Convergence time the convergence time of our distance

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Unformatted text preview: the total bandwidth consumed is (4km2 + 2￿mn)/ADVERT INTERVAL bytes/second. It is easy to see that there is no connected network in which the bandwidth consumed by the simple link-state protocol is lower than the simple distance-vector protocol; the important point is that the former is quadratic in the number of links, while the latter depends on the product of the number of nodes and number of links. Convergence time. The convergence time of our distance vector and path vector protocols can be as large as the length of the longest minimum-cost path in the network multiplied by the advertisement interval. The convergence time of our link-state protocol is roughly one advertisement interval. Robustness to misconfiguration. In a vector protocol, each node advertises costs and/or paths to all destinations. As such, an error or misconfiguration can cause a node to wrongly advertise a good route to a destination that the node does not actually have a good route for. In the worst case, it can cause all the traffic being sent to that destination to be hijacked and possibly “black holed” (i.e., not reach the intended destination). This kind of problem has been observed on the Internet from time to time. In contrast, the link-state protocol only advertises each node’s immediate links. Of course, each node also re-broadcasts the advertisements, but it is harder for any given erroneous node to wreak the same kind of havoc that a small error or misconfiguration in a vector protocol can. In practice, link-state protocols are used in smaller networks typically within a single company (enterprise) network. The routing between different autonomously operating networks in the Internet uses a path vector protocol. Variants of distance vector protocols that guarantee loop-freedom are used in some small networks, including some wireless “mesh” networks built out of short-range (WiFi) radios. ￿ Acknowledgments Thanks to Sari Canelake for several useful comments. CHAPTER 19. NETWORK ROUTING - II 12 ￿ ROUTING AROUND FAILURES Problems and Questions 1. Why does the link-state advertisement include a sequence number? 2. What is the purpose of the hop limit field in packet headers? Is that field used in routing or in forwarding? 3. Describe clearly why the convergence time of our distance vector protocol can be as large as the length of the longest minimum-cost path in the network. 4. Suppose a link connecting two nodes in a network drops packets independently with probability 10%. If we want to detect a link failure with a probability of falsely reporting a failure of ≤ 0.1%, and the HELLO messages are sent once every 10 seconds, then how much time does it take to determine that a link has failed? 5. *PSet* You’ve set up a 6-node connected network topology in your home, with nodes named A, B, . . . , F . Inspecting A’s routing table, you find that some entries have been mysteriously erased (shown with “?” below), but you find the following entr...
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