Defining Bitcoin Layer 2 and an Inclusive Perspective on L2

Technical Perspective Definition of L2

To define L2 from a technical standpoint, the key is to distinguish it clearly from L1 and centralized solutions. I believe there are two main points:

  1. L2 does not create new block space. Technically, solutions that create new block spaces are fundamentally L1.
  2. L2 must utilize L1 for data availability and security. Otherwise, it is no different from a fully centralized solution.

However, the market doesn't solely rely on a technical perspective to define this, as there's also a significant focus on the ecosystem perspective.

Ecosystem Perspective Definition of L2

From the ecosystem perspective, the key is to examine what capabilities of L1 are utilized or inherited by L2. Taking Bitcoin as an example, let's analyze the inheritable and extendable directions on Bitcoin.

BTC Assets

This is a common narrative among all L2s, exploring how the trillion-scale BTC assets can be utilized in additional scenarios, whether in trading or staking, presenting vast imaginative possibilities. Transferring a blockchain system's assets to another system requires a bridge, where trust and security are paramount.

All solutions creating use cases for BTC assets through a bridge can be considered Bitcoin L2. Even BTC ETFs could be seen as a form of Bitcoin L2, representing a fully centralized, custodial bridge secured by regulatory oversight. Thus, the debate is not about decentralization but about trust. Decentralized solutions can reduce the cost of trust for users, offering opportunities for new projects. However, constructing secure, decentralized bridges on Bitcoin is a significant challenge. L2 solutions must also leverage Bitcoin's other features to enhance bridge security.

As Bitcoin's extension protocols evolve, including Ordinals and protocols built on top of Ordinals (BRC20, etc.), Atomicals, RGB, Taproot assets, and more, the variety of new assets on Bitcoin will increase. Ensuring bridges are scalable to quickly support new asset types poses a huge challenge.

Bitcoin Block Space

As the most decentralized blockchain network, Bitcoin's block space value is yet to be fully realized. The surge in Ordinals inscriptions signifies discovering the value of Bitcoin as a Data Availability (DA) layer. The Ordinals protocol provides a scalable data format standard for parsing, displaying, and exchanging inscribed data on Bitcoin.

Exploring how Bitcoin's extension protocols and L2s can effectively utilize Bitcoin's block space is a direction worth exploring.

Bitcoin's Programmable Capabilities

Bitcoin Script offers limited programming capabilities, primarily in the form of time locks, hash locks, and private key locks. Taproot enhances the complexity of Bitcoin Script, opening possibilities like bitvm. However, a key challenge is Bitcoin Script's statelessness, unable to read or accumulate Bitcoin's state, relying solely on inputs. Using Bitcoin's script for arbitration remains an exploration area.

Innovations in cryptography, including protocols that use key exchanges to create secure mechanisms, like the Lightning Network, and the anticipated "extractable one-time signature".

Bitcoin's State

Bitcoin's state includes:

  1. Bitcoin's timestamps.
  2. Bitcoin's block nonce randomness.
  3. Bitcoin's UTXO and the ownership of UTXO.
  4. Bitcoin's blocks and the new assets and information attached to UTXOs.

We can analyze different Bitcoin extension protocols and L2 projects in terms of how they extend Bitcoin's capabilities.

Extending Bitcoin

Bridge + Programmable Environment

Since Bitcoin's programming capabilities are limited, one method is to transfer Bitcoin assets to another programming environment (like EVM) and utilize the programmability of EVM to provide application scenarios for Bitcoin assets. Notable examples include BEVM and Merlin, focusing on the design of the bridge:

  1. Whether L2 can utilize the security provided by L1.
  2. The scalability of the cross-chain solution.

Expanding a Smart Contract Layer on Bitcoin

RGB utilizes Bitcoin's UTXO feature of being usable only once to implement one-time seals (opens in a new tab), also using Bitcoin's block space to announce transaction commitments, offering an Offchain programming environment. Its advantage is a perfect match with the UTXO model, independent of a global state, ensuring privacy. However, this is also its limitation, restricting its programming scenarios. In this direction, CKB's RGB++ makes trade-offs with RGB’s features, providing a richer programming model through the cell model.

Indexer Mode of Offchain Computation

The Indexer mode of inscriptions can be understood as an Offchain computation model, where the asset is defined on-chain, but its legitimacy is ensured by off-chain computations, also providing a global state. Inscriptions can be seen as assets between L1 and L2; if the protocol integrates a migration mechanism between L1 and L2, it enables the asset circulation between them. Allowing the generation and verification logic of inscribed assets, through code inscribed on Bitcoin, is also an extension of Bitcoin's programming capabilities, like bitseed (opens in a new tab).

Stackable L2

If a smart contract is used to implement the indexer of Bitcoin's extension protocol, parsing all UTXOs and attached states on Bitcoin, and allowing developers to deploy applications to the indexer, it's like providing a new smart contract layer for Bitcoin. This is the solution of Rooch. Previously, I called this mode "smart indexer", but the indexer concept feels read-only, hence the new term "Stackable L2", referring to all L2 solutions that include L1's full state in L2. In this scenario, L2 applications can read all states on L1 and also create new states. The assets on L1 and L2 can form new assets through stacking combination, and the security of L2 can be ensured through a modular approach. More details on this concept will be discussed in a later article.

All the above solutions can actually be combined with each other.

Inclusive Perspective of L2

If we abstractly understand L2, setting aside specific implementation methods, we realize it should be viewed as a continuous spectrum. This spectrum ranges from CEX at one end to L1 technologies at the other, with intermediate solutions encompassing the entire range. This spectrum also represents two different growth modes: CEX is almost entirely product-driven and user-driven, while L1, with longer construction periods, prioritizes narratives and blueprints, with L2 falling in between, adopting a hybrid growth mode.

From an inclusive perspective, there's no need to fixate on defining what true L2 is. The various technical terms invented by the industry, such as Validium, Plasma, Sovereign rollup, Op/Zk rollup, Modular Execution layer, Decentralized compute, side chain, L2/L3, etc., can all be included in this view. The industry is exploring new infrastructural needs through various combinations and permutations of these technologies.

Different projects have varying assumptions about new applications, which in turn determines their combination modes and growth patterns. They might be a slight deviation from L1 or a shift towards CEX. The future is uncertain, and it's challenging to predict which model will prevail at this stage. However, one thing is certain: after years of exploration, the industry has established large-scale L1s and CEXs, and now it's time to fill the gap with large-scale intermediary layers.


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