The NQA trace test monitors the forwarding path between the NQA client and a destination and collects statistics about devices along the forwarding path. The trace test functions similarly to the tracert command. The difference is that the trace test provides detailed output information. For example, information about each hop contains the average delay time, packet loss ratio, and the time when the last packet was received. Figure 1 illustrates a network on which an NQA trace test is performed. The NQA trace test process is as follows:
The NQA client (Device A) constructs a UDP packet with a TTL of 1 and sends the packet to destinationDevice B.
After Device C at the first hop receives the UDP packet, it finds that the TTL of the packet expires. Then, Device C discards the packet and replies with an ICMP Time Exceeded message.
After the NQA client receives the ICMP Time Exceeded message, it records the first-hop IP address, increases the TTL of the same UDP packet to 2, and sends the UDP packet.
After Device D at the second hop receives the UDP packet with a TTL of 2, it finds that the TTL of the packet expires. Device D discards the packet and replies with an ICMP Time Exceeded message.
This process continues until the packet reaches the last hop router, which replies with an ICMP Port Unreachable message to the NQA client.
Based on the ICMP message returned from each hop, the NQA client collects information about the forwarding path between the NQA client and the destination and statistics about each router along the forwarding path. The data obtained is used to evaluate the network status.