These steps for forwarding work as long as there are

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Unformatted text preview: ude a checksum that covers the packet’s header, and possibly also the data being sent. These steps for forwarding work as long as there are no failures in the network. In the next lecture, we will expand these steps to combat problems caused by failures, packet losses, and other changes in the network that might cause packets to loop around in the network forever. We will use a “hop limit” field in the packet header to detect and discard packets that are being repeatedly forwarded by the nodes without finding their way to the intended destination. SECTION 18.3. OVERVIEW OF ROUTING ￿ 18.3 5 Overview of Routing If you don’t know where you are going, any road will take you there. —Lewis Carroll Routing is the process by which the switches construct their routing tables. At a high level, most routing protocols have three components: 1. Determining neighbors: For each node, which directly linked nodes are currently both reachable and running? We call such nodes neighbors of the node in the topology. A node may not be able to reach a directly linked node either because the link has failed or because the node itself has failed for some reason. A link may fail to deliver all packets (e.g., because a backhoe cuts cables), or may exhibit a high packet loss rate that prevents all or most of its packets from being delivered. For now, we will assume that each node knows who its neighbors are. In the next lecture, we will discuss a common approach, called the HELLO protocol, by which each node determines who its current neighbors are. The basic idea if for each node to send periodic “HELLO” messages on all its live links; any node receiving a HELLO knows that the sender of the message is currently alive and a valid neighbor. 2. Sending advertisements: Each node sends routing advertisements to its neighbors. These advertisements summarize useful information about the network topology. Each node sends these advertisements periodically, for two reasons. First, in vector protocols, periodic adv...
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