Computer Network Basics An overview of computer networking

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Computer Network Basics

• An overview of computer networking which
  introduces many key concepts and terminology.
  Sets the stage for future topics.

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Components of any Computer
Keyboard, Mouse
Computer
Processor (active)
Devices
Memory (passive) (where programs, data live
when running)
Input
Control (brain)
Disk, Network
Output
Datapath (brawn)
Display, Printer
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Communication Devices

• Synchronous communication uses a clock signal
  separate from the data signal- communication can
  only happen during the tick of the timing cycle
• Asynchronous communication does not use a clock
  signal- rather, it employs a start and stop bit
  to begin and end the irregular transmission of
  data

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Connecting to Networks (and Other I/O)

• Bus - shared medium of communication that can
  connect to many devices
• Hierarchy of Buses in a PC

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Operating systems
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Operating Systems Developed for Portable Devices
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A closer look at network structure

• network edge applications and hosts
• network core
• routers
• network of networks

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General Architecture of Computer Networks
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The Network Core

• mesh of interconnected routers
• the fundamental question how is data transferred
  through net?
• circuit switching dedicated circuit per call
  telephone net
• packet-switching data sent thru net in discrete
  chunks

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Connection of Networks
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Network Topology

• a) bus, b) star, c) ring, d) tree structure

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Classification of the networks according to the
connection establishing

• Line switched network
• Packet switched network
• Radiating/data disseminating systems
• Point-to-point connected networks

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Wired media

• Telephone line
• Thin Coax
• Thick Coax
• Unshielded Twisted Pair (UTP)
• Shielded Twisted Pair (STP)
• Fibre

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(Data) Reliability

• A network service is (data) reliable if the
  sender application can rely on the error-free and
  ordered delivery of the data to the destination
• In the Internet the reliability can obtained
  mainly by acknowledgements and retransmission
• In such a way the losses in the underlying layers
  can be retrieved

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Flow-control and Congestion Prevention

• Flow-control to protect the receiver against the
  overload
• I.e. the sender (source) sends more data than
  the receiver can process
• it is mainly necessary in link and transport
  level
• Congestion prevention to prevent the
  intermediate nodes against the overload
• it is mainly necessary in network level

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Overload and Congestion

• Overload Too many packets occur in a subnetwork
  in the same time, which prevent each other and in
  such a way the throughput decreases
• Congestion the queues in the routers are too
  long, the buffers are full.
• As a consequence some packages are dropped if the
  buffers of the routers are overloaded
• In extreme case grid-lock, lock-up

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Deadlock

• Deadlock the most serious situation of the
  congestion, the routers wait for each other
• Direct store and forward deadlock the buffers of
  two neighbouring routers are full with the
  packets to be sent to the other router
• Indirect store and forward deadlock the deadlock
  occurred not between two neighbouring routers but
  in a subnetwork, where any of the routers has not
  free buffer space for accepting packets

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Review Networking Definitions

• Network physical connection that allows two
  computers to communicate
• Packet unit of transfer, bits carried over the
  network
• Network carries packets from on CPU to another
• Destination gets interrupt when packet arrives
• Protocol agreement between two parties as to how
  information is to be transmitted
• Broadcast Network Shared Communication Medium
• Delivery How does a receiver know who packet is
  for?
• Put header on front of packet Destination
  Packet
• Everyone gets packet, discards if not the target
• Arbitration Act of negotiating use of shared
  medium
• Point-to-point network a network in which every
  physical wire is connected to only two computers
• Switch a bridge that transforms a shared-bus
  (broadcast) configuration into a point-to-point
  network
• Router a device that acts as a junction between
  two networks to transfer data packets among them

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The Need for a Protocol Architecture

• Procedures to exchange data between devices can
  be complex
• High degree of cooperation required between
  communicating systems
• destination addressing, path
• readiness to receive
• file formats, structure of data
• how commands are sent/received and acknowledged
• etc.

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Layered Protocol Architecture

• Modules arranged in a vertical stack
• Each layer in stack
• Performs related functions
• Relies on lower layer for more primitive
  functions
• Provides services to next higher layer
• Communicates with corresponding peer layer of
  neighboring system using a protocol

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Network Layering

• Layering building complex services from simpler
  ones
• Each layer provides services needed by higher
  layers by utilizing services provided by lower
  layers
• The physical/link layer is pretty limited
• Packets are of limited size (called the Maximum
  Transfer Unit or MTU often 200-1500 bytes in
  size)
• Routing is limited to within a physical link
  (wire) or perhaps through a switch
• Our goal in the following is to show how to
  construct a secure, ordered, message service
  routed to anywhere

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Key Features of a Protocol

• Set of rules or conventions to exchange blocks of
  formatted data
• Syntax data format
• Semantics control information (coordination,
  error handling)
• Timing speed matching, sequencing
• Actions what happens when an event occurs

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Operation of Protocols
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The OSI Model

• Physical Layer
• (Data) Link Layer
• Network Layer
• Transport Layer
• Session Layer
• Presentation Layer
• Application Layer

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Physical Layer

• Transmission of energy onto the medium
• Collection of energy from the medium
• This layer is concerned with the physical
  transmission of raw bits
• This bits are transmitted through mechanical,
  electrical, and procedural interfaces which
  include
• interface card standard
• modem standards
• certain portions of the ISDN and LAN MAN standards

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(Data) Link Layer

• Transmission of frames over one link or network
• Often subdivided into the MAC and LLC
• It receives bits from the physical layer,
  converting bits to frames
• frame boundaries
• Using protocols (e.g. HDLC), this layer corrects
  errors that might have occurred during
  transmission across a link
• In addition this layer provides an error-free
  transmission channel to the next layer known as
  the network layer error control
• ARQ
• duplicates
• Flow control

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Network Layer I

• The previous two layers were concerned with
  getting error-free data across a link
• The network layer establishes connections between
  nodes, routes data packets through the network,
  and accounts for them
• End-to-end transmission of packets (possibly over
  multiple links)
• Controls the operation of the subnet
• Routing
• static
• dynamic
• Congestion control
• At this stage, there may be congestion due to
  many packets waiting to be routed
• Some packets may be lost during congestion

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Network Layer II

• Accounting
• packets
• bytes
• etc.
• Internetworking
• This layer is also concerned with internetworking
  where there is talking between technologies,
  such as the traditional Internet connected to ATM
• segmentation
• addressing
• sequencing
• accounting
• Broadcast subnets thin network layer

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Transport Layer I

• This layer presumes the ability to pass through a
  network and provides additional services to
  end-users, such as and-to-and packet reliability
• End-to-end delivery of a complete message
  (end-to-end communication path, usually reliable)
• Isolation from hardware
• Multiplexing/demultiplexing
• Divide message into packets
• Reassemble (possibly out of order packets) into
  the original message of the distant end

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Transport Layer II

• End-to-end flow control
• Acknowledgments
• Types of service
• error-free, point-to-point, in sequence, flow
  controlled
• no correctness guarantees
• no sequencing
• Establishing/terminating connections
• naming/addressing
• intra-host addressing (process, ports)

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Session Layer

• This layer enables users to establish sessions
  across a network between machines
• In addition, it offers session management
  services
• Set up and management of end-to-end conversation
• Establish and terminate sessions
• superset of connections
• Assignment of logical ports
• Dialogue control
• Token management
• for critical operations
• Synchronization
• checkpoints/restarts

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Presentation Layer

• This layer is concerned with the syntax and
  semantics of messages, code conversions between
  machines, and other data conversion services
• Some of these services are data compression and
  data encryption
• Interface between lower layers and application
• Formatting
• Syntax semantics of messages
• Data encoding (e.g. ASCII to EBCDIC)
• Compression
• Encryption/Decryption
• Authentication

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Application Layer

• This layer provides support for the user's
  network applications
• Some application layer services have been
  standardized, e.g.
• File Transfer and Management (FTAM)
• Message Handling Services for electronic mail
  (X.400)
• Directory Services (X.500)
• Electronic Data Interchange (EDI)
• Program youre running,applications
• file transfer, access management
• e-mail
• virtual terminals
• WWW

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The OSI Protocol Stack
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Operation of the model
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Names of the Nodes, Connections and Data Units
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Communication among the layers

• Connection oriented network service (virtual
  circuits, eg. ATM)
• Reliable transport service
• Unreliable transport service
• Connectionless network service (datagram service,
  eg. IP)
• Reliable transport service (eg. TCP)
• Unreliable transport service (eg. UDP)

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Network Tools

• Repeater connects network segments logically to
  one network
• Hub multiport repeater
• Bridge datalink level connection of two networks
• Switch multiport bridge
• Router connects networks that are compatible in
  transport level
• subnetworks are connected to the interfaces of
  the repeater
• Gateway (proxy server) router between two
  individual network. The Way Out

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Physical Layer Devices

• Repeater
• Hub
• dumb
• level-1 hub
• multi-port repeater

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Data Link Layer Devices

• Bridge
• Cascaded vs. Backbone
• Single
• Multiple
• Switch (switched hub)

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Routers

• Provide link between networks
• Accommodate network differences
• Addressing schemes
• Maximum packet sizes
• Hardware and software interfaces
• Network reliability
• Congestion/Traffic Management

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Devices of the Network Connection
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Architectural Implementation of the LANs

• Ethernet (IEEE 802.3)
• FDDI
• Gigabit Ethernet
• Token Bus (IEEE 802.4)
• Token Ring (IEEE 802.5)

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Characteristics of High-Speed LANs
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Wide Area Network Connections

• Solutions for connecting LANs to the Internet
• Ethernet (ring or star topology)
• Managed Leased Line Network (MLLN)
• ATM (Asynchronous Transfer Mode)
• Switched line
• ISDN line

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Soft and Hard States

• State the data collection, which are necessary
  for keeping the connection between two protocol
  entities
• Hard state
• If the connection is established once, it is
  never timed out, even if it is not in usage
• To cancel the connection one of the participants
  of the connection must explicitly close it
• The history of the state is stored
• Soft state
• To keep the connection the participants must send
  occasionally keep-alive messages, since without
  keep-alive message the state information is timed
  out after a certain period
• The state is called as soft since in the
  ordinary operation the state can change easily
• The history of the state is not stored

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Packet switching versus circuit switching

• Is packet switching best in every case?

• Great for bursty data
• resource sharing
• no call setup (less start-up delay)
• However
• Packets can experience delays, so not for
  real-time applications
• excessive congestion leads to packet delay and
  loss
• protocols (like TCP) are needed for reliable data
  transfer, and congestion control

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Performance Considerations

• Before continue, need some performance metrics
• Overhead CPU time to put packet on wire
• Throughput Maximum number of bytes per second
• Depends on wire speed, but also limited by
  slowest router (routing delay) or by congestion
  at routers
• Latency time until first bit of packet arrives
  at receiver
• Raw transfer time overhead at each routing hop
• Contributions to Latency
• Wire latency depends on speed of light on wire
• about 11.5 ns/foot
• Router latency depends on internals of router
• Could be lt 1 ms (for a good router)

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Delay in packet-switched networks

• Nodal processing
• check bit errors
• determine output link
• Queueing
• time waiting at output link for transmission
• depends on congestion level of router

• packets experience delay on end-to-end path
• four sources of delay at each hop

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Delay in packet-switched networks

• Propagation delay
• d length of physical link
• s propagation speed in medium (2x108 m/sec)
• propagation delay d/s

• Transmission delay
• Rlink bandwidth (bps)
• Lpacket length (bits)
• time to send bits into link L/R

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Queueing delay (revisited)

• Rlink bandwidth (bps)
• Lpacket length (bits)
• aaverage packet arrival rate

traffic intensity La/R

• La/R 0 average queueing delay small
• La/R -gt 1 delays become large
• La/R gt 1 more work arriving than can be
  serviced, average delay infinite!

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Internet protocol stack

• Application supporting network applications
• ftp, smtp, http
• Transport host-host data transfer
• tcp, udp
• Network routing of datagrams from source to
  destination
• ip, routing protocols
• Network access data transfer between neighboring
  network elements
• ppp, ethernet
• Physical bits on the wire

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Layering logical communication

• E.g. transport
• take data from app
• add addressing, reliability check info to form
  datagram
• send datagram to peer
• wait for peer to ack receipt
• analogy post office

transport
transport
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Layering physical communication
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Protocol layering and data

• Each layer takes data from above
• adds header information to create new data unit
• passes new data unit to layer below

source
destination
message
segment
datagram
frame
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IP over ATM

• ATM Adaptation Layer (AAL) interface to upper
  layers
• end-system
• segmentation/reassembly
• ATM Layer cell switching
• Physical

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The Internet Protocol Stack
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Network Protocols

• Protocol Agreement between two parties as to how
  information is to be transmitted
• Example system calls are the protocol between
  the operating system and application
• Networking examples many levels
• Physical level mechanical and electrical network
  (e.g. how are 0 and 1 represented)
• Link level packet formats/error control (for
  instance, the CSMA/CD protocol)
• Network level network routing, addressing
• Transport Level reliable message delivery
• Protocols on todays Internet

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Building a messaging service

• Process to process communication
• Basic routing gets packets from machine?machine
• What we really want is routing from
  process?process
• Example ssh, email, ftp, web browsing
• Several IP protocols include notion of a port,
  which is a 16-bit identifiers used in addition to
  IP addresses
• A communication channel (connection) defined by 5
  items source address, source port, dest
  address, dest port, protocol
• UDP The User Datagram Protocol
• UDP layered on top of basic IP (IP Protocol 17)
• Unreliable, unordered, user-to-user communication

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Building a messaging service (cont)

• UDP The Unreliable Datagram Protocol
• Datagram an unreliable, unordered, packet sent
  from source user ? dest user (Call it UDP/IP)
• Important aspect low overhead!
• Often used for high-bandwidth video streams
• Many uses of UDP considered anti-social none
  of the well-behaved aspects of (say) TCP/IP
• But we need ordered messages
• Create ordered messages on top of unordered ones
• IP can reorder packets! P0,P1 might arrive as
  P1,P0
• How to fix this? Assign sequence numbers to
  packets
• 0,1,2,3,4..
• If packets arrive out of order, reorder before
  delivering to user application
• For instance, hold onto 3 until 2 arrives, etc.
• Sequence numbers are specific to particular
  connection

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TCP/IP packet, Ethernet frame

• Application sends message

• TCP breaks into 64KB segments, adds 20B header

• IP adds 20B header, sends to network

• If Ethernet, broken into 1500B frames with
  headers, trailers (24B)

• All Headers, trailers have length field,
  destination, ...