Studying Black Holes in the Internet with Hubble

We present Hubble, a system that operates continuously to find Internet reachability problems in which routes exist to a destination but packets are unable to reach the destination. Hubble monitors at a 15 minute granularity the data-path to prefixes that cover 89% of the Internet's edge address space. Key enabling techniques include a hybrid passive/active monitoring approach and the synthesis of multiple information sources that include historical data. With these techniques, we estimate that Hubble discovers 85% of the reachability problems that would be found with a pervasive probing approach, while issuing only 5.5% as many probes. We also present the results of a three week study conducted with Hubble. We find that the extent of reachability problems, both in number and duration, is much greater than we expected, with problems persisting for hours and even days, and many of the problems do not correlate with BGP updates. In many cases, a multi-homed AS is reachable through one provider, but probes through another terminate; using spoofed packets, we isolated the direction of failure in 84% of cases we analyzed and found all problems to be exclusively on the forward path from the provider to the destination. A snapshot of the problems Hubble is currently monitoring can be found at http://hubble.cs.washington.edu.