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Introduction to Networking Charles Severance

Credits Illustrations: Mauro Toselli Editorial Support: Sue Blumenberg Cover Design: Aimee Andrion The SketchNote illustrations were drawn on an iPad using Paper by www.ﬁftythree.com using a dedicated stylus pencil. The illus- trations were converted from PNG to SVG and EPS vector formats using www.vectormagic.com. The technical ﬁgures for the book were drawn with OmniGrafﬂe. Printing History 2015-May-25 Original Printing - CreateSpace Copyright Details This book is Copyright Charles R. Severance. The paper/print version of this book is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License. Permission is speciﬁcally granted to make copies as necessary of all or part of the materials in this book as part of a course or course packet. http://creativecommons.org/licenses/by-nc/4.0 Electronic copies of this book in EPUB, PDF, and HTML are licensed under a Creative Commons Attribution 4.0 International License. http://creativecommons.org/licenses/by/4.0/ If you are interested in translating this book into a language other than English, please contact me. I am willing to give commercial print distribution rights for a complete and responsible transla- tion.

Preface The goal of this book is to provide a basic understanding of the technical design and architecture of the Internet. The book is aimed at all audiences – even those with absolutely no prior tech- nical experience or math skills. The Internet is an amazingly beau- tiful design and should be understood by all who use it. While this book is not about the Network+ or CCNA certiﬁcations, I hope it serves as a way to give students interested in these certiﬁcations a good starting point. I want to thank Pamela Fox of Khan Academy for coming up with the idea of an introductory network technology course using open materials. I initially developed this material as a single week’s lecture in the SI502 - Networked Computing course that I taught at the Univer- sity of Michigan School of Information starting in 2008. I reﬁned and expanded the material to be three weeks of the Internet His- tory, Technology, and Security (IHTS) course that I have taught to over 100,000 students on Coursera starting in 2012. This book adds further detail to create a standalone text that can be read for enjoyment or used to support an introductory course that fo- cuses on the Internet’s architecture. This book has been particularly fun in that it is a collaboration with my friends Mauro Toselli (@xlontrax) and Sue Blumenberg. I met Mauro and Sue in 2012 when they became volunteer Community Teaching Assistants (CTAs) for my IHTS course on Coursera. Over the past three years we have become friends and colleagues. It is a great example of how open education can bring people to- gether. There is supporting material for this book at http://www.net-intro.com/ If you like the book, let us know. Send us a tweet with your thoughts. You can also send a tweet if you ﬁnd an error in the book. Charles R. Severance (@drchuck) www.dr-chuck.com Ann Arbor, MI USA May 20, 2015

Contents 1 Introduction 1 1.1 Communicating at a Distance . . . . . . . . . . . . . . . . 1 1.2 Computers Communicate Differently . . . . . . . . . . . 4 1.3 Early Wide Area Store-and-Forward Networks . . . . . . 5 1.4 Packets and Routers . . . . . . . . . . . . . . . . . . . . . . 6 1.5 Addressing and Packets . . . . . . . . . . . . . . . . . . . . 7 1.6 Putting It All Together . . . . . . . . . . . . . . . . . . . . . 8 1.7 Glossary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 1.8 Questions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 2 Network Architecture 13 2.1 The Link Layer . . . . . . . . . . . . . . . . . . . . . . . . . . 14 2.2 The Internetwork Layer (IP) . . . . . . . . . . . . . . . . . 16 2.3 The Transport Layer (TCP) . . . . . . . . . . . . . . . . . . 18 2.4 The Application Layer . . . . . . . . . . . . . . . . . . . . . 20 2.5 Stacking the Layers . . . . . . . . . . . . . . . . . . . . . . 21 2.6 Glossary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 2.7 Questions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 3 Link Layer 25 3.1 Sharing the Air . . . . . . . . . . . . . . . . . . . . . . . . . . 26 3.2 Courtesy and Coordination . . . . . . . . . . . . . . . . . . 28 3.3 Coordination in Other Link Layers . . . . . . . . . . . . . 29 3.4 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30 v

vi CONTENTS 3.5 Glossary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31 3.6 Questions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31 4 Internetworking Layer (IP) 35 4.1 Internet Protocol (IP) Addresses . . . . . . . . . . . . . . . 37 4.2 How Routers Determine the Routes . . . . . . . . . . . . 39 4.3 When Things Get Worse and Better . . . . . . . . . . . . 39 4.4 Determining Your Route . . . . . . . . . . . . . . . . . . . . 41 4.5 Getting an IP Address . . . . . . . . . . . . . . . . . . . . . 45 4.6 A Different Kind of Address Reuse . . . . . . . . . . . . . 47 4.7 Global IP Address Allocation . . . . . . . . . . . . . . . . . 48 4.8 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49 4.9 Glossary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49 4.10Questions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50 5 The Domain Name System 57 5.1 Allocating Domain Names . . . . . . . . . . . . . . . . . . 58 5.2 Reading Domain Names . . . . . . . . . . . . . . . . . . . . 59 5.3 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59 5.4 Glossary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60 5.5 Questions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60 6 Transport Layer 63 6.1 Packet Headers . . . . . . . . . . . . . . . . . . . . . . . . . 64 6.2 Packet Reassembly and Retransmission . . . . . . . . . 65 6.3 The Transport Layer In Operation . . . . . . . . . . . . . . 67 6.4 Application Clients and Servers . . . . . . . . . . . . . . . 68 6.5 Server Applications and Ports . . . . . . . . . . . . . . . . 68 6.6 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70 6.7 Glossary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70 6.8 Questions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70

CONTENTS 7 Application Layer vii 73 7.1 Client and Server Applications . . . . . . . . . . . . . . . 73 7.2 Application Layer Protocols . . . . . . . . . . . . . . . . . . 75 7.3 Exploring the HTTP Protocol . . . . . . . . . . . . . . . . . 76 7.4 The IMAP Protocol for Retrieving Mail . . . . . . . . . . . 80 7.5 Flow Control . . . . . . . . . . . . . . . . . . . . . . . . . . . 81 7.6 Writing Networked Applications . . . . . . . . . . . . . . . 83 7.7 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84 7.8 Glossary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84 7.9 Questions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85 8 Secure Transport Layer 89 8.1 Encrypting and Decrypting Data . . . . . . . . . . . . . . 90 8.2 Two Kinds of Secrets . . . . . . . . . . . . . . . . . . . . . . 91 8.3 Secure Sockets Layer (SSL) . . . . . . . . . . . . . . . . . 92 8.4 Encrypting Web Browser Trafﬁc . . . . . . . . . . . . . . . 93 8.5 Certiﬁcates and Certiﬁcate Authorities . . . . . . . . . . 94 8.6 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95 8.7 Glossary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96 8.8 Questions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97 9 The OSI Model 101 9.1 Physical (Layer 1) . . . . . . . . . . . . . . . . . . . . . . . . 102 9.2 Data Link (Layer 2) . . . . . . . . . . . . . . . . . . . . . . . 102 9.3 Network (Layer 3) . . . . . . . . . . . . . . . . . . . . . . . . 102 9.4 Transport (Layer 4) . . . . . . . . . . . . . . . . . . . . . . . 103 9.5 Session (Layer 5) . . . . . . . . . . . . . . . . . . . . . . . . 103 9.6 Presentation (Layer 6) . . . . . . . . . . . . . . . . . . . . . 103 9.7 Application (Layer 7) . . . . . . . . . . . . . . . . . . . . . . 103 9.8 Comparing the OSI and TCP/IP Models . . . . . . . . . . 104 9.9 Link Layer (TCP/IP) . . . . . . . . . . . . . . . . . . . . . . . 104 9.10Internetwork Layer (TCP/IP) . . . . . . . . . . . . . . . . . 105

viii CONTENTS 9.11Transport Layer (TCP/IP) . . . . . . . . . . . . . . . . . . . . 105 9.12Application Layer (TCP/IP) . . . . . . . . . . . . . . . . . . 105 9.13Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . 106 9.14Glossary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 106 9.15Questions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 106 10Wrap Up 109

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