QUIC, as the transport layer of the next-generation Web stack (HTTP/3), natively provides security and performance improvements over TCP-based stacks. However, since QUIC provides end-to-end encryption for both data and packet headers, in-network assistance like Performance-Enhancing Proxy (PEP) is unavailable for QUIC. To achieve the similar optimization as TCP, some works seek to collaborate endpoints and middleboxes to provide in-network assistance for QUIC. But involving both host and in-network devices increases the difficulty of deployment in the Internet. In this paper, by analyzing the QUIC standard, implementations, and the locality of application traffic, we identify opportunities for transparent middleboxes to measure RTT and infer packet loss for QUIC connections, despite the absence of plaintext ACK information. We then propose PEMI as a concrete system that continuously measures RTT and infers lost packets, enabling fast retransmissions for QUIC. PEMI enables performance enhancement for QUIC in a completely transparent manner, without requiring any explicit cooperation from the endpoints. To keep fairness, PEMI employs a delay-based congestion control and utilizes feedback-based methods to enforce CWND. Extensive evaluation results, including Mininet and trace-driven dynamic experiments, show that PEMI can significantly improve the performance of QUIC. For example, in the Mininet experiments, PEMI increases the goodput of file transfers by up to 2.5$\times$, and reduces the 90th percentile jitter of RTC frames by 20-75%.

翻译：QUIC作为下一代Web协议栈（HTTP/3）的传输层，相较于基于TCP的协议栈天然具备安全性与性能优势。然而，由于QUIC对数据包载荷与头部均实施端到端加密，传统TCP网络中性能增强代理（PEP）等网络内辅助机制无法直接适用于QUIC。为实现类似TCP的优化效果，现有研究尝试通过终端与中间盒协同为QUIC提供网络内辅助，但终端与网络设备的双重参与显著增加了互联网实际部署难度。本文通过分析QUIC标准规范、实际实现及应用流量的局部性特征，发现即使缺乏明文ACK信息，透明中间盒仍具备测量QUIC连接往返时延（RTT）与推断数据包丢失的可行性。基于此，我们提出PEMI系统，通过持续测量RTT与推断丢包事件，为QUIC实现快速重传机制。PEMI以完全透明的方式增强QUIC性能，无需终端节点的显式协作。为保障公平性，PEMI采用基于时延的拥塞控制机制，并利用反馈方法动态调整拥塞窗口（CWND）。通过Mininet仿真与轨迹驱动动态实验的广泛评估表明，PEMI能显著提升QUIC性能。例如在Mininet实验中，PEMI将文件传输的有效吞吐量最高提升至2.5$\times$，并将实时通信（RTC）帧的90百分位抖动降低20-75%。