xmodem: Only increment sequence number on ACK

[nekromant]

Otherwise a double fault causes transfer to be terminated
because of a sequence error

Signed-off-by: Andrew Andrianov andrew@ncrmnt.org

[nekromant]

That was sitting in the closet for a while. If I remember xmodem correctly we have to increment sequence only on ACK. If we get NACK, we resend 'as is'. Otherwise, things got broken after two successive NACKs for me.

[jquast]

understood, I agree that you are correct, that sequence number only increments on ACK, from http://textfiles.com/programming/ymodem.txt

Where [sequence] is then next window sequence number (between H000 and H003) after the [sequence] of the last good block. The receiver will discard up to 3 Xmodem packets received after the NAK is transmitted until it receives the packet with the sequence number that had previously been nak'ed by the receiver. The receiver will not send a second NAK until another packet with the same sequence number is received which is also invalid or a timeout has occurred.

5. When the transmitter receives a NAK[sequence], it will complete transmission of any XMODEM block currently being transmitted and then begin re- transmission starting with the block which was nak'ed.