Distributed Systems: Advantages & Networking Structures

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