Internetworking Concept and Architectural Model

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Internetworking Concept and Architectural Model

• Chapter 3

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• Looked at low-level details of transmission
  across individual data networks
• Raise level of abstraction
• System that hides underlying details while
  providing universal communication services
• Result high level abstraction framework for all
  design decisions
• Later chapters
• Use the abstraction to build layers of internet
  communication software
• Show how applications use the comm system

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Application-Level Interconnection

• Two approaches to hiding details
• Have applications do the work
• Hide details in the operating system
• Application-level connections require application
  gateways
• Application running on each computer in the
  network
• Understands details of network connections

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• Seems natural at first, but
• New functionality means new application program
  for every computer
• New network hardware means modifying all existing
  programs
• With hundreds or thousands of networks, no way to
  write all possible application programs
• Intermediate application programs between the
  source and destination must operate correctly

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Network-Level Interconnection

• Alternative to application-level
• Provides a mechanism to deliver small data
  packets from source to destination
• No intermediate application programs

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• Several advantages
• Efficient (scheme maps onto underlying HW)
• Separates data communication from applications
• Flexible (allows general-purpose comm facilities)
• Modifying network does not affect applications
• Key is abstraction of internetworking
• Internetwork (internet) concept is powerful
• Detaches comm from network technologies
• Hides low-level details from user
• Drives software design decisions
• Helps handle physical addresses and routes

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• Motivation for internetworking comes from two
  observations
• No single network hardware technology can satisfy
  all needs
• Example LAN vs WAN
• Users desire universal interconnection
• Goal
• Build unified interconnection of networks to
  support universal communication
• Each network has own technologies
• Software between networks and application
  programs gives impression of single large network

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Internet Properties

• Universal service
• Hidden underlying architecture
• No forced network interconnection topology
• Adding network should be easy
• Intermediate networks not directly connected to
  source or destination computers
• Set of shared, universal machine identifiers
• Operations and applications independent of
  underlying network technologies

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Routers

• Use a special computer to connect networks
• Called internet gateways or internet routers
• IP router IP gateway used interchangeably
• Routers must know the topology beyond the
  networks to which it connects
• Key
• Routers use the destination network, not the
  destination computer, for forwarding packets

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• Will study in detail in later chapters
• For now
• Assume it is possible and practical for routers
  to do their job
• Also assume that only routers connect physical
  networks

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Users View

• Want user to see only one network to which all
  machines are connected
• Network level interconnections means
• Applications that communicate run without change
  on any computer
• Internet SW worries about physical connections
• TCP/IP internet protocols treat all networks
  equally

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Internetworking Summary

• Internet is more than collection of networks
  interconnected by computers
• Need conventions that allow communication
• Need universal identifiers and set of procedures
  for moving data to destination
• IP routers (or gateways) connect networks and
  forward packets

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Details As Yet Unanswered

• Form of internet addresses?
• How addresses relate to Ethernet, ATM, etc.?
• What does a packet look like?
• What if packets come too fast for a computer or
  router to handle?
• How do routers learn routes?
• How do multiple applications executing
  con-currently on one computer send/receive
  packets to multiple destinations without problems?

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