We represent a data network as a set of links shared by a dynamic number of competing flows. These flows are generated within sessions and correspond to the transfer of a random volume of date on a pre-defined network route. The evolution of the stochastic process describing the number of flows on all routes, and the performance of the data transfers, depend on how link bandwidth is allocated between concurrent flows. We use some key properties of Whittle networks to characterize the class of bandwidth allocations which are insensitive in the sense that the stationary distribution of this stochastic process does not depend on any traffic characteristics (session structure, data volume distribution) except the traffic intensity on each route. This insensitivity property presents the practical interest of allowing the development of robust engineering rules independently of precise traffic statistics.