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The TCP/IP Guide  9  TCP/IP Application Layer Protocols, Services and Applications (OSI Layers 5, 6 and 7)       9  TCP/IP Key Applications and Application Protocols            9  TCP/IP File and Message Transfer Applications and Protocols (FTP, TFTP, Electronic Mail, USENET, HTTP/WWW, Gopher)                 9  TCP/IP General File Transfer Protocols (FTP and TFTP)                      9  File Transfer Protocol (FTP)                           9  FTP Concepts and General Operation

FTP Data Connection Management, Normal (Active) and Passive Data Connections and Port Usage 1 2 FTP Data Representation: Data Types, Data Structures and Format Control

Once a data connection has been established between the Server-DTP and the User-DTP, data is sent directly from the client to the server, or the server to the client, depending on the specific command issued. Since control information is sent using the distinct control channel, the entire data channel can be used for data communication. (Of course, these two logical channels are combined at lower layers along with all other TCP and UDP connections on both devices, so it's not like this represents a performance improvement over a single channel. Just wanted to make that clear.)

FTP defines three different transmission modes (also called transfer modes) that specify exactly how data is sent from one device to another over an opened data channel: stream mode, block mode, and compressed mode.

In this mode, data is sent simply as a continuous stream of unstructured bytes. The sending device simply starts pushing data across the TCP data connection to the recipient. No message format with distinct header fields is used, making this method quite different from the way many other protocols send information in discrete chunks. It relies strongly on the data streaming and reliable transport services of TCP. Since there is no header structure, the end of the file is indicated simply by the sending device closing the data connection when it is done.

Of the three methods, stream mode is the one that is by far the most widely used in real FTP implementations. There are likely three reasons for this. First, it is the default and also the simplest method, so it is the easiest to implement and one that is required for compatibility. Second, it is the most general, because it treats all files as simple streams of byte without paying attention to their content. Third, it is the most efficient method because no bytes are wasted on “overhead” such as headers.

FTP Data Connection Management, Normal (Active) and Passive Data Connections and Port Usage 1 2 FTP Data Representation: Data Types, Data Structures and Format Control