Scaling Noncustodial Mining Payouts with CTV

These pools are broadly distinguished as custodial and non-custodial based on how they handle mining rewards. Custodial pools consolidate mined bitcoins into a central wallet before distributing them to participants, as evidenced by transaction patterns seen on platforms like mempool.space. In contrast, non-custodial pools distribute rewards directly to miners’ wallets through transactions containing multiple outputs, ensuring that funds are never held in a single operator-controlled account. Ocean is one of the rare examples of a non-custodial pool, but it faces significant hurdles due to firmware-imposed constraints on mining hardware such as Antminer, which limits the number of outputs a coinbase transaction can have. Overcoming these restrictions requires intricate workarounds, including hardware fingerprinting and managing multiple work templates per user.

Open-source ASIC firmware solutions, such as Bitaxe’s ESP-miner, illustrate the potential for overcoming these constraints, permitting larger and more flexible coinbase transactions dictated only by memory limitations rather than manufacturer-imposed caps. To further mitigate hardware limitations, the community has proposed incorporating CheckTemplateVerify (CTV), a consensus-level Bitcoin protocol enhancement. CTV allows the inclusion of small commitments within the coinbase transaction, which can later expand into complex payout distributions, circumventing firmware restrictions on output numbers. The Coinbase Playground prototype exemplifies this approach by demonstrating scalable, non-custodial payouts and optimized fee management, though it introduces new complexities such as dependency on external data availability and the need for additional claiming transactions. Research into advanced techniques involving nested CTV transactions, cryptographic primitives like musig keys, and Commitment Scheme From Signatures (CSFS) promises even greater privacy and efficiency for non-custodial reward distribution.

Integrating innovations like Ark alongside transaction trees offers a compelling strategy for reducing blockchain transaction costs, particularly beneficial for small-scale miners who often accumulate numerous small UTXOs. By employing OP_CTV, payments can be efficiently structured as a transaction tree—each leaf being a VTXO—that aggregates into larger, consolidated outputs. This aggregation reduces both the on-chain footprint and the cost of offboarding accumulated rewards, addressing fee inefficiencies that typically erode small miners’ profitability. Detailed documentation, such as this overview, elaborates on this promising trajectory, underscoring the potential for transaction trees to foster a more economically inclusive mining environment.

The implementation of CTV and similar mechanisms is not without critique. Detractors argue that increasing block space usage for mining payouts does not inherently scale or maximize fee revenue under normal conditions. Instead, strategies utilizing Ark for mining rewards present a more scalable alternative by reducing average block space requirements and supporting non-interactive payouts, where the pool coordinator need not pre-sign each miner’s transaction. Such enhancements could improve practical payout execution while maintaining or improving scalability. However, there is an ongoing caution against overstating the benefits of protocol changes like a CTV soft fork; proponents are advised to maintain realistic expectations and avoid exaggerated claims, as credibility is paramount when advocating for changes to Bitcoin's foundational protocols.

Technical discussions at venues such as the btc++ conference have brought to light the difficulties faced by operations like Ocean in executing non-custodial fanout transactions under restrictive firmware regimes. Workarounds include hardware-specific strategies and the use of open-source firmware, which allow for larger and more adaptable coinbase transactions. Documentation and community resources, including sections in the Bitcoin Wiki like "Covenants Uses," provide guidance on leveraging tools such as the Coinbase Playground to optimize block space usage and potentially increase transaction fee revenues. Alternative payout mechanisms, such as the use of ark on-chain addresses, also offer flexibility in navigating firmware-imposed transaction output limits.

Examining the broader economic context, there is growing concern about the sustainability of mining incentives as block subsidies diminish. With periods of low network activity resulting in empty mempools and minimal fees, the viability of purely fee-driven mining rewards is questioned. Concepts like fee smoothing and congestion control are suggested as essential tools for ensuring steady miner incentives and network security in a future with reduced block subsidies. Solutions must balance efficiency, decentralization, and transparency, recognizing that hostility towards novel approaches can hinder necessary advancements in mining economics and decentralization.

The debate around the adoption of CTV for mining pool payouts centers on its actual impact on scalability and decentralization. Critics note that CTV may simply defer rather than reduce the ultimate block space consumption of payouts, challenging claims of improved scalability. While Layer 2 solutions are considered for facilitating payouts, concerns remain about their cost-effectiveness, especially regarding unilateral exits. Nevertheless, CTV provides a mechanism for congestion control, allowing for dynamic scheduling of payout transactions and offering participants flexibility in fee selection and timing. This approach ensures that immediate blockspace is preserved for maximized fee revenue,