The Merge – Why Geth Syncing is Still Required After the Merge

The syncing functionality of the geth light node does not currently (Nov 2022) work post-merge, because the historical execution layer blocks are no longer sufficient to light-verify the chain. There is an open PR to add support for standalone beacon chain light syncing in geth here: https://github.com/ethereum/go-ethereum/pull/25901

This is only the first step to restoring the expected functionality of the geth light node. I expect it won't be until early 2023 when the expected functionality is restored.

The Merge was the implementation of the Bellatrix consensus (layer) specs, the Paris execution (layer) specs, and the Engine API.

Bellatrix: https://github.com/ethereum/consensus-specs/tree/dev/specs/bellatrix

Paris: https://github.com/ethereum/execution-specs/blob/master/network-upgrades/mainnet-upgrades/paris.md

• Paris mainly consists of EIP-3675.

The interaction between the consensus layer and execution layer is specified by the Engine API: https://github.com/ethereum/execution-apis/tree/main/src/engine

Q1

The execution layer has blocks all the way since genesis; beacon blocks only started having transactions in Bellatrix. So each layer can be considered to be a "chain".

On the question of transactions duplicated since the Merge, from the specs alone, EIP-3675 does not specify any removal or optimizations of transactions at the execution layer. As the rationale of EIP-3675 states: "This EIP was designed to minimize the complexity of hot-swapping the live consensus of the Ethereum network. Both the safety of the operation and time to production were taken into consideration." So the answer to Q1 is it depends on the implementation of the execution client. In practice, transactions are probably duplicated.

Q2

Yes, the Bellatrix spec states the extensions and additions. The BeaconBlockBody and BeaconState are extended with ExecutionPayload and ExecutionPayloadHeader respectively.

Q3

The consensus layer (CL) and execution layer (EL) largely operate as they did before the Merge. For example, the EL manages the mempool and gathering up transactions for a block, and it syncs from genesis.

Engine API test vectors for The Merge based on a discussion of the Engine API is helpful to see how the layers interact. The CL calls engine_forkchoiceUpdatedV1 so that the EL prepares a payload. The CL calls engine_getPayloadV1 to get a payload. The CL then tells the EL to execute the payload by calling engine_newPayloadV1. Assuming that the payload is valid, the CL then tells the EL that the payload is the head of the chain by calling engine_forkchoiceUpdatedV1.