1. A method for filtering packets in a real time stream of incoming packets, comprising: creating a key from a field in a packet having a header, the field being specified by a set of packet classification rules; transforming the key into a spectral domain key; comparing the spectral domain key to a spectral envelope, the spectral envelope comprising both a phase envelope and an amplitude envelope and also corresponding to the set of packet classification rules; obtaining a negative decision whenever the spectral domain key falls outside at least one of the phase envelope and the amplitude envelope; obtaining a positive decision whenever the spectral domain key falls within both the phase envelope and the amplitude envelope; and for said positive decision, directing the packet to a packet filter for determining a specific packet classification rule from the set of packet classification rules that the packet matches.