1. Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition, Chapter 16: Distributed System Structures

2. 16.2 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Outline n Motivation n Types of Network-Based Operating Systems n Network Structure n Network Topology n Communication Structure n Communication Protocols n Robustness n Design Issues n An Example: Networking

3. 16.3 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Chapter Objectives n To provide a high-level overview of distributed systems and the networks that interconnect them n To discuss the general structure of distributed OS

4. 16.4 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Motivation n Distributed system: collection of loosely coupled processors interconnected by a communications network l Processors variously called nodes, computers, machines, hosts  Site : location of the processor n Reasons for distributed systems l Resource sharing  sharing and printing files at remote sites  processing information in a distributed database  using remote specialized hardware devices l Computation speedup – load sharing l Reliability – detect and recover from site failure, function transfer, reintegrate failed site l Communication – message passing

5. 16.5 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition A Distributed System

6. 16.6 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Types of Distributed Operating Systems n Network OS n Distributed OS

7. 16.7 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Network-Operating Systems n Users are aware of multiplicity of machines n Access to resources of various machines is done explicitly: l Remote logging into the appropriate remote machine (telnet, ssh) l Remote Desktop (Microsoft Windows) l Transferring data from remote machines to local machines, via the File Transfer Protocol (FTP) mechanism

8. 16.8 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Distributed-Operating Systems n Users not aware of multiplicity of machines l Access to remote resources similar to access to local resources n Data Migration – transfer data by transferring entire file, or transferring only those portions of the file necessary for the immediate task n Computation Migration – transfer the computation, rather than the data, across the system

9. 16.9 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Distributed-Operating Systems (Cont.) n Process Migration – execute an entire process, or parts of it, at different sites l Load balancing – distribute processes across network to even the workload l Computation speedup – subprocesses can run concurrently on different sites l Hardware preference – process execution may require specialized processor l Software preference – required software may be available at only a particular site l Data access – run process remotely, rather than transfer all data locally

10. 16.10 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Network Structure n Local-Area Network (LAN) – designed to cover small geographical area l Multiaccess bus, ring, or star network l Speed  10 – 100 megabits/second l Broadcast is fast and cheap l Nodes:  usually workstations and/or PCs  a few (usually one or two) mainframes

11. 16.11 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Depiction of Typical LAN

12. 16.12 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Network Types (Cont.) n Wide-Area Network (WAN) – links geographically separated sites l Point-to-point connections over long-haul lines (often leased from a phone company) l Speed  1.544 – 45 megabits/second l Broadcast usually requires multiple messages l Nodes:  usually a high percentage of mainframes

13. 16.13 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Communication Processors in a Wide-Area Network

14. 16.14 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Network Topology n Sites in the system can be physically connected in a variety of ways with respect to the following criteria: l Installation cost - How expensive is it to link the various sites in the system? l Communication cost - How long does it take to send a message from site A to site B ? l Reliability - If a link or a site in the system fails, can the remaining sites still communicate with each other? n The various topologies are depicted as graphs whose nodes correspond to sites l An edge from node A to node B : a direct connection between the two sites n The following items depict various network topologies

15. 16.15 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Network Topology

16. 16.16 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Communication Structure n Naming and name resolution - How do two processes locate each other to communicate? n Routing strategies - How are messages sent through the network? n Connection strategies - How do two processes send a sequence of messages? n Contention - The network is a shared resource, so how do we resolve conflicting demands for its use? The design of a communication network must address four basic issues:

17. 16.17 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Naming and Name Resolution n Name systems in the network n Address messages with the process-id n Identify processes on remote systems by pair n Domain name service (DNS) – specifies the naming structure of the hosts, as well as name to address resolution (Internet)

18. 16.18 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Routing Strategies n Fixed routing - A path from A to B is specified in advance; path changes only if a hardware failure disables it l Since the shortest path is usually chosen, communication costs are minimized l Fixed routing cannot adapt to load changes l Ensures that messages will be delivered in the order in which they were sent n Virtual circuit - A path from A to B is fixed for the duration of one session l Different sessions may have different paths l Partial remedy to adapting to load changes l Ensures that messages will be delivered in the order in which they were sent

19. 16.19 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Routing Strategies (Cont.) n Dynamic routing - A path form A to B is chosen only when a message is sent l Usually a site sends a message to another site on the link least used at that particular time l Adapts to load changes by avoiding routing messages on heavily used path l Messages may arrive out of order  can be remedied by appending a sequence number to each message

20. 16.20 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Connection Strategies n Circuit switching - permanent physical link established during communication (i.e., telephone system) n Message switching - temporary link established during one message transfer (i.e., post-office mailing system) n Packet switching - Messages of variable length are divided into fixed-length packets l Each packet may take a different path through the network l The packets must be reassembled into messages as they arrive n Comparison l Circuit switching requires setup time, but incurs less overhead for shipping each message, and may waste network bandwidth l Message and packet switching require less setup time, but incur more overhead per message

21. 16.21 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Contention n Techniques to avoid repeated collisions when several sites want to transmit information over a link simultaneously include: n CSMA/CD - Carrier sense with multiple access (CSMA); collision detection (CD) l A site determines whether another message is currently being transmitted over that link l If two or more sites begin transmitting at exactly the same time, then they will register a CD and stop transmitting l When the system is very busy, many collisions may occur, and thus performance may be degraded n CSMA/CD is used successfully in the Ethernet system, the most common network system

22. 16.22 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Contention (Cont.) n Token passing - A unique message type (a token) continuously circulates in the system (usually a ring structure) l A site that wants to transmit information must wait until the token arrives l When the site completes its round of message passing, it retransmits the token l used by some IBM and HP/Apollo systems n Message slots - A number of fixed-length message slots continuously circulate in the system (usually a ring structure) l Only fixed-sized messages: a single logical message may have to be broken down into a number of smaller packets l adopted in the experimental Cambridge Digital Communication Ring

23. 16.23 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Communication Protocol n Physical layer – handles the mechanical and electrical details of the physical transmission of a bit stream n Data-link layer – handles the frames, or fixed-length parts of packets l including any error detection and recovery that occurred in the physical layer n Network layer – provides connections and routes packets in the communication network l including handling the address of outgoing packets, decoding the address of incoming packets, and maintaining routing information for proper response to changing load levels

24. 16.24 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Communication Protocol (Cont.) n Transport layer – responsible for low-level network access and for message transfer between clients l including partitioning messages into packets, maintaining packet order, controlling flow, and generating physical addresses n Session layer – implements sessions, or process-to- process communications protocols n Presentation layer – resolves the differences in formats among the various sites in the network l including character conversions, and half duplex/full duplex (echoing) n Application layer – interacts directly with the users’ deals with file transfer, remote-login protocols and electronic mail, as well as schemas for distributed databases

25. 16.25 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Communication via ISO Network Model

26. 16.26 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition The ISO Protocol Layer

27. 16.27 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition The ISO Network Message

28. 16.28 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition The TCP/IP Protocol Layers

29. 16.29 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Robustness n Failure detection n Reconfiguration

30. 16.30 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Failure Detection n Detecting hardware failure is difficult n To detect link failure, a handshaking protocol can be used l Assume Site A and Site B have established a link l At fixed intervals, each site will exchange an I-am-up message indicating that they are up and running  If Site A does not receive a message within the fixed interval, it assumes either (a) the other site is not up or (b) the message was lost l Site A can now send an Are-you-up? message to Site B  If Site A does not receive a reply, it can repeat the message or try an alternate route to Site B

31. 16.31 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Failure Detection (cont) n If Site A does not ultimately receive a reply from Site B, it concludes some type of failure has occurred n Types of failures: - Site B is down - The direct link between A and B is down - The alternate link from A to B is down - The message has been lost n However, Site A cannot determine exactly why the failure has occurred

32. 16.32 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Reconfiguration n When Site A determines a failure has occurred, it must reconfigure the system: 1. If the link from A to B has failed, this must be broadcast to every site in the system 2. If a site has failed, every other site must also be notified indicating that the services offered by the failed site are no longer available n When the link or the site becomes available again, this information must again be broadcast to all other sites

33. 16.33 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Design Issues n Transparency – the distributed system should appear as a conventional, centralized system to the user n Fault tolerance – the distributed system should continue to function in the face of failure n Scalability – as demands increase, the system should easily accept the addition of new resources to accommodate the increased demand n Clusters – a collection of semi-autonomous machines that acts as a single system

34. 16.34 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition Example: Networking n The transmission of a network packet between hosts on an Ethernet network n Every host has a unique IP address and a corresponding Ethernet (MAC) address l Communication requires both addresses n Domain Name Service (DNS): to acquire IP addresses n Address Resolution Protocol (ARP): to map IP addresses to MAC addresses l If the hosts are on the same network, ARP can be used l If the hosts are on different networks, the sending host will send the packet to a router which routes the packet to the destination network

35. 16.35 Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition An Ethernet Packet

36. Silberschatz, Galvin and Gagne ©2009 Operating System Concepts – 8 th Edition, End of Chapter 16