MAC-Layer Anycasting in Ad Hoc Networks

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INTRODUCTION
Wireless Ad hoc networks are infrastructureless multi-hop
networks in which nodes behave as mobile routers. Routing
protocols attempt to choose “optimal” routes based on some optimality
criteria (e.g., number of hops). However, in the process
of selecting an optimal route, the routing protocol is often faced
with the decision to choose between two equally good routes.
Ties are often broken randomly. MAC-layer anycasting is a proposal
that aims to utilize the knowledge of instantaneous channel
condition in selecting the suitable downstream neighbor on
shorter time scales. The observation that routes chosen by the
network layer are “optimal” on a longer time scale, and ignores
the possibility of transient variations in page link conditions, motivates
our work on MAC-layer anycasting.



II. PRELIMINARIES
Routing protocols can be broadly classified into “sourcerouted”
or “table-driven” protocols [1],[2]. In source routing
[3], the sender of a packet completely specifies the route that
the packet must traverse to reach its final destination. Johnson
et al. proposed dynamic source routing (DSR) [3] in which the
sender node floods a route request (RREQ) probe in search of
a route to the destination. Intermediate nodes that forward this
request probe, append their identifiers to the probe. The probe
that arrives first at the destination is assumed to have arrived on
the optimal path. DSR uses this path for subsequent communication.



III. MAC-LAYER ANYCASTING
MAC-layer anycasting can be envisioned as an enhancement
to existing MAC and routing protocols. In the rest of this paper,
we would call a routing protocol “basic” if it has not been
“enhanced” with the anycasting features. One possible architecture
to implement MAC-layer anycasting is shown in Figure 1.
This section discusses the framework of MAC layer anycasting,
in the context of a generic MAC and routing protocol. We also
propose a simple variation, named Ordered Anycasting2.



IV. APPLICATIONS OF ANYCASTING
This section discusses the shortcomings of existing MAC and
routing protocols. We investigate example scenarios to understand
consequences of these shortcomings, and show how anycasting
can be useful.



VI. CONCLUSION
We propose MAC-layer anycasting for ad hoc wireless networks.
The network layer specifies multiple downstream nodes,
from which the MAC layer chooses a suitable node, based on
instantaneous network conditions. We believe that MAC-layer
anycasting can be combined with different routing schemes – the
routing protocol can form the anycast group based on its metric
of choice. Anycasting can then be applied to this anycast group.
We illustrate cases in which anycasting may offer performance
gain. We discussed some specifics of implementation and discussed
the performance tradeoffs that arise due to MAC-layer
anycasting. Evaluating the performance of anycasting through
simulations is a topic for future work.