ICMP, what is it for?

I sometimes write about networking related topics as a means to teach myself and then when I forget I can search our own site for a refresher course. In this case ICMP it is one of the four core protocols, the other three being TCP, IP and UDP. Currently we are trying to hunt down a network issue effecting RDP sessions and that is prompting us to brush up on our understanding of protocols.

Here is a depiction of the OSI reference model:

Application Layer Protocols like HTTP, SMTP, FTP
Presentation Layer Protocols like JPEG, MPEG
Session Layer Protocols like NFS, SQL, RPC
Transport Layer Protocols like TCP, and UDP
Network Layer Protocols like IP and ICMP
Data Link Layer Protocols like HDLC and PPP
Physical Layer No protocols at this layer

ICMP is therefore on the Network Layer with IP, actually encapsulated in the IP datagram. A ping request to see if an IP is on a specific computer is a common use of the ICMP ping packet. I will install wireshark today on one of the servers and take a look a packet and dissected and write about it as a way to become more familiar with what a sniffer might tell us.