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Internet Control Message Protocol (ICMP) is a TCP/IP network layer protocol that provides troubleshooting, control and error message services. ICMP is most frequently used in operating systems for networked computers, where it transmits error messages.
ICMP for Internet Protocol version 4 is called ICMPv4 and for Internet Protocol version 6 is called ICMPv6.
Internet Control Message Protocol is also known as RFC 792.
An ICMP message is created as a result of errors in an IP datagram or for diagnostic routing purposes. These errors are reported to the originating datagram's source IP address. An ICMP message is encapsulated directly within a single IP datagram and reports errors in the processing of datagrams.
An ICMP header begins after the IPv4 header. An ICMP packet has an eight-byte header, followed by a variable-sized data section. The first four bytes of the header are fixed:
The remaining four bytes of the header vary based on the ICMP type and code.
The error message associated with ICMP includes a data section that holds the entire IP header along with the first eight bytes of the packet that generated the error message. An ICMP datagram is then encapsulated in a new datagram.