The Previous Chapter

The previous chapter discussed the purposes and detailed operation of the Flow Control Protocol. This protocol requires each device to implement credit-based link flow control for each virtual channel on each port. Flow control guarantees that transmitters will never send Transaction Layer Packets (TLPs) that the receiver can't accept. This prevents receive buffer over-runs and eliminates the need for inefficient disconnects, retries, and wait-states on the link. Flow Control also helps enable compliance with PCI Express ordering rules by maintaining separate Virtual Channel Flow Control buffers for three types of transactions: Posted (P), Non-Posted (NP) and Completions (Cpl).