From Free net encyclopedia
The Destination Unreachable message is an ICMP message which is generated by the router to inform the client that the destination host is unreachable. The error will not be generated if the original datagram has a multicast destinstion address. Reasons for this message may include the physical connection to the host does not exist (distance is infinite), the indicated protocol or port is not active, or the data must be fragmented but the 'don't fragment' flag is on.
| 00 |
01 |
02 |
03 |
04 |
05 |
06 |
07 |
08 |
09 |
10 |
11 |
12 |
13 |
14 |
15 |
16 |
17 |
18 |
19 |
20 |
21 |
22 |
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24 |
25 |
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31 |
| Type = 3 |
Code |
Header Checksum |
|
Empty |
<p align="center">Next-Hop MTU |
</table>
|
IP Header + First 8 Bytes of Original
Datagram's Data |
</table>
The type field (bits 0-7) must be set to 3. The code field (bits 8-15) is used to specify the type of error, and can be any of the following:
| Code |
Description |
| 0 |
Network unreachable error. |
| 1 |
Host unreachable error. |
| 2 |
Protocol unreachable error (the designated
transport protocol is not supported). |
| 3 |
Port unreachable error (the designated
protocol is unable to inform the host of the incoming message). |
| 4 |
The datagram is too big. Packet fragmentation
is required but the 'don't fragment' (DF) flag is on. |
| 5 |
Source route failed error. |
| 6 |
Destination network unknown error. |
| 7 |
Destination host unknown error. |
| 8 |
Source host isolated error (military use
only). |
| 9 |
The destination network is administratively
prohibited. |
| 10 |
The destination host is administratively
prohibited. |
| 11 |
The network is unreachable for Type Of
Service. |
| 12 |
The host is unreachable for Type Of Service. |
| 13 |
Communication administratively prohibited
(administrative filtering prevents packet from being forwarded). |
| 14 |
Host precedence violation (indicates the requested precedence is not permitted for the combination of host or network and port). |
| 15 |
Precedence cutoff in effect (precedence of
datagram is below the level set by the network administrators). |
The Next-Hop MTU field (bits 48-63) contains the MTU of the next-hop
network if a code 4 error occurs. The additional data (bits 64-95) is included to allow the
client to match the reply with the request the caused the destination
unreachable reply.
References