+---+ +---+ |S1 |====== \ Forward --> / =======|R1 | +---+ \\ // +---+ \\ // +---+ +-----+ +-----+ +---+ |S2 |=======| A |--------------------------->| B |=======|R2 | +---+ | |<---------------------------| | +---+ +-----+ +-----+ (...) // \\ (...) // <-- Backward \\ +---+ // \\ +---+ |Sn |====== / \ ======|Rn | +---+ +---+