Charles M. Kozierok The TCP-IP Guide

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over the material I had completed, and realized that most of the TCP/IP material was done.

I felt TCP/IP was a topic that was particularly important in the Internet era, and one that I

could cover as a standalone Guide. A few more months’ work, and the TCP/IP Guide was

born.

That’s the story of the creation of an improbable TCP/IP Guide by an improbable author. ☺

You may look at the 1600-odd pages in this Guide and wonder at how this could be only a

part of the document that I originally began. But that is in fact the case, and should give you

an idea of just how large the complete project would have been had I waited to publish it in

whole. Perfectionists should never start open-ended projects!

So, after nearly three years in all, the TCP/IP Guide is complete, and I am pretty pleased

with how it came out. I feel that the breadth and depth of the coverage is unparalleled in any

other single TCP/IP resource, and I have worked very hard to make my descriptions under-

standable and enjoyable to read. I spent many weeks designing the layout of the Guide,

and many more creating and fine-tuning hundreds of examples and diagrams to help illus-

trate most of the technologies and concepts that underlie TCP/IP. Add to this the many

benefits of electronic publication, such as a clickable table of contents, hyperlinks and a

real-time search index, and I think this Guide will be of value to both beginners and

advanced users alike. I hope you find it useful.

Goals of The TCP/IP Guide

Every author who sets out to write a book or other document has certain objectives that he

or she hopes to accomplish when the work is completed. This is why you can go into a

library or bookstore, pick up several books that cover the same subject, and discover that

they are surprisingly different—not just in their content or scope, but in their entire approach

to the material.

I too had a number of goals when I set out to write this Guide; you certainly don’t need to

know them in order to read and appreciate the material, but knowing what I had in mind

while I was writing may help you while you are reading. And if you are reading this infor-

mation prior to licensing The TCP/IP Guide, knowing what I strove for in writing the Guide

may help you decide if this is the right resource for you.

My overall goal in writing this Guide was to create a resource that would allow anyone to

obtain a deep understanding of how TCP/IP technologies really work. To accomplish this, I

had a number of specific objectives that guided my writing efforts:

☯ Comprehensiveness: Like most authors writing a resource that covers a large

subject, I wanted the TCP/IP Guide to be comprehensive. Of course no single

document can cover everything, so I have had to limit the scope of the material.

However, I feel I cover more about TCP/IP as a whole than any other single book or

other resource.

☯ Comprehensibility: Creating a resource that is comprehensive is important, but I felt

that it was even more important that the Guide be comprehensible. Over the past few

years, I’ve had the opportunity to review many hundreds of books, guides, Web sites

and papers related to networking. I have found that even though most of them are

generally high in quality, too many use unexplained technical jargon or assume

extensive prior knowledge of networking concepts and technologies on the part of the

reader. I worked very hard to ensure that my descriptions, even of very complex

concepts, can be understood by almost every student of networking.

☯ Rationale: It’s certainly important to know how every TCP/IP protocol functions.

However, to gain a true understanding of complex material, one also needs to under-

stand the reasons behind why things are what they are. In writing this material I have

always tried to explain not just the what but also the why of TCP/IP. I have anticipated

and answered questions that I believed would commonly arise in the mind of someone

learning about this technology.

☯ Illustrations: A picture is worth a thousand words, as they say. There are many

concepts that no amount of verbiage will adequately explain, while a simple illustration

will do the trick. For this reason, I spent many months creating over 300 full-color

diagrams (some simple and some not so simple!) to complement the written material

in The TCP/IP Guide.

☯ User-Friendliness: I have intentionally broken many of the rules of conventional book

authorship, in creating a document that uses a conversational, first-person style, and

no small amount of humor where appropriate. My intention was to make you feel at

home while you read material that can be quite technically difficult. I want you to think

of me like a friend sitting next to you at your computer explaining how TCP/IP works,

rather than a professor preaching at you from a pulpit.

☯ Organization: Many networking books consist of dozens of subjects just listed one

after the other, leaving the reader to wonder how everything fits together. When I first

began this Guide, I spent weeks just organizing it, with the result being a hierarchical

structure that indicates clearly how subjects are inter-related. I also carefully laid out

each individual section to ensure that it covered its topic in a way that made sense.

☯ Multiple Levels of Detail: I realized that some people reading a TCP/IP Guide might

only want a quick summary of the operation of its constituent protocols, while others

want to learn all the nuances of how everything works. I have provided the full details

that most readers will want, while also providing overview topics in each section that

summarize each technology for quick perusal. This gives you the option of either

skimming the surface or “diving deep”, as you choose.

☯ Platform Independence: I have endeavored whenever possible to avoid describing

TCP/IP in terms specific to any hardware or software platform. Even though I use a PC

for most of my computing and UNIX for other aspects, most of the material is not

particular to any type of device or operating system (though I do focus more on

networks of smaller computers than larger ones).

How successful was I in achieving these goals? I’d like to think I did a pretty good job, but

ultimately, you will be the judge!

Scope of The TCP/IP Guide

The first step to dealing with a problem is recognizing that you have one. So, I have to come

clean with you, my reader. I have a problem: an addiction to… detail. ☺ Every time I set out

to write about a particular protocol, technology or concept, I start with a modest goal

regarding how much I want to write. I always begin knowing that I really need to control

myself, to prevent my project from going on forever. But as I explore each subject, I learn

more and more, and I start to say to myself things like…“this is important, I simply must

include coverage for it” and… “if I am going to cover subject #1, I also should cover subject

#2, because they are related”. This is how I turned a six-month project into a multi-year

ordeal. ☺

However, even though self-control in this area is a weakness for me, even I realized I could

not possibly cover everything related to TCP/IP in this Guide. Consider that the TCP/IP

suite contains dozens of protocols and technologies that have each had thick books written

about them. I was willing to spend years on this project but not decades. ☺ Thus, I had to

limit the scope of this Guide somewhat, both to preserve what remains of my sanity and to

spare you from having to wade through a ridiculously large document.

Here are a few different points that will help explain decisions that I made to limit the scope

of The TCP/IP Guide:

☯ Theory versus Practice: This is primarily a reference resource on the TCP/IP

protocol suite. The material here is designed to allow a student to learn the nuts and

bolts of how TCP/IP works. I do discuss quite a number of “real-world” practical issues

related to how TCP/IP internetworks operate, but this is not my primary focus here. If

you want to really understand what TCP/IP is and what makes it work, you’ve come to

the right place. If all you want is simple instructions on how to connect a few PCs

together in your home using TCP/IP, this probably isn’t the Guide for you. ☺

☯ Current versus Future Protocols: Most of the emphasis in this Guide is on the

present state of the art in TCP/IP. The suite is always changing, and there are

constantly new protocols being written and revisions to existing protocols being

published. I have not provided extensive coverage of technologies still in development,

to try to keep the size of the Guide manageable, and to reduce the number of changes

I will have to make to keep the material up-to-date.

The one exception to this general rule of thumb is version 6 of the Internet Protocol.

IPv6 represents a significant change to the core of how most of TCP/IP operates.

While not universally deployed yet, IPv6 is sufficiently far along in its development that

I feel any student of TCP/IP needs to know what it is and how it works, and understand

its significance. Thus, I have included a detailed section describing it, and also

mentioned how it impacts the operation of several other key protocols such as ICMP,

DNS and DHCP.

☯ Application Coverage: There are many thousands of different applications that run

on TCP/IP internetworks, and I could not possibly hope to describe all of them. The

scope of this Guide is limited to the most important, “classical” TCP/IP applications

and application protocols, such as electronic mail, general file transfer and the World

Wide Web.

☯ TCP/IP versus The Internet: The TCP/IP protocol suite and the Internet are very

closely related in many ways, as you will discover as you read this Guide. In fact, they

are often tied together so much that it is hard to discuss one without the other.

However, the Internet as a whole is an enormous subject, and trying to describe it in

general terms would have substantially increased the size of this Guide. Thus, I

describe Internet issues only within the context of explanations of TCP/IP technol-

ogies.

For example, while I cover the World Wide Web in this Guide, I discuss its generalities

only briefly. I focus my technical discussions on how the Hypertext Transfer Protocol

(HTTP) that implements it works. I don’t talk all about how to set up a Web site, or how

to choose a Web browser, or any of those sorts of details. These are covered in a

dazzling array of different books, papers and of course, Web sites. ☺

☯ Limited TCP/IP Security Coverage: Security is a very important and very large topic,

especially in modern networking. This Guide does include a fairly detailed section on

the operation of the IP Security protocols (IPSec), and also touches upon security

issues in describing several other protocols and technologies. However, it is not

specifically geared towards detailed discussions of security considerations. It is

possible that in the future I will add more security-related information, but for now, if

your primary interest is TCP/IP security matters, you will want to supplement this

Guide with other materials.

☯ Small Computer Orientation: In general terms, TCP/IP technologies can be used to

connect together any types of devices that have the appropriate hardware and

software. There are some issues, however, where explanations require me to focus on

how specific types of underlying networks and devices work; this is especially true of

some of my diagrams. In these cases, my preference has generally been to show how

TCP/IP is used to connect together typical small computers such as PCs, Macintoshes

and UNIX workstations, which are what most people use.

Structure and Organization of The TCP/IP Guide

You may have already noticed that the TCP/IP Guide is organized in a way that is very

different from most books. I believe this structure will help you not only find the information

you need easily, it will even assist you in understanding the content. However, because it is

unorthodox, I want to take a few moments to explain what I have done and why.

In reviewing other networking books and materials before I wrote this Guide, I was consis-

tently disappointed--not in the descriptions of the various subjects, but in the way they were

organized. Invariably, these resources would cover a lot of material, but it would be

presented as a poorly-structured set of seemingly unrelated chapters—“subject salad” as I

sometimes call it. A typical networking book would have dozens of little chapters cobbled

together and presented almost as if they were independent of each other. The reader had to

figure out how they all connected, and also had to wade through several repeated descrip-

tions of similar concepts, due to aspects that were common between certain technologies.

This was all exacerbated by the inability to hyperlink between related topics in a print book.

TCP/IP is a complex topic largely because so many of the concepts and technologies are

closely related. TCP/IP as a whole consists of a number of defining concepts and a large

number of individual protocols, many of which interact in a specific way. Many of those

protocols in turn consist of either subprotocols or descriptive elements that are related to

each other in a particular fashion. For example, you can’t easily describe IP datagram

formatting without making reference to IP addressing, which in turn is related to IP routing,

which is of course related to IP routing protocols, and so forth.

For this reason, I didn’t want to just make this book consist of several dozen chapters

slapped together as peers and leave you to figure out how they fit together. So, I spent

several weeks just trying to come up with a way to describe TCP/IP technologies in a

coherent, organized way that made sense. My primary goal was to come up with a structure

that would clearly show how all the various protocols, concepts and technologies were

interrelated, to help you understand the subject as a whole, and not just as a collection of

parts.

The result of this effort is that rather than using a set of dozens of chapters all at the same

level, this Guide uses a hierarchical structure with many levels. The Guide contains several

sections on various high-level subjects, which we will explore in a moment. Most of these

contain subsections that describe components or more detailed elements within the higher-

level subject. These in turn can contain their own subsections and so on, possibly going

down several layers in detail.

The Guide is formatted so that each section header and section starts at the top of a page.

This facilitates printing small sections of information (for use as handouts in a classroom

setting, for example) and also helps make navigating using hyperlinks easier. A special

horizontal divider graphic is used to clearly mark the end of each section.

Another organizational feature of this Guide is that I have made extensive use of overview

topics. Most sections that include multiple individual topics begin with an overview topic that

provides a high-level look at the contents of that section. These serve two purposes. First,

they introduce the subject and provide a context for understanding the detailed discussions

that follow it, for those who want to read about the protocol or technology in detail. Second,

they serve as a “short cut” for those who do not wish to read an entire section. Some large

sections containing subsections will have an overview for the section as a whole, and an

overview for some of the subsections as well. This is by design, to provide you with flexi-

bility in choosing what you want to read.

Some of you might find that this all of this takes a bit of getting used to, but it has advan-

tages compared to a “flat” set of chapters. The hierarchical structure itself helps you

understand how the many protocols, concepts and other bits and pieces of TCP/IP fit

together. It shows you what aspects of TCP/IP are closely and more distantly related. The

hierarchy and the use of overview topics allows you to explore a particular subject of

interest in the manner you choose, by enabling you to easily choose some subsections to

delve into while skipping others that you may already understand or simply not wish to

know.

So, where to begin in exploring this large Guide? Well, you can just read the entire thing

from cover to cover, but most people lack the time and patience to do that! The key to taking

advantage of this Guide’s hierarchical structure is to make use of the nested bookmarks

that should appear on the left side of your Adobe PDF reader. These let you clearly see the

structure of each part of the Guide and let you instantly jump to any topic of interest. The

next two topics describe in more detail features of the Guide and how to optimize your use

of Adobe Acrobat Reader.

Chapters of The TCP/IP Guide

Due to the hierarchical structure just described, there are only six “top-level” sections in this

Guide, which I sometimes called chapters. You will probably find that it usually makes the

most sense to begin with these higher-level sections and work your way down to the more

detailed topics they contain. Here is a brief description of each, to get you started:

☯ Introduction and “Guide To The Guide”: You’re reading it! ☺

☯ Networking Fundamentals: This is a background chapter that provides lots of useful

general information on networks for those who may be very new to the entire subject.

It has subsections that explain what networking is, discuss key networking character-

istics, describe networking standards, provide information on binary mathematics and

much more. If you are experienced with networks and computers in general you may

wish to skip this (or at least, skim it and only read the topics that interest you).

☯ The Open System Interconnection (OSI) Reference Model: A description of the

important OSI Reference Model, which is used extensively to explain networking archi-

tectures and protocol layering concepts. Unless you already know and understand the

OSI model, this section is worth reading before getting into the “meat” of the Guide.

☯ TCP/IP Protocol Suite and Architecture: This is the first section that is specifically

about TCP/IP; it introduces the protocol suite and discusses it in general terms. This

includes a look at its history, a description of the TCP/IP architectural model and an

overview of key protocols. It should be the place to start reading about TCP/IP for

virtually everyone using this Guide, as it provides necessary background information

and a context for understanding the more detailed chapters below.

☯ TCP/IP Lower-Layer (Interface, Internet and Transport) Protocols (OSI Layers 2, 3 and

4): This large chapter describes all of the TCP/IP protocols that operate at layers 2

through 4 of the OSI model, and is organized by layer for easier reference by function.

This chapter encompasses about half the entire Guide; it includes all of the most

important “core” TCP/IP protocols, including IP, IPv6, ICMP, UDP, TCP and much

more.

☯ TCP/IP Application Layer Protocols, Services and Applications (OSI Layers 5, 6 and

7): This chapter is also very large, as it covers all the TCP/IP application protocols and

services that correspond to the upper three layers of the OSI Reference Model. This

includes protocols such as DNS, DHCP, SNMP, FTP and HTTP, as well as applications

such as electronic mail, the World Wide Web and diagnostic utilities.

As you can see, most of this Guide is actually contained in the last two chapters, since TCP/

IP is primarily defined by the functions of its various protocols.

TCP/IP Guide Features, Inserts and Icons

I created The TCP/IP Guide to provide you with an unparalleled breadth and depth of infor-

mation on TCP/IP. This meant including a lot of content in this Guide—it has over 600

sections, subsections and individual topics, and is over 1,600 pages long. However, I recog-

nized as I was writing this tome that the real goal is not just to provide a lot of detailed

information, but also to present it in such a way that it can be easily understood by you, the

reader. This requires more than just writing large amounts of text and putting it all into a

large file.

For this reason, I have incorporated a number of special features into this Guide to help

make it easier for you to “digest”. ☺ These include tools that will help you navigate through

the information, others that let you quickly jump to related topics, and graphical elements

that explain difficult topics or simply highlight important text elements. Most of these

features are also designed to exploit the many advantages of electronic documents, such

as hyperlinking and the ability to embed full-color graphics.

Special Features

Here is a description of some of these special features, in no particular order:

☯ Hierarchical Table of Contents: The TCP/IP Guide uses a hierarchical structure,

which I described in detail in the previous topic discussing the Guide’s organization. A

complete Table of Contents has of course been included, which reflects this hierar-

chical structure. It is fully hyperlinked, so you can quickly jump to any topic of interest

just by clicking your mouse on the topic’s title.

For your convenience, there is also a link to the Table of Contents in the footer of each

page. Just click on the word “Contents”” that appears after the name and version

number of the Guide.

☯ Hierarchical Bookmark List: The Table of Contents mentioned just above is also

implemented as a set of hierarchical bookmarks, which should automatically appear in

a window pane on the left side of your screen when you open the Guide in Adobe

Acrobat. These bookmarks are convenient, since they remain open while you read the

Guide. You can use them to check your current location in the document, and to jump

to any section or topic. See the next topic for details on how to find these bookmarks if

they do not appear automatically when you open the Guide.

☯ Figures: The TCP/IP Guide contains over 300 full-color illustrations, which support the

text descriptions of TCP/IP technologies and protocols, and help make sense of

difficult concepts. Each figure is cross-referenced with a hyperlink from an appropriate

place in the text. Most include brief descriptions that allow you to understand what the

figure means without even having to read the full text of the topic where the figure is

located.

☯ Tables: The Guide has over 300 tables, which present large amounts of information in

an organized and readable manner, or highlight examples for your convenience.

These tables use alternating white and colored rows for enhanced readability. Like the

figures, these tables are cross-referenced from the text using hyperlinks.

☯ Lists of Figures and Tables: I have provided a List of Figures and a List of Tables to

allow you to easily find any figure or table using either its number or title. Like the Table

of Contents, all entries on these lists are hyperlinked for your convenience.

☯ Hyperlinks: In addition to the hyperlinks in the Table of Contents and lists mentioned

above and the cross-references to figures and tables, there are over 3,000 additional

hyperlinks within the actual text of the Guide. These links are shown in blue text; when

you see one, clicking it will take you to the topic discussing whatever subject matter

was highlighted by the link. This lets you quickly find information relevant to under-

standing the topic you are currently exploring.

Note that all hyperlinks that appear in the main part of this Guide are internal links that

go to other places within the Guide; external links are shown separately (see below).

☯ Highlighting: I use a consistent highlighting mechanism to make the text easier to

read. When new terms are introduced they are shown in italics; I also italicize the

names of fields, messages, options and other protocol constructs to set them apart

from regular text. Emphasized words are shown in bold italics. Document titles are

italicized and underlined. There are also a few places where colored text is used for

emphasis.

☯ Full Text Search: The TCP/IP Guide does not have a conventional index of topics like

most books because it has something better—the ability to search for any string and

have the PDF file viewer software automatically take you to each place where it occurs

in the document. This is described in detail in the next topic.

Inserts and Icons

Most of the text discussions in The TCP/IP Guide are shown as free-form text as you would

expect in any document. However, there are certain situations in which I want to provide

you with a piece of information that is either not directly related to the subject of the main

text, or is deserving of your special attention. For these purposes, the Guide uses a set of

special inserts, each of which is clearly marked by a particular icon, and called out from the

text using a pair of horizontal bars. There are five different inserts used in the Guide, which

I have shown and described below.

Note: This insert used when I need to clarify or explain something that I feel you

need to know, but which is either not directly related to the topic under discussion,

or is sufficiently “tangential” that it needs to be separated from the main text to

avoid disrupting its flow. Examples include terminology explanations, “sidebar” historical

discussions, anecdotes, and clarifications relating to how I am describing particular

concepts.

Key Concept: I tried very hard in crafting this Guide to provide a variety of ways to

present information, to better suit the learning styles of different readers. To that end,

I have created hundreds of these inserts, which summarize and highlight the most

important concepts and essential pieces of knowledge in The TCP/IP Guide. They can be

very useful for quickly distilling the essence of a topic without reading an entire explanation,

or for refreshing your memory of a topic you have already read. Obviously, however, they

contain few details, so you should not assume that you fully understand a topic or concept

just by reading this sort of summary.

Background Information: When you see this insert near the front of a section or a

particular description, it indicates that I am recommending that you read another

part of the Guide before proceeding with that section. While I have attempted to

make each section able to stand alone to a certain degree, some just make more sense if

read in a particular order (usually the order in which the sections are listed within their

parent section).

Related Information: This insert mentions another section or topic within The

TCP/IP Guide that contains information related to the section where the insert is

located, but which is not background material. It may help you understand the

content you are reading, or put it into better context.

On The Web: This insert is used when I need to link to a relevant document or

resource on the World Wide Web. The insert contains a brief description of the

linked item and its URL, highlighted in blue italics.

Be sure to also read the next topic, which provides some very useful advice on how best to

use the Adobe Acrobat (PDF) Reader software. This will help you get the most of the

features I have just described.

Tips For Using Adobe Acrobat Reader To View The TCP/

IP Guide

The TCP/IP Guide is packaged as an Adobe Acrobat portable document format (PDF) file.

PDF files are normally viewed using a software program called the Adobe Acrobat Reader

(or just Adobe Reader). Since Acrobat has become the industry standard for distributing

electronic documents, you have probably used it to view PDF files many times in the past.

Some of you, however, may be new to PDF files, and most others have probably never

used it to view a document of this size. I felt it would be helpful to provide you with a few

suggestions for how best to use the software to view this Guide.

Obviously, I cannot begin to explain all the features and options in the Adobe Acrobat

software, nor would I wish to try. You should consult the built-in help features of the program

for general assistance on how to use it. My interest here is only in highlighting some specific

Acrobat tools that I think will help you in best navigating the Guide and benefitting from its

features.

So, here are some specific tips to getting the most of The TCP/IP Guide using the Adobe

Acrobat Reader. These are based on version 6 of the software, running on a Windows PC

platform. Note that earlier versions of the software may not have all of the features I discuss

below. Versions running on other operating systems likely have the same features, but may

be accessed using different menu options or key sequences.

Basic Navigation

To move through the document:

☯ Use the “Up Arrow” and “Down Arrow” keys to go up or down one line.

☯ Use the “Page Up” and “Page Down” keys to go up or down one screenful of

information.

☯ Use the “Left Arrow” and “Right Arrow” keys to go up or down one full page. (This may

be the same as a full screen if you have the viewing mode set to display exactly one

full page at a time on the screen.)

☯ Use the “Home” key to go to the first page, or “End” to go to the end of the Guide.

You can also use the scroll-wheel on most mice to navigate through the document, or the

scroll bar on the right side of the document. There are also arrow buttons for navigation, at

the bottom of the page on either side of the page number display, and the “View Menu” has

a “Go To” submenu with navigation options as well.

Hyperlinks

Hyperlinks are shown in blue text; when you move the pointer over one, it will change from

a hand to a pointing finger. Just click the link to go to its target.

To return to the place where the link was located, select the “View” menu, then “Go To” and

“Previous View”, or more easily, press “Alt+Left Arrow”. The small green button pointing to

the left at the bottom of the screen does the same thing. This is very useful when you want

to follow a hyperlink quickly to check a related piece of information and then return back to

the place where you were reading. You can do this multiple times as well—if you click three

hyperlinks in sequence, you can go back three times to each hyperlink in sequence.