Networks and Distributed Systems

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Networks and Distributed Systems

• Sarah Diesburg
• Operating Systems
• COP 4610

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Technology Trends
Decade Technology per machine Sales volume Users per machine
50s 10M 100 1000s
60s Mainframe 1M 10K 100s
70s Mini computers 100K 1M 10s
80s PC 10K 100M 1
90s 00s Portables lt1K gt10B 1/10
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Distributed Systems

• Allow physically separate computers to work
  together
• Easier and cheaper to mass-produce simple
  computers
• Off-the-shelf components
• A company can incrementally
• increase the computing power

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Promises of Distributed Systems

• Higher availability
• If one machine goes down, use another
• Better reliability
• A user is able to store data in multiple
  locations
• More security
• Each simple component is easier to make secure

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Reality of Distributed Systems

• Worse availability
• A system may depend on many or all machines being
  up
• Worse reliability
• One can lose data if any machine crashes
• Worse security
• Security is as strong as the weakest component
• Coordination is difficult because machines can
  only use the network medium

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Network Technologies

• Definitions
• Network physical connection that allows two
  computers to communicate
• Packet a unit of transfer
• A sequence of bits carried over the network
• Protocol An agreement between two parties as to
  how information is to be transmitted

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Broadcast Networks

• A broadcast network uses a shared communication
  medium
• e.g. wireless, Ethernet, cellular phone network
• The sender needs to specify the destination in
  the packet header
• So the receiver knows which packet to receive
• If a machine were not the intended destination
• Discard the packet

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Arbitration

• Concerns the way to share a given resource
• In Aloha network (1970s)
• Packets were sent through radios on Hawaiian
  Islands

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Aloha Network

• Arbitration blind broadcast, with a checksum at
  the end of a packet
• Packets might become garbled in the case of
  simultaneous transmissions

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Aloha Network

• Arbitration blind broadcast, with a checksum at
  the end of a packet
• Packets might become garbled in the case of
  simultaneous transmissions

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Aloha Network

• Arbitration blind broadcast, with a checksum at
  the end of a packet
• Packets might become garbled in the case of
  simultaneous transmissions

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Blind Broadcast

• Receiver
• If a packet is garbled
• discard
• else
• sends an acknowledgement
• Sender
• If the acknowledgement does not arrive
• resend the packet

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Ethernet (introduced in the early 80s)

• By Xerox
• First practical local area network
• Uses wire (as opposed to radio)
• Broadcast network
• Key advance a new way for arbitration

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Ethernets Arbitration Techniques

• Carrier sensing Ethernet does not send unless
  the network is idle
• Collision detection sender checks if packet is
  trampled
• If so, abort, wait, and retry
• Adaptive randomized waiting a sender picks a
  bigger wait time (plus some random duration)
  after a collision

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The Internet

• A generalization of interconnected local area
  networks
• Uses machines to interconnect various networks
• Routers, gateways, bridges, repeaters
• Act like switches
• Packets are copied as they
• transmitted across different
• networks

LAN 2
LAN 1
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Routing

• Concerns how a packet can reach its destination
• Typically, a packet has to go through multiple
  hops before getting to a destination
• Each hop is a router, which directs a packet to
  the next hop
• Routing is achieved through routing tables

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Routing Table Updates

• Each routing entry contains a cost
• ltdestination, next hop, hopsgt
• Neighbors periodically exchange routing table
  entries
• If the neighbor has a cheaper route, use that one
  instead

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Point-to-Point Networks

• Instead of sharing a common network medium, all
  nodes in the network can be connected directly to
  a router/switch

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Point-to-Point Networks

• Higher link performance (no collisions)
• Greater aggregate bandwidth than a single link

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Point-to-Point Networks

• Network capacity can be upgraded incrementally
• Lower latency (no arbitration)

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Issues in Point-to-Point Networks

• Congestion occurs when everyone sends to the same
  output link on a switch

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Solutions

• 1. No flow control Packets get dropped when
  the receiving buffer is full
• Downloading large files across the Internet can
  make many people unhappy

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Solutions

• 2. Flow control between switches a switch does
  not send until the buffer space is available in
  the next switch
• Problem cross traffic

Crossbar
buffers
buffers
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Solutions

• 3. Per-flow flow control a separate set of
  buffers is allocated for each end-to-end stream
• Problem fairness

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