1. An apparatus for evaluating available bandwidth in a data transfer path that transfers data between data communication nodes of a data network, comprising: a filter for coupling to the data transfer path, said filter configured to use a filter operation during real-time operation of the data transfer path in which received data is processed at a rate at which it is received to produce real-time information for use in estimating the available bandwidth; logic coupled to said filter for analyzing said real-time information to determine an estimate of the available bandwidth; and an adjuster coupled to said logic and said filter for selectively adjusting input parameters to the filter in response to said logic, said input parameters controlling whether an estimate is produced which reflects slowly changing conditions in the network or rapidly changing conditions in the network.