In one embodiment, a rail extension comprises: an energy absorber comprising a polymer body, and vehicle attachment tabs extending from one end of the energy absorber and configured to attach to a vehicle rail, and an attachment tab extending from another end of the energy absorber and configured to attach to a bumper beam. The particles generally have the same size and shape. The admission control module may, as a result of dynamic changes to network topology capacity, re-compute the link utilization for effected active flows. At least some of the data providers are creatable on a same thread as that of the data model. The optical fiber comprises several pairs of silica layers around the core, each pair comprising an inner layer of refractive index ni and an outer layer of refractive index ne, the refractive index ne of the outer layer being lower that the refractive index ni of the inner layer of the same pair. , EF is a bivalent connecting group formed by the fluorination of E, etc.