Replica Placement for Route Diversity in Tree-Based Routing Distributed Hash Tables

Distributed hash tables (DHTs) share storage and routing responsibility among all nodes in a peer-to-peer network. These networks have bounded path length unlike unstructured networks. Unfortunately, nodes can deny access to keys or misroute lookups. We address both of these problems through replica placement. We characterize tree-based routing DHTs and define MaxDisjoint, a replica placement that creates route diversity for these DHTs. We prove that this placement creates disjoint routes and find the replication degree necessary to produce a desired number of disjoint routes. Using simulations of Pastry (a tree-based routing DHT), we evaluate the impact of MaxDisjoint on routing robustness compared to other placements when nodes are compromised at random or in a contiguous run. Furthermore, we consider another route diversity mechanism that we call neighbor set routing and show that, when used with our replica placement, it can successfully route messages to a correct replica even with a quarter of the nodes in the system compromised at random. Finally, we demonstrate a family of replica query strategies that can trade off response time and system load. We present a hybrid query strategy that keeps response time low without producing too high a load.