Place a light rope on each messenger. one end contains a confirmation token around the messenger’s waist and a piece of paper, the other end is tied to a horse. When messenger arrives -he signs the paper with a predetermined codeword and unties the rope. A horse on the other end of the rope is whipped into running at a time it takes to cross the valley, successfully or unsuccessfully dragging the token and signed codeword BACK to the original side, . New messengers arriving, indicate to receiver that previous rope token paper was not received. when new messengers stop arriving, receiver knows last rope made it through. SUCCESS = Original messengers stay on receiver side with attack message, receipt of ropes with attached tokens and signatures. No new messengers sent/arrive. Attack = 100%