A node will ask for block number N before asking for block N+1. Yes, mostly block N+1 will be discarded if block number N doesn't exist, but that's irrelevant. Because it's not that a node gets blocks only by being the receiving end. It can contact all the other nodes around it, and ask for block number N. If all these nodes don't have block number N, well, then it doesn't exist!
Notice that on a broader scale every node asks its peers for node number N, and verifies that this block follows the rules of the protocol. That's part of the consensus.
If we're talking about mining, the situation is different. You can imagine two competing miners creating blocks. If a miner doesn't have block N, he'll keep trying to mine that block. If he receives block number N after having mined it, the miner just ignores it, and we have a temporary fork in the blockchain with two branches, and block N+1 will have two different versions. Now what determines which branch survives is basically the branch that is mined faster. An arbitrary consensus rule is that the longer chain is always the correct chain. When a longer chain exists, all miners agree that this chain is the correct one.
The reason why this works, is because the probability of mining a block is wide enough over 10 minutes in Bitcoin (or block time in general).
This is why it's wrong to create blockchains with very short time. Because if the time is very short (like 10 seconds), there will be tons of forks before the blockchain stabilizes.
This is why it's wrong to create blockchains with very short time. Because if the time is very short (like 10 seconds), there will be tons of forks before the blockchain stabilizes.
