期刊文献+
共找到136篇文章
< 1 2 7 >
每页显示 20 50 100
Analytical Comparison of Resource Search Algorithms in Non-DHT Mobile Peer-to-Peer Networks 被引量:1
1
作者 Ajay Arunachalam Vinayakumar Ravi +2 位作者 Moez Krichen Roobaea Alroobaea Jehad Saad Alqurni 《Computers, Materials & Continua》 SCIE EI 2021年第7期983-1001,共19页
One of the key challenges in ad-hoc networks is the resource discovery problem.How efciently&quickly the queried resource/object can be resolved in such a highly dynamic self-evolving network is the underlying que... One of the key challenges in ad-hoc networks is the resource discovery problem.How efciently&quickly the queried resource/object can be resolved in such a highly dynamic self-evolving network is the underlying question?Broadcasting is a basic technique in the Mobile Ad-hoc Networks(MANETs),and it refers to sending a packet from one node to every other node within the transmission range.Flooding is a type of broadcast where the received packet is retransmitted once by every node.The naive ooding technique oods the network with query messages,while the random walk scheme operates by contacting subsets of each node’s neighbors at every step,thereby restricting the search space.Many earlier works have mainly focused on the simulation-based analysis of ooding technique,and its variants,in a wired network scenario.Although,there have been some empirical studies in peer-to-peer(P2P)networks,the analytical results are still lacking,especially in the context of mobile P2P networks.In this article,we mathematically model different widely used existing search techniques,and compare with the proposed improved random walk method,a simple lightweight approach suitable for the non-DHT architecture.We provide analytical expressions to measure the performance of the different ooding-based search techniques,and our proposed technique.We analytically derive 3 relevant key performance measures,i.e.,the avg.number of steps needed to nd a resource,the probability of locating a resource,and the avg.number of messages generated during the entire search process. 展开更多
关键词 Mathematical model MANET p2p networks p2p MANET UNSTRUCTURED search algorithms peer-to-peer AD-HOC ooding random walk resource discovery content discovery mobile peer-to-peer broadcast peer
下载PDF
FPSblo:A Blockchain Network Transmission Model Utilizing Farthest Point Sampling
2
作者 Longle Cheng Xiru Li +4 位作者 Shiyu Fang Wansu Pan He Zhao Haibo Tan Xiaofeng Li 《Computers, Materials & Continua》 SCIE EI 2024年第2期2491-2509,共19页
Peer-to-peer(P2P)overlay networks provide message transmission capabilities for blockchain systems.Improving data transmission efficiency in P2P networks can greatly enhance the performance of blockchain systems.Howev... Peer-to-peer(P2P)overlay networks provide message transmission capabilities for blockchain systems.Improving data transmission efficiency in P2P networks can greatly enhance the performance of blockchain systems.However,traditional blockchain P2P networks face a common challenge where there is often a mismatch between the upper-layer traffic requirements and the underlying physical network topology.This mismatch results in redundant data transmission and inefficient routing,severely constraining the scalability of blockchain systems.To address these pressing issues,we propose FPSblo,an efficient transmission method for blockchain networks.Our inspiration for FPSblo stems from the Farthest Point Sampling(FPS)algorithm,a well-established technique widely utilized in point cloud image processing.In this work,we analogize blockchain nodes to points in a point cloud image and select a representative set of nodes to prioritize message forwarding so that messages reach the network edge quickly and are evenly distributed.Moreover,we compare our model with the Kadcast transmission model,which is a classic improvement model for blockchain P2P transmission networks,the experimental findings show that the FPSblo model reduces 34.8%of transmission redundancy and reduces the overload rate by 37.6%.By conducting experimental analysis,the FPS-BT model enhances the transmission capabilities of the P2P network in blockchain. 展开更多
关键词 Blockchain p2p networks SCALABILITY farthest point sampling
下载PDF
Monitoring Peer-to-Peer Botnets:Requirements,Challenges,and Future Works
3
作者 Arkan Hammoodi Hasan Kabla Mohammed Anbar +2 位作者 Selvakumar Manickam Alwan Ahmed Abdulrahman Alwan Shankar Karuppayah 《Computers, Materials & Continua》 SCIE EI 2023年第5期3375-3398,共24页
The cyber-criminal compromises end-hosts(bots)to configure a network of bots(botnet).The cyber-criminals are also looking for an evolved architecture that makes their techniques more resilient and stealthier such as P... The cyber-criminal compromises end-hosts(bots)to configure a network of bots(botnet).The cyber-criminals are also looking for an evolved architecture that makes their techniques more resilient and stealthier such as Peer-to-Peer(P2P)networks.The P2P botnets leverage the privileges of the decentralized nature of P2P networks.Consequently,the P2P botnets exploit the resilience of this architecture to be arduous against take-down procedures.Some P2P botnets are smarter to be stealthy in their Commandand-Control mechanisms(C2)and elude the standard discovery mechanisms.Therefore,the other side of this cyberwar is the monitor.The P2P botnet monitoring is an exacting mission because the monitoring must care about many aspects simultaneously.Some aspects pertain to the existing monitoring approaches,some pertain to the nature of P2P networks,and some to counter the botnets,i.e.,the anti-monitoring mechanisms.All these challenges should be considered in P2P botnet monitoring.To begin with,this paper provides an anatomy of P2P botnets.Thereafter,this paper exhaustively reviews the existing monitoring approaches of P2P botnets and thoroughly discusses each to reveal its advantages and disadvantages.In addition,this paper groups the monitoring approaches into three groups:passive,active,and hybrid monitoring approaches.Furthermore,this paper also discusses the functional and non-functional requirements of advanced monitoring.In conclusion,this paper ends by epitomizing the challenges of various aspects and gives future avenues for better monitoring of P2P botnets. 展开更多
关键词 p2p networks BOTNET p2p botnet botnet monitoring HONEYPOT crawlers
下载PDF
Mathematical Model Validation of Search Protocols in MP2P Networks 被引量:1
4
作者 Ajay Arunachalam Vinayakumar Ravi +2 位作者 Moez Krichen Roobaea Alroobaea Saeed Rubaiee 《Computers, Materials & Continua》 SCIE EI 2021年第8期1807-1829,共23页
Broadcasting is a basic technique in Mobile ad-hoc network(MANET),and it refers to sending a packet from one node to every other node within the transmission range.Flooding is a type of broadcast where the received pa... Broadcasting is a basic technique in Mobile ad-hoc network(MANET),and it refers to sending a packet from one node to every other node within the transmission range.Flooding is a type of broadcast where the received packet is retransmitted once by every node.The naive flooding technique,floods the network with query messages,while the random walk technique operates by contacting the subsets of every node’s neighbors at each step,thereby restricting the search space.One of the key challenges in an ad-hoc network is the resource or content discovery problem which is about locating the queried resource.Many earlier works have mainly focused on the simulation-based analysis of flooding,and its variants under a wired network.Although,there have been some empirical studies in peer-to-peer(P2P)networks,the analytical results are still lacking,especially in the context of P2P systems running over MANET.In this paper,we describe how P2P resource discovery protocols perform badly over MANETs.To address the limitations,we propose a new protocol named ABRW(Address Broadcast Random Walk),which is a lightweight search approach,designed considering the underlay topology aimed to better suit the unstructured architecture.We provide the mathematical model,measuring the performance of our proposed search scheme with different widely popular benchmarked search techniques.Further,we also derive three relevant search performance metrics,i.e.,mean no.of steps needed to find a resource,the probability of finding a resource,and the mean no.of message overhead.We validated the analytical expressions through simulations.The simulation results closely matched with our analyticalmodel,justifying our findings.Our proposed search algorithm under such highly dynamic self-evolving networks performed better,as it reduced the search latency,decreased the overall message overhead,and still equally had a good success rate. 展开更多
关键词 Search protocols random walk MANET p2p networks p2p MANET mathematical model peer-to-peer wireless ad hoc network FLOODING mobile p2p UNSTRUCTURED p2p NS-2 network simulator
下载PDF
A Scalable and Robust DHT Protocol for Structured P2P Network
5
作者 Xiao Shu Xining Li 《International Journal of Communications, Network and System Sciences》 2012年第12期802-809,共8页
Distributed Hash Tables (DHTs) were originated from the design of structured peer-to-peer (P2P) systems. A DHT provides a key-based lookup service similar to a hash table. In this paper, we present the detailed design... Distributed Hash Tables (DHTs) were originated from the design of structured peer-to-peer (P2P) systems. A DHT provides a key-based lookup service similar to a hash table. In this paper, we present the detailed design of a new DHT protocol, Tambour. The novelty of the protocol is that it uses parallel lookup to reduce retrive latency and bounds communication overhead to a dynamically adjusted routing table. Tambour estimates the probabilities of routing entries' liveness based on statistics of node lifetime history and evicts dead entries after lookup failures. When the network is unstable, more routing entries will be evicted in a given period of time, and the routing tables will be getting smaller which minimize the number of timeouts for later lookup requests. An experimental prototype of Tambour has been simulated and compared against two popular DHT protocols. Results show that Tambour outperforms the compared systems in terms of bandwith cost, lookup latency and the overall efficiency. 展开更多
关键词 p2p network Distributed HASH TABLE SMALL-WORLD Distribution Parallel Lookups
下载PDF
基于Super-peer的P2P流媒体服务系统构架的研究 被引量:3
6
作者 蒙应杰 王阳 赵雅洁 《计算机应用与软件》 CSCD 北大核心 2006年第10期33-35,共3页
在分析比对现有P2P流媒体服务系统构架的基础上,提出了一种新的P2P流媒体服务系统构架,在构架中引入了带有冗余的Super-peer结构,使得该流媒体服务系统可以达到较高的效率和健壮性。
关键词 p2p网络 Super—peer 流媒体 多媒体系统
下载PDF
基于Super Peer的P2P e-Learning模型 被引量:1
7
作者 陈金儿 王让定 《计算机工程与应用》 CSCD 北大核心 2007年第11期204-207,共4页
在分析各种P2P网络特点的基础上,提出了分布式环境中基于超节点的P2P e-Learning模型。对特定peer组中超节点的加入和超节点的服务进行了具体描述。针对e-Learning的特性,给出了模型中的数据抽象和本体描述,并对该P2P网络模型在文件传... 在分析各种P2P网络特点的基础上,提出了分布式环境中基于超节点的P2P e-Learning模型。对特定peer组中超节点的加入和超节点的服务进行了具体描述。针对e-Learning的特性,给出了模型中的数据抽象和本体描述,并对该P2P网络模型在文件传输方面的性能做了分析。 展开更多
关键词 超节点 peer to peer(p2p) 远程教育
下载PDF
P2P网络Peer访问Web Service的研究和实现 被引量:4
8
作者 翟丽芳 张卫 《微电子学与计算机》 CSCD 北大核心 2004年第5期30-32,40,共4页
文章首先简要介绍了P2P网络和WebService的技术背景,接着提出了P2P网络与WebService集成的思想,为此提出WebServiceBroker的概念,从而实现P2P网络Peer透明访问WebService。
关键词 p2p(peer to peer) WEB Serivice WEB Service Broker(WSB)en i c'P BIokel'(Ⅵ’SB)
下载PDF
一种基于Super Peer结构的P2P视频点播系统的设计与实现 被引量:3
9
作者 赵振华 张新家 《科学技术与工程》 2008年第7期1866-1870,共5页
分析各种P2P网络的特点,为了使用户在视频点播过程中获得快速、稳定的流媒体数据,提出了一种采用Super Peer结构的P2P视频点播设计方案。讨论了以该方案为基础的拓扑树的构建、拓扑树的变更等关键技术,并对该系统进行了性能分析,证明其... 分析各种P2P网络的特点,为了使用户在视频点播过程中获得快速、稳定的流媒体数据,提出了一种采用Super Peer结构的P2P视频点播设计方案。讨论了以该方案为基础的拓扑树的构建、拓扑树的变更等关键技术,并对该系统进行了性能分析,证明其具有较好的可靠性和稳定性。 展开更多
关键词 Super peer p2p 视频点播 流媒体
下载PDF
Deadline-aware network coding for video on demand service over P2P networks 被引量:13
10
作者 CHI Hui-cheng ZHANG Qian 《Journal of Zhejiang University-Science A(Applied Physics & Engineering)》 SCIE EI CAS CSCD 2006年第5期755-763,共9页
We are interested in providing Video-on-Demand (VoD) streaming service to a large population of clients using peer-to-peer (P2P) approach. Given the asynchronous demands from multiple clients, continuously changing of... We are interested in providing Video-on-Demand (VoD) streaming service to a large population of clients using peer-to-peer (P2P) approach. Given the asynchronous demands from multiple clients, continuously changing of the buffered contents, and the continuous video display requirement, how to collaborate with potential partners to get expected data for future content delivery are very important and challenging. In this paper, we develop a novel scheduling algorithm based on deadline- aware network coding (DNC) to fully exploit the network resource for efficient VoD service. DNC generalizes the existing net- work coding (NC) paradigm, an elegant solution for ubiquitous data distribution. Yet, with deadline awareness, DNC improves the network throughput and meanwhile avoid missing the play deadline in high probability, which is a major deficiency of the con- ventional NC. Extensive simulation results demonstrated that DNC achieves high streaming continuity even in tight network conditions. 展开更多
关键词 Video on Demand (VoD) peer-to-peer (p2p) network CODING (NC) Deadline-aware network CODING (DNC)
下载PDF
METrust: A Mutual Evaluation-based Trust Model for P2P Networks 被引量:2
11
作者 Chun-Ling Cheng Xiao-Long Xu Bing-Zhen Gao 《International Journal of Automation and computing》 EI 2012年第1期63-71,共9页
It is necessary to construct an effective trust model to build trust relationship between peers in peer-to-peer (P2P) network and enhance the security and reliability of P2P systems. The current trust models only fo... It is necessary to construct an effective trust model to build trust relationship between peers in peer-to-peer (P2P) network and enhance the security and reliability of P2P systems. The current trust models only focus on the consumers' evaluation to a transaction, which may be abused by malicious peers to exaggerate or slander the provider deliberately. In this paper, we propose a novel trust model based on mutual evaluation, called METrust, to suppress the peers' malicious behavior, such as dishonest evaluation and strategic attack. METrust considers the factors including mutual evaluation, similarity risk, time window, incentive, and punishment mechanism. The trust value is composed of the direct trust value and the recommendation trust value. In order to inhibit dishonest evaluation, both participants should give evaluation information based on peers' own experiences about the transaction while computing the direct trust value. In view of this, the mutual evaluation consistency factor and its time decay function are proposed. Besides, to reduce the risk of computing the recommendation trust based on the recommendations of friend peers, the similarity risk is introduced to measure the uncertainty of the similarity computing, while similarity is used to measure credibility. The experimental results show that METrust is effective, and it has advantages in the inhibition of the various malicious behaviors. 展开更多
关键词 peer-to-peer p2p network REPUTATION trust model mutual evaluation similarity risk
下载PDF
Cluster based hierarchical resource searching model in P2P network 被引量:1
12
作者 Yang Ruijuan Liu Jian Tian Jingwen 《Journal of Systems Engineering and Electronics》 SCIE EI CSCD 2007年第1期188-194,共7页
For the problem of large network load generated by the Gnutella resource-searching model in Peer to Peer (P2P) network, a improved model to decrease the network expense is proposed, which establishes a duster in P2P... For the problem of large network load generated by the Gnutella resource-searching model in Peer to Peer (P2P) network, a improved model to decrease the network expense is proposed, which establishes a duster in P2P network, auto-organizes logical layers, and applies a hybrid mechanism of directional searching and flooding. The performance analysis and simulation results show that the proposed hierarchical searching model has availably reduced the generated message load and that its searching-response time performance is as fairly good as that of the Gnutella model. 展开更多
关键词 Communication and information system Resource-searching model in p2p network GNUTELLA CLUSTER Hierarchical network
下载PDF
An Anonymity Scheme Based on Pseudonym in P2P Networks 被引量:1
13
作者 Peng Hao Lu Songnian +2 位作者 Li Jianhua Zhang Aixin Zhao Dandan 《China Communications》 SCIE CSCD 2010年第6期157-160,共4页
In this paper, we proposed an anonymity scheme based on pseudonym where peers are motivated not to share their identity. Compared with precious scheme such as RuP (Reputation using Pseudonyms), our scheme can reduce... In this paper, we proposed an anonymity scheme based on pseudonym where peers are motivated not to share their identity. Compared with precious scheme such as RuP (Reputation using Pseudonyms), our scheme can reduce its overhead and minimize the trusted center's involvement. 展开更多
关键词 ANONYMITY p2p networks pseudonym
下载PDF
Deep Q-Learning Based Optimal Query Routing Approach for Unstructured P2P Network 被引量:1
14
作者 Mohammad Shoab Abdullah Shawan Alotaibi 《Computers, Materials & Continua》 SCIE EI 2022年第3期5765-5781,共17页
Deep Reinforcement Learning(DRL)is a class of Machine Learning(ML)that combines Deep Learning with Reinforcement Learning and provides a framework by which a system can learn from its previous actions in an environmen... Deep Reinforcement Learning(DRL)is a class of Machine Learning(ML)that combines Deep Learning with Reinforcement Learning and provides a framework by which a system can learn from its previous actions in an environment to select its efforts in the future efficiently.DRL has been used in many application fields,including games,robots,networks,etc.for creating autonomous systems that improve themselves with experience.It is well acknowledged that DRL is well suited to solve optimization problems in distributed systems in general and network routing especially.Therefore,a novel query routing approach called Deep Reinforcement Learning based Route Selection(DRLRS)is proposed for unstructured P2P networks based on a Deep Q-Learning algorithm.The main objective of this approach is to achieve better retrieval effectiveness with reduced searching cost by less number of connected peers,exchangedmessages,and reduced time.The simulation results shows a significantly improve searching a resource with compression to k-Random Walker and Directed BFS.Here,retrieval effectiveness,search cost in terms of connected peers,and average overhead are 1.28,106,149,respectively. 展开更多
关键词 Reinforcement learning deep q-learning unstructured p2p network query routing
下载PDF
Hybrid ants-like search algorithms for P2P media streaming distribution in ad hoc networks
15
作者 ZUO Dong-hong DU Xu YANG Zong-kai 《Journal of Zhejiang University-Science A(Applied Physics & Engineering)》 SCIE EI CAS CSCD 2007年第8期1191-1198,共8页
Media streaming delivery in wireless ad hoc networks is challenging due to the stringent resource restrictions,po-tential high loss rate and the decentralized architecture. To support long and high-quality streams,one... Media streaming delivery in wireless ad hoc networks is challenging due to the stringent resource restrictions,po-tential high loss rate and the decentralized architecture. To support long and high-quality streams,one viable approach is that a media stream is partitioned into segments,and then the segments are replicated in a network and served in a peer-to-peer(P2P) fashion. However,the searching strategy for segments is one key problem with the approach. This paper proposes a hybrid ants-like search algorithm(HASA) for P2P media streaming distribution in ad hoc networks. It takes the advantages of random walks and ants-like algorithms for searching in unstructured P2P networks,such as low transmitting latency,less jitter times,and low unnecessary traffic. We quantify the performance of our scheme in terms of response time,jitter times,and network messages for media streaming distribution. Simulation results showed that it can effectively improve the search efficiency for P2P media streaming distribution in ad hoc networks. 展开更多
关键词 Ad hoc networks Media streaming distribution Search algorithms peer to peer p2p
下载PDF
Greedysearch based service location in P2P networks
16
作者 Zhu Cheng Liu Zhong Zhang Weiming Yang Dongsheng 《Journal of Systems Engineering and Electronics》 SCIE EI CSCD 2005年第4期886-893,共8页
A model is built to analyze the performance of service location based on greedy search in P2P networks. Hops and relative QoS index of the node found in a service location process are used to evaluate the performance ... A model is built to analyze the performance of service location based on greedy search in P2P networks. Hops and relative QoS index of the node found in a service location process are used to evaluate the performance as well as the probability of locating the top 5% nodes with highest QoS level. Both model and simulation results show that, the performance of greedy search based service location improves significantly with the increase of the average degree of the network. It is found that, if changes of both overlay topology and QoS level of nodes can be ignored during a location process, greedy-search based service location has high probability of finding the nodes with relatively high QoS in small number of hops in a big overlay network. Model extension under arbitrary network degree distribution is also studied. 展开更多
关键词 greedy-search service location p2p network.
下载PDF
Peer selecting model based on FCM for wireless distributed P2P files sharing systems
17
作者 李曦 纪红 郑瑞明 《Journal of Harbin Institute of Technology(New Series)》 EI CAS 2010年第5期593-599,共7页
IIn order to improve the performance of wireless distributed peer-to-peer(P2P)files sharing systems,a general system architecture and a novel peer selecting model based on fuzzy cognitive maps(FCM)are proposed in this... IIn order to improve the performance of wireless distributed peer-to-peer(P2P)files sharing systems,a general system architecture and a novel peer selecting model based on fuzzy cognitive maps(FCM)are proposed in this paper.The new model provides an effective approach on choosing an optimal peer from several resource discovering results for the best file transfer.Compared with the traditional min-hops scheme that uses hops as the only selecting criterion,the proposed model uses FCM to investigate the complex relationships among various relative factors in wireless environments and gives an overall evaluation score on the candidate.It also has strong scalability for being independent of specified P2P resource discovering protocols.Furthermore,a complete implementation is explained in concrete modules.The simulation results show that the proposed model is effective and feasible compared with min-hops scheme,with the success transfer rate increased by at least 20% and transfer time improved as high as 34%. 展开更多
关键词 wireless p2p files sharing systems peer selecting model fuzzy cognitive maps
下载PDF
INCENTIVE MECHANISMS FOR TRUSTWORTHY ROUTING BASED ON GAME MODEL OF THE STRATEGIES OF NODES IN P2P NETWORKS
18
作者 Wang Haoyun Zhang Shunyi +1 位作者 Li Jun Chen Zhixian 《Journal of Electronics(China)》 2009年第6期842-852,共11页
The trustworthiness and security of routing in the existing Peer-to-Peer (P2P) networks can not be ensured because of the diversity of the strategies of P2P nodes. This paper firstly uses game theory to establish game... The trustworthiness and security of routing in the existing Peer-to-Peer (P2P) networks can not be ensured because of the diversity of the strategies of P2P nodes. This paper firstly uses game theory to establish game model of the strategies and profits of various types of routing nodes. Then,two incentive mechanisms for the corresponding stages of P2P trustworthy routing are proposed,namely trust associated mechanism and trust compensated mechanism. Simulation results show that the incentive mechanisms proposed in this paper will encourage cooperation actions of good nodes and restrain malicious actions of bad nodes,which ensure the trustworthiness of routing consequently. 展开更多
关键词 peer-to-peer p2p network ROUTING Node GAME Strategy
下载PDF
Epidemiological modeling of passive worm propagation on the P2P file-sharing network
19
作者 Feng Chaosheng Qin Zhiguang +1 位作者 Laurissa Tokarchuk Muhammad Adeel 《High Technology Letters》 EI CAS 2011年第3期230-235,共6页
The peer-to-peer(P2P) file-sharing network as a vehicle of disseminating files has become very popular. The appearance of dozens of kinds of passive worms on this network has, however, made it unsecured. This proble... The peer-to-peer(P2P) file-sharing network as a vehicle of disseminating files has become very popular. The appearance of dozens of kinds of passive worms on this network has, however, made it unsecured. This problem has been paid attention and a few of models for passive worm propagation has been presented. Unfortunately, the dynamic properties of this network are ignored in these models. Given the fact, the characteristics of both this network and the passive worm are identified, and on this basis a new mathematical model of passive worm propagation on the P2P network is presented in applying epidemiology in this paper. Note that the dynamic properties of this network are considered in the presented model. The model has been validated by large scale simulation experiments, which demonstrates that the presented model may be used for analyzing the behaviors of passive worms and predicting the trend of their propagation. 展开更多
关键词 passive worms PROPAGATION modeling p2p networks FILE-SHARING simulation
下载PDF
Research on Video on Demand Based on P2P Network
20
作者 Min Cui 《Intelligent Information Management》 2010年第5期338-341,共4页
A P2P approaches to extend the ability of Video on Demand systems to serve more users. In the proposed system users share with each other the media data obtained and the media server is no longer the only source to ge... A P2P approaches to extend the ability of Video on Demand systems to serve more users. In the proposed system users share with each other the media data obtained and the media server is no longer the only source to get data from, thereby, the load on the media server could be greatly alleviated and the overall system capacity increases and more users could be served. The P2P streaming system introduces efficient searching;data transfer dynamically monitoring and initial buffering to maintain a high quality of playback. Its provider selection policy helps to reduce the load of the underlying network by avoiding remote data transfer. 展开更多
关键词 p2p peer to peer Video on DEMAND STREAMING Media SUPER-peer CACHE REPLACEMENT Algorithm
下载PDF
上一页 1 2 7 下一页 到第
使用帮助 返回顶部