Sep 12, 2018

The network doesn't have a chain. A chain exists only in the databases of individual nodes. There is no way to know if a block obtained from any node is on any chain at all unless you validate its full history. There is also no way to know that a block on a validated chain is on the longest chain unless you validate each candidate chain. And given that you may not see every chain, you can't actually know even if you validate every chain you see. The best you can do is validate each chain you see and from that obtain the strongest block. In libbitcoin this is the top block of the confirmed chain.