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Negotiations generally consist of one or more turns in which offers – as tentative settlement proposals – as well as other messages are communicated. While the exchange of offers is a constituting feature of negotiations, the particular rules governing these offer sequences can vary (Cranor and Resnick, 2000). The interaction or negotiation protocol is the set of rules that governs the interaction between the participants in a negotiation, these participants can either be exclusively human, exclusively software agents, or combinations of both. The protocol determines the possible states in a negotiation, the actions particular participants can execute in each of these states, and the events that cause state transitions (Jennings et al., 2001). In face-to-face negotiations or technology-mediated negotiations between humans – e.g. video conferencing, telephone, e-mail, or fax – such interaction protocols are less strict and rigorous as people easily can deviate from protocols, e.g. through interrupting each other, withdrawing offers, or finding agreements after negotiations were broken off – for example when just after the deadline a party proposes an acceptable offer. In case of human interaction general social codes – e.g. not to interrupt a person when speaking – or norms particularly related to the domain of negotiation – e.g. that offers once put on the ’negotiation table’ should not be withdrawn – establish and impose a form of tacit interaction protocol causing some type of sanctions if violated (Bartos, 1977).

However, the more automation is used in negotiation and the more activities are delegated to software agents, the fewer flexibility concerning the negotiation procedure remains. Accordingly in automated negotiation – where the negotiation tasks of a human are completely assumed by a software agent – the flexibility is lowest compared to other alternatives and fully specified inter- action protocols (so-called closed protocols) are not only necessary (Kersten and Lai, 2007), but

9_{If the negotiation object is allowed to be manipulated by the software agents the interaction protocol also has}

to determine the rules for negotiation object manipulation, i.e. the rules that govern the addition or elimination of issues and options during the negotiation process.

also can be enforced by the software implementation of the interaction protocol itself. This soft- ware implementation of the interaction protocol can restrict the possible actions of the software agents for example by baring participants from actions in certain states or otherwise completely ignoring actions out of the bounds of the protocol (Cranor and Resnick, 2000).

3.1.2.1 Chronology of communication

One important attribute of the interaction protocol is the timing of offers. Offers can either be proposed simultaneously by the parties such that no party knows the current offer of the opponent in the current round before sending the own offer, or sequentially such that offers can be formulated in response to the opponent’s offers. In the later case the protocol has to determine which party has to start the offering sequence. Examples for simultaneous offering protocols can be found in the early game theoretic approaches on strategic bargaining like the Nash demand game (Nash, 1953), the Nash-Zeuthen bargaining game formulated by Harsanyi (1956), or the closely related iterated prisoner’s dilemma (Axelrod, 1980a,b). Sequential offering protocols were proposed by Cross (1965) first, and later adopted by St˚ahl (1972) and Rubinstein (1982). The alternating turn protocol – where software agents alternate in taking turns where they can perform actions – is dominant in the recent literature on simulation of automated negotiation. Only three of the reviewed studies use other protocols than the alternating turn protocol. Wollkind et al. (2004) use the Nash-Zeuthen bargaining protocol, while in two other studies (Henderson et al., 2005; Nawa, 2006) turn taking is not absolutely sequential, as one turn by one agent is not necessarily followed by a turn of the opponent, but agents can decide to repeat messages or pause in taking their turns in these simulations.

3.1.2.2 Configuration of offers

In single-issue negotiations offer configuration poses no problem. An offer by definition – to be a proposal for settlement – can be any feasible option for this single issue. In case of multi-issue negotiation problems, however, the protocol has to determine what constitutes an offer. The two extreme values for this attribute are package offers, where offers have to include proposals for all issues, and agenda setting, where issues are negotiated one after the other according to a specified order called agenda, just like a series of single-issue negotiations. In the later case an agenda has to be determined, which can be either done by the protocol or endogenously agreed on in a meta-negotiation of the negotiation agents (Raiffa, 1982). Clearly any combination in between these extremes is also feasible, for example different offers could cover options for all, some, or only one of the issues of the negotiation problem.

Most of the interaction protocols used in the reviewed simulation studies require package offers in multi-issue negotiations as they allow for trade-offs between issues, which in turn make logrolling procedures possible. In logrolling worse options in lower priority issues are traded for better options in higher priority issues, which finally should lead to mutually beneficial settlements (Pruitt, 1981). An other approach, dominant in game theory, is the aggregation of preferences over multiple issues to one single utility value of a package which reduces the multi-issue to a single-issue problem (Lang and Rosenthal, 2001). In recent years, however, game theorists started analyzing multi-issue negotiations with Rubinstein’s (1982) alternating offer protocol without

3.1. Current achievements in simulation of automated negotiation 55

aggregating the packages to utility values but in an issue-by-issue fashion under fixed agenda (Fershtman, 1990). Other studies investigate how such agenda for issue-by-issue negotiations can be determined (Busch and Horstmann, 1999a). Furthermore agenda protocols and package offer protocols are compared for very specific bargaining problems (non-cooperative, zero-sum scenarios with costs of delay) (Lang and Rosenthal, 2001; Inderst, 2000).

Agenda protocols at the moment are used with analytical models in game theory only, in simu- lation studies most often only one issue is considered. When the negotiation problem consists of more than one issue, package offering is imposed by the protocol in all but one of the 26 studies that simulate multi-issue problems, so that package offers, consisting of options for each of the negotiated issues, are required by the protocol. Wasfy and Honsi (1998) use a slightly different protocol. In their approach, agents exchange offers in only one of the issues of the multi-issue negotiation problem. The current demand of the agents is stored and agreement is reached when the demands of the agents are equal in all issues – so it is a procedure of single-issue offers in a multi-issue negotiation problem, however not following an agenda. In their approach issues for which consistent demands are achieved during the negotiation are considered as solved and the negotiation continues on the remaining issues of the negotiation problem only.

3.1.2.3 Progression of the negotiation

The progression of negotiation determines whether the protocol demands parties to start from inconsistent points and reach an agreement through concession making, or whether they start from a common basis and try to improve this basis through a number of tentative agreements that dominate the previous ones until no Pareto-improvements are left (or can be found in negotiations that settle for inefficient solutions). These two approaches are illustrated in Figure 3.9.

(a) Concession-based progress (b) Improvement-based progress

Figure 3.9: Concession-based and improvement-based progress in negotiations

Teich et al. (1994) call the first approach concession-based models and the second improvement- seeking models. Concession-based progression is the predominant value for this attribute of the interaction protocol in the reviewed simulation studies of automated negotiation. Schol- ars argue, that when negotiators have conflicting interests they have to make concessions from their initial starting point if any agreement is to be reached. An alternative to this predomi-

nant perspective, that one has to start negotiations with an extremely high demand and then continuously lower this demand by concessions, is the improvement-based progression. The improvement-based approach to negotiations is closely related to the ’single negotiation text’ procedure proposed by Fisher (1978). The agents first determine a common basis and use this as a reference point from which to move to other solutions preferred by both. Only Klein et al. (2003) use a pure improvement-based negotiation protocol, where a mediator proposes possible Pareto-improvements to the status quo to the agents, which they can either accept or reject. It is necessary to determine this progression attribute by the protocol as otherwise any tentative agreement which serves as a basis for later improvement in improvement-based approaches would be rendered as a final agreement and terminate negotiations in a concession-based protocol. While it is impossible to use both forms of progress simultaneously in a negotiation, they can be used sequentially. One form of such a combination is implemented in the negotiation support system Inspire (Kersten and Noronha, 1999a), which provides the opportunity for Pareto- improvement in a post-settlement phase of negotiations, after the negotiators reached a first agreement typically by concession-based procedures. Such post-settlement phases, however, are not implemented in negotiation protocols for automated simulation yet, but only one form of progression in negotiation is followed, most often the concession-based progress as mentioned above.

3.1.2.4 Types of actions

It is necessary to determine the types of actions the protocol allows the software agents to execute. While various kinds of actions are definitively important in negotiations, the most important action is the communication of offers and counter-offers (Tutzauer, 1992). However, there can also occur actions other than offer-exchange in negotiations. Two actions necessary for the termination of a negotiation are the transmission of acceptance and termination messages, other actions could be rejections of offers (Bartos, 1977) – leading to non-alternating offer sequences – or the submission of messages containing (logical) argumentation.

While in bilateral negotiations the disputing parties – buyer and seller, employee and employer, management and union, etc. – have a symmetric set of possible actions, procedures closely related to negotiation such as the ultimatum game or most forms of auctions limit the option to perform certain actions to only one side. In the ultimatum game only the first player is allowed to formulate take-it-or-leave-it offers which the second player can either accept or reject, and in most auction protocols – here the double auction is an exception – also only one side is allowed to make offers. This side is the buyer side in English, Dutch, or Vickrey auctions – to name only the most commonly known –, but it could also be the seller side as it is the case in a reverse auction for example.10

In four of the reviewed studies (Somefun et al., 2004, 2006; Wollkind et al., 2004; Zeng and Sycara, 1998) the set of possible actions of the agents is limited to the exchange of offers. In these studies an agreement is determined by the protocol and reached when the offers of the software agents are consistent. To avoid the necessity of exit messages in three studies it is guaranteed

10_{Consult Klemperer (2004) and G¨uth et al. (1982) for information on auctions and the ultimatum game,}

3.1. Current achievements in simulation of automated negotiation 57

that the agents always reach an agreement, and (Wollkind et al., 2004) use the Zeuthen-Nash bargaining protocol where the protocol determines the end of the negotiation.11 _{Wasfy and}

Honsi (1998) allow agents to send, messages about their power, which is determined randomly by the environment and directly affects the opponent’s situation and strategy, in addition to their offers. The software agents of Klein et al. (2003) can only accept or reject offers proposed by a mediator as a Pareto-improvement of the status quo, but cannot themselves formulate offers. As this study uses an improvement-based protocol the agents start negotiation from a common starting point, thereby agreement is guaranteed and no exit messages are necessary. If there is no progress in the negotiations the agents stay with the best solution available so far. 13 studies use protocols where the set of possible actions of the agents includes offer-exchange and acceptance of the opponent’s offer. In the studies of Chen et al. (2005b) and Chao et al. (2006) agents, with their own offer, provide some information about their satisfaction with the opponent’s last offer. In the other 16 studies the possible actions of the agents are offer-exchange, and termination of negotiation by both agreement and break off.

3.1.2.5 Abandonment of negotiations

If a negotiation does not result in an agreement the abandonment of negotiations can be induced by the protocol after a given period of time – the negotiation deadline – or by a break-off probability at the end of each turn. Alternatively the termination of the negotiation can also be the result of an endogenous decision of the software agents. Clearly a combination of these options is also feasible, where the software implementation of the protocol breaks off negotiations at a deadline unless the agents did this before.

In most (22) of the studies a commonly known deadline determined by the protocol and executed by the protocol or the agent is used to terminate negotiations, in two studies break-off probabili- ties after each round were used for this purpose. In six other studies agreement is guaranteed by the experimental settings and as all negotiations end with an agreement abandonment of negoti- ations is not discussed. In four studies the protocol terminates negotiations unless agents did so before. As mentioned above (Wollkind et al., 2004) uses the Zeuthen-Nash bargaining procedure where the termination criteria is defined by a protocol rule (no concessions of both parties in one round) and (Klein et al., 2003) use an improvement-based approach where negotiation end if no improvements can be found any more, in these two studies protocol rules exclusively determine when the negotiation ends. Only in the study of (Cheng et al., 2006) the decision to break off negotiations is made endogenously and exclusively by the software agents, here agents decide to break off negotiations when there is no progress for a certain period of time.