ICMP
From Hackepedia
ICMP stands for Internet Control Messaging Protocol. It resides on layer 3 of OSI even though it is carried by IP which also resides on that layer. It is seen as an extension of IP. ICMP is explained in RFC 792.
Tools that make direct use of ICMP are ping and traceroute.
A helpful reference when dealing with ICMP is reading the header file found in /usr/include/netinet/ip_icmp.h.
