The Client Server Model and Software Design

MCSE Lab, NTUT, TAIWAN 1

Mobile Computing & Software Engineering Lab

The Client Server Model and Software Design

Prof. Chuan-Ming LiuComputer Science and Information Engineering

National Taipei University of TechnologyTaipei, TAIWAN

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IntroductionShort review for OSI modelPeer-to-peer communication

Client-server paradigm

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LayeringDivide a task into pieces and then solve each piece independently (or nearly so).Establishing a well-defined interface between layers makes porting easier. Major Advantages:♦Code Reuse♦Extensibility

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Layering Example: Federal ExpressLetter in envelope, address on outsideFedX guy adds addressing information, barcode.Local office drives to airport and delivers to hub.Sent via airplane to nearest city.Delivered to right officeDelivered to right person

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FedX LayersLetter Addressed

Envelope

Letter Addressed Envelope

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Layered Software SystemsNetwork software

Operating systems

Windowing systems

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Unix is a Layered SystemApplications

Libraries

System Calls

Kernel

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OSI Reference ModelThe International Standards Organization(ISO) proposal for the standardization of the various protocols used in computer networks (specifically those networks used to connect open systems) is called the Open Systems Interconnection Reference Model(1984), or simply the OSI model.

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OSI Model Although the OSI model is a just a model (not a specification), it is generally regarded as the most complete model (as well it should be - nearly all of the popular network protocol suites in use today were developed before the OSI model was defined).

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OSI <-> Network SoftwareAlthough this course is about network programming (and not about networking in general), an understanding of a complete network model is essential.

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OSI 7 Layer Model:7 Application6 Presentation5 Session4 Transport3 Network2 Data-Link1 Physical

High level protocols

Low level protocols

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Simplified Network ModelProcess

Transport

Network

Data Link

Process

Transport

Network

Data Link

Peer-to-peerProtocols

Interface Protocols

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What’s a Protocol?An agreed upon convention for communication.

both endpoints need to understand the protocol.

Protocols must be formally defined and unambiguous! We will study lots of existing protocols and perhaps develop a few of our own.

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Interface andPeer-to-peer

Protocols

Interface protocols describe the communication between layers on the same endpoint.Peer-to-peer protocols describe communication between peers at the same layer.

Process

Transport

Network

Data Link

Process

Transport

Network

Data Link

Interface Protocols

Peer-to-peerProtocols

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Physical LayerCoordinates the functions required to transmit a bit stream over a physical mediumConcerns

Physical characteristics of interfaces and mediumRepresentation of bitsData rate (Transmission rate): bits/secSynchronization of bits

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Physical LayerLine configuration

Point-to-pointMultipoint

Physical topologymesh, star, ring, or bus.Transmission modesimplex, half-duplex, or full-duplex

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Physical Layer

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Data Link LayerTransforms the physical layer to a reliable linkMakes the physical layer appear error free to upper layerResponsible for

FramingframesPhysical addressing (physical address)

Header defines the sender and/or receiverReceiver is the device connected to the next

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Data Link LayerFlow controlError controlAccess controlMulti-home: computer having two or more NICs

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Data Link Layer

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Node-to-node Delivery

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Network LayerResponsible for the source-to-destination delivery of a packetResponsibilities:

Logical addressing (IP address)Routing

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Network Layer

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End-to-end Delivery

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Transport LayerSource-to-destination (end-to-end) delivery of the entire messageFunctions include:

Service-point addressing (port)Segmentation and reassemblyConnection controlconnectionless v.s. connection-orientedFlow controlError control

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Transport Layer

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Reliable end-to-end delivery of a message

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Session LayerNetwork dialog controller

establishes, maintains, and synchronizes the interaction between communicating systems

Functions include:Dialog controlSynchronization (checkpoint)

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Session Layer

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Presentation LayerConcerned with the syntax an semantics of the information exchanged between two systemsResponsibilities include

TranslationEncryptionCompression

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Presentation Layer

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Application LayerInterface for users to access the networkServices include

Network virtual terminalFile transfer, access, and management (FTAM)Mail servicesDirectory services

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Application Layer

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Summary of Layers

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TCP/IP Protocol SuitePhysical LayerData link LayerNetwork LayerTransport LayerApplication Layer

No specific protocol defined in physical and data link layers in TCP/IP

Correspond to OSI model

Last three layers in OSI

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TCP/IP and OSI model

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Network LayerInternetwork layerSupport

IP: Internetworking ProtocolARP: Address Resolution ProtocolRARP: Reverse Address Resolution ProtocolICMP: Internet Control Message ProtocolIGMP: Internet Group Message protocol

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Internetworking Protocol (IP)Transmission mechanismDatagram: data unit to be sent in IPUnreliable and connectionlessBest-effort delivery serviceHost-to-host protocol

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Transport LayerTCP and UDP: delivery of a message from a process to another process

User Datagram Protocol (UDP)Transmission Control Protocol (TCP)

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TCP/IP Provides basic mechanisms used to transfer dataAllows a programmer to establish communication between applicationsProvides peer-to-peer communicationOne organizational method to use TCP/IP is the client-server paradigm

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MotivationScenario: A user tries to start two programs on separate machines and have them communicate.

Program 1 startsSend message to its peer

Not running; RefuseNo response; exit

Program 2 starts

No connection can be set up

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Client-Server ModelOne side in any pair of communicating application must start execution and wait for the other side to contact it.Since the client-server model places responsibility for rendezvous problem on application, TCP/IP does not need to provide mechanisms that automatically create a running program when a message arrives. Instead, a program must be waiting to accept communication before any request arrive.

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Terminology and ConceptsClients and ServersPrivilege and ComplexityStandard v.s. Nonstandard Client SoftwareParameterization of ClientsConnectionless v.s. Connection-oriented ServersStateless v.s. Stateful ServersIdentifying a Client

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Clients and ServersClient* An application that initiates peer-to-peer communication, e.g. web browser* Easier to build than servers* System privileges usually unnecessary ServerProgram that waits for incoming communication requests from a client

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Privilege and ComplexityServer software often needs to access to objects that OS protectsServer can not rely on the usual OS checksince its privilege status allow to access any fileSecurity issues:

AuthenticationAuthorizationData SecurityPrivacyProtection

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Privilege and ComplexityThe combination of special privileges and concurrent operation usually makes servers more difficult to design and implementation

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Standard v.s. Nonstandard Client Software

Standard application servicesDefined by TCP/IPAssigned well-known, universally recognized protocol port

Non-standard application servicesAll other services which is not standardOr, locally-defined application services

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Be aware of the standard when outside the local environmentStandard application service examples

Remote login, TELNET protocolE-mail client, SMTP or POP protocolFile transfer client, FTP protocolWeb browser, HTTP protocol

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Non-standard application service examples

Music or video transferVoice communicationDistributed database access

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Parameterization of ClientsGenerality for client softwaree.g. TELNET protocol using port numberFully parameterized clientapplication allows more input parameters

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Parameterization of ClientsWhen designing client application, include parameters that allow the user to fully specify the destination machine and port number.

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Connectionless v.s. Connection-Oriented Servers

TCP/IP provides two types of interaction between client and server:

Connectionless styleConnection-oriented style

The distinction is critical TCP

UDP

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Connection-Oriented ServersTCP:* full reliability* verifying the data arrives* automatically retransmits segments* checksum over the data* data arriving in order* no duplicated packets* control the flow* reporting the underlying network problem to sender

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Connectionless ServersUDP* no guarantees about reliable delivery* software contains code to detect and correct errors occurred by transmission* work well when the underlying network running well* aware of testing when using UDP

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Connectionless v.s.Connection-Oriented Servers

TCP is preferable to UDPTCP simplifies programmingTCP relieves the programmers of responsibility for detecting and correcting errorsAdding reliability to UDP is a nontrivial work

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Connectionless v.s.Connection-Oriented Servers

Application programs use UDP only ifApplication protocol specifies the UDP must be usedApplication protocol relies on hardware broadcast or multicast for deliveryOverhead for reliability is unnecessary

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Stateless v.s. Stateful ServersState informationA server maintains about the status of ongoing interaction with clientsServers that do not keep any state information are called stateless servers; otherwise, called stateful servers.

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Stateless v.s. Stateful ServersKeeping a small amount of information in a server can reduce the size of messages that the client and server exchange and allow the server to response quicklyThe point for statefulness is efficiencyThe motivation for statelessness lies in protocol reliability

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Stateless File Server Example

disk

File server

client

Waits for client to access

stores or extracts data from the server

message

Operation (read or write)Name of the filePosition in the fileNumber of bytes to transferPresent only in write operation

opnamepossizedata

DescriptionItem

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Stateful File Server Example

disk

File server

client

Waits for client to access

stores or extracts data from the server

reads

Operation (read or write)Name of the filePosition in the fileNumber of bytes to transferPresent only in write operation

opnamepossizedata

DescriptionItem

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Stateful File Server Example

readreadwriteread

045638

125

test.program.ctcp.book.text

dept.budget.txtteris.txt

1234

Last Operation

Current Position

File NameClient

State information table

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Identifying a ClientStateful servers use two general approaches to identify clients

EndpointsHandles

Endpoint identificationOperate automaticallyRelies on transport protocol, not application protocolEndpoint information may change

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Identifying a Client

server

Transport protocol

IP address and port number

using the endpointinformation to look up the state table

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Identifying a ClientHandles

remains constant across multiple transport connectionsA small integerIndependent of the underlying transport protocol

Change of transport connection does not invalidate handlesVisibility to the application – drawback

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Identifying a ClientBack to stateful server

Can not retain state forever and application protocol requires terminationUsing endpoint identification can be confused by a crash

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The point of state is efficientReducing the amount of data transferredIntuitive designDifficulty

Maintain the correctness when delay, duplication allowedState information may be incorrect when computer restarts

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In general, in a real internet, where machines crash and reboot, and messages can be lost, delayed, duplicated, or delivered out of order, stateful designs lead to complex application protocols that are difficult to design , understand, and implement correctly.

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Statelessness is a Protocol Issues

Statelessness or not centers on the application protocol more than implementationThe issue of statelessness focuses on whether the application protocol assumes the responsibility for reliable deliveryIdempotent – design issue for statelessness operation always has the same result

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Servers as Clients

Programs do not fit exactly into the definition of client or server

Avoid circular dependencies among servers

Internetclient

time server

file server

The Client Server Model and Software Design

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