Disabling min-difficulty blocks on testnet4

This rule allows blocks to be mined at the lowest difficulty level if no blocks have appeared in the preceding 20 minutes. As a result, CPU miners have exploited this feature by consistently mining blocks at minimal difficulty, often choosing to broadcast empty blocks to gain slight advantages in bandwidth and verification time. Such practices, though beneficial for individual miners, detract from the test network’s primary purpose by drastically reducing the number of transactions confirmed per block.

In response to these issues, there is a proposal to eliminate the 20-minute min-difficulty exception through a hard fork planned at block height 201,600, aligning with the epoch 100 boundary. This change is scheduled between August and September of 2027 and aims to enforce standard difficulty rules, maintaining the integrity of the mining process without resetting the difficulty levels arbitrarily. The proposed solution involves using functions like GetNextWorkRequired() and PermittedDifficultyTransition() to ensure a smooth transition. This proposal is still under discussion, particularly concerning the optimal block height for implementation and whether it necessitates a Bitcoin Improvement Proposal (BIP) given its impact on consensus rules. Further details and community perspectives can be explored through resources such as the GitHub pull request (PR), discussions on the Bitcoin developers' mailing list (Mailing list discussion), and the Bitcointalk forum thread (Bitcointalk).

Additionally, the importance of proactive deployment and community engagement in the adoption of new test network versions is highlighted. The history of Testnet4 underscores that significant user, miner, and developer engagement is crucial for any testnet's integration into Bitcoin Core. This is exemplified by the progression from Testnet4's inception to its integration after achieving substantial block production and support from various platforms and mining pools. It emphasizes deploying tangible, improved network versions to garner support and validate enhancements over existing systems rather than waiting for centralized approvals or adhering strictly to previous frameworks.

A new exploitation vector was observed, where the timestamp of the previous block is used to determine the opening of the next min-difficulty window, further complicating the mining landscape under the current rules. This scenario has led to discussions about transitioning to Testnet5, which aims to disable the min-difficulty rule entirely, as detailed in the GitHub pull request (Testnet5 proposal). This transition is seen as necessary due to ongoing manipulations by both CPU and ASIC miners under Testnet4 rules, demonstrating the complex dynamics and unintended consequences of the current min-difficulty rule.