ICMP

ICMP, short for Internet Control Message Protocol, is a network layer protocol used by network devices, including routers, to send error messages and operational information indicating success or failure when communicating with another IP address. ICMP is an integral part of the Internet Protocol Suite, as defined in RFC 792.

ICMP is used for diagnosing and reporting network errors and issues. For example, if a router cannot forward an IP packet due to a problem, it might send an ICMP message back to the sender indicating the issue.

Tools like Ping and Traceroute use ICMP to test the reachability of network devices on an IP network.

  1. Types of Messages:
  2. Echo Request and Echo Reply: Used by the ping command to check network connectivity.
  3. Destination Unreachable: Indicates that a network destination is unreachable.
  4. Time Exceeded: Sent back to the source when a packet has expired (due to a TTL value reaching zero, indicating it has been in the network too long).

Unlike Transmission Control Protocol or User Datagram Protocol|UDP, ICMP is not used for sending and receiving data between end systems. It doesn’t carry application layer data but instead is used for control messaging.

ICMP has a version for IPv6 called ICMPv6, which, in addition to providing similar functionalities as ICMP for IPv4, also handles IPv6-specific messages, like Neighbor Discovery.