ICMP(Internet Control Message Protocol)

ICMP is at the same Protocol layer as IP, its purpose is to transmit information needed to control IP traffic. It is used mainly to provide information about routes to destination addresses. ICMP redirect messages inform hosts about more accurate routes to other systems, whereas ICMP unreachable messages indicate problems with a route. Additionally, ICMP can cause TCP connections to terminate gracefully if the route becomes unavailable. The ping command is a commonly-used ICMP-based service.

From the above figure, ICMP messages generated by router R1, in response to message sent by H0 to H1 and forwarded by R0. This message could, for instance be generated if the MTU of the link between R0 and R1 was smaller than size of the IP packet, and the packet had the Don’t Fragment (DF) bit set in the IP packet header.The ICMP message is returned to H0, since this is the source address specified in the IP packet that suffered the problem.

The figure below shows the encapsulation over an Ethernet LAN using an IP network layer header, and a MAC link layer header and trailer containing the 32-bit checksum: