In RFC2347 TFTP Option Extension, the client has to send back an ACK with block number set to 0 when receiving an OACK. Additionally in RFC2348, it calls for an "The number of octets in a block, specified in ASCII." - the existing code in XeLL specified that number in octal, which doesn't match up with behaviour found in OpenTFTPServer nor anything from the RFC, at least.
In RFC2347 TFTP Option Extension, the client has to send back an ACK with block number set to 0 when receiving an OACK. Additionally in RFC2348, it calls for an "The number of octets in a block, specified in ASCII." - the existing code in XeLL specified that number in octal, which doesn't match up with behaviour found in OpenTFTPServer nor anything from the RFC, at least.