Internet Control Message Protocol

The Internet Control Message Protocol (ICMP), defined in RFC 792, sends control, error message, and other information using the IP datagram delivery facility. There is some disagreement regarding whether ICMP properly belongs in the Internet Layer or in the Transport Layer, as parts of the protocol cross both layers. In this document it is considered to be part of the Internet Layer because it manages the connections across networks. If datagrams arrive too fast for proper processing, a gateway or host can send an ICMP Source Quench back to the sender to temporarily stop it from sending more datagrams. This is how IP manages flow control across the network. If a system routing IP packets determines that a destination is unreachable, it sends a Destination Unreachable message back to the source of the datagram. If a gateway system determines that the destination network or host is unreachable, it sends the message. If the packet reaches the destination host, and that system determines that the destination port is unreachable, then the destination host sends the message. When there is more than one gateway on the same network as a source host, a gateway can send the host an ICMP Redirect message to tell the host to use another gateway. This happens if there is more than one route to a destination and the gateways determine that the host should be using a different route than originally chosen. The ICMP Echo message is used to see if a remote host is present and operational. This message is used in both the ping and trace route programs for diagnostics and troubleshooting.