Decision Making Model

The decision making model in a negotiation-based solution has to make decisions regarding two important aspects of negotiation:

 How to evaluate a received proposal as to whether accept it or not (utility function),

 How to prepare a counter-proposal (tactic/strategy).

The first aspect is studied through the concept and theories related to the utility function of the negotiator, and the second aspect is covered by discussions related to a negotiator’s high level strategy and more specific, low level tactics.

3.6.3.1 Utility Function

To evaluate the received proposals, negotiators need to have clear understanding of their preferences about the negotiation object. These preferences guide their decisions during negotiation. In service selection, the dominant approach to express the preferences is

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based on utility theory; the same approach that we already explained for optimization- based service selection in section 3.5.

Several researchers addressing SLA negotiation have used single attribute linear utility function to evaluate the value of an individual issue (Zulkernine and Martin 2011; Yan et al. 2007; Ardagna and Pernici 2007). This utility function is similar to the equation (3-1) mentioned before (subsection 3.5.1). In the negotiation context, 𝑞_𝑗𝑚𝑎𝑥 and 𝑞_𝑗𝑚𝑖𝑛 are defined as the maximum and minimum admissible values for j-th QoS attribute according to the negotiator’s preferences and constraints. The parametric single attribute utility function (Comuzzi and Pernici 2005), and multi-attribute utility function representing the relative preference with respect to each pair of attributes (Gimpel et al. 2003) have also been discussed in the literature.

As a QoS profile typically involves more than one attribute, the negotiator needs to aggregate the preferences over all the attributes involved to make the decision regarding the acceptance or rejection of a received offer. The more commonly used technique to measure the utility of a profile with multiple attributes is to assign a normalized weight to the utility of each attribute and then calculate the overall utility using a weighted linear additive function; similar to the aforementioned equations (3-2) and (3-3).

3.6.3.2 Negotiation Tactics

To generate an offer during the negotiation process, two main approaches are discussed in the literature: concession and trade-off. The main difference between the two approaches is in the utility of the offer for the negotiator.

A negotiator with the concession approach concedes to the other side of negotiation (opponent) while preparing every new offer. The concession is made by preparing an offer that has a lower utility value for the negotiator itself, and apparently, a higher utility value for the opponent. In order to make the concession, the negotiator needs to make decisions regarding: firstly, the pace of offering concessions throughout the negotiation process, and secondly, the amount of concession in each offer. The offer is then prepared with respect to the decided concession.

Faratin et al. (1998; 2002) and Comuzzi et al. (Comuzzi et al. 2005; Comuzzi and Pernici 2005) have proposed heuristic approaches to define the offer in bilateral negotiations. In

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Faratin’s proposal, three families of tactics are presented to prepare a concessionary offer: time-dependent, resource-dependent, and behavior-dependent (or imitative). The concession made in each tactic depends on the tactic’s influential factor which is: the negotiation time, negotiator’s available recourses and the opponent’s behavior. Faratin’s heuristic functions (1998) are widely adopted by researchers, such as Zulkernine and Martin (2011); Richter et al. (2010); Ardagna and Pernici (2007), due to the clear distinction of tactic families (based on time, resource, and opponent behavior), the clear mathematical representation and the analysis of negotiation convergence for different parameters of the model.

The tactics in Faratin’s proposal are based on the generalized influential factors for any bilateral negotiation context. However, negotiation for composite service selection has specific characteristics that can be used to introduce additional influential factors, and consequently, new tactics. For example, negotiating for a task in the composite service consists of multiple negotiation threads with different service providers for that task. Hence, the requester’s negotiator receives multiple offers at the same time. Considering this characteristic, (Jiuxin et al. 2010) has proposed a new influential factor called the Global Negotiation States factor. This factor can reduce the need for unnecessary negotiations in one-to-many negotiations. In this proposal, the received offers are compared to each other, and then, if all the offers are far from the negotiator’s offer, the negotiator should be ready to make some (big) concessions. Otherwise, if any offer is more desirable than the negotiator’s own offer, negotiator will raise its expectations and prepares the next offer based on the value of the best received offer.

In contrast to the concession approach, a negotiator with the trade-off approach tries to keep its utility value stable at a desirable level (the aspiration level) throughout the negotiation, while generating an offer that has more utility value for the opponent. This can be achieved by trading-off between the values of different issues (Faratin et al. 2002), that is lowering the values of some QoS attributes while demanding more on some others. Such a strategy maximizes the chance of the offer being accepted.

In the trade-off approach, as the negotiator usually has no information about the opponent preferences and utility function, the main challenge is how to determine which offer increases the opponent’s utility value. The trade-off strategy proposed by Faratin et al. (2002) uses the concept of “fuzzy similarity” (Zadeh 1971) to approximate the

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preferences of the opponent. Assuming that the opponent’s last offer reflects its preferences, the negotiator uses it as a reference point and prepares a counter-offer that is most similar to it. In the Yan et al. (2007) proposal, the authors take advantage of the one- to-many negotiations occurring for a composite service. The utility value of all the received offers is calculated, and the one with best utility is used as a reference point for preparing the counter-offer.

3.6.3.3 Negotiation Strategy

Negotiation strategy is another part of the decision model. Conceptualized at a higher level of abstraction than the negotiation tactics, it aims to maximize the utility function of the negotiator for a contract (Faratin et al. 1998), by determining when to use which tactic to prepare the offer, or what combination of tactics to use. More precisely, strategy can be thought of as the pattern of change in the weight of different tactics over time (Zulkernine et al. 2009). Taking it one step further, Di Nitto et al. (2007) states that strategy is not just about how to weight different tactics over time, but it can also address the following factors:

 Changing the importance of negotiation issues over time, such as preferring availability over the response time if the latter cannot be improved so far,

 Changing the severity of the constraint, such as relaxing some constraints on the values of some negotiation issues to reflect more concession when the negotiation time is about to expire.

Deciding on the best strategy for a negotiator involves the challenges addressed mostly in game-theory, microeconomics, and multi-agent systems and has not been the focus of composite service selection community.