Chapter 18:
Communication
Service-Oriented Computing: Semantics, Processes, Agents
– Munindar P. Singh and Michael N. Huhns, Wiley, 2005
Highlights of this Chapter

Agent Communication Languages






Chapter 18
Speech Act Theory
Semantics
Interaction Patterns
Combining ACLs with Web Services
Contract Net Protocol
Business Protocols
Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns
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Interaction and Communication

Interactions occur when agents exist and act in close
proximity:


Communications are the interactions that preserve
autonomy of all participants, but maybe realized
through physical actions that don’t:


resource contention, e.g., bumping into each other
Communications are inevitable when composing services.
Communications can be realized in several ways, e.g.,



Chapter 18
through shared memory
because of shared conventions
by messaging passing
Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns
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A Classification of Message Classifications

Structure-based (syntactic)


Meaning-based (semantic)


distinguish messages based on grammatical forms in natural
language
distinguish messages based on a notion of intrinsic meaning
prohibitive is different from directive, despite syntactic
similarity
Use-based (pragmatic)

Chapter 18
distinguish messages based on their roles in specific classes
of protocols
assertion is different from acknowledgment
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Speech Act Theory


Speech act theory, developed for natural language,
views communication as action
It considers three aspects of a message:

Locution, or how it is phrased, e.g., "It is hot here" or "Turn on
the air conditioner"

Illocution, or how it is meant by the sender or understood by
the receiver, e.g., a request to turn on the air conditioner or an
assertion about the temperature

Perlocution, or how it influences the recipient, e.g., turns on
the air conditioner, opens the window, ignores the speaker
Illocution is the core aspect
Here is an interesting example of speech act theory
Chapter 18
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Speech Act Theory Applied

Classifications of illocutions motivate message types,
but are typically designed for natural language



rely on NL syntax, e.g., they conflate directives and
prohibitives
Most research in speech act theory is about
determining the agents’ beliefs and intentions, e.g.,
how locutions map to illocutions
For Web Services,


Chapter 18
determining the message type is trivial, because it is
explicitly encoded
determining the agents’ beliefs and intentions is impossible,
because the internal details of the agents are not known
Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns
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ACL Semantics
What is the semantics of queries, requests, promises?
 Mentalist: each agent has a knowledge base that its
messages refer to. An agent promises something if it
intended to make that promise
 Public: semantics depends on laws, protocols, and
observable behavior
Evaluation: For open systems, public semantics is
appropriate, because a semantics without compliance
doesn’t make sense
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Informing
How can one agent tell another agent something?
 Send the information in a message (message
passing)
 Write the information in a location where the other
agent is likely to look (shared memory)
 Show or demonstrate to the other agent (teaching)
 Insert or program the information directly into the
other agent (master --> slave; controller -->
controllee; "brain surgery")
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Querying
How can one agent get information from another
agent?
 Ask the other agent a question (message passing)
 Read a location where the other agent is likely to
write something (shared memory)
 Observe the other agent (learning)
 Access the information directly from the other agent
("brain surgery")
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Syntax, Semantics, Pragmatics
For message passing
 Syntax: requires a common language to represent
information and queries, or languages that are
intertranslatable
 Semantics: requires a structured vocabulary and a
shared framework of knowledge-a shared ontology
 Pragmatics:



Chapter 18
knowing whom to communicate with and how to find them
knowing how to initiate and maintain an exchange
knowing the effect of the communication on the recipient
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Agent Communication Languages (ACL)
• KQML: Knowledge Query and Manipulation Language
• FIPA ACL
ACL
Agent
Chapter 18
ACL
Agent
Application
Program
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FIPA



Chapter 18
FIPA is the Foundation for Intelligent Physical
Agents, with website at www.fipa.org
Specifies standards for heterogeneous,
interoperating agent-based systems.
Concerned with agency as it relates to
1. Autonomy (goal-driven)
2. Communal integration; mostly
communication, but also cooperation.
Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns
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FIPA (contd.)


Chapter 18
Mission Statement: The promotion of technologies
and interoperability specifications that facilitate the
end-to-end interworking of intelligent agent systems
in modern commercial and industrial settings.
Goal: through a combination of speech acts,
predicate logic and public ontologies, FIPA can offer
standard ways of interpreting communication
between agents in a way that respects the intended
meaning of the communication. This is much more
ambitious than, for example, XML, which only aims to
standardize the syntactic structure of documents.
Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns
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Structure of Specifications
Chapter 18
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Agent Message Transport

Chapter 18
Agent Message Transport
(AMT) defines a message as an
envelope plus a body.
Together they handle
 Guidelines for various
transport protocols (e.g.,
IIOP, HTTP, WAP)
 Message envelope
representation (e.g., XML
for HTTP, bit-efficient for
WAP).
 FIPA ACL representions
(e.g., string encoding, XML
encoding, bit-efficient
encoding).
Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns
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Interaction Patterns
Query
Requester
Provider
Reply
Synchronous: a blocking query waits for an expected reply
Query
Handle
Requester
Next
Reply
Provider
Next
Reply
Provider maintains state; replies sent individually when requested
Subscribe
Reply
Requester
Reply
Provider
Reply
Asynchronous: a nonblocking subscribe; replies sent as available
Chapter 18
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Combining Agents with Traditional Web Services
Web Service
Client
SOAP
Request
SOAP
Response
Web
Service Agent
Gateway
ACL
Request
ACL
Inform
Agent
Chapter 18
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Planning and Control Needed
How to maintain global coherence without
explicit global control? Important aspects
include how to
 Determine shared goals
 Determine common tasks across services
 Avoid unnecessary conflicts
 Pool knowledge and evidence
Chapter 18
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Coordination
A property of interaction among a set of agents performing some
activity in a shared state. The degree of coordination is the
extent to which they

Avoid extraneous activity


reduce resource contention
avoid livelock
Avoid deadlock

Maintain safety conditions
Cooperation is coordination among nonantagonistic agents.
Typically,

Each agent must maintain a model of the other agents

Each agent must develop a model of future interactions

Chapter 18
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Goals for Multiagent Control
Develop techniques for




Chapter 18
Locating and allocating capabilities and
resources that are dispersed in the
environment
Predicting, avoiding, or resolving contentions
over capabilities and resources
Mediating among more agents, with more
heterogeneity and more complex interactions
Maintaining stability, coherence, and
effectiveness
Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns
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Control Challenges
The difficulty of control depends on some
major characteristics of the overall
system, including:



Chapter 18
agents comprising the system
problems being solved by the agents
individually and/or collectively
critical solution characteristics
Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns
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Control Challenges: Agents
Control gets harder as
agents become more:
quantity

complexity


heterogeneity
Chapter 18
numerous
complex individually (e.g.,
more versatile)
heterogeneous (in
capabilities,
representations,
languages)
Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns
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Control Challenges: Problems
Control gets harder as the
problems agents solve
become more
degree of
interaction
volatility


severity of
failure
Chapter 18

interrelated
rapidly changing, or pursued in
an uncertain and changing
world
unforgiving of control failures
(e.g., involving irreversible
actions)
Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns
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Control Challenges: Solutions
Control gets harder as
solutions to agent
problems are required to
be more
quality /
efficiency
robustness


low overhead
Chapter 18

efficient
robust to changing
circumstances
cheaper or faster to
develop individually and in
concert
Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns
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Technologies for Agent Control


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

Chapter 18
Broker-based

Matchmaker-based

Market-based; auctions 
BDI and commitment 
based
Decision theoretic

Workflow (procedural)
based

Standard operating
procedures
Learning / adaptive
Coordinated planning
Conventions / protocols
Stochastic or physicsbased
Organizations: teams and
coalitions
Constraint satisfaction/
optimization
Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns
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Dimensions of Control
Control how
capabilities
are
Possibly by
Why difficult? Why needed?
Allocated or
scheduled
Markets, CSP,
hierarchy,
planning,
teamwork
Many demands,
scarce supply
Avoid
contention, use
resources well
Located
Brokers,
matchmakers,
markets,
broadcast
Arrival/
departure rate,
variety, scale
Must find
resources to
employ them
Chapter 18
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Control how
capabilities
are
Possibly by
Why difficult? Why needed?
Not wasted
Brokermonitored
requests,
caching,
communication
Reacting to
prices,
replanning, goal
adjustment
Similar tasks
arising various
places
Efficiency
Number of
alternative
choices, self
interest
Avoid contention
Reprogram or
train agents,
evolve/spawn,
inject "friction"
Oscillations or
Adapt/grow with
chaos from
need
sporadic demand,
allocating
processes
Demanded
Supplied
Chapter 18
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Control how
capabilities
are
Possibly by
Described
Provide more or
less detail, lump
or differentiate
capabilities
Initially
allocated
Differentiated
Chapter 18
Why difficult? Why needed?
Rich space of
capabilities,
disagree on better
description,
propagating
descriptions
Organization or Detecting need,
role restructuring assessing
choices,
propagating
Description must
follow use
Prevention
Continuous
(maintain
operation, agent
consistency),
adaptation
response (enlarge
capability
language)
Differentiation
inevitable
Choice
constrains quality
of coordination
Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns
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Task Decomposition
Divide-and-conquer to reduce complexity:
smaller subtasks require less capable agents
and fewer resources
 Task decomposition must consider the
resources and capabilities of the agents, and
potential conflicts among tasks and agents
 The designer decides what operators to
decompose over
 The system decides among alternative
decompositions, if available
Chapter 18
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Task Decomposition Methods



Chapter 18
Inherent (free!): the representation of the
problem contains its decomposition, as in an
AND-OR graph
Composition designer (human does it):
decomposition is programmed during
implementation. (There are few principles for
automatically decomposing tasks)
Hierarchical planning (agents implementing
the services do it): decomposition again
depends heavily on task and operator
representation
Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns
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Task Decomposition Examples

Spatial decomposition: service is info source or decision point:
Agent 1
Agent 3
Agent 2

Functional decomposition: service offers specialized expertise:
Pediatrician
Neurologist
Internist
Cardiologist
Chapter 18
Psychologist
Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns
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Task Distribution Criteria






Chapter 18
Avoid overloading critical resources
Engage services on tasks where expertise matches
Make an agent with a wide view assign tasks to other
agents
Assign overlapping responsibilities to agents to
achieve coherence
Assign highly interdependent tasks to agents in
spatial or semantic proximity. This minimizes
communication and synchronization costs
Reassign tasks if necessary for completing urgent
tasks
Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns
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Task Distribution Mechanisms





Chapter 18
Market mechanisms: tasks are matched to agents by
generalized agreement or mutual selection
(analogous to pricing commodities)
Contract net: announce, bid, and award cycles
Multiagent planning: planning agents have the
responsibility for task assignment
Organizational structure: agents have fixed
responsibilities for particular tasks
Recursive allocation: responsible agent may further
decompose task and allocate the resultant subtasks
Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns
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The Contract Net Protocol
An important generic
protocol




Chapter 18
Manager announces tasks
via a (possibly selective)
multicast
Agents evaluate the
announcement. Some
submit bids
Manager awards a contract
to the most appropriate
agent
Manager and contractor
communicate privately as
necessary
Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns
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Task Announcement Message




Chapter 18
Eligibility specification: criteria that a node
must meet to be eligible to submit a bid
Task abstraction: a brief description of the
task to be executed
Bid specification: a description of the
expected format of the bid
Expiration time: a statement of the time
interval during which the task announcement
is valid
Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns
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Bid and Award Messages


Chapter 18
A bid consists of a node abstraction—a brief
specification of the agent’s capabilities that
are relevant to the task
An award consists of a task specification—the
complete specification of the task
Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns
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Applicability of Contract Net
The Contract Net is



a high-level communication protocol
a way of distributing tasks
a means of self-organization for a group of agents
Best used when



Chapter 18
the application has a well-defined hierarchy of tasks
the problem has a coarse-grained decomposition
the subtasks minimally interact with each other, but
cooperate when they do
Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns
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Commitment Protocols


Protocols enable open systems to be
constructed
Interaction protocols expressed in terms of




Participants’ commitments
Actions for performing operations on commitments
(to create and manipulate them)
Constraints on the above, e.g., captured in
temporal logic
Examples: escrow, payment, RosettaNet (107
request-response PIPs)
Chapter 18
Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns
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Example: NetBill Protocol
FSM Representation
C: rfq
M: offer
C: accept
M: goods
C: pay
Some variations:
 The merchant may start the
protocol by sending a quote
 The customer may send an
accept prior to offer
 The merchant may send the
goods prior to accept
These variations are not allowed by
the FSM
M: receipt
Chapter 18
Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns
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NetBill Enhanced by CMs
M: g o
C:
C:
Meanings:
ods
1. true
rfq
M: offer
acc
2. request
ept
3. offer
M: offer
M:
go
od
s
C: accept
M: goods
C:
4. Cmgoods  accept 
promiseReceipt
pa
y
5. goods  Ccpay 
promiseReceipt
6. goods  pay  Cmreceipt
C: pay
M: receipt
7. goods  pay  receipt
8. goods  promiseReceipt
9. accept
Final state: No open commitments remain
Chapter 18
Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns
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Message Patterns for Commitment
Operations
Ensure that information about commitment operations
flows to the right parties, to enable local decisions
x
y
z
create(x,c)
x
y
create(x,c)
delegate(x,z,c)
assign(x,y,z)
delegate(x,z,c)
discharge(x,c)
Chapter 18
z
assign(x,y,z)
discharge(x,c)
Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns
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Compliance with Protocols
In an open environment, agents are
contributed by different vendors and
serve different interests
 How can an application check if the
agents comply with specified
protocols?



Coordination aspects: traditional techniques
Commitment aspects: representations of
the agents’ commitments in temporal logic
Commitment protocols are specified in
terms of



Chapter 18
Main roles and sphere of commitment
Roles essential for coordination
Domain-specific propositions and actions
Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns
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Verifying Compliance with Commitment
Protocols
Specification



models based on potential causality
commitments based on branching-time TL
Run-time Verification




Chapter 18
respects design autonomy
uses TL model-checking
local verification based on observed
messages
Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns
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Run-Time Compliance Checking

An agent can keep track of




its pending commitments
commitments made by others that are not
satisfied
It uses this local model to see if a
commitment has been violated
An agent who benefits from a
commitment can always determine if it
was violated
Chapter 18
Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns
45
Fish-Market Sample Execution
Bidder B1
Auctioneer A
[0,0,0]
m1 “50”: [1,0,0]
[0,0,0]
[1,1,0]
m2 “50”: [2,0,0]
[3,2,0]
[4,2,2]
m4 “No”: [6,2,4]
[5,3,2]
m6 “40”: [6,2,2]
[8,4,4]
Chapter 18
[2,0,1]
m3 “No”: [1,2,0]
m5 “40”: [5,2,2]
[7,4,2]
Bidder B2
[0,0,0]
[6,2,3]
m7 “Yes”: [5,4,2]
m8 “No”: [6,2,4]
fish
money
Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns
46
Fish-Market Local Observations
Auctioneer A
start
[1,0,0]
Bidder B1
start
s(m1)
s(m1)
[1,0,0]
[2,0,0]
s(m2)
[1,1,0] r(m1)
s(m4)
[2,0,2]
[3,2,0]
[4,2,2]
[5,2,2]
r(m3)
[1,2,0] s(m3)
s(m5)
[5,2,2]
r(m4)
[5,3,2] r(m5)
s(m5)
[5,4,2]
[6,2,2]
s(m6)
[7,4,2]
r(m7)
s(m7)
end
end
Chapter 18
Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns
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Fish-Market Compliance




Chapter 18
Auctioneer can verify if the bidders
comply
An individual bidder cannot verify if the
auctioneer complies
If bidders pool their observations, then
they can verify if the auctioneer
complies
Asymmetry indicates need for third
party
Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns
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Policies and Structure

Spheres of commitment (SoComs)





abstract specifications of societies
made concrete prior to execution
Policies apply on performing social actions
Policies relate to the nesting of SoComs
Role conflicts can occur when agents play
multiple roles, e.g., because of nonunique
nesting
- Munindar Singh and Michael Huhns
Chapter 18 Summary
Chapter 18
Service-Oriented Computing: Semantics, Processes, Agents - Munindar Singh and Michael Huhns
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