Routing is central to networking performance, including: (1) latency in anycast services and websites served from multiple locations,(2) networking expenses and throughput in multi-homed enterprises, (3) the ability to keep traffic domestic when considering data sovereignty. However, understanding and managing how routing affects these services is challenging. Operators use Traffic Engineering (TE) with BGP to optimize network performance, but what they get is the result of all BGP policies throughout the Internet, not just their local choices. Our paper proposes Fenrir, a new system to rediscover recurring routing results. Fenrir can discover changes in network routing, even when it happens multiple hops away from the observer. Fenrir also provides new methods to quantify the degree of routing change, and to identify routing "modes" that may reappear. Second, we show that Fenrir can be applied to many different problems: we use five instances of three different types of systems to illustrate the generalization: anycast catchments showing in a root DNS service, route optimization for two multi-homed enterprises, and website selection for two of the top-10 web services. Each type requires different types of active measurements, data cleaning and weighting. We demonstrate Fenrir's methods of detecting and quantifying change are helpful because they all face similar operational questions: How much effect did traffic engineering have? Did a third-party change alter my routing? In either case, is the current routing new, or is it like a routing mode I saw before?

翻译：路由是影响网络性能的核心因素，包括：(1) 任播服务及多地域部署网站的延迟，(2)多宿主企业的网络开销与吞吐量，(3)考虑数据主权时保持流量国内化的能力。然而，理解并管理路由对这些服务的影响具有挑战性。运营商通常采用边界网关协议（BGP）进行流量工程以优化网络性能，但其实际效果取决于整个互联网中所有BGP策略的综合作用，而不仅限于本地决策。本文提出名为Fenrir的新型系统，用于重新发现周期性出现的路由结果。Fenrir能够探测网络路由的变化，即使该变化发生在观测点多个跃程之外。该系统还提供量化路由变化程度的新方法，并能识别可能重复出现的路由"模式"。其次，我们通过三类系统的五个实例证明Fenrir具有广泛适用性：根域名系统服务中的任播服务范围展示、两家多宿主企业的路由优化，以及两个全球前十网络服务的网站选择。每种类型都需要不同的主动测量方法、数据清洗与加权策略。我们验证了Fenrir在检测和量化路由变化方面的方法具有实用价值，因为这些系统都面临相似的实际问题：流量工程产生了多大效果？第三方变更是否改变了我的路由？无论是哪种情况，当前路由是全新的，还是与我曾见过的路由模式相似？