data carried during TCP control flow I am learning about TCP control flow and came across a question about how much data is carried by the 4th segment. The answer is supposed to be 1200 (2230 - 1030) but I don't quite understand why. 

By definition, I know that the acknowledgement number is telling the other side what is next expected from it. Thus 5 is the server acknowledging everything up to 2230 and telling the client that the server is expecting 2230 next, while 3 is the client acknowledging everything up to 3848 and telling the server that it next expects 3848. But I still don't understand why we're considering segment 5 and segment 2? If it's only "expecting" then how do we know how much data is being carried?
The data that will be carried in the 4th segment is 1200 why?we can know it by simpley look at the sequence number of the 6th packet -a packet's sequence number is in simple words:"im sending data from this certain number"
if we take a look at the 3th packet we can see that it was sent from the client to the server with a sequence number of 1030 and we can see in the 6th packet that the sequence number of it is 2230 which means a total 2230-1030=1200 of data was sent from the clinet to server in the 4th packet