Perhaps GP considers a NACK more semantically easier to understand on an intuitive level, instead of the implicit NACK with "I know you sent 8, but we're really

Perhaps GP considers a NACK more semantically easier to understand on an intuitive level, instead of the implicit NACK with "I know you sent 8, but we're really at 4 so resume from that". But I do agree that it's more efficient for the remote host to just immediately establish "4 was the last point we are ok at".