Transaction rate-limiting

The recent integration of a significant update into Bitcoin Core, specifically through the pull request #34628, marks a pivotal enhancement aimed at optimizing transaction handling and CPU utilization in Bitcoin nodes. This advancement stems from an intensive three-year project initiated due to an incident where a mainnet debug-build node hosted on a VPS was reported for excessive CPU usage caused by slow transaction processing times. The issue was traced back to two primary bugs; the first involved inefficient iterator implementations by Boost with O(N) performance relative to container size, and the second arose from mechanisms intended to mitigate the adverse effects of large transaction bursts on the network.

Historically, Bitcoin's transaction relay mechanism functioned as an amplifier, where each node could significantly multiply outbound transaction data, potentially leading to substantial network congestion and bandwidth usage. This system, though designed ten years ago to manage network load, has shown limitations such as reduced transaction throughput across nodes and increased discrepancies between transaction fees expected by miners and those chosen during transaction relays. Furthermore, the advent of segregated witness and the popularization of small text-based inscriptions have altered transaction patterns, thereby straining the original system.

Efforts to address these inefficiencies led to improvements like the removal of transactions lingering in per-peer queues and increasing the processing rate for backlog clearance. These adjustments were part of a broader strategy to refine transaction handling, culminating in the decision to replace per-peer queues with a single global queue. This change aims to reduce CPU load consistently across all peers and manage large transaction volumes more efficiently. Additionally, new bandwidth rate limits have been introduced to prevent the overload of nodes during peak transaction periods, ensuring better overall network stability and performance.

This comprehensive update is expected to enhance the robustness of Bitcoin nodes, particularly in handling high transaction volumes whether stemming from increased adoption or other factors like spam or deliberate network attacks. The solutions implemented not only address practical issues but also aim to mitigate theoretical vulnerabilities, marking a significant step forward in the evolution of Bitcoin's transaction processing capabilities.