International Journal of Internet Protocol Technology (23 papers in press)
An Information Discovery Scheme for IoS of Agricultural Services using Improved Chord-ONS Model
by Baojun Fu
Abstract: The main research content of this article is intelligent algorithm in data transmission of agricultural Internet of things(IoS). We explain the architecture and workflow of structured P2P based ONS resolution system, and analyze advantages and disadvantages of current P2P-ONS system. Then the improved scheme is put forward which adopts hierarchical P2P network architecture to form ONS resolution system. It proposes a two-level Chord ONS architecture including routing performance analysis, storage and query of system information, nodes joining in or out, and selection of super nodes. The routing strategy based on node performance for super nodes and common nodes is also put forward. By the analysis of simulation results, after introduction of multi-level chord the network discovery service has large improvement in query efficiency, burden and dependence of super node, and link utilization rate, which shows better overall performance than traditional structured P2P network.
Keywords: IoS; EPCIS; Chord-ONS; P2P; super node; discover service.
An Efficient Link Based Model for routing, modulation format and spectrum allocation (RMFSA) with Quality of Transmission (QoT) Consideration in Elastic Optical Networks
by Sridhar Iyer, Shree Prakash Singh
Abstract: Elastic Optical Networks (EONs) based on Orthogonal Frequency Division Multiplexing (OFDM) adjust bandwidth utilization as per the demands, and also provision long transmission reach (TR) and high spectral-efficiency (SE). Hence, compared to the fixed-grid optical networks, in EONs, managing resources and addressing the network design and the control poses new and complex challenges. In this work our main aim is to address the routing, modulation format and spectrum allocation (RMFSA) issue in EONs. In regard to the aforementioned, we formulate a link based Mixed Integer Linear Program (MILP) model which considers the Quality of Transmission (QoT) while determining the route, modulation format/level (MF/ML) and spectrum allocation (SA) for every request. In order to improve SE, the model chooses the highest ML; however, when QoT is considered, TR of path with highest ML is very limited. Hence, model finds trade-off between the ML, number of required Frequency Slots (FSs), and TR, with the objective of minimizing the required amount of network FSs so as to admit all the requests. Finally, we evaluate and analyze the models performance and show that for simplicity, existing schemes impractically assume single MF (i.e., higher SE and ML) compared to fixed-grid networks, thereby ignoring the flexible nature of EONs.
Keywords: EONs; QoT; MILP; OFDM; link based model.
Research on Highly Available Storage System for Open Virtual Experiment Platform
by Hui Yan
Abstract: Open teaching platform for virtual experiment can overcome some disadvantages of traditional virtual experiment systems such as heterogeneity of experiments and realize great interaction and availability. In this paper, a highly available data storage model for open virtual experiment platform is put forward, in which Consistent Hashing Distribution, eventual consistency is used to realize this system and reading and writing control mechanism and exception handling method are adopted to ensure availability of system services. In this system, standardization of experimental procedures and data stream are realized, based on which the efficiency of data communication is increased and development costs of virtual experiments are reduced.
Keywords: Highly Available Storage; Consistent Hashing Distribution; Eventual Consistency; Distributed State Synchronization.
Multi-tenant SaaS Deployment Optimization Algorithm for Cloud Computing Environment
by Ming Cao, Bingjie Yu, Xiantong Liu
Abstract: This article adopts MapReduce and multi-targeted ant colony algorithm(ACO) distribution in parallel to solve large-scaled service dynamic selection in Saas and puts forward a service dynamic selection algorithm based on these technologies. The algorithm integrates cloud calculation technologies such as loading strategy, ACO, MapReduce, and HDFS, which deploys the service to servers as little as possible, to further save the energy target. Meanwhile, it also takes into account the smallest price target deployment and server loading balancing target, which transforms the global optimization service dynamic selection into a multi-targeted service combination optimization problem with QoS restriction. The simulation experiments verify and prove the feasibility, effectiveness and convergence of the improved algorithm.
Keywords: SaaS; multi-tenant; SPP; ACO; MapReduce.
Wireless sensor networks routing algorithm based on particle swarm optimization
by Junhan Yang
Abstract: Routing policy is a key problem in wireless sensor network (WSN), and it aims to find an optimal path to transmit the data of source nodes to the destination nodes. Wireless sensor networks can represented as a graph model, where graph nodes refers the wireless sensors, and graph edges are used to connects various sensors. Furthermore, in this work, we suppose that wireless sensor networks contain three types of nodes, including base station, gateway, and sensor. In the proposed algorithm, each particle is exploited to provide a suitable route from each cluster head to the base station, and the dimension of each particle is equal to the number of gateways in WSN. In the end, experimental results demonstrate that the proposed algorithm is able to effectively prolong network lifetime and increase the number of packets received by base station. Furthermore, the conclusion can be drawn that the proposed method can effectively solve the WSNs routing problem, because particle swarm optimization is able to effectively optimize WSN routing scheme.
Keywords: Wireless sensor networks; Routing algorithm; Particle swarm optimization; Linear programming.rnrn.
Remote Vehicle Communication anti-interference Design Based on Multipath Channel Equalization Suppression
by Chunling Liu, Chunbao Huo, Xinfa Zeng
Abstract: In order to improve the quality of remote vehicle communication and reduce communication distortion and error, this paper proposes a remote vehicle communication anti-interference design method based on multipath channel equalization suppression, and constructs a dynamic channel allocation model for multi-vehicle remote communication. The signal-to-interference ratio of each channel is calculated and the continuous fractional frequency method of vehicle communication signal is used for blind source separation and on this basis, feedback link is added to the output end of the communication signal. The research results show that the method has advantages of good balance performance and vehicle communication code elements output error rate is low, anti-interference ability is strong, and the quality of remote vehicle communication is improved.
Keywords: Remote vehicle communication; Anti-interference; Channel equalization; Car networking.
Intelligent Fuzzy Information Retrieval Based on Ontology Knowledge Base
by Yangxin Yu, Quanyin Zhu, Liuyang Wang
Abstract: Intelligent fuzzy information retrieval (IR) based on ontology knowledge base has become one of the most active research directions in the field of intelligent IR system. How to use ontology knowledge base to further improve its retrieval performance and intelligence has become the main research goal of intelligent fuzzy IR system based on ontology knowledge base. IR using knowledge organization can improve the quality between returning document and users initial query. The intelligent fuzzy information retrieval model proposed in this paper can provide a coded knowledge base structure for IR, which is composed of multiple related ontologies, and the relationship between ontology is expressed as fuzzy relationship. In this kind of knowledge organization, a new method is used to extend users initial query and index document set, which independently represents ontology and the relationship between concepts. The experimental results show that the proposed model has a better overall performance compared with the other classical IR method. The prospects for future development and suggestions for possible extensions of the ontology knowledge base-based intelligent fuzzy IR systems are also discussed.
Keywords: ontology knowledge base; intelligent; fuzzy IR; knowledge organization; user query.
A Buffering Mechanism for IP mobility support in 6LoWPAN-WSN Under Critical Environment
by Riaz Khan, Ajaz Hussain Mir
Abstract: With the advent of Internet of Things (IoT), IP mobility has become an inexorable service to Wireless Sensor Networks (WSN). Existing mobility support protocols suffer extra packet loss due to conventional mobility operations with extra signaling cost, hence are not applicable to critical mobile applications. Therefore, this paper proposes a Packet Buffering Mechanism (PBM) to reduce extra packet loss occur during Layer 2 (L2) mobility. In the mechanism, Hand-Off (HO) is anticipated and triggered before the actual movement of the Mobile Node (MN) and domain gateway routers assisting mobility will buffer packets for the MN during L2 mobility when the MN is not able to hear from any Access Network (AN). As soon as the MN is attached to new link, the buffered packets get delivered to it. Due to the advance HO initiation leads to Layer 3 (L3) HO is performed before L2 HO, thus eliminating L2 channel scanning process. Analysis and evaluation of PBM shows performance improvements in critical WSN domains. The average HO delay in the proposed scheme remains 15ms and 50ms against the MN speed and Wireless Link Delay (WLD) respectively whereas the average packet loss reduced to 2-4 % for both the cases when compared with the existing solutions.
Keywords: IP Mobility; PMIPv6; Handover; Handoff Delay; Packet Loss; Signaling Cost; 6LoWPAN; IoT; RSS; Mobility Speed.
Algorithm for the Most Surviving Nodes Selection Based on Adaptive Forwarding Control under the Impact of Network Storm
by Zhichao Wang, HongWei Shi, Yan Wei
Abstract: In order to improve the survivability of wireless network nodes under the impact of network storm to improve the adaptive forwarding performance of network and control the nodes selection optimization, an algorithm for the most surviving nodes selection based on adaptive forwarding control under the impact of network storm is proposed in this paper. In this algorithm, a node distribution model under the impact of network storm is constructed; the automatic regulation and control method is adopted for optimal localization and deployment of target nodes; based on node location, an output link forwarding routing protocol is constructed for interference wave filtering and adaptive forwarding control of output package, so as to select the most surviving nodes and improve the survival rate of nodes. The simulation result shows that this method can realize 100% success rate of data forwarding, reduce the average transmission power by 5KW than that before optimization, and stabilize the transmission power. The calculation cost and time delay are 5-20ms lower than that of other methods, which proves the high efficiency of this method.
Keywords: impact of network storm; surviving node; selection; forwarding routing protocol.
Storage management in DONA content routers
by Sonia Mettali
Abstract: Information-centric networking (ICNs) as an
emerging paradigm for the Future introduces a radical change in
Internet communications. It allows information access regardless
of their location and it defines innovative concepts such as
forwarding, name based routing, security and caching. Many ICN
architectures have been proposed over the past few years such
as DONA , NetInf, CCN, NDN, Publish-Subscribe Architecture,
etc. In this paper, we focus especially on DONA, one of the first
ICN architecture, and we analyse the required memory space to
store routing states. Our study shows that todays technologies are
not able to satisfy the content routing needs. Thus, we propose
an enhancement of DONA called BADONA to deal with this
problem. It uses the Bloom filter to drastically reduce the usage
of the memory space. We evaluate our proposal performances
and compare it with two ICN solutions: CCN, DHTChord to find
out how these models scale when they are deployed over large
networks. The obtained simulation results show that our solution
reduces considerably the memory usage, the decreasing ratio
reaches the value of 40 with the standard Bloom filter version.
Moreover, BADONA outperformes the other existing schemes.
Keywords: Information Centric Networking; DONA; BloomrnFilter; memory space usage; Routing Information storage.
A flexible and scalable Ethernet architecture for telecommunication transport networks
by A. S. M. Asadujjaman, Mohammad Shah Alam
Abstract: This paper presents a novel wireless channel condition aware and floodless Ethernet architecture for telecommunication carrier networks, namely FFCSEA. The distinct advantages of FFCSEA are: (i) efficiency and resilience for wireless transmission links that are capable of adaptive modulation, (ii) scalability of a single Ethernet network for a large number of nodes (i.e. eNodeB), (iii) backward compatibility with existing Ethernet technology (iv) fast response time compared to SDN controller based proxy. By introducing a multi-instance ITU-T G.8032 (ERPS) ring design and an optimized proactive learning based proxy mechanism, FFCSEA achieves flexibility (i.e. efficiency and resilience during microwave adaptive modulation changes) and scalability of the transport network. Simulation and analytical results validate comparative advantage of FFCSEA over existing protocols both in terms of flexibility and scalability. The analysis and design presented in this paper will be useful for both traditional networks and SDN.
Keywords: Ethernet Ring; Flexibility; Scalability; SDN; Autoconfiguration; Adaptive Modulation; Microwave; Transport Network; X2 interface; Backwards Compatibility.
A Cloud-Fog Scalable Video Streaming Architecture for Multimedia Internet of Things (IoT) Devices
by Tz-Heng Hsu
Abstract: Cloud computing can be used to provide guaranteed quality of video services. On the other hand, fog computing can lower the transmission costs by using collaborative machine-to-machine and near-user edge communications. In this paper, a cloud-fog scalable video streaming architecture for multimedia Internet of things (Iot) devices is proposed, where fog nodes can collaborate with the rented cloud servers for efficiently delivering the requiring video frames to multimedia Iot devices. The proposed architecture strengthens the stability of the video streaming transmission and improves the quality of service (QoS) of multimedia Iot services with cloud-fog computing techniques.
Keywords: Internet of Things (IoT); Video Streaming; Fog Computing.
An Energy Efficient Authentication Scheme for Wireless Body Area Networks Based on the Bilinear Pairings
by Songsong Dai, Linmei Jiang, Shan He, Donghui Guo
Abstract: With the popularity of wearable electronic devices, much attention has been paid to wireless body area networks (WBANs) for health monitoring. However, by the limited energy of electronic devices, WBANs not only need to consider the security and privacy preservation but also to consider the low power consumption implementation. Recently, Wang and Zhang presented an authentication scheme for WBANs. Unfortunately, in this paper, we will show their scheme is vulnerable to impersonation attack. Then we present a new authentication scheme for WBANs based on the bilinear pairings. Our scheme not only provides mutual authentication and client anonymity but also resists well-known cryptographical attacks. We also use the BAN logic to demonstrate the validity of our scheme. Compared with Wang-Zhang scheme, the energy consumption of our scheme is reduced by 1.79%.
Keywords: Mutual authentication; Wireless body area network; Energy efficient; Bilinear pairing.
The performance and QoE analysis of B2-DASH algorithm
by Tanapat Anusas-amornkul, Surawut Moonsin
Abstract: Dynamic Adaptive Streaming over HTTP (DASH) protocol provides several video bit rates for each video segment depending on the current network conditions and it can be used for several platforms. However, the adaptive algorithm in DASH can be further improved to gain better performance. Therefore, Bandwidth and Buffer-based (B2)-DASH algorithm is proposed to enhance DASH algorithm using both buffer and bandwidth-based approaches to provide high average video bit rate in fluctuated network conditions. In this paper, the B2-DASH is compared with FDASH algorithm, which is the best adaptive algorithm among others. This work compares the performance in three scenarios, i.e. Low Switching Rate, Fast Switching Rate, and Wi-Fi networks. The performance metrics are average video bit rates, number of interruptions, and resolution changes. B2-DASH parameters are analyzed and the recommended parameter settings are presented for the best performance for each scenario. In term of user experiences for video streaming, Quality of Experience (QoE) is analyzed for both B2-DASH and FDASH algorithms.
From the analysis, B2-DASH outperformed FDASH in all scenarios. The number of resolution changes was enhanced about 22% for Fast Switching Rate and Wi-Fi network scenarios. The parameter adjustment process is necessary when the actual bandwidth is highly fluctuated in order to give better performance to B2-DASH algorithm. Quality of Experience (QoE) was calculated for the FDASH and B2-DASH algorithms and B2-DASH algorithm improved the QoE about 13% over FDASH in the Wi-Fi network scenario.
Keywords: B2-DASH; QoE Analysis; Adaptive Video Streaming Algorithm.
Bandwidth Management Framework for Smart Homes Using SDN: ISP Perspective
by Hung-Chin Jang, Jian-Ting Lin
Abstract: With the increasing number of Internet of Things (IoT) devices and the advance of smart home technologies, not only smart environments like smart homes, smart cities and smart countries become feasible, but also all these devices consume more or less bandwidth in data transmission. In this paper, we propose an SDN (Software Defined Networking) based QoS (Quality of Service) aware bandwidth management framework for thousands of IoT enabled smart homes from ISP perspective. The operational scenario of this research assumes that an ISP should support thousands of smart homes. Each smart home equips with tens of IoT devices with a broad spectrum of functional capabilities, and each smart home exploits a variety of services. The overall system architecture consists of into SDN Smart Home Cloud at ISP side, and massive SDN enabled Smart Homes. The Smart Home Cloud interconnects with Smart Homes through OpenFlow protocol. With this architecture, we first prioritized smart home services into categories by adapting 3GPP LTE QoS Class Identifier (QCI). Then we propose a Bandwidth Allocation for Smart Home (BASH) strategy to calculate appropriate bandwidth for each service category according to distinct QoS requirements. ISP can thus use BASH to optimize bandwidth allocation of aggregated and classified services of smart homes. The experiments are conducted under an SDN-based network environment constructed by Linux based OpenvSwitch, Ryu controller, and Mininet. The experiment results show that the proposed framework can effectively enhance QoS and outperform the traditional ISP bandwidth allocation strategy in terms of average transfer ratio, average throughput, average delay and average jitter.
Keywords: Software-Defined Networking (SDN); Quality of Service (QoS); Internet Service Provider (ISP); smart home; bandwidth management.
Energy Aware Multi Objective Genetic Algorithm for Task Scheduling In Cloud Computing
by Hima Bindu G B, Ramani K, Shoba Bindu C
Abstract: In the recent years, the utilization of cloud is expanded to all the fields. Cloud computing has the advantage to provide anything as a service to the clients. One of the major issues in cloud computing is energy consumption at the time of task allocation to the virtual machines. This paper address the problem of task scheduling by considering the energy consumption of tasks in the cloud computing. Existing research works only focused on optimizing the time and cost of the resource without paying attention to the energy consumption. This paper proposes a new algorithm for multi objective task scheduling in clouds, and develop a genetic algorithm to optimize the energy consumption. Simulation results proves the efficiency of the proposed method.
Keywords: Energy consumption; Virtual machines; Tasks; Genetic algorithm; Optimization.
Bio-inspired Approaches for OFDM Based Cognitive Radio
by Naziha Ali Saoucha, Badr Benmammar
Abstract: Link adaptation algorithms design for OFDM based cognitive radio networks is a challenging task. The main concern is to provide a high Quality of Service for the secondary user while the mutual interference between this last and the primary user persists within a tolerable range. This issue can be formulated as a multiobjective optimization constraint problem. To tackle this optimization problem in a multiobjective constraint framework, in this paper we exploit three of the most recent powerful bio-inspired algorithms: firefly, bat, and cuckoo search. Simulation results revealed that, in contrast to the classical genetic algorithm and particle swarm optimization based link adaptation, our proposed algorithms exhibit better performance in terms of convergence speed and solution quality with saving rates reaching over 98.93% and 46.60%, respectively.
Keywords: cognitive radio; OFDM; QoS; interference; firefly; bat; cuckoo; particle swarm
optimization(PSO); genetic algorithm (GA); binary.
Modeling and Analysis of Real Time and Reliability for WSN based CPS
by Junhua Zhang, Yi Zhu, Fangxiong Xiao
Abstract: WSN based CPS can be used to collect information from remote environment and control it. Delay and dependability directly affect the running quality of WSN based CPS. In this paper, we present a specified process calculus to model the running of the system, including its real time and reliability character. Then we design a group of algorithms to calculate the reliability of the system under certain responsible time constraint. Using these methods, we can formally depict the running of WSN based CPS, and can assess real time and reliability of WSN based CPS quantitatively. We illustrate our ideas through bank night automatic monitoring and emergency system, and we can improve the systems reliability from 0.58 to 0.87 and higher gradually.
Keywords: WSN based CPS; real time; reliability; process calculus.
Geographic Information System Query Optimization Algorithm Based on Redundant Data Deletion and Filtering Technology
by Chunyang Lu, Feng WEN
Abstract: In order to reduce the interference of redundant information during geographic information system query and improve the accuracy of information query, a geographic information system query optimization algorithm based on redundant data deletion and filtering technology is proposed. In this method, a distributed data query structure model of geographic information system is constructed; adaptive group matching of query intervals is conducted through the segment fusion method, and then key information points after group matching are located through the association rule feature information fusion method; spatial state features of geographic information are analyzed in the data clustering center of location; redundant data are acquired at the data management application layer of geographic information system, and redundant data deletion processing is conducted to the acquired redundant data through the adaptive matching detection filter method to realize automatic filtering and deletion of redundant data during geographic information system query. Finally, the high-order cumulant feature extraction method is adopted to achieve accurate information query of geographic information system. Simulation results show that in geographic information system query under redundant data infection, this method can provide accuracy 8.75% higher than that provided by the FCM method and 12.98% higher than that provided by the support vector machine algorithm, which indicates that this method can provide strong anti-interference ability of query and cost less overhead.
Keywords: Redundant data; infection; geographic information system; query; feature extraction.
Modified QUIC Protocol (ModQUIC) for Improved Network Performance and Comparison with QUIC and TCP
by Prashant Kharat, Muralidhar Kulkarni
Abstract: Congestion control mechanism is solely responsible for maintaining the performance of streaming data. However, when there is no congestion, a regular delivery window update is followed as a step by step process. The process can be improved by individual window update along with acknowledgment (ACK) as feedback to the server even in the absence of congestion. To achieve maximum throughput and minimum delay, we have suggested modification in the existing handshaking mechanism of Quick UDP Internet Connections (QUIC) protocol. This paper presents an investigation of QUIC protocol performance and proposes a Modified QUIC (ModQUIC): a modification in existing handshaking mechanism of QUIC to reduce control overhead. Chromium server-client model testbed setup results, show that the proposed technique gives stable output and improves performance in terms of overall throughput and delay over QUIC and Transmission Control Protocol (TCP). Performance has been tested for limited (2 Mbps) and sufficient (10 Mbps) link bandwidth in presence of loss. The validation of results has been carried out
with the help of linear regression model. The result show a throughput improvement of 35.66% and 51.93% over QUIC and TCP respectively and also the delay is reduced by 3% to 5% over QUIC and TCP.
Keywords: ModQUIC; QUIC; Network performance; Congestion control.
Special Issue on: Future of IoT (Internet of Things) Information Digital Era and Societal Challenges
ATM PIN GENERATION - A Formal Mathematical Model to generate PIN Using Regular Grammar, Context Free Grammar and recognition through Finite State Machine, Pushdown Automata
by VAITHYASUBRAMANIAN SUBRAMANIAN
Abstract: A Secret Passcode composed of numbers in authenticating the genuine user is recognized as Personal Identification Number (PIN). A PIN is utilized to authenticate the identity of the legitimate person to access Debit/Credit card in an ATM or online. In general Personal Identification Number is used to validate the legitimate user for transactions like Credit card, Debit card; online and also in accessing mobile telephones. In banking sectors, as a part of two factor authentication ATM card assumes noteworthy part, PIN serves as a legalized application to distinguish the authenticated user. PIN commonly contains four digits; to generate this four digit access code new behavioral model has been proposed. This paper exhibits PIN generation technique made out of a finite number of states utilizing "Regular Grammar" and "Context Free Grammar". Programming Language Python generating PIN of 4 digit numbers is also offered in this paper. To perceive the produced 4-digit ATM PIN computational Mathematical Model known as a Finite State Machine and "Pushdown Automata" has been utilized.
Keywords: Automated Teller Machine - Personal Identification Number – Access Code –– Mathematical Model – Computational Model - Regular Grammar - Context Free Grammar - Finite State machine - Pushdown Automata.
Analysis of factors affecting Customer Satisfaction in E-Commerce applications using Condorcet AHP method
by Sweta Bhattacharya, V. Raju
Abstract: The paper proposes a new approach by combining Condorcet Voting Theory with Analytical Hierarchy Process (AHP) to analyze various criteria and alternatives affecting customer satisfaction in E-commerce systems. Customer satisfaction depends on multiple criteria and alternatives and choosing the most prioritized one leads to a multi-criteria decision making (MCDM) problem. AHP method has been successfully used to resolve MCDM problems in various sectors of human life. But the major weakness of AHP lies in the use of subjective assessment and importance scale point which leads to judge mental errors in results. In this paper Condorcet voting method is used to derive the importance of the criteria involved in customer satisfaction instead of subjective assessment and quantitative ratio method is used instead of the standard importance scale point eliminating the weaknesses of the traditional AHP technique. The derived consistency ratio values provide support to the consistency accuracy and consistency of the results. The comparative analysis of the priority vectors highlight Trust and Support as the most significant criteria affecting customer satisfaction in e-commerce. The structured analysis of the criteria in this paper thus provides immense insight and hence would benefit business personnels to frame strategies pertinent to the most significant criteria and enhance customer satisfaction in e-commerce systems.
Keywords: E-commerce; Customer Satisfaction; MCDM; Analytical Hierarchy Process; AHP; Condorcet Voting Theory.
Security analysis of a Cloud Authentication Protocol using applied pi-calculus
by Norbert Oláh, Andrea Huszti
Abstract: Nowadays cloud computing is the most promising model within information technology. One of the most important issues is to achieve secure user authentication. Vulnerability of an authentication protocol results in successful attacks against confidentiality and integrity of user data stored and processed in the cloud. In our suggested protocol a person uses a static password and a one-time password for identity verification. Shared control among the cloud servers is provided by applying a Merkle-tree for storing one-time passwords distributed. A security analysis is carried out in case of outsider adversaries. We show that our authentication protocol fulfills typical security requirements of a key exchange protocol, i.e. authentication of the participants, key secrecy, key freshness and confirmation that both parties know the new key in the Dolev-Yao model.
Keywords: Cloud computing; two-factor authentication; applied pi calculus; Merkle-tree.