1. UNIT 2 LESSON 8 CS PRINCIPLES

2. UNIT 2 LESSON 8 OBJECTIVES Students will be able to: Describe how routers develop routing tables to determine how to send packets. Simulate the job of a router. Develop a router table to efficiently transmit packets from one destination to another across the Internet.

3. ROUTERS Routers, like many systems on the Internet, are governed by protocols. In order to determine where a packet should be sent so that it can reach its desired destination, the router uses a static or dynamic routing protocol such as Routing Information Protocol (RIP) or Open Shortest Path First (OSPF), which both create and maintain a routing table.

4. ROUTERS (CONT) Routers are only able to communicate with their directly connected neighbors, so in order to generate an efficient table for sending packets they communicate with their neighbors, comparing who has the best way to reach a particular destination. As routers learn about more paths, they are able to share this information with their neighbors to help update and maintain their routing tables.

5. HOW DOES A ROUTER LEARN WHICH WAY TO ROUTE PACKETS? A router is connected to a few other routers but the Internet is much bigger. If a packet is supposed to get to destination X, but the router is only connected to other routers A, B, and C, how do you know which one you should forward the packet to in order to reach X as fast as possible? There might be a lot of Internet between you, your neighbors, and X. Even though the cost or distance to your neighbor B might be the greatest of your other neighbors, you can’t know what lies beyond B. It might be the best way to send traffic to X, but how can you know?

6. HOW DOES A ROUTER LEARN WHICH WAY TO ROUTE PACKETS? Like the problems we considered yesterday, a router wants to find the shortest path through the network to any possible location on the Internet. Unlike the algorithm from yesterday, today we recognize that a router cannot see how the entire network is connected ahead of time; not only is this not possible because the Internet is too big, but also the network changes all the time, new routers come online, cables get cut accidentally, etc. Instead, a router can only observe traffic between it and its direct connections. The question, then, is how can a router learn about the network in order to optimally route packets?

7. IMPORTANCE OF ORDER: If tables are immobile, it is sufficient to create circles of chairs. This activity relies on each student communicating with a specific set of individuals. The activity guides are designed with the assumption that students will be sitting in alphabetical order, and the lesson will be much harder to facilitate if this not the case.

8. GROUPS OF EIGHT: This activity has been designed with eight routers, because that’s enough to make the shortest paths between nodes interesting and to give students an idea of the kind of interactions that routers have. Using the current set of routers and costs, all students will find that there is a more efficient route between themselves and one of their neighbors than through the direct connection. With smaller groups, these connections are much more subtle to the student.