Analysis on the “Robust Yet Fragile” Nature of Internet: Load, Capacity and the Cascading Failure Avalanche Effect

´Robust yet fragile (RYF)´ is a major feature of Internet survivability. The cascading failure, which is caused by attacks or configuration mistakes, is one of the most serious incidents impact on the network survivability. Taking aim at this incident, this paper proposes a simple ´load-capacity´ model based on the ´self-organized criticality´ and the ´highly optimized tolerance´ theories in complex system. The model is the abstract of the network topology, the dynamics in network protocols and the complex patterns of network behaviors (load). In this model, the network capacity reflects the expectation of survivability. Our research shows that the RYF nature in network is closely related to abnormal network load pattern.