The objective of this work is to help the user of a simulation software to prepare a simulation run. To this end, the user has to supply the simulator with numerous data of various kinds and our goal is to help him to define a set of coherent data. In this paper, we indicate on the example of a stream specification how constraint propagation techniques can be used to reach this objective. After a brief description of the basic principles of constraint propagation, we give an algorithm for the good specification of a stream. This algorithm relies on the transformation of the equations linking the various properties of a stream into a set of constraints expressing necessary conditions for a stream being correctly specified. By the end, we show how to advise the user on specifying a stream from constraint propagation. Future works will consist in extending constraint propagation to the assignment of the other variables required for a simulation run.