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Researchers have shown that the Internet exhibits path inflation – end-to-end paths can be significantly longer than necessary. We present a trace-driven study of 65 ISPs that characterizes the root causes of path inflation, namely topology and routing policy choices within an ISP, between pairs of ISPs, and across the global Internet. To do so, we develop and validate novel techniques to infer intra-domain and peering policies from end-to-end measurements. We provide the first measured characterization of ISP peering policies. In addition to “early-exit,” we observe a significant degree of helpful non-early-exit, load-balancing, and other policies in use between peers. We find that traffic engineering (the explicit addition of policy constraints on top of topology constraints) is widespread in both intraand inter-domain routing. However, intra-domain traffic engineering has minimal impact on path inflation, while peering policies and inter-domain routing lead to significant inflation. We argue that the underlying cause of inter-domain path inflation is the lack of BGP policy controls to provide convenient engineering of good paths across ISPs.

研究人员已经表明，互联网存在路径膨胀现象——端到端路径可能比实际需要的长得多。我们对65个互联网服务提供商（ISP）进行了一项基于跟踪数据的研究，该研究描述了路径膨胀的根本原因，即在一个ISP内部、成对的ISP之间以及整个全球互联网中拓扑结构和路由策略的选择。为此，我们开发并验证了从端到端测量中推断域内和对等互联策略的新技术。我们首次对ISP对等互联策略进行了实测描述。除了“提前退出”策略，我们还观察到对等体之间大量有益的非提前退出、负载均衡及其他正在使用的策略。我们发现流量工程（在拓扑约束基础上明确添加策略约束）在域内和域间路由中都很普遍。然而，域内流量工程对路径膨胀的影响极小，而对等互联策略和域间路由则会导致显著的膨胀。我们认为，域间路径膨胀的根本原因是缺乏边界网关协议（BGP）策略控制，无法为跨ISP的良好路径提供便捷的工程设计。