A physical layer chip of a first physical port of a network device receives a packet and sends a first time stamp and the packet to a Media Access Control (MAC) chip of the first physical port. The MAC chip of the first physical port adds the first time stamp to the packet and sends the packet. A MAC chip of a second physical port receives the packet, extracts the first time stamp of the packet, and sends the packet to a physical layer chip of the second physical port. The MAC chip of the second physical port receives a second time stamp of the packet. The network device calculates a processing delay for the packet. The processing delay is a value obtained by subtracting the first time stamp from the second time stamp.