The next technology after 5G, i.e., Beyond 5G, has been expected to realize sustainable networks for creating new values in our society by adding new functionalities (i.e., ultra-low power consumption, ultra-security and reliability, autonomy, and scalability) to the 5G features (i.e., high speed and high capacity, low latency, and multiple simultaneous connections). Network functions virtualization (NFV) plays a key role to realize Beyond 5G. NFV is a new paradigm to achieve flexible and agile network services by decoupling network functions from proprietary hardware and running them on generic hardware as virtual network functions (VNFs). One of the challenging issues of NFV networking is optimal resource allocation where physical resources (e.g., physical servers and links) are appropriately allocated to a logical network (i.e., slice) or logical path (i.e., service path) according to users' demand. In addition, adaptability to spatio-temporal changes of users' demand and resiliency against failures of equipment/software are also important issues. In this research, we tackle these issues by combining mathematical methods (i.e., mathematical optimization and graph neural networks) and practical technologies (e.g., cloud-native network function (CNF)).

1. Practicality of In-Kernel/User-space Packet Processing Empowered by Lightweight Neural Network and Decision Tree
   Takanori Hara, Masahiro Sasabe.
   Computer Networks, 240(110188), pp.1-18, February 2024 (accepted on 7 January 2024). [publisher's version]
2. Capacitated Shortest Path Tour Based Service Chaining Adaptive to Changes of Service Demand and Network Topology
   Takanori Hara, Masahiro Sasabe.
   IEEE Transactions on Network and Service Management, pp.1-15, January 2024 (accepted on 6 January 2024). [publisher's version]
3. Resource-Efficient and Availability-Aware Service Chaining and VNF Placement with VNF Diversity and Redundancy
   Takanori Hara, Masahiro Sasabe, Kento Sugihara, Shoji Kasahara.
   IEICE Transactions on Communications, E107-B(1), pp.105-116, January 2024 (accepted on 28 September 2023). [publisher's version]
4. Speedy and Efficient Service Chaining and Function Placement Based on Lagrangian Heuristics for Capacitated Shortest Path Tour Problem
   Takanori Hara, Masahiro Sasabe.
   Journal of Network and Systems Management, 31(24), pp.1-34, December 2022 (accepted on 21 December 2022). [paper (941KB pdf)] [publisher's version]
5. Capacitated Shortest Path Tour Problem Based Integer Linear Programming for Service Chaining and Function Placement in NFV Networks
   Masahiro Sasabe, Takanori Hara.
   IEEE Transactions on Network and Service Management, 18(1), pp.104-117, March 2021 (accepted on 8 December 2020). [paper (2MB pdf)] [publisher's version]

6. On Practicality of Kernel Packet Processing Empowered by Lightweight Neural Network and Decision Tree
   Takanori Hara, Masahiro Sasabe.
   Proc. of 14th International Conference on Network of the Future, October 2023 (accepted on 3 July 2023). [publisher's version]
7. Deep Reinforcement Learning with Graph Neural Networks for Capacitated Shortest Path Tour Based Service Chaining
   Takanori Hara, Masahiro Sasabe.
   Proc. of 18th International Conference on Network and Service Management (CNSM), pp.19-27, October 2022 (acceptance rate: 0.19 (18/93)). [paper (985KB pdf)] [publisher's version]
8. Lagrangian Heuristics for Capacitated Shortest Path Tour Problem Based Online Service Chaining
   Takanori Hara, Masahiro Sasabe.
   Proc. of IEEE/IFIP Network Operations and Management Symposium (NOMS), pp.1-9, April 2022 (acceptance rate: 0.25 (43/171)). [paper (463KB pdf)] [publisher's version]
9. Shortest Path Tour Problem Based Integer Linear Programming for Service Chaining in NFV Networks
   Masahiro Sasabe, Takanori Hara.
   Proc. of 6th IEEE Conference on Network Softwarization (NetSoft), pp.114-121, June 2020 (acceptance rate: 0.20 (18/95)). [paper (4MB pdf)] [publisher's version]

We have been supported by various social systems, e.g., the Internet and transportation networks. It is natural for individuals to desire better quality of experience (QoE), e.g., watching high quality video without delay and jitter, and faster travel to the destination. On the other hand, the resources of systems are limited. For example, in case of content distribution services, such resources can be the computation capacity of servers and network capacity. From the viewpoint of the overall systems, global optimization, e.g., improving the total QoE among all users, under the resource constraint is important. In this research, we aim to achieve the selfish yet optimal control, which can achieve the global optimization even under users' selfish decision making by appropriately controlling the perceived information at individuals.

1. Multi-Agent Distributed Route Selection under Consideration of Time Dependency among Agents' Road Usage for Vehicular Networks
   Takanori Hara, Masahiro Sasabe, Shoji Kasahara.
   IEICE Transactions on Communications, E105-B(2), pp.140-150, February 2022 (accepted on 14 July 2021). [paper (1MB pdf)] [publisher's version]
2. Selfish Yet Optimal Routing by Adjusting Perceived Traffic Information of Road Networks
   Takanori Hara, Masahiro Sasabe, Shoji Kasahara.
   IEEE Open Journal of Intelligent Transportation Systems, 1, pp.120-133, September 2020 (accepted on 24 August 2020). [publisher's version]

3. Distributed Route Selection under Consideration of Time Dependency among Agents' Road Usage for Vehicular Networks
   Takanori Hara, Masahiro Sasabe, Shoji Kasahara.
   Proc. of International Conference on Emerging Technologies for Communications (ICETC), pp.1-4, December 2020 (Best Paper Award). [paper (594KB pdf)] [publisher's version]

Software update and video streaming require massive and/or long-term content distribution. Specifically, new release of content causes steep increase of requests, making the distribution server a bottleneck. To tackle this problem, several systems, e.g., Windows update, recently apply peer-to-peer (P2P) file distribution where clients called peers upload retrieved fragments of the whole content, i.e., pieces, to other peers. However, some peers will not be willing to upload pieces to others, which are called free riders, due to communication overhead. tit-for-tat (TFT) strategy in game theory can suppress such free riding behavior by encouraging equivalent exchange of pieces among each pair of peers. In this research, we analyze the optimal design of the TFT-based P2P file distribution through mathematical optimization (e.g., linear optimization (programming) and integer linear optimization) and analytical models (e.g., flood model).

1. Analysis of Minimum Distribution Time of Two-Class Tit-for-Tat-Based P2P File Distribution
   Masahiro Sasabe, Masaki Kiyomitsu.
   Computer Networks, 214, pp.109142: 1-16, September 2022 (accepted on 28 June 2022). [paper (2MB pdf)] [publisher's version]
2. Analysis of Minimum Distribution Time of Tit-for-Tat-Based P2P File Distribution: Linear Programming Based Approach
   Masahiro Sasabe.
   Peer-to-Peer Networking and Applications, 14(4), pp.2127-2138, July 2021 (First Online: 17 May 2021). [paper (1MB pdf)] [publisher's version]
3. Optimality Analysis of Locality-Aware Tit-for-Tat-Based P2P File Distribution
   Yohei Nishi, Masahiro Sasabe, Shoji Kasahara.
   Peer-to-Peer Networking and Applications, 13(5), pp.1688-1703, September 2020 (First Online: 11 June 2020). [paper (580KB pdf)] [publisher's version]
4. Topological Influence on Optimality of Tit-for-Tat Based P2P Content Distribution
   Masahiro Sasabe.
   Peer-to-Peer Networking and Applications, 13(1), pp.243-254, January 2020 (First Online: 17 May 2019). [paper (810KB pdf)] [publisher's version]

5. Impact of Locality-awareness on Tit-for-Tat-based P2P File Distribution
   Yohei Nishi, Masahiro Sasabe, Shoji Kasahara.
   Proc. of IEEE Consumer Communications & Networking Conference (CCNC), pp.1-6, January 2020 (acceptance rate: 0.40 (88/219)). [paper (721KB pdf)] [publisher's version]

In the 2011 Great East Japan Earthquake, both fixed and mobile communication networks had been unavailable for long time and in wide areas, due to damage of communication infrastructures. As a result, it has been reported that there were many cases where evacuees and rescuers could not collect and distribute important information, e.g., damage information, evacuation information, and government information. In this research, we aim to achieve an evacuation assisting system that can speedily and safely navigate evacuees to appropriate refuges. Specifically, we tackle this problem by combining several functions (e.g., automatic road-state estimation and evacuation guiding under implicit collaboration among evacuees and their mobile devices, road network state information sharing among evacuees, and road network risk analysis using geographical big-data.)

1. Path Reachability Including Distance-Constrained Detours
   Masahiro Sasabe, Miyu Otani, Takanori Hara, Shoji Kasahara.
   Proceedings of the Institution of Mechanical Engineers, Part O: Journal of Risk and Reliability, 238(1), pp.79-92, February 2024 (accepted on 2 October 2022). [publisher's version]
2. On Robustness Against Evacuees' Unexpected Movement in Automatic Evacuation Guiding
   Jun Kawahara, Takanori Hara, Masahiro Sasabe.
   Computers and Electrical Engineering, 105, pp.108531: 1-17, January 2023 (accepted on 2 December 2022). [publisher's version]
3. Capacitated Refuge Assignment for Speedy and Reliable Evacuation
   Takanori Hara, Masahiro Sasabe, Taiki Matsuda, Shoji Kasahara.
   ISPRS International Journal of Geo-Information, 9(7), pp.442: 1-19, July 2020 (accepted on 15 July 2020). [publisher's version]
4. Road Network Risk Analysis Considering People Flow under Ordinary and Evacuation Situations
   Masahiro Sasabe, Kodai Fujii, Shoji Kasahara.
   Environment and Planning B: Urban Analytics and City Science, 47(5), pp.759-774, June 2020 (First Online: 10 October 2018). [paper (2MB pdf)] [supplemental (1MB pdf)] [publisher's version]

In the Bitcoin system, each miner (i.e., a special node that creates new blocks and broadcasts these blocks to the other miners) has to win both competitive block mining, i.e,. Proof of Work (PoW), and block diffusion over the Bitcoin network so as to acquire the system rewards. In this research, we focus on block diffusion delay attacks, where multiple attackers collude with a specific miner to disturb the propagation of blocks generated by competing miners. With the help of mathematical epidemiology, we develop a scalar (resp. network) standby-interrupted-retrieved-attackable (SIRA) models, which capture the information diffusion with interruption under a well-mixed population (resp. arbitrary networks). To acquire the rewards effectively, multiple miners tend to form a group called a pool to conduct PoW collaboratively. The pool manager will distribute rewards to members according to their contribution (called share). It has been pointed out that some malicious miners can realize block withholding (BWH) attacks that sabotage the mining process and gain more rewards by hiding found blocks and reporting their contribution. In this research, we aim to achieve early detection of BWH attacks with the help of machine learning approaches, e.g., support vector machine (SVM).

1. Block Diffusion Delay Attack and Its Countermeasures in a Bitcoin Network
   Masahiro Sasabe, Masanari Yamamoto, Yuanyu Zhang, Shoji Kasahara.
   International Journal of Network Management, 32(3), pp.1-21, May 2022 (accepted on 7 October 2021). [paper (538KB pdf)] [publisher's version]
2. Mathematical Epidemiological Analysis of Dynamics of Delay Attacks on Pull-based Competitive Information Diffusion
   Masahiro Sasabe.
   Computer Networks, 180, pp.107383: 1-9, October 2020 (accepted on 26 June 2020). [paper (2MB pdf)] [publisher's version]

We have been faced with the new era of IoT, where various devices are connected to the Internet in addition to the conventional PCs and mobile phones. Most of the IoT devices are managed and controlled through wireless communications. On the other hand, most radio frequency bands have already been allocated to various existing systems. Thus, it is important to solve the spectrum exhaustion problem and to improve the spectrum utilization. To tackle this problem, cognitive radio has been studied, in which unlicensed users, i.e., secondary users (SUs), utilize the spectrum of a licensed user, i.e., primary user (PU), while avoiding interference in PU's communications. In this research, we aim to achieve a distributed cooperative control mechanism that can consider both cooperation among SUs for improving detection performance of PU's communication and competitive relationship among SUs to acquire individual communication opportunities.