To use this protocol, we can make the corresponding ICMP settings. For example, in Windows XP, first open "Network Connection", right click on "Network Connection" with Internet Connection Firewall enabled, and select "Properties" to open the Properties window. Next, select the "Advanced" tab and click the "Settings" button in the lower right corner. Then, select the "ICMP" tab in the Advanced Settings window where you can make the appropriate settings, including allowing incoming echo requests and more.
We often use the ICMP protocol in the network, but we are not aware of it. For example, the Ping command that we often use to check the network does not work. This "Ping" process is actually the process of working with the ICMP protocol. There are other network commands such as tracert Tracert commands that are also based on the ICMP protocol.
ICMP application pingPing is a tool used to query the network, it is the system's own, running in the kernel, calling the ICMP protocol.
The working principle of ping: ping sends a request by calling echo, and queries the connectivity of the network layer by receiving echo-reply. The result of the ping will give the time of transmission and TTL data, and also the route from the host to the destination host. This is because the ICMP ping request datagram will put its IP address into the packet every time it passes through a router, and the destination host will copy the IP list to the ICMP packet and send it back to the packet. Host. However, the routing list that the IP header can record is very limited. If you want to view the complete routing information, you need to use the traceroute tool.
Traceroute is a tool used to detect the routing of the host to the destination host, and can obtain the IP address of the router.
The working principle of traceroute: after receiving the IP of the destination host, it first sends a UDP packet with TTL=1 to the destination host, and the first router after receiving the packet automatically reduces the TTL by 1. After the TTL becomes 0, the router discards the packet and simultaneously generates a host unreachable ICMP packet to the host. After receiving the packet, the host sends a packet with TTL=2 to the host, and then stimulates the second router to send the ICMP packet to the host, and so on until it reaches the destination host. In this way, traceroute gets all the router IPs, thus avoiding the problem that the IP header can only record limited routing IP.
How do you know that UDP has not reached the destination host? This involves a tricky problem. The TCP and UDP protocols have a port number definition, while a normal network program only monitors a small number of ports with smaller numbers, say 80, say 23, and so on. The traceroute sends the UDP packet with the port number "30000 (true metamorphosis). Therefore, when the destination host arrives, the destination host can only send an ICMP datagram with an unreachable port to the host. After receiving this report, the host knows that the host is up, so it is no exaggeration to say that Traceroute is a liar.
Another role of traceroute is to determine the path MTU. All you have to do is send the packet and set the "no fragmentation" flag. The length of the first packet sent is exactly equal to the egress MTU, and the length of the packet is reduced each time an ICMP "cannot fragment" error is received.
Three cases of ICMP unreachable:The TTL is 0, the port is unreachable (the corresponding port number cannot be found), and the IP needs to be fragmented but the non-fragmented flag is set.
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