ICMP(Internet Control Message Protocol)
The Internet Control Message Protocol's (ICMP) job is to report errors that may have occurred in processing IP datagrams.
ICMP is an integral part of IP and and its messages are encapsulated within an IP datagram.
Note: there are 6 messaging formats used by ICMP which will be discussed later.
The ICMP messages are indicated by the Type Field and are:
0 - Echo Reply
3 - Destination unreachable
8 - Echo
11 - Time exceeded
12 - Parameter problem
13 - Timestamp
14 - Timestamp reply
15 - Information request (obsolete)
16 - Information reply (obsolete)
17 - Address mask request
18 - Address mask reply
The most well-known uses of ICMP are the ping and traceroute (tracert in Window) commands. The ping command sends out a special ICMP echo request message to a destination. If the destination is alive, it will respond with the ICMP echo replay messageTraceroute uses the Timestamp services of ICMP to perform its task of tracing a route to a destination. The Timestamp message and Timestamp Reply measure the roundtrip time that is taken to go from the source to the destination. Traceroute lists the path and the roundtrip time to each router taken from the source to the destination