Stale blocks and propagation

Wonder

1. Why does a valid block sometimes get discarded?
2. How does network latency favor large mining operations over small ones?
3. What happens when two miners find blocks at the exact same height?

Explore

1. When a miner finds a valid block, it relays it to peers, usually as a compact block. Bandwidth limits and validation delays mean this broadcast takes time to reach every node in the network. During this interval, other miners who have not yet received the new tip continue mining on top of the previous chain state, unaware that a competing branch has already been found.
2. If one of these lagging miners discovers their own valid block at the same height before receiving the initial broadcast, a temporary fork occurs. Both blocks possess correct proof-of-work and are technically valid. However, only one can enter the active canonical chain. The next block found by any miner will attach to one of them, extending that branch; the other becomes stale.
3. Propagation latency directly increases the probability of this collision. Slower propagation raises the stale rate, wasting computational power intended for ledger security on internal competition instead. This dynamic favors large, well-connected miners who can propagate their blocks faster than smaller, isolated operators, creating a structural advantage tied to infrastructure rather than just hashpower.
4. To mitigate losses from staling, some miners engage in ‘validationless mining.’ They check only the incoming block header’s proof-of-work and immediately begin building on it while downloading full transaction data for later validation. The shortcut can backfire: in July 2015, shortly after a soft fork activated, miners doing this built five more blocks on top of an invalid one, a six-block fork that had to be abandoned.
5. Compact blocks (BIP152) and dedicated relay networks like FIBRE have significantly reduced propagation times by sending short transaction identifiers and only the transactions a peer is missing. These optimizations lower the stale rate across the network, making it harder for large pools to exploit latency advantages. However, the tension between block size, validation time, and propagation speed remains a central constraint in consensus design.
6. Selfish mining, described by Eyal and Sirer in 2013, demonstrated that a miner with substantial hashpower could gain an advantage by withholding blocks strategically rather than broadcasting them immediately. While one-block reorganizations still occur occasionally due to natural latency, deeper reorgs are rare because the economic cost of sustained adversarial behavior outweighs potential gains for most participants.

Key facts

Exercise nine, six, three, one

Fact check

Dates, figures and claims checked against the published sources listed below.

• BIP152: Compact Block Relay
• Eyal, I. & Sirer, E.G., 'Majority is not Enough: Bitcoin Mining is Vulnerable' (2013)
• Decker, S. & Wattenhofer, R., 'Information Propagation in the Bitcoin Network' (2013)