Thursday, November 15, 2012

Notes - Network Layer Design Issues

Store and Forward Packet Switching

  • Context of Network Layer Protocols
    • major components of network are the ISP's equipment
    • Host directly connected to ISP's routers
  • Store and Forward Packet Switching
    • Second host 
      • connected to private router
      • private router has line to ISP's equipment
      • for purposes of understanding, can still treat private routers as part of the isp network because they run on the same algorithms
    • Host with packet to send transmits to nearest router, either to own LAN, or over point to point link to ISP
    • packet is stored until fully arrived and checksum verified
    • then forward to next router until reach host
Services Provided to the Transport Layer
  • Network layer provides services to the transport layer at the network/transport layer interface
    • services must be independent of router technology
    • transport layer should be shielded from number type and topology of routers
    • network addresses should be made available and use a uniform numbering plan
  • One camp
    • routers job is moving packets and nothing else
    • Internet Community
    • network is inherently unreliable so hosts should accept this fact
    • primitives SEND PACKET and RECEIVE PACKET
    • end to end argument
      • no packet ordering and flow control should be done because hosts are going to be doing it anyway
  • Other Camp
    • represented by telephone companies
    • quality of service is dominant factor
  • controversy, X.25 and Frame Relay are connection oriented
  • since ARPANET, connectionless grown in popularity
  • IP Protocol shows prominence of connectionless technologies
  • however internet is coming back to connection oriented services
  • MPLS(MultiProtocol Label Switching)
Implementation of Connectionless Service
  • Two different organizations are possible depending on type of service
  • packets are 
    • injected into network individually 
    • routed indpendently
    • no advanced setup
    • called datagrams, datagram network
  • connection oriented service, path from source router must be established all the way to destination router, VC(virtual circuit) needs to be set up
    • called virtual circuit network
  • In connectionless
    • every router has internal routing table
    • use outgoing lines to send packets
    • stored briefly at link and checksum verified
    • then forward to next router
    • packets don't always follow the same path, since routing algorithm can change during transmission
      • causes timing of packets to be different at arrival
Implementation of Connection-Oriented Service
  • require virtual circuit network
  • avoid choosing new routes
    • route is part of connection setup and is stored in the router
    • when connection released vc also terminated
  • Avoiding conflicts is key, label switching
    • MPLS(MultiProtocol Label Switching)
    • hidden from customers
    • MPLS header wrapped around IP packets, used for long term connections
Comparison of Virtual Circuit and Datagram Networks

  • trade off is setup time, vc requires significantly more resources
  • router is very fast, only forwards info with datagram network, what is currently in use
  • table space in router memory an issue
  • vc ensures quality of resources and avoids congestion
  • vc has vulnerability to crash if any router in path goes down, but datagram just resends packets

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