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Welcome to the ninth edition of Computer Networking: A Top-Down Approach. Since the publication of the first edition years ago, our book has been adopted for use at thousands of colleges and universities, translated into languages, and used by many hundreds of thousands of students and practitioners worldwide. We’ve heard from many of these readers and have been overwhelmed by the positive response. We think one important reason for this success has been that our book continues to offer a fresh and timely approach to computer networking instruction. We’ve made changes in this ninth edition, but we’ve also kept unchanged what we believe (and the instructors and students who have used our book have confirmed) to be the most important aspects of this book: its top-down approach, its focus on the Internet, its modern treatment of computer networking, its attention to both principles and practice, and its accessible style and approach. Nevertheless, the ninth edition has been revised and updated substantially. Readers of earlier editions of our book may recall that in moving from the sixth to the seventh edition, we deepened our coverage of the network layer, expanding material which had been previously covered in a single chapter into a new chapter focused on the so-called “data plane” component of the network layer (Chapter 4) and a new chapter focused on the network layer’s “control plane” (Chapter 5). That change turned out to be prescient, as software-defined networking (SDN), one of the most important and exciting advances in networking in decades, has been rapidly adopted in practice—so much so that it’s hard to imagine an introduction to modern computer networking that doesn’t cover SDN. In moving from the seventh to the eighth edition, we significantly updated the application layer (Chapter 2), introduced a modern treatment of the transport layer in Chapter 3 by covering the (then new) QUIC protocol, and significantly expanded our coverage of cellular networks in Chapter 7. These changes have also proved prescient as the past five years since the eighth edition have seen important advances in both applicationlayer infrastructure and transport-layer protocols. Additionally, with the introduction of , cellular networks, and the software-i-zation of cellular networks, the pace of change in wireless and mobile networking has only accelerated since the eighth edition.