Some people may feel strange when it comes to ICMP. In fact, ICMP is closely related to us. In all levels of the network architecture, control is required, and different levels have different division of labor and control content. The IP layer control function is the most complex, mainly responsible for error control, congestion control, etc. Any control is established in On the basis of information, in the network system based on IP datagram, the gateway must handle the transmission of datagrams by itself, and the IP protocol itself has no intrinsic mechanism to obtain error information and process. In order to deal with these errors, TCP/IP designed the ICMP protocol. When a gateway finds a transmission error, it immediately sends an ICMP message to the source host, reports the error message, and causes the source host to take appropriate action. It is an error. And control message protocol, not only for transmitting error messages, but also for transmitting control messages.
What is the ICMP protocol? Perhaps many Internet people have never heard of it. In fact, the ICMP protocol is closely related to our online life, but we have never noticed it. As long as there are network, router, IP and other places, there will be ICMP protocol, the following small series for everyone to introduce the ICMP protocol.
The ICMP protocol is a connectionless protocol for transmitting error reporting control information. It is a very important protocol and it is extremely important for network security.
It is a sub-protocol of the TCP/IP protocol family and belongs to the network layer protocol. It is mainly used to transfer control information between the host and the router, including reporting errors, exchange limited control, and status information. ICMP messages are automatically sent when IP data cannot be accessed and the IP router cannot forward packets at the current transmission rate. The value of the ICMP packet in the header protocol type field (Protocol 8bit) of the IP frame structure is 1.
As shown in the following figure, the ICMP packet has an 8-byte long header, where the first 4 bytes are in a fixed format, including an 8-bit type field, an 8-bit code field, and a 16-bit checksum; the last 4 bytes. Different values ​​are taken depending on the type of ICMP packet.
ICMP provides consistent and error-prone information. The sent error message is returned to the device that sent the original data, because only the sending device is the logical recipient of the error message. The sending device can then determine the type of error that occurred based on the ICMP message and determine how to better retransmit the failed packet. But the only function of ICMP is to report the problem instead of correcting the error, and the task of correcting the error is done by the sender.
We often use the ICMP protocol in the network. For example, we often use the Ping command to check the network (both in Linux and Windows). 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.
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