Revisiting why Kad lookup fails

Kad is one of the most popular peer-to-peer (P2P) networks deployed on today's Internet. Its reliability is dependent on not only to the usability of the file-sharing service, but also to the capability to support other Internet services. However, Kad can only attain around a 91% lookup success ratio today. We build a measurement system called Anthill to analyze Kad's performance quantitatively, and find that Kad's failures can be classified into four types: packet loss, selective Denial of Service (sDoS) nodes, search sequence miss, and publish/search space miss. The first two are due to environment changes, the third is caused by the detachment of routing and content operations in Kad, and the last one shows the limitations of the Kademlia DHT algorithm under Kad's current configuration. Based on the analysis, we propose corresponding approaches for Kad, which achieve a success ratio of 99.8%, with only moderate communication overhead.