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Internetworking
An Introduction


Prof. David Bernstein
James Madison University

Computer Science Department
bernstdh@jmu.edu

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Internetworking
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  • The Concept:
    • Interconnect multiple distinct networks
  • Things to Consider:
    • Important Terminology
    • What's Needed?
Terminology - General
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  • Internet:
    • A collection of interconnected communications networks
  • Intranet:
    • An internet used by a single organization
  • Subnetwork:
    • A constituent network (in an internet)
  • End System (ES):
    • A device on an internet that supports end-user applications/services
  • Intermediate System (IS):
    • A device that connects two (or more) networks
Terminology - Hardware
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  • Bridge:
    • An IS that connects two subnetworks that use the same protocols (and, hence does not need to modify PDUs but only needs to resolve addresses)
    • (Typically operates at the data link layer of OSI, unlike a hub that operates at the physical level)
  • Router:
    • An IS that connects two (usually dissimilar) networks
    • (Typically operates at the network layer of OSI)
What's Required for Internetworking?
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  • Encapsulation
  • Addressing
  • Routing
  • Fragmentation/Segmentation and Reassembly
  • Ordered Delivery
  • Flow Control
  • Error Control
Encapsulation
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  • Defined:
    • The addition of control information to data to create a protocol data unit (PDU)
  • Examples of Control Information:
    • Address
    • Error Detection/Correction Information
    • Protocol headers/trailers
Addressing
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  • An Observation:
    • To transmit data between devices across networks we must be able to identify the devices
  • Issues that Arise:
    • Scope
    • Multiplexing
    • Mode
Addressing (cont.) - Scope
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  • Some Observations:
    • Different layers of the architecture may use addresses
    • Addresses at different layers may conflict
  • Types:
    • Global - an address that is unique and applicable in any system
    • Local - enables an individual network to deliver data (e.g., MAC addresses on IEEE802/Ethernet; host addresses on ATM)
Addressing (cont.) - Multiplexing
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  • The Issue:
    • One might want to communicate with a particular application on a particular device
  • A Common Solution:
    • Ports (which can be thought of as sending a letter to a particular person at an address)
Addressing (cont.) - Modes
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  • The Issue:
    • An address can refer to a single device/port or multiple devices/ports
  • Modes:
    • Unicast
    • Multicast
    • Broadcast
Routing
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  • The Issue:
    • There might be multiple paths that can be used to transmit information form a sender to a receiver
  • An Observation:
    • The issues that arise here are similar to those that arise in all data communications that involve "complicated" networks
Fragmentation/Segmentation and Reassembly
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  • Defined:
    • Dividing data into smaller units and then re-assembling
  • Rationale:
    • Lower layers often place limits on the size of data
    • Improved error control (e.g., fewer errors, smaller re-transmissions)
    • More equitable/fair access (e.g., reduces hogging)
Ordered Delivery
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  • The Issue:
    • Since PDUs might take different paths, they might not arrive in the same order in which they were transmitted
    • This can be a particular problem in connection-oriented communications
  • An Obvious Approach:
    • Use a sequence number
Flow Control
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  • The Issue:
    • Since different networks have different performance characteristics, it may be necessary to limit data rates
  • Common Approaches:
    • Stop and wait (i.e., each PDU must be acknowledged before the next is transmitted)
    • Sliding window (i.e., multiple PDUs can be sent without an acknowledgment)
Error Control
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  • The Issue:
    • Since different networks have different error properties it may be necessary to add error control to internetwork communications
  • An Observation:
    • The issues that arise here are similar to those that arise in all data communications (e.g., detection, correction, re-transmission)
There's Always More to Learn
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