Buffer credits

Buffer credits are also called buffer-to-buffer credits . They ensure communication in a Fiber Channel Storage Area Network (SAN), especially when there are large distances between the devices. The number of buffer credits required does not only depend on the distance. The transmission rate of the Fiber Channel line also plays a role.

Since the signals cannot run faster than the speed of light, but each arriving data packet must be confirmed with an ACK at the receiving end, a number of packets is agreed between the two endpoints that can find space in the input buffer. This means that the sending device can send this number of data packets without waiting for confirmation for each individual packet.

The sending device keeps a record of the number of data packets en route, i.e. those that were sent but whose confirmation of receipt has not yet been returned. The "buffer credits" are reduced by one for each data packet sent, and increased by one for each confirmed data packet. If the "buffer credits" have reached zero, the sending is interrupted until the confirmations are received.