ACCORDING TO NETWORKING MODELS NETWORK ARE CLASSIFFIED INTO THREE

PEER TO PEER

Peer-to-peer (P2P) computing or networking is a distributed application architecture that partitions tasks or work loads between peers. Peers are equally privileged, equipotent participants in 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 network bandwidth, directly available to other network participants, without the need for central coordination by servers or stable hosts.^[1] Peers are both suppliers and consumers of resources, in contrast to the traditional client–server model where only servers supply, and clients consume.
The peer-to-peer application structure was popularized by file sharing systems like Napster. The peer-to-peer computing paradigm has inspired new structures and philosophies in other areas of human interaction. In such social contexts, peer-to-peer as a meme refers to the egalitarian social networking that is currently emerging throughout society, enabled by Internet technologies in general.

Architecture of P2P systems

Peer-to-peer systems often implement an abstract overlay network, built at Application Layer, on top of the native or physical network topology. Such overlays are used for indexing and peer discovery and make the P2P system independent from the physical network topology. Content is typically exchanged directly over the underlying Internet Protocol (IP) network. Anonymous peer-to-peer systems are an exception, and implement extra routing layers to obscure the identity of the source or destination of queries.
In structured peer-to-peer networks, peers (and, sometimes, resources) are organized following specific 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 Chord system (MIT).^[2]
Unstructured peer-to-peer networks do not provide any algorithm for organization or optimization of network connections.^{[citation needed]}. In particular, three models of unstructured architecture are defined. In pure peer-to-peer systems the entire network consists solely of equipotent peers. There is only one routing layer, as there are no preferred nodes with any special infrastructure function. Hybrid peer-to-peer systems allow such infrastructure nodes to exist, often called supernodes.^[3] In centralized peer-to-peer systems, a central server is used for indexing functions and to bootstrap the entire system.^{[citation needed]}. Although this has similarities with a structured architecture, the connections between peers are not determined by any algorithm. The first prominent and popular peer-to-peer file sharing system, Napster, was an example of the centralized model. Gnutella and Freenet, on the other hand, are examples of the decentralized model. Kazaa is an example of the hybrid model.
P2P networks are typically used for connecting nodes via largely ad hoc connections.^{[citation needed]} Data, including digital formats such as audio files, and real time data such as telephony traffic, is passed using P2P technology.
A pure P2P network does not have the notion of clients or servers but only equal peer nodes that simultaneously function as both "clients" and "servers" to the other nodes on the network. This model of network arrangement differs from the client–server model where communication is usually to and from a central server. A typical example of a file transfer that does not use the P2P model is the File Transfer Protocol (FTP) service in which the client and server programs are distinct: the clients initiate the transfer, and the servers satisfy these requests.
The P2P overlay network consists of all the participating peers as network nodes. There are links between any two nodes that know each other: i.e. if a participating peer knows the location of another peer in the P2P network, then there is a directed edge from the former node to the latter in 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.

 Structured systems

Structured P2P networks employ a globally consistent protocol to ensure that any node can efficiently route a search to some peer that has the desired file, even if the file is extremely rare. Such a guarantee necessitates a more structured pattern of overlay links. By far the most common type of structured P2P network is the distributed hash table (DHT), in which a variant of consistent hashing is used to assign ownership of each file to a particular peer, in a way analogous to a traditional hash table's assignment of each key to a particular array slot. Distributed hash tables

Distributed hash tables

Distributed hash tables (DHTs) are a class of decentralized distributed systems that provide a lookup service similar to a hash table: (key, value) pairs are stored in the DHT, and any participating node can efficiently retrieve the value associated with a given key. Responsibility for maintaining the mapping from keys to values is distributed among the nodes, in such a way that a change in the set of participants causes a minimal amount of disruption. This allows DHTs to scale to extremely large numbers of nodes and to handle continual node arrivals, departures, and failures.
DHTs form an infrastructure that can be used to build peer-to-peer networks. Notable distributed networks that use DHTs include BitTorrent's distributed tracker, the Kad network, the Storm botnet, YaCy, and the Coral Content Distribution Network.
Some prominent research projects include the Chord project, the PAST storage utility, the P-Grid, a self-organized and emerging overlay network and the CoopNet content distribution system (see below for external links related to these projects).
DHT-based networks have been widely utilized for accomplishing efficient resource discovery^[4][5] for grid computing systems, as it aids in resource management and scheduling of applications. Resource discovery activity involve searching for the appropriate resource types that match the user’s application requirements. Recent advances in the domain of decentralized resource discovery have been based on extending the existing DHTs with the capability of multi-dimensional data organization and query routing. Majority of the efforts have looked at embedding spatial database indices such as the Space Filling Curves (SFCs) including the Hilbert curves, Z-curves, k-d tree, MX-CIF Quad tree and R*-tree for managing, routing, and indexing of complex Grid resource query objects over DHT networks. Spatial indices are well suited for handling the complexity of Grid resource queries. Although some spatial indices can have issues as regards to routing load-balance in case of a skewed data set, all the spatial indices are more scalable in terms of the number of hops traversed and messages generated while searching and routing Grid resource queries.

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 copy existing links of another node and then form its own links over time. In an unstructured P2P network, if a peer wants to find a desired piece of data in the network, the query has to be flooded through the network to find as many peers as possible that share the data. The main disadvantage with such networks is that the queries may not always be resolved. Popular content is likely to be available at several peers and any peer searching for it is likely to find the same thing. But if a peer is looking for rare data shared by only a few other peers, then it is highly unlikely that search will be successful. Since there is no correlation between a peer and the content managed by it, there is no guarantee that flooding will find a peer that has the desired data. Flooding also causes a high amount of signaling traffic in the network and hence such networks typically have very poor search efficiency. Many of the popular P2P networks are unstructured.
In pure P2P networks: Peers act as equals, merging the roles of clients and server. In such networks, there is no central server managing the network, neither is there a central router. Some examples of pure P2P Application Layer networks designed for peer-to-peer file sharing are Gnutella (pre v0.4) and Freenet.
There also exist hybrid P2P systems, which distribute their clients into two groups: client nodes and overlay nodes. Typically, each client is able to act according to the momentary need of the network and can become part of the respective overlay network used to coordinate the P2P structure. This division between normal and 'better' nodes is done in order to address the scaling problems on early pure P2P networks. Examples for such networks are for example Gnutella (after v0.4) or G2.
Another type of hybrid P2P network are networks using on the one hand central server(s) or bootstrapping mechanisms, on the other hand P2P for their data transfers. These networks are in general called 'centralized networks' because of their lack of ability to work without their central server(s). An example for such a network is the eDonkey network (eD2k).

 Indexing and resource discovery

Older peer-to-peer networks duplicate resources across each node in the network configured to carry that type of information. This allows local searching, but requires much traffic.
Modern networks use central coordinating servers and directed search requests. Central servers are typically used for listing potential peers (Tor), coordinating their activities (Folding@home), and searching (Napster, eMule). Decentralized searching was first done by flooding search requests out across peers. More efficient directed search strategies, including supernodes and distributed hash tables, are now used.
Many P2P systems use stronger peers (super-peers, super-nodes) as servers and client-peers are connected in a star-like fashion to a single super-peer.

Peer-to-peer-like systems

In modern definitions of peer-to-peer technology, the term implies the general architectural concepts outlined in this article. However, the basic concept of peer-to-peer computing was envisioned in earlier software systems and networking discussions, reaching back to principles stated in the first Request for Comments, RFC 1.^[6]
A distributed messaging system that is often likened as an early peer-to-peer architecture is the USENET network news system that is in principle a client–server model from the user or client perspective, when they read or post news articles. However, news servers communicate with one another as peers to propagate Usenet news articles over the entire group of network servers. The same consideration applies to SMTP email in the sense that the core email relaying network of Mail transfer agents has a peer-to-peer character, while the periphery of e-mail clients and their direct connections is strictly a client–server relationship. Tim Berners-Lee's vision for the World Wide Web, as evidenced by his WorldWideWeb editor/browser, was close to a peer-to-peer design in that it assumed each user of the web would be an active editor and contributor creating and linking content to form an interlinked web of links. This contrasts to the broadcasting-like structure of the web as it has developed over the years.

 Advantages and weaknesses

In P2P networks, clients provide resources, which may include bandwidth, storage space, and computing power. As nodes arrive and demand on the system increases, the total capacity of the system also increases. In contrast, in a typical client–server architecture, clients share only their demands with the system, but not their resources. In this case, as more clients join the system, less resources are available to serve each client.
The distributed nature of P2P networks also increases robustness,^{[citation needed]} and—in pure P2P systems—by enabling peers to find the data without relying on a centralized index server^{[citation needed]}. In the latter case, there is no single point of failure in the system.^{[citation needed]}
As with most network systems, unsecure and unsigned codes may allow remote access to files on a victim's computer or even compromise the entire network.^{[citation needed]} In the past this has happened for example to the FastTrack network when anti P2P companies managed to introduce faked chunks into downloads and downloaded files (mostly MP3 files) were unusable afterwards or even contained malicious code.^{[citation needed]} Consequently, the P2P networks of today have seen an enormous increase of their security and file verification mechanisms. Modern hashing, chunk verification and different encryption methods have made most networks resistant to almost any type of attack, even when major parts of the respective network have been replaced by faked or nonfunctional hosts.
Internet service providers (ISPs) have been known to throttle P2P file-sharing traffic due to the high-bandwidth usage.^[7] Compared to Web browsing, e-mail or many other uses of the internet, where data is only transferred in short intervals and relative small quantities, P2P file-sharing often consists of relatively heavy bandwidth usage due to ongoing file transfers and swarm/network coordination packets. As a reaction to this bandwidth throttling several P2P applications started implementing protocol obfuscation, such as the BitTorrent protocol encryption. Techniques for achieving "protocol obfuscation" involves removing otherwise easily identifiable properties of protocols, such as deterministic byte sequences and packet sizes, by making the data look as if it was random.^[8]
A possible solution to this is called P2P caching, where a ISP stores the part of files most accessed by P2P clients in order to save access to the Internet.

 Social and economic impact

Main article: Peer-to-peer (meme)

The concept of P2P is increasingly evolving to an expanded usage as the relational dynamic active in distributed networks, i.e., not just computer to computer, but human to human. Yochai Benkler has coined the term commons-based peer production to denote collaborative projects such as free and open source software and Wikipedia. Associated with peer production are the concepts of:

• peer governance (referring to the manner in which peer production projects are managed)
• peer property (referring to the new type of licenses which recognize individual authorship but not exclusive property rights, such as the GNU General Public License and the Creative Commons licenses)
• peer distribution (or the manner in which products, particularly peer-produced products, are distributed)

Some researchers have explored the benefits of enabling virtual communities to self-organize and introduce incentives for resource sharing and cooperation, arguing that the social aspect missing from today's peer-to-peer systems should be seen both as a goal and a means for self-organized virtual communities to be built and fostered.^[9] Ongoing research efforts for designing effective incentive mechanisms in P2P systems, based on principles from game theory are beginning to take on a more psychological and information-processing direction.

 Applications

There are numerous applications of peer-to-peer networks. The most commonly known is for content distribution

 Content delivery

• Many file sharing networks, such as Gnutella, G2 and FastTrack popularized peer-to-peer technologies. From 2004, it is the largest contributor of network traffic on the Internet.
• Peer-to-peer content delivery networks (P2P-CDN) (Giraffic, Kontiki, Ignite, RedSwoosh).
• Software publication and distribution (Linux, several games); via file sharing networks.
• Streaming media. P2PTV and PDTP. Applications include TVUPlayer, Joost, CoolStreaming, Cybersky-TV, PPLive, LiveStation
• Spotify uses a peer-to-peer network along with streaming servers to stream music to its desktop music player.
• Peercasting for multicasting streams. See PeerCast, IceShare, FreeCast, Rawflow
• Pennsylvania State University, MIT and Simon Fraser University are carrying on a project called LionShare designed for facilitating file sharing among educational institutions globally.
• Osiris (Serverless Portal System) allows its users to create anonymous and autonomous web portals distributed via P2P network.

Networking

• Domain Name System, for Internet information retrieval. ee Comparison of DNS server software
• cloud computing
• Dalesa a peer-to-peer web cache for LANs (based on IP multicasting).

 Science

• In bioinformatics, drug candidate identification. The first such program was begun in 2001 the Centre for Computational Drug Discovery at the University of Oxford in cooperation with the National Foundation for Cancer Research. There are now several similar programs running under the United Devices Cancer Research Project.
• The sciencenet P2P search engine.
• BOINC

Search

• YaCy, a free distributed search engine, built on principles of peer-to-peer networks.

[Communications networks

• Skype, one of the most widely used internet phone applications is using P2P technology.
• VoIP (using application layer protocols such as SIP)
• Instant messaging and online chat
• Completely decentralized networks of peers: Usenet (1979) and WWIVnet (1987).

 General

• Research like the Chord project, the PAST storage utility, the P-Grid, and the CoopNet content distribution system.
• JXTA, for Peer applications. See Collanos Workplace (Teamwork software), Sixearch

 Miscellaneous

• The U.S. Department of Defense has started research on P2P networks as part of its modern network warfare strategy.^[10] In May, 2003 Dr. Tether. Director of Defense Advanced Research Project Agency testified that U.S. Military is using P2P networks.
• Kato et al.’s studies indicate over 200 companies with approximately $400 million USD are investing in P2P network. Besides File Sharing, companies are also interested in Distributing Computing, Content Distribution.
• Wireless community network, Netsukuku
• An earlier generation of peer-to-peer systems were called "metacomputing" or were classed as "middleware". These include: Legion, Globus

Historical perspective

Tim Berners-Lee's vision for the World Wide Web was close to a P2P network in that it assumed each user of the web would be an active editor and contributor, creating and linking content to form an interlinked "web" of links.^{[citation needed]} This contrasts to the current broadcasting-like structure of the web.^{[citation needed]}
Some networks and channels such as Napster, OpenNAP and IRC serving channels use a client–server structure for some tasks (e.g., searching) and a P2P structure for others. Networks such as Gnutella or Freenet use a P2P structure for nearly all tasks, with the exception of finding peers to connect to when first setting up.
P2P architecture embodies one of the key technical concepts of the Internet, described in the first Internet Request for Comments, RFC 1, "Host Software" dated April 7, 1969. More recently, the concept has achieved recognition in the general public in the context of the absence of central indexing servers in architectures used for exchanging multimedia files.

Network neutrality controversy

Peer-to-peer applications present one of the core issues in the network neutrality controversy. In October 2007, Comcast, one of the largest broadband Internet providers in the USA, started blocking P2P applications such as BitTorrent. Their rationale was that P2P is mostly used to share illegal content, and their infrastructure is not designed for continuous, high-bandwidth traffic. Critics point out that P2P networking has legitimate uses, and that this is another way that large providers are trying to control use and content on the Internet, and direct people towards a client-server-based application architecture. The client-server model provides financial barriers-to-entry to small publishers and individuals, and is quite inefficient for sharing large files.


SERVER-CLIENT

The client–server model of computing is a distributed application structure that partitions tasks or workloads between the providers of a resource or service, called servers, and service requesters, called clients.^[1] Often clients and servers communicate over a computer network on separate hardware, but both client and server may reside in the same system. A server machine is a host that is running one or more server programs which share their resources with clients. A client does not share any of its resources, but requests a server's content or service function. Clients therefore initiate communication sessions with servers which await incoming requests.

Description

The client–server characteristic describes the relationship of cooperating programs in an application. The server component provides a function or service to one or many clients, which initiate requests for such services.
Functions such as email exchange, web access and database access, are built on the client–server model. Users accessing banking services from their computer use a web browser client to send a request to a web server at a bank. That program may in turn forward the request to its own database client program that sends a request to a database server at another bank computer to retrieve the account information. The balance is returned to the bank database client, which in turn serves it back to the web browser client displaying the results to the user. The client–server model has become one of the central ideas of network computing. Many business applications being written today use the client–server model. So do the Internet's main application protocols, such as HTTP, SMTP, Telnet, and DNS.

The interaction between client and server is often described using sequence diagrams. Sequence diagrams are standardized in the Unified Modeling Language.
Specific types of clients include web browsers, email clients, and online chat clients.
Specific types of servers include web servers, ftp servers, application servers, database servers, name servers, mail servers, file servers, print servers, and terminal servers. Most web services are also types of servers.

Comparison to peer-to-peer architecture

In peer-to-peer architectures, each host or instance of the program can simultaneously act as both a client and a server, and each has equivalent responsibilities and status.
Both client–server and peer-to-peer architectures are in wide usage today. Details may be found in Comparison of Centralized (Client-Server) and Decentralized (Peer-to-Peer) Networking.

Advantages

• In most cases, a client–server architecture enables the roles and responsibilities of a computing system to be distributed among several independent computers that are known to each other only through a network. This creates an additional advantage to this architecture: greater ease of maintenance. For example, it is possible to replace, repair, upgrade, or even relocate a server while its clients remain both unaware and unaffected by that change.
• All data is stored on the servers, which generally have far greater security controls than most clients.^{[citation needed]} Servers can better control access and resources, to guarantee that only those clients with the appropriate permissions may access and change data.
• Since data storage is centralized, updates to that data are far easier to administer in comparison to a P2P paradigm. In the latter, data updates may need to be distributed and applied to each peer in the network, which is both time-consuming and error-prone,^{[citation needed]} as there can be thousands or even millions of peers.
• Many mature client–server technologies are already available which were designed to ensure security, friendliness of the user interface, and ease of use.^{[citation needed]}
• It functions with multiple different clients of different capabilities.

 Disadvantages

• As the number of simultaneous client requests to a given server increases, the server can become overloaded.^{[citation needed]} Contrast that to a P2P network, where its aggregated bandwidth actually increases as nodes are added, since the P2P network's overall bandwidth can be roughly computed as the sum of the bandwidths of every node in that network.
• The client–server paradigm lacks the robustness of a good P2P network.^{[citation needed]} Under client–server, should a critical server fail, clients’ requests cannot be fulfilled. In P2P networks, resources are usually distributed among many nodes. Even if one or more nodes depart and abandon a downloading file, for example, the remaining nodes should still have the data needed to complete the download.

HOST -TERMINAL
A host-terminal emulation program which permits secure host linkage communication between a host within a protected network and a client outside the network. The client establishes in advance a receiving connection with a relay device, for receiving data compliant to a second protocol (Step S1). Subsequently, the client establishes a transmitting connection with the relay device, for transmitting data compliant to the second protocol (Step S2), and transmits data to the relay device via the transmitting connection (Step S3). The relay device converts the data to a first protocol (Step S4), and transmits the converted data to the host (Step S5). On completion of data processing by the host (Step S6), the processing result is transmitted to the relay device by means of the first protocol (Step S7). The processing result is converted to the second protocol in the relay device (Step S8) and transferred to the client (Step S9).