It has been nearly ten years since the fifth edition of Computer Networks: A Systems Approach was published. Much has changed in that time, most notably, the explosion of the cloud and smartphone apps onto the scene. In many ways, this is reminiscent of the dramatic effect the Web was having on the Internet when we published the first edition of the book in 1996.

The sixth edition adapts to the times, but keeps the Systems Approach as its North Star. In broad strokes, we update and improve this new edition in four main ways:

  • We refresh the examples to reflect the current state of the world. This includes deleting anachronisms (e.g., dial-up modem), using popular applications (e.g., Netflix, Spotify) to motivate the problems being addressed, and updating the numbers to represent the state-of-the-art technology (e.g., 10-Gbps Ethernet).

  • We connect the dots between the original research that led to the development of technologies like multicast, real-time video streaming, and quality-of-service, and the now-familiar cloud applications like Zoom, Netflix, and Spotify. This is in keeping with our emphasis on the design process and not just the end result, which is especially important today since so much of the Internet’s functionality is primarily delivered by proprietary commercial services.

  • We place the Internet in the broader context of the Cloud, and just as importantly, in the context of the commercial forces that are shaping the Cloud. This has minimal impact on the technical details presented throughout the book, but it is discussed in a new Perspective section at the end of each chapter. We hope one side-effect of this discussion is to foster an appreciation for the Internet’s continuous evolution, and the opportunity for innovation this represents.

  • We distill the important principles of network design in a series of Key Takeaways throughout the book. Each takeaway is a concise statement of either a general system design rule or a fundamental networking concept, drawing on the examples presented in the surrounding text. Pedagogically, these takeaways correspond to the high-level learning objectives for the book.

What is “A Systems Approach”?

The “Systems Approach” refers to the field of design and implementation of computer systems. The term is used commonly by computer science researchers and practitioners who study the issues that arise when building complex computing systems such as operating systems, networks, distributed applications, etc. The key to the systems approach is a “big picture” view: you need to look at how the components of a system interact with each other to achieve an overall result, rather than simply optimizing each component. In the networking context, that often means going beyond the traditional layered view to see how an issue is best tackled in a way that might touch several layers. A good example of this is congestion control, which touches multiple layers and affects both end systems and network devices. We treat congestion, a system-wide issue, in its own chapter rather than trying to shoehorn it into the transport layer, for example. The Systems Approach has a strong focus on real-world implementation, with the Internet being the obvious example of a widely-deployed, complex networking system.

New Material in the Sixth Edition

More specifically, the sixth edition includes the following major changes:

  • New Perspective Section in Chapter 1 introduces the recurring Cloudification theme.

  • New Section 2.8 describes the Access Network, including Passive Optical Networks (PON) and 5G’s Radio Access Networks (RAN).

  • Refactored topics across Sections 3.1 (Switching Basics) and 3.2 (Switched Ethernet), including expanded coverage of VLANs.

  • Section 3.5 updated to include descriptions of Bare-Metal Switches and Software-Defined Networks (SDN).

  • New Perspective Section in Chapter 3 describes virtual network overlays, VXLAN, and the role of overlays in the Cloud.

  • Refactored topics across Sections 4.1 (Global Internet) and 4.2 (IP Version 6).

  • New Perspective Section in Chapter 4 describes how the Cloud impacts the Internet’s structure.

  • Section 5.2 expanded to include a discussion of QUIC.

  • Section 5.3 expanded to include a description of gRPC.

  • Sections 6.3 and 6.4 updated to include descriptions of TCP CUBIC, DCTCP, and BBR.

  • Section 6.4 expanded to include a description of Active Queue Management (AQM).

  • Section 7.1 expanded to include a description of Protocol Buffers.

  • Section 7.2 expanded to include a description of HTTP Adaptive Streaming.

  • New Section 8.1 introduces the duality of Threats and Trust.

  • Refactored topics across Sections 8.3 (Key Predistribution) and 8.4 (Authentication Protocols).

  • New Perspective Section in Chapter 8 describes Decentralized Identity Management and the role of Blockchains.

  • Section 9.1 updated to include a description of HTTP/2, along with a discussion of REST, gRPC, and Cloud Services.

  • Section 9.3 expanded to include a description of modern Network Management Systems including the use of OpenConfig and gNMI.


To construct a networking course around the material in this book, it can be helpful to understand the overall organization, which can be characterized as having three major parts:

  • Conceptual and foundational material, that is, the big ideas at the heart of networking.

  • Core protocols and algorithms that illustrate how the foundational ideas are put to practice.

  • Advanced material that might or might not fit in any single semester course.

This characterization can be applied at the Chapter level: Chapter 1 is foundational, Chapters 2, 3, 5, and 9 are core, and Chapters 4, 6, 7, and 8 cover more advanced topics.

This characterization can also be applied at the Section level, where roughly speaking, each Chapter advances from basic concepts to specific technology to advanced techniques. For example, Chapter 3 starts by introducing the basics of switched networks (3.1), then covers the specifics of Switched Ethernet and the IP Internet (3.2-3.4), and concludes with an optional discussion of SDN (3.5). Similarly, Chapter 6 starts with foundational ideas (6.1-6.2), then explores TCP congestion control (6.3), and concludes with optional advanced material (6.4-6.5).

Code Examples

Since the first edition of this book, we have used code examples to illustrate how various protocols and algorithms might be implemented. Originally, the code came from the $x$-kernel, a framework for protocol implementation and research developed in the 1990s, and you could compile and run every code fragment in the book. As the $x$-kernel code is no longer maintained, we no longer check that all the code in the book compiles or runs. At this point the code should only be used as a guide to assist the reader in understanding the topic at hand.


We would like to acknowledge the following people for their help with new content:

  • Larry Brakmo: TCP Congestion Control

  • Carmelo Cascone: Bare-Metal Switches

  • Charles Chan: Bare-Metal Switches

  • Jude Nelson: Decentralized Identity

  • Oguz Sunay: Cellular Networks

  • Thomas Vachuska: Network Management

Along with the following individuals (GitHub users) for their various contributions and bug fixes:

  • Mohammed Al-Ameen

  • Mike Appelman

  • Andy Bavier

  • Manuel Berfelde

  • Brian Bohe

  • John Craton

  • Peter DeLong

  • Aaron Gember-Jacobson

  • Chris Goldsworthy

  • John Hartman

  • Ethan Lam

  • Diego López León

  • Matteo Scandolo

  • Mike Wawrzoniak

  • 罗泽轩 (spacewander)

  • Arnaud (arvdrpoo)

  • Desmond (kingdido999)

  • Guo (ZJUGuoShuai)

  • Hellman (eshellman)

  • Xtao (vertextao)

  • Joep (joepeding)

  • Seth (springbov)

  • miluchen (miluchen)

  • Takashi Idobe (Takashiidobe)

  • Beth (Innominata88)

  • Mohit Mayank (cueo)

Finally, we would like to thank the following reviewers for their many helpful comments and suggestions. Their impact was significant.

  • Mark J. Indelicato, Rochester Institute of Technology

  • Michael Yonshik Choi, Illinois Institute of Technology

  • Sarvesh Kulkarni, Villanova University

  • Alexander L. Wijesinha, Towson University

Open Source

Source for this book is available at, and is available under terms of the Creative Commons (CC BY 4.0) license. The community is invited to contribute corrections, improvements, updates, and new material under the same terms. Like many open source software projects, this one has been seeded with once restricted content: the fifth edition of Peterson and Davie, copyrighted by Elsevier. Our hope is that open sourcing this material will both make it widely available and serve as an attractor for new content: updating what’s already there, expanding it to cover new topics, and augmenting the text with additional teaching collateral.

If you make use of this work, the attribution should include the following information:

Title: Computer Networks: A Systems Approach
Authors: Larry Peterson and Bruce Davie
Copyright: Elsevier, 2012
License: CC BY 4.0

How to Contribute

We hope that, if you use this material, you are also willing to contribute back to it. If you are new to open source, you might check out this How to Contribute to Open Source ( guide. Among other things, you’ll learn about posting Issues that you’d like to see addressed, and issuing Pull Requests to merge your improvements back into GitHub. We hope you’ve derived value from Computer Networks: A Systems Approach over the years, and we’re eager to have you join us in this new venture.

Larry & Bruce
October 2020