NGN (Next Generation Network) architectures are constantly evolving towards solutions that allow the operator to provide QoS-guaranteed services in heterogeneous, multi-domain and multi-services networks. In this context, DiffServ-aware Traffic Engineering (DS-TE) is one of the most advanced technologies, which performs traffic engineering (IE) in a differentiated service environment by applying routing constrains with class granularity. This approach requires the definition of an efficient bandwidth constraint model to allocate the available link resources in a per Class Type (CT) basis, which influence both resource utilization and provided QoS. We analyze this problem and propose an algorithm to find the optimal mapping of service types into Class Types and the optimal bandwidth allocation for each of these. Herein, the optimality is expressed in terms of a cost function weighting both the expected satisfaction of the quality of service targets and the network resource utilization. The effectiveness of the proposed solution is analyzed considering a real context with expect near future combination of service requests from the end-users.