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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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ICMP Version 4 (ICMPv4) Informational Message Types and Formats
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ICMPv4 Parameter Problem Messages
(Page 1 of 2)

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.

ICMPv4 Parameter Problem Message Format

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


Table 94: ICMPv4 Parameter Problem Message Format

Field Name

Size (bytes)

Description

Type

1

Type: Identifies the ICMP message type; for Parameter Problem messages this value is 12.

Code

1

Code: Identifies the “subtype” of problem being communicated. See Table 95 for more information on this field for Parameter Problem messages.

Checksum

2

Checksum: 16-bit checksum field for the ICMP header, as described in the topic on the ICMP common message format.

Pointer

1

Pointer: An offset that points to the byte location in the datagram that caused the Parameter Problem message to be generated. The device receiving the ICMP message can use this value to get an idea of which field in the original message had the problem.

This field is used only when the Code value is 0.

Unused

3

Unused: 3 bytes that are left blank and not used.

Original Datagram Portion

Variable

Original Datagram Portion: The full IP header and the first 8 bytes of the payload of the datagram that prompted this error message to be sent.



Figure 145: ICMPv4 Parameter Problem Message Format

 


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