Internet Control Message Protocol

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 16 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.