<i>"Basically, what I think Google and some other companies find attractive about SDN is the ability to affect policy more directly from a single location with one view of the network as a whole."</i><p>That's "Software defined networking", which is a lot like virtual circuits. The first time A tries to send to B, a request is sent to some central server which decides if A can talk to B, how A gets to reach B, and whether it's desirable to listen in or MITM the conversation. The central server then sets up the routing, but doesn't handle the traffic.<p>This is amazingly like Tymnet, circa 1969.<p>Amazon has something like this internally, which they use as a big patchboard for their servers. This gives them freedom in allocating IP addresses. It's am example of "There is no problem in computer science which cannot be solved by adding another layer of indirection."