Chapter 2. Policy Routing Theory
| Traditional IPv4 routing is summarized as "All routing is a destination-driven process." When a router looks at an IPv4 packet it cares only about the destination address in the header of the packet. It uses this destination address to make a decision on where to forward the packet. This scenario works fine for simple networks where all of the machines in the network only need to get out to some place. Think of a standard driveway on a house. It starts at the house and goes to the road. When you get to the road you have two choices: turn right or turn left. The choice you make depends on where you want to go. Thus you make a destination-based routing choice. In the simplest case your driveway connects to a dead-end road and you will always turn the same way. This would be the single default route scenario. But to somewhat extend the analogy, what if your driveway opened onto a 6-way intersection? And to complicate the picture further, one of the intersecting roads is a highway on-ramp that only permits sedans, one of the other roads is a gravel road only usable by tractors, and so on. In these cases you need to make a decision based on what you are driving as well as where you are going. These are the types of network setups where you route packets differently, depending not only on the destination addresses but also on other packet fields such as source address, IP protocol, transport protocol ports, or even packet payload. This type of routing is called Policy Routing. |
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