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Back to Basics: TCP/IP Fundamentals
by Thomas W. Shinder
As I talked about in an earlier
article, a solid grounding in TCP/IP is pivotal to your success in networking.
In the past, you had to be expert at a variety of transport protocols in order
to get a handle on your network. Heterogeneous networks that included NetBEUI,
IPX/SPX, and TCP/IP were the norm. However, the playing field has been leveled
with the ascendance of TCP/IP as the transport protocol for all modern
As I talked about in an earlier article, a solid grounding in TCP/IP is pivotal to your success in networking. In the past, you had to be expert at a variety of transport protocols in order to get a handle on your network. Heterogeneous networks that included NetBEUI, IPX/SPX, and TCP/IP were the norm. However, the playing field has been leveled with the ascendance of TCP/IP as the transport protocol for all modern networks.
In our book, Troubleshooting Windows 2000 TCP/IP, we take the subject of the TCP/IP protocol suite and approach it from the ground up. The book starts with a thorough review of the foundations of the TCP/IP protocol stack, starting with the OSI and DoD models, and expand on that foundation.
An Unintended Study Guide
The vast majority of the book focuses on how things go wrong, and how to figure out what has gone wrong and how to fix it. Although we did not write the book as an MCSE Study Guide, it has turned out to be extremely helpful for a large number of test candidates. We are frankly very surprised at the wonderful reviews our book has received at amazon.com, and how many people have stated how much it has helped them on the Microsoft MCSE Exam 70-216, Administering and Supporting a Windows 2000 Network. However, since the exam focuses a lot on troubleshooting, I suppose this makes sense.
Also, the book focuses on the major subjects covered in the exam preparation guide, which you can find at the Microsoft website www.microsoft.com/mcp. We tried to cover as much as possible about what might go wrong with the various Windows 2000 Networking Services, such as WINS, DNS, DHCP, RRAS. We also go into some common problems when dealing with IP addressing issues, hardware and interface problems, and core routing troubles.
Just the FAQs
Each of the chapters includes a FAQ in which we seek to address some of the common questions related to the protocols and services discussed in the chapter. These FAQs at located at the end of each chapter, and are geared at answering some of the questions which may have come to mind when you read the chapter's material.
Here's an example of the FAQ that is at the end of Chapter 1 - TCP/IP Overview
Q: Why do some books specify that certain software components, such as redirectors, operate at the Application layer, while others say that redirectors work at the Presentation layer?
A: There are a few reasons for the discrepancy. First, there are many different types of network redirectors, some of which are part of the operating system, and others (such as the Novell Client 32 software for connecting a Windows machine to a NetWare network) made by third parties. Additionally, some books reference the OSI networking model, which consists of seven layers, while others are basing their statements on the DoD model, which only has four. A component that operates at the Presentation layer of the OSI model would be operating at the Application (or Application/Process) layer of the DoD model.
Q: It's called TCP/IP. What are all those other protocols, and what are they for?
A: TCP and IP are the "core" protocols (sometimes called the "protocol stack"), but an entire suite of useful protocols has grown up around them. Some of these provide for basic functionality in performing such common network tasks as transferring files between two computers (FTP) or running applications on a remote computer (Telnet). Others are used for information gathering (SNMP, NETSTAT, IPCONFIG), and many are troubleshooting tools that also allow you to perform basic configuration tasks (ARP, ROUTE).
Q: What is the difference between TCP and UDP if they both operate at the Transport layer?
A: Although both TCP and UDP are Transport layer protocols and provide the same basic function, TCP is a connection-oriented protocol, which means a session is established before data is transmitted, and acknowledgments are sent back to the sending computer to verify that the data did arrive and was accurate and complete. UDP is connectionless; no session, or one-to-one connection, is established prior to data transmission. This makes UDP the faster of the two, and TCP the more reliable.
Q: What is the purpose of a networking model? How will knowing this theoretical stuff help me in administering my TCP/IP network?
A: The models give us a way to understand the process that takes place when computers communicate with each other across the network, the order in which tasks are processed, and which protocols are responsible for handling which duties. Understanding the models will help you to narrow down the source of your TCP/IP connectivity problems. For example, if you know that the data is being sent but is not arriving at the correct destination, you will know to start troubleshooting by examining what is happening at the Network layer, since that's where addressing and routing takes place.
Q: Why do we need three different networking models? Why can't everyone use the same one?
A: Actually, that was the plan when the ISO developed the Open Systems Interconnection model. It was to be the common standard used by all vendors and software developers in describing the network communication process. The DoD model actually predates the OSI, and the seven-layer OSI model builds on (and further breaks down) the components of the DoD model. However, individual vendors such as Microsoft still use their own models, which map more closely to their software (such as the Windows NT/2000 model), although they also use the OSI model as a guideline.
Q: What is a gateway, and why would I need one?
A: The word gateway has many different meanings in the IT world. A protocol translating gateway translates between different protocols. Think of it as the United Nations interpreter of the networking world. If the president of the United States needs to exchange information with the president of France, but neither speaks the other's language, they can call in someone who is fluent in both to help them get their messages across. Similarly, if a mainframe system and a Windows 2000 computer need to communicate with one another-perhaps the mainframe has important files that need to be accessed by the PC-but they don't know how to "talk" to each other, you can install a gateway to clear up the confusion. The gateway is even more skilled than the interpreter is; it actually fools the mainframe into believing it's communicating with another mainframe, and makes the PC think it is having a "conversation" with a fellow PC. Gateway is also the term used to refer to the address of a router that connects your network to another, acting as the gateway to the "outside world.
TCP/IP is Your Friend For Life
If you feel a little fuzzy on your TCP/IP knowledge and ability to apply that knowledge, then you need to get cooking! Remember that when you troubleshoot networking problems, you should always start from the bottom of the OSI model. That means you need to understand more than just the application layer protocols and the applications themselves! Make TCP/IP your friend for life, and you'll be rewarded many times over. You'll solve problems that other would be network administrators shake their heads at. And in the process, you'll get more pay, and more respect, than you would have had if you weren't the TCP/IP Pro!