1 / 27

Operating Systems CMPSCI 377 Lecture 19: Network Structures

Operating Systems CMPSCI 377 Lecture 19: Network Structures. Emery Berger University of Massachusetts Amherst. Next Few Classes. Networking basics Distributed services e-mail, www, telnet Distributed operating systems Distributed file systems. Distributed Systems.

giovanna
Download Presentation

Operating Systems CMPSCI 377 Lecture 19: Network Structures

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript


  1. Operating SystemsCMPSCI 377Lecture 19: Network Structures Emery Berger University of Massachusetts Amherst

  2. Next Few Classes • Networking basics • Distributed services • e-mail, www, telnet • Distributed operating systems • Distributed file systems

  3. 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... P2 P1 P4 P3

  4. 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

  5. Advantages of Distributed Systems • Resource sharing • Computational speedup • Reliability • Communication

  6. 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

  7. Advantages, continued • Computational speedup • n processors = n times computational power • SETI@home • Problems must be decomposable into subproblems • Trivial = embarrassingly parallel • Coordination & communication required between cooperating processes • Synchronization • Exchange of results

  8. 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

  9. Advantages, continued • Communication • Users/processes on different systems can communicate • Mail, transaction processing systems like airlines & banks, www

  10. Distributed Systems Issues • Operating systems support for distribution • Communication & networks • Transparency • Security • Reliability • Performance & scalability • Programming models

  11. 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)

  12. Network Topologies • Connection of nodes impacts: • Maximum & average communication time • Fault tolerance • Expense • Two basic topologies: • Point-to-point • Bus

  13. Point-to-Point Network Topologies • Fully-connected • Each message takes one “hop” • Node failure – no effect on communication with others • Expensive – impractical for WANs

  14. 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

  15. 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

  16. 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

  17. Point-to-Point Network Topologies • One-directional ring • Given n nodes, max hops? • Inexpensive • Fault-tolerant?

  18. Point-to-Point Network Topologies • Bi-directional ring • Given n nodes, max hops? • Inexpensive • Fault-tolerant? One node? Two?

  19. Point-to-Point Network Topologies • Doubly-connected ring: nodes connected to neighbors & one more distant • Given n nodes, max hops? • Fault-tolerant? • More expensive

  20. 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

  21. Bus Network Topologies • Ring bus – single shared circular link • Same technology & tradeoffs as linear bus

  22. 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

  23. Communication Protocols • Protocol: agreed-upon rules for communication • Protocol stack: layers that comprise networking software • Each layer N provides service to layer N+1

  24. 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”

  25. TCP/IP Protocol Stack • Internet standard protocol stack • TCP: reliable protocol – packets received in order • UDP (user datagram protocol) – unreliable • No guarantee of delivery

  26. 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

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

More Related