2015 Bitcoin Garbage Transaction Attack: Network Resilience and Block Size Controversy

The Bitcoin network experienced a series of spam transaction attacks in the summer of 2015, which became an early focal point of the block size controversy. The attackers were a group advocating for an increase in the block size limit, arguing that the 1MB limit was too small and easily filled with spam transactions. They argued that increasing the block size could raise the cost of filling blocks, thereby enhancing Bitcoin's resilience against attacks.

The attack was initiated by a Bitcoin wallet and exchange named CoinWallet.eu and was carried out in four phases. The first round of the attack began on June 22 but failed to achieve the expected results due to technical issues. The second round of the attack unfolded on June 29, causing some users to experience delays in their transactions. The third round of the attack occurred on July 7 and was the most intense, costing approximately $8,000 in fees, using diverse strategies to generate a large number of junk transactions. The fourth and final round of the attack took place in September, employing a different approach by triggering a large number of transactions through the public disclosure of private keys.

These attacks had a significant impact on the Bitcoin network. According to an academic study, approximately 23.41% of transactions were identified as spam transactions during the peak of the attacks over a 10-day period. The attacks also caused the average transaction fee for normal transactions to rise by 51%, and processing delays increased by 7 times.

The consequences of the attack prompted the Bitcoin network to make some changes:

1. Miners raised the block size limit policy from 250KB or 750KB to 1MB.
2. In October 2015, the minimum relay fee in Bitcoin Core increased by 5 times.
3. Introduced a memory pool limit and a default memory pool size limit of 300MB.
4. Intensified the debate over block size limits.

Although small block supporters ultimately gained the upper hand in this debate, discussions on how to define and handle spam transactions continue. Compared to 2015, the "spam" transaction fees in recent years have reached hundreds of millions of dollars, far exceeding the attack cost of about 10,000 dollars at that time. These historical events provide valuable references for our understanding of the evolution of the Bitcoin network and the challenges it faces.