How can this be? Well, the Border Gateway Protocol (BGP) never
promised that it would find the shortest route between any two sites;
it only tries to find some route. To make matters more complex, BGP’s
routes are heavily influenced by policy issues, such as who is paying
whom to carry their traffic. This often happens, for example, at peering
points between major backbone ISPs. In short, that the triangle inequality
does not hold in the Internet should not come as a surprise.
Howdowe exploit this observation? The first step is to realize that there
is a fundamental tradeoff between the scalability and optimality of a routing
algorithm. On the one hand, BGP scales to very large networks, but
often does not select the best possible route and is slow to adapt to network
outages. On the other hand, if you were only worried about finding
the best route among a handful of sites, you could do a much better job of
monitoring the quality of every path you might use, thereby allowing you
to select the best possible route at any moment in time.