Guide to OSI and TCPIP Models by Mohammed M. Alani

Author:Mohammed M. Alani
Language: eng
Format: epub
Publisher: Springer International Publishing, Cham

The Type field is used to define the type of the ICMP message, for example, error message. There are almost forty standard message types that were designed to accomplish multiple functions starting from echo request and reply (used in PING command), to destination unreachable (to report lost or undelivered packets), and many more.

The Code field is used to define the subtype of the message within the specified ICMP message type. For example, message type 3 (Destination Unreachable) has 16 different subtypes like destination network unreachable, destination host unreachable, destination protocol unreachable, …etc.

The Checksum field is a 16-bit checksum that is calculated in a manner similar to the IP header checksum. While the IP header checksum is used to detect errors in the IP header only, the ICMP checksum provides bit error detection coverage for the entire ICMP message.

Mainly, the contents of the Data depend on the type and code of the ICMP message. For some messages, the Data field contains the IP header and the first 64 bits of the packet that triggered the error that caused the ICMP message to be sent. In certain cases, such as the echo request or reply, data field contains three timestamps; originate, receive, and transmit timestamps.

The main duties of ICMP are the following:1.Report network errors such as a host or a network being unreachable.

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