Takuya Yoshihiro

Single backup table schemes for shortest-path routing

We introduce a new recovery scheme that needs only one extra backup routing table for networks employing shortest-path routing. By precomputing this backup table, the network recovers from any single link failure immediately after the failure occurs. To compute the backup routing table for this scheme, we use an almost linear time algorithm to solve the {r, v}-problem, which is a variation of the best swap problem presented by Nardelli et al. We further show that the same solution can be computed in exactly linear time if the underlying graph is unweighted.

Avoiding Routing Loops on the Internet

AbstractSuppose that some particular link in the Internet is currently congested. A natural solution is to try to make packets bypass that link. This can be done by increasing the cost of that link intentionally, say from a1 to a2, since the Internet uses shortest-path routing. Unfortunately, however, this often causes temporary loops for packet traveling, called routing loops. In this paper we show that routing loops can be avoided by increasing the cost of the link not directly from a1 to a2 but through an intermediate value, a3, i.e., from a1 to a3 and then to a2. We may need several intermediate values. We show that in this case the greedy strategy, namely, raising the cost as much as possible in each step, is optimal.

A Single Backup-Table Rerouting Scheme for Fast Failure Protection in OSPF

We propose a practical link protection scheme, called Single Backup-table Rerouting, (SBR) as an extension for Open Shortest Path First (OSPF). SBR protects against any single link failure as soon as the failure occurs if the topology of every area in OSPF is two-link-connected. An efficient algorithm to compute a set of backup tables is provided for networks with symmetric link costs. The foremost feature of SBR is that the backup process is fully distributed, so no message exchange is required and the modification of OSPF is minor. OSPF is extended with the following: only one extra backup routing table, a 2-bit flag at each traffic packet, and a process for handling the backup table. There are no changes to the message format of OSPF. In this paper, we present the practical link protection scheme by fitting SBR into several OSPF specific mechanisms such as OSPF areas, Equal Costs Multipath (ECMP), and virtual links with proofs of their correctness. Furthermore, together with a loop-free routing technique for link-state routing, SBR guarantees the consistency of every route against a single link failure, even during the path recomputation phase, until it converges to the new shortest paths.

A Probability-based Evolutionary Algorithm with Mutations to Learn Bayesian Networks

Bayesian networks are regarded as one of the essential tools to analyze causal relationship between events from data. To learn the structure of highly-reliable Bayesian networks from data as quickly as possible is one of the important problems that several studies have been tried to achieve. In recent years, probability-based evolutionary algorithms have been proposed as a new efficient approach to learn Bayesian networks. In this paper, we target on one of the probability-based evolutionary algorithms called PBIL (Probability-Based Incremental Learning), and propose a new mutation operator. Through performance evaluation, we found that the proposed mutation operator has a good performance in learning Bayesian networks.

Loop-Free Link Stability Metrics for Proactive Routing in Wireless Ad Hoc Networks

To improve communication quality in wireless ad hoc networks, it is definitely important to select stable links as communication paths. To this end, various mobility metrics are presented in the literature. However, the routing loop problem, which is inevitably involved in link-state proactive routing scheme, has not been discussed well. In this paper, we propose a simple dynamic metrics in respect of link stability, which is loop-free throughout dynamic metric transition. Through a theoretical analysis of loop-freeness, we present our new strategy to achieve loop-freeness with sufficiently low cost for ad hoc networks.

Collision-Free Channel Assignment Is Possible in IEEE802.11-Based Wireless Mesh Networks

IEEE802.11-based Wireless Mesh Networks (WMNs) has been deeply studied as a low-cost wireless mesh networks. However, they still suffer from severe interference due to hidden terminals, which significantly degrades the performance to an impractical level. Several studies tried to use multiple frequency channels to avoid collisions among wireless links. However, static channel assignment requires so many orthogonal channels to achieve collision-freedom, resulting in that we still suffer from severe collisions as long as we use the well-populated IEEE802.11 standards. In this paper, we propose a new static channel assignment scheme CASCA (CSMA-aware Static Channel Assignment) that achieves collision-free channel assignment under 3-4 available channels by incorporating a CSMA-aware interference model as well as partially a routing functionality in it. CASCA enables us to realize collision-free WMNs using the license-free IEEE802.11 devices.

Practical fast scheduling and routing over slotted CSMA for wireless mesh networks

Although Wireless Mesh Networks (WMNs) over CSMA and TDMA MAC have been extensively studied, CSMA still suffers from heavy interference due to hidden terminals, while TDMA has a practical difficulty in accurate time synchronization. Slotted CSMA, in which we assume that CSMA runs within relatively large time-divided slots, could be one of the solutions since it works on loosely synchronized time slots and simultaneously avoids major collision under a properly designed feasible schedule. However, if we apply a schedule generally used in TDMA, there are several inconveniences: First, we need heavy overhead of centralized computation or negotiation among nodes in distributed scheduling. Second, end-to-end packet delivery delay grows large since TDMA requires a large number of slots to achieve collision-free scheduling. In this paper, we propose a new scheduling algorithm for slotted CSMA called CATBS (CSMA-Aware Time-Boundable Scheduling) that requires a smaller number of slots for feasible scheduling, and runs within a bounded time. Incorporating the scheduling, we design a routing protocol that enables flexible paths recomputation with small communication overhead. We consequently achieve a new fully distributed and robust network architecture of WMNs that provides practically high throughput and low delivery delay, which can be implemented on the off-the-shelf IEEE802.11 interfaces.

Combining local channel selection with routing metrics in multi-channel Wireless Mesh Networks

Dynamic metric is a well-known method to improve performance of WMNs (Wireless Mesh Networks). By quantifying quality of links, and by computing optimal paths based on the values, dynamic metrics enable packets to be forwarded with high-quality links while avoiding low-quality links. ETX [1] would be the best-known dynamic metric, which measures the average retransmission count on a link via periodically transmitted probe packets. Many dynamic metrics have been proposed so far, which include those for multi-channel WMNs such as WCETT [2] and MIC [3]. They proved that dynamic metrics improve network performance in multi-channel environments by pursuing relatively slow transition of link quality.

Reducing Routing Loops under Dynamic Metrics in Wireless Mesh Networks

Many dynamic metrics for wireless mesh networks (WMNs) to improve communication quality are proposed so far. However, all of them have possibility to create temporary routing loops which may cause severe congestion and disruption of communications. In order that WMNs are regarded as a reliable infrastructure of the next generation, such routing loops are strongly desired to be liminated since they bring the possibility of ervice disruption. In this paper, we propose a mechanism called Loop-free Metric Range (LMR) to make existing dynamic metrics loop-free by restricting the range of metric values to change. LMR is applicable to a major part of existing etrics such as ETX, ETT, MIC, and so on without any message overhead. We show the mechanisms of LMR, theoretical results on the range of loop-free etric values, and simulation results on the effectiveness of LMR.

A queue-length based distributed scheduling for CSMA-driven Wireless Mesh Networks

Although Wireless Mesh Networks using CSMA MAC protocol (CSMA-driven WMNs) have been extensively studied, severe collisions mainly due to hidden terminals prevent us from realizing practical networks. In this paper, we propose a new distributed scheduling scheme for CSMA-driven WMNs that significantly reduce collision caused by hidden terminals. In our scheduling, we propagate queue-length information within two-hop neighborhood via data and Ack frames, and avoid collision by allowing transmission to only the node that has the longest queue length among its two-hop neighbors. Our scheduling scheme is fully distributed and requires low overhead so that it works within the framework of CSMA. Through evaluation, we found that the proposed scheduling mechanism significantly reduces the number of collisions, resulting in significant improvement of communication quality of CSMA-driven WMNs.