“No man but a blockhead ever wrote, except for money.” - Samuel Johnson
The textbook world is changing. On the one hand, open source software and creative-commons licensing have been great successes; on the other hand, unauthorized PDFs of popular textbooks are widely available, and it is time to consider flowing with rather than fighting the tide. Hence this open-access textbook, released for free under the Creative Commons license described below. Mene, mene, tekel pharsin.
Perhaps the last straw, for me, was patent 8195571 for a roundabout method to force students to purchase textbooks. (A simpler strategy might be to include the price of the book in the course.) At some point, faculty have to be advocates for their students rather than, well, Hirudinea.
This is not to say that I have anything against for-profit publishing. It is just that this particular book does not – and will not – belong to that category; the online edition will always be free. In this it is in good company: there is Wikipedia, there is Gnu/Linux, and there is an increasing number of other free online textbooks out there. The market inefficiencies of traditional publishing are sobering: the return to authors of advanced textbooks is at best modest, and costs to users are quite high. (None of this is meant to imply there will never be a print edition; when I started this project it seemed inconceivable that a print publisher would ever agree to having the online edition remain free, but times are changing.)
The official book website (potentially subject to change) is intronetworks.cs.luc.edu. The book is available there as online html, as a zipped archive of html files, in .pdf format, and in other formats as may prove useful.
This text is released under the Creative Commons license Attribution-NonCommercial-NoDerivs. This text is like a conventional book, in other words, except that it is free. You may copy the work and distribute it to others for any noncommercial use, but all reuse requires attribution.
Creation of derivative works also requires permission. It is not entirely clear, however, what would be considered a derivative work, beyond the traditional examples of abridgement and translation. Any supplemental materials like exams, slides or coverage of additional topics would be new, independent works, and would require no permission. Even the inclusion in such supplements of modest amounts of material from this book would have a strong claim to Fair Use. In the open-source software world, the right to make derivative works is exercised whenever the software is modified, but it is hard to see how this applies to textbooks. The bottom line is that if you have a situation you’re concerned about in this regard, let me know and I’ll probably be happy to grant permission.
The work may not be used for commercial purposes without permission. Free permission is likely to be granted for use and distribution of all or part of the work in for-profit and commercial training programs, provided there is no direct charge to recipients for the work and provided the free nature of the work is made clear to recipients (eg by including this preface). However, such permission must always be requested. Alternatively, participants in commercial programs may be instructed to download the work individually.
The Creative Commons license does not precisely spell out what constitutes “noncommercial” use. The author considers any sale of this book, even by a non-profit organization and even if the price just covers expenses, to be commercial use.
0.2 Classroom Use¶
This book is meant as a serious and more-or-less thorough text for an introductory college or graduate course in computer networks, carefully researched, with consistent notation and style, and complete with diagrams and exercises. My intent is to create a text that covers to a reasonable extent why the Internet is the way it is, to avoid the endless dreary focus on TLA’s (Three-Letter Acronyms), and to remain not too mathematical. For the last, I have avoided calculus, linear algebra, and, for that matter, quadratic terms (though some inequalities do sneak in at times). That said, the book includes a large number of back-of-the-envelope calculations – in settings as concrete as I could make them – illustrating various networking concepts.
Overall, I tried to find a happy medium between practical matters and underlying principles. My goal has been to create a book that is useful to a broad audience, including those interested in network management, in high-performance networking, in software development, or just in how the Internet is put together.
One of the best ways to gain insight into why a certain design choice was made is to look at a few alternative implementations. To that end, this book includes coverage of some topics one may never encounter in practice, but which may be useful as points of comparision. These topics arguably include ATM (3.5 Asynchronous Transfer Mode: ATM), SCTP (12.21.2 SCTP) and even 10 Mbps Ethernet (2.1 10-Mbps Classic Ethernet).
The book can also be used as a networks supplement or companion to other resources for a variety of other courses that overlap to some greater or lesser degree with networking. At Loyola, this book has been used – sometimes coupled with a second textbook – in courses in computer security, network management, telecommunications, and even introduction-to-computing courses for non-majors. Another possibility is an alternative or nontraditional presentation of networking itself. It is when used in concert with other works, in particular, that this book’s being free is of marked advantage.
Finally, I hope the book may also be useful as a reference work. To this end, I have attempted to ensure that the indexing and cross-referencing is sufficient to support the drop-in reader. Similarly, obscure notation is kept to a minimum.
Much is sometimes made, in the world of networking textbooks, about top-down versus bottom-up sequencing. This book is not really either, although the chapters are mostly numbered in bottom-up fashion. Instead, the first chapter provides a relatively complete overview of the LAN, IP and transport network layers (along with a few other things), allowing subsequent chapters to refer to all network layers without forward reference, and, more importantly, allowing the chapters to be covered in a variety of different orders. As a practical matter, when I use this text to teach Loyola’s Introduction to Computer Networks course, I cover the IP/routing and TCP material more or less in parallel.
A distinctive feature of the book is the extensive coverage of TCP: TCP dynamics, newer versions of TCP such as TCP Cubic, and a chapter on using the ns-2 simulator to explore actual TCP behavior. This has its roots in a longstanding goal to find better ways to present competition and congestion in the classroom. Another feature is the detailed chapter on queuing disciplines.
One thing this book makes little attempt to cover in detail is the application layer; the token example included is SNMP. While SNMP actually makes a pretty good example of a self-contained application, my recommendation to instructors who wish to cover more familiar examples is to combine this text with the appropriate application documentation.
Although the book is continuously updated, I try very hard to ensure that all editions are classroom-compatible. To this end, section renumbering is avoided to the extent practical, and existing exercises are never renumbered. New exercises are regularly inserted, but with fractional (floating point) numbers. Existing integral exercise numbers are sometimes given a trailing .0, to reduce confusion between exercise 12.0, say, and 12.5.
For those interested in using the book for a “traditional” networks course, I with some trepidation offer the following set of core material. In solidarity with those who prefer alternatives to a bottom-up ordering, I emphasize that this represents a set and not a sequence.
- 1 An Overview of Networks
- Selected sections from 2 Ethernet, particularly switched Ethernet
- Selected sections from 3.7 Wi-Fi
- Selected sections from 5 Packets
- 6 Abstract Sliding Windows
- 7 IP version 4 and/or 8 IP version 6
- Selected sections from 9 Routing-Update Algorithms and 10 Large-Scale IP Routing
- 11 UDP Transport
- 12 TCP Transport
- 13 TCP Reno and Congestion Management
With some care in the topic-selection details, the above can be covered in one semester along with a survey of selected important network applications, or the basics of network programming, or the introductory configuration of switches and routers, or coverage of additional material from this book, or some other set of additional topics. Of course, non-traditional networks courses may focus on a quite different sets of topics.
Instructors who make use of this book in a course, as either a primary or a secondary text, are encouraged to let me know, as this helps support continued work on the book. Comments – from anyone – on clarity, completeness, consistency and correctness are also much appreciated. I can be contacted at pld AT cs.luc.edu, or via the book comment form.
0.3 Progress Notes¶
Edition 1.0 was declared complete as of March 2014; the current edition is 1.9.0.
The intronetworks.cs.luc.edu website carries both edition 1.0 and also the current 1.9.0 edition.
As of December 2014 chapters on network management and security have been added; this completes the set of chapters originally envisioned.
At this point I am actively seeking reviewers – either for style or for technical accuracy.
0.4 Technical considerations¶
The book was prepared in reStructuredText using the linux Sphinx package, which can produce multiple formats from the same source. That said, the primary format is html. The table-of-contents sidebar and the text sidebars work best there. The html version also provides a “Quick search” box, though it only works for all-alphabetic strings; strings with hyphens such as “wi-fi” and “Diffie-Hellman” fail. The index is an effective alternative.
This book uses a modest set of unicode special characters. Unfortunately, some of these characters are not universally available in all browsers. The comma-separated characters in the first line, below, appear to have the most limited support.
The characters above should look roughly as they do in the following image:
If they do not, there are two options for browser-based viewing. If the second and third rows above display successfully, there is a unicode-safer version of the book (both online and zipped) available at intronetworks.cs.luc.edu that has the characters in the first row above replaced by those in the second row.
The other alternative is to add an appropriate font. Generally Firefox and Internet Explorer display the necessary characters out of the box, but Chrome does not. The Chrome situation can usually be fixed by adding a font and then tweaking the Chrome font settings. I have had good luck with Symbola (at shapecatcher.com/unicodefonts.html and other places). To install the font, extract the .ttf file and double-click on it. Then to adjust Chrome, go to Settings → Show advanced settings → Customize fonts (button), and change at a minimum the default Sans-serif font to Symbola. Then restart Chrome.
If no available browser properly displays the symbols above, I recommend the pdf or epub formats. The unicode-safer version, however, should work on most systems.
The diagrams in the body of the text are now in the process of being migrated to the vector-graphics .svg format, although the majority are still in .png format and a few diagrams rendered with line-drawing characters appear in the exercises. Some browsers still (2016) do not properly support zooming in on .svg images, but hopefully that is coming, and in any event .svg images are usually crisper than .png.