Internet Control Message Protocol (ICMP), documented in RFC 792, is a required protocol tightly integrated with IP. ICMP messages, delivered in IP packets, are used for out-of-band messages related to network operation or mis-operation. Of course, since ICMP uses IP, ICMP packet delivery is unreliable, so hosts can't count on receiving ICMP packets for any network problem. Some of ICMP's functions are to:

• Announce network errors, such as a host or entire portion of the network being unreachable, due to some type of failure. A TCP or UDP packet directed at a port number with no receiver attached is also reported via ICMP.
• Announce network congestion. When a router begins buffering too many packets, due to an inability to transmit them as fast as they are being received, it will generate ICMP Source Quench messages. Directed at the sender, these messages should cause the rate of packet transmission to be slowed. Of course, generating too many Source Quench messages would cause even more network congestion, so they are used sparingly.
• Assist Troubleshooting. ICMP supports an Echo function, which just sends a packet on a round--trip between two hosts. Ping, a common network management tool, is based on this feature. Ping will transmit a series of packets, measuring average round--trip times and computing loss percentages.
• Announce Timeouts. If an IP packet's TTL field drops to zero, the router discarding the packet will often generate an ICMP packet announcing this fact. TraceRoute is a tool which maps network routes by sending packets with small TTL values and watching the ICMP timeout announcements.

List of permitted control messages:

• 0 - Echo Reply
• 1 - Reserved
• 2 - Reserved
• 3 - Destination Unreachable
• 4 - Source Quench
• 5 - Redirect Message
• 6 - Alternate Host Address
• 7 - Reserved
• 8 - Echo Request
• 9 - Router Advertisement
• 10 - Router Solicitation
• 11 - Time Exceeded
• 12 - Parameter Problem
• 13 - Timestamp
• 14 - Timestamp Reply
• 15 - Information Request
• 16 - Information Reply
• 17 - Address Mask Request
• 18 - Address Mask Reply
• 19 - Reserved for security
• 20-29 - Reserved for robustness experiment
• 30 - Traceroute
• 31 - Datagram Conversion Error
• 32 - Mobile Host Redirect
• 33 - IPv6 Where-Are-You
• 34 - IPv6 Here-I-Am
• 35 - Mobile Registration Request
• 36 - Mobile Registration Reply
• 37 - Domain Name Request
• 38 - Domain Name Reply
• 39 - SKIP Algorithm Discovery Protocol
• 40 - Photuris, Security failures
• 41-255 - Reserved