Method for delivering data streams over communication networks is disclosed, the method comprising determining reference points in a stream of data being locations in the stream where predefined number of characters fulfill a predetermined criterion; registering digital signatures being values returned from a predetermined function taken over predefined ranges of content, the ranges are in correlation with the reference points; using the digital signatures to locate locally stored content, and using the reference points or creating a dictionary and using it to synchronize between currently received pieces of data and between locally stored matching content. Communication server implementing the method is also disclosed, and further disclosed are communication systems comprising at least one said server.