Skip certificates
A leader goes dark in the middle of its window. Watch validators time out, vote Skip, and close the slots with certificates so the chain can move on.
In this scenario the validator that leads slots 4-7 goes offline at 1.0 s and stays down until 5.0 s. Because it also holds a lot of stake, it happens to lead the first window too, so its outage starts during slots 0-3.
Alpenglow has no global clock to tell everyone "the slot is over". Instead each validator runs a local timeout per slot, set when the window's parent became ready: Δtimeout (900 ms) plus one Δblock (400 ms) for each later slot in the window.
Press Start.
Where it stops
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The leader goes offline
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The first Skip certificate
Validators' timeouts for the dead leader's slots are about to fire. Predict what happens to those slots.
The leader is offline for slots 2 and 3 of its window. What happens to those slots?
- The chain halts until the leader comes back at 5 s
- Validators time out, vote Skip, and a 60% Skip certificate closes each slot
- The next leader takes over immediately and produces the missing blocks
- Two forks form and the heavier one wins later
When a timeout fires for a slot the validator has not voted in, Votor runs
TrySkipWindow: it votes Skip for every remaining slot of the window and sets theBadWindowflag. Once 60% of stake has voted Skip (or SkipFallback), anyone can form a Skip certificate. An empty slot is now final, and the chain can move on without the leader. -
ParentReady for the next window
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The hero transaction lands
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Finalized
What you saw
- Alpenglow validators use local timeouts, not a global clock, to decide a slot is dead.
- A timeout triggers Skip votes for the rest of the window; 60% of stake makes a Skip certificate and the empty slot is final.
- ParentReady tells the next leader what to build on: notarized parent + Skip certificates for every slot between.
Try next
- Paste the scenario as custom JSON and lower
params.alpenglow.timeout_msbelow 700: healthy leaders start getting skipped, because a block is only voted on once all of its slices have arrived. - Lesson Lockouts and why partitions hurt TowerBFT looks at a fault that stalls both protocols.