Computer Networks (CSC 345)

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Computer Networks(CSC 345)

• Fall 2004
• Professor Haimeng Zhang
• IVERS 234F
• zhang_at_cord.edu
• x4742

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Course Objectives

• Motivation What is the Internet and how it works
• To present a comprehensive view of the principles
  and fundamental concepts in Computer Networks
• To learn about the basics in design and
  implementation of network protocols
• To provide an understanding of the components of
  a network and how they are connected.
• To acquire some hands-on experience

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Course Requirements

• Prerequisites
• Programming experience with C/C, equivalent to
  CSC 225
• Good to have the knowledge on OS
• Required Textbook
• Douglas Comer Computer Networks and Internets
  with Internet Applications, 4th ed. Prentice
  Hall, 2004
• Reference Book
• R. Stevens, TCP/IP Illustrated, Volume 1 The
  Protocols, Addison-Wesley, 1994.
• Supplementary class notes available on line
• Course web page http//www.cord.edu/faculty/zhang
  /cs345/cs345.html

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Course Organization

• Lectures TH 1030am 1210pm, IVERS 218
• Homework assignments once every two weeks
• Programming project one group project
• Reading project individual project
• Midterm
• Final

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Course Outline

• Introduction
• Fundamental concepts
• Basic definitions
• Network architecture
• Communication Basics
• Media and signals
• Asynchronous and synchronous communication
• Relationship among bandwidth, throughput, and
  noise
• Frequency-division and time-division multiplexing

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Course Outline (Continued)

• Networking and network technologies
• Packing switching
• Framing, parity, and error detection
• Local and wide area technologies
• Network addressing
• Connection, wiring and extension (repeaters,
  bridges, hubs, switches)
• Forwarding and measuring of delay and throughput

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Course Outline (Continued)

• Internets and Internetworking
• Motivation and concept
• Internet Protocol (IP) datagram format and
  addressing
• Internet routers and routing
• Address binding (ARP)
• Internet Control Message Protocol (ICMP)
• User Datagram Protocol (UDP)
• Transmission Control Protocol (TCP)

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Course Outline (Continued)

• Network Applications
• Domain Name System (DNS)
• File Transfer Protocol (FTP)
• Remote Login Protocol (TELNET)
• Email Transfer (SMTP)
• Web technologies and protocol (HTTP)
• Putting all pieces together

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Schedule of Topics

• Signals, media, bandwidth, throughput and
  multiplexing 2 weeks
• Packet transmission concepts, technologies 5
  weeks
• Internetworking fundamentals 5 weeks
• Internet applications 2 weeks

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What is a Computer Network?

• A collection of computers (PCs, workstations) and
  other devices (e.g. printers, credit card
  readers) are all interconnected
• Components
• Hosts (computers)
• Links (coaxial cable, twisted pair, optical
  fiber, radio, satellite)
• Switches/routers (intermediate systems)
• Goal provide ubiquitous access to resources
  (e.g., database servers, Web), allow remote users
  to communicate (e.g., email)
• User runs applications

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What is a Computer Network?

• Major Network Categories
• The global Internet
• Internal corporate networks
• The worldwide telephone system

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What is a Computer Network?

• Telecommunications spans two concerns
• Voice and video communication versus
• Data communication
• At least one party is a computer
• The two are converging

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What is a Computer Network?
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What is a Computer Network?
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What is a Computer Network?
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What is a Computer Network?
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What is a Computer Network?
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What is a Computer Network?
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What is a Computer Network?

• In summary, a network is a system of hardware,
  software and transmission components that
  collectively allow two application programs on
  two different stations connected to the network
  to communicate well

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What is a Computer Network?

• Direct links (connectivity)
• Point-to-point communication
• Multiple-access

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What is a Computer Network?

• Switched Networks
• Circuit - switched network public telephone
  network
• Packet switched network Internet (collection of
  networks)

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Circuit-Switching

• Set up a connection path (circuit) between the
  source and the destination (permanent for the
  lifetime of the connection)
• All bytes follow the same dedicated path
• Used in telephony
• Advantages dedicated resources
• Disadvantages not very efficient (lower
  utilization, e.g., a person talks lt 35 of the
  time during a call)
• While A talks to C, B cannot talk to D on the
  same line.

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Packet-Switching

• Packets from different sources are interleaved
• Efficient use of resources (since they are used
  on a demand) statistical multiplexing. Nobody
  reserves a lane on a freeway
• Can accommodate bursty traffic (as opposed to
  circuit-switching where transmission is at
  constant rate).

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Features of a Packet-Switching

• Store and forward intermediate nodes (e.g.,
  routers) store (buffer) incoming packets, process
  them and forward them to the appropriate outgoing
  link.
• Allows for flexibility and robustness. Packets
  can travel through alternate paths (adaptive
  routing).
• Undesired situations such congestion, long delays
  may occur.

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Packet Switched Networks Example

• Packets can travel on different networks/links
  that may have different line speeds

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Packet-Switched Networks Topologies
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What is the Internet?

• In the 60s and 70s the Internet (ARPANET) was a
  small network connecting universities, research
  labs and government agencies. Main application
  email, FTP. Motivation share research
• Today it is a global, non-regulated
  communications network with millions of hosts and
  users. Main applications Web, multimedia
  (audio/video), email. Motivation
  commercialization
• A large number of different network technologies
  and standards exist LANs, WANs, B-ISDN, Optical
  Nets, Wireless, Satellite.

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The Internet Today-- Complicated

• A huge and arbitrary collection of heterogeneous
  nets. A network of networks!
• More than 70 million hosts
• Growing exponentially doubling every 18 months
• Hierarchically structured
• LANs (e.g., Ethernet)
• CANs (e.g., FDDI)
• National/global (e.g., ATM or optical backbone)
• Fully distributed operation (i.e., no centralized
  system or computer)

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An Internet
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Probing the Network-Example

• Concordia campus network
• http//www.cord.edu/faculty/dduncan/cordnet.ht
  m
• Minnesota State Network
• http//graphs.onvoy.com/infrastructure
• Ping - sends message that is echoed by remote
  computer
• Traceroute - reports path to remote computer

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Internet Today

• Packet - switched network
• Packets
• Data are chopped up into small blocks called
  packets (e.g., 4500 bytes)
• Each packet carries extra information to allow it
  to reach its destination
• Each intermediate node processes the packet and
  forward it to the next node

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Issues

• Resource sharing (i.e., accommodate many users
  over the same link or through the same router)
• Addressing and routing (i.e., how does an email
  message finds its way to the receiver)
• Reliability and recovery guarantee end-to-end
  delivery
• Traffic management monitoring and policing the
  network! Regulate traffic

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Network Performance

• There is a number of measures that characterize
  and capture the performance of a network
• It is not enough that networks work
• They must work well
• Quality of service (QoS) defines quantitative
  measures of service quality
• Speed
• Delay (Latency)
• Reliability
• Security (not a QoS measure but crucial)

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Network Performance

• Speed
• Bits per second (bps)
• Multiples of 1,000 (not 1,024)
• Kilobits per second (kbps) ? Note the lower case
  k
• Megabits per second (Mbps)
• Gigabits per second (Gbps)
• Terabits per second (Tbps)
• Related to link bandwidth

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Network Performance

• Congestion and Latency
• Congestion because traffic chronically or
  momentarily exceeds capacity
• Latency delay measured in milliseconds (ms),
  microseconds ( ).
• Especially bad for some services such as voice
  communication or highly interactive applications

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Network Performance

• Delay
• Transmission time time it takes to transmit a
  packet (depends on the link speed) packet size/
  speed
• Propagation delay time for a bit to travel
  across a link (depends on the distance, physical
  medium)
• Queuing delay waiting time inside a buffer
• Processing delay time to process a packet
• RTT (round-trip time) time for a bit to travel
  to the destination and come back

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Network Performance

• Example consider a 100 Mbps link which is 4,000
  miles long, if data travels at 40,000 miles/sec
  and a packet is 1MB ( Bytes
  
• bits), then
• Transmission delay 1MB/100 Mbps
• ms 0.080 sec
• Propagation delay 4,000/40,000 0.1 sec

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Reliability and Recovery

• Reliability
• Availability percentage of time the network is
  available to users for transmission and reception
• Error rate percentage of lost or damaged
  messages or bits. (For example, bit error rate of
  )
• Examples
• Bit errors (bits are flipped, e.g., due to
  electrical signal interference.)
• Packet loss (packets may be dropped due to
  insufficient buffer space.)
• Packet delays (e.g., due to large queue size)
• Nodes or links can fail (go down)
• Malicious users

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Reliability and Recovery

• As a consequence
• Packets delivered to the wrong destination
• Long delays on packets
• Packets delivered out-of-order
• Duplicate packets
• Recovery
• Implement error-control mechanism
• Hop by hop (I.e., between nodes)
• End-to-end (source-to-destination).
• Retransmissions
• End-to-end security (e.g., encryption,
  authentication)

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User Applications

• Users run application programs (web, email, ftp)
  at the hosts interconnected through a network
• Hosts need to communicate in a meaningful way.
  User should not be concerned with the underlying
  network
• Network supports process-to-process (uni- or
  bi-directional) communication among the hosts
• Applications need to take into consideration
  limitations imposed by the networks physical
  characteristics

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What is a Protocol?

• Set of rules that specify the format and meaning
  of messages exchanged between computers across a
  network
• Format is sometimes called syntax
• Meaning is sometimes called semantics
• Example from everyday life traffic laws!

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One Or Many Protocols?

• Computer communication across a network is a very
  hard problem
• Complexity requires multiple protocols, each of
  which manages a part of the problem
• May be simple or complex must all work together

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Protocol Suites

• A set of related protocols that are designed for
  compatibility is called a protocol suite
• Protocol suite designers
• Analyze communication problem
• Divide problems into subproblems
• Design a protocol for each subproblem

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Layered Protocol Design

• Layering model is a solution to the problem of
  complexity in network protocols
• Model suggests dividing the network protocol
  into layers, each of which solves part of the
  network communication problem
• These layers have several constraints, which
  ease the design problem
• Network protocol designed to have a protocol or
  protocols for each layer

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Layered Network Architecture

• Application data need to be transformed into
  packets (the basic transmission unit)
• Peer entities in layer N1 communicate with each
  other by communication services provided by layer
  N (below them)
• Each layer has specific tasks and functionality.
  It also provides services to the layers above and
  below it
• Peer entities communicate by exchanging messages

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ISO 7-Layer Reference Model

• International Organization for Standards (ISO)
  defined a 7-layer reference model as a guide to
  the design of a network protocol suite

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ISO 7-Layer Reference Model

• Layers are named and numbered reference to
  layer n'' often means the nth layer of the ISO
  7-layer reference model
• many modern protocols do not exactly fit the ISO
  model, and the ISO protocol suite is mostly of
  historic interest

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ISO 7-Layer Reference Model

• Layer 7 Application
• Application-specific protocols such as FTP and
  SMTP (electronic mail)
• Layer 6 Presentation
• Common formats for representation of data
• Layer 5 Session
• Management of sessions such as login to a remote
  computer
• Layer 4 Transport
• Reliable delivery of data between computers

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ISO 7-Layer Reference Model

• Layer 3 Network
• Address assignment and data delivery across a
  physical network
• Layer 2 Data Link
• Format of data in frames and delivery of frames
  through network interface
• Layer 1 Physical
• Basic network hardware media transmission

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Layering Principle
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Layering Principle

• Application data need to be transformed into
  packets (the basic transmission unit)
• Peer entities in layer N1 communicate with each
  other by communication services provided by layer
  N (below them)
• Each layer has specific tasks and functionality.
  It also provides services to the layers above and
  below it
• Peer entities communicate by exchanging messages

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Data Communications

• On the sender, each layer
• Accepts an outgoing message from the layer above
• Adds a header and other processing
• Passes resulting message to next lower layer
• On the receiver, each layer
• Receives an incoming message from the layer below
  
• Removes the header for that layer and performs
  other processing
• Passes the resulting message to the next higher
  layer

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Data Communications

• The software at each layer communicates with the
  corresponding layer through information stored in
  headers
• Each layer adds its header to the front of the
  message from the next higher layer
• Headers are nested at the front of the message as
  the message traverses the network

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Data Communications
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Internet Protocol Architecture

• Originally it was based on the ISO reference
  model
• Currently, Internet is mostly based on the TCP/IP
  protocol suite (designed in late 70s)
• TCP/IP became popular as it was bundled with the
  UNIX/C environment
• ISO is still influential in designing networks
• Other architectures ATM. Frame Relay

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Reading Materials

• Textbook
• Chapters 1, 2 and Sections 3.1, 3.2 of Chapter 3
• Chapter 16