The Internet Control Message Protocol (ICMP), defined in the RFC792, prepares information of general type over problems with IP-data or serves actions as simple form of the time stamps or echo.

8	16	32 bits
Type	Code	Checksum
Type and Code dependently

Type	Message type: Type Meaning Reference 3 Destination Unreachable [RFC792] 4 Source Quench [RFC792] 5 Redirect [RFC792] 6 Alternate Host Address [JBP] 8 / 0 Echo Request / Echo Reply [RFC792] 9 / 10 Router Advertisement / Router Solicitation [RFC1256] 11 Time Exceeded [RFC792] 12 Parameter Problem [RFC792] 13 / 14 Timestamp Request / Timestamp Reply [RFC792] 15 / 16 Information Request / Information Reply [RFC792] 17 / 18 Adress Mask Request / Adress Mask Reply [RFC950] 19 "Reserved" (for Security) [Solo] 20..29 "Reserved" (for Robustness Experiment) [ZSu] 30 Traceroute [RFC1393] 31 Datagram Conversion Error [RFC1475] 32 Mobile Host Redirect [Johnson] 33 / 34 IPv6 Where-Are-You / IPv6 I-Am-Here [Simpson] 35 / 36 Mobile Registration Request / Mobile Registration Reply [Simpson] 37 / 38 Domain Name Request / Domain Name Reply [RFC1788] 39 SKIP [Markson] 40 Photuris [RFC2521] 41..255 "Reserved" for future use ...
Code	Type specific piece of information
Checksum	Checksum over ICMP-packet plus a so-called pseudo-header as protection against lacking segments (Source IP-Adresse, Destination IP-Adresse, Transport protocol, ICMP-length)
Type and Code dependently	according to Type and Code (see RFC792 / RFC 1700)