Artiﬁcial Intelligence and Law (2006) 13: 133–151
DOI 10.1007/s10506-006-9005-1

 Springer 2006

Towards a multi-agent system for regulated
information exchange in crime investigations
PIETER DIJKSTRA1, FLORIS BEX1, HENRY PRAKKEN1,2
and KEES DE VEY MESTDAGH1
1

Centre for Law & ICT, Faculty of Law, University of Groningen, Groningen, The Netherlands
Department of Information and Computing Sciences, Utrecht University, Utrecht, The Netherlands(E-mail: p.dijkstra@rug.nl)
2

Abstract. This paper outlines a multi-agent architecture for regulated information exchange ofcrime investigation data between police forces. Interactions between police oﬃcers about information exchange are analysed as negotiation dialogues with embedded persuasion dialogues. An
architecture is then proposed consisting of two agents, a requesting agent and a responding agent,
and a communication language and protocol with which these agents can interact to promote
optimal information exchange while respecting the law. Finally, dialogue policies are deﬁned forthe individual agents, specifying their behaviour within a negotiation. Essentially, when deciding toaccept or reject an oﬀer or to make a counteroﬀer, an agent ﬁrst determines whether it is obligatory
or permitted to perform the actions speciﬁed in the oﬀer. If permitted but not obligatory, the agent
next determines whether it is in his interest to accept the oﬀer.

1. Introduction
In many organisations information has to be exchanged, an activity which isoften regulated by law. For instance, in the European Union exchange of any
personal data is regulated by privacy law and in the Netherlands exchange ofcrime investigation data between police departments is regulated by a special
act. Typically, organisations must balance the goal to exchange as muchinformation as possible with the obligation to stay within the law. It is
important to investigate to what extent this balancing act can be supportedwith advanced information technology. Application of regulations can besupported with legal knowledge-based systems but the distributed nature ofmany organisations suggests that it may be worthwhile to combine knowledgebased technology with multi-agent technology.
This paper explores this idea in the context of the exchange of crime
investigation data between Dutch police departments. In particular, we
propose an architecture of two agents, a requesting agent and a responding

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agent, and we deﬁne a communication language and protocol with whichthese agents can interact to promote optimal information exchange while
respecting the law. We model the overall structure of their interactions as anegotiation dialogue about whether a certain body of information will be
exchanged which may shift to a persuasion dialogue about whether suchexchange is allowed or sensible. Finally, we deﬁne dialogue policies for theindividual agents, specifying their behaviour within a negotiation. Essentially, when deciding to accept or reject an oﬀer or to make a counteroﬀer, an
agent ﬁrst reasons about the law and then about the interests that are atstake: he ﬁrst determines whether it is obligatory or permitted to perform theactions speciﬁed in the oﬀer; if permitted but not obligatory, the agent next
determines whether it is in his interests to accept the oﬀer. The policies are
speciﬁed within the overall assumption that the agents take the overall goalof optimal and lawful information exchange into account.
The architecture proposed in this paper assumes and combines several
formalisms and ideas from the literature. The main original contributions arean account of regulated information exchange as negotiation with embedded
persuasion and a speciﬁcation of dialogue policies for negotiation within thisapplication domain. In order to focus on these original contributions, theelements taken from the literature will be summarised or referred to only; forfull descriptions of the technical details the reader will be referred to theliterature.
This research is part of an ongoing research project ANITA (Administrative Normative Information Transaction Agents), which aims at
performing the fundamental research needed to develop a multi-agent system
for regulated information exchange in the police intelligence domain (De Vey
Mestdagh 2003).
This paper is organised as follows. First in Section 2 we discuss the
problem of regulated information exchange and how it manifests itself incrime investigation exchange between Dutch police forces. In Section 3 we
present two example interactions between information exchanging police
oﬀers, after which in Section 4 we list the main requirements for a multi-agent
architecture in this domain. In Section 5 we then propose an architecture thatmeets these requirements, which we then apply to the examples in Section 6.
2. The problem of regulated information exchange
Information exchange is often regulated by legal norms and by the policies ofthe exchanging institutions. This regulation of information exchange serves
several goals. On the one hand, the privacy of the persons who are thesubjects of the information must be protected. On the other hand, thelegitimate interests of the exchanging institutions must be served. TheseTOWARDS A MULTI-AGENT SYSTEM

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interests of institutions vary from obtaining as much information as possible
from other institutions to further their own objectives, to not providing
information to other institutions in order to protect their own objectives.
Several types of conﬂicts of interest arise from these diverging goals. In
most cases there is a central institution (for example, the state, the mothercompany) that is mainly interested in both optimal and legitimate information exchange, because it has to give account of the eﬀectiveness and lawfulness of its operations to the outside world (for example, the parliament,
the shareholders). Besides the central institution there often also are distributed local institutions with their own interests and objectives. The central
institutions take the interests at the local level into account by formulating
legal norms and central policies; these give room for ﬁne tuning in local
policies and individual decisions by granting discretionary authority to local
institutions. In many cases such discretionary authority is again constrainedby general obligations (do’s and don’ts). Some typical norms resulting fromthe need to reconcile conﬂicting interests are: ‘Information must be
exchanged if this is necessary for the execution of the other’s appointed task’and ‘It is allowed to refuse to exchange information if such refusal is necessary for the execution of one’s own appointed task’ (e.g. sections 14 and13a of the Dutch Police Registers Act).
It can easily be seen that such norms may give rise to interesting dialogues
between oﬃcials of diﬀerent local institutions. Ideally, these dialogues
guarantee that an optimal and legitimate balance is found in the exchange of
information in institutions characterized by diﬀering and in some cases
conﬂicting interests. However, in practice this ideal is not always realised.
For example, it is well known that police departments are very reluctant toshare crime investigation information with other departments, even if the
sharing of information is allowed. One of the ultimate research goals of the
ANITA project is to investigate whether such problems can be tackled byproviding automated support for information-exchanging police oﬃcers.

3. Examples
As a possible solution to the above-mentioned problems we investigate theuse of a multi-agent architecture. The idea is that the overall goals of anorganisation (optimal and lawful information exchange) are promoted by thedesigns of the individual agents and the ways they interact. We illustrate thiswith a case study in crime investigation in the Dutch police organisation. The
Dutch police organisation is divided into separate departments, that each
operate in its own region. In order to solve crime cases, departments often
need information held by other departments. Information exchange between

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police departments is governed by national and international privacy regulations and these regulations are supplemented by local rules of the departments. In consequence, when exchanging information with each other, police
oﬃcers often have to interact in several ways to make sure they conform to
the regulations and at the same time serve local interests.
We illustrate such interactions between police oﬃcers with two examples
from police practice. We have been assured by police oﬃcers that interactions
like these are exemplary. A very typical interaction is about the exchange of
information acquired from informants (about 80% of police information on
heavy crime in the departments we examined is obtained from informants).
Police departments are very cautious about the exchange of this kind of
information, since crime suspects who are confronted with information obtained from informants may ﬁnd out who supplied the information, and thismay endanger the safety of the informant and the continuity of the investigation performed by the department that supplied the information. Therefore, in most cases the department that ‘runs’ the informant will not be
willing to supply the information unless the receiving department oﬀers
certain guarantees.
EXAMPLE 1: agent a working in police region a requests information about
trading in explosive materials from agent b working in region b.
A: Give me all information about trading in explosive materials.
B: I will not give you this information.
A: Why don’t you give me the information?
B: Because I am not allowed to do so.
A: Why are you not allowed to share the information?
B: Because it is protected.
A: You may be right in general but in this case it is allowed to share the information
because this is a matter of national importance.
B: Ok, I admit that in this case it is allowed to share the information, so I retract that
I am not allowed to give you the information. I will give you the information on
the following condition: the given information may not be exchanged with otherpolice oﬃcers.
A: I agree with this condition.

EXAMPLE 2: agent a working in police region a requests information abouta suspect from agent b working in region b.
A: Tell me all you know about suspect X.
B: No I won’t.
A: Why don’t you want to tell me about suspect X?
B: Because I need to protect my informant.
A: Why do you need to protect your informant?

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B: I need to protect my informant because your use of my information about suspect
X could disclose my informant’s identity and though endangering the continuityof several of my investigations.
A: You don’t need to protect your informant since I will only use the information
about suspect X statistically for policy reasons and not in individual investigations.
B: Ok, in that case I don’t need to protect my informant. I will give the information
under the following condition: the given information may not be exchanged withother police oﬃcers.
A: I agree with this condition.

4. Requirements for the multi-agent architecture
In this section we sketch the requirements for a multi-agent architecture forregulated information exchange that can be applied to the above-sketchedpolice scenario.

4.1. KNOWLEDGE
As seen in the previous sections the agents must have knowledge of the relevant
regulations for information exchange and the information stored in the local
database to solve crime cases constrained by those regulations. Knowledge ofregional interpretations of national regulations must also be available to theregional agent. This knowledge is only accessible locally and will not be shared
between the agents. Furthermore, the agents must have knowledge of the likely
consequences of their communicating acts for the realisation of their goals.
Finally, in order to represent knowledge and being able to share information
the agents must have an ontology of the domain. Ideally all agents use the same
ontology so problems with diﬀerent ontologies can be avoided.

4.2. REASONING
As usual in legal domains, most of the available knowledge is defeasible.
Also, as can be seen from the examples in the previous section, the interaction
between agents often involves argumentation. Therefore, the agents should
be capable of generating and evaluating arguments for and against certain
claims. Finally, to generate conditional oﬀers, the agents must be able to dosome form of hypothetical reasoning.

4.3. GOALSAs described above, the agents in our problem domain have individual goals.
In our case there are two agent roles: a requesting agent (denoted by a) who

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wants to collect as much information as possible for the purpose of his crime
investigations, and a responding agent (denoted by b) who wants to protect
his own information resources and crime investigations. In addition both
agents want to contribute to the overall goals of the police organisation,
which are the optimal and lawful exchange of information. The agents andtheir interactions should be designed in such a way that their behaviour
agrees with their goals.

4.4. COMMUNICATION
Of course, the agents should be able to exchange information but other typesof interactions should also be possible. Above we noted that the responding
agent’s goals sometimes lead him to state conditions under which he is willingto give information. Therefore, the agents must be able to negotiate with eachother. Also, the responding agent may be mistaken in believing that he mustor should not give the requested information. Therefore, the agents must beable to engage in persuasion dialogues to reach a better information state. To
enable such interactions, a suitable dialogue protocol must be implemented.
Also, the agents must be given policies, or tactics, for their behaviour in the
dialogues. These policies should be designed to further the agent’s goals.
Since these goals include those of the overall institution, the agents’ policies
should induce a fair degree of cooperativeness.
In fact, there are (at least) two ways to model the relation between thethree types of (information-seeking, negotiation and persuasion) dialogues,
depending on two ways to interpret the start of a dialogue in this domain.
i. The ﬁrst interpretation is that each dialogue starts as an informationseeking dialogue. It then shifts to another type of dialogue, either
persuasion or negotiation. It shifts to a persuasion dialogue if the
responding agent b states he will not grant the request since doing sowould have negative consequences for his investigations and the requesting a starts to persuade b that he is wrong about this. The dialogue instead shifts to negotiation if a promises to do or refrain from doing
something on the condition that b gives him the information. Further
shifts may occur, for instance, from a persuasion to a negotiation dialogue or vice versa.
ii. The second interpretation is that each dialogue starts as a negotiation,
viz. as a request to give information about something. Such a dialogue
may shift to persuasion if b rejects the request on the grounds that
granting it would have negative consequences for his investigations anda tries to persuade b that he is wrong about this. After the persuasion

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terminates the interrupted negotiation resumes. If that terminates successfully, a (trivial) information-seeking dialogue starts; its termination
also terminates the overall interaction.
We contend that most interactions in our domain will be of type (ii) since
usually the requesting agent will not simply ask a question but will inquire
whether the other agent is willing to provide him with a certain body of
information. This seems more like negotiation than like information-seeking.
In the remainder of this paper we will therefore only focus on interactions of
the second type.
Figure 1 gives a high-level view of the required multi-agent architecture.
Both agents have access to their local information database. If the requesting
agent a wants to access information from region b he has to communicate
with the responding agent b. Using the communication channel the agents
can have dialogues about conditions and when they agree the ﬁnal step whichis the exchange of information.
5. Outline of a computational architecture
In this section we outline a computational architecture that respects therequirements of the previous section. We ﬁrst sketch a dialogue system fornegotiation with embedded persuasion, consisting of a communication
language and a protocol. Then we outline the main components of the

Figure 1. Interaction between information-exchanging agents.

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individual agents: knowledge representation, reasoning, goals and dialogue
policies.

5.1. DIALOGICAL INTERACTION
Dialogue systems have a communication language C with associated protocol
P and a topic language T with associated logic L (possibly nonmonotonic).
The communication language consists of speech acts l(t) where l is a locution
and t an element or subset of T or an argument in L. Our language andprotocol essentially is that of (Van Veenen and Prakken 2005), which follows
the general format of (Prakken 2000) as extended and revised in (Prakken, toappear). The system of (Van Veenen and Prakken 2005) combines a negotiation protocol of (Wooldridge and Parsons 2000) with a persuasion language and protocol of (Prakken to appear). The system of (Van Veenen andPrakken 2005) seems appropriate for present purposes since, as noted abovein Section 3, the agent interactions typically have the form of negotiation
with embedded persuasion. Also, this embedding occurs especially when therequesting agent asks why the responding agent rejects the request for
information, and this is precisely the kind of embedding modelled in (Van
Veenen and Prakken 2005).
5.1.1. Communication language
We ﬁrst present the sublanguages for negotiation and persuasion and thendeﬁne their combination.
In (Van Veenen and Prakken 2005), following (Prakken, to appear), a
communication language has a reply structure: each speech act replies to one
preceding speech act in the dialogue. Moreover, a reply can be of two kinds,
being either an attacking or a surrendering reply. How to attack or surrenderto another speech act is speciﬁed in the following tables.

Table I. A persuasion communication language (Cp)Speech acts

Attacks

Surrenders

Claim pWhy pConcede p

Why pp since Q

Retract p

p since Q

Why q (q 2 Q)

Concede q (q 2 Q)p¢ since Q¢

Concede p

Concede p
Retract p141

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Table II. A negotiation communication language (Cn)Speech acts
Oﬀer pReject p

Attacks

Surrenders

Oﬀer q (q „ p)

Accept p

Reject p

WithdrawOﬀer q (q „ p)

Accept p
Withdraw p

In Table I ‘‘since’’ speech acts state arguments in L (which is a logic for
defeasible argumentation) and p¢ since Q¢ attacks p since Q according to L,
that is, the latter is a counterargument of the former.
The idea of Table II is that an initial request is made as a special kind ofoﬀer, namely, an oﬀer that the other party do something (possibly conjoinedwith an action to-be performed by the oﬀeror). This is for simplicity only:
since the replies to a request are the same as to an oﬀer, having separate
speech acts for both would be an unnecessary complication. Yet for convenience an initial oﬀer will be denoted by ‘request’ in the example below.
The negotiation language is very simple and the distinction between
attacking and surrendering replies would seem to make no sense, but thetables will be extended and combined in a way that makes this distinction
sensible for negotiation also. In Table II the propositional contents of the
locutions typically are (possibly negated) act descriptions, such as ‘‘you tellme all you know about suspect X’’ or ‘‘you do not pass on the information toother police oﬃcers’’. Since the logic of reasoning about actions is not themain concern of this paper, we simply assume that these act descriptions are
expressed in ﬁrst-order logic. More precisely, formulas from Tn are assumed
to be conjunctions of ﬁrst-order literals (atomic formulas or their negations)
without free variables. For present purposes what matters most is that
conjunctions can be used to state conditions for acceptance. For example, if
the requesting agent says ‘request: you tell me all you know about suspect X’then the responding agent could state conditions in a counteroﬀer ‘oﬀer: I tell
you all I know about suspect X  you do not pass on the information to otherpolice oﬃcers’ thus requiring that the requested information is not passed on
to other police oﬃcers. We contend that representing conditional oﬀers witha conjunction is more natural than representing them with conditional
operators, since the content of a conjunctive agreement allows the agents toinfer what they have committed to irrespective of whether the other agent
keeps his part of the agreement.
Table III contains the two combined communication languages, the idea
of Van Veenen and Prakken (2005) is to add to the negotiation language aspeech act that triggers a persuasion dialogue: a new attacking reply is added

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Table III. A combined communication language (Cnp)

Speech acts

Attacks

Oﬀer pOﬀer q (q „ p)

Accept p

Reject p

Withdraw

Reject p

Surrenders

Oﬀer q (q „ p)
Why-reject p

Accept p
Withdraw
Why-reject p

Claim qClaim pWhy pConcede p

Why pp since Q

Retract p

p since Q

Why q (q 2 Q)

Concede q (q 2 Q)p¢ since Q¢

Concede p

Concede p
Retract pin Cn to a ‘reject p’ move, namely ‘why-reject p’. The only possible reply tothis move is claim q, where q is a ground for the rejection. This claim starts a
persuasion dialogue.
5.1.2. Communication protocol
We ﬁrst sketch the individual negotiation and persuasion protocols. They
have the following rules in common. A move is a speech act made by a dialogue participant. If it is not the ﬁrst move, it replies to a unique preceding
move in the dialogue made by the other party, according to the reply structureof C. A dialogue terminates if a player is to move but has no legal moves.
The negotiation protocol is very simple. Agent a begins with an oﬀer, andthen the agents take turns after each move, replying to the last move of theother party. Thus negotiations can be of arbitrary length and terminate afteran accept or withdraw move but they are not guaranteed to terminate.
The persuasion protocol is more elaborate. We sketch the main rules only.
Each persuasion starts with a claim made by agent a. The main protocol rule isthat each move is relevant. Relevance is deﬁned in terms of the notion of
dialogical status of a move. Brieﬂy, a move is in if it has a surrendering reply orelse all its attackers are out; and a move is out if is it not surrendered and has
an attacking reply that is in. As for turntaking, an agent keeps moving until
the dialogical status of the initial move has changed, then the turn switches tothe other agent. Thus each turn of a player consists of zero or more surrenders
followed by zero or one attacker. Also, these rules imply that unlike innegotiation dialogues, in persuasion dialogues postponing replies and making

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alternative replies to earlier moves are allowed. Thus, if the reply structure of a
dialogue is made explicit in a graph, the graph of a negotiation dialogue is alinear structure while that of a persuasion dialogue can be any tree. Finally, allthis implies that a persuasion dialogue terminates if a player is to move but has
no relevant moves: in the present simple protocol this can happen only ifeither the player to move is a and he has retracted his initial claim or the player
to move is b and he has conceded the initial claim.
As for the combined protocol, the main idea is that if a negotiation dialogue shifts to a persuasion dialogue, their relation is one of embedding (cf.
McBurney and Parsons 2002): the embedded persuasion dialogue is undertaken until its termination, after which the embedding negotiation dialogue isresumed. So whenever a persuasion move is allowed by the protocol, nonegotiation move is allowed. In addition, the structural rules of the persuasion system now also hold for negotiation, especially those of relevance andturntaking. This allows for alternative explanations for rejections and also
for accepting an oﬀer (perhaps conditionally) that was ﬁrst rejected forreasons that could not be upheld in a persuasion dialogue. For a more detailed speciﬁcation of the combined protocol the reader is referred to (Van
Veenen and Prakken 2005).

5.2. THE AGENTS
We next outline the architecture of the individual agents. Since knowledge
representation and reasoning are not the primary focus of this paper, we will
be brief and informal on these aspects, making only those assumptions about
the architecture that are required by the speciﬁcation of the dialogue policies.
5.2.1. Representation of knowledge and goals
The knowledge representation language must allow for the expression of asuitable crime investigation ontology, for the deontic modalities ‘obligatory’,‘permitted’ and ‘forbidden’, for the description of actions and their eﬀects
and for the expression of an agent’s goals. These goals are assumed to be
represented in the agent’s internal knowledge base, together with knowledge
that is relevant to respecting the agent’s goals, such as knowledge on when
exchanging information is obligatory, allowed or forbidden and when
exchanging information is promotes or violates the agent’s interests.
5.2.2. Reasoning engine
We assume that agents are capable of performing argument-based defeasible
reasoning with their internal knowledge. In particular, we assume that they

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are able to construct arguments for certain propositions and then to verify
whether these arguments are justiﬁed, defensible or overruled (cf. Prakken
and Vreeswijk 2002). In the present model the agents especially reason about
whether something is obligatory, permitted or forbidden, and about whether
something is in their interest. With respect to the latter it is useful to remarkthat we assume that any conﬂict between arguments on this issue can be
resolved in the defeasible reasoning process. For instance, if the agent hasconstructed a justiﬁed argument that a particular action does not violate hisinterests, then we assume that he has considered and refuted all counterarguments that, say, the action detracts from one of his goals, or that there isanother way to satisfy the same goal. For a similar approach to practicalreasoning based on argumentation schemes see Atkinson et al. (2005).
We also assume that the agents are able to perform hypothetical reasoning. This is especially useful when the agent has to reason about whether
to make a counteroﬀer to an oﬀer: hypothetical reasoning provides thecandidate conditions to be added to such a counteroﬀer.
5.2.3. Dialogue policies
We next specify policies for what an agent will choose to do at various points
in a dialogue. In our application, two main kinds of dialogue policies need to
be speciﬁed, viz. for negotiation and for persuasion. To our knowledge,
formal dialogue policies for argumentation dialogues were ﬁrst studied by
Parsons et al., who called them ‘‘agent attitudes’’. They only deﬁned policies
for persuasion, while we are particularly interested in policies for negotiationand largely adopt Parsons et al.’s persuasion policies. An important diﬀerence with Parsons et al. is that when a policy requires an agent to constructarguments, in our case the agent only reasons with his initial knowledge baseplus the propositions that he has explicitly conceded to in the persuasion
dialogue, while in Parsons et al. the agent must also reason with everything
the other agent has said, regardless of whether he has conceded to this or not.
We regard the latter as less realistic.
Our negotiation policies consider two issues: the normative issue whether
accepting an oﬀer is obligatory, permitted or forbidden, and the teleologicalissue whether accepting an oﬀer is in the agent’s interest. Of course thesepolicies can be diﬀerent for the requesting and the responding agent. Also,
diﬀerent responding agents can have diﬀerent policies. One agent, forexample, might easily be persuaded to give information and thus agree withevery request for information he receives, while another agent may guard his
secrets more closely, adding extra conditions to the information exchange tomake sure the information does not fall into the wrong hands.
For simplicity we have chosen to specify just one set of policies which we
think best suits our domain (although we will comment on other design

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choices when relevant). As remarked above, in the police domain all agents
must on the one hand protect their own interests, such as their own investigations and informants, but on the other hand they must cooperate toensure optimal exchange of information within the bounds of the law. Our
set of policies is meant to implement these general considerations. Since thenegotiation policies are the main novel contribution of this paper, we will
especially focus on their speciﬁcation.
The following notation will be used in the policies. Recall ﬁrst that Tn is thenegotiation topic language and Tp is the persuasion topic language. The content
of an oﬀer is a conjunction of literals from Tn. For any formula p of Tn and agentaa the notation p denotes the conjunction of all conjuncts in p that describe
actions performed by a. Also, KBd ðaÞ  Tp denotes the set of beliefs of agent a
at dialogue stage d. Note that these beliefs may change during persuasion if theagent concedes or retracts propositions and then brings his internal beliefs inagreement with his (public) commitments. Finally, we assume that each oﬀer
content p is divided into two parts q  c, where q are the essentials of an oﬀer,
which the oﬀeror does not want to be changed in counteroﬀers, and where c are
the conditions of the oﬀer, which the oﬀeror does not mind to be changed incounteroﬀers. For instance, an oﬀer ‘‘send me a ﬁle containing all you know
about mr. X’’ can be decomposed into an essential element ‘‘inform me about
all you know about mr. X’’ and a condition ‘‘send me the information in a ﬁle’’.
The oﬀeror thus indicates that he does not want to receive counteroﬀers ‘‘I willtell you how to ﬁnd out everything about mr. X’’ but that he does not mind
receiving a counteroﬀer ‘‘I will tell you everything I know about mr. X over thephone but I will not send you a ﬁle’’. We will now give the diﬀerent policies,
starting with the negotiation policies.
5.2.3.1. Negotiation policies. Responding to an oﬀer
We now specify the policy agent a should apply in responding to an oﬀer
in a dialogue. Note that the oﬀer can be the initial one (i.e., a request) or acounteroﬀer.
When agent a receives an oﬀer p=q  c, where q are the essentials of theoﬀer and c the conditions, he should perform the following actions:• Determine whether KB(a) supports a (justiﬁed/defensible) argument fora d
the conclusion that p is obligatory. If there is such an argument, thenaccept the oﬀer. Otherwise,• Determine whether KB(a) supports a (justiﬁed/defensible) argument fora d
the conclusion that p is forbidden. If there is such an argument, thenreject the oﬀer. Otherwise,• Determine whether KB(a) supports a (justiﬁed/defensible) argument fora d
the conclusion that p violates a’s interests. If there no such argument,
then accept the oﬀer. Otherwise:

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– Find a subset-minimal setc0  Tn such that KBd (a) [ c¢ supports a(justiﬁed/defensible) argument for the conclusion that p does not violate
a’s interests and KBd(a) does not support a (justiﬁed/defensible)
argument for the conclusion that (q  c¢) is forbidden.
– If there is one such set c¢, then make a counteroﬀer q  c¢.
– If there are more than one such sets, make a counteroﬀer q  c¢, where c¢is the subset-minimal set that has the most elements in common with c.
– Otherwise reject the oﬀer p.
The negotiation policy contains a number of design choices. Of course, inother domains a choice for other types of agents can be made. For example, theagent could also reject the request for information if there is no justiﬁedargument which says he is obliged to give the information. However, thisaction is not in line with the overall goal of the system, which says that if at all
possible, information should be exchanged. Furthermore, in our design of thepolicy we have not yet speciﬁed whether the agent needs justiﬁed, defensible orjust well-formed arguments for his conclusions. This can have a signiﬁcant
impact on the behaviour of the agent. For example, an agent who accepts wellformed arguments that say that it is not forbidden to give a certain piece of
information will be much easier persuaded than an agent who only accepts
justiﬁed arguments for the conclusion that giving the information is not forbidden. So the ﬁrst agent, who only needs a well-formed argument, will not be
as protective of his information as the second agent, who needs a justiﬁedargument. Diﬀerent policies are also possible with respect to making acounteroﬀer. We have described a policy where having as few extra conditions
as possible is more important than making a counteroﬀer which includes theconditions oﬀered by the opponent. In our policy it is thus possible to delete
some or all conditions of the opponent’s oﬀer. Another policy would be apolicy where an agent can only add conditions to the original oﬀer. Anotherchoice option is in how an agent should internally reason about whether anoﬀer is in the agent’s interests. Is the reasoning only about his own actions or
also about the actions of the other agent? The diﬀerent design options mentioned here are mostly empirical questions, to be answered by domain analysis.
Responding to a reject
The next policy describes responding to a reject.

• First respond with a why-reject move. If the resulting persuasion
subdialogue is won, then it is the other agent’s turn. Otherwise,• if the reject move responded to the initial oﬀer then reply with awithdraw, while• if the reject move responded to a counteroﬀer, backtrack to the target ofthe reject move.

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147– If an alternative counteroﬀer exists that satisﬁes the policy for
responding to an oﬀer then make it.
– Otherwise reply with a withdraw.
5.2.3.2. Persuasion policies. We now turn to a less formal speciﬁcation of
persuasion policies, which determine how an agent should respond to arguments and why moves. Other persuasion policies can be developed similarly.
Recall that our policies assume that an agent a reasons with KBd(a), which is
his initial internal knowledge base, possibly modiﬁed by his concede andretract moves during the dialogue. We will ﬁrst explain how an agent canreact to an argument.
Responding to arguments
Can the agent construct a (justiﬁed/defensible) counterargument?

• If the agent can construct such an argument, it should be moved in the
dialogue.
• If the agent cannot construct such an argument and there is a premise pof the opponent’s argument for which the agent has no (justiﬁed/
defensible) argument, then the agent should ask a why p question.
• If the agent cannot construct such an argument and for all of thepremises of the opponent’s argument has a justiﬁed argument thenconcede to the conclusion of the opponent’s argument.
Responding to why moves
Say that the requesting agent asks a why p question in response to a claim
or argument move by the responding agent. Can the responding agent construct a (justiﬁed/defensible) argument for p?

• If the agent can construct such an argument, it should be moved in the
dialogue.
• If the agent cannot construct such an argument, he should retract his
claim p or the conclusion of the argument of which p is a premise.
A few things should be noted regarding the above policies. Firstly, as in thenegotiation policies, we have not speciﬁed what kind of arguments (justiﬁed ordefensible) an agent needs for his decisions. Secondly, the agent is cooperativein that he only asks why p questions if he does not have an argument for p.
Clearly, several other policies are possible. One option we want to explorein future research is to make policies partly domain-speciﬁc. For example, thesecond part of the policy for responding to arguments could be reﬁned suchthat premises are never challenged when they are about subject X and/or

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when they are claimed by person Y, who is considered to be a reliable sourceof information concerning X.
6. Illustration of the proposed architecture
We now illustrate the outlined architecture with a more formal reconstruction of example 1. We assume that the language is closed under negation andmaterial implication but we make no further assumptions on the language or
logic. We also assume that agent b needs defensible arguments. The knowledge of the agent are all the rules and facts known by him about a requested
piece of information. Up to step 9 each move replies to the immediately
preceding move.
The responding agent b has the following knowledge:
KB0 ðbÞ ¼ fð:e ) :iÞ; ð:i ) gÞ; ðp ) :aÞ; p; ng

wheree = ‘‘the information may be exchanged with other police oﬃcers’’,
i = ‘‘the informant is in danger’’, g = ‘‘you give all information about
trading in explosive materials’’, p = ‘‘the information is protected’’,a = ‘‘agent b is allowed to give information about trading in explosive
materials’’, n = ‘‘the information is a matter of national importance’’.
The dialogue from example 1 (Section 3) will go as follows:
1. A: request g (Give me all information about trading in explosive materials).
According to the negotiation policy outlined in section 5.2.3.1, agent b willﬁrst have to look whether he is obliged to give the information or not. He
cannot construct a defensible argument from his knowledge stating that
he is obliged to give the information, so b applies the next step in his policy, which is ﬁnding out whether b is allowed to give the information ornot. b can construct an argument ‘‘:a since p’’ which states that b is notallowed to give the information, so b rejects the request.
2. B: reject g (I will not give you this information)
3. A: why-reject g (Why don’t you give me the information?) b now starts
a persuasion dialogue. Until it is terminated no negotiation moves are allowed by the protocol.
4. B: claim:a (I am not allowed to give you the information)
5. A: why:a (Why are you not allowed to give me the information?)
According to agent b’s persuasion policy, he has to move defensible arguments so he moves argument:a since p.
6. B::a since p (I am not allowed to give the information since the information is protected) Say that agent a rebuts p ) :a and defeats argument p ) :a.

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149

7. A: a since n (The information is allowed to be shared because this is amatter of national importance)b cannot ask a ‘‘why n’’ question here because b can construct a (trivial) defensible argument for n since n is partof his knowledge base, so agent b has to concede to a.
8. B: concede a (Ok, I admit that in this case the information is allowed
to be shared)B retracts:a from his commitments because his commitments have to stay consistent.
9. B: retract:a (I retract the fact that I am not allowed to give you theinformation)With this move b backtracks to A3, now surrendering tothat move. This move terminates the persuasion dialogue so that negotiation moves are allowed again by the protocol. b now knows he is not
obliged to give the information but that he is allowed to give the information. b now has to look if there are extra conditions serving his interests
under which the information can be given. Through backward chaining
through the rulesð: i )gÞ and ð: e ) :iÞ, b arrives at the possible extra
condition :e and internally veriﬁes that this extra condition does notmake the oﬀer he has in mind violate his interests. With B6 the responding agent then backtracks to A1, this time replying with a counteroﬀer.
10. B: oﬀer g ^ :e (I will give you the information on the following condition: the given information may not be exchanged with other police
oﬃcers). We assume that the local interests of agent a are not violated
and therefore he accepts the oﬀer.
11. A: accept g^:e (I agree with this condition)

7. Conclusion
In this paper we have proposed a multi-agent architecture for regulated
information exchange and we have illustrated it with examples of information exchange between police forces in the context of crime investigation. The
architecture combines and adapts several elements from the literature: adefeasible-argumentation mechanism for the agents’ internal reasoning
behaviour, a communication language and protocol for negotiation withembedded persuasion about reasons for rejections of oﬀers, and dialogue
policies for one of the agents of our architecture. Our dialogue policies for
persuasion arguably improve those of Parsons et al. in one respect and wehave added dialogue policies for negotiation. Also, we have proposed a novel
view on the nature of dialogues in the context of regulated informationexchange, viz. as negotiation with embedded persuasion.
As for related research, Trevor Bench-Capon has in Bench-Capon et al.
(1998) proposed an information-seeking protocol where one of the preconditions for answering a question is that doing so is permitted. Thus the

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question of lawfulness of providing the information is modelled as an aspectof the dialogue protocol. Arguably a drawback of this approach is thatusually interaction protocols are meant to promote coherence and rationalityof a dialogue; they are not meant to promote lawfulness of dialogues undersome arbitrary normative system. We therefore regard discussions about
lawfulness of speech acts as a domain matter, to be the topic of separate
persuasion dialogues.
Parsons et al. (1998) model negotiation as argumentation: a proposal is
the conclusion of an argument, the premises of which are the grounds tomake the proposal. We think that whether it is good to communicate thereasons for a proposal is context-dependent: for instance, if a buyer says‘‘please sell me this since I need it badly’’, the seller is likely to oﬀer a higher
price. Therefore, arguments for or against a proposal can better be exchanged in a separate persuasion dialogue.
Doutre et al. (2005) propose a model for regulated information exchange
in the medical domain. They model it as information seeking with embedded
persuasion about whether providing the requested information is permitted.
As for future research, the informal and semiformal parts of our architecture
speciﬁcation should, of course, be made fully formal and then implemented.
With respect to the negotiation policies we intend to investigate alternative
design choices on the various aspects discussed in Section 5.6.3. Also, wewant to investigate other combination patterns of dialogue types, including
also information-seeking. As for the negotiation part of the dialogue system,
we aim to investigate whether besides arguing about rejections, other ways toargue in negotiation, such as those studied by Parsons et al. (1998), occur in
our application domain and should therefore be modelled in our architecture.
Acknowledgments
This research was supported by the Netherlands Organisation for Scientiﬁc Research(NWO) under project numbers 634.000.017 and 634.000.429. Henry Prakken was
also partially supported by the EU under IST-FP6-002307 (ASPIC).

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