Khi phân tích định tuyến có nhiều điều quan trọng cần nhớ. Đầu tiên là để biết được định tuyến lưu lượng truy cập. Bạn phải nhìn thấy và xác minh các đường dẫn thực tế rằng lưu lượng truy cập | User Datagram Protocol 157 UDP Header Figure 5-1 shows the UDP header. The fields of the UDP header are defined in the sections that follow. Source Port The source port is a 16-bit number greater than 1023 that is chosen by a user of a UDP based application. These ports are known as ephemeral ports because they are only used for the lifetime of the connection ephemeral means shortlived . Ephemeral source port numbers are chosen by the IP stack and used by the transport layer for the delivery of data to upper-layer applications. They function essentially the same as the Ethernet Ethertype and the IP Protocol ID field. When a source host sends data using UDP to a destination host it chooses an unused source port greater than 1023 to be placed into the UDP Source Port field. The application on the destination station in turn uses this port number in the Destination Port field when it sends data back to the host. Destination Port The destination port is also a 16-bit number which may be greater or less than 1023. In the early ages of TCP IP most applications used what are called well-known port numbers. Unlike ephemeral ports applications will keep a well-known port open for as long as the application is running. Well-known port numbers are in the range of 0 to 1023. Hosts using these applications would use a source ephemeral port greater than 1023 and a destination port well-known between 0 and 1023. For example DNS uses UDP port 53. A DNS server allocates the use of port 53 on the local server. When a client sends a DNS request to that server it will use a source ephemeral port and a destination port of 53. When the DNS server replies to the request it will use the client s ephemeral port as the destination UDP port and port 53 as the source port. So many applications now use TCP and UDP that the defining line between which ports applications use is blurred. It is typical to see applications using ports ranging from 0 to 65535. Table 5-1 shows some typical UDP .
