The Internet Control Message Protocol (ICMP) is one of the core protocols of the Internet protocol suite. It is chiefly used by networked computers operating systems to send error messages—indicating, for instance, that a requested service is not available or that a host or router could not be reached.

ICMP differs in purpose from Transmission Control Protocol and User Datagram Protocol in that it is usually not used directly by user network applications. One exception is the Ping tool, which sends ICMP Echo Request messages (and receives Echo Response messages) to determine whether a host is reachable and how long packets take to get to and from that host.

= Technical overview =

The Internet Control Message Protocol (ICMP) is part of the Internet protocol suite as defined in RFC 792. ICMP messages are typically generated in response to errors in Internet Protocol datagrams (as specified in RFC 1122) or for diagnostic or routing purposes.

The version of ICMP for Internet Protocol version 4 is also known as ICMPv4, as it is part of IPv4. IPv6 has an equivalent protocol.

ICMP messages are constructed at the IP layer, usually from a normal IP datagram which has generated an ICMP response. IP encapsulates the appropriate ICMP message with a new IP header (to get the ICMP message back to the original sending host), and transmits the resulting datagram in the usual manner.

For example, every machine (such as intermediate Routers) that forwards an IP datagram has to decrement the time to live (TTL) field of the IP header by one; if the TTL reaches 0, an ICMP Time to live exceeded in transit message is sent to the source of the datagram.

Each ICMP message is encapsulated directly within a single IP datagram, and thus, like User Datagram Protocol, ICMP does not guarantee delivery.

Although ICMP messages are contained within standard IP datagrams, ICMP messages are usually processed as a special case distinguished from normal IP processing, rather than processed as a normal sub-protocol of IP. In many cases, it is necessary to inspect the contents of the ICMP message, and deliver the appropriate error message to the application which generated the original IP packet, the one which prompted the sending of the ICMP message.

Many commonly used network utilities are based on ICMP messages. The Traceroute command is implemented by transmitting UDP datagrams with specially set IP TTL header fields, and looking for ICMP Time to live exceeded in transit (above) and Destination unreachable messages generated in response. The related Ping utility (well known on Unix) is implemented using the ICMP Echo and Echo reply messages.

List of permitted control messages (incomplete list): 0 - ICMP Echo Reply 1 - Reserved 2 - Reserved 3 - ICMP Destination Unreachable 4 - ICMP Source Quench 5 - ICMP Redirect Message 6 - Alternate Host Address 7 - Reserved 8 - ICMP Echo Request 9 - Router Advertisement 10 - Router Solicitation 11 - ICMP_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 (protocol), Security failures 41-255 - Reserved (Source: [http://www.iana.org/assignments/icmp-parameters IANA ICMP Parameters])

=See also=

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