Blocks from the future
We identified two issues relating to "blocks from the future".
- Blocks from the near future
- Blocks from the far future
While blocks from the near future have been known to occur on mainnet as a result of clock skew/misconfiguration, there are no known instances on mainnet of blocks from the far future. In both cases, restarting an affected node would resolve the issue.
What is meant by a Block from the Future?
A node considers a block to be from the future if its slot is ahead of the current slot. Ouroboros Praos mandates that all chains containing blocks from the future (at that time) are ignored during chain selection. As Praos assumes that all nodes have access to perfectly synchronized clocks, this will never cause nodes to disregard blocks that have been minted by other honest nodes. In an actual real-world deployment, this assumption is unrealistic due to the imperfections of protocols like NTP as well as leap seconds.
The issues that were identified meant that blocks from the future could potentially be used by malicious actors to create denial-of-service attacks.
Both issues were fixed by Cardano node 8.8 or later, and were eradicated at the Chang hard fork.