This repository contains the software artifacts which have been used to measure instant ACK with the QUIC Interop Runner testbed and the Internet. This is a release version from the official artifacts repository. NOTE: If you use our tools, please cite our paper as follows: ReACKed QUICer: Measuring the Performance of Instant Acknowledgments in QUIC HandshakesJ. Mücke, M. Nawrocki, R. Hiesgen, T. C. Schmidt, and and M. WählischProc. of ACM Internet Measurement Conference (IMC), Madrid, Spain: ACM, 2024https://doi.org/10.1145/3646547.3689022 Abstract In this paper, we present a detailed performance analysis of QUIC instant ACK, a standard-compliant approach to reduce waiting times during the QUIC connection setup in common CDN deployments. To understand the root causes of the performance properties, we combine numerical analysis and the emulation of eight QUIC implementations using the QUIC Interop Runner. Our experiments comprehensively cover packet loss and non-loss scenarios, different round trip times, and TLS certificate sizes. To clarify instant ACK deployments in the wild, we conduct active measurements of 1M popular domain names. For almost all domain names under control of Cloudflare, Cloudflare uses instant ACK, which in fact improves performance. We also find, however, that instant ACK may lead to unnecessary retransmissions or longer waiting times under some network conditions, raising awareness of drawbacks of instant ACK in the future.