Anypath routing

In many networks, it is less costly to transmit a packet to any node in a set of neighbors than to one specific neighbor. A well-known instance is with unreliable wireless links, where the probability that at least one node out ofnreceives a packet increases with n. This observation was previously exploited, by modifying single-path routing to assign to each node group of candidate next-hops for a particular destination. However, single-path metrics not do reflect the cost of forwarding when a sender has multiple candidate relays, and they result in routing decisions that are in many cases suboptimal. This dissertation addresses the shortest anypath routing problem: how to assign a set of candidate relays at each node for a given destination, such that the cost of forwarding a packet to the destination is minimized. The key is the following tradeoff: on the one hand, increasing the number of candidate relays decreases the forwarding cost, but on the other, this increases the likelihood of "veering" away from the most direct trajectory. Solving the shortest anypath problem requires us first to formalize the notions of anycast link cost and remaining path cost, that generalize the link cost and path cost of single-path routing. Unlike with single-path routing, a packet can travel across an anypath route in many different ways; the cost of this route is naturally defined as the expected cost of all possible traversals. We introduce an algorithm that provably computes the shortest anypath route between each node and a destination. It is based on the Bellman-Ford algorithm and so is amenable to implementation in a distributed setting. This algorithm works for all physical cost metrics; we show that there exist certain "non-physical" cost metrics under which the shortest anypath route may contain cycles. We also explore the interaction between the relay selection policy, that is, the way in which the effective relay is chosen among many receivers, and the cost of optimal routes. We also explore the robustness of anypath routes, and find that they are significantly more stable in the face of topology changes and imperfect information than are single-path and multipath routes. The principles of anypath routing are general and can be applied in many settings. Our application focus in this dissertation is on low-power, low-rate wireless communication in embedded wireless networks. We introduce novel ways in which low-power link-layers can take advantage of anycast forwarding to reduce transmission energy or latency. We describe the design and implementation of a complete anypath routing protocol; evaluation on a 50-node wireless testbed demonstrates that anypath routing is robust, stable, and increases energy efficiency of low-power nodes by a significant factor over the equivalent system using single-path routing. Finally, we describe a novel error-correctionmechanism for multi-hop wireless communication based on packet combining. Packet combining allows to exploit corrupt packets received at different nodes by jointly decoding independent, noisy versions of an original packet.

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