I think there should be more slides about flow control in the lecture. About XON/XOFF flow control.

Lost Slide: Communication Buffer Availability
(1) Credit-based flow control
- upstream knows how many buffers are downstream
- Downstream passes back credits to upstream
- Significant upstream signaling (esp. for small flits)

(2) On/Off(XON/XOFF) flow control
- Downstream has on/off signal to upstream

(3)Ack/Nack flow control
- Upstream optimistically sends downstream
- Buffer cannot be deallocated until ACK/NACK received
- Inefficiently utilizes buffer space