We’re now moving on to cover interdomain routing or routing between ASes. Recall that internet routing consists of routing between tens of thousands of independently operated networks, or autonomous systems. Each of these networks operate in their own self-interest, and have independent economic and performance objectives, and yet they must cooperate to provide global connectivity so that when you’re sitting at home, you can retrieve content that might be hosted at the Georgia Tech network. Now, each independently operated network is called an autonomous system, or AS. And each AS advertises reachability to some destination by sending what are called route advertisements or announcements. The protocol that ASes use to exchange these route advertisements is called the Border Gateway Protocol, or simply, BGP. A route advertisement has many important attributes, but for now, let’s just talk about three. Now a router here, let’s say on the Comcast network, might receive a route advertisement, typically from its neighboring AS. That route advertisement might contain a destination prefix, such as the IP prefix for Georgia Tech. Then it might contain what’s called a next hop IP address, which is the IP address of the router that Comcast router must send traffic to, to send traffic along that route. Typically that next hop IP address is the IP address for the first router in the neighboring network. And Comcast router knows how to reach that next hop IP address because its border router and the border router in the neighboring AS are on the same subnet. Typically this might be a /30 subnet, therefore this IP address is reachable from Comcast’s border. A third important attribute is whats called the AS path which is a sequence of what are called AS numbers, that describe the route to the destination. Now strictly speaking, the AS path is nothing more than the sequence of ASes that the route traversed to reach the recipient AS. So for example, Georgia Tech’s AS number is 2637 and Abilene’s is 10578. So the AS path that Comcast would hear if it received a route advertisement from Abilene for Georgia Tech, would be 10578 followed by 2637. So in the remainder of the lesson we’ll look at other BGP route attributes. But these are essentially the three most important because they describe how to stitch together an interdomain path to a global destination. So we have the destination IP prefix for the destination that a router might want to send traffic to. The next hop, which is the IP address for the router for the next hop along the path. And finally, the AS path, which is the sequence of ASes that the route traversed en route to the AS that’s hearing the announcement. The last AS number on the AS path is often called the origin AS, because that is the AS that originated the advertisement for this IP prefix. In this case, the origin AS is 2637, or Georgia Tech, because it is the AS that originated the announcement for this prefix.