Computer Networking An Introduction to Computer Networks and Layered Architectures

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Computer NetworkingAn Introduction to Computer
Networks and Layered Architectures

• Dr Sandra I. Woolley

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Contents

• Introduction to computer networks
• Layered architectures
• OSI and TCP/IP layer models
• Overview of TCP/IP
• Application protocols and TCP/IP utilities
• OSI Open Systems Interconnection
• TCP/IP - Transmission Control Protocol/Internet
  Protocol

8P8C (RJ45) network connectors http//www.flickr.c
om/photos/kluzz/1694878799/
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What is a Communication Network?

• A communication network is a set of equipment and
  facilities that provides a communication service.
• Examples of equipment are routers, servers,
  switches, multiplexers, hubs and modems.
• Examples of facilities are copper wires, coaxial
  cables, optical fiber, ducts, conduits, etc.

A cloud is usually used to represent a computer
network.
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Network Protocols

• Communications between computers requires very
  specific unambiguous rules.
• A protocol is a set of rules that governs how two
  or more communicating parties are to interact.
• For example,
• Internet Protocol (IP)
• Transmission Control Protocol (TCP)
• HyperText Transfer Protocol (HTTP)
• Simple Mail Transfer Protocol (SMTP)

http//www.flickr.com/photos/kairin/68086104/
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Standards Bodies

• Internet Engineering Task Force
• Internet standards development
• Request for Comments (RFCs) www.ietf.org
• International Telecommunications Union
• International telecom standards
• IEEE 802 Committee
• Local area and metropolitan area network
  standards
• Industry Organizations
• MPLS Forum, WiFi Alliance, World Wide Web
  Consortium

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Packet vs. Circuit Switching

• Architectures appear and disappear over time
• Telegraph (message switching)
• Telephone (circuit switching)
• Internet (packet switching)
• The trend is toward packet switching
• Cellular voice networks are packet-based.
• However, large packet flows are easier to manage
  with circuit-like methods.

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Computer Network Evolution

• 1950s Telegraph technology adapted to computers
  
• 1960s Dumb terminals access shared host computer
• SABRE airline reservation system
• 1970s Computers connect directly to each other
• ARPANET packet switching network
• Ethernet local area network
• TCP/IP internet protocols
• 1980s and 1990s New applications and Internet
  growth
• Commercialization of Internet
• E-mail, file transfer, web, P2P, . . .
• Internet traffic surpasses voice traffic

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ARPANET

• The Advanced Research Projects Agency Network
  (ARPANET) of the U.S. Department of Defense was
  the world's first operational packet switching
  network, and the progenitor of the global
  Internet.
• Previous data communications used circuit
  switching, where a dedicated circuit is required
  for the duration of each communication.
• With packet switching links could be shared and
  packets routed independently.

Heart, F., McKenzie, A., McQuillian, J., and
Walden, D., ARPANET Completion Report, Bolt,
Beranek and Newman, Burlington, MA, January 4,
1978.
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Ethernet Local Area Network

• In 1980s, affordable workstations available.
• Need for low-cost, high-speed networks
• To interconnect local workstations
• To access local shared resources (printers,
  storage, servers)
• Low cost, high-speed communications with low
  error rate possible using coaxial cable.
• Ethernet is the standard for high-speed wired
  access to computer networks.

Category 5 Ethernet cable http//www.flickr.com/ph
otos/zinkwazi/474203018/
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Ethernet Medium Access Control

• Network interface cards (NICs) connect
  workstations to a LAN (Local Area Network.)
• Each NIC has a globally unique address.
• Frames are broadcast into coaxial cable.
• NICs listen to the medium for frames with their
  address.
• Transmitting NICs listen for collisions with
  other stations, and abort and reschedule
  retransmissions.

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Internet

• An internet is a network of networks.
• The Internet has a capital I ( or does it? See
  right.)
• Routers (gateways) interconnect different
  networks.
• Host computers prepare Internet Protocol (IP)
  packets and transmit them over their attached
  network.
• Routers forward IP packets across networks.
• IP provides a best-effort service.

0814 AM Aug. 16, 2004 PT Effective with this
sentence, Wired News will no longer capitalize
the "I" in internet. At the same time, Web
becomes web and Net becomes net.
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Internet Addressing and Routing

• IP uses a hierarchical address space Network ID
  Host ID
• IP packets (datagrams) are routed according to
  the Network ID
• IP routers use routing tables to direct the
  transfer of packets.

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Internet Names and IP Addresses

• Routing is done based on IP addresses. IPv4 has
  32-bit addresses. We will consider IPv6 later.
• Addresses have a dotted-decimal notation. E.g.,
• 128.100.11.1
• Hosts are also identified by name
• Easier to remember
• Names often have a hierarchical name structure,
  e.g.www.eee.bham.ac.uk
• Domain Name System (DNS) provides conversion
  between names and addresses.
• Domain names can be purchased, for example, from
  UK2.net.

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Internet Transport Protocols

• Host computers run two transport protocols on top
  of IP to enable process-to-process
  communications.
• User Datagram Protocol (UDP) enables best-effort
  transfer.
• Transmission Control Protocol (TCP) enables
  reliable transfer.
• All Internet applications run on TCP or UDP. For
  example,
• TCP HTTP (web) SMTP (e-mail) FTP (file
  transfer)
• UDP DNS, RTP (voice multimedia)

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Example in Textbook

• Study the example in the recommended text that
  describes what happens when a user clicks on a
  URL.
• Note DNS domain name service uses UDP, HTTP uses
  TCP. Also note the use of port numbers
  ephemeral port numbers and well-known port
  numbers.
• Browser software uses HyperText Transfer Protocol
  (HTTP) to send request for document
• HTTP server waits for requests by listening to a
  well-known port number (80 for HTTP)
• HTTP client sends request messages through an
  ephemeral port number, e.g. 1127
• HTTP needs a Transmission Control Protocol (TCP)
  connection between the HTTP client and the HTTP
  server to transfer messages reliably

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Layer Models
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Layers, Services and Protocols

• The overall communications process between two or
  more machines connected across one or more
  networks is very complex.
• Layering puts similar communication functions
  into groups that are manageable.
• Each layer provides a service to the layer above.
• Each layer operates according to a protocol.

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The OSI Reference Model

• Open Systems Interconnection
• Network architecture
• Definition of layers
• Design of protocols for each layer
• By the 1970s every computer vendor had developed
  its own proprietary layered network architecture.
• Computers from different vendors could not be
  networked together.
• Open Systems Interconnection (OSI) was an
  international effort by the International
  Organization for Standardization (ISO) to enable
  multivendor computer interconnection.

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The OSI Reference Model
Application A
Application B
Application layer
Application layer
7
Presentation layer
Presentation layer
6
Session layer
Session layer
6
Transport layer
Transport layer
4
Communication network
Network layer
Network layer
Network layer
Network layer
3
Data Link layer
Data Link layer
Data link layer
Data link layer
2
Physical layer
Physical layer
Physical layer
Physical layer
1
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The OSI Layers

• Application layer e-mail, file transfer,
  network management, etc.
• Presentation layer independent representation
  of data.
• Session layer dialogue control.
• Transport layer end-to-end transfer of data -
  controls error control, flow control, connection
  set-up and release.
• Network layer provides packet transfer of
  packets across the network. The key aspect is
  routing.
• Data link layer transfers frames between
  network nodes. Inserts address and check bits.
  Performs flow control.
• Physical layer Transfer of bits over the
  channel. Set up and release of physical
  connection.

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Peer-to-Peer Communication Terminology

• Processes at layer n (any of the 7 layers) are
  referred to as layer n entities.
• Layer n1 entities make use of layer n services
  below via a software port called the layer
  service access point (SAP).
• Entities exchange protocol data units (PDUs).
• PDUs contain a header (which contains protocol
  control information) and a service data unit
  (SDU) (information requiring communication).

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Layer Services

• The layer n SDU is the layer n 1 PDU and is
  encapsulated in the layer n PDU.
• The service provided by a layer can be
  connection-oriented or connectionless.

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Headers and Trailers
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Segmentation and Reassembly

• A layer may impose a limit on the size of a data
  block that it can transfer for implementation or
  other reasons.
• Thus a layer-n SDU may be too large to be handled
  as a single unit by layer-(n-1)
• Sender side SDU is segmented into multiple PDUs
• Receiver side SDU is reassembled from sequence
  of PDUs

(a)
Segmentation
n-SDU
n-PDU
n-PDU
n-PDU
Reassembly
(b)
n-SDU
n-PDU
n-PDU
n-PDU
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The Internet and TCP/IP
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TCP/IP Network Architecture

• While the OSI standards were being developed the
  TCP/IP (Transmission Control Protocol/Internet
  Protocol) network architecture emerged as an
  alternative.
• TCP/IP was distributed freely as part of Berkeley
  UNIX.
• Numerous applications were developed at various
  universities and a market for networking software
  emerged.
• This led to a global Internet and the dominance
  of the TCP/IP network architecture.
• IP provides a connectionless best-effort service
  for packets of information.

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TCP/IP Network Architecture

• The TCP/IP model does not require strict
  layering.
• E.g. Applications can run directly over the
  internet layer.

Application Layer
Application Layer
Transport Layer
Transport Layer
Internet Layer
Internet Layer
Network Interface
Network Interface
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How the Layers Work Together (Encapsulation
Example)
TCP Header contains source destination port
numbers
IP Header contains source and destination IP
addresses transport protocol type
Ethernet Header contains source destination MAC
addresses network protocol type
Ethernet header
FCS Frame Check Sequence
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TCP/IP Network Architecture

• The TCP/IP network architecture is a set of
  protocols that allows communication across
  multiple diverse networks.
• It has four layers. The TCP/IP application layer
  incorporates the functions of the top three OSI
  layers.
• Application layer programs (email, file transfer
  etc.) are intended to run directly over the
  transport layer.
• Two basic types of service are offered in the
  transport layer
• Transmission Control Protocol (TCP) - reliable
  connection-oriented transfer
• User Datagram Protocol (UDP) - best-effort
  connectionless transfer.
• The internet layer handles the transfer of
  information across multiple networks (e.g.,
  routing and congestion control). A key aspect of
  the internet layer is the definition of a
  globally unique address for machines. The
  internet layer provides a single service of best
  effort connectionless packet transfer.
• The network interface layer enables IP
  communication across different networks.

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Internet Protocol Approach

• IP packets transfer information across Internet
• Host A IP ? router? router? router? Host B IP
• IP layer in each router determines next hop
  (router)
• Network interfaces transfer IP packets across
  networks

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TCP/IP Protocols
HTTP
RTP
DNS
SMTP
Reliable stream service
TCP
UDP
Best-effort connectionless packet transfer
(ICMP, ARP)
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IP Utilities

• PING - a simple application to determine if a
  host is reachable.
• Makes use of Internet Control Message Protocol
  (ICMP) messages. The purpose of ICMP is to
  inform sending hosts about errors encountered by
  destination hosts or routers.
• Traceroute - to determine the route that a
  packet will take to another host.
• Makes use of ICMP and UDP. The sender sends a
  UDP datagram with TTL 1 (Time to live in hops
  1) and an invalid port no. to the destination
  host. The first router sets TTL to 0, discards
  the datagram and sends an ICMP Time Exceeded
  message to the sender. This identifies the first
  machine in the route. TTL is incrementally
  increased until the destination is reached. The
  destination then returns an ICMP Port Unreachable
  message to the sender.
• Netstat - provides information about the network
  status.
• Tcpdump - capture and observe packet exchanges in
  a link.

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Private Study Recommendations

• Read chapters 1 and 2 (Section 2.4 on sockets is
  NOT assessed).
• Study the nytimes example in the text.
• Experiment with Wireshark (previously
  Ethereal).
• Experiment with utilities, for example
• ping at command prompt -Type "command" in "Run"
  box and then "ping" at prompt instructions will
  appear)
• traceroute See visual traceroute or 3D
  traceroute applications at www.snapfiles.com
  (tracert is the command line instruction)
• Visit www.mycooltools.com for VisualRoute and
  Myspeed

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Thank You