《计算机网络和 与因特网》课件第四章文件传输和 与远程文件访问.pptVIP

Computer Networks and Internets 《计算机网络与因特网》课件 林坤辉;PART IV Network Applications;FTP;Control Connection (1);Control Connection (2);Data Connection (3);Communication is achieved through commands and responses.;The client must define the type of file to be transferred,the structure of the data,and the transmission mode.;ASCII file: this is the default format for transferring text files. Each character is encoded using NVT ASCII. The sender transforms the file form its own representation into NVT ASCII characters and the receiver transforms the NVT ASCII characters to its own representation.;EBCDIC file: EBCDIC: Extended Binary-Coded Decimal Interchange Code 扩充的二—十进制交换码. if one or both ends of the connection use EBCDIC encoding,the file can be transferred using EBCDIC encoding.;Image file: this is the default format for transferring binary files. the file is sent as continuous streams of bits without any interpretation or encoding. This is mostly used to transfer binary files such as compiled programs.;File structure(default): it is a continuous stream of bytes. Record structure: the file is divided into records.this can be used only by with text files. Page structure: the file is divided into pages, with each page having a page number and a page header.the pages can be stored or accessed randomly or sequentially. ;Stream mode: this is the default mode. Data are delivered from FTP to TCP as a continuous stream of bytes. Block mode: data can be delivered from FTP to TCP in blocks.each block is preceded by a 3-byte header. Compressed mode: if the file is big,the data can be compressed. the compression method used is normally run-length encoding(游程长度编码). In a binary file,null characters are usually compressed.;FTP uses the control connection to establish a communication between the client control process and the server control process.;Access commands;these commands let the user