139、The transfer of the contents of a file held on the file system of one computer to a file on another computer is a common requirement n many distributed/networked applications.In some applications the two computers involved may both be large servers each running a different operating system with a different file system and character set.In another application, one of the computers may be a server and the other item of equipment such as a cable modem or a set-top box which does not have a hard disk.Hence in this case all the data that is transferred must have been formatted specifically for running in the cable modem or set-top box.Clearly, therefore, the file transfer protocol associated with the second type of application can be much simpler than the first.Hence to meet the different requirements of these two types of application, there are two Internet application protocols associated with file transfer.The first is called the file transfer protocol ( )and the second the trivial file transfer protocol ( ).FTP is widely used Internet application protocol that has been designed to enable a user at a terminal to initiate the transfer of the contents of a named file from one computer to another using the TCP/IP protocol suite.The two computers may use different operating systems with different file systems and, possibly different character sets.It also supports the transfer of a number of different file types such as character and binary.It is specified in RFC959.We describe first how the file contents are represented and then the operation of the protocol itself.we conclude with some examples.(5)、what is main pint of this text?