1. A method for performance evaluation in a network providing communication services by means of a generalized processor sharing packet scheduling discipline, said services comprising classes of services having relative priorities, the method being performed by a computer system comprising at least one processor and at least one non-transitory computer readable medium encoded with program instructions that, when executed, cause the system to perform the method, the method comprising the steps of: evaluating, by the at least one processor, performance of a given class of services as a function of a load produced by other classes of services having a priority higher than or equal to said given class as well as of perturbation effect exerted on said given class by other classes having priority higher than said given class; and determining, by the at least one processor, said load and said perturbation effect as weighed sums of contributions of each of said other classes to said load and said perturbation effect.