Lecture A4 Network / InternetComputing and Art : Nature, Power, and LimitsCC 3.12: Fall 2007FunctionaliaInstructorChipp Jansen, [email protected] Web Pagehttp://www.sci.brooklyn.cuny.edu/~chipp/cc3.12/Today:•Network Concepts•Internet Specifics•Clients and Servers•IP Address and Domain Name ServersWhat is a network?• when computers talk to each other, this is called a networkpurpose: to share resources, physical (hardware), digital (software and data)• open system = “a system whose architecture is not a secret” • Internet is a distributed network- robust to failure, or attackARPAnetComponents of a Network• the network can have different kinds of computers and peripherals attached to it routerprintercameratoasterToaster?2001 - British Design - queried the local weather via the network, burned an appropriate symbol on your toast.WAN vs. LANWhat is a LAN?What is a WAN?What kind of network is the internet?WAN vs. LANWhat is a LAN?• networks in which computers are physically connected to each other in the close geographical proximity are called local area networks (LANs) What is a WAN?• other networks are called wide area networks (WANs) What kind of network is the internet?• the internet is a wide area networkNetwork Topologies• the way in which the computers are connected to each other is called the network’s topology Hub- StarKinds of Network Topologies• bus (linear model; inexpensive to run cables, but not robust to node failure) (back-bones of the internet)Kinds of Network Topologies• ring (example: IBM token ring)Kinds of Network Topologies• star (can be expensive to run cables, but robust to node failure)Kinds of Network Topologies• hub (efficient; internet model) (scalable, centralized point of failure)hubworkstationworkstationworkstationprinterfile-serverNetwork Architecture• Open System Interconnection (OSI) reference model or “7-layer” model1. application layer 2. presentation layer 3. session layer4. transport layer 5. network layer 6. data link layer 7. physical layer• “All People Seem To Need Data Processing.”What is a layer? •layers allow us to break-down a large task (i.e. communicating over a network) into smaller tasks•provides abstraction•a collection of related functions that provides services to the layer above it and receives service from the layer below itpresentationapplicationsessionprovidesservicereceivesserviceOSI / “7-layer” model1. application layer •displays data, communicates with lower layers via presentation layer•programmers writing applications (i.e. web-browser) use this layer’s functionality•end-user’s use the application itself2. presentation layer 3. session layer4. transport layer 5. network layer 6. data link layer 7. physical layerOSI / “7-layer” model1. application layer 2. presentation layer •converts application layer data to forms understandable by other layers, and back; translates the “meaning” of the bits•“special-packaging”• encryption3. session layer4. transport layer 5. network layer 6. data link layer 7. physical layerOSI / “7-layer” model1. application layer 2. presentation layer 3. session layer•exchange of data between applications – “dialog” – and synchronization between applications•manages and terminates the connections between the local and remote application4. transport layer 5. network layer 6. data link layer 7. physical layerOSI / “7-layer” model1. application layer 2. presentation layer 3. session layer4. transport layer •transfer of data through network; effects flow control; provides some error recovery •TCP (transmission control protocol) and UDP (universal datagram protocol) operate at this layer•“post-office” - handles / classifies the parcels5. network layer 6. data link layer 7. physical layerOSI / “7-layer” model1. application layer 2. presentation layer 3. session layer4. transport layer 5. network layer •physical routing of data from one computer to another; facilitates sender finding receiver •IP, Internet Protocol - addressing•Routers operate at the network level•fragmentation - can break a package up into smaller packages (packets), handles resending (packet switched)•“shipping network” - sacks of mail, trucks, airplanes 6. data link layer 7. physical layerPacket-switchingSending units in smaller chunks:1. More efficient: line of people at a busy pay-phone2. React to failure and congestion: avoiding traffic jams3. Improves reliability: part of message will arrive, and can resendOSI / “7-layer” model1. application layer 2. presentation layer 3. session layer4. transport layer 5. network layer 6. data link layer •manages transmissions of low-level data; detects and corrects transmission errors •point-to-point links7. physical layerOSI / “7-layer” model1. application layer 2. presentation layer 3. session layer4. transport layer 5. network layer 6. data link layer 7. physical layer •sends electronic signals, or “bits” – 0’s and 1’s) •(usually linked to above layer)•wired ethernet network vs. wireless networknetwork information units and formats • “information units” are passed from one layer to another • “headers” are added as information passes from upper to lower layer • terminology is defined below (it is often not used clearly or precisely): – frame - information unit whose source and destination are the data link layer – packet - information unit whose source and destination are the network layer – segment - information unit whose source and destination are the transport layer – message - information unit whose source and destination are the application, presentation or session layers – datagram - information unit in a “connectionless” networknetwork/internet protocols • protocol = set of rules for how computers communicate with each other • IP: internet protocol, i.e., moving data via TCP or UDP • TCP: transmission control protocol (computer computer); can re-transmit if errors • UDP: user datagram protocol; no error checking, fast messaging • HTTP: hypertext transfer protocol (computer browser) • FTP: file transfer protocol • SMTP: simple mail transfer protocolinternet addresses• IP address = Internet Protocol address • every computer on the internet has a unique address • dotted quad notation = four numbers separated by dots (.); e.g., 146.245.250.131 (which is
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