Internet Control Message Protocol, ICMP, How ICMP Work, ICMP Header, ICMP Message Header

When you send data from one device to another remote device, the IP Datagram often travels through one or more routers. There can be errors at routers while they try to forward the IP Datagram to its final destination. The Internet Control Message Protocol (ICMP) protocol is used to report problems with delivery of IP Datagrams within an IP network. ICMP is also used for other diagnosis and troubleshooting functions.

An Internet Control Message Protocol (ICMP) packet header is 8 Bytes long. The first four bytes always have the same meaning, and the contents of the remaining four depend on the ICMP packet type. An ICMP header follows the IP header in an IP packet and Internet Control Message Protocol (ICMP) is considered to be an integral part of IP.

The most common ICMP messages

Echo Request and Echo Reply: Internet Control Message Protocol (ICMP) is often used during test the connectivity between devices. We can use the ping (Packet InterNet Grouper) command to check connectivity from one device with another device and the ping command is using Internet Control Message Protocol (ICMP). Ping command sends an IP datagram to the IP address of the device we are trying to check the connectivity and requests the destination device to return the data sent in a response datagram. Ping command uses Internet Control Message Protocol (ICMP) Echo Request and Echo Reply.

Source Quench: If a device is sending large amounts of data to another remote device, the volume can flood the router with data. The router can use Internet Control Message Protocol (ICMP) to send a Source Quench message to the source IP to ask it to slow down the rate at which it is sending data.

Destination Unreachable: If a router receives a datagram that cannot be delivered, Internet Control Message Protocol (ICMP) returns a Destination Unreachable message to the source IP.

Time Exceeded: Internet Control Message Protocol (ICMP) sends this message to the source IP if a datagram is discarded because TTL reaches zero. One reason is the destination device is too many router hops away to reach with the current TTL value or a routing loop (An undesirable condition when the IP Datagrams loop infinitely between the routers, without reaching the destination).

Internet Control Message Protocol (ICMP) Message Header

Internet Control Message Protocol ICMP header

Internet Control Message Protocol (ICMP) Header

Type and Code: The following table shows the values which are possible for the Type and Code fields in the Internet Control Message Protocol (ICMP) header.

Type	Code	Description
0	0	Echo reply (used to ping)
1 and 2		Reserved
3	0	Destination network unreachable
	1	Destination host unreachable
	2	Destination protocol unreachable
	3	Destination port unreachable
	4	Fragmentation required, and DF set
	5	Source route failed
	6	Destination network unknown
	7	Destination host unknown
	8	Source host isolated
	9	Network administratively prohibited
	10	Host administratively prohibited
	11	Network unreachable for Type Of Service
	12	Host unreachable for Type Of Service
	13	Administratively prohibited
4	0	Traffic Congestion Control
5	0	Redirect Datagram for the Network
	1	Redirect Datagram for the Host
	2	Redirect Datagram for the Type Of Service & network
	3	Redirect Datagram for the Type Of Service & host
6		Alternate Host Address
7		Reserved
8	0	Echo request
9	0	Router Advertisement
10	0	Router discovery/selection/solicitation
11	0	TTL expired
11	1	Fragment reassembly time exceeded
12	0	Pointer indicates the error
	1	Missing a required option
	2	Bad length
13	0	Timestamp
14	0	Timestamp reply
15	0	Information Request
16	0	Information Reply
17	0	Address Mask Request
18	0	Address Mask Reply
19		Reserved for security
20 through 29		Reserved for robustness experiment
30	0	Information Request
31		Datagram Conversion Error
32		Mobile Host Redirect
33		Where-Are-You
34		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		ICMP for experimental mobility protocols such as Seamoby
42 through 255		Reserved

Checksum: The checksum field in the Internet Control Message Protocol (ICMP) message contains error checking data calculated from the Internet Control Message Protocol (ICMP) header+data, with value 0 for this field.

ID: The ID field in the Internet Control Message Protocol (ICMP) message contains an ID value, should be returned in case of ECHO REPLY.

Sequence number: The sequence number field in the Internet Control Message Protocol (ICMP) message is the sequence number for each host, generally this starts at 1 and is incremented by 1 for each packet.

You have learned Internet Control Message Protocol (ICMP), use of Internet Control Message Protocol (ICMP), Internet Control Message Protocol (ICMP) message header and the fields in Internet Control Message Protocol (ICMP) message. Click "Next" to continue.