Networks

1
Networks

• Nature seems to reach many of her ends by long
  circuitous routes.
• - Lotze

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Computer Network Fundamentals

• A Computer Network is a
• group of computers connected such that they
  allow for remote communication among themselves

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Network Terminology

• Node
• anything connected to the network,
• usually a computer, but it could be a printer or
  a scanner
• Link
• physical medium such as coax cable, fiber optic
  cable etc. connecting the machines
• Segment
• any portion of a network that is separated by a
  switch, bridge or a router from another part of a
  network.
• Backbone
• the main physical cabling network connecting all
  the segments
• holds and supports the physical network, just as
  the name suggests in most cases, only routers or
  gateways of individual networks are connected to
  the backbone, not the host itself
• Topology
• The way (or arrangement of) each node is
  physically connected to the network

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Hierarchical vs. Peer-to-peer

• Client-Server components are a typical example of
  hierarchical network
• client computers rely on the resources of
  another machine (usually termed the server) to
  manage the network, share information and host
  the peripheral devices etc.
• Peer-to-Peer usually done at home or office
  where fewer than half-a dozen machines are
  connected directly via cables
• where each machine takes an equal role in
  managing and sharing information among the rest
• Usually not efficient as hierarchical for larger
  networks

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Direct and Switched Networks

• A Computer Network can be
• Direct Linked
• Nodes interconnected by cables directly
  point-to-point
• One link per node
• Indirect or Switched
• Certain nodes connected to two or more links act
  as forwarding agents, or switches

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Network Classifications

• Computer Networks are classified based on
• Topology
• The way in which they are physically connected
• Ownership
• Private (closed network), Public (Open network)
• Geography
• WAN, LAN, MAN
• Transmission
• Circuit switched, Packet Data, Asynchronous
  Transfer Mode (ATM), Frame Relay
• For example A subset of PCC computers could be
  configured in a star topology with private LAN
  using Frame Relay mode of transmission

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Main Network Classifications

• Two main classifications we will look at are
• Topology
• Ring, Bus, Star, Tree, Mesh, Hybrid
• Geography
• LAN often machines hooked up within a building
• WAN connects cities domestically and across the
  world

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Network Topology Star

• Star
• A central hub machine
• All other machines connect to it
• Pros
• Easily expand to add more nodes to the network
• Cons
• Hub is crucial and can bring the whole network
  down

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Network Topology Ring

• Ring
• Useful when nodes are close to each other
• Pros
• Easy network management
• Less attenuation
• Cons
• Failure of one node breaks the network

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Network Topology Bus

• Bus
• Useful for high-speed connectivity, especially
  LAN
• Pros
• High reliability as nodes are independent
• Cons
• Hard to isolate problem
• Limited number of nodes

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Connecting Networks Bridge

• Two compatible network topologies can be directly
  connected using a Bridge to form a larger network

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Connecting Networks Routers

• Non-compatible networks are connected such that
  they form a network of networks, referred to as
  an internet, using a Router which knows how to
  handle each of the two connecting networks they
  remain independent and can communicate only via
  the Router. Router is a sophisticated device that
  analyzes network outages, capacity, data paths
  etc in order to efficient route the data between
  networks.

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An internet

• An internet is just an interconnection of two
  independent networks in two topologies
• The network of 3 computers in your home when
  hooked up to a network of computers at your work
  can form an internet.
• There are no standardized tools and
  communication rules governing such an
  interconnection.
• You can write your own proprietary software
  for sharing data between these networks that you
  created.

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The Internet

• The Internet (with capital I) is a special
  network of networks involving LANs and WANS all
  over the world, connecting millions of machines,
  using specialized network protocols

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Internet The Big Picture
Interaction and Communication among Computer
Systems
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Metaphorically.

• The Internet can be thought of as a fast,
  high-capacity Interstate highway system that
  forms the backbone for transportation of data,
  connected by access ramps to smaller slower
  country roads that compose the smaller local
  network domains

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Birth of the Internet

• In the Cold War days of 1960s U.S. DoD initiated
  research through DARPA for connecting government
  and research computers for what was then known as
  C3 -Command, Control, and Communication - with
  important features like
• Resistant to point-of-failure attacks
• Highly fault tolerant
• Fairly reliable and fast
• Standardized and easy to expand (scalability)
• What is now known as the Internet was born as
  ARPANET on Labor day, 1969, connecting 4 host
  computers at UCLA, UCSB, Stanford Research
  Institute and University of Utah.

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Internet Topology

• Many colleges and universities collaborated on
  research and set up their own indigenous
  internets leading to the establishment of
  conceptual domains
• This led to haphazard growth making it necessary
  to standardize connections and addressing
• Internet is a mish-mash of domains having Bus,
  Star, Mesh, some hybrid topology interconnected
  by gateway routers

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Internet Domains

• A domain is an autonomous system configured and
  controlled by its owner
• Each domain has a name as well as an address
  which needs to be registered with ICANN
• A newly registered domain can attach to the
  Internet via gateways
• Gateways are essentially routers with certain
  special functionality

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Network Protocols

• Network Protocols are a collection of rules
  governing network activities to be followed by
  all computers in the network, such as
• Data/Message format for sending and receiving
• Addressing scheme to locate networks and machines
  in the network
• Transmission format and privileges
• Routing format and specifications
• Because there is no centralized governing agency
  or board of directors or regents, protocols play
  a vital role in the smooth and efficient
  operation of networks

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Common Network Protocols

• Token Ring Protocol
• For controlling transmission rights in a network
  configured in ring topology
• Ethernet Protocol
• Developed in the 70s uses CSMA/CD (Carrier Sense
  Multiple Access with Collision Detection) to
  regulate network traffic efficiently
• Internet Protocol (IP)
• For formatting, controlling, and regulating
  Internet traffic
• File Transfer Protocol (FTP)
• For controlling the transferring of files across
  the Internet
• HyperText Transfer Protocol (HTTP)
• For transferring hypertext (text, images, audio,
  video) across the Internet
• Simple Mail Transfer Protocol (SMTP)
• For transferring email across the Internet
• Post Office Protocol (POP)
• For accessing local mail server and transferring
  email

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Token Ring Protocol

• Developed by IBM for ring topology networks
• Transmits message only in one direction in the
  ring
• Destination machine gets a copy of the message
  and forwards a copy along the ring
• When Originating machine gets its copy, it knows
  the message was received by the destination
  machine

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Ensuring cooperation among machines in the ring

• Token ring system will work best when each
  sending machine is cooperative and allows other
  machines to transmit as well
• To ensure this, a token is passed along the ring
• Each machine either simply forwards the token if
  it has nothing to transmit, or holds on to it
  indicating it has to transmit
• The sending machine that grabs this token, holds
  on to it till its message has been sent and
  acknowledged, then passes the token along

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Ethernet with CSMA/CD

• No tokens used
• Nodes can transmit whenever they want, after
  waiting for silence in the network
• If two nodes want to transmit at the same time,
  it is treated as a collision
• Then, both nodes reset for a short amount of time
  before trying to resend

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For the end user

• Does it matter what protocol is used by the
  underlying network??
• Well, not really.
• These protocols are transparent to the user.
• But, when setting up applications that need
  networking and communication between networked
  nodes, then
• knowing more about the underlying network is
  crucial for efficiency.

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Internet Protocol IP

• Each computer on the network is identified by a
  unique address, known as the IP address
• Each message, be it text, image, or sound, is
  broken up into packets of about 1500 characters
  and transmitted packet-by-packet by the sender or
  source machine
• Each packet has headers showing source IP
  address, destination IP address, and its Sequence
  Number
• Headers help Routers or Hosts route the packets
  to the destination machine
• Destination machines reassemble all the packets
  it receives using another set of rules, the
  Transmission Control protocol (TCP) that verifies
  if any packets got lost along the way

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IP Address

• An IP address is used for Network Layer
  identification of hosts and routers on a TCP/IP
  network.
• The address consists of a 32-bit binary number of
  4 octets (00000000.00000000.00000000.00000000 )
• Each of the 4 octets is a byte or an 8-bit binary
  number
• Q What is the max value per octet in decimal?
• A 28 256
• The address is usually displayed in the decimal
  format of quads 100.100.100.100, which is called
  dotted decimal notation or dotted quads
• Example124.223.165.45
• Q What is the highest value for such a dotted
  decimal address?
• A 256.256.256.256

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Network classes

• Network CLASS is determined by the high-order bit
• Class 1st Octet Range  High
  Order Bits
• A 1-127 one byte
• B 128-191 two bytes
• C 192-223 three bytes
• D 224-239 reserved for Multicasting
• E 240-255 reserved for future use
• Certain addresses are reserved and will be used
  for Network self-diagnostics, loop back,
  broadcast etc. (like 127 for class A)

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Computing the class of an IP address

• Look at the first four bits of the address.
• 0000 through 0111 is class A
• 1000 through 1011 is class B
• 1100 through 1101 is class C
• 1110 is class D
• 1111 is class E
• All hosts on a network have the same network
  prefix.
• Some special IP addresses
• All 0s This computer
• All 1s All hosts on this network dont
  broadcast outside of this network

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Class C Addresses

• Most of the addresses we normally use are class C
  addresses
• First three octets represent the Network I.D.
• Last octet represents a Host connected to this
  Network
• Example Addison-Wesley Publishing Companys IP
  address is 197.207.177.xxx
• The last octet xxx represents the host address,
  and implies there can be up to 256 machines
  connected to the network I.D. 197.207.177
• Q For a machine with the class C address
  200.121.111. 23, what is the host address, and
  what is the network I.D?
• A Network I.D is 200.121.111, host I.D is 23

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Name Resolution

• Humans dont like to deal with numbers a lot, so,
  we set up names that are easier to work with
• TLD Top-level domains that we already know are
  .com, .edu, .org, .net, .gov
• Domain names need to be registered with ICANN,
  that associates a name with an IP address for
  that domain.
• Example pcc.edu
• Individual machines within that domain can then
  have another identifying name
• Example faculty.pcc.edu
• DNS Domain Name Servers maintain a look-up table
  associating IP address with host names.

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Package Transmission Example

• Three-level hierarchy
• User level
• Shipping company level
• Airline
• Each level has sender and receiver roles to play
• Each level need not know the detailed workings of
  the other level - abstraction

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Internet Architecture

• 4-Layered Architecture
• Application
• Transport
• Network
• Link

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IP Application Layer

• Application layer consists of software that
  require Internet communication to carry out their
  task
• They are implemented as Utility software and are
  usually a collection of routines to get a task
  done
• Example FTP, SMTP, Telnet
• Application layer uses the Transport layer to
  send and receive messages via the Internet

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IP Transport Layer

• Transport layer receives messages from
  Application layer and formats it properly
• Break the message into chunks of equal sized
  packets
• Attach header information on each packet like
  source address, destination address, size of the
  packet and sequence number
• After formatting the message, Transport layer
  sends the packets down to the Network layer
• It is also responsible for doing the reverse when
  it receives a message from the Network layer
  below it

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IP Network Layer

• Network layer is responsible for forwarding
  packets correctly from one network to another
  within the Internet till the message reaches the
  final destination
• This layer must deal with the different network
  topologies within the Internet and know how to
  pass messages between them
• It does this by adding the intermediate addresses
  or stops for the messages along the way in
  other words, it tries to find a path from source
  to destination by routing it correctly

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IP Link Layer

• The physical links like coax cables and the
  topology of the networks form the core of this
  layer
• Depending on the network, link layer follows
  appropriate procedure till packets are delivered
  to the destination link layer
• If it is a ring topology, it waits for the token
  and follows the protocol to pass the message
• It also receives messages and sends it up to its
  Network layer

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TCP Transmission Control Protocol

• TCP is a transport layer protocol that is
  responsible for reliable transmission of data
• UDP (User Datagram Protocol) is another transport
  layer protocol
• Just like you have the choice of shipping company
  for sending your package, each having its own
  unique characteristics, but provides the same
  service,
• Transport layer has a choice of protocols to
  ensure safe transmission and delivery of
  data/message across the Internet.

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TCP and UDP

• TCP establishes connection with destination first
  before sending any packets
• Reliable TCP works at both ends of the
  connection the sender end waits for
  acknowledgement from receiver end and retransmits
  any lost packets
• Slightly slower and less efficient as a result of
  retransmission

• UDP is connectionless just blindly sends the
  data even if the destination machine is not
  operational
• Not Reliable Does not wait for acknowledgement
  and retransmit lost packets
• More streamlined

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Network Layer protocols

• It is the routing layer and the layer that is
  responsible for network addressing.
• IP (Internet Protocol) is the most common network
  layer protocol. Others include Novell's IPX and
  IBM's APPN (Advanced Peer-to-Peer Networking).
• Network layer protocols offer best-effort
  services, as opposed to transport layer
  protocols, which provide reliable data delivery
  services.
• Network layer protocols such as IP are
  connectionless, as opposed to transport layer
  services, which are connection-oriented.
• A common way to think of the network layer is as
  a service provider to the transport layer.
• In the TCP/IP protocol suite, IP provides
  connectionless (unreliable) packet delivery
  services, while TCP provides transport layer,
  connection-oriented services.

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Internet Applications

• Two major ones in your textbook are
• E-mail
• World Wide Web (Web)

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E-mail

• A machine is designated as that domains mail
  server by the domains local authority
• Every email sent from within a domain first goes
  to its domain mail server then gets forwarded to
  the destination
• Every email sent to a domain is received by the
  domains mail server where it is held until it is
  downloaded by the recipient

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World Wide Web

• Propagate multimedia documents (hypertext) on the
  Internet
• Web because a document can reference another
  and so on, spanning the whole Internet, forming a
  network-wide web
• Client Server Architecture
• Client applications like browser reside on client
  machines and can request documents
• Web Server handles these requests and provides
  copies of the requested documents

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Uniform Resource Locator (URL)

• Each document on the web has a unique address to
  help locate it, called the URL
• Protocol (http)
• Mnemonic Address (host IP address in name form)
• Directory, and file name

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HyperText Markup Language

• Hypertext document is similar to regular text
  document i.e., ASCII or Unicode encoded
• Difference is that hypertext has tags or markers
  that instruct how that text should be formatted
  and presented to present a specified appearance
  on screen
• The system of pre-defined tags is known as HTML
• HTML is not really a high-level programming
  language, just a markup language indicating how
  the document should appear

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Summary

• Networks are a group of computers physically
  linked to share data and resources
• Networks can be classified based on topology,
  geography, ownership, transmission
• Network Protocols define how networks should
  handle data as there is no centralized
  controlling authority
• The Internet is a special network of networks
• 4-layered software architecture
• Uses TCP/IP protocol suite for transmission of
  data
• Special form of addressing, known as IP
  addressing