For ICMP Type 3 “Destination Unreachable” Message (RFC 792) with support for PMTUD (RFC 1191)
Contains 2 unused bytes followed by a Next-Hop MTU field indicating the maximum transmission unit
of the next-hop network on which fragmentation is required.
For ICMP Type 40 (PHOTURIS) Message (RFC 2521)
Contains 2 “Reserved” bytes followed by the Security Parameters Index used
for a security association between two peers. Also includes a 2-byte pointer
field indicating where in the message the error was detected.
For ICMP Type 30 “Traceroute” Message (RFC 1393)
Contains a 16-bit ID Number field used by the source to match responses to outgoing requests
followed by 2 unused bytes to make a total of 4 bytes.
Represents an ICMP header as defined in RFC 792.
The header consists of a type and code field identifying the message type,
a checksum for error detection, and a data field whose format depends on the message type.
For ICMP Type 12 “Parameter Problem” Message (RFC 792)
Contains a pointer to the byte in the original datagram that caused the error
and 3 bytes of unused padding to make the field a total of 4 bytes.
Represents an ICMPv6 header as defined in RFC 4443.
The header consists of a type and code field identifying the message type,
a checksum for error detection, and a data field whose format depends on the message type.