An Evaluation of Caching in Nation Scale, Normally Isolated Mobile Ad Hoc Networks

Nation Scale Mobile Ad Hoc Network, or NSHoc, was developed to enable smartphone users to request and receive content via opportunistic encounters in nation scale topologies. It provides a novel solution for large scale delay tolerant communications when network infrastructure is down after natural disasters, inaccessible due to extreme censorship or nonexistent in remote areas. Prior work demonstrated that NSHoc can successfully deliver content, including unpopular content, with no existing knowledge of network members and in sparse topologies where individual nodes may remain isolated for minutes or even hours at a time. However, significant questions regarding the caching properties of NSHoc were left unanswered. In this paper we present an analysis of caching in NSHoc, the first analysis of caching in a nation scale MANET. Our results show that NSHoc can deliver content with only a small performance loss even when half of the users are immobile and that it is resilient against black hole attacks. However, NSHoc is vulnerable to content flooding attacks. We found that the percentage of requests fulfilled dropped from 97% to 78% when only 5% of users were flooding the network with unpopular content. We demonstrate the effectiveness of two defenses against content flooding attacks: friends first caching and statistical checking of content caches prior to exchanging content. Both of these defenses provide complete protection against least popular content flooding attacks, restoring the percentage of requests fulfilled to 97%.