4.3.3.1 Destination Unreachable

If a router cannot forward a packet because it has no routes at all (including no default route) to the destination specified in the packet, then the router MUST generate a Destination Unreachable, Code 0 (Network Unreachable) ICMP message. If the router does have routes to the destination network specified in the packet but the TOS specified for the routes is neither the default TOS (0000) nor the TOS of the packet that the router is attempting to route, then the router MUST generate a Destination Unreachable, Code 11 (Network Unreachable for TOS) ICMP message.

If a packet is to be forwarded to a host on a network that is directly connected to the router (i.e., the router is the last-hop router) and the router has ascertained that there is no path to the destination host then the router MUST generate a Destination Unreachable, Code 1 (Host Unreachable) ICMP message. If a packet is to be forwarded to a host that is on a network that is directly connected to the router and the router cannot forward the packet because no route to the destination has a TOS that is either equal to the TOS requested in the packet or is the default TOS (0000) then the router MUST generate a Destination Unreachable, Code 12 (Host Unreachable for TOS) ICMP message.

DISCUSSION

The intent is that a router generates the "generic" host/network unreachable if it has no path at all (including default routes) to the destination. If the router has one or more paths to the destination, but none of those paths have an acceptable TOS, then the router generates the "unreachable for TOS" message.