Optimistic mining tip (slow block propagation)

Validationless mining, a practice where miners work on a new chain tip without fully validating it, poses significant risks to blockchain security. This approach, also known as SPV (Simplified Payment Verification) mining or spy mining, involves miners validating only the block's header before proceeding to mine on top of it. This method can lead to the extension of an invalid chain, either through malicious actions or inadvertently due to changes in consensus rules. A notable incident occurred during the BIP 66 soft fork, which resulted in multiple invalid blocks being created, leading to invalid chains that extended for as many as six blocks. Such practices highlight the vulnerabilities introduced by validationless mining, including the potential for fake confirmations and the undermining of system integrity.

The practice also raises concerns regarding the incentive structure within mining. High costs associated with trustless validation may push mining pools towards seeking alternatives to ensure they are not expending resources on an invalid chain. This could include reliance on a state's legal system to enforce validity, rather than technical verification methods. To mitigate the risks associated with validationless mining, it is crucial to make full validation as accessible and cost-effective as possible. This involves optimizing block propagation and validation times for both common scenarios and attack scenarios, through measures such as compact blocks and mempool synchronization, alongside strategies to prevent exploitation of long block validation times, as suggested in BIP 54.

Furthermore, a proposal has been made advocating for miners to voluntarily signal their level of block validation. This gentleman's agreement aims to mitigate risks by informing the network of the extent of each miner’s validation efforts. The motivation behind this proposal is twofold: to support thin clients and to address the delays in the mining process that unfairly benefit larger miners over smaller ones. By deferring validation, there is a potential to reduce centralization pressures and enhance the overall security of the Bitcoin network, allowing for a more balanced distribution of rewards and minimizing the collateral damage from bypassed validation processes.