TCP is designed to provide reliable data transfer between a pair of devices on an IP internetwork. An important impediment to the safe transmission of data is the risk of errors being introduced into a TCP segment during its travel across the network. To provide basic protection against errors in transmission, TCP includes a 16-bit checksum field in its header.
The Foglight Experience Monitor agent employs an algorithm that calculates the checksum for every segment that it analyzes. The calculated checksum is then compared to the checksum found in the header. If there is a discrepancy, then this metric is incremented.
If this metric is zero, that indicates there is likely a problem with the span port or network taps that are collecting traffic for the appliance. If at the same time, the metric is greater than zero, that indicates that the appliance is seeing only one-way traffic from server to client, and is not seeing the client-to-server traffic.