OSes: 14. Dist. System Structures 1 Operating Systems v Objectives –introduce the basic notions behind a networked/distributed system Certificate Program

OSes: 14. Dist. System Structures 1

Operating SystemsOperating Systems

ObjectivesObjectives– introduce the basic notions behind a introduce the basic notions behind a

networked/distributed system networked/distributed system

Certificate Program in Software DevelopmentCSE-TC and CSIM, AITSeptember -- November, 2003

14. Distributed System Structures

(S&G 6th ed., Ch. 15)


OverviewOverview

1. Background1. Background

2. Network Topologies2. Network Topologies

3. Network Types3. Network Types

4. Communication Network Issues4. Communication Network Issues

5. Partitioning the Network5. Partitioning the Network

6. Robustness6. Robustness

7. Design Strategies7. Design Strategies

8. Networking Example8. Networking Example


1. Background: A Distributed System1. Background: A Distributed System

Figure 15.1, p.540


1.1. Motivation1.1. Motivation

Resource sharingResource sharing– sharing and printing files at remote sitessharing and printing files at remote sites– processing information in a distributed databaseprocessing information in a distributed database– using remote specialized hardware devicesusing remote specialized hardware devices

Computation speedupComputation speedup– load sharingload sharing

continued


ReliabilityReliability– detect and recover from site failure, function detect and recover from site failure, function

transfer, reintegrate failed sitetransfer, reintegrate failed site

CommunicationCommunication– message passing (simpest form)message passing (simpest form)– higher-level capabilities: FTP, rlogin, RPChigher-level capabilities: FTP, rlogin, RPC


1.2. Network Operating Systems1.2. Network Operating Systems

Users are aware of multiplicity of machines. Users are aware of multiplicity of machines. Access to resources of various machines is Access to resources of various machines is done explicitly by:done explicitly by:– remote logging into the appropriate remote remote logging into the appropriate remote

machinemachine

– transferring data from remote machines to local transferring data from remote machines to local machines, via the File Transfer Protocol (FTP) machines, via the File Transfer Protocol (FTP) mechanismmechanism


1.3. Distributed Operating Systems1.3. Distributed Operating Systems

Users unaware of multiplicity of machinesUsers unaware of multiplicity of machines– access to remote resources similar to access to access to remote resources similar to access to

local resourceslocal resources

Data migrationData migration– transfer data by transferring entire file, or transfer data by transferring entire file, or

transferring only those portions of the file transferring only those portions of the file necessary for the immediate tasknecessary for the immediate task

continued


Computation migrationComputation migration– transfer the computation, rather than the data, transfer the computation, rather than the data,

across the systemacross the system e.g. Remote Procedure Calls (RPCs)e.g. Remote Procedure Calls (RPCs)

continued


Process migration – execute an entire Process migration – execute an entire process, or parts of it, at different sitesprocess, or parts of it, at different sites– load balancingload balancing

distribute processes across network to even the distribute processes across network to even the workloadworkload

– computation speedupcomputation speedup subprocesses can run concurrently on different sitessubprocesses can run concurrently on different sites

– explicit vs. implicitexplicit vs. implicit

continued


– hardware preferencehardware preference process execution may require specialized processorprocess execution may require specialized processor

– software preferencesoftware preference required software may be available at only a particular required software may be available at only a particular

sitesite

– data accessdata access run process remotely, rather than transfer all data locallyrun process remotely, rather than transfer all data locally


Some CriteriaSome Criteria– Basic costBasic cost. How expensive is it to link the various . How expensive is it to link the various

sites in the system?sites in the system?

– Communication costCommunication cost. How long does it take to send a . How long does it take to send a message from site message from site AA to site to site BB??

– ReliabilityReliability. If a link or a site in the system fails, can . If a link or a site in the system fails, can the remaining sites still communicate with each other?the remaining sites still communicate with each other?

1.4. Connecting Sites1.4. Connecting Sitesused in section 2


The various topologies are depicted as The various topologies are depicted as graphs whose nodes correspond to sites. graphs whose nodes correspond to sites.

An edge from node An edge from node AA to node to node BB corresponds to a direct connection between corresponds to a direct connection between the two sites.the two sites.

2. Network Topologies2. Network Topologies


2.1.2.1.Network Network TopologyTopologyDiagramsDiagrams

Figure 15.2, p.547


3. Network Types3. Network Types

Local-Area NetworkLocal-Area Network (LAN) – designed to (LAN) – designed to cover small geographical area.cover small geographical area.– multiaccess bus, ring, or star networkmultiaccess bus, ring, or star network– ~10 megabits/second, or higher~10 megabits/second, or higher– broadcast is fast and cheapbroadcast is fast and cheap– nodes: nodes:

usually workstations and/or personal computers usually workstations and/or personal computers a few (usually one or two) mainframesa few (usually one or two) mainframes

continued


A typical LAN:A typical LAN:Figure 15.3, p.550


Wide-Area NetworkWide-Area Network (WAN) – links (WAN) – links geographically separated sites. geographically separated sites. – point-to-point connections over long-haul lines point-to-point connections over long-haul lines

(often leased from a phone company)(often leased from a phone company)– ~100 kilobits/second.~100 kilobits/second.– broadcast usually requires multiple messagesbroadcast usually requires multiple messages– nodes: nodes:

usually a high percentage of mainframesusually a high percentage of mainframes

continued


A typical A typical WAN:WAN:

Figure 15.5p.551


4. Communication Network Issues4. Communication Network Issues

Naming and name resolutionNaming and name resolution– how do two processes locate each other to how do two processes locate each other to

communicate?communicate?

Routing strategiesRouting strategies– how are messages sent through the how are messages sent through the

network?network?

continued

More detailsin the next few slides


Connection strategiesConnection strategies– how do two processes send a sequence of how do two processes send a sequence of

messages?messages?

ContentionContention– the network is a shared resource, so how do we the network is a shared resource, so how do we

resolve conflicting demands for its use?resolve conflicting demands for its use?


4.1. Naming and Name Resolution4.1. Naming and Name Resolution

Name systems in the networkName systems in the network– fine for LANsfine for LANs

Address messages with the process IDsAddress messages with the process IDs– fine for process to process comms.fine for process to process comms.

Identify processes on remote systems Identify processes on remote systems by pairs:by pairs:

<host-name, PIDs><host-name, PIDs>

continued


Domain name serviceDomain name service (DNS) (DNS)– specifies the naming structure of the hosts, as specifies the naming structure of the hosts, as

well as name to address resolution (Internet)well as name to address resolution (Internet) e.g. from a hierarchical name "ratree.psu.ac.th" e.g. from a hierarchical name "ratree.psu.ac.th"

to a dotted decimal 127.50.2.7to a dotted decimal 127.50.2.7


4.2. Routing Strategies4.2. Routing Strategies

Fixed routingFixed routing. A path from . A path from AA to to BB is is specified in advancespecified in advance; path changes only if a ; path changes only if a hardware failure disables it. hardware failure disables it. – since the shortest path is usually chosen, since the shortest path is usually chosen,

communication costs are minimizedcommunication costs are minimized– fixed routing cannot adapt to load changesfixed routing cannot adapt to load changes– ensures that messages will be delivered in the ensures that messages will be delivered in the

order in which they were sentorder in which they were sent

continued


Virtual circuitVirtual circuit. A path from . A path from AA to to BB is fixed is fixed for the for the duration of one sessionduration of one session. Different . Different sessions involving messages from sessions involving messages from AA to to BB may have different paths. may have different paths. – partial remedy to adapting to load changespartial remedy to adapting to load changes– ensures that messages will be delivered in the ensures that messages will be delivered in the

order in which they were sent order in which they were sent

continued


Dynamic routingDynamic routing. The path used to send a . The path used to send a message form site message form site AA to site to site BB is is chosen only when chosen only when a message is senta message is sent. . – usually a site sends a message to another site on the usually a site sends a message to another site on the

link least used at that particular timelink least used at that particular time– adapts to load changes by avoiding routing messages adapts to load changes by avoiding routing messages

on heavily used pathon heavily used path– messages may arrive out of order. This problem can messages may arrive out of order. This problem can

be remedied by appending a sequence number to each be remedied by appending a sequence number to each message. message.


4.3. Connection Strategies4.3. Connection Strategies

Circuit switchingCircuit switching– a permanent physical link is established for the a permanent physical link is established for the

duration of the communicationduration of the communication e.g. the telephone system; TCPe.g. the telephone system; TCP

Message switchingMessage switching..– a temporary link is established for the duration of a temporary link is established for the duration of

one message transferone message transfer e.g. the post-office mailing system; UDPe.g. the post-office mailing system; UDP

continued


Packet switchingPacket switching– messages of variable length are divided into messages of variable length are divided into

fixed-length packets which are sent to the fixed-length packets which are sent to the destinationdestination

– each packet may take a different path through each packet may take a different path through the networkthe network

– the packets must be reassembled into messages the packets must be reassembled into messages as they arriveas they arrive

continued

messagespackets


Circuit switching requires setup time, but Circuit switching requires setup time, but incurs less overhead for shipping each incurs less overhead for shipping each message, and may waste network message, and may waste network bandwidthbandwidth

Message and packet switching require less Message and packet switching require less setup time, but incur more overhead per setup time, but incur more overhead per message.message.


4.4. Contention4.4. Contention

CSMA/CDCSMA/CD. Carrier sense with multiple access . Carrier sense with multiple access (CSMA); collision detection (CD)(CSMA); collision detection (CD)– a site determines whether another message is a site determines whether another message is

currently being transmitted over that link. If two or currently being transmitted over that link. If two or more sites begin transmitting at exactly the same more sites begin transmitting at exactly the same time, then they will register a CD and will stop time, then they will register a CD and will stop transmittingtransmitting

– When the system is very busy, many collisions may When the system is very busy, many collisions may occur, and thus performance may be degraded.occur, and thus performance may be degraded.

continued


CSMA/CD is used successfully in the CSMA/CD is used successfully in the Ethernet system, the most common network Ethernet system, the most common network system.system.

continued


Token passingToken passing– a unique message type, known as a token, a unique message type, known as a token,

continuously circulates in the systemcontinuously circulates in the system usually a ring structureusually a ring structure

– a site that wants to transmit information must a site that wants to transmit information must wait until the token arrives. When the site wait until the token arrives. When the site completes its round of message passing, it completes its round of message passing, it retransmits the tokenretransmits the token

– used by the IBM and Apollo systems used by the IBM and Apollo systems

continued


Message slotsMessage slots– a number of fixed-length message slots continuously a number of fixed-length message slots continuously

circulate in the systemcirculate in the system usually a ring structureusually a ring structure

– since a slot can contain only fixed-sized messages, a since a slot can contain only fixed-sized messages, a single logical message may have to be broken down single logical message may have to be broken down into a number of smaller packets, each of which is into a number of smaller packets, each of which is sent in a separate slotsent in a separate slot

– adopted in the experimental Cambridge Digital adopted in the experimental Cambridge Digital Communication RingCommunication Ring

X X

slots


5. Partitioning the Network5. Partitioning the Network

1. Physical layer1. Physical layer– handles the mechanical and electrical details handles the mechanical and electrical details

of the physical transmission of a bit streamof the physical transmission of a bit stream

2. Data-link layer2. Data-link layer– handles the handles the framesframes, or fixed-length parts of , or fixed-length parts of

packets, including any error detection and packets, including any error detection and recovery that occurred in the physical layerrecovery that occurred in the physical layer

continued

7 layers


3. Network layer3. Network layer– provides connections and routes packets in the provides connections and routes packets in the

communication networkcommunication network handling the address of outgoing packetshandling the address of outgoing packets decoding the address of incoming packetsdecoding the address of incoming packets maintaining routing info. for proper response to maintaining routing info. for proper response to

changing load levelschanging load levels

continued


4. Transport layer4. Transport layer– responsible for low-level network access and for responsible for low-level network access and for messagemessage

transfer between clients (hosts)transfer between clients (hosts) partitioning messages into packetspartitioning messages into packets maintaining packet order, controlling flowmaintaining packet order, controlling flow generating physical addresses.generating physical addresses.

5. Session layer5. Session layer– implements sessions, or implements sessions, or process-to-processprocess-to-process

communications protocolscommunications protocols

continued


6. Presentation layer6. Presentation layer– resolves the differences in resolves the differences in formatsformats among the various among the various

sites in the networksites in the network character conversionscharacter conversions half duplex/full duplex (echoing).half duplex/full duplex (echoing).

7. Application layer7. Application layer– interacts directly with the users’ deals with file transfer, interacts directly with the users’ deals with file transfer,

remote-login protocols and e-mailremote-login protocols and e-mail– schemas for distributed databases.schemas for distributed databases.


5.1. The ISO Network Model5.1. The ISO Network ModelFigure 15.5, p.559


5.2.5.2.The ISO The ISO Protocol Protocol LayerLayer

Figure 15.6p.560

Summarises the slides on the seven layers


5.3.5.3.The ISO The ISO Network Network MessageMessage

Figure 15.7p.561

header


5.4. The TCP/IP Protocol Layers5.4. The TCP/IP Protocol Layers

Figure 15.8, p.562


6. Robustness6. Robustness

Failure detectionFailure detection– many types of failure: host, link, routing, loss many types of failure: host, link, routing, loss

of message, excessive delays, etc.of message, excessive delays, etc.

ReconfigurationReconfiguration– main aim: to "keep going" in the face of partial main aim: to "keep going" in the face of partial

failurefailure


6.1. Failure Detection6.1. Failure Detection

Detecting hardware failure is difficult.Detecting hardware failure is difficult. To detect a link failure, a handshaking To detect a link failure, a handshaking

protocol can be used.protocol can be used.

continued

AA BB"I-am-up""I-am-up"

"ok""ok"

AA BB"Are you up?""Are you up?"

"yes""yes"


Assume Site A and Site B have established a Assume Site A and Site B have established a link. At fixed intervals, each site will exchange link. At fixed intervals, each site will exchange an an I-am-upI-am-up message indicating that they are up message indicating that they are up and running.and running.

If Site A does not receive a message within the If Site A does not receive a message within the fixed interval, it assumes eitherfixed interval, it assumes either– a) the other site is not up or a) the other site is not up or

– b) the message was lostb) the message was lost

continued


Site A can now send an Site A can now send an Are-you-up?Are-you-up? message to Site message to Site B.B.

If Site A does not receive a reply after a fixed If Site A does not receive a reply after a fixed interval, it can repeat the message or try an alternate interval, it can repeat the message or try an alternate route to Site B.route to Site B.

If Site A does not ultimately receive a reply from Site If Site A does not ultimately receive a reply from Site B, it concludes some type of failure has occurred.B, it concludes some type of failure has occurred.

continued


Types of failures:Types of failures:– site B is downsite B is down– the direct link between A and B is downthe direct link between A and B is down– the alternate link from A to B is downthe alternate link from A to B is down– the message has been lostthe message has been lost

However, Site A cannot determine exactly However, Site A cannot determine exactly whywhy the failure has occurred. the failure has occurred.


6.2. Reconfiguration6.2. Reconfiguration

When Site A determines a failure has occurred, When Site A determines a failure has occurred, it must reconfigure the system:it must reconfigure the system:

– 1. If the link from A to B has failed, this must be 1. If the link from A to B has failed, this must be broadcast to every site in the systembroadcast to every site in the system

– 2. If a site has failed, every other site must also be 2. If a site has failed, every other site must also be notified indicating that the services offered by the notified indicating that the services offered by the failed site are no longer availablefailed site are no longer available

continued


When the link or the site becomes available When the link or the site becomes available again, this information must again be again, this information must again be broadcast to all other sites.broadcast to all other sites.


7. Design Issues7. Design Issues

TransparencyTransparency– the distributed system should appear as a the distributed system should appear as a

conventional, centralized system to the userconventional, centralized system to the user

Fault toleranceFault tolerance– the distributed system should continue to the distributed system should continue to

function in the face of failurefunction in the face of failure

continued


ScalabilityScalability– as demands increase, the system should easily as demands increase, the system should easily

accept the addition of new resources to accept the addition of new resources to accommodate the increased demandaccommodate the increased demand

ClustersClusters– a collection of semi-autonomous machines that a collection of semi-autonomous machines that

acts as a single systemacts as a single system


8. Networking Example8. Networking Example

The transmission of a network packet The transmission of a network packet between hosts on an Ethernet network.between hosts on an Ethernet network.

Every host has a unique IP address and a Every host has a unique IP address and a corresponding Ethernet (MAC) address.corresponding Ethernet (MAC) address.

Communication requires both addresses.Communication requires both addresses.

continued


Domain Name Service (DNS) can be used to Domain Name Service (DNS) can be used to acquire IP addresses.acquire IP addresses.

Address Resolution Protocol (ARP) is used to map Address Resolution Protocol (ARP) is used to map MAC addresses to IP addresses.MAC addresses to IP addresses.

If the hosts are on the same network, ARP can be If the hosts are on the same network, ARP can be used. If the hosts are on different networks, the used. If the hosts are on different networks, the sending host will send the packet to a sending host will send the packet to a routerrouter which which routes the packet to the destination network.routes the packet to the destination network.


8.1. An Ethernet Packet8.1. An Ethernet PacketFigure 15.9

p.568

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OSes: 14. Dist. System Structures 1 Operating Systems v Objectives –introduce the basic notions behind a networked/distributed system Certificate Program