Internet Control Message Protocol

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Internet Control Message Protocol

Internet Control Message Protocol (ICMP) is part of the Internet layer and is responsible for reporting errors and messages regarding the delivery of IP datagrams. It can also send “source quench” and other self tuning signals during the transfer of data between two machines without the intervention of the user. These signals are designed to fine-tune and optimize the transfer of data automatically. ICMP is the protocol that warns you when a destination host is unreachable, or how long it took to get to a destination host. In Figure 2.24, ICMP is represented by a policeman. If it helps, think of ICMP as the Internet Control Military Police, the protocol that’s always watching over your shoulder.

ICMP messages can be broken down into two basic categories: the reporting of errors and the sending of queries. Error messages include the following:

Destination unreachable
Redirect
Source quench
Time exceeded

The Destination unreachable error message is generated by ICMP when an IP datagram is sent out and the destination machine either cannot be located or does not support the designated protocol. For instance, a sending machine may receive a Destination host unreachable message when trying to communicate through a router that does not know to which network to send a datagram.

The first important thing to realize about Redirect messages is that these are only sent by routers in a TCP/IP environment, not individual machines. A machine may have more than one default gateway defined for redundancy. If a router detects a better route to a particular destination, it forwards the first packet it receives, but sends a redirect message to the machine to update its route tables. In this way, the machine can use the better route to reach the remote network.

Sometimes a machine has to drop incoming datagrams because it has received so many it can’t process them all. In this case, a machine can send a Source quench message to the source, indicating it needs to slow up transmission. The Source quench message can also be sent by a router if it is in between the source and destination machines and is encountering trouble routing all the packets in time. Upon receiving a Source quench message, the source machine immediately reduces its transmissions. However, it continues to try to increase the amount of data as time progresses to the original amount of data it was sending before.

The Time exceeded error message is sent by a router whenever it drops a packet due to the expiration of the TTL. This error message is sent to the source address to notify the machine of a possible infinite routing loop or that the TTL is set too low to get to the destination.

ICMP also includes general message queries. The two most commonly used are the following:

Echo request
Echo reply

The most familiar tool for verifying that an IP address on a network actually exists is the Personal Internet Groper (PING) utility. This utility uses the ICMP echo request and reply mechanisms. The echo request is a simple directed datagram that asks for acknowledgment that a particular IP address exists on the network. If a machine with this IP address exists and receives the request, it is designed to send an ICMP echo reply. This reply is sent back to the destination address to notify the source machine of its existence. The PING utility reports the existence of the IP address and how long it took to get there.

ICMP serves a number of functions, but primarily acts as the messenger for what is happening during the communication process. (Remember, you should think of ICMP as standing for the Internet Control Military Police.) For instance, in the mail example, if Bob improperly formats his address, ICMP (the police) come knocking on Bob’s door to notify him of his error. Or if Bob sends so many letters to Kim that Kim’s mailbox cannot hold them all, causing a considerable overflow at the post office, ICMP (the police) knock on Bob’s door and politely ask him to reduce his transmissions. In the phone call scenario, if Bob dials the wrong number, ICMP (the police) are right there to warn Bob about the error of his ways. Or if Bob talks Kim’s poor ear off and doesn’t let her get a word in edgewise, ICMP (the police) kindly step in on Kim’s behalf and remind Bob that conversations are supposed to work both ways.

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