J Comb Optim
total length in the given network as shown in Fig. 1b. Using these algorithms, we will
find the first path P
1
= (s, a, b, c, t) and the second path P
2
= (s, d, e, h, t) as shown
by bold lines in Fig. 1b. The total length of P
1
and P
2
is 4 + 13 = 17. However, the
optimal answer should be P
1
= (s, d, e, c, t) and P
2
= (s, a, f, g, t) with total length
6 + 6 = 12.
We propose a distributed algorithm in Zhang et al. (2014) to find k disjoint s ∼ t
paths with correctness guarantee. However, the algorithm does not consider the length
of the routing paths.
In Srinivas and Modiano (2003), Bhandari (1997), Khuller and Schieber (1989),
Iwama et al. (1997), Suurballe (1974), Chen et al. (2004), and Hashiguchi et al. ( 2011),
some centralized algorithms are given for finding k disjoint s ∼ t paths in graphs.
All these algorithm guarantee correctness. In particular, the algorithms in Srinivas
and Modiano (2003), Bhandari (1997), Suurballe (1974), and Hashiguchi et al. (2011)
will find k disjoint s ∼ t paths with the minimum total length, i.e., they guarantee
both correctness and optimality. If we use these centralized algorithms to solve the
MIN-k-DP problem in a WSN, we have to collect every node’s neighbor list to build
the topology of the whole network. The communication cost of such operation is very
high for a large-scale network.
The works in Bley (2003),
Itai et al. (1982), and Ronen and Perl (1984) consider the
problem of finding the maximum number of length-bounded disjoint s ∼ t paths in
graphs. A length-bounded path is a path whose length is no more than a user-specified
threshold l.InItai et al. (1982), this problem is proven to be NP-Hard for l ≥ 5. In Bley
(2003), this problem is further proven to be APX-Complete for l ≥ 5, which means
that there is no Polynomial Time Approximation Scheme (PTAS) for this problem.
A heuristic algorithm for this problem is given in Ronen and Perl (1984) without
approximation bound.
So far, all the existing algorithms which can solve the MIN-k-DP problem with
correctness and optimality guarantee are centralized algorithms. These centralized
algorithms need to build the topology of the whole network, which makes them not
applicable in WSNs. On the other hand, all the distributed algorithms can not guarantee
correctness or optimality. Therefore, we have two challenges to solve the MIN-k-
DP problem in WSNs: (1) to guarantee correctness and optimality, (2) to maintain
efficiency.
3 Preliminaries
The given sensor network can be expressed as a weighted undirected graph G =
(V, E), where V ={v | v is a sensor node }, E ={(u,v) | there is a wireless link
between u ∈ V and v ∈ V }. Function l : E → R
+
defines the lengths of the edges in
E, which denotes the energy cost or transmission delay of the corresponding links. We
assume that all the links are bidirectional, i.e., u can hear v if v can hear u. Hereinafter,
we do not distinguish the terms “network” and “graph”. For simplicity, we use l(u,v)
rather than l((u,v))to denote the length of edge (u,v).
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