[Network layer] Analysis Network Layer-Internet Control Packet protocol ICMP

[Network layer] Analysis Network Layer-Internet Control Packet Protocol ICMP (Internet Control Message Protocol) 1) Message format: the ICMP packet is encapsulated into the data part of the IP packet, and the data part of the ICMP packet is the first eight bytes (IP address and port number) of the IP header and data part of the IP datagram to report errors 2) ICMP Type: Error Control Message, inquiry Message 1> inaccessibility: the router cannot correctly deliver the datagram. For example, the UDP packet header contains an unknown Port Number 2> source point suppression: Route congestion discard packet 3> time exceeds: TTL = 0 4> Parameter Problem: packet header field problem 5> route change (redirection): Let the host know that the next time the datagram should be sent to another vro, it is discovered that it has arrived at a host. Instead of going through the default route, it should go through another route. 3) ICMP error messages are not generated in the following situations. 1> ICMP error messages (however, ICMP Query Packets may generate ICMP error messages) 2> the destination address is the IP datagram of the broadcast address or multicast address 3> as the datagram of The Link Layer 4> not the first part of the IP segment 5> the source address is not the datagram of a single host. That is, the source address cannot be zero, loop address, broadcast address, or multicast address. These rules are used to prevent the broadcast storms caused by the past Allow ICMP error messages to respond to broadcast groups. 4) ICMP inquiry messages include four types of return requests and answers, timestamp requests and answers, mask address requests and answers, and router inquiries and passes. 1> ICMP return request message: Send an inquiry to the host and receive the ICMP return response message to test whether the target site is accessible and whether the status of the Response Message is known. 2> ICMP timestamp request packets 5) ping command details [Use Network Layer directly at the application layer] inter-network detection (Packet Internet Groper): test the connectivity between two hosts. Requests and replies are sent to the reply packet when ICMP is used. Ping 127.0.0.1 is different from ping the NIC address of the Local Machine: the difference is that 6) Traceroute or Tracert is used to track the path from the source to the end of a group: send IP datagram (including UDP datagram that cannot be delivered ), the first TTL = 1 get an error control packet with an ICMP time exceeding the second IP datagram TTL = 2 get an error control packet with an ICMP time exceeding ...... The last IP packet is an undeliverable transport layer UDP, which returns an error report packet that is inaccessible to the ICMP Destination.