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Overview

The Nuffle Fast Finality Layer (NFFL, formerly SFFL) aims to provide a fast settlement layer that allows participating networks to quickly access information from other networks in a safe way.

In order to achieve this, NFFL leverages both NEAR and EigenLayer, providing not only a way for protocols to provide interoperability features by verifying state attestations secured by staked ETH.

The architecture is comprised of two off-chain actors, the Operators and the Aggregator, the AVS nodes, and multiple on-chain contracts:

  • on Ethereum Mainnet, there's the NFFL AVS contract set, which interacts directly with EigenLayer contracts.
  • on rollup networks, there are NFFL verifier contracts to check network state attestations.
  • on NEAR, there is a NEAR DA contract for each participating rollup network which serves as a medium for storing historical block data.

Architecture​

Below is a diagram representation of NFFL's architecture. Let's consider, as an example, HelloProtocol, a very primitive protocol in which users want to send and receive hello from one network to another. In abstract terms, this is the base feature of every bridging protocol. It's a good idea to refer to this diagram whenever any of the interactions seems unclear.

Full Architecture Overview

Ethereum​

NFFL's is enabled to provide the fast finality guarantees by leveraging the architecture of EigenLayer Actively Valildated Service. EigenLayer AVS allows the coordination of validators in distributed network towards a common goal. EigenLayer allows validators to reuse Ethereum stake as the mechanism for incentivization.

In order for NFFL to have economic security, users must first restake ETH into EigenLayer, becoming Restakers. This means the NFFL architecture actually starts on Ethereum, as the EigenLayer core contracts live there.

The NFFL has a set of smart contracts which, in EigenLayer terms, are called middleware. The middleware contracts are directly connected to the EigenLayer core contracts. The AVS contracts facilitate the interaction between the off-chain node with the EigenLayer protocol. Operations such as registering as an Operator (a validator) and validating task resolutions are handled by the AVS contracts. Restakers can delegate their restaked Eth to Operators, who will then validate the AVS on their behalf.

NEAR Data Availability​

To settle rollup transactions on NEAR, participating rollups must post block data to NEAR. NEAR DA is the data availability solution used for hosting the rollup block data.

The data posted to NEAR DA is indexed by the AVS nodes. The nodes compare the posted blocks with their own full nodes' data and agree on the network state.

The role of posting block data to NEAR DA is separated from the rollup sequencer, into a Relayer role. This Relayer constantly posts block data to NEAR DA, providing a fast and public ledger to the current network state. The separation of the Relayer allows for participating rollups to use the fast finality layer without any modifications to their sequencer implementation.

Since the AVS nodes compare the data posted by the Relayer with their local full nodes, this doesn't mean the AVS will necessarily agree with it even if the Relayer acts maliciously

There's an example Relayer implementation, but it should slightly change depending on the specific network and stack, as it should ideally be operated by the sequencer.

NFFL off-chain nodes​

NFFL nodes attest to the state root of rollup after executing the block. The individual signatures are aggregated off-chain and submitted to Ethereum and participating rollups.

An NFFL Operator, runs a full node for each of the participant networks (including Ethereum), as well as a NEAR full node and a NEAR DA indexer. The simplified flow can be described through the following:

  1. The indexer captures a block posted to NEAR DA for one of the networks and sends it to the operator node.
  2. The operator node retrieves and parses the block.
  3. The operator node checks if the block is the same as the one in their self-hosted network full node.
    1. If the blocks do not match, then the block posted by the Relayer are wrong and the NFFL nodes do not sign any state root.
  4. The operator node, through their BLS keypair, signs a message attesting that, for the network in question in that block height, the state root is the one that was fetched.
  5. The operator sends the signed message to the Aggregator.

The Aggregator collects BLS signatures from multiple NFFL nodes. When the desired quorum of operator power (i.e. restaked amount) is reached, then all of the signatures are aggregated into one and made available through an API.

This aggregated signature, when validated by a program that has access to the operator set, is the equivalent of "A sufficient amount of operators have agreed that, for network N, at block height H, the state root is S". By verifying the aggregated signatures a rollup can be sure of other rollup, allowing seamless cross-chain interoperability!

Apart from voting on the state roots of rollups, NFFL operators also track operator set updates on the AVS contracts and emit attestations for those in a somehow similar process - instead of expecting block data externally, it simply subscribes to Ethereum updates through its full node. The importance of that will be discussed in Network Registry.

For more details on the messaging flow, please read Messaging and Checkpoints.

Network Registry​

The Registry contract is a vital component of the NFFL architecture. There is a Registry contract on each rollup. This contract is used to verify the state root attestations.

In order to verify the signature, the Registry contract have access to the AVS operator set - otherwise, it can't know if a signer is an operator or not, much less whether the attestation has passed quorum or not.

In fact NFFL Registry contracts have two roles - 1) store a copy of the operator set and 2) verifying attestations. To keep the operator set up to date, the Registry contract accepts attestations for operator set updates.

The operator set relies on the AVS attestations to be up-to-date - the AVS operators themselves agree on each operator set delta. This is an easily verifiable 'task' in terms of slashing, and implements the cross-chain messaging necessary for communicating this from Ethereum to the other participant networks.

The operator set update is an Aggregator task by initially, but it would not be restricted to it - any user can submit it. Changes to this mechanism, especially in terms of economic incentives, are planned.

Checkpoint Tasks​

As defined in the EigenLayer AVS guidelines, AVS operation should be represented in terms of units of work called Tasks. These tasks are defined in the AVS contracts on Ethereum. The AVS payment and slashing are based on the operators fulfilling these tasks in a correct manner.

In the NFFL architecture, the attestations defined above are not defined as a Task - rather, they are defined as Messages. The Task for NFFL nodes is defined as the amalgamation of all the Message types, specifically the unit of work required from all validators is to attest on the aggregation (more specifically Merkleization) of messages in a time range.

Using the merkleization of aggregated messages, the existance or non-existance of a message in a time range is verified on Ethereum through the task response. The verification of the aggregated message is used for slashing and payment processing.

For more details on checkpoints, refer to Messaging and Checkpoints.

User Flow​

Finally - how can NFFL be used by a user or protocol? The integration is actually quite simple. Let's follow the HelloProtocol example: consider a user had sent a "hello!" message on Network #2 to Network #1, recording it on Network #2's state.

Eventually, the block in which the message was submitted gets considered in NFFL and a state root attestation was collected for the Network #2's state. Through it, anyone can submit the attestation to any network, not only Network #1, making Network #2's state available on it.

The HelloProtocol (off-chain) app would then keep track of NFFL's state and, as such, would be able to fetch this attestation as soon as it's available. This complexity can be simply abstracted from the user.

When the attestation is done, the protocol lets the user consume the "hello!" on Network #1 by sumbitting a transaction that indicates the storage proof of the message on Network #2 and the attestation from NFFL. Again, the UX is not really impacted - the proof should also be generated in the background.

This data is then relayed by the HelloProtocol contract to the NFFL Registry contract, which validates the attestation and checks the storage proof - and there is our "hello!"!

In easier and UX terms, all of the parts of this integration, that may seem strange to the user, can be simply abstracted from them. In implementation terms, it's a matter of fetching the attestation and the proof, as well as linking the protocol's contracts to NFFL's and relaying the fetched data.