Overlay networks provide exactly this opportunity. Overlay nodes can be
programmed to support the new capability or feature and then depend on
conventional nodes to provide the underlying connectivity. Over time, if the
idea deployed in the overlay proves useful, there may be economic motivation
to migrate the functionality into the base system—that is, add it to
the feature set of commercial routers. On the other hand, the functionality
may be complex enough that an overlay layer may be exactly where it
belongs.