Use ’em or lose ’em: On unidirectional links in reactive routing protocols

Abstract

In reactive unicast routing protocols, Route Discovery aims to include only bidirectional links in discovered routing paths. This is typically accomplished by having routers maintain a “blacklist” of links recently confirmed (through Route Reply processing) to be unidirectional – which is then used for excluding subsequent Route Discovery control messages received over these links from being processed and forwarded.This paper first presents an analytical model, which allows to study the impact of unidirectional links being present in a network, on the performance of reactive routing protocols. Next, this paper identifies that despite the use of a “blacklist”, the Route Discovery process may result in discovery of false forward routes, i.e.,routes containing unidirectional links – and proposes a counter-measure denoted Forward Bidirectionality Check. This paper further proposes a Loop Exploration mechanism, allowing to properly include unidirectional links in a discovered routing topology – with the goal of providing bidirectional connectivity even in absence of bidirectional paths in the network.Finally, each of these proposed mechanisms are subjected to extensive network simulations in static scenarios. When the fraction of unidirectional links is moderate (15–50%), simulations find Forward Bidirectionality Check to significantly increase the probability that bidirectional routing paths can be discovered by a reactive routing protocol, while incurring only an insignificant additional overhead. Further, in networks with a significant fraction of unidirectional links ( ≥ 50%), simulations reveal that Loop Exploration preserves the ability of a reactive routing protocol to establish bidirectional communication (possibly through non-bidirectional paths), but at the expense of a substantial additional overhead.