OSPF LSAck packet

[Loubnate]

Hello everyone,

my question is about LSAck in OSPF, i have already learned the different types of LSAck (Implicit/Explicit) and the different senarios for each one and i have a general idea about the reliable transport protocol, but i'm little bit confused about LSAck type explicit , which consists on sending an "empty" packet containing just the LSAck header, for the first time i thought that the packet contains : 

LSAck header + LSAck body ( 0000000...) but Zeros significate >> interruption so body >> 00000... false 

i wanna really know what does the "empty packet"means especially for the LSAck body and how the reliable transport protocol will generate the packet .

[Peter Paluch]

Hello,

I believe that the terminology is slightly confusing, and this might be what you are looking at.

First of all, there is no such thing as an explicit and implicit LSAck. There is only an explicit and implicit acknowledgment for an LSA (do not confuse with LSAck). An implicit acknowledgment is simply done by a router sending back the same LSA it wants to acknowledge. This is usually seen on broadcast OSPF network types when a DR sends floods back an updated LSA it just received. An explicit acknowledgment is done by sending an LSAck back to the sender of the LSA we want to acknowledge.

Second, the "empty packet" or "empty body" you are talking about actually refers to the LSA header inside an LSAck. When a router sends an LSAck to acknowledge a received LSA, it will insert that LSA's header into the payload of the LSAck it is sending back. It won't insert the full LSA body - that wouldn't make sense. In essence, the router sends back an "empty LSA" - an LSA without its body, just with its header - inserted into an LSAck packet back to the neighbor that sent the updated LSA.

So, to sum up: There are implicit and explicit acknowledgments in OSPF; implicit acknowledgment is done by sending back the whole LSA while explicit acknowledgment is done by sending an LSAck back that contains the headers of the LSAs being acknowledged.

Feel welcome to ask further!

Best regards,
Peter

[Loubnate]

Hi Peter,

Thank you  for responding,i  really appreciate , now i can get the difference between the Lsack and the acknowledgement 

and the most important point was about the lsa acknowledgement packet because i didn't find these details in documents,

your are saying " it will insert that LSA's header into the payload of the LSAck it is sending back. It won't insert the full LSA body - that wouldn't make sense. "

you meant only the header will be transferred instead of sending the whole packet so no need for the empty body, thank you so much it seems logic

if you can suggest me a document i'll be grateful 

Thank you.