Thursday, November 22, 2012

Notes - Internetworking

The following are notes from Tanenbaum's Computer Networks 5th Edition.
  • when multiple networks are connected together called internetwork or internet
How Networks Differ
  • networks differ in many ways
  • differences can be somewhat papered over
    • gateway connecting networks can generate separate packets for each destination in lieu of better network support for multicasting
    • QoS is difficult to ensure between multiple networks
    • security difficult as well but encryption can be layered on top of one another
How Networks can be Connected
  • build devices that translate packets, or add a layer of indirection and build a common layer on top of different networks
  • TCP/IP protocol is foundation of modern internet
  • Example
    • 802.11, MPLS, and Ethernet networks want to be connected
    • between MPLS and 802.11 
      • virtual circuit must be set up because 802.11 is connectionless, MPLS is connection oriented
    • between vc and ethernet
      • packet may be too large to be carried, so packet must be divided into fragments
  • routers that can handle these different protocols is called a multiprotocl router
    • translates protocols or leave the connection for a higher protocol layer
    • higher level requires all networks implement that application example TCP
  • encapsulating an IPv6 packet through IPv4
  • used to connect isolated hosts and networks
  • overlay on the network
  • limitation of tunnels is an advantage with VPNs (Virtual Private Networks)
    • overlay used to provide a measure of security
Internetwork Routing
  • two level routing algorithm
  • within each network an intradomain, interior gateway protocol is used for routing
  • across networks use interdomain or exterior gateway protocol
    • often referred to as BGP(Border Gateway Protocol)
  • AS(Autonomous System)
    • network is operated independently
    • i.e. ISP  network
  • routing policy
    • routing based on political agreements
Packet Fragmentation
  • Limitations on packet size
    • Hardware(size of a frame)
    • Operating System(all buffers are 512 bytes)
    • Protocols (the number of bits in packet length field)
    • Compliance(to a standard or political agreement)
    • Desire to reduce error induced retransmissions
    • desire to prevent packets from occupying channel for a long time
  • Max payload is 1500 bytes for ethernet
  • 2272 bytes for 802.11
  • Path MTU(Path Maximum transmission unit)
    • know protocols from source to destination send packets small size
    • MTU discovery, send packet length, if not possible return with try smaller size that is acceptable packet, until it reaches destination
  • if MTU not possible break up into fragments
  • first fragmentation strategy
    • oversize packet arrives, router breaks it up, addressed to exit router where it is recombined
    • end of packet bit provided because reassembly may not be in order
  • second fragmentation strategy
    • split up and recombine only at destination not at each transmitted router

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