July 2002
Beginner
360 pages
11h 19m
English
Content preview from IPv6 Essentials
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ICMP Error Messages
Every ICMP message can have a slightly different header depending on the kind of error report or information it carries. The following sections outline the structure of each type of ICMPv6 message.
Destination Unreachable
A Destination Unreachable message is generated if an IP datagram cannot be delivered. A Type field with the value 1 identifies this message. The ICMP message is sent to the source address of the invoking packet. The format of the Destination Unreachable message is shown in Figure 4-2.
Figure 4-2. Format of the Destination Unreachable message
The Type field is set to one, which is the value for the Destination Unreachable message. The Code field supplies more information about the reason why the datagram was not delivered. The possible codes are listed in Table 4-3. The data portion of the ICMP message contains parts of the original message—as much as will fit into the ICMP message.
Table 4-3. Code values of the Destination Unreachable message (type 1)
|
Code |
Description |
|---|---|
|
0 |
No route to destination This message is generated if a router cannot forward a packet because it does not have a route in its table for a destination network. This can only happen if the router does not have an entry for a default route. |
|
1 |
Communication with destination administratively prohibited This type of message can, for example, be sent by a firewall that cannot ... |
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