Internet Control Message Protocol (ICMP)

Internet Control Message Protocol (ICMP) works at the Network layer and is used by IP for many different services. ICMP is a management protocol and messaging service provider for IP. Its messages are carried as IP datagrams. RFC 1256 is an annex to ICMP, which affords host’s extended capability in discovering routes to gateways.

ICMP packets have the following characteristics:

• They can provide hosts with information about network problems.
• They are encapsulated within IP datagrams.

The following are some common events and messages that ICMP relates to:

Destination Unreachable If a router can’t send an IP datagram any further, it uses ICMP to send a message back to the sender, advising it of the situation.

ICMP error message is sent to the sending host from the remote router.

When Host A sends a packet destined for Host B, the Lab_B router will send an ICMP destination unreachable message back to the sending device (Host A in this example).

Buffer Full If a router’s memory buffer for receiving incoming datagrams is full, it will use ICMP to send out this message until the congestion abates.

Hops Each IP datagram is allotted a certain number of routers, called hops, to pass through. If it reaches its limit of hops before arriving at its destination, the last router to receive that datagram deletes it. The executioner router then uses ICMP to send an obituary message, informing the sending machine of the demise of its datagram.

Ping Packet Internet Groper (Ping) uses ICMP echo request and reply messages to check the physical and logical connectivity of machines on an internetwork.

Traceroute Using ICMP time-outs, Traceroute is used to discover the path a packet takes as it traverses an internetwork.

Note:- Both Ping and Traceroute (also just called Trace; Microsoft Windows uses tracert) allow you to verify address configurations in your internetwork. 

The Ping program uses the alphabet in the data portion of the packet as just a payload, 100 bytes by default, unless, of course, you are pinging from a Windows device, which thinks the alphabet stops at the letter W and doesn’t’ include X, Y, or Z and then starts at A again.