TAILIEUCHUNG - TCP/IP Analysis and Troubleshooting Toolkit phần 3

giao thức lớp mạng của chúng tôi, phải xác định làm thế nào để chuyển tiếp dữ liệu này lên mạng 2 lớp. Ví dụ, nếu khách hàng có hai kết nối mạng IP, nó phải chọn một kết nối là con đường tốt nhất đến đích. Giới thiệu Nghị định thư Phân tích 25 | CHAPTER 3 Inside the Internet Protocol Now that Part I of the book has discussed the building blocks of communication the OSI model and some techniques on how to analyze those communications it s time to start discussing what makes communications happen the protocols. This next part of the book discusses the core TCP IP protocols that allow all other upper-layer protocols to operate. The approach is once again layer by layer starting with Internet Protocol IP in this chapter and moving on to the most complex core protocol Transmission Control Protocol TCP . It is important to understand how the bottom layered protocols such as IP work before you begin trying to understand the higher-layer protocols. Since each of the higher-layer protocols such as UDP and TCP use the services of the lower-layer protocols such as IP it s important to understand the impact of each layer s responsibility to the entire communications process. Such understanding is critical in troubleshooting situations. I begin this chapter with a review of Layer 2 communications and how the limitations of Layer 2 create a need for Layer 3 protocols. Then instead of generic layer descriptions I begin the discussion of the actual functions of the protocols at each layer starting in this chapter with IP and other supporting protocols at the network layer. 69 70 Chapter 3 Reviewing Layer 2 Communications The next sections provide a quick review of what Part I revealed about the functions of Layer 2. Multiplexing Layer 2 packets encapsulate the data of all other upper-layer protocols. As is discussed in Chapter 1 Ethertypes are used by Layer 2 to determine the destination Layer 3 protocol of the data. For example Figure 3-1 shows a decoded IP packet. Notice how even though you tend to think of Ethernet as a bottom-layer protocol it resides at Layer 2 the analyzer shows it listed under the Ethernet Header section of the decode panel. Analyzers decode the packet detail in reverse showing you first the lower

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