Miner Signaling (BIP 9 & 8)
Miner Signaling: BIP 9 & BIP 8
To deploy a Soft Fork safely, the network needs to coordinate when the new rules should start being enforced. This coordination is done through Miner Signaling, where miners use bits in the Block Header to communicate their readiness.
1. Version Bits (BIP 9)
BIP 9 allowed multiple upgrades to be signaled simultaneously using the 32-bit nVersion field in the block header.
The Activation Lifecycle:
- Defined: The upgrade is written in code with a start time and a timeout.
- Started: Miners can begin setting a specific bit (e.g., bit 1) in their blocks.
- Locked-in: Once a threshold (usually 95% of blocks in a 2016-block window) signal "Ready," the upgrade is locked in.
- Active: After one more window, the nodes start enforcing the new rules.
2. The Problem with BIP 9
The main weakness of BIP 9 was that if miners refused to signal (even if users wanted the upgrade), the upgrade would Timeout and fail. This gave miners a "veto" over protocol improvements.
3. BIP 8: LOT=true
BIP 8 was designed to solve the miner veto. It introduced the Lock-on-Timeout (LOT) parameter. * LOT=false: Works like BIP 9. If miners don't signal, it fails. * LOT=true: If the timeout is reached and the threshold wasn't met, the nodes force the activation anyway. This forces miners to upgrade or risk mining invalid blocks.
4. Taproot Activation (Speedy Trial)
For the 2021 Taproot upgrade, a compromise called "Speedy Trial" was used. It was a short-duration BIP 9 attempt. If it failed quickly, the community would have decided whether to use a BIP 8 (LOT=true) follow-up. Fortunately, miners signaled 90% within weeks, and Taproot activated smoothly.
| Feature | BIP 9 | BIP 8 (LOT=true) |
|---|---|---|
| Threshold | 95% | 90-95% |
| Miners can stop it? | Yes | No (only delay it) |
| User Power | Low | High |
[!NOTE] Miner signaling is not a "vote." It is a readiness signal. Miners are signaling that they have upgraded their software and are ready to validate the new rules without accidentally splitting the chain.
Next, we will look at how users can take control of this process through a UASF.
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