Emery Berger

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Operating SystemsCMPSCI 377Lecture 19 Network
Structures

• Emery Berger
• University of Massachusetts Amherst

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Next Few Classes

• Networking basics
• Distributed services
• e-mail, www, telnet
• Distributed operating systems
• Distributed file systems

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Distributed Systems

• distributed system set of physically separate
  processors connected by one or more communication
  links
• no shared clock or memory
• Many systems today distributed in some way
• e-mail, file servers, network printers, remote
  backup, web...

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P1
P4
P3
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Parallel vs. Distributed Systems

• Tightly-coupled systems parallel processing
• Processors share clock, memory, run one OS
• Frequent communication
• Loosely-coupled systems distributed computing
• Each processor has own memory, runs independent
  OS
• Infrequent communication

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Advantages of Distributed Systems

• Resource sharing
• Computational speedup
• Reliability
• Communication

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Advantages of Distributed Systems

• Resource sharing
• Resources need not be replicated
• Shared files
• Expensive (scarce) resources can be shared
• Color laser printers
• Processors present same environment to user
• Keeping files on file server

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Advantages, continued

• Computational speedup
• n processors n times computational power
• SETI_at_home
• Problems must be decomposable into subproblems
• Trivial embarrassingly parallel
• Coordination communication required between
  cooperating processes
• Synchronization
• Exchange of results

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Advantages, continued

• Reliability
• Replication of resources provides fault tolerance
• One node crashes, user works on another one
• Performance degradation but system available
• Must avoid single point of failure
• Single, centralized component of system
• Example central file servers

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Advantages, continued

• Communication
• Users/processes on different systems can
  communicate
• Mail, transaction processing systems like
  airlines banks, www

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Distributed Systems Issues

• Operating systems support for distribution
• Communication networks
• Transparency
• Security
• Reliability
• Performance scalability
• Programming models

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Networks

• Goal provide efficient, correct, robust message
  passing between two separate nodes
• Local area network (LAN) connects nodes in
  single building, fast reliable (Ethernet)
• Media twisted-pair, coax, fiber
• Bandwidth 10-100MB/s
• Wide area network (WAN) connects nodes across
  large geographic area (Internet)
• Media fiber, microwave links, satellite channels
• Bandwidth 1.544MB/s (T1), 45 MB/s (T3)

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Network Topologies

• Connection of nodes impacts
• Maximum average communication time
• Fault tolerance
• Expense
• Two basic topologies
• Point-to-point
• Bus

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Point-to-Point Network Topologies

• Fully-connected
• Each message takes one hop
• Node failure no effect on communication with
  others
• Expensive impractical for WANs

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Point-to-Point Network Topologies

• Partially connected
• Links between some, but not all nodes
• Less expensive, less tolerant to failures
• Single node failure can partition network
• Sending message takes several hops
• Needs routing algorithms

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Point-to-Point Network Topologies

• Tree structure network hierarchy
• Messages fast between direct descendants
• Max message cost?
• Not failure tolerant
• Any interior node fails network partitioned

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Point-to-Point Network Topologies

• Star network all nodes connect to central node
• Each message takes how many hops?
• Not failure tolerant
• Inexpensive sometimes used for LANs

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Point-to-Point Network Topologies

• One-directional ring
• Given n nodes, max hops?
• Inexpensive
• Fault-tolerant?

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Point-to-Point Network Topologies

• Bi-directional ring
• Given n nodes, max hops?
• Inexpensive
• Fault-tolerant? One node? Two?

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Point-to-Point Network Topologies

• Doubly-connected ring nodes connected to
  neighbors one more distant
• Given n nodes, max hops?
• Fault-tolerant?
• More expensive

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Bus Network Topologies

• Bus nodes connect to common network
• Linear bus single shared link
• Nodes connect directly to each other via bus
• Inexpensive (linear in of nodes)
• Tolerant of node failures
• Ethernet LAN

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Bus Network Topologies

• Ring bus single shared circular link
• Same technology tradeoffs as linear bus

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Principles ofNetwork Communication

• Data broken into packets
• Basic unit of transfer
• Packets sent through network
• Computers routers at switching points control
  packet flow
• Road analogy
• Packets cars
• Network roads
• Computer traffic lights (intersection)
• Too many packets on shared link/node traffic jam

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Communication Protocols

• Protocol agreed-upon rules for communication
• Protocol stack layers that comprise networking
  software
• Each layer N provides service to layer N1

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Traditional Layers

• Application layer applications that use the net
• Presentation layer data format
  conversion(big/little endian)
• Session layer implements communication
  strategy(e.g., RPC)
• Transport layer reliable end-to-end
  communication
• Network layer routing congestion control
• Data link control layer reliable point-to-point
  communication over unreliable channel
• Physical layer electrical/optical signaling
  across wire

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TCP/IP Protocol Stack

• Internet standard protocol stack
• TCP reliable protocol packets received in
  order
• UDP (user datagram protocol) unreliable
• No guarantee of delivery

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Packet Format

• Contains all info needed to recreate original
  message
• Packets may arrive out of order need sequence
  number
• Data segment contains headers for higher protocol
  layers application data

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Summary

• Virtually all computer systems contain
  distributed components
• Networks connect them
• Key tradeoffs
• Speed
• Reliability
• Expense