Dear all, At todays meeting, the claim was made (at least twice) that adding dynamically computed metrics to IS-IS is "just a feature". I strongly disagree with this assessment -- it's an open research problem, and a difficult one at that.
Any interesting metric (packet loss, delay, etc.) will cause a negative feedback loop, which will lead to oscillations. In Babel, there are three mechanisms that cope with the oscillations caused by feedback loops: 1. the protocol is loop-avoiding, which means that even when oscillations happen they don't normally cause packets to be lost; 2. the protocol uses delayed updates, which means that even when a metric is varying the amount of network traffic remains controlled; 3. the protocol uses a hysteresis mechanism which limits the frequency of oscillations. IS-IS is fundamentally based on the notion that a topology change is propagated throughout the network in a timely manner and SPF is recomputed by all nodes -- it has no loop-avoidance mechanism other then timely reconvergence. If implemented naively, a dynamically computed metric will cause repeated flooding, repeated SPF computation, and repeated transient loops. I'm sure these issues can be solved, and I'm pretty confident that Henning can tell you how; at any rate, it would be a very interesting research project. However, it is a difficult one, and the three techniques used in Babel do not apply directly to a link-state protocol. -- Juliusz _______________________________________________ homenet mailing list homenet@ietf.org https://www.ietf.org/mailman/listinfo/homenet
