The ICMPv6 Destination Unreachable, Packet Too Big and Time Exceeded messages described in the previous topics in this section are used to indicate specific error conditions to the original sender of a datagram. Recognizing that a router or host may encounter some other problem in processing a datagram that is not covered by any of these message types, ICMPv6 includes a generic error message type, just as ICMPv4 did. This is called the ICMPv6 Parameter Problem message.

As the name suggests, a Parameter Problem message indicates that a device found a problem with a parameter (another name for a datagram field) while attempting to work its way through the header (or headers) in an IPv6 datagram. This message is only generated when the error encountered is serious enough that the device could not make sense of the datagram and had to discard it. That is, if an error is found that a device is able to recover from and not drop the datagram, no Parameter Problem message is created.

As was the case for the ICMPv4 version of this message, the ICMPv6 message was designed to be generic, so it can indicate an error in basically any field in the original datagram. A special Pointer field is used that points to the place in that datagram where the error was encountered. By looking at the structure of the original message (which as you may recall is included, up to a certain size, in the ICMP message format) the original device can tell which field contained the problem. The Code value is also used to communicate additional general information about the nature of the problem.