REPORTS ON SYSTEMS AND COMMUNICATIONS Opportunistic Information Dissemination in Mobile Ad-hoc Networks: The Profit of Global Synchrony

The topic of this paper is the study of Information Dissemination in Mobile Ad-hoc Networks by means of deterministic protocols. In this problem, a piece of information initially held by a source node has to be propagated to a given set of destination nodes. We consider multihop dynamic networks, in which n nodes move in the plane, join, and leave. Our connectivity model only requires that, within α time slots, some node that has the information must be connected to some node without it for at least β time slots. In this context, the protocols considered make use of opportunistic communication during this connectivity interval to make progress in the dissemination. The protocols studied are classified into three classes: oblivious (the transmission schedule of a node is only a function of its ID), quasi-oblivious (the transmission schedule may also depend on a global time), and adaptive. A collection of interesting complexity gaps among protocol-classes is observed in this work. In order to guarantee any progress towards solving the problem, it is shown that β must be at least n − 1 in general, but that this bound becomes β ∈ Ω(n/ log n) if an oblivious protocol is used. Since quasi-oblivious protocols can guarantee progress with β ∈ O(n), this represents a significant gap, almost linear on β, between oblivious and quasi-oblivious protocols. Regarding the time to complete the dissemination, a lower bound of Ω(nα+n/ log n) is proved for oblivious protocols, while a constructive O(nα + n log n) upper bound is shown for the same class (which is tight up to a polylogarithmic factor). It is also proved that quasi-oblivious protocols achieve O(nα + n), which is shown to be optimal by giving a matching lower bound for adaptive protocols. These results show that the gap in time complexity between oblivious and quasi-oblivious, and hence adaptive, protocols is almost linear. This gap is what we call the profit of global synchrony, since it represents the gain the network obtains from global synchrony with respect to not having it. We note that the global synchrony required by the efficient quasioblivious protocol proposed is simply achieved by piggybacking in the messages sent the time at the source node, as a global reference. ⋆ Research partially supported by Comunidad de Madrid grant S2009TIC-1692 and Spanish MICINN grant TIN2008-06735-C02-01. ⋆⋆ Research partially supported by ANR VERSO Shaman project. ⋆ ⋆ ⋆ Research partially supported by EU Marie Curie European Reintegration Grant IRG 210021; NSF grant CCF 0632838; and Comunidad de Madrid grant S-0505/TIC/0285.