  BGP prefix hijacking is a critical threat to the resilience and security of
communications in the Internet. While several mechanisms have been proposed to
prevent, detect or mitigate hijacking events, it has not been studied how to
accurately quantify the impact of an ongoing hijack. When detecting a hijack,
existing methods do not estimate how many networks in the Internet are affected
(before and/or after its mitigation). In this paper, we study fundamental and
practical aspects of the problem of estimating the impact of an ongoing hijack
through network measurements. We derive analytical results for the involved
trade-offs and limits, and investigate the performance of different measurement
approaches (control/data-plane measurements) and use of public measurement
infrastructure. Our findings provide useful insights for the design of accurate
hijack impact estimation methodologies. Based on these insights, we design (i)
a lightweight and practical estimation methodology that employs ping
measurements, and (ii) an estimator that employs public infrastructure
measurements and eliminates correlations between them to improve the accuracy.
We validate the proposed methodologies and findings against results from
hijacking experiments we conduct in the real Internet.
