Greedy Heaviest Observed Subtree GHOST Protocol Blockchain

GHOST Protocol Blockchain

A consensus method called the Greedy Heaviest Observed Subtree Protocol (GHOST) was created to solve problems in blockchain networks, especially ones with quick block generation rates. In a paper titled “Accelerating Bitcoin’s Transaction Processing,” published in December 2013, Yonatan Sompolinsky and Aviv Zohar first put up this idea. Trees, not chains, are where quick money grows.

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How GHOST Protocol Works

By making the “longest chain rule” more inclusive, GHOST seeks to address these problems. GHOST takes into account the “heaviest” chain rather than just going with the longest chain.

Fundamentals of GHOST

GHOST reinterprets the “best” or “canonical” chain as the “heaviest” chain principle. The total cumulative labour (or endorsements in Proof-of-Stake) of a chain and its observable “subtrees” are more important than its linear length (number of blocks). A node takes into account the amount of computation required to create each valid block in the “subtree” rooted at each fork, including legitimate “uncle” blocks that are not immediately on the main chain, when determining which branch of a fork to follow.

Orphaned (Uncle) Blocks

In order to identify the legitimate chain, GHOST takes these orphaned blocks into account. Some newly orphaned blocks may be referred to as “uncles” when a new block is mined. Their existence in later blocks adds to the chain’s total “weight” or “heaviness” and shows that the effort required to mine them is still valued. The number of blocks that reference these orphaned blocks determines their partial weight.

The GHOST rule is a recursive selection method

The procedure starts from the genesis block and proceeds to the child block at each fork, which is the root of the “heaviest subtree” (the branch with the most cumulative work, including its uncles). Until a leaf block the end of the selected chain is reached, this procedure keeps going. This preference for the subtree with the most accumulated labour is referred to as the “greedy” aspect.

The Problem GHOST Solves: Orphaned Blocks and Network Latency

Multiple miners (or validators in Proof-of-Stake systems) can generate valid blocks at the same time in decentralised blockchain networks. This frequently results in brief “forks” in the blockchain, where various regions of the network may consider different legitimate chains to be the “longest” due to network latency.

The conventional “Longest Chain Rule” (as used by Bitcoin): The “longest chain rule” is the main principle followed by Bitcoin and several early Proof-of-Work (PoW) blockchains. The network eventually converges on the chain with the most built-up Proof-of-Work (i.e., the most blocks) if a fork takes place. Any blocks that are mined on the fork’s “losing” branch are simply thrown away and become orphaned blocks, sometimes referred to as stale blocks or uncle blocks in the context of Ethereum. There is no compensation for miners that worked on these rejected blocks.

Issues with orphaned blocks

• Wasted Work: Miners’ computational work (hash power) on orphaned blocks is inefficiently wasted.
• Decreased Throughput: The likelihood of forks and orphaned blocks rises sharply when block times are extremely short (for example, Ethereum’s 12–15 seconds as opposed to Bitcoin’s 10 minutes). Because transactions in orphaned blocks are not part of the main chain, this lowers the effective transaction throughput.
• Centralization Risk: Mining successive blocks and creating a longer chain more quickly are more likely to occur in large mining pools that possess a sizable amount of the network’s hash power. This could result in mining centralization since it penalizes smaller miners whose blocks may be orphaned more frequently.

GHOST in Ethereum (Pre-Merge)

During its Proof-of-Work stage, Ethereum deployed a restricted and altered version of the GHOST system.

Uncle Blocks in Ethereum

A block in Ethereum may contain two or more “uncle” blocks. An uncle block was typically within a specific “generation” distance (e.g., 2 to 7 blocks deep) and required to be a direct offspring of an ancestor of the present block, but not an ancestor itself.

Uncle Rewards

Ethereum gave miners rewards for both mining blocks and adding legitimate uncle blocks in order to discourage effort waste and preserve decentralisation. Even if their block wasn’t on the canonical chain, the miner who mined the uncle block itself also got a piece of the block reward, and the miner of the main block got a smaller reward for including an uncle.

Advantages for Ethereum’s Proof of Work Phase

This change allowed Ethereum to reach a faster block time (about 12–15 seconds) than Bitcoin (10 minutes). By lowering the amount of mining effort wasted, increasing transaction throughput by enabling more blocks including uncles to contribute to the chain’s validity, and lowering the risk of centralization by making it profitable for smaller miners to participate, it achieved these goals while preserving security and decentralization.

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Evolution of GHOST in Ethereum 2.0 (Proof-of-Stake)

• Ethereum switched to Proof-of-Stake (PoS) during The Merge, however the fundamental concept of GHOST remains in its consensus process. The Latest Message Driven Greediest Heaviest Observed SubTree (LMD GHOST) is a fork-choice rule used by Ethereum’s current PoS consensus (Gasper).

• Latest Message-Driven (LMD): In a proof-of-stake (PoS) system, validators cast “votes” (attestations) for blocks they think should be at the head of the chain in place of the “work” that comes from mining. To prevent a validator from obtaining excessive influence by voting on several forks, LMD ensures that only the validator’s most recent vote is taken into account when choosing the heaviest chain.

• The idea is still the same for the Heaviest Observed Subtree (in PoS): the chain with the highest “weight” (total of stakes from validators’ most recent votes) over its whole observed subtree including legitimate but non-canonical branches is selected as the canonical chain.

• Combination with Casper: Casper the Friendly Finality Gadget (Casper FFG) and LMD GHOST work together. Casper FFG offers “cryptoeconomic finality,” which means that once a block is finalized, it is nearly impossible to go back without incurring a significant financial penalty for validators, whereas LMD GHOST chooses the “head” of the chain at any given time.

Advantages of GHOST Protocol

With its advanced chain selection rule, the GHOST protocol provides a number of advantages:

• Increased Throughput: Even if a transaction isn’t on the longest chain, it can still be included in the effective blockchain history by accounting for uncle blocks. A higher block creation rate is made possible by this.
• Decreased Centralisation: Since smaller miners’ labour is not entirely thrown away, rewarding uncle blocks encourages them to keep mining. This promotes a more democratic consensus and keeps mining pools from seizing too much power.
• Faster Confirmation Times: It enables shorter block intervals without appreciably sacrificing security, which speeds up the completion of transactions.
• Enhanced Security: The network becomes more resistant to some attacks (such as selfish mining) that attempt to take advantage of network latency by including a larger portion of the network’s computational power into the chain selection process. It makes it more difficult for bad actors to manipulate the blockchain and produce forks.
• Effective Resource Use: GHOST optimises network resources by taking orphaned blocks into account and preventing miners’ computing efforts from being squandered.
• Decreased Fork Persistence: By concentrating on the heaviest subtree, the protocol makes it easier to resolve forks and rapidly converge on a single valid chain, which lessens the possibility of persistent forks.

Challenges of GHOST Protocol Implementation

Notwithstanding its benefits, GHOST protocol implementation has certain drawbacks:

• Increased Complexity: The consensus method becomes more complex when GHOST is implemented since it needs extra logic to handle orphaned blocks and assess the heaviest subtree. This can make it more difficult to implement, debug, and maintain.
• Increased Computational Overhead: In order to identify the heaviest subtree, the protocol requires nodes to assess both the main chain and orphaned blocks, which raises the computational overhead.
• Higher Storage Requirements: The blockchain must store more data as a result of taking orphaned blocks into account, which could raise node storage needs and increase operational load.
• Complex Fork Resolution: Because GHOST’s method involves analysing the heaviest subtree, which can be computationally demanding and cause delays, it may complicate fork resolution, particularly when dealing with big or deep forks.
• Transaction Reorganisation Risks: Should the heaviest subtree change, transactions included in orphaned blocks may be reorganised.
• Possibility of Delayed Finality: Including orphaned blocks and analyzing the heaviest subtree can occasionally cause delays in transaction completion.