Xon/Xoff is a software based flow control method. Its very simple and is designed for ASCII based comms links. When enabled the following 2 bytes/characters are sent by the pother side to disable and enable you transmitting:
Xoff = Ctrl-S = 0x13 = You must stop sending data
Xon = Ctrl-Q = 0x11 = You can start sending data again
When transferrign binary data you have the problem that your binary data may contain these bytes values. You can work around that by detecting and converting them (e.g. to 2specail byte values say) before you transmit and then converting them back at the far end, or simply use and encoding method to encode your data to ASCII.