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:
- 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.
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.
|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|
|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|
|9||0||Normal router advertisement||Query|
|9||16||Router does not route common traffic||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|
|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.
TCP/IP Illustrated, Volume1, The Protocols
Author:W. Richard Stevens, Publisher: Addison Wesley. ISBN 0201633469