What is
ICMP (Internet Control Message Protocol)?
The Internet Control Message Protocol (ICMP)
is an error reporting protocol that is an integral part of the
IP protocol. ICMP communicate control data, information data,
and error recovery data across the network. Problems that are less severe than
transmission errors result in error conditions that can be reported. For
example, suppose some of the physical paths in Internet fail
causing the Internet to be partitioned into two sets of networks with no path
between the sets. A datagram sent from a host in one set to a host in other
cannot be delivered.
The TCP/IP suite includes a protocol called ICMP that IP
uses to send error messages when condition such as the one described above
arises. The protocol is required for a standard implementation of IP. We will
see that the two protocols are co-dependent. IP uses ICMP when it sends an
error message, and ICMP uses IP to transport messages.
Following is a brief description of some of the error
messages defined by ICMP protocol:
§ Source Quench A router or host whose receive communication buffers are nearly full
normally triggers this message. A source quench message is sent to the sending
host, the receiver is simply requesting the sending host to reduce the rate at
which it is transmitting until advised otherwise.
§ Time Exceeded A time-exceeded message is sent in two cases. Whenever a router
reduces the TIL field in a data gram to zero. The router discards the datagram
and sends a time exceeded message. In addition, a time exceeded message is sent
by a host if the reassembly timer expires before all fragments from a given
datagram arrive,
§ Route Redirect A router sends this message to a host that is requesting its routing
services. When a host creates a datagram destined for a network, the host sends
the datagram to a router, which forwards the datagram to its destination. If a
router determines that a host has incorrectly sent a datagram that should be
sent to a different router, the router uses route redirect message to cause the
host to change its route. In this manner, a route redirect message improves the
efficiency of the routing process by informing the req4esting host of a shorter
path to the desired destination.
§ Host Unreachable Whenever a gateway or a router determines that a datagram cannot be
delivered to its final destination (due to link failure or bandwidth
congestion), an ICMP host unreachable message is sent to the originating node
on the network. Normally, the message includes the reason the host cannot be
reached.
§ Fragmentation and Reassembly The largest datagram the IP protocol can handle is
64 Kbytes. The maximum datagram size is dictated by the width of the total
length field in the IP header. Realistically, most underlying data link
technologies cannot accommodate this data size. For example, the maximum size of
the data frame supported by Ethernet is 1,514 bytes. Unless something is done
about situations like this, IP has to discard data that is delivered to it from
upper layer protocols with sizes exceeding the maximum tolerable size by the
data link layer. To circumvent this difficulty, IP is built to provide data
fragmentation and reassembly.
- Echo request/Echo reply –
These two ICMP messages are exchanged between ICMP software on any two
hosts in a bid to check connectivity between them. The ping command is an
example of a diagnostic command commonly used by network users to check
for the reach ability of a certain host. Whenever ping is invoked at the
command line, ICMP echo request message is sent to the target host. If the
target host is operational and connected to the network, it responds with
an echo reply message as proof of reach ability. In other words, the reply
carries the same data as the request.
- Address Mask
Request/Reply A host broadcasts an address mask request when
it boots, and routers that receive the request send an address mask reply
that contains the correct 2-bit subnet mask being used on
the network.
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