The TCP/IP Guide - ICMPv6 Destination Unreachable Messages

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There are a number of different reasons why a destination may be unreachable. To provide additional information about the nature of the problem to the device that originally tried to send the datagram, a value is placed in the message's Code field. One interesting difference between ICMPv4 and ICMPv6 Destination Unreachable messages is that there are many fewer Code values for ICMPv6. The ICMPv6 Code values were “streamlined”, mainly because several of the ICMPv4 codes were related to relatively obscure features that aren't applicable to ICMPv6.

Table 103 shows the different Code values, corresponding message subtypes and a brief explanation of each:

**Table 103: ICMPv6 *Destination Unreachable* Message Subtypes**
Code Value	Message Subtype	Description
0	No Route To Destination	The datagram was not delivered because it could not be routed to the destination. Since this means the datagram could not be sent to the destination device's local network, this is basically equivalent to the “Network Unreachable” message subtype in ICMPv4.
1	Communication With Destination Administratively Prohibited	The datagram could not be forwarded due to filtering that blocks the message based on its contents. Equivalent to the message subtype with the same name (and Code value 13) in ICMPv4.
3	Address Unreachable	There was a problem attempting to deliver the datagram to the host specified in the destination address. This code is equivalent to the ICMPv4 “Host Unreachable” code and usually means the destination address was bad or there was a problem with resolving it into a layer two address.
4	Port Unreachable	The destination port specified in the UDP or TCP header was invalid or does not exist on the destination host.

Note that Code value 2 is not used. Also, Destination Unreachable messages are only sent when there is a fundamental problem with delivering a particular datagram; they are not sent when a datagram is dropped simply due to congestion of a router.

It is up to the recipient of an ICMPv6 Destination Unreachable message to decide what to do with it. However, just as the original datagram may not reach its destination, the same is true of the Destination Unreachable message itself. Therefore, a device cannot rely on the receipt of one of these error messages to inform it of every delivery problem. This is especially true given that it is possible some unreachable destination problems may not be detectable.