ICMP (Internet Control Message Protocol)

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Internet Control Message Protocol (ICMP) defined by RFC 792 and RFC 1122 is used for network error reporting and generating messages that require attention. The errors reported by ICMP are generally related to datagram processing. ICMP only reports errors involving fragment 0 of any fragmented datagrams. The IP, UDP or TCP layer will usually take action based on ICMP messages. ICMP generally belongs to the IP layer of TCP/IP but relies on IP for support at the network layer. ICMP messages are encapsulated inside IP datagrams.
ICMP will report the following network information:

  • Timeouts
  • Network congestion
  • Network errors such as an unreachable host or network.

The ping command is also supported by ICMP, and this can be used to debug network problems.
ICMP Messages:
The ICMP message consists of an 8 bit type, an 8 bit code, an 8 bit checksum, and contents which vary depending on code and type. The below table is a list of ICMP messages showing the type and code of the messages and their meanings.
Type      Codes        Description                                                         Purpose
0            0                Echo reply                                                           Query
3            0                Network Unreachable                                         Error
3            1                Host Unreachable                                                Error
3            2                Protocol Unreachable                                          Error
3            3                Protocol Unreachable                                          Error
3            4                Fragmentation needed with don’t fragment bit set Error
3            5                Source route failed                                             Error
3            6                Destination network unknown                           Error
3            7                Destination host unknown                                  Error
3            8                Source host isolated                                            Error
3            9                Destination network administratively prohibited Error
3            10              Destination host administratively prohibited     Error
3            11              Network Unreachable for TOS                           Error
3            12              Host Unreachable for TOS Error
3            13              Communication administratively prohibited by filtering Error
3            14             Host precedence violation                                  Error
3            15              Precedence cutoff in effect                                 Error
4            0                Source quench                                                     Error
5            0                Redirect for network                                           Error
5            1                Redirect for host                                                 Error
5            2                Redirect for type of service and network           Error
5            3                Redirect for type of service and host                  Error
8            0                Echo request                                                       Query
9            0                Normal router advertisement                               Query
9            1                6 Router does not route common traffic             Query
10          0                Router Solicitation                                              Query
11          0                Time to live is zero during transit                        Error
11          1                Time to live is zero during reassembly                Error
12          0                IP header bad                                                      Error
12          1                Required option missing                                      Error
12          2                Bad length                                                           Error
13          0                Timestamp request                                               Query
14          0                Timestamp reply                                                  Query
15          0                Information request                                             Query
16          0                Information reply                                                Query
17          0                Address mask request                                         Query
18          0                Address mask request                                         Query

ICMP is used for many different functions, the most important of which is error reporting. Some of these are “port unreachable”, “host unreachable”, “network unreachable”, “destination network unknown”, and “destination host unknown”. Some not related to errors are:
  • Timestamp request and reply allows one system to ask another one for the current time.
  • Address mask and reply is used by a diskless workstation to get its subnet mask at boot time.
  • Echo request and echo reply is used by the ping program to test to see if another unit will respond.

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