networks 1 © 2000 franz kurfess course overview principles of operating systems introduction ...

Networks Networks 11 © 2000 Franz Kurfess

Course OverviewPrinciples of Operating Systems

Course OverviewPrinciples of Operating Systems

Introduction Computer System

Structures Operating System

Structures Processes Process Synchronization Deadlocks CPU Scheduling

Memory Management Virtual Memory File Management Security Networks Distributed Systems Case Studies Conclusions


Chapter Overview Networking

Chapter Overview Networking

Motivation Objectives Topology Network Types Communication Design Strategies

Examples Client/Server Model Middleware Important Concepts and

Terms Chapter Summary


MotivationMotivation resource sharing

users can access all resources available on the machines connected to the network

computation speedup computation-intensive tasks can be partitioned into subtasks and

distributed over several computers on the network specialized or more powerful computers can be used instead of or in

addition to the local machine of the user

reliability the overall system can continue to operate even if some of its

components fail

communication fast exchange of information remote procedure calls


ObjectivesObjectives

be aware of benefits and problems of computers connected via networks

know relevant network topologies and network types understand the issues involved in communication of

computers via networks apply networking concepts to the client/server model


TerminologyTerminology site

indicates the location of computer systems

host specific system at a site frequently implies the execution of a program or the

availability of services

local resources all the resources available directly within a specific

computer system system local sometimes also refers to a particular site, not one

single computer system


Terminology cont. Terminology cont. remote resources

mainly for exchange of information sometimes specialized resource types

network operating system users are aware of the individual machines in the network resources are accessible via login or explicit transfer of data

distributed operating system users are unaware of the underlying machines and networks remote resources are accessible in the same way as local

resources


Computer NetworkComputer Network

set of computers linked to each other through some type of network physical link virtual link

indirect via other computers

logical link more abstract level, independent of the physical realization


Computer Network DiagramComputer Network Diagram

Network

logicallink

physicallink


Network TopologyNetwork Topology

describes the interconnection structure of the network fully connected partially connected hierarchical network

tree structure

star network ring network bus network

unless otherwise noted, the topology refers to the physical interconnections


Comparison CriteriaComparison Criteria

important properties of the configuration site refers to one or several computers with a single access

point to the network link is a connection between two sites

basic cost setup of the communication between two sites

communication cost costs of transferring a message from one site to another time required for the transfer

reliability effects of a failure in a link or site


Fully ConnectedFully Connected

each site is directly linked with all the others basic cost

high: requires a direct link between every two sites quadratic w.r.t. the number of sites

communication cost low: no intermediate steps

reliability very high if messages can be rerouted


Fully Connected DiagramFully Connected Diagram


Partially ConnectedPartially Connected

direct links exist between some pairs of sites basic cost

lower than for the fully connected network

communication cost higher than for the fully connected network

reliability lower than for the fully connected network


Partially Connected DiagramPartially Connected Diagram


Hierarchical NetworkHierarchical Network the sites are organized as a tree

often used for corporate networks headquarters -> main offices -> regional offices

basic cost much lower than fully, usually lower than partially conn.

communication cost higher than for the fully connected network acceptable if communication patterns match the hierarchical

structure

reliability medium: the failure of a site or link partitions the network into

disjoint subtrees


Hierarchical Network DiagramHierarchical Network Diagram


Star NetworkStar Network all sites in the network are linked to a central hub

the hub may be a special device for communication only, or a regular site

basic cost linear with the number of sites

communication cost very low if there isn’t too much traffic central hub may become a bottleneck

reliability depends on the central hub: if it fails, the network is completely

partitioned the failure of a site or link affects only that site


Star Network DiagramStar Network Diagram


Ring NetworkRing Network

each site is connected to its two neighbors links can be uni- or bi-directional

basic cost linear with the number of sites

communication cost linear with the number of sites n-1 transfers for unidirectional, n/2 transfers for bi-

directional links in the worst case

reliability low: the failure of one site or link partitions the network


Ring Network DiagramRing Network Diagram


Directional Ring Network Diagram

Directional Ring Network Diagram

unidirectional bi-directional


Bus NetworkBus Network

single shared link (bus) multiple simultaneous access must be coordinated

all sites are directly connected to the bus basic cost

linear with the number of sites

communication cost low as long as traffic is not too high network contention can become a problem for high traffic

situations

reliability the bus is the critical component


Bus Network DiagramBus Network Diagram

linear bus ring bus


Hybrid NetworksHybrid Networks

combination of different network topologies different topologies usually use different protocols

requires routers or bridges to translate between different protocols and routing mechanisms

more or less unavoidable in practice Ethernet LAN hierarchical WAN


Hybrid Network DiagramHybrid Network Diagrambridge node


Network TypesNetwork Types

geographical distribution of networks local area networks (LANs)

single or adjacent buildings

wide area networks (WANs) sites distributed over a large geographical area

has a major impact in the design of networks speed, reliability, protocol, security


Local Area NetworksLocal Area Networks

connect sites within a small geographical area make resources available to all sites

applications, files, special devices

high-speed communication links Mega-Bit to Giga-Bit per second transfer rate wire, optical fiber, infrared requires expensive links

gateways provide access to other networks other LANs WANs Internet


LAN ExamplesLAN Examples

Ethernet usually wire (shielded or twisted-pair cable) TCP/IP protocol variations

10BaseT 100BaseT

FDDI optical fiber token ring protocol 100 MBit/s


Wide Area NetworksWide Area Networks

connect geographically distributed sites exchange of information access to special purpose computers

not useful for access to peripheral devices

links must cross long distances speed, reliability, security problems may involve other organizations


WAN TechnologyWAN Technology

link technology telephone lines microwave links satellite connections

specialized communication processors interface to LANs (protocol, transfer rate) transmission of information routing billing, usage statistics


WAN ExamplesWAN Examples

Arpanet Internet company networks


Internet as WAN ExampleInternet as WAN Example

LANs are connected to regional networks regional network are connected with routers and

high-speed links special purpose links telephone connections

connections from LANs to the Internet telephone modems (up to 50 KBit/s) ISDN (up to 128 KBit/s) T1 connections (1.544 MBit/s)

can be combined into multiple links


CommunicationCommunication

internal workings of networks naming routing packet strategies connection strategies contention


NamingNaming the entities to exchange information are processes processes must specify the recipient

sometimes also the sender names are more appropriate for human users

must be unique, at least within the horizon of the user computers use numbers internally

hierarchical numbering scheme for unique addresses mapping of names to numbers (addresses)

single file with all names and addresses impractical for large systems

distributed scheme more complicated to implement


Domain Name ServiceDomain Name Service

naming scheme used in the Internet specifies the naming structure of sites and hosts conversion from names to addresses

logical names consist of several fields separated by periods www.cis.njit.edu

refers to a host named www in the CIS department at NJIT


Name Resolution Name Resolution

the name is parsed in reverse order edu indicates that the requested host is in an educational

institution, and the name server for the edu domain is contacted for the address of njit.edu refers

the edu name server returns the address of the host acting as name server for njit.edu refers

the njit.edu returns the address of the cis.njit.edu name server, or directly the address of the requested host

this results in an Internet address (IP-address) of the form 128.132.55.116

caching improves access speed and reliability


RoutingRouting

in WANs there are usually multiple potential connection from host A to host B

a routing table contains information about possible communication paths alternative routes speed, costs

routing schemes fixed virtual dynamic


Fixed RoutingFixed Routing

the path from host A to host B is specified in advance

it does not change unless severe obstacles are encountered e.g. hardware failure

used for heavily used connections minimization of communication costs or transfer time


Virtual RoutingVirtual Routing

the path from A to B is fixed for the duration of one session

different paths for different sessions are possible more flexible than fixed routing reasonably easy to administrate

parts of a message take the same path


Dynamic RoutingDynamic Routing

the path for a message from A to B is chosen at the time of sending the message

different messages from the same session may take different paths composition of messages at the receiver can become

complicated messages may arrive out of order

takes into account the traffic conditions link failures, load changes


RouterRouter

responsible for routing messages can be a computer, or a special purpose device

has routing tables for the networks it is connected to possibly cached copies of the name files from the name

server

examines the destination IP address and decides where to send the message


Packet StrategiesPacket Strategies

variable-length messages are commonly implemented through fixed-length packets datagrams, frames are alternative names for packets

connections are established to transfer packets constituting a message to increase reliability, acknowledgment packets can be

sent


Connection StrategiesConnection Strategies

establishing communication between processes that want to exchange information

circuit switching a connection is established for the whole duration of the

session similar to the telephone system inefficient resource utilization

message switching packet switching


Circuit SwitchingCircuit Switching

a connection is established for the whole duration of the session similar to the telephone system

inefficient resource utilization the communication line is reserved even if there is no

activity

little overhead for individual messages


Message SwitchingMessage Switching

a connection is established for the transfer of one particular message similar to letters sent through the postal service

better resource utilization many messages can use the same link no waste due to inactivity of individual processes

more overhead for messages each message must contain administrative information

receiver, sender, etc.


Packet SwitchingPacket Switching

individual packets from one message are sent separately

very good resource utilization better load balancing efficient treatment of packets since all are of the same

size

overhead for individual packets receiver, sender

packets must be reassembled into messages may not be suitable for time-sensitive information


ContentionContention

several hosts may want to use a link simultaneously mainly relevant for bus topologies

without coordination, data will be scrambled and becomes useless

solutions collision detection token passing message slots


Collision DetectionCollision Detection

a host can only use a link if it is free if two hosts start transmitting at the same time, a

collision will occur the collision must be detected the hosts will try again with some random delay

many collisions in high-traffic situations limited number of hosts on a network segment used in the Ethernet protocol

CSMA/CD: carrier sense with multiple access/collision detection


Token PassingToken Passing

a unique message (token) circulates in the network normally a ring topology

a host may transmit only if it has the token must wait until the token arrives forwards the token after the transmission is finished precautions must be taken for a lost token

constant performance, independent of traffic load worse than CSMA/CD in low traffic, better for high traffic

used in IBM Token Ring networks


Message SlotsMessage Slots

a number of fixed-length message slots circulate in the system normally a ring topology variable-size messages may have to be partitioned

a host must wait until an empty slot arrives used int the Cambridge Digital Communication Ring

experimental system little experience with performance


Network DesignNetwork Design

complex problem involving various levels of abstraction names, IP addresses, messages, packets

coordination between different protocols one approach is to partition the problem into several

layers ISO/OSI protocol layers ISO/OSI network model


OSI ModelOSI Model

Open Systems Interconnection (OSI) reference model

developed by the International Organization for Standardization (ISO)

is commonly used as abstract model TCP/IP is much more widely used in practice

less complex more mature


OSI LayersOSI Layers

Physical

Data Link

Network

Transport

Session

Presentation

Applicationinteraction with the userfile transfer, remote login, email, distributed data bases

conversion of different formats and data representations;characters (ASCII, ISO), transmission modes

communication protocols between processesremote login, file and mail transfer (at the process level)

transfer of data between processesmessage handling, error recovery, flow control

independence from network technologiesnetwork routing, addressing, connection management

reliable transfer of information on physical linkspacket handling, error detection and recovery on lower level

transmission of a bit stream over the physical mediummechanical and electrical network interface


Network ExampleNetwork Example

exchange of information between hosts on different Ethernet networks

TCP/IP TCP: transmission control protocol IP: Internet protocol very widely used

available for practically all computer systems

simpler than the ISO/OSI model four layers instead of seven

Ethernet very popular LAN


TCP/IP ExampleTCP/IP Example

physical

network accessprotocol 1

IP

TCP

application

Network 1

physical

network accessprotocol 1

IP

TCP

application

Network 2

Router

Host A Host B

IP

NAP 1 NAP 2

logical connection


Client/Server ComputingClient/Server Computing

applications are separated into tasks client tasks

user interface, presentation, some processing

server tasks data management, storage, computation-intensive processing

cooperation between client and server clients request services from servers servers return results to clients

network environment frequently LAN, sometimes WAN


Client/Server DiagramClient/Server Diagram

[Stallings 98]

ServersClients

Network(LAN, WAN)


Client/Server ArchitectureClient/Server Architecture

[Stallings 98]

Hardware Platform

Operating System

Communication

Application Logic(Client Side)

PresentationServices

Hardware Platform

Operating System

Communication

Application Logic(Server Side)

ClientServer

Request

Response

Protocol


Client/Server Example: Data Base

Client/Server Example: Data Base

[Stallings 98]

Hardware Platform

Operating System

Application Logic(Client Side)

Data BaseLogic

Hardware Platform

Operating System

Communication

Application Logic(Server Side)

Client

Server

Request

Response

Protocol

PresentationServices

Communication

Data BaseLogic & DBMS


Client/Server UsageClient/Server Usage

a data base is a good example for client/server centralized maintenance possibly large storage space requirements computation-intensive operations

sorting, searching, joins

powerful server with large hard disk(s) client provides user interface, smaller computations potential problem: balance between client & server

searching should not be done by the client network traffic to transfer the whole data base computation power required


Client/Server AdvantagesClient/Server Advantages

lower cost than mainframes better utilization of PCs, workstations distribution or centralization of critical services

possible, as needed

resource sharing load distribution


Client/Server ProblemsClient/Server Problems

client administration different platforms (hardware, OS, language)

software distribution reliable verifiable

security network information

integration of various systems legacy systems multi-vendor, multi-platform environments


MiddlewareMiddleware

tools and methods that provide a uniform access mechanism to systems across all platforms standardized interfaces and protocols

examples Common Object Request Broker Architecture (CORBA) Common Object Model (COM), Object Linking and

Embedding (OLE) Java


Middleware DiagramMiddleware Diagram

Middleware(distributed system services)

Application

Platform

[Stallings 98]

APIs

PlatformInterfaces


Middleware ExampleMiddleware Example

ObjectRequest Broker

DBMSServlet

OODBMSServlet

CORBAServlet

Server Middleware

WebServer

CustomServer

Servers

Web Browser

CORBAClient

CustomClient

CustomServlet

Data Base ServerClients


Tiered ArchitecturesTiered Architectures

distinction between various types of networked computer system

drive towards a global shared information space (WWW) dynamic, executable content platform-independent implementation

integration of existing systems


Tiered Architectures cont.Tiered Architectures cont.

utilization of resources processing power of underutilized computers access to networked resources

total cost of ownership (TCO) basic costs for infrastructure cost per computer system


Single-Tiered ArchitectureSingle-Tiered Architecture

typical mainframe with directly connected terminals all resources are available through the mainframe advantages and problems

easy to manage high cost low flexibility connectivity

usually star network with mainframe as central hub


Single-Tier DiagramSingle-Tier Diagram

mainframe

terminals


Two-Tiered ArchitectureTwo-Tiered Architecture typical client/server model

server provides various services clients have limited processing power

advantages and problems lower cost better use of desktop processing power higher flexibility

access to services on various servers

more difficult to manage and program connectivity

flexible, usually LAN

separation of business logic from client processing


Two-Tier DiagramTwo-Tier Diagram

servers

PCs, workstations


Three-Tiered ArchitectureThree-Tiered Architecture integration of existing systems (“legacy” systems)

frequently via middleware reduction of management and program development

problems through platform-independent methods CORBA, Java

advantages and problems reasonable to manage and program

platform-independent applications intermediate components

high flexibility integration of legacy systems

LAN-based connectivity


Three-Tier DiagramThree-Tier Diagram

PCs, workstations

servers

mainframes


Important Concepts and TermsImportant Concepts and Terms application layer bus computer system CSMA/CD distributed operating system Ethernet fully connected hierarchical network host ISO/OSI model Internet local machine middleware

network operating system node operating system partially connected physical layer processes, tasks remote resource ring service site star TCP/IP tier topology


Chapter SummaryChapter Summary

most computer systems are connected to networks local area network (LAN) wide area network (WAN), especially Internet

networking enables communication, collaboration, and access to shared resources

networks require infrastructure interconnections (links) protocols

a client/server model is frequently used in networks middleware integrates various platforms and applications

on networks

networks 1 © 2000 franz kurfess course overview principles of operating systems introduction ...

Documents

network resources

network link

franz kurfess network

networks remote resources

franz kurfess objectives

relevant network topologies

franz kurfess terminology

communication of computers