Introduction%20to%20Computer%20Networks

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Introduction to Computer Networks

• Chapter 1 Foundation
• University of Ilam
• By
• Dr. Mozafar Bag-Mohammadi

2
Introduction

• Building a network to support diverse ranges of
  applications
• Distributed computing.
• Multimedia.
• Telecommunication.
• E-commerce, etc.
• What kind of technology do we need?
• Hardware.
• Software.

3
Design goals

• Connectivity
• Scalability
• Simplicity
• For designers.
• Most importantly for users.
• Efficiency
• cost
• performance
• Support for common user services.

4
Building Blocks

• Nodes PC, special-purpose hardware
• hosts
• switches, routers and gateways
• Links coaxial cable, optical fiber
• point-to-point
• multiple access

5
Switched Networks
A network can be defined recursively as...

• two or more nodes connected by a link, or

• two or more networks connected by two or more
  nodes

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Strategies

• Circuit switching carry bit streams
• Connection oriented.
• Original telephone network
• Dedicated resource.
• Packet switching store-and-forward messages
• Connectionless (IP) or connection oriented (ATM)
• Shared resource.
• Packet switching is the focus of computer
  Networks.

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Addressing and Routing

• Address byte-string that identifies a node
• usually unique
• Routing process of forwarding messages to the
  destination node based on its destination address
• Types of addresses
• unicast node-specific
• broadcast all nodes on the network
• multicast some subset of nodes on the network

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Multiplexing (resource sharing)

• Time-Division Multiplexing (TDM)
• Frequency-Division Multiplexing (FDM)

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Statistical Multiplexing

• On-demand time-division
• Schedule link on a per-packet basis
• Packets from different sources interleaved on
  link
• scheduling
• fairness, quality of service
• Buffer packets that are contending for the link
• Buffer (queue) overflow is called congestion

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Packet Switching

• A node in a packet switching network

Node
incoming links
outgoing links
Memory
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Inter-Process Communication

• Turn host-to-host connectivity into
  process-to-process communication regardless where
  the process are.
• Give a unified view and fill gaps between what
  applications expect and what the underlying
  technology provides.

Host
Host
Application
Host
Channel
Application
Host
Host
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IPC Abstractions

• Request/Reply (Client-server)
• Guarantee delivering data, and might protect
  privacy and integrity.
• distributed file systems (NFS)
• digital libraries (web)
• File Transfer (FTP)
• Stream-Based- sequence or stream of bits.
• Video on demand
• sequence of frames. Delay constrained, but can
  be fetched before hand.
• For example, a 1/4 NTSC with 352x240 pixels and
  24 bit color. (352 x 240 x 24)/8247.5KB
• Assuming 30 frame per second gt 7500KBps 60Mbps
• Video Conferencing-
• tightly delay bounded. VIC From Berkeley.
• Both application can tolerate packet loss.

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Reliability in the network?

• What Goes Wrong in the Network?
• Bit-level errors (electrical interference), a bit
  is corrupted or a burst error.
• Packet-level errors (congestion)
• Messages are delayed
• Messages are deliver out-of-order
• Packet loss
• Third parties eavesdrop
• Link and node failures

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Performance Metrics

• Bandwidth (throughput)
• data transmitted per time unit
• link versus end-to-end
• notation
• KB 210 bytes
• Mbps 106 bits per second
• Latency (delay)
• time to send message from point A to point B
• one-way versus round-trip time (RTT)
• components
• Latency Propagation Transmit Queue
• Propagation Distance / c (light speed)
• Transmit Size / Bandwidth

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Bandwidth versus Latency

• Relative importance
• Latency bounded- sending 1-byte by client, 1ms vs
  100ms dominates sending a message on a 1Mbps or
  100Mbps link
• Bandwidth Bounded- sending 25MB image 1Mbps vs
  100Mbps dominates 1ms vs 100ms delayed channel.
• Infinite bandwidth
• RTT dominates
• Throughput TransferSize / TransferTime
• TransferTime RTT 1/Bandwidth x TransferSize

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Delay x Bandwidth Product

• Amount of data in flight or in the pipe
• Example 100ms x 45Mbps 560KB

We are usually more interested in 2 times of this
value Since it take RTT to hear from receiver.
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Layering

• Use abstractions to hide complexity and decompose
  to manageable components.
• Abstraction naturally lead to layering
• Alternative abstractions at each layer

Application programs
Request/reply
Message stream
channel
channel
Host-to-host connectivity
Hardware
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Layering

• Advantages
• Modularity protocols easier to manage and
  maintain
• Abstract functionality lower layers can be
  changed without affecting the upper layers
• Reuse upper layers can reuse the functionality
  provided by lower layers
• Disadvantages
• Information hiding inefficient implementations

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Protocols

• Building blocks of a network architecture, or
  layer abstraction.
• Each protocol object has two different interfaces
• service interface operations on this protocol
• peer-to-peer interface messages exchanged with
  peer

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Interfaces
Host 2
Host 1
Service
High-level
High-level
interface
object
object
Protocol
Protocol
Peer-to-peer
interface
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Protocol Machinery

• Protocol Graph
• Nodes are protocols and edge are depends on.
• most peer-to-peer communication is indirect
• peer-to-peer is direct only at hardware level

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Protocol Machinery (cont)

• Multiplexing and Demultiplexing (demux key)
• Encapsulation (header/body)

Host 2
Host 1
Application
Application
program
program
Data
Data
RRP
RRP
RRP
Data
RRP
Data
HHP
HHP
RRP
Data
HHP
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ISO OSI Reference Model

• ISO International Standard Organization
• OSI Open System Interconnection
• Started to 1978 first standard 1979
• ARPANET started in 1969 TCP/IP protocols ready
  by 1974
• Goal a general open standard
• allow vendors to enter the market by using their
  own implementation and protocols

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ISO Architecture
End host
End host
Telnet, FTP, TFTP MSB, integer Manage TCP
streams Message, P2P(process) Packet,
routing Frame, CRC Raw bit pipe
Application
Application
Presentation
Presentation
Session
Session
Transport
Transport
Network
Network
Network
Network
Data link
Data link
Data link
Data link
Physical
Physical
Physical
Physical
One or more nodes
within the network

• The last 3 protocols are implemented in all
  elements in the
• Network.

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Encapsulation

• A layer can use only the service provided by the
  layer immediate below it
• Each layer may change and add a header to data
  packet

data
data
data
data
data
data
data
data
data
data
data
data
data
data
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OSI Model Concepts

• Service says what a layer does
• Interface says how to access the service
• Protocol says how is the service implemented
• a set of rules and formats that govern the
  communication between two peers

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Physical Layer (1)

• Service move the information between two systems
  connected by a physical link
• Interface specifies how to send a bit
• Protocols coding scheme used to represent a bit,
  voltage levels, duration of a bit
• Examples coaxial cable, optical fiber links
  transmitters, receivers

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Datalink Layer (2)

• Service
• framing, i.e., attach frame separators
• send data frames between peers
• others
• arbitrate the access to common physical media
• ensure reliable transmission
• provide flow control
• Interface send a data unit (packet) to a machine
  connected to the same physical media
• Protocols physical layer addresses, implement
  Medium Access Control (MAC) (e.g., CSMA/CD)

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Network Layer (3)

• Service
• deliver a packet to specified destination
• perform segmentation/reassemble
• others
• packet scheduling
• buffer management
• Interface send a packet to a specified
  destination
• Protocols define global unique addresses
  construct routing tables

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Transport Layer (4)

• Services
• provide an error-free and flow-controlled
  end-to-end connection
• multiplex multiple transport connections to one
  network connection
• split one transport connection in multiple
  network connections
• Interface send a packet to specified destination
• Protocols implement reliability and flow control
• Examples TCP and UDP

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Session Layer (5)

• Service
• full-duplex
• access management, e.g., token control
• synchronization, e.g., provide check points for
  long transfers
• Interface depends on service
• Protocols token management insert checkpoints,

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Presentation Layer (6)

• Service convert data between various
  representations
• Interface depends on service
• Protocol define data formats, and rules to
  convert from one format to another

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Application Layer (7)

• Service any service provided to the end user
• Interface depends on the application
• Protocol depends on the application
• Examples FTP, Telnet, WWW browser

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Internet Architecture

• Defined by Internet Engineering Task Force
  (IETF). Developed in mid 60s in the ARPANET
  project.
• No assumption about the network tech.

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Internet Architecture

• Hourglass Design, IP is the focal point. Delivery
  is separated from end-to-end process channel.
• No restrict layering
• Application vs Application Protocol (FTP, HTTP)

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OSI vs. TCP/IP

• OSI conceptually define services, interfaces,
  protocols
• Internet provide a successful implementation

Application
Application
Presentation
Session
Transport
Transport
Network
Internet
Datalink
Host-to- network
Physical
OSI
TCP