L18 - MIT 6.02 DRAFT Lecture Notes Fall 2010(Last update...

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MIT 6.02 DRAFT Lecture Notes Fall 2010 (Last update: November 16, 2010) Comments, questions or bug reports? Please contact [email protected] C HAPTER 18 Network Routing - I Without Any Failures This lecture and the next one discuss the key technical ideas in network routing. We start by describing the problem, and break it down into a set of sub-problems and solve them. The key ideas that you should understand by the end are: 1. Addressing. 2. Forwarding. 3. Distributed routing protocols: distance-vector and link-state protocols. 4. How routing protocols handle adapt to failures and find usable paths. 18.1 The Problem As explained in earlier lectures, sharing is fundamental to all practical network designs. We construct networks by interconnecting nodes (switches and end points) using point-to- point links and shared media. An example of a network topology is shown in Figure 18-1; the picture shows the “backbone” of the Internet2 network, which connects a large number of academic institutions in the U.S., as of early 2010. The problem we’re going to discuss at length is this: what should the switches (and end points) in a packet-switched network do to ensure that a packet sent from some sender, S , in the network reaches its intended destination, D ? The word “ensure” is a strong one, as it implies some sort of guarantee. Given that packets could get lost for all sorts of reasons (queue overflows at switches, repeated colli- sions over shared media, and the like), we aren’t going to worry about guaranteed delivery just yet. 1 Here, we are going to consider so-called best-effort delivery: i.e., the switches will “do their best” to try to find a way to get packets from S to D , but there are no guaran- tees. Indeed, we will see that in the face of a wide range of failures that we will encounter, providing even reasonable best-effort delivery will be hard enough. 1 Subsequent lectures will address how to improve delivery reliability. 1
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2 CHAPTER 18. NETWORK ROUTING - I WITHOUT ANY FAILURES Figure 18-1: Topology of the Internet2 research and education network in the United States as of early 2010. To solve this problem, we will model the network topology as a graph , a structure with nodes (vertices) connected by links (edges), as shown at the top of Figure 18-2. The nodes correspond to either switches or end points. The problem of finding paths in the network is challenging for the following reasons: 1. Distributed information: Each node only knows about its local connectivity, i.e., its immediate neighbors in the topology (and even determining that reliably needs a little bit of work, as we’ll see). The network has to come up with a way to provide network-wide connectivity starting from this distributed information. 2. Efficiency: The paths found by the network should be reasonably good; they shouldn’t be inordinately long in length, for that will increase the latency of pack- ets. For concreteness, we will assume that links have costs (these costs could model link latency, for example), and that we are interested in finding a path between any source and destination that minimizes the total cost. We will assume that all link
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