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

Block Mode

This is a more “conventional” data transmission mode in which data is broken into data blocks and encapsulated into individual FTP blocks, or records. Each record has a three-byte header that indicates its length and contains information about the data blocks being sent. A special algorithm is used to keep track of the transmitted data and to detect and restart an interrupted transfer.

Compressed Mode

A transmission mode where a relatively simple compression technique called run-length encoding is used to detect repeated patterns in the data being sent, and then represent them in such a way that the overall message takes fewer bytes. The compressed information is then sent in a way similar to block mode, using a header+payload record format.

Compressed mode seems on the surface to be useful. In practice, however, compression is often implemented in other places in a typical networking software stack, making it unnecessary in FTP. For example, if you are transferring a file over the Internet using an analog modem, your modem normally performs compression down at layer one. Large files on FTP servers are also often already compressed using something like the ZIP format, meaning further compression would serve no purpose.

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