1. A classification engine capable of receiving a plurality of protocol data units (PDUs) and performing a tree-based classification on the PDUs, the classification engine comprising: input means for receiving the PDUs; parsing means capable of parsing the PDUs to generate an abstracted protocol structure for at least one of the PDUs; classifier capable of performing the tree-based classification, said classifier being capable of enforcing policy using the abstracted protocol structure; and output means for transmitting the PDUs.