The CoDel queue management algorithm

“Bufferbloat” can be thought of as the buffering of too many packets in flight between two network end points, resulting in excessive delays and confusion of TCP’s flow control algorithms. It may seem like a simple problem, but the simple solution—make buffers smaller—turns out not to work. A true solution to bufferbloat requires a deeper understanding of what is going on, combined with improved software across the net. A new paper from Kathleen Nichols and Van Jacobson provides some of that understanding and an algorithm for making things better—an algorithm that has been implemented first in Linux…”

“…One of the key insights in the design of CoDel is that there is only one parameter that really matters: how long it takes a packet to make its way through the queue and be sent on toward its destination. And, in particular, CoDel is interested in the minimum delay time over a time interval of interest. If that minimum is too high, it indicates a standing backlog of packets in the queue that is never being cleared, and that, in turn, indicates that too much buffering is going on. So CoDel works by adding a timestamp to each packet as it is received and queued. When the packet reaches the head of the queue, the time spent in the queue is calculated; it is a simple calculation of a single value, with no locking required, so it will be fast…”

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