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X/YMODEM Protocol Reference 10-27-87 4
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2. YMODEM MINIMUM REQUIREMENTS
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All programs claiming to support YMODEM must meet the following minimum
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requirements:
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+ The sending program shall send the pathname (file name) in block 0.
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+ The pathname shall be a null terminated ASCII string as described
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below.
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+ The receiving program shall use this pathname for the received file
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name, unless explicitly overridden.
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+ The sending program shall use CRC-16 in response to a "C" pathname
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nak, otherwise use 8 bit checksum.
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+ The receiving program must accept any mixture of 128 and 1024 byte
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blocks within each file it receives. Sending programs may switch
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between 1024 and 128 byte blocks at the end of file(s), and when the
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frequency of retransmissions so suggests.
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+ The sending program must not change the length of an unacknowledged
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block.
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+ At the end of each file, the sending program shall send EOT up to ten
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times until it receives an ACK character. (This is part of the
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XMODEM spec.)
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+ The end of a transfer session shall be signified by a null (empty)
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pathname.
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Programs not meeting all of these requirements are not YMODEM compatible,
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and shall not be described as supporting YMODEM.
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Meeting these MINIMUM requirements does not guarantee reliable file
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transfers under stress. Particular attention is called to XMODEM's single
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character supervisory messages that are easily corrupted by transmission
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errors.
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Chapter 2
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X/YMODEM Protocol Reference 10-27-87 5
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3. WHY YMODEM?
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Since its development half a decade ago, the Ward Christensen modem
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protocol has enabled a wide variety of computer systems to interchange
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data. There is hardly a communications program that doesn't at least
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claim to support this protocol.
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Advances in computing, modems and networking have revealed a number of
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weaknesses in the original protocol:
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+ The short block length caused throughput to suffer when used with
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timesharing systems, packet switched networks, satellite circuits,
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and buffered (error correcting) modems.
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+ The 8 bit arithmetic checksum and other aspects allowed line
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impairments to interfere with dependable, accurate transfers.
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+ Only one file could be sent per command. The file name had to be
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given twice, first to the sending program and then again to the
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receiving program.
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+ The transmitted file could accumulate as many as 127 extraneous
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bytes.
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+ The modification date of the file was lost.
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A number of other protocols have been developed over the years, but none
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have displaced XMODEM to date:
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