The Internet ushered in a new era of communication, and it has engendered an ever-growing set of applications that have transformed our lives. It is remarkable that all this has taken place with an Internet architecture that has remained unchanged for over forty years. While much research has been done on promising new Internet architectures, there is little hope for their adoption because the pain of transitioning to a new architecture is too great. When it comes to Internet architecture, we know much more about where we want to go, than we do about how to get there. This project looks not at what new architectures we should create, but how we could more easily deploy them. The goal is to create an Internet that is more evolvable and dynamic, so that what has served us so well over the past forty years can evolve to meet our needs over the next forty and more. <br/><br/>Some view the forty years of architectural stagnation as inevitable. After all, it has long been a central tenet that the Internet needs a "narrow waist" at the internetworking layer (L3), a single uniform protocol adopted by everyone; given this assumption, changing this layer is inevitably hard. This project rejects this dogma, attributing the requirement of a narrow waist to a fundamental flaw in the original Internet architecture: the conflation between the interdomain dataplane and the intradomain dataplanes. As an alternative, this project separates these two dataplanes. Furthermore, in copying the Internet?s use of layering, in which L3 is an overlay on L2, and L2 is an overlay on L1, this project implements the interdomain dataplane (referred to as L3.5) as an overlay on top of the intradomain dataplane (L3). This decouples the two dataplanes, allowing them to evolve independently. This project uses this approach to create an architectural framework that enables all clean-slate architectures to be incrementally deployable and coexist, making the Internet fully extensible. Moreover, rather than being some radical clean-slate architecture that could never be deployed, the framework itself is incrementally deployable.<br/><br/>Enabling architectural extensibility could greatly change the nature of the Internet. Today, an Internet architecture must try to meet all the needs of all applications. Once the Internet becomes fully extensible, with an ever-changing set of coexisting architectures, each architecture can be designed to more fully meet only some of the needs of some applications. This would hopefully lead to a broader ecosystem of architectures which, in their union, can better meet all the needs of all applications.<br/><br/>This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.