3.2 ICMP
The acronym ICMP stands for Internet Control Message Protocol (RFC792) and is used to
exchange error and information messages.
● Error message
Informs the sender of the IP frame that when forwarding the frame an error or a parameter
problem occurred.
● Information message
Can contain information about the time measurement, the address mask, the reachability of
the destination or for finding the router.
Structure of the ICMP data packet
0 4 8 12 16 20 24 28 31
ICMP packet type
Type of message
Code
Further details of the
message
Checksum
Data (optional)
● ICMP packet type
The most important ICMP packet types are as follows:
– Redirect
The router informs the host in one of its subnets that there is a better route to the
destination. This ICMP packet type is dealt with in more detail in the following description.
– Destination Unreachable
IP frame cannot be delivered.
– Time Exceeded
Time limit exceeded
– Echo-Request
Echo request, better known as ping.
● Code
The code describes the ICMP packet type in greater detail. The selection depends on the
selected ICMP packet type. With "Destination Unreachable,", for example "Code 1" host
cannot be reached.
You will find a full list of the ICMP packet types and codes on the website of IANA.
Technical basics
3.2 ICMP
SCALANCE S615 Web Based Management
Configuration Manual, 11/2019, C79000-G8976-C388-08 33
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