Figure 1: Encapsulation of ICMP messages in IP packets.
This is an INFORMATIONAL section on ICMP, it provides background information for the applications PING and TRACEROUTE.
The Internet Control Message Protocol is a protocol for the exchange of error messages and other vital information between (Physical) Internet entities such as hosts and routers.
ICMP is a network layer protocol, often it is placed next to the IP protocol. Normally, ICMP is used by the IP network layer to exhange e.g. error messages. However, applications may also use ICMP, two such popular applications are PING and TRACEROUTE. These applications allow user-intitiated network diagnostics (as will be exercised in the training).
ICMP is not 'supported' by all equipment. In particular firewalls are often configured as to block ICMP messages. Hence, hosts residing at the other side of such a firewall cannot be reached by ICMP messages.
ICMP messages are transmitted as encapsulated IP datagrams, that is,
and ICMP message is transmitted as data of an IP packet (see
figure below).
Figure 1: Encapsulation of ICMP messages in IP packets.
The ICMP message itself consists of a header and data, the length of the data depends on the message type and code. The figure below shows the structure of an ICMP message.
Figure 2: ICMP message format.
A number of possible ICMP message types is given in the Table below
(for a full list we refer to the sources listed at the end of this page).
| type | code | description | Query/Error |
| 0 | 0 | Echo reply (see e.g. PING) | Query |
| 3 | 1 | Host unreachable | Error |
| 3 | 3 | Port unreachable (see e.g. TRACEROUTE) | Error |
| 8 | 0 | Echo request (see e.g. PING) | Query |
| 11 | 0 | Time-to-live equals 0 during transit (see e.g. TRACEROUTE) | Error |
| 12 | 0 | Bad IP header | Error |
| 13 | 0 | Timestamp request | Query |
| 14 | 0 | Timestamp reply | Query |
As can be seen in the table ICMP message can either be an error message or a query message. In general ICMP error messages types are treated differently than ICMP query message types, as a rule of thumb the following rules apply:
This last rule is very important, because if it would not be obeyed situations may occur in which floods of ICMP messages are being generated causing severe congestion problems.
For those who are interested in further readings on ICMP we refer to: