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 TCP and UDP in that it is not used to send and receive data between end systems. It is usually not used directly by user network applications, with some notable exceptions being the ping tool and traceroute.
Internet control message protocol is part of the Internet protocol suite as defined in RFC 792. ICMP messages are typically generated in response to errors in IP 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, ICMPv6.
ICMP messages are constructed at the IP layer, usually from a normal IP datagram that 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 UDP, ICMP is unreliable.
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 that generated the original IP packet, the one that 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 is implemented using the ICMP “Echo request” and “Echo reply” messages.
Links:
http://candle.ctit.utwente.nl/wp5/tel-sys/exercises/icmp/icmp.html
http://en.wikipedia.org/wiki/Internet_Control_Message_Protocol
An increasing number of people are using the Internet and, many for the first time, are using the tools and utilities that at one time were only available on a limited number of computer systems (and only for really intense users!). One sign of this growth in use has been the significant number of TCP/IP and Internet books, articles, courses, and even TV shows that have become available in the last several years; there are so many such books that publishers are reluctant to authorize more because bookstores have reached their limit of shelf space! This memo provides a broad overview of the Internet and TCP/IP, with an emphasis on history, terms, and concepts. It is meant as a brief guide and starting point, referring to many other sources for more detailed information.
