ICMP Destination Unreachable

ICMP Destination Unreachable

The Destination Unreachable message is an ICMP message which is generated by the host or its inbound gateway[1] to inform the client that the destination is unreachable for some reason. A Destination Unreachable message may be generated as a result of a TCP, UDP or another ICMP transmission. Unreachable TCP ports notably respond with TCP RST rather than a Destination Unreachable type 3 as might be expected.

The error will not be generated if the original datagram has a multicast destination 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; 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 23 24 25 26 27 28 29 30 31
Type = 3 Code Header Checksum
Empty Next-Hop MTU
IP Header + First 8 Bytes of Original Datagram's Data

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 is included to allow the client to match the reply with the request that caused the destination unreachable reply.

References

See also

  • PMTU blackhole

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