Sau đó các công ty viễn thông bắt đầu bán các dịch vụ dữ liệu, nói cách khác, đường dây thuê. Các công ty điện thoại có thể bán một DS0 dịch vụ tại 64 kbps. Tuy nhiên, khi nó lần đầu tiên xuất hiện, họ thường cung cấp dịch vụ 56-kbps. Tại sao? Vâng, | 152 Chapter 6 Fundamentals of TCP and UDP Figure 6-2 shows a figure of the example network with Jessie running three applications A UDP-based ad application A TCP-based wire-transfer application A TCP web server application Figure 6-2 Hannah Sending Packets to Jessie with Three Applications Hannah Jessie I Received Three Packets Each from the Same MAC and IP Address. What Application Should Get the Data in Each Packet Web Server Ad Application Wire Application Eth IP UDP Ad Data Eth Eth IP TCP Wire Transfer Data Eth Eth IP TCP Web Page Data Eth Jessie needs to know which application to give the data to but all three packets are from the same Ethernet and IP address. You might think that Jessie could look at whether the packet contains a UDP or a TCP header but as you see in the figure two applications wire transfer and web both are using TCP. TCP and UDP solve this problem by using a port number field in the TCP or UDP header respectively. Each of Hannah s TCP and UDP segments uses a different destination port number so that Jessie knows which application to give the data to. Figure 6-3 shows an example. Multiplexing relies on the use of a concept called a socket. A socket consists of three things an IP address a transport protocol and a port number. So for a web server application on Jessie the socket would be TCP port 80 because by default web servers use the well-known port 80. When Hannah s web browser connected to the web server Hannah used a socket as well possibly one like this TCP 1030 . Why 1030 Well Hannah just needs a port number that is unique on Hannah so Hannah saw that port 1030 was available and used it. In fact hosts typically allocate dynamic port numbers starting at 1024 because the ports below 1024 are reserved for well-known applications such as web services. The Transmission Control Protocol 153 Figure 6-3 Hannah Sending Packets to Jessie with Three Applications Using Port Numbers to Multiplex Destination Port 800 Eth IP TCP
