The Internet Control Message Protocol (ICMP) is one of the core protocols of the Internet Protocol Suite. It is chiefly used by the operating systems of networked computers to send error messages indicating, for example, that a requested service is not available or that a host or router could not be reached. ICMP can also be used to relay query messages.^[1] It is assigned protocol number 1.^[2]

ICMP^[3] differs from transport protocols such as TCP and UDP in that it is not typically used to exchange data between systems, nor is it regularly employed by end-user network applications (with the exception of some diagnostic tools like ping and traceroute).

ICMP for Internet Protocol version 4 (IPv4) is also known as ICMPv4. IPv6 has a similar protocol, ICMPv6.

Technical details[link]

The Internet Control Message Protocol is part of the Internet Protocol Suite, as defined in RFC 792. ICMP messages are typically used for diagnostic or control purposes or generated in response to errors in IP operations (as specified in RFC 1122). ICMP errors are directed to the source IP address of the originating packet.^[1]

For example, every device (such as an intermediate router) forwarding an IP datagram first decrements the time to live (TTL) field in the IP header by one. If the resulting TTL is 0, the packet is discarded and an ICMP Time To Live exceeded in transit message is sent to the datagram's source address.

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 tracert (traceroute), Pathping commands are 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.

ICMP segment structure[link]

Header[link]

The ICMP header starts after the IPv4 header. All ICMP packets will have an 8-byte header and variable-sized data section. The first 4 bytes of the header will be consistent. The first byte is for the ICMP type. The second byte is for the ICMP code. The third and fourth bytes are a checksum of the entire ICMP message. The contents of the remaining 4 bytes of the header will vary based on the ICMP type and code.^[1]

ICMP error messages contain a data section that includes the entire IP header plus the first 8 bytes of data from the IP datagram that caused the error message. The ICMP datagram is then encapsulated in a new IP datagram.^[1]

• Type – ICMP type as specified below.
• Code – Subtype to the given type.
• Checksum – Error checking data. Calculated from the ICMP header+data, with value 0 for this field. The checksum algorithm is specified in RFC 1071.
• Rest of Header – Four byte field. Will vary based on the ICMP type and code.

List of permitted control messages (incomplete list)[link]

(Sources: IANA ICMP Parameters [1] and Computer Networking – A Top-Down Approach by Kurose and Ross) //

See also[link]

• Path MTU Discovery
• ICMPv6
• IRDP
• Smurf attack
• TCP
• ping
• traceroute
• ICMP tunnel

References[link]

External links[link]

• RFCs
  • RFC 792, Internet Control Message Protocol
  • RFC 1122, Requirements for Internet Hosts – Communication Layers
  • RFC 1716, Towards Requirements for IP Router
  • RFC 1812, Requirements for IP Version 4 Routers
• IANA ICMP parameters
• IANA protocol numbers