for multi-round negotiations.
• FIPA-Auction (English & Dutch): Protocols for competitive resource allocation.
• FIPA-Brokering: An agent acts as an intermediary between a requester and a provider.
• FIPA-Subscribe: For setting up persistent notification streams.

The message envelope is a separate wrapper containing meta-information required for physical message transport between agent platforms, independent of the ACL content. It includes:

• To/From: Physical transport addresses.
• Intended-Receiver: The logical agent identifier.
• Encoding & Date: Technical details for payload interpretation.
• Acl-representation: Specifies the format of the enclosed ACL message (e.g., XML, string).
• Transport-specific parameters. This separation allows FIPA ACL to be transport-agnostic, operable over HTTP, IIOP, or other middleware, ensuring interoperability across heterogeneous distributed systems.

FIPA specifies supporting services for a functional multi-agent system. These are not part of ACL itself but are essential for its operation:

• Agent Management System (AMS): The white pages service. It is responsible for agent lifecycle (create, suspend, resume, kill) and maintains a directory of agent identifiers (AID). Every agent must register with the AMS.
• Directory Facilitator (DF): The yellow pages service. Agents register their services and capabilities here. It allows for service discovery through keyword or ontology-based search, enabling dynamic formation of agent communities.
• Message Transport Service (MTS): The abstract layer responsible for delivering envelopes between agents, often implemented by the underlying agent platform (e.g., JADE).

A predefined conversation pattern that specifies the permissible sequences of ACL messages between agents to achieve a specific interaction goal. FIPA standardized numerous protocols including:

• Request: A simple query-response pattern.
• Query: A request for information.
• Subscribe: For event notification.
• Contract Net: For task allocation.
• Iterated Contract Net: For multi-round allocation.
• Auction Protocols: English, Dutch, and Vickrey. These protocols define the communicative acts (performatives) that can be sent at each state.

The core unit of an ACL message, defined by its performative (e.g., inform, request, cfp, propose). Each performative has precise semantics dictating the sender's intentions and the expected mental states of the participants. For example:

• inform: The sender believes a proposition is true and wants the receiver to also believe it.
• request: The sender wants the receiver to perform an action.
• cfp: The sender is calling for proposals to perform a task. The semantics are grounded in a formal theory of agency, often based on Belief-Desire-Intention (BDI) models.

The formal language used within the :content parameter of an ACL message to express the proposition or action being communicated. FIPA ACL is content-language independent. Common content languages used with ACL include:

• FIPA SL (Semantic Language): A formal logic-based language for expressing beliefs, desires, and actions.
• KIF (Knowledge Interchange Format): A logic-oriented language for knowledge representation.
• XML/RDF: For web-oriented, semantic web-compatible content.
• Prolog-like syntax: For simpler rule-based expressions. The content language must be agreed upon by communicating agents, typically specified in the message's :language parameter.

The infrastructure services defined by FIPA that enable agents to find and communicate with each other, which is a prerequisite for using ACL. Key components include:

• Agent Platform (AP): The runtime environment.
• Agent Management System (AMS): The white-page service that handles agent lifecycle (creation, deletion, authentication).
• Directory Facilitator (DF): The yellow-page service where agents register their services and search for others.
• Message Transport Service (MTS): Handles the physical delivery of ACL messages between agents, potentially across different platforms.

A formal, shared specification of the concepts, relationships, and rules within a specific domain of discourse. In FIPA ACL, the :ontology parameter of a message references the ontology used to interpret the terms in the :content. This ensures semantic interoperability; agents can agree on the meaning of terms like "price," "delivery date," or "task specification." Ontologies are typically expressed in languages like OWL (Web Ontology Language) or FIPA's own ontology service specification. Without a shared ontology, agents may exchange syntactically valid but semantically meaningless messages.