5.2.3 Local Delivery Decision

When a router receives an IP packet, it must decide whether the packet is addressed to the router (and should be delivered locally) or the packet is addressed to another system (and should be handled by the forwarder). There is also a hybrid case, where certain IP broadcasts and IP multicasts are both delivered locally and forwarded. A router MUST determine which of the these three cases applies using the following rules.

• An unexpired source route option is one whose pointer value does not point past the last entry in the source route. If the packet contains an unexpired source route option, the pointer in the option is advanced until either the pointer does point past the last address in the option or else the next address is not one of the router's own addresses. In the latter (normal) case, the packet is forwarded (and not delivered locally) regardless of the rules below.

• The packet is delivered locally and not considered for forwarding in the following cases:
  • The packet's destination address exactly matches one of the router's IP addresses,

  • The packet's destination address is a limited broadcast address ({-1, -1}), or

  • The packet's destination is an IP multicast address which is never forwarded (such as 224.0.0.1 or 224.0.0.2) and (at least) one of the logical interfaces associated with the physical interface on which the packet arrived is a member of the destination multicast group.

• The packet is passed to the forwarder AND delivered locally in the following cases:
  • The packet's destination address is an IP broadcast address that addresses at least one of the router's logical interfaces but does not address any of the logical interfaces associated with the physical interface on which the packet arrived

  • The packet's destination is an IP multicast address which is permitted to be forwarded (unlike 224.0.0.1 and 224.0.0.2) and (at least) one of the logical interfaces associated with the physical interface on which the packet arrived is a member of the destination multicast group.

• The packet is delivered locally if the packet's destination address is an IP broadcast address (other than a limited broadcast address) that addresses at least one of the logical interfaces associated with the physical interface on which the packet arrived. The packet is ALSO passed to the forwarder unless the link on which the packet arrived uses an IP encapsulation that does not encapsulate broadcasts differently than unicasts (e.g., by using different Link Layer destination addresses).

• The packet is passed to the forwarder in all other cases.

DISCUSSION

The purpose of the requirement in the last sentence of the fourth bullet is to deal with a directed broadcast to another network prefix on the same physical cable. Normally, this works as expected: the sender sends the broadcast to the router as a Link Layer unicast. The router notes that it arrived as a unicast, and therefore must be destined for a different network prefix than the sender sent it on. Therefore, the router can safely send it as a Link Layer broadcast out the same (physical) interface over which it arrived. However, if the router can't tell whether the packet was received as a Link Layer unicast, the sentence ensures that the router does the safe but wrong thing rather than the unsafe but right thing.

IMPLEMENTATION

As described in Section [5.3.4], packets received as Link Layer broadcasts are generally not forwarded. It may be advantageous to avoid passing to the forwarder packets it would later discard because of the rules in that section.

Some Link Layers (either because of the hardware or because of special code in the drivers) can deliver to the router copies of all Link Layer broadcasts and multicasts it transmits. Use of this feature can simplify the implementation of cases where a packet has to both be passed to the forwarder and delivered locally, since forwarding the packet will automatically cause the router to receive a copy of the packet that it can then deliver locally. One must use care in these circumstances to prevent treating a received loop-back packet as a normal packet that was received (and then being subject to the rules of forwarding, etc.).

Even without such a Link Layer, it is of course hardly necessary to make a copy of an entire packet to queue it both for forwarding and for local delivery, though care must be taken with fragments, since reassembly is performed on locally delivered packets but not on forwarded packets. One simple scheme is to associate a flag with each packet on the router's output queue that indicates whether it should be queued for local delivery after it has been sent.