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FTP General Data Communication and Transmission Modes
(Page 2 of 2)
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.
Key Concept: FTP includes three different transmission modes: stream, block and compressed. In stream mode, data is sent as a continuous sequence of bytes; in block mode, data is formatted into blocks with headers; and in compressed mode, bytes are compacted using run-length encoding. Stream mode is the one most commonly used. |
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The TCP/IP Guide (http://www.TCPIPGuide.com)
Version 3.0 - Version Date: September 20, 2005
© Copyright 2001-2005 Charles M. Kozierok. All Rights Reserved.
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