"A collection of interconnected networks" defines what the Internet is, but not what it is not. Events threaten Internet fragmentation: politics suggest countries or ISPs may secede or be de-peered, disputes between ISPs result in persistent unreachability between their customers, and architectural changes risk breaking the "one" Internet. Understanding such threats benefits from a testable definition of what the Internet is and is not, enabling discussion and quantification of partial connectivity. We provide a conceptual definition giving an idealized asymptote of connectivity. It implies peninsulas of persistent, partial connectivity, and islands when one or more computers are partitioned from the main Internet. We provide algorithms to measure, operationally, the number, size, and duration of peninsulas and islands. We apply these algorithms in rigorous measurement from two complementary measurement systems, one observing 5M networks from a few locations, and the other a few destinations from 10k locations. Results show that peninsulas (partial connectivity) are about as common as Internet outages, quantifying this long-observed problem. Root causes show that most peninsula events (45%) are routing transients, but most peninsula-time (90%) is from a few long-lived events (7%). Our analysis helps interpret DNSmon, a system monitoring the DNS root, separating measurement error and persistent problems from underlying differences and operationally important transients. Finally, our definition confirms the international nature of the Internet: no single country can unilaterally claim to be "the Internet", but countries can choose to leave.

翻译：“互联网络的集合”定义了互联网是什么，但未定义它不是什么。一些事件威胁着互联网的碎片化：政治因素表明国家或ISP可能退出或被解除对等互联，ISP之间的争端导致其用户之间持续不可达，架构变化则可能破坏“单一”互联网。理解此类威胁需要一个可检验的互联网定义，明确其“是什么”与“不是什么”，从而支持对部分连通性的讨论与量化。我们提出一个概念性定义，给出了连通性的理想化渐近线。该定义暗示了持久性部分连通的“半岛”，以及当一台或多台计算机与主互联网隔离时的“岛屿”。我们提供了算法，可从操作层面测量半岛和岛屿的数量、规模及持续时间。我们运用互补的两套测量系统严格应用这些算法：一套从少数位置观测500万个网络，另一套从1万个位置观测少数目标。结果表明，半岛（部分连通性）的出现频率与互联网中断相当，量化了这一长期被观察到的现象。根本原因显示，大部分半岛事件（45%）由路由瞬态引起，但大部分半岛持续时间（90%）源于少数长期事件（7%）。我们的分析有助于解读监控DNS根系统的DNSmon，可区分测量误差、持续性问题与底层差异及操作重要的瞬态。最后，该定义证实了互联网的国际性：没有任何国家能单方面声称“互联网就是自身”，但各国可以选择退出。