How do you configure ICMP header fields for different types of messages?

1 ICMP Packet Format

An ICMP packet consists of an 8-byte header and a variable-length data section. The header contains four fields: type, code, checksum, and rest of header. The type field specifies the type of ICMP message, such as echo request (8), echo reply (0), destination unreachable (3), or time exceeded (11). The code field provides further details about the type of message, such as the reason for the error or the sub-type of the message. The checksum field is used to verify the integrity of the packet and is calculated by adding the 16-bit words of the packet and taking the one's complement. The rest of header field is used for different purposes depending on the type and code of the message, such as the identifier and sequence number for echo messages, or the unused bits for error messages.

2 ICMP Echo Request and Reply

One of the most common uses of ICMP is to test the reachability and latency of a destination host or network by sending an echo request and receiving an echo reply. An echo request is an ICMP packet with type 8 and code 0, and an echo reply is an ICMP packet with type 0 and code 0. The rest of header field for both messages contains a 16-bit identifier and a 16-bit sequence number that are used to match the request and reply. The data section of both messages usually contains a timestamp and some arbitrary data that is copied from the request to the reply. To configure an ICMP echo request or reply, you need to set the type, code, identifier, sequence number, and checksum fields in the header, and optionally fill the data section with some data.

3 ICMP Error Messages

Another important use of ICMP is to report errors that occur during the processing or forwarding of IP datagrams. ICMP error messages have a type field that ranges from 3 to 5 or from 11 to 12, and a code field that indicates the specific cause of the error, such as network unreachable (3,0), host unreachable (3,1), protocol unreachable (3,2), or fragmentation needed (3,4). The rest of header field for error messages is usually set to zero, except for the fragmentation needed message, which contains the maximum transmission unit (MTU) of the next-hop network. The data section of error messages contains the original IP header and the first 8 bytes of the original IP payload that caused the error. To configure an ICMP error message, you need to set the type, code, rest of header, and checksum fields in the header, and copy the original IP header and payload in the data section.

4 ICMP Informational Messages

ICMP also supports some informational messages that are used to exchange routing or configuration information between hosts or routers. ICMP informational messages have a type field that ranges from 9 to 10 or from 13 to 18, and a code field that is usually set to zero. The rest of header field and the data section for informational messages vary depending on the type and purpose of the message, such as the router advertisement (9), router solicitation (10), timestamp request (13), timestamp reply (14), address mask request (17), or address mask reply (18). To configure an ICMP informational message, you need to set the type, code, rest of header, and checksum fields in the header, and fill the data section with the appropriate information.

5 ICMP Extensions

ICMP has been extended to support more features and functionalities, such as multipath routing, traceroute, or path MTU discovery. ICMP extensions are optional and are indicated by a length field in the rest of header field of the original ICMP message. ICMP extensions consist of one or more extension objects that are appended to the original ICMP message. Each extension object has a class, a type, a length, and a value. The class and type fields identify the type of extension object, such as interface information (2,1), interface name (2,2), or MTU (2,3). The length field specifies the length of the extension object in bytes, and the value field contains the data for the extension object. To configure an ICMP extension, you need to set the length field in the rest of header field of the original ICMP message, and add one or more extension objects after the original ICMP message.