The new QUIC protocol can be implemented in user space, and various implementations already exist. While they follow the same specification and general interoperability is given, differences in performance, functionality, but also security (e.g., due to bugs) can be expected. Therefore, knowledge about the implementation of an endpoint on the Internet can help researchers, operators and users to better analyze connections, evaluations and findings. We provide an approach to identify used libraries of QUIC servers based on CONNECTION_CLOSE frames and transport parameter orders. We apply our methodology to Internet-wide scans and identify at least one deployment for 18 QUIC libraries. In total, we can identify the library of 8.8 M IPv4 and 2.5 M IPv6 addresses.

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