LSU(Link State Update) is also known as the Link State Acknowledgements.
Link State Update packets are use for flooding of the Link State Acknowledgements and sending Link State Acknowledgement response to the Link State Request packets.
It sent directly connected neighbors who previously requested the Link State Acknowledgements in form of the Link State Request packets.
In the case of the flooding, neighbors routers are only responsible for the re-encapsulation of the received Link State Acknowledgement informations in the new Link State Update packets.
