Networks

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Networks

• CSC 212

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What is a Computer Network?

• A Computer Network is two or more computers
  connected together through some communication
  medium for the purposes of sharing data and/or
  devices.

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Network Types

• Three main types or categories of networks
• Local Area Network (LAN)
• Wide Area Network (WAN)
• Metropolitan Area Networks (MAN)
• We will focus on LANs and WANs

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Local Area Networks

• Smaller scope Building or small campus.
• Typically owned by same organization as attached
  devices.
• Internal data rates very high.
• Low propagation delay
• Less overall traffic
• Most common setup is Ethernet

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Wide Area Networks

• Large geographical area.
• Most well-known example The Internet
• Rely on common carrier circuits.
• Alternative technologies for networks
• Circuit switching.
• Packet switching.

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Circuit Switching

• Dedicated communications path established for the
  duration of the conversation.
• On each link, a logical channel is dedicated to
  the connection.
• At each node, incoming data are routed or
  switched to the appropriate outgoing channel
  without delay
• Example Telephone network.

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Public Circuit SwitchedNetwork
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Packet Switching

• No dedicated path
• Divide message into small chunks (called packets)
  of data
• Send individual packets out onto the network
• Because no dedicated path, each packet may take a
  different route to destination
• Packets passed from node to node between source
  and destination.
• The entire packet is received, stored briefly,
  and then transmitted to the next node.

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Packet Switching Operation

• A station breaks long message into packets.
• Packets sent one at a time to the network.

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Networking Basics

• A computer network is a set of independent
  computer systems connected by telecommunication
  links
• Network applications are considered to work on a
  layered architecture

Application Layer
Layer
Layer
Layer 2
Physical Layer
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Physical Layer

• What is the physical medium for transmitting
  information?
• Voice sound waves
• We can use radio waves, electromagnetic waves,
  electrical impulses
• Every medium has a bandwidth and a latency
• Bandwidth how much data can be sent in a given
  period of time (usually measured in bits per
  second bps)
• Latency how much delay between source and
  destination

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How Do You Connect to the Internet?

• Modem/Dial-Up?
• Cable?
• DSL?

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Modems Back in my day...

• In a computer system, information is represented
  digitally
• Voice-oriented telephone systems are analog
  systems
• So how can we send digital information over a
  phone line?
• MODULATOR / DEMODULATOR or MODEM

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Transmission media

• Twisted-pair copper wire type of wiring used in
  telephone communications
• Coaxial cable like the wires that bring cable TV
  into your house
• Fiber-optic cable
• Wireless radio, microwave, infrared signals

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From the computer science perspective

• All we really care about is that data can be
  translated from 1s and 0s at one computer,
  transported somehow to another computer, and
  decoded into the same sequence of 1s and 0s.
• Strictly speaking transmissions over networks are
  never guaranteed to be completely error-free

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Simplified CommunicationsModel
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Between-node communication

• Lets look at the simplest useful network
  possible 2 nodes, 1 link. Consider
• a) What happens if the physical layer lets us
  down? How do we detect the problem, how do we
  react?
• b) What happens if we send information too fast
  to a receiver?
• c) How do we agree on what 1's and 0's mean?

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Between-node communication (2)

• a) Detecting correcting transmission problems
• Message acknowledgments (or ACK's)
• Waiting for the acknowledgment timeout
• If an error is detected then we can request that
  the sender repeat the message.
• What is the problem with this?
• Are there alternative ways?
• integrity checks" so that the most likely types
  of physical layer errors are caught (i.e. parity
  bit, CRC checks)

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Error Checking

• Checksums one simple way of detecting errors.
• Add up the data and send that along with the
  message
• Receiver does same process, and if the calculated
  checksum doesn't match the received one, then an
  error must have occurred
• Potential problem?

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Error Recovery

• Sometimes resending a message is extremely
  expensive
• When would this be true?
• Rather than just detect an error, can we repair
  the error?
• To a certain degree yes, and these are known as
  error recovery mechanisms

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Between-node communication (3)

• b) What if information is sent too quickly?
• Flow Control giving senders/receivers capacity
  for influencing the rate data is sent
• Why is this necessary?

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Between-node communication (4)

• c) What does a given sequence of 1s and 0s
  mean?
• Answer whatever you want it to! But if you
  want to communicate, youd better have a common
  understanding!
• Determine a protocol (accepted standard
  mechanism for communication)
• Analogy talking on the phone

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Network Routing

• What if we want communication between gt2 nodes?

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Network Routing

• Usually, in a LAN (local area network) this is
  Ethernet

• Every message is broadcast to every node on the
  line
• Each node reads the address field and politely
  discards message not addressed to it
• Collisions are a problem!!

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Collisions

• How do we deal with them?
• In Ethernet use CSMA/CD
• Carrier Sense Multiple Access with Collision
  Detection
• A mouthful, but very simple
• Detect collisions by examining data sent with
  data received
• If collision, wait random amount of time and
  resend

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CSMA/CD Example
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Network Routing

• How does a node know if a message belongs to it?
• IP ADDRESS 32-bit integer (usually written in
  decimal dotted notation 142.104.96.5
• Each number is 8 bits, therefore what's the range
  of values for each?
• Coming soon IPv6 -gt 128-bit addresses
• Hierarchical addressing scheme
• 142 -gt BC, 104-gt Victoria, 96 -gtEngineering
  Dept., 5-gt Machine X
• IP stands for Internet Protocol

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Network Routing

• In a WAN (wide-area network)
• A message is chopped into packets . Each packet
  hops from one node to another...not necessarily
  using the same path!

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Routing Algorithms

• How do we pick which path through a network to
  take?
• Use a routing algorithm to pick a path
• Many possibilities, most common is Dijkstra's
  Algorithm
• Learn in detail in CSC 450

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Packet-switched vs. Circuit-switched Networks

• Advantages/disadvantages of each?

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Packet-switched vs. Circuit-switched Networks

• Phone calls are circuit-switched
• connection phase (during and after dialing)
• use of that circuit for all communications
  between A and B
• disconnection phase (after hanging up)
• Advantages of circuit switching
• Virtually no overhead during communications phase
• Route is established for the duration of the
  call
• Communications are guaranteed to be in sequence
• Disadvantages
• Delay establishing communication
• What happens if one link goes down

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Packet-switched vs. Circuit-switched Networks

• Computer networks are usually packet switched
• Network data is divided into packets
• Packets have a header which indicates (among
  other things) the sender and receiver.
• Advantages
• No overhead required to initiate a circuit
  packets may flow to any destination without an
  initial delay
• Routing may take advantage of redundant links,
  making it possible to "work around" problems or
  balance network loads
• Disadvantages
• Additional overhead on each packet (the packet
  "header")
• Packets may be delivered out of sequence or lost

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Packet-switched and Circuit-switched Networks

• Can we combine the advantages of each??
• Virtual circuit-switched networks write some
  program that takes care of
• Breaking messages into packets
• Making sure packets arrive in the right order at
  the destination
• Re-assembling the message
• to the user, it behaves like a circuit-switched
  network!
• This is the role of the TCP Transmission
  Control Protocol
• Usually used with IP protocol TCP/IP which is
  the protocol suite of the Internet

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Packet-switched and Circuit-switched Networks

• For some applications, we dont care if the
  packets arrive in order or if one is missed!
• Example?
• For these applications, TCP is wasteful
• Instead we can use UDP User Datagram Protocol
• UDP just sends out packets over the network
  (thus it is unreliable)

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Packet-switched and Circuit-switched Networks
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TCP/IP Protocol ArchitectureModel
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UDP

• User Datagram Protocol (UDP)
• Connectionless service for application level
  procedures.
• Unreliable.
• Delivery and duplication control not guaranteed.
• Reduced overhead.
• Example Network management.
• UDP header format

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UDP Usage

• Inward data collection
• Periodical sampling, monitoring.
• Loss will not cause a disaster.
• Outward data dissemination
• Broadcast messages.
• Distribution of real time clocks.
• Real time application (where delay is
  catastrophic)
• Ex VoIP

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TCP

• Transmission Control Protocol
• Connection-orientated service for application
  level procedures
• Reliable
• Delivery and duplication control guaranteed
  (however no guarantees about delay)
• Relatively high overhead compared to UDP
• Why is there a potential problem with latency or
  delay in TCP vs UDP?

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TCP Usage

• File transfers
• Order of packets matters greatly, and missing
  packets are catastrophic
• Applications where users are logically
  connected to a server for a period of time

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IP Addresses (IPv4)

• IP address consists of network ID and host ID.

Class A
Class B
Class C
Class D
Class E
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IP Addresses - Class A

• Few networks, each with many hosts.
• Network part and host part.
• Network identifies the network (typically a LAN)
  and host identifies a machine on that network
• Class A
• Start with binary 0.
• All 0 reserved.
• 01111111 (127) reserved for loopback (127.0.0.1
  is sometimes referred to as the localhost
  address)
• Range 1.x.x.x to 126.x.x.x
• All allocated.
• Class A addresses are extremely rare and valuable

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IP Addresses - Class B

• Medium number of networks, each with a medium
  number of hosts.
• Start 10.
• Range 128.x.x.x to 191.x.x.x
• Second Octet also included in network address.
• Ex 140.23.234.10
• 140.23 is host
• 234.10 is network
• All allocated.

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IP Addresses - Class C

• Many networks, each with a few hosts.
• Start 110 (in binary).
• Range 192.x.x.x to 223.x.x.x
• All addresses in the range 192.168.0.0 to
  192.168.255.255 are for private addresses
• Routers/WAPs?
• Second and third octet also part of network
  address.
• Ex 232.234.33.10, 232.234.33 is network, 10 is
  host
• Nearly all allocated.
• See IPv6
• In the meantime, check out NAT and other
  workarounds

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Applications Network Services

• The Internet is an internetwork a network of
  networks.
• The World Wide Web is one service available your
  Web Browser connects to a Web Server that
  provides graphics and text (or maybe movies,
  mp3s)
• Other services resource sharing, file servers,
  print servers, email servers

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The client/server model

• The fundamental architecture of most network
  applications today
• This involves a server application that waits
  passively for a message and a client application
  that actively initiates the communication

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Client Software

• Generally, client software
• is invoked by the user
• actively initiates contact with a server
• runs locally on a users computer

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Server Software

• The server software
• Is a program dedicated to providing one service
  (say HTTP, FTP, etc)
• However, a server machine may run multiple server
  applications (so the same machine may be your
  HTTP and your FTP server)
• Can handle multiple remote clients concurrently
• Waits passively for requests from remote clients

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Selecting a Service

• How do we as clients indicate to the server what
  service we wish to use? (be it HTTP, FTP, SMTP,
  POP, SSH, etc)

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Service Identification

• TCP assigns unique 16-bit integer values called
  port numbers to different services
• A client specifies the protocol port number of
  the desired service when sending requests. A
  server specifies the port number and waits
  passively for incoming requests,
• (The complete address of a service is therefore a
  32-bit IP address plus 16-bit port address)

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Common Protocols And Port Numbers

• HTTP 80
• HTTPS 443
• SMTP 25
• FTP 20 (for data) and 21 (for control)
• SSH 22
• Telnet 23
• DNS 53
• POP3 110

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Example service DNS

• Domain Name Service
• It would be annoying to have to remember the IP
  address of a server to see a web page!
• Domain names can be purchased from commercial
  companies a DNS server will correlate a domain
  name (www.csc.uvic.ca) to an IP address
  (142.104.106.242)

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P2P The Future?

• The client/server paradigm is currently dominant
• But a different approach is gaining in popularity
  Peer to Peer or P2P
• In P2P, each computer is just another node,
  each equally important to the network as a whole

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P2P (cont)

• Very useful for file sharing
• no reliance on single server machine
• No single point of attack
• Redundancy reliability
• Most famous from the past Napster
• Currently most well-known BitTorrent

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A (very) brief history of the Internet and the WWW

• Mid 1960s researchers at MIT, RAND Corp. and
  U.Ks NPL Research Lab start kicking around the
  idea of large-scale networking
• 1966 One researcher moves to ARPA (Department
  of Defense) -gtlooking at packet-switched networks
  for wartime communication
• October 1969 first 2 nodes of ARPANET
• 1972 Demonstrated to the research community
  also, Electronic Mail (e-mail) is invented

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A (very) brief history of the Internet and the
WWW (2)

• 1970s Everyone is developing their own
  independent networks!
• ARPA researchers develop internetworking
  protocols so that these networks can communicate
  with each other (TCP/IP, IP Addressing)
• 1984 National Science Foundation builds NSFNet
  to bring the information on this interconnection
  of networks to entire academic professional
  community
• 1995 acceptance of the term Internet by U.S.
  Government

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A (very) brief history of the Internet and the
WWW (3)

• World Wide Web different from the Internet!
• 1989 Tim Berners-Lee at CERN comes up with the
  idea of hypertext wouldnt it be easier to
  look at documents if they were interlinked,
  presented nicely?
• HTML Hypertext Markup Language
• HTTP Hypertext Transfer Protocol
• URL Uniform Resource Locator
• Tim Berners-Lees vision for the future
• the Semantic Web