What Is ICMP?

Date First Published: 15th April 2022

Topic: Computer Networking

Subtopic: Internet Protocols

Article Type: Computer Terms & Definitions

Difficulty: Advanced

Difficulty Level: 8/10

CONTENTS

Other Uses
Control Messages
History

Learn more about what ICMP is in this article.

Not to be confused with IGMP (Internet Group Management Protocol).

Short for Internet Control Message Protocol, ICMP is a network-layer protocol used to communicate error messages between routers, hosts, and other network devices. For example, if a data packet was too large for a router to handle, it would drop the packet and send an ICMP message to the sending end. The most common use of ICMP is to check whether data packets are reaching their destination node within a reasonable period of time. ICMP is a very important protocol for error testing.

Note:

ICMP is known as a connectionless protocol as it is not linked to any transport layer protocol, such as TCP or UDP. As a result, ICMP does not perform a handshake or use a specific port.

Other Uses

Another use of ICMP is diagnosing networks using the traceroute command and ping. The traceroute command shows the routing path between two routers, hosts, or other network devices and the journey from one router to another is known as a hop. In addition, traceroute provides a report of the period of time that is required for each hop. This use of ICMP in traceroute is helpful for identifying sources of delay in a network and information as to whether the routers can effectively transfer the data.

Ping tests the reachability of a computer network by sending a signal to host and receiving a response. It can provide a report of latency, which is the period of time that it takes for a data packet to reach its destination. Ping does not provide data in regard to routing or hops, but it can still give a measure of the latency. Ping uses ICMP for the purpose of sending an ICMP echo message to the specified host.

Control Messages

The type and code of control messages help identify the response of the device. The table below shows a list of control messages with descriptions.

ICMP Type	Description	Status
0	Echo reply	Active
1 and 2	Reserved	Unassigned
3	Destination unreachable	Active
4	Source quench	Deprecated
5	Redirect message	Active
6	Alternate host address	Deprecated
7	Reserved	Unassigned
8	Echo request	Active
9	Router Advertisement	Active
10	Router Solicitation	Active
11	Time exceeded	Active
12	Parameter problem	Active
13	Timestamp request	Active
14	Timestamp reply	Active
15	Information request	Deprecated
16	Information reply	Deprecated
17	Address mark request	Deprecated
18	Address mark reply	Deprecated
19	Reserved for security	Reserved
20-29	Reserved for robustness experiment	Reserved
30	Information request	Deprecated
31	Datagram conversion error	Deprecated
32	Mobile host redirect	Deprecated
33	Where-Are-You	Deprecated
34	Here-I-Am	Deprecated
35	Mobile Registration Request	Deprecated
36	Mobile Registration Reply	Deprecated
37	Domain Name Request	Deprecated
38	Domain Name Reply	Deprecated
39	Domain Name Reply	Deprecated
40	Photuris, Security failures	Active
41	ICMP for experimental mobility protocols such as Seamoby	Experimental
42	Requested Extended Echo	Active
44-252	Reserved	Unassigned
253	RFC3692-style Experiment 1	Experimental
254	RFC3692-style Experiment 2	Experimental
255	Reserved	Reserved

History

ICMP was introduced in 1981 by DARPA (Defence Advanced Research Projects Agency) and was defined in RFC 792 as an extension to the Internet Protocol.

	TCP/IP Protocol
Application layer	BGP• DHCP • DNS • FTP • HTTP • IMAP • LDAP • MGCP • NNTP • NTP • OSPF • POP • PTP • ONC/RPC • RTP • RTSP • RIP • SIP • SMTP • SNMP • SSH • Telnet • XMPP
Transport layer	TCP • UDP • DCCP • SCTP • RSVP • QUIC
Internet layer	IP • ICMP • NDP • ECN • IGMP.
Link layer	Tunnels • PPP • MAC