ICMPv4 Parameter Problem Messages
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Parameter Problem Message Interpretation Codes and The Pointer Field
When a Parameter Problem message is generated due to a specific bad field in the original message, the Pointer field is used as described above to show the location of the problem. This meaning of the Parameter Problem message is the one that was defined in the original ICMP standard, RFC 792, and is associated with Code value 0. There are some cases of a parameter problem where a pointer to a specific field in the original message really wouldn't make sense, so other standards have defined two new Code field values for Parameter Problem messages. Table 95 shows the three Code values and provides a brief explanation of each.
Note that the Pointer field is only 8 bits wide, but since this allows for values up to 256 this is sufficient to allow it to point to any location within the IP header. It is possible for the Pointer field to point to a field within an IP option. Both hosts and routers can generate Parameter Problem messages.
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