Peer to Peer Networking

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内容中心网络原理介绍及其相关研究

内容中心网络原理介绍及其相关研究1研究背景1.1 当前互联网的现状从互联网产生至今，它给人们的生产、生活和学习带来了深刻改变，在感慨互联网技术成功经验的同时，也应该重视现有互联网面临的众多挑战：采用32位地址码的IPv4正面临着地址枯竭的境遇，难以更大规模扩展；网络安全漏洞多，可信度不高；网络服务质量控制能力弱，不能保障高质量的网络服务；网络带宽和性能不能满足用户的需求；传统无线移动通信与互联网属于不同技术体制，难以实现高效的移动互联网等等【1】。

互联网在设计的最初认为网络用户是基本友好的，对商业应用、用户移动性及应用多样性等需求都欠考虑。

互联网管理者和用户对网络的可知性也较差。

伴随着网络规模的逐年扩大而引起的扩展性问题更迅速成为了业界关注的重点。

显然，采用IP分组技术设计的传统互联网已经不能满足人们对网络规模、功能和性能等方面的需求。

因此，通力解决互联网在可扩展性、安全性和可控可管性等问题的需求十分迫切。

1.2下一代互联网架构体系的研究基于TCP/IP的现有互联网也逐渐暴露出许多的不适应和暴露出的问题，为了解决这些问题，当前国内外主要有“演进”和“革命”两种思路：一是不改变互联网IP的主体地位的“演进”方案；二是想要替代IP主体地位的网络“革命”方案。

“演进”即在现有的IPv4协议的互联网基础上上不断改良和完善网络，最终平滑过渡到IPv6的互联网，例如Peer-to-Peer、内容分发网络（Content Delivery Network, CDN）等；另一种“革命”思路以美国FIND/GENI项目为代表，即重新设计全新的互联网体系结构，满足未来互联网的发展需要，例如ICN。

1. 演进型解决方案在网络发展遇到瓶颈，人们对网络的使用需求超出了网络能力时，会有相应的解决方案被提出。

这些方案都是基于现有网络架构做出的改良和完善措施，可以归之为演进型方案。

这些已被采取的措施只能解决互联网面临的部分问题，而且大都以失败告终（见表1-1）。

peer-to-peer体系结构风格

Peer-to-Peer(P2P)体系结构风格**************************************************目录一. 名称二. 基本特征三. 技术特点四. 不变式五. 参考实现六. 典型应用附录：参考文献**************************************************一. 名称Peer-to-Peer，以下简称P2P，翻译成中文有“对等网络计算”、“点对点”等说法。

它是一种近年来在网络应用方面新出现的技术，可以说它是一种从宏观上对分布式系统架构的规范。

依赖网络中参与者的计算能力和带宽，而不是把依赖都聚集在较少的几台服务器上。

P2P通常被用在下载技术中，意思是在自己下载的同时，自己的电脑还要继续做主机上传，这种下载方式，人越多速度越快但缺点是对硬盘损伤比较大（在写的同时还要读），还有对内存占用较多，影响整机速度。

网络上有一种常见的说法Point to Point，也简写为P2P，其实和Peer-to-Peer描述的是同一件事物，但Peer-to-Peer的说法更为专业。

目前，在学术界、工业界对于P2P没有一个统一的定义，以下是几个常用的定义：1、Peer-to-peer is a type of Internet network allowing a group of computer users with the same networking program to connect with eachother for the purposes of directly accessing files from one another's hard drives.2、Peer-to-peer networking (P2P) is an application that runs on a personal computer and shares files with other users across the Internet. P2P networks work by connecting individual computers together to share files instead of having to go through a central server.3、P2P是一种分布式网络，网络的参与者共享他们所拥有的一部分硬件资源（处理能力、存储能力、网络连接能力、打印机等），这些共享资源需要由网络提供服务和内容，能被其它对等节点(Peer)直接访问而无需经过中间实体。

SNA

SNA standard network architecture与 OSI Model 一起，由 IBM 提出的系统网络体系结构（SNA）是一种使用较为普遍的网络体系结构模型。

尽管现在 SNA 模型被认为是一种旧网络模型，但仍然得到了普遍发展。

SNA 的设计采用了 IBM 大型机使用的主机到终端的通信模型。

IBM 扩展 SNA 协议以支持对等网络（peer-to-peer networking）。

这种扩展协议包括高级对等网络（APPN：Advanced Peer to Peer Networking ）和高级程序间通信（APPC：Advanced Program to Program Communication ），其中 APPN 代表了 IBM 第二代 SNA。

APPN 中，IBM 将 SNA 从分级型大型机中心环境移动到点对点（P2P）网络环境。

APPN 的核心是一个支持一个或多个不直接相连的 APPC 系统之间的对等通信、目录服务以及路由选择等服务的 IBM 体系结构。

IBM SNA 模型与 OSI 七层参考模型有很多相似之处。

所不同的是，SNA 模型只有六层，它并没有为物理控制层定义任何特殊协议，物理控制层是通过其它标准实现的。

SNA 各层功能描述如下：•数据链路层（DLC）：定义了同步数据链路控制规程（SDLC）和局域网（LAN）协议，如令牌环（Token Ring）网络通信协议。

SDLC 是 ISO HDSL 和 IEEE 802.2 的基础。

•路径控制（Path Control）：完成多种 OSI 网络层功能，包括路由选择和数据报的分割与重组（SAR）。

•传输控制（Transmission Control）：提供可靠的终端到终端（end-to-end）的连接服务（类似于TCP），以及加密（encrypting）和解密（decrypting）服务。

•数据流控制（Data Flow Control）：管理请求和响应处理、决定通信对象、集合信息以及中断数据流。

SonicWALL PRO 5060多服务高速网络安全平台说明说明书

Remote / Branch Office Telecommuters Partners RemoteUsersWLANZoneEngineeringZoneHRZoneDMZFail-over10/100/1000 MbpsInternetRemoteWLANsSonicWALL VPN ClientSonicWALL GMS• Multi-site WLAN managementand reporting• Integration with overall securitymanagementLegacy WLAN Zone• Basic connectivitySonicWALL WLAN Zone• IPSec, WPA, WEP• Guest services• User roaming• Rogue AP detectionand wireless IDPSonicWALLSonicWALLInternetDistributed WLAN Security and ManagementThe PRO 5060’s distributed WLAN capabilities rival those of the most sophisticated WLAN switch vendors on the market. With SonicWALL, you can easily integrate advanced WLAN services within the organization’s existing network and security architectures. And with over 500 Mbps of IPSec VPN throughput, the PRO 5060 can aggregate a massive number of access points (APs) for large WLAN deployments.Features■Automatic discovery and provisioning enabled by the SonicWALL Discovery Protocol (SDP) and SonicWALLSimple Provisioning Protocol (SSPP)■Utilizes SonicWALL’s easy-to-use Global VPN Client software for secure IPSec wireless communicationsuser or group basis■DHCP over VPN capability for centralized IP management ■Award-winning Global Management System (GMS) andSonicWALL ViewPoint® for comprehensive management and reportingInternetSonicWALL IPS ServiceInternal NetworkExistingFirewall / VPN AppliancePRO 5060 as in-line Intrusion Prevention Service• L2-4 Packet header inspection • Basic access control• Full L2-7 signature-based inspection • Application awarenessIngress Packet Ingress PacketRules, Identity Management Flow ClassifierThe nature of network security threats has evolved. While。

Peer-to-peer networking system using interconnecti

专利名称：Peer-to-peer networking system usinginterconnectivity framework and peer libraryfor interacting with applications发明人：Attila Vass,Howard Berkey,John P.Bates,Payton R. White申请号：US10859430申请日：20040601公开号：US07421708B2公开日：20080902专利内容由知识产权出版社提供专利附图：摘要：An interconnectivity framework, method, and system for communicating in apeer-to-peer network is disclosed. A peer of the interconnectivity framework includes a peer library for publishing, messaging and locating component blocks over the peer-to-peer network and a telespace framework for managing component blocks in response to a requirement of an application to be executed at the peer. The component blocks being obtained by the peer library enable execution of the application at the peer in accordance with the requirement. The requirement defines the type of application so that the appropriate component blocks can be obtained from the peer-to-peer network.A networker is further included to enable communication with specific grids of the peer-to-peer network and to enable the publishing, messaging, and locating of objects published by specific peers of a grid of the peer-to-peer network.申请人：Attila Vass,Howard Berkey,John P. Bates,Payton R. White地址：Foster City CA US,Foster City CA US,Foster City CA US,Foster City CA US国籍：US,US,US,US代理机构：Martine Penilla & Gencarella, LLP更多信息请下载全文后查看。

Peer-To-Peer介绍最近几年，Peer-to-Peer（对等计算，简称P2P）迅速

Peer-T o-Peer 介绍最近几年，Peer-to-Peer (对等计算，简称P2P) 迅速成为计算机界关注的热门话题之一，财富杂志更将P2P列为影响Internet未来的四项科技之一。

“Peer”在英语里有“对等者”和“伙伴”的意义。

因此，从字面上，P2P可以理解为对等互联网。

国内的媒体一般将P2P翻译成“点对点”或者“端对端”，学术界则统一称为对等计算。

P2P可以定义为：网络的参与者共享他们所拥有的一部分硬件资源（处理能力、存储能力、网络连接能力、打印机等），这些共享资源通过网络提供服务和内容，能被其它对等节点(Peer)直接访问而无需经过中间实体。

在此网络中的参与者既是资源（服务和内容）提供者（Server），又是资源获取者（Client）。

客观地说，这种计算模式并不是什么新技术，自从上个世纪70年代网络产生以来就存在了,只不过当时的网络带宽和传播速度限制了这种计算模式的发展。

90年代末，随着高速互联网的普及、个人计算机计算和存储能力的提升，P2P技术重新登上历史舞台并且带来了一场技术上的革命。

许多基于P2P技术的杀手级应用应运而生，给人们的生活带来了极大的便利。

从计算模式上来说，P2P打破了传统的Client/Server (C/S)模式，在网络中的每个结点的地位都是对等的。

每个结点既充当服务器，为其他结点提供服务，同时也享用其他结点提供的服务。

P2P与C/S模式的对比如下图所示:图1 Client/Server模式图2 Peer to Peer 模式P2P技术的特点体现在以下几个方面[1]：∙非中心化：网络中的资源和服务分散在所有结点上，信息的传输和服务的实现都直接在结点之间进行，可以无需中间环节和服务器的介入，避免了可能的瓶颈。

P2P的非中心化基本特点，带来了其在可扩展性、健壮性等方面的优势。

∙可扩展性：在P2P网络中，随着用户的加入，不仅服务的需求增加了，系统整体的资源和服务能力也在同步地扩充，始终能比较容易地满足用户的需要。

网络连接说明 NETWORKING CONNECTIVITY 英语介绍

NETWORKINGCONNECTIVITYExplanation and Working NETWORKING CONNECTIVITY is being described as the process of connecting the varied parts of networks to one another. A computer network possesses the collections of printers, computers and different others type of equipment that are connected together in such a manner so that communication can be held between them.At present, millions of people share information as well as data with each other via using somekind of networks. However, after seeing the given activity one most important question that tends to arises in the mind of an individual is that how the whole process of sharing information with the help of computer networks happen.On the other hand, the given thing will also put another question in a p erson’s mind that is how a machine can decide that specific message belongs to the particular computer. In this article, detail description is being given in relation to the respective aspect. We will explain you about networking connectivity in details.BASICS OF NETWORKING CONNECTIVITYPrior to improving the understanding of the Networking connectivityprocess, it is vital to improving the knowledge of the networks that are in computer networks. The discussion of the same is given below:Peer to Peer networksThe given form of network is used when there are not more than computers are being used in the network. On the other hand, in the respective type of network strict security is not very much essential. The term peer is beingused in this because all the computers that are being used in this have a similar type of status.In addition to this, they also use equal footing with an aim to communicate with each other. In this network, different types of files such as spreadsheet and word processing are shared. On the other hand, different computers on the network tend to share devices such as printers and scanners, etc. They all are connected to any one computer in the network.Client/server networkIt is being regarded as another type of network. In today’s scenario, the given form of network is more popular than the previous one. It can also be said that this network is used for the large network. Here, the server plays the role of storage. In this context, it stores different applications as well as files that are being shared over the network in an effectual way.The performance which is being given by server is higher than the performance of computers. In addition to this, apart from storing the files, the server also plays the role of controller in the network. In accordance with the given context, itcontrols the network access of the different other computers and they are referred from the name of client computers. We are giving you best knowledge related to the topic you have selected.MAJOR DEVICES OF NETWORKING CONNECTIVITYJust like above before getting into the working process of computer networking connectivity, we need to improve the understanding in relation to different major devices that tends to play a vital role in the networking connectivity. The details in relation to the same are given below:Network interface card (NIC):The name itself suggests its meaning. But in simple words, we can say that it is the card which interfaces between computer and network. Thus, it is basically an expansion card which is being installed on the computer. The main aim of this card is to connect the computer with the network. In addition to this, it can also be said that the given device will also provide an electrical, physical and electronic connection to the media in the network.The NIC comes in two forms. Here, either it could be in the form of an expansion card or itcould be built in the motherboard of the computer system. However, it has been examined that in most of the cases network interface card connects to the computers with the help of the expansion slot. The given slot is very much special. This is due to the reason that the respective slot placed on the computer’s motherboard. Due to this, peripheral can directly be plugged into this.On the other hand, it is also examined that NIC possesses LEDs (Light emitting diodes) which assist in the task of diagnosing problems that are associated with the functionality of NIC.Suppose if there are two different types of NIC is there then among two one will be Link LEDs whose main work is to show that when proper connectivity to the active network is being detected. This whole scenario will give an indication the card is receiving proper signals from the hub or the switch. Besides this, Active LED is the most popular LED. This indicates the receipt of framed from or to the network.Hub:It is also being considered as another major component of the networking connectivity. Herein, it can be said that in the star topologyEthernet network, the hub is basically the type of device which connects the different segments of the network together.In this single, single cable is used as a medium with an aim to form a connection between different devices in the network to the hub. In addition to this, it can also be said that any kind of transmission that is being received on the one port will be being sent out to all other port in the hub.This thing will allow CSMA/CD on the transmitter with regard to performing the function of monitoring for the collision. For example, if onesender sends something then in this situation station whose address mentioned in the sender list will receive it. However, it can also be said that the hub is nothing more than the glorified repeaters that do not possess the capability to recognize the frame boundaries as well as data structures. It is due to the presence of a given aspect only hubs play the act of tool which does not possess any intelligence.Switch:The third major component which is used in the networking connectivity is called by the name of the switch. It is just similar to the hub. Here, itforms the connection between the multiple segments of the network. However, one major difference that exists between hub and switch is that hubs do not possess any kind of filter.Thus, whatever thing that hub receives over the network will be sent to all other computers. But, in comparison to this the switch will basically recognize the frame boundaries and give attention to the MAC address of the incoming frames and on the basis of the given address switch will send the file to the correct the address. However, if file send does not possess any recognizable address then in the givensituation it will be again sent to the sender location.In addition to this, in many satiation switch will play the role like a hub. For example, the location of the destination is not known then in the given situation switch will react much like a hub and as a result of this it floods out the frame at each and every port. However, except for the port over which it is being received. The switch is more effective than the hub because it is able to give support to the full wire speed on every port.Bridge:It is basically called by the name of the transparent bridge. It is being typed of a network device that tends to connect the two similar segments of networks together. The main role of the bridge is to keep the traffic separated from the different sides of the bridge. From this, it can be said that the bridge is also function just like its name. Here, information or the packets is being transmitted to the other side of the bridge if it is intended for the station on the other side. In addition to this, it can also be said that the main reason for using the bridge in the networking connectivity is to form the connectionbetween two segments and to divide the bust networks into two segments.Router:It is being regarded as the most popular term which is being used in the networking connectivity. This connects multiple segments of the network in the inter-network. In the networking, a router plays the role of the decision and maker and thus it decides that how in best and possible manner network data can be sent to its respective destination on the network. However, it is to be evaluated that the work of the router is very complex in nature.Here, CPU is dedicated to the task of routing different functions. Further, due to the complexity of router, it is possible for with regard to performing the functions of other types of devices such as firewalls and gateways, etc that is also involved in the network. This can be made possible by implementing the features of the given devices in the software of the router.Gateway:It is the combination of the hardware and software that tends to connect the dissimilar network environment. It is also being regarded as the most complex of the network environment.This is due to the reason that gateway tends to perform translation of multiple layers of the OSI model in an effectual way. For instance, the gateway is the type of device which connects the LAN environment to the mainframe environment. These two environments are completely different from each other. LAN environment complies with the distributed processing whereas the mainframe environment complies with centralized processing.Thus, we can say that networking connectivitycomprises of these all main devices that enable all the things working over the network. But,apart from this, there are other devices also such as modem and wireless access points also that also have a very important role in the network. Along with these two devices, detail description is being given in relation to other devices also that has importance in the computer network.Modem:Modem performs the function of modulation and demodulation and thus it converts digital data into analog data and the vice versa. It is of three basic types such as POTS, DSL, and Cable, etc.Wireless Access Network: The wireless access network gives the opportunity to the mobile user with regard to connecting themselves over the network with the help of wireless devices. It is also similar to hub and switch but it connects multiple wireless devices to form the network.Firewall: It is being regarded as one of the most important devices in networking connectivity. The main role of a firewall is to provide protection to the LAN resources from the attackers that are present on the internet. On the other hand, the firewall also assists in the task of preventing the computers over the network withregard to access the varied types of services on the internet. Additionally, this can also be used as the filter packets that are based on the rules and network administrative sets. The given rules will basically state that what are the specific types of information or the data can be flow on the computer network. Thus, it assists in the process of maintaining the security of computer over the network that is prone to the many types of risks. In simple words, it can also be stated that the firewall is the standalone black box that can be set up in software on the server or router. The firewall should have two basic types ofnetwork. Here, one should be on the public side and others should be at the private side.Transceivers:This is also called by the name of the media converter. On the other hand, transceivers are small devices that can be seen on the network. These devices are very simple in nature that allows NIC (Network interface card) and the other networking devices to form the connection with the different type of Medias. On the other hand, there are many NIC that tends to possess a special converter that tends to give theopportunity to this to perform the functions like hubs and switches, etcMANNER IN WHICH NETWORKING CONNECTIVITY WORKSIn the earlier part, we have improved the knowledge about the major components of the networking connectivity. Hence, in this section discussion is being carried out in relation to the overall process of the computer network. Thus, here the answer is given to the question that is “How Networking connectivity works?”.How Networking connectivity worksThe diagram which is being depicted above simply defines the whole process which is being carried out on the networking connectivityin an effectual manner. The whole process begins with the internet. The internet possesses lots and lots of information that are being shared between people via mediums like the mobile phone and computer etc.The computers that are present in the network tend to have a connection to the network. If one computer sends some information in this situation the information is being received by all the computers that are present in the network.However, each computer in the network will have different MAC address that distinguishes themselves from each other.For example, person A has transferred some file to B over the internet. Then in the given situation, the file sent by A will give an address which is called by the name of MAC.After the given phase, the data sent will be transferred over the internet. In this phase, the role of the router comes. The main function of a router is to route the specific data packets into a particular location as per their MAC address.From this, it can be said that for computer network router plays the function like the human brain and thus it gives direction to the data packets. However, after going through with the router phase the data which is being sent by the person A will go to the switch. The main function of the switch is to further provide direction to the data packets. Thus, it segregates the data as per the address which is being written over it.After the function of switches accomplished the data is being sent to their specific destination and after the packets clear the switch path then it will arrive at the network interface which isbasically a card that forms the interface between the network and the computer.After that packets are being opened and information about the URL address is being taken. However, after this, the data packet encounters the firewall. The main aim of the firewall is to restrict the access of virus and other ineffective information over the internet.Once the data will clear the firewall step, then in the given situation the data will transfer through LAN or the wireless network. Thus, it can be said that it is the end of computer networkprocess. Here, finally, the data packets will be sent to the respective user.Here, the information which is being sent by A is being received by B. It is the very simple way to improve the understanding regarding the whole computer networking process.BENEFITS OF NETWORKINGAs we have seen that how computer networking makes the whole process of transferring information from one place to another quite simple. In accordance with the given context,there are different benefits examined that are related to the concept of networking.It allows cost effective resource sharing: It is being regarded as one of a most significant benefit that is associated with the concept like networking. In this regard, it can be said that with the help of networking an individual can share information from one place to another without investing much money on the same. It could be proved as the beneficial aspect for the companies. This is due to the reason that the money saved from this activity can be used by firm for some other important purpose.Improves storage efficiency as well as volumes: This happen when the business enterprise tends use network for storing different information. Here, individual can store huge data over network.CONCLUSIONIt can be stated from the whole study that in order to improve the knowledge about the whole working process of networking connectivity. It is important for the individual that it should improve its understanding of different major terminologies that are associated with the same. This is due to the reason that if the given thing isnot improved then in this situation it will become very difficult for the individual with regard to understanding the whole process of network working in an effectual way.。

学术英语课后答案 unit1

学术英语理工教师手册Unit 1 Choosing a TopicI Teaching ObjectivesIn this unit , you will learn how to:1.choose a particular topic for your research2.formulate a research question3.write a working title for your research essay4.enhance your language skills related with reading and listening materials presented in this unit II. Teaching Procedures1.Deciding on a topicTask 1Answers may vary.Task 21 No, because they all seem like a subject rather than a topic, a subject which cannot be addressed even by a whole book, let alone by a1500-wordessay.2Each of them can be broken down into various and more specific aspects. For example, cancer can be classified into breast cancer, lung cancer, liver cancer and so on. Breast cancer can have such specific topics for research as causes for breast cancer, effects of breast cancer and prevention or diagnosis of breast cancer.3 Actually the topics of each field are endless. Take breast cancer for example, we can have the topics like:Why Women Suffer from Breast Cancer More Than Men?A New Way to Find Breast TumorsSome Risks of Getting Breast Cancer in Daily LifeBreast Cancer and Its Direct Biological ImpactBreast Cancer—the Symptoms & DiagnosisBreastfeeding and Breast CancerTask 31 Text 1 illustrates how hackers or unauthorized users use one way or another to get inside a computer, while Text2 describes the various electronic threats a computer may face.2 Both focus on the vulnerability of a computer.3 Text 1 analyzes the ways of computer hackers, while Text 2 describes security problems of a computer.4 Text 1: The way hackers “get inside” a computerText 2: Electronic threats a computer facesYes, I think they are interesting, important, manageable and adequate.Task 41Lecture1:Ten Commandments of Computer EthicsLecture 2:How to Deal with Computer HackersLecture 3:How I Begin to Develop Computer Applications2Answersmay vary.Task 5Answers may vary.2 Formulating a research questionTask 1Text 3Research question 1: How many types of cloud services are there and what are they? Research question 2: What is green computing?Research question 3: What are advantages of the cloud computing?Text 4Research question 1: What is the Web 3.0?Research question 2: What are advantages and disadvantages of the cloud computing? Research question 3: What security benefits can the cloud computing provide?Task 22 Topic2: Threats of Artificial IntelligenceResearch questions:1) What are the threats of artificial intelligence?2) How can human beings control those threats?3) What are the difficulties to control those threats?3 Topic3: The Potentials of NanotechnologyResearch questions:1) What are its potentials in medicine?2) What are its potentials in space exploration?3) What are its potentials in communications?4 Topic4: Global Warming and Its EffectsResearch questions:1) How does it affect the pattern of climates?2) How does it affect economic activities?3) How does it affect human behavior?Task 3Answers may vary.3 Writing a working titleTask 1Answers may vary.Task 21 Lecture 4 is about the security problems of cloud computing, while Lecture 5 is about the definition and nature of cloud computing, hence it is more elementary than Lecture 4.2 The four all focus on cloud computing. Although Lecture 4 and Text 4 address the same topic, the former is less optimistic while the latter has more confidence in the security of cloud computing. Text3 illustrates the various advantages of cloud computing.3 Lecture 4: Cloud Computing SecurityLecture 5: What Is Cloud Computing?Task 3Answers may vary.4 Enhancing your academic languageReading: Text 11.Match the words with their definitions.1g 2a 3e 4b 5c 6d 7j 8f 9h 10i2. Complete the following expressions or sentences by using the target words listed below with the help of the Chinese in brackets. Change the form if necessary.1 symbolic 2distributed 3site 4complex 5identify6fairly 7straightforward 8capability 9target 10attempt11process 12parameter 13interpretation 14technical15range 16exploit 17networking 18involve19 instance 20specification 21accompany 22predictable 23profile3. Read the sentences in the box. Pay attention to the parts in bold.Now complete the paragraph by translating the Chinese in brackets. You may refer to the expressions and the sentence patterns listed above.ranging from（从……到）arise from some misunderstandings（来自于对……误解）leaves a lot of problems unsolved（留下很多问题没有得到解决）opens a path for（打开了通道）requires a different frame of mind（需要有新的思想）4.Translate the following sentences from Text 1 into Chinese.1) 有些人声称黑客是那些超越知识疆界而不造成危害的好人（或即使造成危害，但并非故意而为），而“骇客”才是真正的坏人。

计算机网络第五版(英文版)

– 物联网高速接入和应用技术
• Co-operator: EPFL • 正申请上海市科委项目
This is a Bilingual Course
• Why we give this bilingual course?
– To meet the needs of excellent engineer training program and internationalization
– Final exam (40%), middle exam (20%), experiments (20%), reports/problems (10%), and others (10%) – After study Chapter 4, there will be mid-term exam
– Deployment Models
• • • • Private Cloud Community Cloud Public Cloud Hybrid Cloud
Source: Wikipedia
1.2 Uses of Computer Networks (5)
• Client-server module
Source: David Lazer et al., “Computational Social Science”, SCIENCE, 323, 721-724 (2009)
Exploring Study (1)
1. Use complex network theory and dynamics of human behavior (DHB) to analyze social networks and optimize social networking service (SNS). 2. Research on opinion evolution and interference model on Internet, specially on social networking service (SNS).

网络组建对等网(Peer-to-Peer)通信模式

网络组建对等网（Peer-to-Peer）通信模式对等网（Peer-to-Peer）通信是非结构化的访问资源。

网络中的所有设备可直接访问数据、软件和其他网络资源。

每一台网络计算机与其他联网的计算机是对等的，它们没有层次的划分。

如图1-10所示为一对等网网络结构的实例，从图中我们可以看出每台计算机都连接了外部设备，如打印机、传真机以及扫描仪，并且每台计算机都可以使用网络上的其他任何外部设备。

图1-10 对等网络结构对于客户/服务器模式来说，对等网既存在一定的优点，也有其局限性，用户在实际应用时，要充分考虑各方面的情况进行选择，不要只看到其优点而忽视了其局限性。

1．对等网的优点对等网作为一种简单的网络，有以下4个主要优点：●相比较容易实现和操作对等网是一组具有网络功能允许对等的资源共享的计算机群。

因此，建立一个对等网只需获得和安装局域网的一个或多个集线器、计算机、连接导线以及提供资源访问的操作系统就可以了。

●成本低对等网不需要昂贵、复杂、精密的服务器和服务器需要的特殊管理和环境条件。

实际上，没有精密的服务器也消除了人员配备和训练及维护费用，同时也不需要为服务器建立一个温度、湿度可调节的房间。

从理论上说，每一台计算机只需要由使用它的用户来维护。

●不需要专门的昂贵的网络操作系统对等网可使用人们熟悉的操作系统来建立，例如Windows 2000/XP、Windows Server 2003等，不需要专门的昂贵的网络操作系统。

●架构简单、易于维护对等网没有层次依赖，因此，它比基于服务器的网络由更大的容错性，更易于维护。

从理论上说，客户/服务器网络中的服务器是一个单故障点。

单故障点是可以影响整个网络的弱点。

而在对等网中，任何计算机发生故障只会使网络连接资源的一个集变为不可使用。

2．对等网的局限性虽然对等网存在许多优点，但同时也有许多弱点，它在安全性、性能和管理方面存在很大的局限性。

其局限性包括以下几方面：●用户必须保留多个口令，以便进入他们需要访问的计算机。

信息技术专业英语-第8章

一台受感染的计算机会使整个网络非常脆弱因此杀毒软件必须快速有效地隔离受感染的文件甚至是隔离受感染的计算机这是最坏的情况了
信息技术专业英语
第八章
English for Information Technology
Chapter 8.rk and Computer Security 1. Lead in 2. Passage A (Text, Words and Phrases, Notes, Chinese Version) 3. Exercises (Exercise 1, Exercise 2, Exercise 3, Exercise 4, Exercise5, Exercise6)
A white hat hacker：A white hat hacker is a computer security specialist who breaks into protected systems and networks to test and check their security. White hat hackers use their skills to improve security by exposing vulnerabilities before malicious hackers (known as black hat hackers) can detect and exploit them.
Network and Computer Security A network topology represents its layout or structure from the point of view of data flow. In socalled bus networks, for example, all of the computers share and communicate across one common conduit, whereas in a star network, all data flows through one centralized device. Common types of network topologies include bus, star, ring and mesh.

Research Interests

Bin Wu2916 S. Shields Ave. #1N Email: bwu3@ Chicago, IL60616 Cell Phone: 312-203-5012 ObjectiveTo find a challenging full-time research/development position in the areas of P2P networking and Web protocol design.Education (GPA: 3.75/4.0) Ph.D. in Computer Science expected 06/2006 University of Illinois at Chicago Chicago,ILMaster in Computer Science 2002 University of Illinois at Chicago Chicago,ILMaster of Engineering 1995 ZheJiang University ZheJiang, China Bachelor of Science 1992 ZheJiang University ZheJiang, China Research InterestsDistributed Systems, Peer-to-Peer Networks, Networks Protocols and Algorithms, Web-BasedMedication Systems.Publications1. B. Wu, A. D. Kshemkalyani, “Objective-Optimal Algorithms for Long-Term Web Prefetching,”IEEE Transactions on Computers, December 2005.2. B. Wu, A.D. Kshemkalyani, “Objective-Greedy Algorithms for Long-Term Web Prefetching”,3rd IEEE Conference on Network Computing and Applications (NCA), 61-68, August 2004.3. A.D. Kshemkalyani, B. Wu, “Nonintrusive Snapshots Using Thin Slices”, The 2005 IFIP InternationalConference on Embedded And Ubiquitous Computing (EUC-05), Japan, 6-9 December 2005.4. B. Wu, A.D. Kshemkalyani, “Evaluation of Analysis Models for Unguided Search in UnstructuredP2P Networks”, IFIP International Symposium on Network-Centric Ubiquitous Systems (NCUS2006), LNCS 4097, Springer, 163-172, 2006.5. B. Wu, A.D. Kshemkalyani, “Analysis Models for Blind Search in Unstructured Overlays”, 5th IEEESymposium on Network Computing and Applications (NCA), 223-226, 2006.Reviewer•Computer Network (COMNET) Journal.•4th IEEE International Conference of Peer-to-Peer (P2P) Network.•7th International Workshop on Distributed Computing (IWDC), 2005.Honors and Certificates•UIC Provost’s Award for Graduate Research, 2005.•UIC University Fellowship, 1999-2000 Academic Year.Technical Skills•Language: , , , ASP, C/C++, C#, Java, Visual Basic 6, DHTML, VB Script, Java Bean, JavaScript, XML/XSLT, SQL/Transact-SQL, WML•Operating Systems: Unix, Windows 2000 Server/98/XP•Programming Platforms: Microsoft Visual Studio, Microsoft Visual , Microsoft Office, J-Builder•Programming Skills: MFC, OLEDB, ActiveX, COM/COM+, IIS 5.0/6.0 Programming (Server Extension, ISAPI Filter)•DBMS: SQL Server 2000, Oracle 8/8i, MS Access/FoxProWorking Experience1. Software Engineer University of Illinois Medical College 01/2000-PresentAs a core team member, I designed and developed the AvidCare Tele-Medicine system. This web-based system is used for physicians and medical professionals to provide medical services to registered patients. Physicians log on to the AvidCare Tele-Medicine system to monitor and advise theirpatients who may reside all over the world. In addition, this system organizes patients, physicians, carecenters and monitoring devices with full consideration of security, efficiency, scalability and robustness.This system has been submitted to FDA examination. My contributions to this project include: •Designed the system database using SQL Server 2000.•Designed and implemented the Web application for AvidCare Tele-Medicine system•Developed an Expert Diagnostic Toolkit.•Built a real time Alarm Paging system.•Implemented a wireless version of the Tele-Medicine system with WAP protocol and WML language.2. Research Assistant Department of Mechanical Engineering at UIC 01/1998-01/2000Modified and enhanced an academic program for combustion computation (OPPDIF by Sandia Lab) to perform micro-gravity combustion radiation computation for a NASA founded micro-gravityproject.3. Software Developer ZheJiang University, China 04/1995-11/1997•Designed, implemented and maintained a real-time monitoring and control network (MCN) for Power Industrial processes. The tasks of data collection, data analysis and generationof control commands are distributed over the network which is composed of datacollectors, PCs and industrial control units. Compared with centralized control systemsthat were widely used in industry processes, our distributed version was especiallyadvantageous with regards to cost, reliability and maintenance. Since the time this projectwas completed in 1997, it has been installed and used in a large number of power plantsand industrial companies in China.•Designed a Computer Aided Instruction Tool Kit using C++, VB, Microsoft Access and Microsoft Graphic User Interface. This set of software tools are used by course instructorsor professors to create a series of interactive lectures in the form of user-friendly computerprograms that help students better understand course materials with various approaches.ReferencesAvailable on demand。

精选最新版2020年安全专业模拟复习题库588题(含答案)

2020年安全专业考试复习题库588题【含答案】一、选择题1．7、下面对于IIS的wwwroot目录的权限配置，说法正确的是：A、一个目录在操作系统上被分配的权限与在IIS中的被分配的权限无关B、如果wwwroot目录下存在脚本，则必须打开脚本资源访问权限C、如果打开了目录浏览权限，如果缺省网页不存在（index.html）时，客户端就可以直接浏览到wwwroot目录的目录结构D、此目录必须具备读取权限，否则通过ie就不能访问到此站点参考答案：CD2．按照业务横向将支撑系统划分，可分为以下安全域：（）。

A、业务支撑系统安全域、网管系统安全域、企业信息化系统安全域。

B、集团网管系统安全子域、省公司网管系统安全子域、地市分公司的网管系统安全子域。

C、互联接口区、核心生产区、日常维护管理区（维护终端）、第三方接入区（漫游区）、DMZ区。

参考答案：A3．每一个安全域总体上可以体现为以下的层面：（）。

A、接口层B、核心层C、系统层D、网络层参考答案：ABC4．设备日志应支持记录用户对设备的操作,记录需要包括（）。

A、用户账号、操作时间、操作内容以及操作结果。

B、操作系统、操作时间、操作内容以及操作结果。

C、操作次数、操作时间、操作内容以及操作结果。

D、登陆次数、操作时间、操作内容以及操作结果。

参考答案：A5．18职责分离的主要目的是？A．不允许任何一个人可以从头到尾整个控制某一交易或者活动；B．不同部门的雇员不可以在一起工作；C．对于所有的资源都必须有保护措施；D．对于所有的设备都必须有操作控制措施。

参考答案：A6．12．FINGER服务使用哪个TCP端口？A．69B.119C.79D.70参考答案：C7．8．MD5产生的散列值是多少位？A．56B．64C．128D．160参考答案：C8．1.TCP/IP协议的4层概念模型是？A.应用层、传输层、网络层和网络接口层B.应用层、传输层、网络层和物理层C.应用层、数据链路层、网络层和网络接口层D.会话层、数据链路层、网络层和网络接口层参考答案：A9．7、WindowsDNS安全动态更新的方法，是其他DNS服务器所不具备，下面说法正确的是：A、安全动态更新是基于WindowsAD服务，通过AD的权限控制来完成B、通过AD可以对DNS管理的区域等对象进行基于用户的权限分配C、在DHCP的网络环境里提供域名服务，此方法比较适用D、以上说法均不正确参考答案：ABC10．6、相对于BIND，WindowsDNS存在很多的不足，处在一个被取代的趋势，下面说法正确的是：A、在认证、加密、访问控制上存在缺陷B、windows代码不公开，不像BIND经过严格测试评估C、在软件开发上，BIND投入很大，对DNS这个领域的发展有很大的影响D、BIND有很高的使用率参考答案：ABCD11．What is a PRIMARY reason for designing the security kernel to be as small as possible?A. The operating system cannot be easily penetrated by users.B. Changes to the kernel are not required as frequently.C. Due to its compactness, the kernel is easier to formally verify.D. System performance and execution are enhanced.Answer: C12．9、下面那些方法，可以实现对IIS重要文件的保护或隐藏？A、通过修改注册表，将缺省配置文件改名，并转移路径B、将wwwroot目录，更改到非系统分区C、修改日志文件的缺省位置D、将脚本文件和静态网页存放到不同目录，并分配不同权限参考答案：ABCD13．风险评估的内容包括：（）A、识别网络和信息系统等信息资产的价值。

Peer-to-PeerProtocols and Systems

KaZaA: File Search
search(Xena.mp3) --> 123.2.22.50 123.2.22.50 Query Replies search(Xena.mp3) --> 123.2.0.18
Where is Xena.mp3?
123.2.0.18
28
KaZaA: Fetching
24
KaZaA: Overview
• “Smart” Query Flooding:
– Join: on startup, client contacts a “supernode” ... may at some point become one itself – Publish: send list of files to supernode – Search: send query to supernode, supernodes flood query amongst themselves. – Fetch: get the file directly from peer(s); can fetch simultaneously from multiple peers
20
• TTL limits propagation
Gnutella: Search
“I have XenaEpisode1.mpg”
Reply “I have XenaEpisode1.mpg”
Query “Where is XenaEpisode1.mpg?”
21
Gnutella: Discussion
15
Napster: Search
Query “Where is the Xena Theme song?” Fetch search(Xena) Reply

对等网组建方法及流程

对等网组建方法及流程Building a peer-to-peer network involves a series of methods and processes that require careful consideration and planning. 对等网的组建涉及一系列方法和过程，需要仔细考虑和规划。

When establishing a peer-to-peer network, it is essential to first define the objectives and goals of the network. These may include the need for decentralized communication, sharing of resources, or establishing a secure and scalable network. 在建立对等网络时，首先要定义网络的目标和目标。

这些可能包括分散式通信的需求、资源共享或建立一个安全和可扩展的网络。

Once the goals are established, the next step is to identify the appropriate peer-to-peer network architecture. This involves determining the type of peer-to-peer network, whether it is a pure peer-to-peer network, a hybrid peer-to-peer network, or a structured peer-to-peer network. 一旦目标确定，下一步就是确定合适的对等网络架构。

这涉及确定对等网络的类型，无论是纯对等网络、混合对等网络还是结构化对等网络。

The method of building a peer-to-peer network also requires careful consideration of the network topology. This includes determining how peers will be connected and how data will be routed within the network. 对等网络的构建方法还需要仔细考虑网络的拓扑结构。

D-Link DWL-G122 快速说明书

DWL-G122If any of the above items are missing, please contact your reseller.Windows XP, Windows2000, Windows Me, Windows 98SEYou must have at least the following:•You will need a computer with an available USB port to connect the DWL-G122Wireless USB adapter.•At least a 300 MHz processor and 32 MB of memory•An 802.11b/g Access Point (for Infrastructure Mode) or another 802.11b/gwireless adapter (for Ad-Hoc; Peer-to-Peer networking mode.)•Properly installed and working USB Controller.Check Your Package ContentsThese are the items included with your DWL-G122 purchase:If the CD Autorun function does not automatically Go to Start > Run on your computer, then type“D:\PC\Driver\Setup.exe .” If it does start, proceed to the next screen.Turn on the computer andInsert the D- Link Air Plus TM G DWL-G122Driver CD in the CD-ROM drive.The step-by-step instructions that follow are shown in Windows XP . The steps and screens are similar for the other Windows operating systems.ComputerDo the DWL-G122The letter CD-ROM drive may be a different drive letter.Computer (cont.)Installing the DWL-G122 Wireless USBAdapter to Your ComputerAntennaUsed to wirelessly connect to 802.11bnetworks.Product OverviewTurn on Your ComputerWhen you restart your computer this Found New Hardware Wizard (Windows XP) screen will appear:Digital Signature NotYour Installation is Complete!After you’ve continued in Windows XP (or after the computer restarts in the other Windows operating systems), the D-Link Air Plus TM G DWL-G122 Configuration Utility will automatically start and the utility icon will appear in the bottom right hand corner of the desktop screen (systray). If this icon appears GREEN , then you have successfully installed the DWL-G122, are connected to awireless network and are ready to communicate!For Windows XP , if you wish to use the Air Plus USB Utility , pleaseperform the following steps.For Windows Me and 98SE, this screen may appear.Your Installation is Complete! (Cont.)Connecting to a Wireless NetworkC onnecting to a Wireless Network (cont.)Appendix B - Link Info Using the Configuration UtilityFor Windows Operating SystemsE C DF B AG H IJ K L MNAppendix B (cont.)ConfigurationOPlease refer to the manual on your driver CD for more in depth information.。

peer的用法

peer的用法1. Peer can be used as a noun to refer to a person who is equal to another in abilities, qualifications, age, or status. For example, "She received recognition from her peers for her outstanding performance."2. Peer can also be used as a verb to mean looking closely or carefully. For example, "The students were asked to peer through the microscope to observe the cells."3. Peer can be used as a prefix to form words such as peer-reviewed, which refers to a process where experts in a specific field review and evaluate the work of others before it is published or accepted.4. In the context of computer networks, peer refers to a computer or device that is connected to the same network and can communicate with other devices on that network. For example, "The Internet relies on peers to share and distribute data."5. Peer can also be used in the phrase "peer pressure," which refers to the influence exerted by a group or individual to conform to their behavior, attitudes, or values. For example, "She succumbed to peer pressure and started smoking."Overall, the term "peer" can be used in various contexts, including social, professional, technological, and psychological, to denote equality, observation, evaluation, networking, and influence.。

mmcn

A Measurement Study of Peer-to-Peer File Sharing SystemsStefan Saroiu,P.Krishna Gummadi,Steven D.GribbleDept.of Computer Science and Engineering,Univ.of Washington,Seattle,WA,98195-2350ABSTRACTThe popularity of peer-to-peer multimediaﬁle sharing applications such as Gnutella and Napster has created a ﬂurry of recent research activity into peer-to-peer architectures.We believe that the proper evaluation of a peer-to-peer system must take into account the characteristics of the peers that choose to participate.Surprisingly, however,few of the peer-to-peer architectures currently being developed are evaluated with respect to such considerations.In this paper,we remedy this situation by performing a detailed measurement study of the two popular peer-to-peerﬁle sharing systems,namely Napster and Gnutella.In particular,our measurement study seeks to precisely characterize the population of end-user hosts that participate in these two systems.This characterization includes the bottleneck bandwidths between these hosts and the Internet at large,IP-level latencies to send packets to these hosts,howoften hosts connect and disconnect from the system,howmany ﬁles hosts share and download,the degree of cooperation between the hosts,and several correlations between these characteristics.Our measurements showthat there is signiﬁcant heterogeneity and lack of cooperation across peers participating in these systems.Keywords:Peer-to-Peer,Network Measurements,Wide-Area Systems,Internet Services,Broadband1.INTRODUCTIONThe popularity of peer-to-peerﬁle sharing applications such as Gnutella and Napster has created aﬂurry of recent research activity into peer-to-peer architectures.1–6Although the exact deﬁnition of“peer-to-peer”is debatable,these systems typically lack dedicated,centralized infrastructure,but rather depend on the voluntary participation of peers to contribute resources out of which the infrastructure is constructed.Membership in a peer-to-peer system is ad-hoc and dynamic:as such,the challenge of such systems is toﬁgure out a mechanism and architecture for organizing the peers in such a way so that they can cooperate to provide a useful service to the community of users.For example,in aﬁle sharing application,one challenge is organizing peers into a cooperative,global index so that all content can be quickly and eﬃciently located by any peer in the system.2–4,6 In order to evaluate a proposed peer-to-peer system,the characteristics of the peers that choose to participate in the system must be understood and taken into account.For example,if some peers in aﬁle-sharing system have low-bandwidth,high-latency bottleneck network connections to the Internet,the system must be careful to avoid delegating large or popular portions of the distributed index to those peers,for fear of overwhelming them and making that portion of the index unavailable to other peers.Similarly,the typical duration that peers choose to remain connected to the infrastructure has implications for the degree of redundancy necessary to keep data or index metadata highly available.In short,the system must take into account the suitability of a given peer for a speciﬁc task before explicitly or implicitly delegating that task to the peer.Surprisingly,however,few of the architectures currently being developed are evaluated with respect to such considerations.We believe that this is,in part,due to a lack of information about the characteristics of hosts that choose to participate in peer-to-peer systems.We are aware of a single previous study7that measures only one such characteristic,namely the number ofﬁles peers share.In this paper,we remedy this situation by performing a detailed measurement study of the two most popular peer-to-peerﬁle sharing systems,namely Napster and Gnutella.The hosts that choose to participate in these systems are typically end-users’home or oﬃce machines,located at the“edge”of the Internet.Our measurement study seeks to precisely characterize the population of end-user hosts that participate in these two systems.This characterization includes the bottleneck bandwidths between these hosts and the Internet at large,IP-level latencies to send packets to these hosts,howoften hosts connect and disconnect from the system,howmanyﬁles hosts share and dow nload,and correlations betw een these characteristics.Our The authors may be contacted at{tzoompy,gummadi,gribble}@measurements—four days for Napster andThere are amount of heterogeneity of sharing vary between three any similar peer-to-peer peers tend to deliberately responsibility depends on peers to tell the truth,or2.METHODOLOGYThe methodology behind our measurements is quite simple.For each of the Napster and Gnutella systems,we proceeded in two steps.First,we periodically crawled each system in order to gather instantaneous snapshots of large subsets of the systems’user population.The information gathered in these snapshots include the IP address and port number of the users’client software,as well as some information about the users as reported by their software.Second,immediately after gathering a snapshot,we actively probed the users in the snapshot over a period of several days to directly measure various properties about them,such as their bottleneck bandwidth.In this section of the paper,weﬁrst give a brief overview of the architectures of Napster and Gnutella. Following this,we then describe the software infrastructure that we built to gather our measurements,including the Napster crawler,the Gnutella crawler,and the active measurement tools used to probe the users discovered.2.1.The Napster and Gnutella ArchitecturesBoth Napster and Gnutella have similar goals:to facilitate the location and exchange ofﬁles(typically images, audio,or video)amongst a large group of independent users connected through the Internet.In these systems,ﬁles are stored on the computers of the individual users or peers,and exchanged through a direct connection between the downloading and uploading peers,over an HTTP-style protocol.All peers in this system are symmetric:they all have the ability to function both as a client and a server.This symmetry distinguishes peer-to-peer systems from many conventional distributed system architectures.Though the process of exchanging ﬁles is similar in both systems,Napster and Gnutella diﬀer substantially in howpeers locateﬁles(Figure1).In Napster,a large cluster of dedicated central servers maintain an index of theﬁles that are currently being shared by active peers.Each peer maintains a connection to one of the central servers,through which theﬁle location queries are sent.The servers then cooperate to process the query and return a list of matchingﬁles and locations.On receiving the results,the peer may choose to initiate aﬁle exchange directly from another peer. In addition to maintaining an index of sharedﬁles,the centralized servers also monitor the state of each peer in the system,keeping track of metadata such as the peers’reported connection bandwidth and the duration that the peer has remained connected to the system.This metadata is returned with the results of a query,so that the initiating peer has some information to distinguish possible download sites.There are no centralized servers in Gnutella,however.Instead,Gnutella peers form an overlay network by forging point-to-point connections with a set of neighbors.To locate aﬁle,a peer initiates a controlledﬂood of the network by sending a query packet to all of its neighbors.Upon receiving a query packet,a peer checks ifany locally storedﬁles match the query.If so,the peer sends a query response packet back towards the query originator.Whether or not aﬁle match is found,the peer continues toﬂood the query through the overlay.To help maintain the overlay as the users enter and leave the system,the Gnutella protocol includes ping and pong messages that help peers to discover other nodes.Pings and pongs behave similarly to query/query-response packets:any peer that sees a ping message sends a pong back towards the originator,and forwards the ping onwards to its own set of neighbors.Ping and query packets thusﬂood through the network;the scope ofﬂooding is controlled with a time-to-live(TTL)ﬁeld that is decremented on each hop.Peers occasionally forge newneighbor connections w ith other peers discovered through the ping/pong mechanism.Note that it is possible to have several disjoint Gnutella overlays of Gnutella simultaneously coexisting in the Internet;this contrasts with Napster,in which peers are always connected to the same cluster of central servers.2.2.Crawling the Peer-to-Peer SystemsWe nowdescribe the design and implementation of our Napster and Gnutella craw lers.2.2.1.The Napster CrawlerBecause we did not have direct access to indexes maintained by the central Napster servers,the only way we could discover the set of peers participating in the system at any time was by issuing queries forﬁles,and keeping a list of peers referenced in the queries’responses.To discover the largest possible set of peers,we issued queries with the names of popular song artists drawn from a long list downloaded from the web.The Napster server cluster consists of approximately160servers;each peer establishes a connection with only one server.When a peer issues a query,the server the peer is connected toﬁrst reportsﬁles shared by “local users”on the same server,and later reports matchingﬁles shared by“remote users”on other servers in the cluster.For each crawl,we established a large number of connections to a single server,and issued many queries in parallel;this reduced the amount of time taken to gather data to3-4minutes per crawl,giving us a nearly instantaneous snapshot of peers connected to that server.For each peer that we discovered during the crawl,we then queried the Napster server to gather the following metadata:(1)the bandwidth of the peer’s connection as reported by the peer herself,(2)the number ofﬁles currently being shared by the peer,(3)the current number of uploads and the number of downloads in progress by the peer,(4)the names and sizes of all theﬁles being shared by the peer,and(5)the IP address of the peer.To get an estimate of the fraction of the total user population we captured,we separated the local and remote peers returned in our queries’responses,and compared them to statistics periodically broadcast by the particular Napster server that we queried.From these statistics,we veriﬁed that each crawl typically captured between40%and60%of the local peers on the crawled server.Furthermore,this40-60%of the peers that we captured contributed between80-95%of the total(local)ﬁles reported to the server.Thus,we feel that our crawler captured a representative and signiﬁcant fraction of the set of peers.Our crawler did not capture any peers that do not share any of the popular content in our queries.This introduces a bias in our results,particularly in our measurements that report the number ofﬁles being shared by users.However,the statistics reported by the Napster server revealed that the distributions of number of uploads,number of downloads,number ofﬁles shared,and bandwidths reported for all remote users were quite similar to those that we observed from our captured local users.2.2.2.The Gnutella CrawlerThe goal of our Gnutella crawler is the same as our Napster crawler:to gather nearly instantaneous snap-shots of a signiﬁcant subset of the Gnutella population,as well as metadata about peers in captured subset as reported by the Gnutella system itself.Our crawler exploits the ping/pong messages in the protocol to discover hosts.First,the crawler connects to several well-known,popular peers(such as or ).Then,it begins an iterative process of sending ping messages with large TTLs to known peers,adding newly discovered peers to its list of known peers based on the contents of received pong messages.In addition to the IP address of a peer,each pong message contains metadata about the peer, including the number and total size ofﬁles being shared.We allowed our crawler to continue iterating for approximately two minutes,after which it would typically gather between8,000and10,000unique peers(Figure2).According to measurements reported by Clip2,8this corresponds to at least25%to50%of the total population of peers in the system at any time.After twoFigure2:Number of Gnutella hosts captured by our crawler over timeminutes,we would terminate the crawler,save the crawling results to aﬁle and begin another crawl iteration to gather our next snapshot of the Gnutella population.Unlike our Napster measurements,in which we were more likely to capture hosts sharing popular songs, we have no reason to suspect any bias in our measurements of the Gnutella user population.Furthermore, to ensure that the crawling process does not alter the behavior of the system in any way,our crawler neither forwarded any Gnutella protocol messages nor answered any queries.2.2.3.Crawler StatisticsBoth the Napster and Gnutella crawlers were written in Java,and ran using the IBM Java1.18JRE on Linux 2.2.16.The crawlers ran in parallel on a small number of dual-processor Pentium III700MHz computers with 2GB RAM,and four40GB SCSI disks.Our Napster trace captured four days of activity,from Sunday May 6th,2001through Wednesday May9th,2001.We recorded a total of509,538Napster peers on546,401unique IP addresses.Our Gnutella trace spanned eight days(Sunday May6th,2001through Monday May14th,2001) and captured1,239,487Gnutella peers on1,180,205unique IP-addresses.2.3.Directly Measured Peer CharacteristicsFor each gathered peer population snapshot,we directly measured additional properties of the peers.Our goal was to capture data that would enable us to reason about the fundamental characteristics of the users(both as individuals and as a population)participating in any peer-to-peerﬁle sharing system.The data collected includes the distributions of bottleneck bandwidths and latencies between peers and our measurement infrastructure, the number of sharedﬁles per peer,the distribution of peers across DNS domains,and the“lifetime”of the peers in the system,i.e.,howfrequently peers connect to the systems,and howlong they remain connected.tency MeasurementsGiven the list of peers’IP-addresses obtained by the crawlers,we measured the round-trip latency between the peers and our measurement machines.For this,we used a simple tool that measures the RTT of a40-byte TCP packet exchanged between a peer and our measurement host.Our interest in latencies of the peers is due to the well known feature of TCP congestion control which discriminates againstﬂows with large round-trip times. This,coupled with the fact that the average size ofﬁles exchanged is in the order of2-4MB,makes latency a very important consideration when selecting amongst multiple peers sharing the sameﬁle.Although we realize that the latency to any particular peer is dependent on the location of the host from which it is measured,we feel the distribution of latencies over the entire population of peers from a given host might be similar(but not identical)from diﬀerent hosts,and hence,could be of interest.2.3.2.Lifetime MeasurementsTo gather measurements of the lifetime characteristics of peers,we needed a tool that would periodically probea large set of peers from both systems to detect when they were participating in the system.Every peer in bothNapster and Gnutella connects to the system using a unique IP-address/port-number pair;to download aﬁle, peers connect to each other using these pairs.There are therefore three possible states for any participating peer in either Napster or Gnutella:1.oﬄine:the peer is either not connected to the Internet or is not responding to TCP SYN packets becauseit is behind aﬁrewall or NAT proxy.2.inactive:the peer is connected to the Internet and is responding to TCP SYN packets,but it is discon-nected from the peer-to-peer system and hence responds with TCP RST’s.3.active:the peer is actively participating in the peer-to-peer system,and is accepting incoming TCPconnections.We developed a simple tool(which we call LF)using Savage’s“Sting”platform.9To detect the state of a host,LF sends a TCP SYN-packet to the peer and then waits for up to twenty seconds to receive any packets from it.If no packet arrives,we mark the peer as oﬄine.If we receive a TCP RST packet,we mark the peer as inactive.If we receive a TCP SYN/ACK,we label the host as active,and send back a RST packet to terminate the connection.We chose to manipulate TCP packets directly rather than use OS socket calls to achieve greater scalability;this enabled us to monitor the lifetimes of tens of thousands of hosts per workstation.Because we identify a host by its IP address,one limitation in the lifetime characterization of peers our inability of distinguishing hosts sharing dynamic IP addresses(e.g.DHCP).2.3.3.Bottleneck Bandwidth MeasurementsAnother characteristic of peers that we wanted to gather was the speed of their connections to the Internet. This is not a precisely deﬁned concept:the rate at which content can be downloaded from a peer depends on the bottleneck bandwidth between the downloader and the peer,the available bandwidth along the path,and the latency between the peers.The central Napster servers can provide the connection bandwidth of any peer as reported by the peer itself. However,as we will show later,a substantial percentage of the Napster peers(as high as25%)choose not to report their bandwidths.Furthermore,there is a clear incentive for a peer to discourage other peers from downloadingﬁles by falsely reporting a low bandwidth.The same incentive to lie exists in Gnutella;in addition to this,in Gnutella,bandwidth is reported only as part of a successful response to a query,so peers that share no data or whose content does not match any queries never report their bandwidths.Because of this,we decided to actively probe the bandwidths of peers.There are two diﬃcult problems with measuring the available bandwidth to and from a large number of hosts:ﬁrst,available bandwidth can signiﬁcantlyﬂuctuate over short periods of time,and second,available bandwidth is determined by measuring the loss rate of an open TCP connection.Instead,we decided to use the bottleneck link bandwidth as aﬁrst-order approximation to the available bandwidth;because our workstations are connected by a gigabit link to the Abilene network,it is likely that the bottleneck link between our workstations and any peer in these systems is last-hop link to the peer itself.This is particularly likely since,as we will show later,most peers are connected to the system using low-speed modems or broadband connections such as cable modems or DSL.Thus,if we could characterize the bottleneck bandwidth between our measurement infrastructure and the peers,we would have a fairly accurate upper bound on the rate at which information could be downloaded from these peers.Bottleneck link bandwidth between two diﬀerent hosts equals the capacity of the slowest hop along the path between the two hosts.Thus,by deﬁnition,bottleneck link bandwidth is a physical property of the network that remains constant over time for an individual path.Although various bottleneck link bandwidth measurement tools are available,10–13for a number of reasons that are beyond the scope of this paper,all of these tools were unsatisfactory for our purposes.Hence,we developed our own tool(called SProbe)14based on the same underlying packet-pair dispersion technique as some of the above-mentioned tools.Unlike other tools,however,SProbe uses tricks inspired by Sting9to actively measure both upstream and downstream bottleneck bandwidths using only a few TCP packets.Our tool also proactively detects cross-traﬃc that interferes with the accuracy of the packet-pair technique,improving the overall accuracy of our measurements.∗By comparing the reported bandwidths of the peers with our measured ∗For more information about SProbe,refer to .Figure3.Left:CDFs of upstream and downstream bottleneck bandwidths for Gnutella peers;Right:CDFs of down-stream bottleneck bandwidths for Napster and Gnutella peers.bandwidths,we were able to verify the consistency and accuracy of SProbe,as we will demonstrate in Section3.5.2.3.4.A Summary of the Active MeasurementsFor the lifetime measurements,we monitored17,125Gnutella peers over a period of60hours and7,000Napster peers over a period of25hours.For each Gnutella peer,we determined its status(oﬄine,inactive or active) once every seven minutes,and for each Napster peer,once every two minutes.For Gnutella,we attempted to measure bottleneck bandwidths and latencies to a random set of595,974 unique peers(i.e.,unique IP-address/port-number pairs).We were successful in gathering downstream bot-tleneck bandwidth measurements to223,552of these peers,the remainder of which were either oﬄine or had signiﬁcant cross-traﬃc.We measured upstream bottleneck bandwidths from16,252of the peers(for various reasons,upstream bottleneck bandwidth measurements from hosts are much harder to obtain than downstream measurements to hosts14).Finally,we were able to measure latency to339,502peers.For Napster,we attempted to measure downstream bottleneck bandwidths to4,079unique peers.We successfully measured2,049peers.In several cases,our active measurements were regarded as intrusive by several monitored systems.Un-fortunately,e-mail complaints received by the computing staﬀat the University of Washington forced us to prematurely terminate our crawls,hence the lower number of monitored Napster hosts.Nevertheless,we suc-cessfully captured a signiﬁcant number of data points for us to believe that our results and conclusions are representative for the entire Napster population.3.MEASUREMENT RESULTSOur measurement results are organized according to a number of basic questions addressing the capabilities and behavior of peers.In particular,we attempt to address how many peers are capable of being servers,how many behave like clients,howmany are w illing to cooperate,and also howw ell the Gnutella netw ork behaves in the face of random or malicious failures.3.1.How Many Peers Fit the High-Bandwidth,Low-Latency Proﬁle of a Server?One particularly relevant characteristic of peer-to-peerﬁle sharing systems is the percentage of peers in the system having server-like characteristics.More speciﬁcally,we are interested in understanding what percentage of the participating peers exhibit the server-like characteristics with respect to their bandwidths and latencies. Peers worthy of being servers must have high-bandwidth Internet connections,they should remain highly avail-able,and the latency of access to the peers should generally be low.If there is a high degree of heterogeneity amongst the peers,a well-designed system should pay careful attention to delegating routing and content-serving responsibilities,favoring server-like peers.3.1.1.Downstream and Upstream Measured Bottleneck Link BandwidthsToﬁt the proﬁle of a high-bandwidth server,a participating peer must have a high upstream bottleneck link bandwidth,since this value determines the rate at which a server can serve content.On the left,Figure3Figure 4.Left:Reported bandwidths For Napster peers;Right:Reported bandwidths for Napster peers,excluding peers that reported “unknown”.presents cumulative distribution functions (CDFs)of upstream and downstream bottleneck bandwidths for Gnutella peers.†From this graph,we see that while 78%of the participating peers have downstream bottleneck bandwidths of at least 100Kbps,only 8%of the peers have upstream bottleneck bandwidths of at least 10Mbps.Moreover,22%of the participating peers have upstream bottleneck bandwidths of 100Kbps or less.Not only are these peers unsuitable to provide content and data,they are particularly susceptible to being swamped by a relatively small number of connections.The left graph in Figure 3reveals asymmetry in the upstream and downstream bottleneck bandwidths of Gnutella peers.On average,a peer tends to have higher downstream than upstream bottleneck bandwidth;this is not surprising,because a large fraction of peers depend on asymmetric links such as ADSL,cable modems or regular modems using the V.90protocol.15Although this asymmetry is beneﬁcial to peers that download content,it is both undesirable and detrimental to peers that serve content:in theory,the download capacity of the system exceeds its upload capacity.We observed a similar asymmetry in the Napster network.The right graph in Figure 3presents CDFs of downstream bottleneck bandwidths for Napster and Gnutella peers.As this graph illustrates,the percentage of Napster users connected with modems (of 64Kbps or less)is about 25%,while the percentage of Gnutella users with similar connectivity is as low as 8%.At the same time,50%of the users in Napster and 60%of the users in Gnutella use broadband connections (Cable,DSL,T1or T3).Furthermore,only about 20%of the users in Napster and 30%of the users in Gnutella have very high bandwidth connections (at least 3Mbps).Overall,Gnutella users on average tend to have higher downstream bottleneck bandwidths than Napster users.Based on our experience,we attribute this diﬀerence to two factors:(1)the current ﬂooding-based Gnutella protocol is too high of a burden on low bandwidth connections,discouraging them from participating,and (2)although unveriﬁable,there is a widespread belief that Gnutella is more popular to technically-savvy users,who tend to have faster Internet connections.3.1.2.Reported Bandwidths for Napster PeersFigure 4illustrates the breakdown of Napster peers with respect to their voluntarily reported bandwidths;the bandwidth that is reported is selected by the user during the installation of the Napster client software.(Peers that report “Unknown”bandwidth have been excluded in the right graph.)As Figure 4shows,a signiﬁcant percent of the Napster users (22%)report “Unknown”.These users are either unaware of their connection bandwidths,or they have no incentive to accurately report their true bandwidth.Indeed,knowing a peer’s connection speed is more valuable to others rather than to the peer itself;a peer that reports high bandwidth is more likely to receive download requests from other peers,consuming network resources.Thus,users have an incentive to misreport their Internet connection speeds.A well-designed system therefore must either directly measure the bandwidths rather than relying on a user’s input,or create the right†“Upstream”denotes traﬃc from the peer to the measurement node;“downstream”denotes traﬃc from the mea-surement node to the peer.Figure 5.Left:Measured latencies to Gnutella peers;Right:Correlation between Gnutella peers’downstream bottleneck bandwidth and latency.incentives for the users to report accurate information to the system.Finally both Figures 3and 4conﬁrm that the most popular forms of Internet access for Napster and Gnutella peers are cable modems and DSLs (bottleneck bandwidths between 1Mbps and 3.5Mbps).3.1.3.Measured Latencies for Gnutella PeersFigure 5(left)shows a CDF of the measured latencies from our measurement nodes to Gnutella peers.Approx-imately 20%of the peers have latencies of at least 280ms,whereas another 20%have latencies of at most 70ms:the closest 20%of the peers are four times closer than the furthest 20%.From this,we can deduce that in a peer-to-peer system where peers’connections are forged in an unstructured,ad-hoc way,a substantial fraction of the connections will suﬀer from high-latency.On the right,Figure 5shows the correlation between downstream bottleneck bandwidth and the latency of individual Gnutella peers (on a log-log scale).This graph illustrates the presence of two clusters;a smaller one situated at (20-60Kbps,100-1,000ms)and a larger one at over (1,000Kbps,60-300ms).These clusters correspond to the set of modems and broadband connections,respectively.The negatively sloped lower-bound evident in the low-bandwidth region of the graph corresponds to the non-negligible transmission delay of our measurement packets through the low-bandwidth links.An interesting artifact evident in this graph is the presence of two pronounced horizontal bands.These bands correspond to peers situated on the North American East Coast and in Europe,respectively.Although the latencies presented in this graph are relative to our location (Seattle,WA),these results can be extended to conclude that there are three large classes of latencies that a peer interacts with:(1)latencies to peers on the same part of the continent,(2)latencies to peers on the opposite part of a continent and (3)latencies to trans-oceanic peers.As Figure 5shows,the bandwidths of the peers ﬂuctuate signiﬁcantly within each of these three latency classes.3.2.How Many Peers Fit the High-Availability Proﬁle of a Server?Server worthiness is characterized not only by high-bandwidth and low-latency network connectivity,but also by the availability of the server.If,peers tend to be unavailable frequently,this will have signiﬁcant implications about the degree of replication necessary to ensure that content is consistently accessible on this system.On the left,Figure 6shows the distribution of uptimes of peers for both Gnutella and Napster.Uptime is measured as the percentage of time that the peer is available and responding to traﬃc.The “Internet host uptime”curves represent the uptime as measured at the IP-level,i.e.,peers that are in the inactive or active states,as deﬁned in Section 2.3.2.The “Gnutella/Napster host uptime”curves represent the uptime of peers in the active state,and therefore responding to application-level requests.For all curves,we have eliminated peers that had 0%uptime (peers that were never up throughout our lifetime experiment).The IP-level uptime characteristics of peers are quite similar for both systems;this implies that the set of。