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Table Of Contents  The TCP/IP Guide
 9  TCP/IP Lower-Layer (Interface, Internet and Transport) Protocols (OSI Layers 2, 3 and 4)
      9  TCP/IP Internet Layer (OSI Network Layer) Protocols
           9  Internet Control Message Protocol (ICMP/ICMPv4 and ICMPv6)
                9  ICMP Message Types and Formats
                     9  ICMP Version 4 (ICMPv4) Error Message Types and Formats

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ICMPv4 Redirect Messages
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ICMPv4 Parameter Problem Messages
(Page 2 of 2)

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.


Table 95: ICMPv4 Parameter Problem Message Interpretation Codes

Code Value

Message Subtype

Description

0

Pointer Indicates The Error

This is the normal use of the Parameter Problem message; when this Code value is used, the Pointer field indicates the location of the problem.

1

Missing A Required Option

The IP datagram needed to have an option in it that was missing. Since the option was missing there is no way to point to it, of course. J

2

Bad Length

The length of the datagram overall was incorrect, indicating a general problem with the message as a whole. Again, the Pointer field makes no sense here.


Key Concept: The ICMPv4 Parameter Problem message is a generic “catch all” that can be used to convey an error of any type in an IP datagram. A special Pointer field is normally used to indicate to the recipient of the message where the problem was in the original datagram.


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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