Peer 2 Peer Architecture

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Peer 2 peer architecture

Peer-to-peer (P2P) computing or networking is a distributed application architecture that

partitions tasks or workloads among peers. Peers are equally privileged, equipotent participantsin the application. They are said to form a peer-to-peer network of nodes.

Peers make a portion of their resources, such as processing power, disk storage or networkbandwidth, directly available to other network participants, without the need for centralcoordination by servers or stable hosts.[1] Peers are both suppliers and consumers of resources, incontrast to the traditional clientservermodel where only servers supply (send), and clientsconsume (receive).

The peer-to-peer application structure was popularized by file sharing systems likeNapster. Theconcept has inspired new structures andphilosophies in many areas of human interaction. Peer-to-peer networking is not restricted to technology, but covers also socialprocesses with a peer-to-peer dynamic. In such context,social peer-to-peer processesare currently emerging

throughout society.

Architecture of P2P systemsPeer-to-peer systems often implement an abstract overlay network, built at Application Layer, ontop of the native or physical network topology. Such overlays are used for indexing and peerdiscovery and make the P2P system independent from the physical network topology. Content istypically exchanged directly over the underlying Internet Protocol (IP) network. Anonymouspeer-to-peersystems are an exception, and implement extra routing layers to obscure the identityof the source or destination of queries.

Instructuredpeer-to-peer networks, peers (and, sometimes, resources) are organized followingspecific criteria and algorithms, which lead to overlays with specific topologies and properties.They typically use distributed hash table-based (DHT) indexing, such as in the Chordsystem(MIT).[2]

Unstructured peer-to-peernetworks do not impose any structure on the overlay networks. Peersin these networks connect in an ad-hoc fashion[3]. Ideally, unstructured P2P systems would haveabsolutely no centralized system, but in practice there are several types of unstructured systemswith various degrees of centralization. Three categories can easily be seen.

Inpure peer-to-peersystems the entire network consists solely ofequipotent peers. Thereis only one routing layer, as there are no preferred nodes with any special infrastructurefunction.

Hybrid peer-to-peersystems allow such infrastructure nodes to exist, often called

supernodes.[4]

In centralized peer-to-peersystems, a central server is used for indexing functions and tobootstrap the entire system. Although this has similarities with a structured architecture,the connections between peers are not determined by any algorithm.

The first prominent and popularpeer-to-peer file sharingsystem, Napster, was an example of thecentralized model.[5]Freenet and early implementations of the gnutellaprotocol, on the other
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hand, are examples of the decentralized model. Modern gnutella implementations,Gnutella2, aswell as the now deprecated Kazaa network are examples of the hybrid model.

A pure P2P network does not have the notion ofclientsor servers but only equalpeernodes thatsimultaneously function as both "clients" and "servers" to the other nodes on the network. Thismodel of network arrangement differs from theclientservermodel where communication is

usually to and from a central server. A typical example of a file transfer that does not use the P2Pmodel is the File Transfer Protocol (FTP) service in which the client and server programs aredistinct: the clients initiate the transfer, and the servers satisfy these requests.

The P2P overlay networkconsists of all the participating peers as network nodes. There are linksbetween any two nodes that know each other: i.e. if a participating peer knows the location ofanother peer in the P2P network, then there is a directed edge from the former node to the latterin the overlay network. Based on how the nodes in the overlay network are linked to each other,we can classify the P2P networks as unstructured or structured.

Unstructured systems

An unstructured P2P network is formed when the overlay links are established arbitrarily. Such

networks can be easily constructed as a new peer that wants to join the network can copyexisting links of another node and then form its own links over time. In an unstructured P2Pnetwork, if a peer wants to find a desired piece of data in the network, the query has to beflooded through the network to find as many peers as possible that share the data. The maindisadvantage with such networks is that the queries may not always be resolved. Popular contentis likely to be available at several peers and any peer searching for it is likely to find the samething. But if a peer is looking for rare data shared by only a few other peers, then it is highlyunlikely that search will be successful. Since there is no correlation between a peer and thecontent managed by it, there is no guarantee that flooding will find a peer that has the desireddata. Flooding also causes a high amount of signaling traffic in the network and hence suchnetworks typically have very poor search efficiency. Many of the popular P2P networks areunstructured.

Inpure P2P networks: Peers act as equals, merging the roles of clients and server. In suchnetworks, there is no central server managing the network, neither is there a central router. Someexamples of pure P2P Application Layernetworks designed forpeer-to-peer file sharingaregnutella (pre v0.4) and Freenet.

There also exist hybridP2P systems, which distribute their clients into two groups: client nodesand overlay nodes. Typically, each client is able to act according to the momentary need of thenetwork and can become part of the respectiveoverlay networkused to coordinate the P2Pstructure. This division between normal and 'better' nodes is done in order to address the scalingproblems on early pure P2P networks. As examples for such networks can be named modernimplementations of gnutella (after v0.4) and Gnutella2.

Another type of hybrid P2P network are networks using on the one hand central server(s) orbootstrapping mechanisms, on the other hand P2P for their data transfers. These networks are ingeneral called 'centralized networks' because of their lack of ability to work without their centralserver(s). An example for such a network is theeDonkey network(often also called eD2k).

Advantages and weaknessesIn P2P networks, clients provide resources, which may includebandwidth, storage space, andcomputing power. This property is one of the major advantages of using P2P networks because it
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makes the setup and running costs very small for the original content distributor. As nodes arriveand demand on the system increases, the total capacity of the system also increases, and thelikelihood of failure decreases. If one peer on the network fails to function properly, the wholenetwork is not compromised or damaged. In contrast, in a typical clientserver architecture,clients share only their demands with the system, but not their resources. In this case, as moreclients join the system, fewer resources are available to serve each client, and if the central serverfails, the entire network is taken down. The decentralized nature of P2P networks increasesrobustness because it removes thesingle point of failurethat can be inherent in a client-serverbased system.[11]

Another important property of peer-to-peer systems is the lack of a system administrator. Thisleads to a network that is easier and faster to setup and keep running because a full staff is notrequired to ensure efficiency and stability. Decentralized networks introduce new security issuesbecause they are designed so that each user is responsible for controlling their data andresources. Peer-to-peer networks, along with almost all network systems, are vulnerable tounsecure and unsigned codes that may allow remote access to files on a victim's computer oreven compromise the entire network. A user may encounter harmful data by downloading a filethat was originally uploaded as a virus disguised in an .exe, .mp3, .avi, or any other filetype. Thistype of security issue is due to the lack of an administrator that maintains the list of files beingdistributed.

Harmful data can also be distributed on P2P networks by modifying files that are already beingdistributed on the network. This type of security breach is created by the fact that users areconnecting to untrusted sources, as opposed to a maintained server. In the past this has happenedto the FastTracknetwork when the RIAA managed to introduce faked chunks into downloadsand downloaded files (mostly MP3 files). Files infected with the RIAA virus were unusableafterwards or even contained malicious code. The RIAA is also known to have uploaded fakemusic and movies to P2P networks in order to deter illegal file sharing.[12]Consequently, the P2Pnetworks of today have seen an enormous increase of their security and file verificationmechanisms. Modern hashing, chunk verificationand different encryption methods have made

most networks resistant to almost any type of attack, even when major parts of the respectivenetwork have been replaced by faked or nonfunctional hosts.

There are both advantages and disadvantages in P2P networks related to the topic of data backup,recovery, and availability. In a centralized network, the system administrators are the only forcescontrolling the availability of files being shared. If the administrators decide to no longerdistribute a file, they simply have to remove it from their servers, and it will no longer beavailable to users. Along with leaving the users powerless in deciding what is distributedthroughout the community, this makes the entire system vulnerable to threats and requests fromthe government and other large forces. For example, YouTube has been pressured by the RIAA,MPAA, and entertainment industry to filter out copyrighted content. Although server-clientnetworks are able to monitor and manage content availability, they can have more stability in the

availability of the content they choose to host. A client should not have trouble accessing obscurecontent that is being shared on a stable centralized network. P2P networks, however, are moreunreliable in sharing unpopular files because sharing files in a P2P network requires that at leastone node in the network has the requested data, and that node must be able to connect to the noderequesting the data. This requirement is occasionally hard to meet because users may delete orstop sharing data at any point.

In this sense, the community of users in a P2P network is completely responsible for decidingwhat content is available. Unpopular files will eventually disappear and become unavailable as
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more people stop sharing them. Popular files, however, will be highly and easily distributed.Popular files on a P2P network actually have more stability and availability than files on centralnetworks. In a centralized network, only the loss of connection between the clients and server issimple enough to cause a failure, but in P2P networks, the connections between every node mustbe lost in order to fail to share data. In a centralized system, the administrators are responsiblefor all data recovery and backups, while in P2P systems, each node requires its own backupsystem. Because of the lack of central authority in P2P networks, forces such as the recordingindustry, RIAA, MPAA, and the government are unable to delete or stop the sharing of contenton P2P systems.

CLIENT-SERVER COMPUTING

client-server application is one in which a complex application is decomposed

into a part that runs on a server and one which runs on personal computers

(the clients). Besides doing part of the computing for the application, the PC

client provides a natural point and click graphical interface to the application.

ADVANTAGES OF CLIENT-SERVER

Advantages often cited include:

Centralization - access, resources, and data security are controlled

through the server

Scalability - any element can be upgraded when needed

Flexibility - new technology can be easily integrated into the system

Interoperability - all components (clients, network, servers) work

together

Accessibility - server can be accessed remotely and across multiple

platforms

Ease of application development

Lower total costs than mainframe legacy systems.


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User friendly - familiar point and click interface

DISADVANTAGES OF CLIENT-SERVER

Disadvantages often cited include:

Dependability - when the server goes

down, operations cease

Lack of mature tools - it is a relatively new

technology and needed tools are lacking

e.g.. Automated client software

distribution

Lack of scalability - network operating

systems (e.g.. Novell Netware, Windows NT

Server) are not very scalable.

Higher than anticipated costs

Can cause network congestion

CLIENT-SERVER ARCHITECTURES

There are basically two types of client-server

architectures

Two tier architectures

Three tier architectures

The choice between the two should be made

based on combination of:


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Schedule for project implementation

Expected system changes and

enhancements

TWO-TIER ARCHITECTURES

Application components are distributed

between the server and client software In addition to part of the application software,

the server also stores the data, and all data

accesses are through the server.

The presentation (to the user) is handled

strictly by the client software.


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The PC clients assume the bulk of the

responsibility for the application logic.

The server assumes the bulk of the

responsibility for data integrity checks, query

capabilities, data extraction and most of the

data intensive tasks, including sending the

appropriate data to the appropriate clients.

SQL is a standard used on the clients to

request appropriate subsets of data from the

server.

Data returned from the server to the clients is

manipulated by the client software for

reporting, business analysis, and what ifanalysis.

TWO-TIER ARCHITECTURES, ADVANTAGES

The commonly cited advantages of two-tier

systems include:

Fast application development time

Available tools are robust and lend

themselves to fast prototyping to insure


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user needs a met accurately and

completely.

Conducive to environments with

homogeneous clients, homogeneous

applications, and static business rules.

TWO-TIER ARCHITECTURES, DISADVANTAGES

The commonly cited disadvantages of two-tier

systems include:

Not suitable for dispersed, heterogeneous

environments with rapidly changing

business rules.

Because the bulk of the application logic is

on the client, there is the problem of client

software version control and new version

redistribution.

Security can be complicated because auser may require separate passwords for

each SQL server accessed.


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Client tools and SQL middleware in two-tier

environments tend to be proprietary.

The volatility of the client /server tool

market raises questions about the long-term viability of any proprietary tool.

Organizations should be wary about

committing to proprietary tools

THREE-TIER ARCHITECTURES

3-tier architectures attempt to overcome some

of the limitations of the 2-tier architecture by


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separating presentation, processing, and data

into 3 separate and distinct entities.

The software in the client handles the

presentation (to the user) using similar tools as

in the 2-tier architecture.

When data or processing are required by thepresentation client, a call is made to the

middle-tier functionality server.


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This tier performs calculations, does reports,

and makes any needed client calls to other

servers (e.g.. a data base server).

Middle tier servers are usually coded in a

highly portable, non-proprietary language such

as C or C++.

Middle tier servers may be multithreaded and

can be accessed by multiple clients.

The calling mechanism from client to server andfrom server to server is by means of RPCs

Because the bulk of 3-tier implementations use

RPCs and 2-tier implementations use SQL

messaging, a careful examination of the merits

of these respective request/response

mechanisms is warranted.

THREE-TIER ARCHITECTURES, ADVANTAGES

Commonly cited advantages include:


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RPC calls from the presentation client to

the middle tier provide greater generality

(i.e. flexibility) than the SQL calls usually

used in the 2-tier architecture.

The 3-tier clients do not have to understand SQL.

This allows the organization, names, or even the

overall structure of back end data to changes

without requiring changes in the PC-based

presentation clients. This allows clients to access

even non-relational databases and greatlysimplifies introduction of new database

technologies

Having separate functionality servers

allows for the parallel development of

individual tiers by application specialists.

Provides for more flexible resource

allocation. Can reduce network traffic by

having the functionality servers strip data

to the precise structure needed before

sending it to the clients.

Can use reusable program modules fordifferent applications, thus reducing

development and migration costs.


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3-tier standards such as OSF/DCE (Open

Software Foundation Distributed

Computing Environment) offers a variety of

features to support distributed applicationdevelopment.

THREE-TIER ARCHITECTURES, DISADVANTAGES

Often cited disadvantages of 3-tier

architectures include:

Creates an increased need for network

traffic management, server load balancing,

and fault tolerance.

Current tools are relatively immature andare more complex.

Maintenance tools are currently inadequate

for maintaining server libraries. This is a

potential obstacle for simplifying

maintenance and promoting code reuse

throughout the organization.

Documents

Peer 2 Peer Architecture