Networks and Distributed Systems.The Rise of Networks.Until the 1.The 1. 98. 0s, however, saw two important developments powerful microprocessors and higher speed networks LANsWANs now up and over a gigabit in speed.More recent developments including research are scalable cluster based computer e.Beowulf and peer to peer networks e.Bit.Torrent, SETIHome, etc.The result is networked computers.It is now feasible to design and implement computing systems with large numbers of networked computers.Advantages to this include shared resources, such as hardware file stores, printers, CPU, etc and software files, databases, etc, to speed up computation partitioned computations can have parts excecuting concurrently within a system, reliability if one site fails, others may be able to continue and easier communication.Network Protocols.Seperate machines are connected via a physical network, and computers pass messages to each other using an appropriate protocol.Networking operating system services interface to the network via the kernel and contains complex algorithms to deal with transparency, scheduling, security, fault tolerance, etc.They provide peer to peer communication services and user applications directly access the services, or the middleware.Middleware is a high level of OS services.Protocol is a well known set of rules and forats to be sued for communications between processes in order to perform a given task.Protocols specify the sequence and format of messages to be exchanged.We can consider a protocol that allows the receiver to request a re transmission across a data link on message error This protocol is inefficient due to a cross over of data and control messages.Steps 6 and 7 are not needed.Protocol Layers.Protocol layers exist to reduce design complexity and improve portability and support for change.Networks are organised as series of layers or levels each built on the one below.The purpose of each layer is to offer services required by higher levels and to shield higher layers from the implementation details of lower layers.Each layer has an associated protocol, which has two interfaces a service interface operations on this protocol, callable by the level above and a peer to peer interface, messages exchanged with the peer at the same level.Kilauea Mount Etna Mount Yasur Mount Nyiragongo and Nyamuragira Piton de la Fournaise Erta Ale.Error Cyclic Redundancy Check Bittorrent Ip' title='Error Cyclic Redundancy Check Bittorrent Ip' />P2P PeertoPeer Het systeem dat gegevens van persoon naar persoon verstuurt.Kazaa, Edonkey, Morpheus zijn er goede en bekende voorbeelden van.Vol.No. 3, May, 2004.Mathematical and Natural Sciences.Study on Bilinear Scheme and Application to Threedimensional Convective Equation Itaru Hataue and Yosuke.Error Cyclic Redundancy Check Bittorrent Ip' title='Error Cyclic Redundancy Check Bittorrent Ip' />No data is directly transferred from layer n on one machine to layer n on another machine.Data and control pass from higher to lower layers, across the physical medium and then back up the network stack on the other machine.As a message is passed down the stack, each layer adds a header and possibly a tail such as a checksum, although this is most common at the bottom of the stack and passes it to the next layer.As a message is received up the stack, the headers are removed and the message routed accordingly.As you move down the stack, layers may have maximum packet sizes, so some layers may have to split up the packet and add a header on each packet before passing each individual packet to the lower layers.The TCPIP reference model originated from research in 1974 using the ARPANET predecessor to the modern Internet.TCPIP does not have session or presentation.Some protocol design issues include identification multiple applications are generating many messages and each layer needs to be able to uniquely identify the sender and intended recipient of each message, data transfer alternatives simplex, half and full duplex, EDC error control error detection and correction depends on environment e.SNR and application requirements, message order preservation and swamping of a slow receiver by a fast sender babbling idiot problem, when a failed node swamps a databus with nonsense.We can also consider two types of protocols, connection vs.Connection orrientated services are where a connection is created and then messages are received in the order they are sent, a real world analogy is the POTS.Connectionless services are where data is sent independently.It is dispatched before the route is known and it may not arrive in the order sent, a real world analogy here is the postal system.OSI Reference Model.The ISO Open Systems Interconnections OSI reference model deals with connecting open networked systems.The key principes of the OSI are layers are created when different levels of abstraction are neededthe layer should perform a well defined functionthe layer function should be chosen with international standards in mindlayer boundaries should be chosen to minimise information flow between layersthe number of layers should be large enough to prevent distinct functions being thrown together, but small enough to prevent the whole architecture being unweildy.Layer.Description.Examples.Application APPProtocols that are designed to meet the communication requirements of specific applications, often defining the interface to a service.HTTP, FTP, SMTP, CORBA IIOPPresentation SECURITYProtocols at this level transmit data in a network representation that is independent of the representations used in individual computers, which may differ.SSL, CORBA Data Representation.Session ERRORSAt this level, reliability and adaptation are performed, such as detection of failures and automatic recovery.Transport TRANSThis is the lowest level at which messages rather than packets are handled.Messages are addressed to communication ports attached to processes.Protocols in this layer be be connection orrientated or connectionless.TCP, UDPNetwork ROUTINGTransfers data packets between computers in a specific network.In a WAN or an inter network this involves generation of a route passing through routers.In a single LAN, no routing is required. 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