[Conflicting] Design Goals
scalability -- the architecture must be able to cope with faster and faster links: today, 1Gbit or OC-48. Tomorrow, OC-192 and more.

Note, I said the architecture - in reality, we barely know how to do forwarding at those speeds...

portability -- must adapt to different HW platforms, from off-the shelf PC, to dedicated capture hardware and/or processing nodes (e.g. network processors).

ease of reprogramming/reconfiguration. We cannot possibly compute all metrics on all traffic, so ease of adaptation is a major requirement.