Since the local delivery case is well covered by [INTRO:2], the following assumes that the IP datagram was queued for forwarding. If the destination is an IP unicast address:
(5) The forwarder determines the next hop IP address for the packet, usually by looking up the packet's destination in the router's routing table. This procedure is described in more detail in Section [5.2.4]. This procedure also decides which network interface should be used to send the packet. (6) The forwarder verifies that forwarding the packet is permitted. The source and destination addresses should be valid, as described in Section [5.3.7] and Section [5.3.4] If the router supports administrative constraints on forwarding, such as those described in Section [5.3.9], those constraints must be satisfied. (7) The forwarder decrements (by at least one) and checks the packet's TTL, as described in Section [5.3.1]. (8) The forwarder performs any IP option processing that could not be completed in step 3. (9) The forwarder performs any necessary IP fragmentation, as described in Section [126.96.36.199]. Since this step occurs after outbound interface selection (step 5), all fragments of the same datagram will be transmitted out the same interface. (10) The forwarder determines the Link Layer address of the packet's next hop. The mechanisms for doing this are Link Layer- dependent (see chapter 3). (11) The forwarder encapsulates the IP datagram (or each of the fragments thereof) in an appropriate Link Layer frame and queues it for output on the interface selected in step 5. (12) The forwarder sends an ICMP redirect if necessary, as described in Section [188.8.131.52].