What is an Orphan Block?

The blockchain ecosystem relies heavily on miners to keep the system running. However, problems can arise when two or more miners mine a new block at the same time, leading to confusion about which block to add to the main chain.

While these blocks may look the same, they can differ in how they are treated by various blockchain protocols.

There are three types of blocks that can be encountered in the blockchain ecosystem: orphan, ommer (uncle), and stale blocks. Orphan blocks are blocks that are not considered valid because their parent block is unknown. In contrast, ommer blocks are valid since their parent block is known and are rewarded. Stale blocks are valid but are no longer part of the main chain.

What is an Orphan Block?

In older versions of the Bitcoin Core software, orphan blocks were blocks without a known or existing parent block. However, since the release of Bitcoin Core v.0.10 in early 2015, true orphan blocks are no longer possible. Despite this, the term "orphan block" is still commonly used in the cryptocurrency industry to refer to valid mined blocks that have been discarded.

When people use the term "orphan block," they are typically referring to a stale block, which is a valid block that is no longer part of the longest and valid blockchain due to its lower difficulty level. The block reward and transaction fees of a stale block are no longer spendable on the valid blockchain, which means that the miner who found the block does not receive the reward or fees. This situation can cause issues for mining pools that use payout strategies other than "proportional."

Technically, calling a stale block an orphan block is incorrect since it does have a parent block. In the Bitcoin source code and more technical discussions, orphan blocks and stale blocks are two distinct types of blocks. However, in general discussions, people often use the term "orphan block" to refer to stale blocks.

Stale blocks are well-formed blocks that are no longer part of the longest and most well-formed blockchain because they do not have sufficient proof-of-work. If a miner or mining pool forks the blockchain and demonstrates that they have done the most work, establishing a new main chain, the miners who failed to switch to this chain would be working on stale blocks and would not be eligible for a mining reward.

As previously noted, the term "orphan block" is commonly used to refer to blocks that have been rejected by the network, despite not accurately reflecting a familial relationship. In reality, an orphan block would refer to a block that lacks information about its parent blocks, resulting in an incomplete block hash. As the block hash includes information about preceding blocks, an orphan block would be anomalous in a network that relies on validating all preceding blocks.

A block lacking parental block information is likely to have been tampered with. Nonetheless, an unaccepted block is typically called an orphan block for simplicity's sake.

Ommer Blocks

An ommer block, which is also sometimes referred to as an "uncle block", is a valid block that is not included in the blockchain's longest chain. This can occur in a blockchain that uses a proof-of-work consensus algorithm, such as Bitcoin. When miners compete to solve a block, there can be instances where multiple miners solve the block at the same time. In these cases, the network will only accept one of the blocks, and the other blocks become ommer blocks.

Ommer blocks are similar to orphan blocks, which are also blocks that are not included in the blockchain's longest chain. However, ommer blocks are different in that they do have a known parent block. In fact, ommer blocks are created by including a reference to a previous block that is not the immediate parent. This can happen when two miners solve a block at almost the same time, and one of the blocks includes a reference to the other block as its parent.

Ommer blocks were first introduced in the Ethereum blockchain when it used a proof-of-work consensus algorithm, prior to the transition to proof-of-stake in mid-September 2022. In Ethereum, ommer blocks served a specific purpose: to incentivize miners to continue mining even when they were unable to solve a block before other miners. Miners who created ommer blocks received a smaller reward than those who created valid blocks, but still received a reward. This helped prevent miners from leaving the network when they realized they could not solve a block before their competitors.

In addition to incentivizing miners, ommer blocks can also improve the security of the blockchain network. By including references to ommer blocks, the network can create a larger and more robust blockchain. This can help to prevent attacks on the network, as attackers would need to control a significant portion of the network's computing power to alter the blockchain.

How to Prevent Stale Blocks (Orphan Blocks)?

Stale blocks can be a problem for miners because they do not receive the block reward or transaction fees associated with the stale block. Fortunately, there are several strategies that can help to prevent stale blocks:

Faster Block Time: By reducing the time between block creations, the likelihood of two miners creating a block simultaneously is reduced, which in turn reduces the chance of stale blocks.
Lower Latency: Miners can reduce the likelihood of stale blocks by improving their network latency. A lower latency network allows miners to receive information about new blocks more quickly, reducing the chance of creating a stale block.
Higher Block Propagation Speed: By improving the speed at which new blocks are propagated across the network, miners can reduce the chance of creating a stale block. This can be accomplished by using better network infrastructure, such as dedicated high-speed lines or data centers.
Mining Pools: Joining a mining pool can help to reduce the likelihood of finding stale blocks. By collectively mining blocks and sharing the rewards among members, mining pools can help to reduce the chance of a single miner finding a stale block.
Improved Mining Software: Using mining software that is optimized for speed and efficiency can help to reduce the chance of stale blocks. For example, some mining software can prioritize the creation of a block over receiving new transactions, reducing the likelihood of creating a stale block.

What is an Orphan Block?

What is an Orphan Block?

Ommer Blocks

How to Prevent Stale Blocks (Orphan Blocks)?

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