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Building Internet Firewalls

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C. TCP/IP Fundamentals

You need a good understanding of TCP/IP to be able to follow the details of the discussions of packet filtering in this book. If you are not already familiar with TCP/IP , we strongly recommend that you read at least this appendix. This appendix is adapted from Chapters 1 and 2 of TCP/IP Network Administration by Craig Hunt (O'Reilly & Associates, 1992). See that book for complete information about administering TCP/IP -based services.

C.1 Introduction to TCP/IP

The name TCP/IP refers to an entire suite of data communications protocols. The suite gets its name from two of the protocols that belong to it: the Transmission Control Protocol and the Internet Protocol. Although there are many other protocols in the suite, TCP and IP are certainly two of the most important.

C.1.1 TCP/IP Features

The popularity of the TCP/IP protocols on the Internet did not grow rapidly just because the protocols were there, or because military agencies mandated their use. They met an important need (worldwide data communication) at the right time, and they had several important features that allowed them to meet this need. These are:

  • Open protocol standards, freely available and developed independently from any specific computer hardware or operating system. Because it is so widely supported, TCP/IP is ideal for uniting different hardware and software, even if you don't communicate over the Internet.

  • Independence from specific physical network hardware. This allows TCP/IP to integrate many different kinds of networks. TCP/IP can be run over an Ethernet, a token ring, a dial-up line, an X.25 net, and virtually any other kind of physical transmission media.

  • A common addressing scheme that allows any TCP/IP device to uniquely address any other device in the entire network, even if the network is as large as the worldwide Internet.

  • Standardized high-level protocols for consistent, widely available user services.

C.1.2 Protocol Standards

The open nature of TCP/IP protocols requires publicly available standards documents. All protocols in the TCP/IP protocol suite are defined in one of three Internet standards publications. A number of the protocols have been adopted as Military Standards ( MIL STD ). Others were published as Internet Engineering Notes ( IEN ) - though the IEN form of publication has now been abandoned. But most information about TCP/IP protocols is published as Requests for Comments ( RFC ). RFC s contain the latest versions of the specifications of all standard TCP/IP protocols.[1] As the name "Request for Comments" implies, the style and content of these documents is much less rigid than most standards documents. RFC s contain a wide range of interesting and useful information, and are not limited to the formal specification of data communications protocols.

[1] Interested in finding out how Internet standards are created? Read The Internet Standards Process , RFC 1310.

As a network system administrator, you will no doubt read many of the RFC s yourself. Some contain practical advice and guidance that is simple to understand. Other RFC s contain protocol implementation specifications defined in terminology that is unique to data communications.