The Internet is in the midst of a transformation, one that moves away from bundled proprietary devices, and instead embraces disaggregating network hardware (which becomes commodity) from the software that controls it (which scales in the cloud). The transformation is generally known as Software-Defined Networking (SDN), but because it is disrupting the marketplace, it is challenging to untangle business positioning from technical fundamentals, from short-term engineering decisions. This book provides such an untangling, where the most important thing we hope readers take away is an understanding of an SDN-based network as a scalable distributed system running on commodity hardware.

Anyone who has taken an introductory networking class recognizes the protocol stack as the canonical framework for describing the network. Whether that stack has seven layers or just three, it shapes and constrains the way we think about computer networks. Textbooks are organized accordingly. SDN suggests an alternative world-view, one that comes with a new software stack. This book is organized around that new stack, with the goal of presenting a top-to-bottom tour of SDN without leaving any significant gaps that the reader might suspect can only be filled with magic or proprietary code. We invite you do the hands-on programming exercises included at the end of the book to prove to yourself that the software stack is both real and complete.

An important aspect of meeting this goal is to use open source. We do this in large part by taking advantage of two community-based organizations that are leading the way. One is the Open Compute Project (OCP), which is actively specifying and certifying commodity hardware (e.g., bare-metal switches) upon which the SDN software stack runs. The second is the Open Networking Foundation (ONF), which is actively implementing a suite of software components that can be integrated into an end-to-end solution. There are many other players in this space—from incumbent vendors to network operators, startups, standards bodies, and other open source projects—each offering varied interpretations of what SDN is and is not. We discuss these other perspectives and explain how they fit into the larger scheme of things, but we do not let them deter us from describing the full breadth of SDN. Only time will tell where the SDN journey takes us, but we believe it is important to understand the scope of the opportunity.

This book assumes a general understanding of the Internet, although a deeper appreciation for the role switches and routers play forwarding ethernet frames and IP packets is helpful. Links to related background information are included to help bridge any gaps.

This book is a work-in-progress, with updates, clarifications, and new material added continuously. For example, the latest version (v2.0) includes new chapters on Network Virtualization (Chapter 8) and Access Networks (Chapter 9). We are eager to hear your feedback and suggestions on how we can continue to improve this book.


The software described in this book is due to the hard work of the ONF engineering team and the open source community that works with them. We acknowledge their contributions, with a special thank-you to Yi Tseng, Max Pudelko, and Charles Chan for their contributions to the tutorials that this book includes as hands-on exercises. We also thank Charles Chan, Jennifer Rexford, Nick McKeown, Kentaro Ebisawa, Motonori Shindo, and Sina Ebrahimi for their comments and feedback.

The cover photo of the Ueno Station (Tokyo) is by Athena Lam on Unsplash.

Larry Peterson, Carmelo Cascone, Brian O’Connor, Thomas Vachuska, and Bruce Davie
November 2021