Latency is a key indicator of Internet service performance. Continuously tracking the latency of client requests enables service operators to quickly identify bottlenecks, perform adaptive resource allocation or routing, and mitigate attacks. Passively measuring the response latency at intermediate vantage points is attractive since it provides insight into the experience of real clients without requiring client instrumentation or incurring probing overheads. This paper presents PIRATE, a passive approach to measure response latencies when only the client-to-server traffic is visible, even when transport headers are encrypted. PIRATE estimates the time gap between causal pairs - two requests such that the response to the first triggered the second - as a proxy for the client-side response latency. Our experiments with a realistic web application show that PIRATE can estimate the response latencies measured at the client application layer to within 1 percent. A PIRATE-enhanced layer-4 load balancer (with DSR) cuts tail latencies by 37 percent.

翻译：延迟是衡量互联网服务性能的关键指标。持续追踪客户端请求的延迟，能够帮助服务运营商快速识别瓶颈、执行自适应资源分配或路由调整，并有效缓解攻击。在中间观测点被动测量响应延迟具有显著优势，因为它能够在不要求客户端植入监测代码或产生探测开销的情况下，深入了解真实用户的体验。本文提出了一种被动测量方法PIRATE，该方法即使在传输层头部加密的情况下，也能在仅可见客户端到服务器流量时测量响应延迟。PIRATE通过估计因果对（即两个请求，其中第一个请求的响应触发了第二个请求）之间的时间间隔，作为客户端响应延迟的代理指标。我们在一个真实的Web应用上进行的实验表明，PIRATE能够将应用层客户端测量的响应延迟估计误差控制在1%以内。一个集成了PIRATE的第四层负载均衡器（采用DSR架构）能够将尾部延迟降低37%。