A system protects against loss of communication during network attacks. In a first implementation, a system (120) models the behavior of normal users in a network in response to an application of a first packet filtering technique. The system (120) receives a group of packets from a first user subsequent to the application of the first packet filtering technique and creates one or more models reflecting the behavior of the first user based on the received packets. In another implementation, a system (130) receives a stream of packets subsequent to a filtering technique being applied, partitions the packets into groups, where each group corresponds to more than one packet, and classifies each group of packets as a normal group or an attack group using one or more models. Each model reflects a normal response to an application of the filtering technique. The system (130) forwards groups classified as normal groups, thus preventing network attacks from choking off all communication in the network.