Each newly received link state advertisement must be acknowledged. This is usually done by sending Link State Acknowledgment packets. However, acknowledgments can also be accomplished implicitly by sending Link State Update packets (see step 7a of Section 13).
Many acknowledgments may be grouped together into a single Link State Acknowledgment packet. Such a packet is sent back out the interface that has received the advertisements. The packet can be sent in one of two ways: delayed and sent on an interval timer, or sent directly (as a unicast) to a particular neighbor. The particular acknowledgment strategy used depends on the circumstances surrounding the receipt of the advertisement.
Sending delayed acknowledgments accomplishes several things: it facilitates the packaging of multiple acknowledgments in a single Link State Acknowledgment packet; it enables a single Link State Acknowledgment packet to indicate acknowledgments to several neighbors at once (through multicasting); and it randomizes the Link State Acknowledgment packets sent by the various routers attached to a multi-access network. The fixed interval between a router's delayed transmissions must be short (less than RxmtInterval) or needless retransmissions will ensue.
Direct acknowledgments are sent to a particular neighbor in response to the receipt of duplicate link state advertisements. These acknowledgments are sent as unicasts, and are sent immediately when the duplicate is received.
The precise procedure for sending Link State Acknowledgment packets is described in Table 19. The circumstances surrounding the receipt of the advertisement are listed in the left column. The acknowledgment action then taken is listed in one of the two right columns. This action depends on the state of the concerned interface; interfaces in state Backup behave differently from interfaces in all other states. Delayed acknowledgments must be delivered to all adjacent routers associated with the interface. On broadcast networks, this is accomplished by sending the delayed Link State Acknowledgment packets as multicasts. The Destination IP address used depends on the state of the interface. If the state is DR or Backup, the destination AllSPFRouters is used. In other states, the destination AllDRouters is used. On non-broadcast networks, delayed Link State Acknowledgment packets must be unicast separately over each adjacency (i.e., neighbor whose state is >= Exchange).
The reasoning behind sending the above packets as multicasts is best explained by an example. Consider the network configuration depicted in Figure 15. Suppose RT4 has been elected as Designated Router, and RT3 as Backup Designated Router for the network N3. When Router RT4 floods a new advertisement to Network N3, it is received by routers RT1, RT2, and RT3. These routers will not flood the advertisement back onto net N3, but they still must ensure that their topological databases remain synchronized with their adjacent neighbors. So RT1, RT2, and RT4 are waiting to see an acknowledgment from RT3. Likewise, RT4 and RT3 are both waiting to see acknowledgments from RT1 and RT2. This is best achieved by sending the acknowledgments as multicasts.
The reason that the acknowledgment logic for Backup DRs is slightly different is because they perform differently during the flooding of link state advertisements (see Section 13.3, step 4).