1. The Client Server Model And Software Design
Lecturer: Tran Dang Hoan
Faculty of Information Technology
Vietnam Maritime University

2. Introduction
TCP merely provides basic mechanisms used to transfer data.
TCP/IP allows a programmer to establish communication between two application programs and to pass data back and forth.
TCP/IP provides peer-to-peer communication.
Clien/Server paradigm is a method to build a network application.
Everyone is connected with fat pipes.
It is easy to connect hardware together.
Definition: a distributed system is
A collection of independent computers that appears to its users as a single coherent system.

3. Motivation
Because TCP/IP does not provide any mechanisms that automatically create running programs when a message arrives, a program must be waiting to accept communication before any requests arrive.
To ensure that computers are ready to communicate, most system administrators arrange to have communication programs start automatically whenever OS boots. Each program runs forever.

4. Terminology And Concept (1)
Client is an application that initiates peer-to-peer communication.
Most client software consists of conventional programs.
When client is executed, it contacts a server, sends a request, and awaits a response. When repsonses arrives, the client continues processing.
Server is an application that waits for incoming communication requests from client.
The server receives a client’s request, performs the necessary computation, and returns the results to the client.

5. Terminology And Concepts
Because server often need to access data, computation, or protocol ports that the OS protects, server usually requires special system privileges.
Authentication-verifying the identity of the client.
Authorization-determining whether a given client is permitted to access the service the server supplies.
Data security-guaranteeing that data is not unintentionally revealed or compromised.
Privacy-keeping information about an individual from unauthorized access.
Protection-guaranteeing that network applications cannot abuse system resources.
Server is an application that waits for incoming communication requests from client.
The server receives a client’s request, performs the necessary computation, and returns the results to the client.

6. Terminology And Concepts (2) Standard Vs. Nonstandard Client Software
Standard application services consist of the serivices defined by TCP/IP and those serivices defined by TCP/IP and assigned well-known, universally recognized protocol port identifiers. (Eg: ).
Nonstandard application is locally-defined application serivice (Eg: an instituation’s private database system)
The distinction between standard services and nonstandard appclication is only important when communcating outside the local environment.
Within a given environment, system administrators usually arrange to define serivice names in such a way that users cannot distinguish between local and standard services.
Programmers who build network application that will be used at other sites must understand the distinction.

7. Terminology And Concepts Parameterization of Clients
Some client software provides more generality than others. In particular, some client software allows the user to specify both the remote machine on which a server operates and protocol port number at which the server is listening.
Software that allows a user to specify a protocol port number has more input parameters than the other software, this type of software called fully parameterized client.
An example:
telnet machine-name port
When designing client application software, include parameters that allow the user to fully specify the destination machine and destination protocol port number.

8. Connectionless Vs. Connection-Oriented Servers
The two types of interaction correspond directly to the two major transport protocols that the TCP/IP protocol suite supplies. TCP: connection-oriented, UDP: connectionless.
OS can be developed for either type of environment.
The distinction between two types of interaction is critical because it determines the level of reliability that underlying system provides.
Connection-oriented interaction: verify that data arrives, that transmits segments that do not. It computes a checksum over data to guarantee that it is not corrupted during transmission. It uses sequence numbers to ensure that the data arrives in order, and automatically eliminates duplicate packets. It provides flow control to ensure that the sender does not transmit data faster than the receiver can consume it. TCP informs both the client and server if the underlying network becomes inoperable for any reasons.
By contrast, clients and servers that use UDP do not have any guarantees about reliable delivery. When a client sends a request, the request may be lost, duplicated, delayed, or delivered out of order. Similarly, a response the server sends back to a client may be lost, duplicated, delayed, or delivered out of order. The client and server application programs must take appropriate actions to detect and correct such errors.

9. Connectionless Vs. Connection-Oriented Servers-continued
Application programs only use UDP if:
(1) The application protocol specifies that UDP must be used
(2) The application relies on hardware broadcast or multicast, or
(3) The application cannot tolerate the computational overhead or delay required for TCP virtual circuits.
Summary:
When designing clien-server applications, beginners are strongly advised to use TCP because it provides reliable, connection-oriented communication. Programs only use UDP if the application protocol handles reliablity, the application requires hardware broadcast or multcicast, or the application cannot tolerate virtual circiut overhead.

10. Stateless Vs. Stateful Server
Server maintains information about the status of ongoing interaction with clients is called stateful server.
Server that do not keep any state information are called stateless server.
Why we need stateful servers?
State information allow a server to remember what client requested previously and to compute an incremental response as each new request arrives=>efficiency, server give response quickly
Why we need stateless servers?
State information in a server can become incorrect if messages are lost, duplicated, or delivered out of order or if the client computer crashes and reboots. If server uses incorrect information when computing a response, it may respond incorrectly.

11. Servers As Clients A program can be both client and server.
Server may need to access network services that require it to act as a client.
File server needs to obtain the time of the day so it can stamp files with the time of access. Suppose that the system on which it operates does not have a time-of-day clock. To obtain a time-of-day server, the server acts as a client by sending a request to a time-of-day server.

12. Concurrent Processing in Client-Server Software
Concurrent Processing is the power of client-server software, but also makes difficult to design and built.
Concurrency refers to real or apparent simultaneous computing.
Concurrent Processing in:
Multi-user computer system:can achieve concurrency by time-sharing, a method to switch a single processor among multiple computations quickly enough to give the appearance of simultaneous
Multiprocessing: a design in which multiple processors performs multiple computations simultaneously.

13. Concurrency Processing:Continued
Most client software achivieves concurrent operation because
the underlying operating system allows users to execute client programs concurrently
Users on many machines each execute client software simultaneously.
An individual client program operates like any conventional program; it does not manage concurrency explicitly.

14. Concurrency in Servers
A single server must handle incoming requests concurrently.
The process defines the fundamental unit of computation. The most essential information associated with a process is an instruction pointer that specifies the address at which the process is executing.
Application programmers builds programs for a concurrent environment without knowing whether the underlying hardware consists of uniprocessor or multiprocessor