By focusing on the recent swing to the centralized approach by the software defined network (SDN), this paper presents a novel network architecture for refactoring the current distributed Internet protocol (IP) by not only utilizing the SDN itself but also implementing its cooperation with the optical transport layer. The first IP refactoring is for flexible network topology reconfiguration: the global routing and explicit routing functions are transferred from the distributed routers to the centralized SDN. The second IP refactoring is for cost-efficient maintenance migration: we introduce a resource portable IP router that can behave as a shared backup router by cooperating with the optical transport path switching. Extensive evaluations show that our architecture makes the current IP network easier to configure and more scalable. We also validate the feasibility of our proposal.

Although SDN provides desirable characteristics such as the manageability, flexibility and extensibility of the networks, it has a considerable disadvantage in its reliability due to its centralized architecture. To protect SDN-enabled networks under large-scale, unexpected link failures, we propose ResilientFlow that deploys distributed modules called Control Channel Maintenance Module (CCMM) for every switch and controllers. The CCMMs makes switches able to maintain their own control channels, which are core and fundamental part of SDN. In this paper, we design, implement, and evaluate the ResilientFlow.

Smart OSPF (S-OSPF), a load balancing, shortest-path-based routing scheme, was introduced to improve the routing performances of networks running on OSPF assuming that exact traffic demands are known. S-OSPF distributes traffic from a source node to neighbor nodes, and after reaching the neighbor nodes, traffic is routed according to the OSPF protocol. However, in practice, exact traffic demands are difficult to obtain, and the distribution of unequal traffic to multiple neighbor nodes requires complex functionalities at the source. This paper investigates non-split S-OSPF with the hose model, in which only the total amount of traffic that each node injects into the network and the total amount of traffic each node receives from the network are known, for the first time, with the goal of minimizing the network congestion ratio (maximum link utilization over all links). In non-split S-OSPF, traffic from a source node to a destination node is not split over multiple routes, in other words, it goes via only one neighbor node to the destination node. The routing decision with the hose model is formulated as an integer linear programming (ILP) problem. Since the ILP problem is difficult to solve in a practical time, this paper proposes a heuristic algorithm. In the routing decision process, the proposed algorithm gives the highest priority to the node pair that has the highest product of the total amount of injected traffic by one node and total amount of received traffic by the other node in the pair, where both traffic volumes are specified in the hose model, and enables a source node to select the neighbor node that minimizes network congestion ratio for the worst case traffic condition specified by the hose model. The non-split S-OSPF scheme's network congestion ratios are compared with those of the split S-OSPF and classical shortest path routing (SPR) schemes. Numerical results show that the non-split S-OSPF scheme offers lower network congestion ratios than the classical SPR scheme, and achieves network congestion ratios comparable to the split S-OSPF scheme for larger networks. To validate the non-split S-OSPF scheme, using a testbed network experimentally, we develop prototypes of the non-split S-OSPF path computation server and the non-split S-OSPF router. The functionalities of these prototypes are demonstrated in a non-split S-OSPF network.

Connection setup on various computer networks is now achieved by GMPLS. This technology is based on the source-routing approach, which requires the source node to store metric information of the entire network prior to computing a route. Thus all metric information must be distributed to all network nodes and kept up-to-date. However, as metric information become more diverse and generalized, it is hard to update all information due to the huge update overhead. Emerging network services and applications require the network to support diverse metrics for achieving various communication qualities. Increasing the number of metrics supported by the network causes excessive processing of metric update messages. To reduce the number of metric update messages, another scheme is required. This paper proposes a connection setup scheme that uses flooding-based signaling rather than the distribution of metric information. The proposed scheme requires only flooding of signaling messages with requested metric information, no routing protocol is required. Evaluations confirm that the proposed scheme achieves connection establishment without excessive overhead. Our analysis shows that the proposed scheme greatly reduces the number of control messages compared to the conventional scheme, while their blocking probabilities are comparable.

This letter proposes a scheme to update metrics without loops while minimizing routing instability time in an Open Shortest Path First (OSPF) network. The original OSPF network enters the transient state when metrics are being updated to improve the routing performance, and in this state packets may fall into loops. This may cause packet loss and inefficient network resource utilization. To avoid transient loops, a conventional scheme gives each router a priority that reflects the optimum time for metric update. However, when the updated metrics include both larger and smaller values than the preceding ones, two sequential updating processes, one for larger values and one for smaller values, are required. It takes time to converge on the final metric values in the conventional scheme, given that the interval time between the two processes is not insignificant. The second process starts only when the first process is confirmed to be completed. The interval time including the confirmation time and the time needed to reconfigure the metrics in all routers, lengthens the transient state duration; from several seconds to several tens of seconds. This causes routing instability. The proposed scheme transforms the set of updated metrics into an equivalent set of metrics that are either all larger or all smaller (if changed at all) than the ones before the update. The set of equivalent metrics yield exactly the same results in terms of routing as the conventional scheme, i.e. the result desired by the network operator. The non-mixture update requires only one updating process and so eliminates the interval time. Numerical results indicate that the probability that the proposed scheme can achieve non-mixture update is more than 67% in the networks examined.

A key traffic engineering problem in the Open Shortest Path First (OSPF)-based network is the determination of optimal link weights. From the network operators' point of view, there are two approaches to determining a set of link weights: Start-time Optimization (SO) and Run-time Optimization (RO). We previously presented a Preventive Start-time Optimization (PSO) scheme that determines an appropriate set of link weights at start time. It can counter both unexpected network congestion and network instability and thus overcomes the drawbacks of SO and RO, respectively. The previous work adopts a preventive start-time optimization algorithm with limited candidates, named PSO-L (PSO for Limited candidates). Although PSO-L relaxes the worst-case congestion, it does not confirm the optimal worst-case performance. To pursue this optimality, this paper proposes a preventive start-time optimization algorithm with a wide range of candidates, named PSO-W (PSO for Wide-range candidates). PSO-W upgrades the objective function of SO that determines the set of link weights at start time by considering all possible single link failures; its goal is to minimize the worst-case congestion. Numerical results via simulations show that PSO-W effectively relaxes the worst-case network congestion compared to SO, while it avoids the network instability caused by the run-time changes of link weights caused by RO. At the same time, PSO-W yields performance superior to that of PSO-L.

This paper presents an IP performance management system having the triple frameworks of performance measurement, topology monitoring and data analysis. The system infers the causal location of the performance degradation with a network tomographic approach. Since the Internet is still highly prone to performance deterioration due to congestion, router failure, and so forth, not only detecting performance deterioration, but also monitoring topology and locating the performance-degraded segments in real-time is vital to ensure that Internet Service Providers can mitigate or prevent such performance deterioration. The system is implemented and evaluated through a real-world experiment and its considerable potential for practical network operations is demonstrated.

With quick topology changes due to mobile node movement or signal fading in MANET environments, conventional routing restoration processes are costly to implement and may incur high flooding of network traffic overhead and long routing path latency. Adopting the traditional shortest path tree (SPT) recomputation and restoration schemes used in Internet routing protocols will not work effectively for MANET. An object of the next generation SPT restoration system is to provide a cost-effective solution using an adaptive learning control system, wherein the SPT restoration engine is able to skip over the heavy SPT computation. We proposed a novel SPT restoration scheme, called Cognitive Shortest Path Tree Restoration (CSPTR). CSPTR is designed based on the Network Simplex Method (NSM) and Sensitivity Analysis (SA) techniques to provide a comprehensive and low-cost link failure healing process. NSM is used to derive the shortest paths for each node, while the use of SA can greatly reduce the effort of unnecessary recomputation of the SPT when network topology changes. In practice, a SA range table is used to enable the learning capability of CSPTR. The performance of computing and communication overheads are compared with other two well-known schemes, such as Dijstra's algorithm and incremental OSPF. Results show that CSPTR can greatly eliminate unnecessary SPT recomputation and reduce large amounts of the flooding overheads.

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.

It is important to monitor routing protocols to ensure IP networks and their operations can maintain sufficient level of stability and reliability because IP routing is an essential part of such networks. In this paper, we focus on Open Shortest Path First (OSPF), a widely deployed intra-domain routing protocol. Routers running OSPF advertise their link states on Link State Advertisements (LSAs) as soon as they detect changes in their link states. In IP network operations, it is important for operators to ascertain the location and type of a failure in order to deal with failures adequately. We therefore studied IP network failure identification based on the monitoring of OSPF LSAs. There are three issues to consider in regard to identifying network failures by monitoring LSAs. The first is that multiple LSAs are flooded by a single failure. The second is the LSA delay, and the third is that multiple failures may occur simultaneously. In this paper, we propose a method of network failure identification based on a detailed analysis of OSPF LSA flooding that takes into account the above three issues.

In the Next-Generation Network (NGN), accommodating a wide variety of customer networks through virtual private network (VPN) technologies is one of the key issues. In particular, a core network provider has to provide bandwidth-assured and secured data transmission for individual private networks while performing optimal and flexible path selection. We present hierarchically distributed path computation elements (HDPCEs) that enable a virtual private network (VPN) provider to guarantee end-to-end required bandwidth and to maintain the secrecy of the link-state information of each customer from other customers. In previous studies, a VPN provider only considered link states in the provider network and did not consider customer domains connected by the provider network. HDPCEs, which are distributed to customer domains, communicate with an HDPCE for the provider network, and these HDPCEs enable the guarantee of necessary bandwidth for a data transmission from one customer domain to another via a provider network. We propose a new path-selection algorithm in each HDPCE and cooperation scheme to interwork HDPCEs, which are suitable for VPN requirements. In the evaluation, the superiority of HDPCE-based VPN path selection over legacy OSPF-TE-based VPN path selection is demonstrated in two typical VPN models: the dedicated model and shared model.

Recently, integration of multiple network domains, such as optical fiber domains and packet domains, has been required by network providers and users. To achieve this interdomain integration, generalized multiprotocol label switching (GMPLS) is now receiving more attention. One of the main features of a GMPLS network is its multilayered complexity, which sometimes places a large burden on source GMPLS routers to determine optimal routes to destinations in other domains and causes label switching path (LSP)-setup delays. To reduce this source-router burden, we propose hierarchically distributed path computation equipment (HDPCE) that cooperates with each other to determine interdomain routes, reduce setup delay, and conduct flexible interdomain route creation taking individual GMPLS domain routing policies into consideration. Each domain routing policy can be set independently from that of other domains, and this routing information is not revealed to other peer domains because each HDPCE is allocated to every domain, including an interdomain, which has several underlying domains under it. Each underlying domain's HDPCE flexibly chooses three types of routing policies depending on the domain's requirement, and the interdomain HDPCE conducts interdomain route creation in accordance with underlying domain policies. OSPF routing protocol is now being applied to interdomain routing on GMPLS networks. Therefore, we compare the proposed HDPCE-based interdomain route creation with OSPF-based route creation in terms of performance and applicability, and we evaluate the effects of each underlying domain policy on interdomain route creation.

Multilayered network interaction among various networks such as IP/MPLS packet networks and optical fiber networks are now achieved using generalized multiprotocol label switching (GMPLS) technology. One unique feature of GMPLS networks is that GMPLS packet-layer label switching paths (LSPs), such as IP/MPLS LSPs, sometimes tunnel through GMPLS lower layer LSPs such as optical fiber/lambda LSPs. One problem that occurs in this situation is protecting an important primary packet LSP by using a protection LSP that is physically separated from the primary LSP. The packet router has difficulty recognizing lower layer LSPs that are totally disjointed from the primary LSP. This is because, in a GMPLS's packet layer, a source router only differentiates one lower layer LSP from another, and does not check the disjointedness of segments through which the lower layer path passes. Sometimes, different lower LSPs pass through the same optical fiber, and a malfunction of one optical fiber sometimes causes many lower layer LSPs to malfunction at the same time. To solve this problem, a shared risk link group (SRLG) is introduced. Network links that belong to the same SRLG share a common physical resource. We apply this SRLG to the proposed hierarchically distributed path computation elements (HDPCEs) and achieve effective disjointed SRLG protection for important primary GMPLS packet paths.

The demand for intra- and interdomain routing for multilayered networks such as those using generalized multiprotocol label switching (GMPLS) is strong. One of the features that is peculiar to GMPLS networks is that because several different domains, such as those of IP, ATM, and optical fiber, are combined with each other hierarchically, various routing policies, which are sometimes independent from underlying domains and sometimes taking the underlying domains' policies into consideration, are required. For example GMPLS's lower layer LSPs like lambda LSP are expected to be established independently before the upper-layer LSPs, like IP and MPLS LSPs, are established in the underlying domains. Another requirement for the GMPLS interdomain routing is lightening the burden for selecting the interdomain route, because there are a lot of demands to interconnect many GMPLS domains. In order to satisfy these demands, we propose a path computation server (PCS) that is special for the intra/interdomain routing of GMPLS networks. As a counterpart of the proposed interdomain routing, it is now becoming popular to apply OSPF to the GMPLS interdomain routing. Therefore, we compared the proposed interdomain routing with OSPF, and show the applicability of the routing to GMPLS networks.

The next-generation IP-based mobile network must accommodate various kinds of wireless access technologies, including W-CDMA. Although the soft handover technique should be supported if W-CDMA is used, redundant paths will be created by the soft handover scheme employed by the 3rd generation mobile communication system. This paper proposes the Network Distributed Soft Handover (NDSHO) method, which achieves soft handover control in an IP network but without any redundant paths. NDSHO continuously optimizes all routing paths by relocating the data copy point dynamically during communication according to the movement of the mobile terminal. To achieve the proposed method, this paper introduces a copy point seamless relocation method and an optimal point selection method which takes advantage of OSPF. Furthermore, we show quantitatively that NDSHO makes more efficient use of system resources than the 3rd generation system.

The cost-effective provision of IP services requires multi-layered traffic engineering to obtain dynamic cooperation between IP and photonic layers. The effective control and management of generalized multi-protocol label-switching (GMPLS) networks is an essential part of this. Huge photonic capacities and the number of IP and photonic networks make it likely that enormous amounts of GMPLS network-related data will have to be managed in the near future. At the same time, routing burdens on individual GMPLS routers are critical because of the strong need for per-path quality of service (QoS). To solve these problems, we propose a hierarchically distributed network-management system (NMS) in which we flexibly allocate a GMPLS subnetwork to each sub-NMS and at the same time conduct QoS routing. The distributed nature of our architecture reduces the burden on the NMS as a whole and also lets us remove the routing-burden from GMPLS routers with minimum effect on management processes.

Because most link-state routing protocols, such as OSPF and IS-IS, calculate routes using the Dijkstra algorithm, which poses scalability problems, implementors often introduce an artificial delay to reduce the number of route calculations. Although this delay directly affects IP packet forwarding, it can be acceptable when the network topology does not change often. However, when the topology of a network changes frequently, this delay can lead to a complete loss of IP reachability for the affected network prefixes during the unstable period. In this paper, we propose the Cached Shortest-path Tree (CST) approach, which speeds up intra-domain routing convergence without extra execution of the Dijkstra algorithm, even if the routing for a network is quite unstable. The basic idea of CST is to cache shortest-path trees (SPTs) of network topologies that appear frequently, and use these SPTs to instantly generate a routing table when the topology after a change matches one in the caches. CST depends on a characteristic that we found from an investigation of routing instability conducted on the WIDE Internet in Japan. That is, under unstable routing conditions, both frequently changing Link State Advertisements (LSAs) and their instances tend to be limited. At the end of this paper, we show CST's effectiveness by a trace-driven simulation.

We evaluate resolution models for resource allocation in a GMPLS distributed control plane for heterogeneous all-optical networks. In a practical regional-to-backbone network environment, the local resolution model is advantageous in resource utilization, protocol compatibility and scalability. We demonstrate a lookup procedure, which inter-works with OSPF-TE and RSVP-TE protocols and allocates resources in the local resolution model.