Counting Flows over Sliding Windows in High Speed Networks

Counting the number of flows present in network traffic is not trivial, given that the naive approach of using a hash table to track the active flows is too slow for the current backbone network speeds. Several algorithms have been proposed in the recent literature that can calculate an approximate count using small amount of memory and few memory accesses per packet. Fewer works have addressed the more complex problem of counting flows over sliding windows, where the main challenge is to continuously expire old information. One of the existing proposals is a straightforward adaptation of the direct bitmaps technique to the sliding window model. We present an algorithm called Countdown Vector that also builds upon the direct bitmaps technique. Our algorithm, however, obtains significant cost reductions both in terms of memory and CPU, by introducing an extra approximation in the mechanism in charge of the expiration of old information.