Common ICMP packet types

The common ICMP packet Echo is the consulting information. The ping command uses this type of ICMP packet. When the ping command is used, an ICMP packet of the Echo-query type will be sent to the target host, and after the target host receives the ICMP packet, the Echo-response type ICMP packet is returned, and the ICMP packet containing the data is queried. Ping Command is an important tool for network troubleshooting. If an IP address can receive a reply through the ping command, other network protocol communication methods may also be successful. Source quench is an error message. If a host transfers data to a destination quickly, but the destination host does not have the matching processing capability, the destination host can send this type of ICMP packet to the destination host, remind the departure host to slow down sending speed (please be gentle ). Destination Unreachable is an error message. If a router receives an IP packet that cannot be further relayed, it will send this type of ICMP packet to the departure host. For example, when an IP packet arrives at the last vro and the vro finds that the Destination host is down, it will send an ICMP packet of the Destination Unreachable type to the Destination host. There may be other reasons why the destination cannot be reached, such as the absence of a relay path, or the unreceived port number. Timeout (Time Exceeded) is an error message. The Time to Live (TTL) in IPv4 and the Hop Limit in IPv6 decrease with the passing router. When the value of this region is reduced to 0, the IP packet times out (Time Exceeded ). Time Exceeded is the ICMP packet sent from the router to the departure host when the TTL is reduced to 0, notifying it of a timeout error. Traceroute uses this type of ICMP packet. The traceroute command is used to find the routers in the IP address route. It sends an IP packet to the destination, and sets TTL to 1 for the first Time, causing the Time Exceeded error of the first router. In this way, the first vroicmp replies to the ICMP packet so that the starting host knows the information of the first vro in the route. The TTL is then set to 2, 3, 4,... until it reaches the target host. In this way, each router along the way will send an ICMP packet to the departure host to report the error. Traceroute prints the ICMP packet information on the screen, which is the information of the relay path. Redirect is an error message. When a router receives an IP packet and compares it with its routing table, it finds that it should not receive the IP packet, and it will send a redirection type ICMP to the departure host, remind the departure host to modify its own routing table. For example, for the following network: If 145.1 is sent to the 145.15 IP packet, the result is received by the Intermediate router through the 145.17 NIC. Then the router will find that according to its own routing table, this IP package should be returned in the original path. Then the router can determine that 145.1 of the routing table may be faulty. Therefore, the router will send a redirect ICMP packet to 145.1.