Why nodes are still unconfirmed with two tips?

Question

As per the understanding, to issue a new transaction, we need to validate 2 unconfirmed transactions called tips, and then we can append a new transaction to those transactions, adding 2 new edges to the DAG and the new transaction will be a new tip and waiting for confirmation by next new transaction.

With this understanding, all white nodes in the above diagram are already validated by two other transactions. Then why these nodes are still unconfirmed? Please correct me where my understanding is going wrong.

Answer 1

I'm the author of the post you mentioned. Your understanding is right, that the white nodes are already validated by two other transactions. This however doesn't mean that they are confirmed yet. In order to be confirmed, the transactions need to gather enough approval weight from other nodes and all white nodes don't have enough approval weight (yet). You can read more about the concept of approval weights in the same article at the chapter "How do we resolve conflicts?" or more specifically in the part about "On Tangle Voting (OTV)". If you have more questions about the concept please let me know.

Answer 2

You have to distinguish between validating the transaction (by attaching a new tip to it) and confirmed transactions.

There are still multiple confirmation models in IOTA research, and as far as I know it is not yet clear what exact consensus model (with which parameters) will be eventually used in IOTA 2.0. (IOTA 1.5 uses a simple coordinator-based approach where every transacion validated by a milestone transaction from the coordinator is automatically considered confirmed).

Why cannot you confirm a transaction as soon as there is a new tip? As every participant in the network can create new tips (at some rate limit), it would be too easy to create two conflicting transactions and then validate both of them by different tips, causing a double spend.

Common models include "all tips except one", or some majority rule.

But in the picture above, I assume it was quickly drawn for illustration, and does not follow any exact rules, as the bottom node in the last confirmed layer is validated by three tips and the second-to-top node in the first unconfirmed layer is validated by four tips (so either the former should be unconfirmed or the latter should be confirmed).