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9. Forwarding

9. Forwarding​

When a router receives a packet addressed to a node other than itself, it must take the zone of the destination and source addresses into account as follows:

  • The zone of the destination address is determined by the scope of the address and arrival interface of the packet. The next-hop interface is chosen by looking up the destination address in a (conceptual) routing table specific to that zone (see Section 10). That routing table is restricted to refer to interfaces belonging to that zone.

  • After the next-hop interface is chosen, the zone of the source address is considered. As with the destination address, the zone of the source address is determined by the scope of the address and arrival interface of the packet. If transmitting the packet on the chosen next-hop interface would cause the packet to leave the zone of the source address, i.e., cross a zone boundary of the scope of the source address, then the packet is discarded. Additionally, if the packet's destination address is a unicast address, an ICMP Destination Unreachable message [4] with Code 2 ("beyond scope of source address") is sent to the source of the original packet. Note that Code 2 is currently left as unassigned in [4], but the IANA will re-assign the value for the new purpose, and [4] will be revised with this change.

Note that even if unicast site-local addresses are deprecated, the above procedure still applies to link-local addresses. Thus, if a router receives a packet with a link-local destination address that is not one of the router's own link-local addresses on the arrival link, the router is expected to try to forward the packet to the destination on that link (subject to successful determination of the destination's link-layer address via the Neighbor Discovery protocol [9]). The forwarded packet may be transmitted back through the arrival interface, or through any other interface attached to the same link.

A node that receives a packet addressed to itself and containing a Routing Header with more than zero Segments Left (Section 4.4 of [3]) first checks the scope of the next address in the Routing Header. If the scope of the next address is smaller than the scope of the original destination address, the node MUST discard the packet. Otherwise, it swaps the original destination address with the next address in the Routing Header. Then the above forwarding rules apply as follows:

  • The zone of the new destination address is determined by the scope of the next address and the arrival interface of the packet. The next-hop interface is chosen as per the first bullet of the rules above.

  • After the next-hop interface is chosen, the zone of the source address is considered as per the second bullet of the rules above.

This check about the scope of the next address ensures that when a packet arrives at its final destination, if that destination is link-local, then the receiving node can know that the packet originated on-link. This will help the receiving node send a "response" packet with the final destination of the received packet as the source address without breaking its source zone.

Note that it is possible, though generally inadvisable, to use a Routing Header to convey a non-global address across its associated zone boundary in the previously used next address field. For example, consider a case in which a link-border node (e.g., a router) receives a packet with the destination being a link-local address, and the source address a global address. If the packet contains a Routing Header where the next address is a global address, the next-hop interface to the global address may belong to a different link than that of the original destination. This is allowed because the scope of the next address is not smaller than the scope of the original destination.

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