Policy-based Routing

Even when we finally manage to build a map of the Internet, there are still some difficulties we need to take into account. The main one is called policy-based routing [24], by which a router is configured to use different criteria than just a distance metric to decide which peer to send a packet to. For example, some inter-AS connections are made available only to packets coming from a pre-selected set of ASes. This may lead to a situation in which the distance measured on the AS-based map is inconsistent with the actual route length.

Figure 4.6: The policy-based routing can falsify the distance calculations

For example, assume that we have built our map of the network presented in Figure 4.6 using the BGP routing tables coming from both AS 64001 and AS 64003. In this way the map we have built is complete. Also, assume that the policy-based link between these two systems can be used only by packets originating from one of them. According to our metric, the distance between machines A and B is 3, as the shortest route between them goes through ASes 64000, 64001, 64003, and 64004. It is inconsistent, however, with the real route of length 4 that packets from AS 64000 or AS 64004 will follow, as these packets are not allowed to make use of the link between AS 64001 and AS 64003. In practice, it is impossible to distinguish between ``public'' and ``policy-based'' inter-AS links given only a routing table.