Figure 64 illustrates the relation ships among TCP IP UDP and ICMP ICMP is used

Figure 64 illustrates the relation ships among tcp ip

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use the same routing infrastructure as IP. Figure 6.4 illustrates the relation- ships among TCP, IP, UDP, and ICMP. ICMP is used to deliver many different types of administrative control messages. Some examples are: Network/host/port unreachable Packet time to live expired chapple06 10/12/04 9:21 AM Page 141
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142 CHAPTER 6 Securing TCP/IP TCP UDP IP ICMP Figure 6.4 Relationships among pro- tocols in the TCP/IP suite Source quench (used when a gateway is overloaded and wishes to pause incoming traffic) Redirect messages (used to reroute traffic) Echo request and echo reply messages (used to determine whether a host is active on the network; these messages are used by the ping command) 6.1.2 Open Systems Interconnection Model The Open Systems Interconnection (OSI) reference model, shown in Fig- ure 6.5, was developed by the International Organization for Standardiza- tion in the late 1970s in an effort to describe the basic functionality of networked data communications. The model consists of seven layers: Application layer, Presentation layer, Session layer, Transport layer, Net- work layer, Data Link layer, and Physical layer. Application Presentation Session Transport Network Data Link Physical Figure 6.5 OSI model chapple06 10/12/04 9:21 AM Page 142
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6.1 Introduction to Transmission Control Protocol/Internet Protocol (TCP/IP) 143 The OSI model uses a process known as encapsulation to sequentially process data through the various model layers until it is ready for transmis- sion across a network medium (e.g., a copper wire or fiber-optic cable). Each layer of the OSI model performs some transformation of the data, either by adding a header that encapsulates the data received from the pre- vious layer or by converting the data into another form (such as from binary data into electrical impulses). When the remote device receives the packet, it also processes it through the layers of the OSI model, but in reverse order. At the conclusion of the process, the destination machine has the same data that was sent by the originating machine. The process of encapsulation is illustrated in Figure 6.6. The beauty of the OSI model lies in the ability of system developers to take advantage of abstraction. A programmer writing software that works at the Application layer doesn’t need to worry about the details of how the lower layers work. If the software communicates with other systems, the pro- grammer may simply view it as communication between the systems at the Application layer. The encapsulation process ensures that the networking protocols take care of the other details. The OSI model is a fundamental principle of networking, and many texts devote entire chapters to fully exploring the model. Keeping with the focus of this text, we’ll briefly describe each layer and provide information on its relevance to informa- tion security practitioners.
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  • Spring '18
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  • Networking, Transmission Control Protocol, Internet Protocol Suite, User Datagram Protocol

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