A denial-of-service attack (DoS attack; UK: /dɒs/ doss US: /dɑːs/ daas^[1]) is defined as a cyberattack in which the attacker aims to render a machine or network resource unavailable to its intended users by temporarily or indefinitely disrupting the services of a host connected to a network. – The Wikipedia definition of denial-of-service attack. 

This concept is fundamentally straightforward. An individual utilizes their own resources to disrupt the operations of other machines within a network.

Denial-of-service attacks have persisted since the inception of the internet. One of the frequently cited early instances of a Distributed Denial-of-Service (DDoS) attack occurred against the Internet Service Provider (ISP) Panix in the mid-1990s. While numerous technical examples existed on earlier internet services, this incident marked one of the first significant instances of such an attack on the modern World Wide Web.

This particular attack involved multiple computers initiating a Transmission Control Protocol (TCP) connection with the ISP’s servers, but failing to complete the handshake protocol necessary to finalize the connection. Consequently, this consumed server resources for managing network connections and inhibited legitimate users from accessing the internet via the ISP’s servers.

Since that initial DDoS attack, they have become as common on the internet as storms in nature, leading to the development of substantial infrastructure designed to defend against such occurrences.

The Blockchain

The blockchain constitutes a fundamental element of Bitcoin and is essential for its functionality as a distributed ledger. Some individuals within this domain argue that so-called “spam” transactions function as a DoS attack on the Bitcoin blockchain. To label it as such, one must delineate the “service” that the blockchain provides as a system and clarify how spam transactions impede that service in a manner not contemplated by the system’s design.

It can be hypothesized that most proponents of the view that spam constitutes a DoS attack might articulate that “the service the blockchain offers is processing financial transactions, and spam occupies space that could be used by legitimate transactions.” However, this characterization does not accurately reflect the specific service provided by the blockchain.

The actual service it offers is the confirmation of any consensus-valid transaction through a real-time auction that periodically settles when a miner discovers a block. If a transaction is consensus-valid and the fee bid is sufficiently high for a miner to include it in a block, the user is utilizing the blockchain’s services precisely as intended.

This approach was a deliberate design choice refined over time during the “Block Size Wars,” culminating in the activation of Segregated Witness and the rejection of the Segwit2x block size increase proposed through a hard fork by significant companies at the time. The blockchain functions by prioritizing transactions with the highest bidding fees, allowing users to compete in this auction for block space allocation while ensuring global restrictions to maintain verifiability and a free market pricing mechanism.

Thus, the inclusion of a transaction some might arbitrarily define as “spam” in this open auction does not constitute a DoS attack on the blockchain; rather, it represents a user effectively utilizing the resource as originally intended, competing in the auction alongside others.

The Relay Network

Many, if not the majority, of Bitcoin nodes provide transaction relay services to the broader network. When users broadcast their transactions to peers, the transactions are subsequently relayed to additional peers, facilitating rapid propagation across the network, especially to mining nodes.

Additionally, block relay entails the propagation of valid blocks as they are discovered, a process that has undergone considerable optimization over the years. Typically, an entire block is not relayed; instead, a “sketch” of the block header alongside the included transactions is transmitted, enabling reconstruction from individual mempools. Therefore, block relay optimizations rely on an effective transaction relay to ensure the distribution of valid and likely-to-be-mined transactions.

If nodes lack transactions already present in their mempool, they must request them from neighboring nodes, which introduces delays in validating the block. Furthermore, nodes may forward these transactions along with block sketches to other peers to prevent gaps, consuming bandwidth inefficiently. When nodes implement transaction filtering, they may impede the efficiency of both transaction relay and block propagation significantly, causing noticeable degradation as more nodes filter out transactions they label as spam.

Such filtering practices deliberately seek to disrupt the services related to propagating transactions to miners and the broader network. This approach not only diminishes transaction relay effectiveness but also degrades block propagation as a consequence for miners opting to include valid transactions that have been filtered. The intention is to create a degradation of service and to manifest that degradation as a penalty directed at miners.

This circumstance genuinely constitutes a DoS attack, as it actively undermines a network service in a manner counter to the system’s intended design.

Conclusion

The ongoing contention between Knotz and Core, characterized by the acrimony between “Spammers” and “Filterers,” epitomizes a largely ineffective and failed DoS attack on the Bitcoin network. Filters do not prevent transactions filtered from being included in blocks. Efforts to disrupt transaction propagation to miners have yielded no successes, and the degradation of block relay has been marginal enough that it does not disincentivize miners.

This scenario is viewed as a compelling testament to Bitcoin’s robustness and resilience against attempts at censorship and disruption within the Bitcoin Network itself.

What lies ahead?

An anonymous author has proposed a BIP to enact a temporary soft fork that would invalidate several methods for including “spam” in Bitcoin transactions for a duration of approximately one year. Recognizing the failure of the DoS attack on the peer-to-peer network, supporters of filtering have shifted towards seeking consensus changes—a course of action anticipated over two years ago.

Will this initiative resolve the issue? Unlikely, as it would merely compel those wishing to submit “spam” to adopt fake ScriptPubKeys to encode their data in unspendable outputs, ultimately bloating the UTXO set.

Even if this fork receives overwhelming support, is successfully activated, and does not lead to a chain split, it would still fall short of achieving its stated goal, leaving “spammers” with no alternative but to engage in potentially the most detrimental method of spamming the network.

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