Y-Modem

send n*CAN
receive CAN+CAN abort transmission
	- when is waiting for the beginning of a block or 
	- when is waiting for an acknowledgement to a block that has been sent.

The receiver initiates batch file transmission by sending a "C" character (for CRC-16).

The sender opens the first file and sends block number 0 with the following information:
	- Only the filename part is required, the file length and all other fields are optional.
	- All unused bytes in block 0 should be set to null.
	- The filename: a null terminated ASCII string.
	- The length: a decimal string counting the number of data bytes in the file. 
	- If the file length is not known, it should be set to the maximum expected value or not sent.
	- other fields: Modification Data, Unix File Mode, Serial Number, future fields.
	- remainder: set to null.

If header block is received with error, a retransmission is requested.
After the header block has been received, 
	- if the can be opened for writing --> send ACK. 
	- else the receiver cancels the transfer --> n*CAN.

The receiver then initiates transfer of the file contents with a "C" character, 
according to the standard XMODEM/CRC protocol.

After the file contents and XMODEM EOT have been transmitted and
acknowledged, the receiver again asks for the next pathname.
Transmission of a null pathname terminates batch file transmission.

Note that transmission of no files at all is not necessarily an error. 

Example: YMODEM Batch Transmission Sessions

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RECEIVER			SENDER								RECEIVER

C (command:rb)		SOH 00 FF "foo.c" NUL[123] CRC CRC	ACK

C					SOH 01 FE Data[128] CRC CRC			ACK
					SOH 02 FC Data[128] CRC CRC			ACK
					SOH 03 FB Data[100] ^Z[28] CRC CRC	ACK
					EOT									NAK
					EOT									ACK

C					SOH 00 FF NUL[128] CRC CRC			ACK

________________________________________________________________
RECEIVER 			SENDER								RECEIVER

C (command:rb)		SOH 00 FF "foo.c" NUL[123] CRC CRC	ACK

C					SOH 01 FE Data[128] CRC CRC			ACK
					SOH 02 FC Data[128] CRC CRC			ACK
					SOH 03 FB Data[100] ^Z[28] CRC CRC	ACK
					EOT									NAK
					EOT									ACK

C					SOH 00 FF "baz.c" NUL[123] CRC CRC	ACK

C					SOH 01 FB Data[100] ^Z[28] CRC CRC	ACK
					EOT									NAK
					EOT									ACK

C					SOH 00 FF NUL[128] CRC CRC			ACK

________________________________________________________________
RECEIVER 			SENDER								RECEIVER

C (command:rb)		SOH 00 FF "foo.c" NUL[123] CRC CRC	ACK

C					STX 01 FD Data[1024] CRC CRC		ACK
					SOH 02 FC Data[128] CRC CRC			ACK
					SOH 03 FB Data[100] ^Z[28] CRC CRC	ACK
					EOT									NAK
					EOT									ACK

C					SOH 00 FF NUL[128] CRC CRC			ACK


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RECEIVER 			SENDER								RECEIVER

					(timeout)							NAK
					SOH 01 FE -data- xx					ACK
					SOH 02 FD -data- xx	(broken)		NAK
					SOH 02 FD -data- xx					ACK
					SOH 03 FC -data- xx					(ACK) (gets garbaged)
					SOH 03 FC -data- xx					ACK
					EOT 								(anything except ACK)
					EOT									ACK