+---+ +---+ |S1 |===== \ Forward --> / =======|R1 | +---+ \\ // +---+ \\ // +---+ +-----+ +-----+ +---+ |S2 |=======| A |------------------------------>| B |=======|R2 | +---+ | |<------------------------------| | +---+ +-----+ +-----+ // <-- Backward \\ +---+ // \\ +---+ |S3 |===== / \ ======|R3 | +---+ +---+