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Chapter 8 ZMODEM Protocol 18
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serial number of the receiving program, or 0.
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8.2 FFFFiiiilllleeee TTTTrrrraaaannnnssssmmmmiiiissssssssiiiioooonnnn
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The sender then sends a ZZZZFFFFIIIILLLLEEEE header with ZMODEM Conversion, Management,
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and Transport options[3] followed by a ZCRCW data subpacket containing the
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file name, file length, modification date, and other information identical
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to that used by YMODEM Batch.
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The receiver examines the file name, length, and date information provided
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by the sender in the context of the specified transfer options, the
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current state of its file system(s), and local security requirements. The
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receiving program should insure the pathname and options are compatible
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with its operating environment and local security requirements.
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The receiver may respond with a ZZZZSSSSKKKKIIIIPPPP header, which makes the sender
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proceed to the next file (if any) in the batch.
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If the receiver has a file with the same name and length,
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it may respond with a ZZZZCCCCRRRRCCCC header, which requires the
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sender to perform a 32 bit CRC on the file and transmit the
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complement of the CRC in a ZZZZCCCCRRRRCCCC header.[4] The receiver
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uses this information to determine whether to accept the
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file or skip it. This sequence is triggered by the ZMCRC
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Management Option.
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A ZZZZRRRRPPPPOOOOSSSS header from the receiver initiates transmission of the file data
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starting at the offset in the file specified in the ZZZZRRRRPPPPOOOOSSSS header.
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Normally the receiver specifies the data transfer to begin begin at
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offset 0 in the file.
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The receiver may start the transfer further down in the
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file. This allows a file transfer interrupted by a loss
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or carrier or system crash to be completed on the next
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connection without requiring the entire file to be
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retransmitted.[5] If downloading a file from a timesharing
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system that becomes sluggish, the transfer can be
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interrupted and resumed later with no loss of data.
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The sender sends a ZZZZDDDDAAAATTTTAAAA binary header (with file position) followed by
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one or more data subpackets.
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__________
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3. See below, under ZFILE header type.
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4. The crc is initialized to 0xFFFFFFFF.
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5. This does not apply to files that have been translated.
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Chapter 8 Rev 10-27-87 Typeset 10-27-87 18
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Chapter 8 ZMODEM Protocol 19
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The receiver compares the file position in the ZZZZDDDDAAAATTTTAAAA header with the
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number of characters successfully received to the file. If they do not
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agree, a ZZZZRRRRPPPPOOOOSSSS error response is generated to force the sender to the
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right position within the file.[6]
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A data subpacket terminated by ZZZZCCCCRRRRCCCCGGGG and CRC does not elicit a response
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unless an error is detected; more data subpacket(s) follow immediately.
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ZZZZCCCCRRRRCCCCQQQQ data subpackets expect a ZZZZAAAACCCCKKKK response with the
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receiver's file offset if no error, otherwise a ZZZZRRRRPPPPOOOOSSSS
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response with the last good file offset. Another data
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subpacket continues immediately. ZZZZCCCCRRRRCCCCQQQQ subpackets are
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not used if the receiver does not indicate FDX ability
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with the CCCCAAAANNNNFFFFDDDDXXXX bit.
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ZZZZCCCCRRRRCCCCWWWW data subpackets expect a response before the next frame is sent.
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If the receiver does not indicate overlapped I/O capability with the
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CCCCAAAANNNNOOOOVVVVIIIIOOOO bit, or sets a buffer size, the sender uses the ZZZZCCCCRRRRCCCCWWWW to allow
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the receiver to write its buffer before sending more data.
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A zero length data frame may be used as an idle
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subpacket to prevent the receiver from timing out in
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case data is not immediately available to the sender.
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In the absence of fatal error, the sender eventually encounters end of
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