The previous topics in this section describe four specific ICMPv4 message types that allow a device to report various error conditions to the original sender of a datagram. However, other error situations may arise also that don't correspond to any of these four specific message types. Typically, the problem results when a device attempts to process the header fields of an IP datagram and finds something in it that doesn't make sense.

If a device finds a problem with any of the parameters in an IP datagram header that is serious enough that it cannot complete processing the header, it must discard the datagram. Like other cases where a datagram must be tossed out, this is serious enough to warrant communication of the problem back to the device that sent the original datagram. This is accomplished in ICMPv4 using the Parameter Problem message type.

This is a “catch all” type of message that can be used to indicate an error in any header field of an IP datagram. The message type does not contain any specific fields or coding to indicate what the problem is. This was done intentionally to keep the Parameter Problem message “generic” and ensure that it could indicate any sort of error. Instead of special error codes, most Parameter Problem messages tell the original source which parameter caused the problem by including a special pointer that indicates which field in the original datagram header caused the problem.

Table 94 and Figure 145 show the format for ICMPv4 Parameter Problem messages.