Time Complexity of Link Reversal Routing
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abstract
Link reversal is a versatile algorithm design paradigm, originally proposed by Gafni and Bertsekas in 1981 for routing and subsequently applied to other problems including mutual exclusion, leader election, and resource allocation. Although these algorithms are well known, until now there have been only preliminary results on time complexity, even for the simplest link reversal algorithm for routing, called Full Reversal. In Full Reversal, a sink reverses all its incident links, whereas in other link reversal algorithms (e.g., Partial Reversal), a sink reverses only some of its incident links. Charron-Bost et al. introduced a generalization, called LR, that includes Full and Partial Reversal as special cases. In this article, we present an exact expression for the time complexity of LR. The expression is stated in terms of simple properties of the initial graph. The result specializes to exact formulas for the time complexity of any node in any initial acyclic directed graph for both Full and Partial Reversal. Having the exact formulas provides insight into the behavior of Full and Partial Reversal on specific graph families. Our first technical insight is to describe the behavior of Full Reversal as a dynamical system and to observe that this system is linear in min-plus algebra. Our second technical insight is to overcome the difficulty posed by the fact that LR is not linear by transforming every execution of LR from an initial graph into an execution of Full Reversal from a different initial graph while maintaining the execution's work and time complexity.
