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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.
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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- Fall '13