Delay in the RX data path
The RX data path consists of a number of components, such as NIM (for instance SFP), PHY and MAC, which create a delay from an Ethernet frame hits the connector until the frame is time-stamped by the FPGA.
Delay in the TX data path
For transmission there is also a delay from the time the SmartNIC releases a frame until the frame is sent on to the wire/fiber. This delay is called the TX path delay.