CS1CS Lecture 9: Getting Connected PDF

CS1CS Computer Systems Lecture 9 Getting Connected Megan Robertson FBCS CITP SFHEA What’s in today’s lecture?  We’re going to look at computer networks, and the larger network that is the Internet. CS1CS Lecture 9: Getting Connected 2 A brief histo...

CS1CS Computer Systems Lecture 9 Getting Connected Megan Robertson FBCS CITP SFHEA What’s in today’s lecture?  We’re going to look at computer networks, and the larger network that is the Internet. CS1CS Lecture 9: Getting Connected 2 A brief history  Making devices talk to each other for the purposes of communication is nothing new CS1CS Lecture 9: Getting Connected The first telegraph  Invented in 1790 by Claude Chappe  Used extensively by Napoleon to send military & administrative messages the length & breadth of France. CS1CS Lecture 9: Getting Connected 4 A heliograph  Used to signal by flashing reflected sunlight  Often used Morse code  Military applications  No power needs…  …but the sun does need to be shining! CS1CS Lecture 9: Getting Connected 5 A Morse key  Used to tap out messages transmitted over wires CS1CS Lecture 9: Getting Connected An early ‘mobile’ telephone  Around 1985… CS1CS Lecture 9: Getting Connected As for computers…  They’ve come a long way too… CS1CS Lecture 9: Getting Connected Modern computers CS1CS Lecture 9: Getting Connected 9 Early Internet  Initially, a few large computers were linked via leased lines, and academics shared time on the computers over them  The US government, although providing some funding for defence-related computing, thought general internet provision ought to be a commercially sponsored affair… CS1CS Lecture 9: Getting Connected 10 Early Internet  ButAl Gore pushed for some government sponsorship and the first baby steps towards what we know today were taken CS1CS Lecture 9: Getting Connected 11  Research Networks outages? December 1969 1960s-1070s How can we avoid having a direct connection between all pairs of computers? How to transport messages efficiently? How can we dynamically August 1972 handle outages? http://som.csudh.edu/fac/lpress/history/ arpamaps/ CS1CS Lecture 9: Getting Connected And in 1977  Heart, F., McKenzie, A., McQuillian, J., and Walden, D., ARPANET Completion Report, Bolt, Beranek and Newman, Burlington, MA, January 4, 1978 CS1CS Lecture 9: Getting Connected 13 The World Wide Web  Invented by Tim Berners-Lee in 1989  And it’s grown apace ever since! CS1CS Lecture 9: Getting Connected 14 The original proposal CS1CS Lecture 9: Getting Connected 15 This is Berner-Lee’s 1st webserver… CS1CS Lecture 9: Getting Connected 16 And now look at it! CS1CS Lecture 9: Getting Connected 17 Why the history?  Simplyput, a lot of things in the way networks work today are that way because of the way in which they developed… … so if you know the history you will understand how they work today! CS1CS Lecture 9: Getting Connected 18 For example Packet switching  You have probably heard about messages flying around divided into ‘packets’ rather than entire, but have you ever wondered why? CS1CS Lecture 9: Getting Connected 19  Store and forward networking Leased Dialup http://en.wikipedia.org/wiki/BITNET Clipart: http://www.clker.com/search/networksym/1 CS1CS Lecture 9: Getting Connected Efficient Message Transmission  Challenge: in a simple approach, like store- and-forward, large messages block small ones  Break each message into packets  Can allow the packets from a single message to travel over different paths, dynamically adjusting for use  Use special-purpose computers, called routers, for the traffic control CS1CS Lecture 9: Getting Connected 21 Think Postcards  ImagineI want to say, “Hello there, have a nice day”… but can only get 10 characters on each card, along with ‘from’ and ‘to’ addresses and a number… CS1CS Lecture 9: Getting Connected 22 Understanding networking Principles CS1CS Lecture 9: Getting Connected 23 Uses of networks CS1CS Lecture 9: Getting Connected 24 The role of networks Fundamentally, the role of a network is to accept data from the sender host and transmit it to the receiving host. CS1CS Lecture 9: Getting Connected The role of networks  Purpose  Benefits  Resource implications  Communications  Working practice  Commercial opportunity  Information sharing  collaboration CS1CS Lecture 9: Getting Connected Why have a network?  Every large organisation will have computers in which their employees use to track inventory, contact suppliers and do a lot of other functions that are vital to business operations… Networks are used so that certain pieces of data can be accessed by all or certain people within the company CS1CS Lecture 9: Getting Connected Sharing resources  Printers CS1CS Lecture 9: Getting Connected Network security  Internal  External CS1CS Lecture 9: Getting Connected Types of networks CS1CS Lecture 9: Getting Connected Types of network CS1CS Lecture 9: Getting Connected Clusters A cluster network is two or more computing devices working together for a common computing purpose e.g. Load-balancing cluster High-availability cluster High-performance cluster CS1CS Lecture 9: Getting Connected Virtualized networks  Virtualizationis the basic act of decoupling an infrastructure service from the physical assets on which that service operates. CS1CS Lecture 9: Getting Connected Virtualized networks CS1CS Lecture 9: Getting Connected Network standards CS1CS Lecture 9: Getting Connected 35 The layered model What is a layered model?  Models help us to visualize different aspects of complex abstract systems  Almost all communication can be broken down into independent layers that work interdependently.  The ‘layers' (and protocols between them) conceptually represent negotiations between aspects of communication: Content, logical (encoding) and physical delivery of messages. CS1CS Lecture 9: Getting Connected What is a layered model? 2 models for network communications  OSI 7-Layer Model International Standards Organization’s Open Systems Interconnection model  TCP/IP Model Developed by the US Department of Defense CS1CS Lecture 9: Getting Connected The OSI layered model  OSI – Open System Interconnection  Layered Approach  Allows better interoperability between software and hardware  Allows design of elaborate but highly reliable protocol stacks CS1CS Lecture 9: Getting Connected The Physical Layer  Defines all electrical and physical specifications for devices.  Major Functions Establishment & Termination of Connections Connection Resolution & Flow Control of Communication Resources Modulation & Conversion between Digital Data  Example – radio, SCSI (Small Computer System Interface), hubs, repeaters CS1CS Lecture 9: Getting Connected The Data Link Layer  Controls data transfer between network entities  Performs error detection & correction  Uses physical/flat Addressing Scheme  Example – Ethernet, bridges & switches CS1CS Lecture 9: Getting Connected The Network Layer  Performs network routing, flow control, segmentation, and error control functions  The router operates at this layer  Uses local addressing scheme  Example – IP, token ring CS1CS Lecture 9: Getting Connected The Transport Layer  Provide transparent transfer of data between end users  Controls reliability of a given link  Some protocols are stateful and connection oriented (cookies)  Tracks packets & retransmits those that fail  Example – TCP / UDP CS1CS Lecture 9: Getting Connected The Session Layer  Provides mechanism for managing the dialogue between end-user application processes  Provides for either duplex or half- duplex operation  Responsible for setting up and tearing down TCP/IP sessions  Example – NetBIOS CS1CS Lecture 9: Getting Connected The Presentation Layer  Little to do with PowerPoint   Controls syntactical differences in data representation within end-user systems  MIME encoding is done at this layer  Example - XML CS1CS Lecture 9: Getting Connected The Application Layer  Provide semantic conversion between associated application processes  Interfaces directly to and performs common application services for the application processes  Example – Telnet, Virtual Terminal CS1CS Lecture 9: Getting Connected TCP/IP layered model  Transmission Control Protocol and Internet Protocol  TCP/IP is a suite of protocols, also known as the Internet Protocol Suite  It was originally developed for the US Department of Defense Advanced Research Project Agency (DARPA) network, but it is now the basis for the Internet CS1CS Lecture 9: Getting Connected 46 TCP/IP Model Layers CS1CS Lecture 9: Getting Connected 47 What does each layer do?  As with the OSI model, the TCP/IP suite uses a layered model.  TCP/IP model has four or five - depending on who you talk to and which books you read!  Some people call it a four-layer suite - Application, Transport, Internet and Network Access, others split the Network Access layer into its Physical and Datalink components. CS1CS Lecture 9: Getting Connected 48 Network Access Layer  The combination of datalink and physical layers deals with pure hardware (wires, satellite links, network interface cards, etc.)  Access methods such as CSMA/CD (carrier sensed multiple access with collision detection)  Ethernet exists at the network access layer - its hardware operates at the physical layer and its medium access control method (CSMA/CD) operates at the datalink layer. CS1CS Lecture 9: Getting Connected 49 Internet Layer  This layer is responsible for the routing and delivery of data across networks.  It allows communication across networks of the same and different types and carries out translations to deal with dissimilar data addressing schemes. IP (Internet Protocol) and ARP (Address Resolution Protocol) are both to be found at the Internet layer. CS1CS Lecture 9: Getting Connected 50 Transport Layer  The transport layer is similar to the OSI transport model, but with elements of the OSI session layer functionality.  The two protocols found at the transport layer are: TCP (Transmission Control Protocol): reliable, connection-oriented protocol that provides error checking and flow control through a virtual link that it establishes and finally terminates. Examples include FTP and Email UDP (User Datagram Protocol): unreliable, connectionless protocol that not error check or offer any flow control. Examples include SNMP CS1CS Lecture 9: Getting Connected 51 Application Layer  This layer is broadly equivalent to the application, presentation and session layers of the OSI model.  It gives an application access to the communication environment.  Examples:  Telnet  HTTP (Hyper Text Transfer Protocol)  SMTP (Simple Mail Transfer Protocol) CS1CS Lecture 9: Getting Connected 52 Network topology CS1CS Lecture 9: Getting Connected 53 Networking infrastructure CS1CS Lecture 9: Getting Connected 54 Network infrastructure  A network infrastructure is an interconnected group of computer systems linked by the various parts of a telecommunications architecture.  Specifically, this infrastructure refers to the organization of its various parts and their configuration — from individual networked computers to routers, cables, wireless access points, switches, backbones, network protocols, and network access methodologies. CS1CS Lecture 9: Getting Connected 55 Simple network infrastructure  The simplest form of network infrastructure typically consists of one or more computers, a network or Internet connection, and a hub to both link the computers to the network connection and tie the various systems to each other. The hub merely links the computers but does not limit data flow to or from any one system. CS1CS Lecture 9: Getting Connected 56 Switches and routers  To control or limit access between systems and regulate information flow, a switch replaces the hub to create network protocols that define how the systems communicate with each other.  To allow the network created by these systems to communicate to others, via the network connection, requires a router, which bridges the networks and basically provides a common language for data exchange, according to the rules of each network. CS1CS Lecture 9: Getting Connected Tying this all together  Most networks are designed as a stack of layers, each one built upon the one below it. Why? Host 1 Host 2 Layer 3 protocol Layer 3 Layer 3 Layer 2/3 interface Layer 2 protocol Layer 2 Layer 2 Layer 1/2 interface Layer 1 protocol Layer 1 Layer 1 Physical Medium CS1CS Lecture 9: Getting Connected Tying this all together  Each layer provides services to the higher levels.  Each layer behaves as a black box.  Layer n on one machine talks to layer n on another machines.  The corresponding layer in the layered structure are called peers. CS1CS Lecture 9: Getting Connected Tying this all together  The communication between peers must follow certain rules, known as protocols.  No data are directly transferred between layers. Actual communication is through a physical medium below layer 1. Unit 43: Networking Infrastructure CS1CS Lecture 9: Getting Connected OSI 7-layer model CS1CS Lecture 9: Getting Connected 61 Reasons for layering  Simplifies the network model  Enables programmers to specialize in a particular level or layer of the networking model  Provides design modularity  Encourages interoperability  Allows for standardized interfaces to be produced by networking vendors CS1CS Lecture 9: Getting Connected 7: Application layer  The layer where users communicate to the computer  Contains protocols and utilities that provides services to network applications  E-mail: Message formats such as RFC 822 SMTP, POP3 (Post Office Protocol Version 3), IMAP (Internet Message Access Protocol)  WWW: HTML (The HyperText Markup Language), XML (eXtensible Markup Language), XSL (eXtensible Style Language) HTTP (The HyperText Transfer Protocol) CS1CS Lecture 9: Getting Connected 6: Presentation layer  The presentation layer prepares the data from the application layer for transmission over the network or from the network to the application layer.  Include protocols specifying how to represent data (MPEG, JPEG, PIC, WAV)  Responsible for data translation, formatting, encryption, compression.  We need these services because different computers use different internal representation for data (integers and characters) CS1CS Lecture 9: Getting Connected 5: Session layer  Enables two applications on the network to have an ongoing conversation  Provide following services Communication setup and teardown Control for data exchange Data synchronization definition Failure recovery  Examples: Structured Query Language (SQL) X Windows AppleTalk Session Protocol (ASP) CS1CS Lecture 9: Getting Connected 4: Transport layer  Provides end-to-end error free data transport services establish a logical connection data segmentation into maximum transmission unit size messaging service for session layer  Protocols in this layer can be connection-oriented : require an acknowledgment of the receipt of data packets. connectionless : do not require an acknowledgment of the receipt of data packets. CS1CS Lecture 9: Getting Connected  Connection oriented protocols sender receiver Synchronize Negotiate connection Synchronize Acknowledge Virtual Circuit Connection Establish Data Transfer CS1CS Lecture 9: Getting Connected Flow control  The segments delivered back to the sender upon their reception  Any segment not acknowledged are retransmitted.  Segments are sequence back into their proper order upon arrival at their destination  Manageable data flow is maintained in order to avoid congestion CS1CS Lecture 9: Getting Connected 68 Windowing  Thequantity of data segment (in bytes) is sent without receiving an acknowledgment (ack) is called a window. CS1CS Lecture 9: Getting Connected 69 Acknowledgement CS1CS Lecture 9: Getting Connected 70 3: Network layer  Provides services to manage devices addressing to tracks the location of devices on the network to determine the best way to move data on the network  The network layer must transport traffic between devices that are not directly connected.  Routers are specified at this layer. CS1CS Lecture 9: Getting Connected 2: Data link layer  Services Identification of the source and destination nodes via their physical address (Media Access Control (MAC) address) Definition of how data is packaged for transport as frames Error detection Flow control of information sent across the link  Has two sublayers: Media Access Control (MAC) 802.3 Logical Link Control (LLC) 802.2 CS1CS Lecture 9: Getting Connected 1: Physical layer  This layer communicates directly with the various types of actual communication media  Services definition of the physical characteristics of the network hardware, including cable and connector Encoding Transmission of signals on the wire CS1CS Lecture 9: Getting Connected Network devices: repeaters  The number of nodes on a network and the length of cable used influence the quality of communication on the network  Attenuation Natural degradation of a transmitted signal over distance  Repeaters work against attenuation by repeating signals that they receive on a network  Why are repeaters Layer 1 devices? CS1CS Lecture 9: Getting Connected Network devices: hubs  Generic connection device used to tie several networking cables together to create a link between different stations on a network CS1CS Lecture 9: Getting Connected More on hubs  Hubs that are plugged into electric power are called active hubs  A hub that merely connects different cables on a network and provides no signal regeneration is called a passive hub and is not a repeater  “Hub” is a generic term applied to many different network-connection devices  If a hub in some way segments or subdivides the traffic on a network, it is an intelligent, or switching hub CS1CS Lecture 9: Getting Connected Network Segmentation  Segmentation - Process of breaking a network into smaller broadcast or collision domains  Ethernet network, which are characterized by IEEE 802.3 standard, define the use of a Carrier Sense Multiple Access with Collision Detection (CSMA/CD) access method Backoff algorithm : Mathematical calculation performed by computers after a collision occurs on a CSMA/CD network Backoff period : Random time interval used after a collision has been detected on an Ethernet network CS1CS Lecture 9: Getting Connected Layer 2 devices: bridges  Operate at the Data Link layer of the OSI model  Filters traffic between network segments by examining the destination MAC address Based on this destination MAC address, the bridge either forwards or discards the frame When a client sends a broadcast frame to the entire network, the bridge will always forward the frame CS1CS Lecture 9: Getting Connected Network segmentation via bridges CS1CS Lecture 9: Getting Connected More on bridges  Transparent Bridges : Also called learning bridges because they build a table of MAC addresses as they receive frames This means that they “learn” which addresses are on which segments Ethernet networks mainly use transparent bridges  Source-routing bridges : Rely on the source of the frame transmission to provide the routing information Usually employed by Token Ring networks  Translation bridges : Can connect networks with different architectures CS1CS Lecture 9: Getting Connected Layer 2 devices: switches  Increase network performance by reducing the number of packets transmitted to the rest of the network  Like bridges, operate at the Data Link layer of the OSI model  In an Ethernet network, computers are usually connected directly to a switch  Virtual circuit Private connections between two points created by a switch that allows the two points to use the entire available bandwidth between those two points without contention CS1CS Lecture 9: Getting Connected Next time…  How do we find things (addressing)?  How do we keep track of things?  How do we keep our networks secure? 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