A         Controller          B
          |(1) INV()     |              |
          |<-------------|              |
          |(2) 200(SDP1) |              |
          |------------->|              |
          |              |(3) INV()     |
          |              |------------->|
          |              |(4) 200(SDP2) |
          |              |<-------------|
          |(5) ACK(SDP2) |              |
          |<-------------|              |
          |              |(6) ACK(SDP1) |
          |              |------------->|