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I did my homework on the IDP devices during the trip to the client, and will do my best to recap it here.
The official datasheet put it bluntly:
Juniper Networks® IDP Series Intrusion Detection and Prevention Appliances provide comprehensive management of unwanted applications and easy-to-use in-line protection that stops network- and application-level attacks before they inflict any damage, minimizing the time and costs associated with maintaining a secure network. Using industry-recognized stateful detection and prevention techniques, the IDP Series provides zero-day protection against worms, trojans, spyware, key loggers, and other malware from penetrating the network or spreading from already infected users.