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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 Source Quench Messages
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ICMPv4 Destination Unreachable Messages
(Page 2 of 4)

ICMPv4 Destination Unreachable Message Format

Table 88 and Figure 139 show the specific format for ICMPv4 Destination Unreachable messages.


Table 88: ICMPv4 Destination Unreachable Message Format

Field Name

Size (bytes)

Description

Type

1

Type: Identifies the ICMP message type; for Destination Unreachable messages this is set to 3.

Code

1

Code: Identifies the “subtype” of unreachable error being communicated. See Table 89 for a full list of codes and what they mean.

Checksum

2

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

Unused

4

Unused: 4 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 139: ICMPv4 Destination Unreachable Message Format

 

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