ICMP (Internet Control Message Protocol)

ICMP (Internet Control Message Protocol) is a network layer protocol that reports the errors and provide information related to IP packet. ICMP is used by network device to send error message indicating, Such that a requested services is not available or that a host is not reachable.

ICMP is commonly use by network tools such ping or traceroute . Consider the below example that explain how ping can be used to test the reachability with the host:

Host A want to test whether it reach Server over the network. Host A will start ping that will send ICMP Echo Request packets to Server. If the Server is reachable, it will respond with ICMP Echo Reply packet. If Host A receive no response from the Server, there might be  problem with the network.

NOTE
ICMP message is encapsulated in IP datagram, which mean that they do not use higher level protocols (such as TCP or UDP) for data transmission.

 

One other ICMP message is Destination unreachable message. below is an example:

Host A send a packet to Host B. due to the Host B down, the router will send ICMP Destination host unreachable message to Host A, for informing the destination host is unreachable:

C:\>ping 192.168.8.11

Pinging 192.168.8.11 with 32 byte of data:
Request timed out.
Request timed out.
Request timed out.
Request timed out.

Ping statistics for 192.168.8.11:
    Packet: Send = 4, Received = 0, Lost = 4 (100% loss)