The paper faces the problem of the runtime negotiation on the Traffic Conditioning Agreement (TCA) and Service Level Agreement (SLA) in IP Differentiated Services (Diff-Serv) networks. A first contribution of the paper with respect to previous literature is to propose a multi-attribute description of QoS targets and traffic profile. A second contribution of this work is to provide a mathematical formalization of the negotiation problem suitable for automated negotiation. The model is tested through simulation to a) verify the convergence of the negotiation process, which is critical to ensure that negotiation can be performed automatically and b) analyze the agreement solutions that are reached by negotiation parties for different values of quality of service and price parameters. Results show that the model has a sound convergence. With this paper?s negotiation model, the primary goal of negotiating parties is to maintain the value of their utility function constant by searching a trade-off between QoS and price. However, the willingness to cooperate of the negotiating parties has a strong impact on the agreement value of negotiation parameters.