Protocols for Secure Communication in Wireless Sensor Networks

132 Chapter 5. Multipath Communication
separation may prove more relevant when a trade-off between cost and security
is to be found, for example.
Most algorithms for constructing multiple paths treat every pair of sender
and receiver separately. Since there is substantial overhead required for constructing
and maintaining multiple paths, this does only pay off for longerlasting
communication relationships between nodes. In WSNs, there are usually
many node pairs exchanging messages at the same time, and communication
relationships are mostly transient. Conventional multipath schemes do not
amortize set-up overhead over all node pairs, and are inefficient for short-term
connections.
There are many possible metrics that could be used for evaluating and selecting
paths. Especially in dynamic (e.g. mobile) networks, it is important
that these metrics can be efficiently evaluated. There is a trade-off between the
overhead induced by the metric evaluation and the penalty that must be paid for
using non-optimal paths. Our attacker model suggests that spatially separated
paths are desirable as such paths can circumvent areas that are under attack.
For efficiency reasons, we would like to amortize the set-up and maintenance
overhead over many communication relationships. Current approaches mainly
emphasize the set-up of an alternate path in case the currently used path fails.
Usually, the replacement path is very close to the replaced path, which means
that most nodes are shared between both. In terms of our attacker model, this
means that if one path is being compromised, the replacement path is probably
also compromised.
The idea for setting up multiple paths we present in this work is based on
the concept of spanning trees, which is fundamental for shortest path construction
(cf. Dijkstra’s algorithm, described in [44]) and multicast. A multicast
tree can be shared by all senders that are located on the tree, thereby reducing
the overhead compared to a separate tree for each sender [12]. We use the
same principles for end-to-end communication. Messages are routed based on
the paths determined by spanning trees, and all senders use the same set of
spanning trees. This leads to suboptimal path lengths, but using different trees
simultaneously provides spatially separated paths.
5.2 Routing on Spanning Trees
5.2.1 The Basic Scheme
We propose a multipath communication scheme that adequately addresses the
requirements of wireless sensor networks. It uses spanning trees as routing