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 perform a filter operation during real-time operation of the data transfer path to produce real-time information for use in estimating the available bandwidth, wherein the available bandwidth at any time t is a maximum increase in data rate between the data communication nodes which can theoretically occur at time t without causing congestion, according to B ⁡ ( t ) = min j ⁢ ( Cj - Sj ⁡ ( t ) ) where: B(t)=the available bandwidth at time (t); Cj=highest possible bit rate at packet level over link j in the data transfer path; Sj(t)=load on given link j at time (t); logic coupled to said filter for analyzing said real-time information; an adjuster comprising a selector coupled to said logic and said filter for selectively adjusting said filter operation in response to said logic, by selecting filter parameters from a first set of filter parameters that provide stability and noise insensitivity, or from a second adaptive set of filter parameters that provide a fast adaptation to a new system state; and a change detector for providing an alarm indication to the selector at an abrupt change in the system state, wherein the filter parameters are adjusted temporarily from the first set of filter parameters to the second adaptive set of filter parameters.