Advertisements flooded out an adjacency are placed on the adjacency's Link state retransmission list. In order to ensure that flooding is reliable, these advertisements are retransmitted until they are acknowledged. The length of time between retransmissions is a configurable per-interface value, RxmtInterval. If this is set too low for an interface, needless retransmissions will ensue. If the value is set too high, the speed of the flooding, in the face of lost packets, may be affected.
Action taken in state
Circumstances Backup All other states
_______________________________________________________________
Advertisement has No acknowledgment No acknowledgment
been flooded back sent. sent.
out receiving in-
terface (see Sec-
tion 13, step 5b).
_______________________________________________________________
Advertisement is Delayed acknowledg- Delayed ack-
more recent than ment sent if adver- nowledgment sent.
database copy, but tisement received
was not flooded from Designated
back out receiving Router, otherwise
interface do nothing
_______________________________________________________________
Advertisement is a Delayed acknowledg- No acknowledgment
duplicate, and was ment sent if adver- sent.
treated as an im- tisement received
plied acknowledg- from Designated
ment (see Section Router, otherwise
13, step 7a). do nothing
_______________________________________________________________
Advertisement is a Direct acknowledg- Direct acknowledg-
duplicate, and was ment sent. ment sent.
not treated as an
implied ack-
nowledgment.
_______________________________________________________________
Advertisement's LS Direct acknowledg- Direct acknowledg-
age is equal to ment sent. ment sent.
MaxAge, and there is
no current instance
of the advertisement
in the link state
database (see
Section 13, step 4).
Table 19: Sending link state acknowledgements.
Several retransmitted advertisements may fit into a single Link State Update packet. When advertisements are to be retransmitted, only the number fitting in a single Link State Update packet should be transmitted. Another packet of retransmissions can be sent when some of the advertisements are acknowledged, or on the next firing of the retransmission timer.
Link State Update Packets carrying retransmissions are always sent as unicasts (directly to the physical address of the neighbor). They are never sent as multicasts. Each advertisement's LS age must be incremented by InfTransDelay (which must be > 0) when copied into the outgoing Link State Update packet (until the LS age field reaches its maximum value of MaxAge).
If the adjacent router goes down, retransmissions may occur until the adjacency is destroyed by OSPF's Hello Protocol. When the adjacency is destroyed, the Link state retransmission list is cleared.