Method and apparatus for controlling the flow of data between servers

Link-based flow control requires each link transmitter to retain packets until such time as they are acknowledged by the link receiver. Depending on the type of acknowledge, the transmitter will then either retry or de-allocate the packets. The NG I/O flow control of the present invention is credit based. At power up, each end of a link determines buffer size, deferring to the smaller implemented buffer if size is different. A transmitter will not send a packet unless buffer space at the receiver is available. Transmitter and receiver also synchronize on width of the link ID counter. Each link transmitter assigns an ordered ID (beginning with zero) to each sent packet. The link receiver tests the checksum of each delivered packet. If the packet tests with no error, the receiver sends a positive acknowledge (ACK) back to the transmitter. Flow control is either contained in its own packet or is appended to a packet assembling for transmission back. Some period of time can be consumed while the receiver attempts to acknowledge the transmitter. In the interests of efficiency, a receiver can ACK out-of-order to de-allocate buffer space for the named transaction and any other sequentially previous transactions outstanding at the transmitter. In exchange for this performance benefits, the receiver assumes responsibility for tracking transmit ID sequence. This audition activity reduces the opportunity for undetected packets. In the case of negative acknowledge (NAK), the transmitter may still de-allocate for sequentially previous transactions. NAK instructs the transmitter to resend the indicated packet and any packets that have been sent since then. Each transmitter has a configurable interval timer for the link acknowledge cycle. Each transmitter has a configurable iteration counter for NAK. Rather than congest the link, a transmitter that has run through either counter is obliged to return the undeliverable packet to its source.