On the Feasibility of Rerouting-Based DDoS Defenses

Large botnet-based flooding attacks have recently demonstrated unprecedented damage. However, the best-known end-to-end availability guarantees against flooding attacks require costly global-scale coordination among autonomous systems (ASes). A recent proposal called routing around congestion (or RAC) attempts to offer strong end-to-end availability to a selected critical flow by dynamically rerouting it to an uncongested detour path without requiring any inter-AS coordination. This paper presents an in-depth analysis of the (in)feasibility of the RAC defense and points out that its rerouting approach, though intriguing, cannot possibly solve the challenging flooding problem. An effective RAC solution should find an inter-domain detour path for its critical flow with the two following desired properties: (1) it guarantees the establishment of an arbitrary detour path of its choice, and (2) it isolates the established detour path from non-critical flows so that the path is used exclusively for its critical flow. However, we show a fundamental trade-off between the two desired properties, and as a result, only one of them can be achieved but not both. Worse yet, we show that failing to achieve either of the two properties makes the RAC defense not just ineffective but nearly unusable. When the newly established detour path is not isolated, a new adaptive adversary can detect it in real time and immediately congest the path, defeating the goals of the RAC defense. Conversely, when the establishment of an arbitrary detour path is not guaranteed, more than 80% of critical flows we test have only a small number (e.g., three or less) of detour paths that can actually be established and disjoint from each other, which significantly restricts the available options for the reliable RAC operation. The first lesson of this study is that BGP-based rerouting solutions in the current inter-domain infrastructure seem to be impractical due to implicit assumptions (e.g., the invisibility of poisoning messages) that are unattainable in BGP's current practice. Second, we learn that the analysis of protocol specifications alone is insufficient for the feasibility study of any new defense proposal and, thus, additional rigorous security analysis and various network evaluations, including real-world testing, are required. Finally, our findings in this paper agree well with the conclusion of the major literature about end-to-end guarantees; that is, strong end-to-end availability should be a security feature of the Internet routing by design, not an ad hoc feature obtained via exploiting current routing protocols.