This document discusses the client-server model and concurrency in computer networks. It explains that in the client-server model, client applications request services from server applications that provide services. Servers typically run continuously and listen on well-known ports, while clients run intermittently to request services. The document also describes how servers can operate concurrently by handling multiple clients simultaneously through child processes.

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Chapter 15
Application Layer
and
Client-Server Model
 CLIENT-SERVER MODEL
 CONCURRENCY
 PROCESSES

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Comparison between OSI and TCP/IP

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15.1 Client-server model
 To make any use of the Internet, application programs should run on
the two endpoints of a network connection.
 The applications are the entities that communicate with each other
to exchange services
 “Client” applications request service
 “Server” applications provide service.

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Client-Server Relationship: Many-to-One
 Servers
 Run all the time (i.e. infinite)
 Provide service to any client
 Typically specialize in providing
a certain type of service, e.g.
Mail.
 Listen to a well-known port and
passively open connection.
 Clients
 Run when needed, then
terminate (i.e. finite)
 Actively Open TCP or UDP
connection with Server’s socket.

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15.1 Concurrency
 Operation mode could be either iterative or concurrent.
 In clients:
 Iterative mode: One client at-a-time in serial
 Concurrent mode: Several clients run at the same time
 In servers:
 Iterative mode: serve one client at-a-time (clients wait in a queue)
 Concurrent mode: serve multiple clients concurrently and
independently.
 In addition, servers could chose the Transport layer protocol:
 UDP, i.e. connectionless
 TCP, i.e. connection-oriented

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Server types
Transport Protocol  Operation Mode

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Connectionless iterative server
 Clients’ request arrive inside
UDP datagrams and wait in a
queue for the server
 Server processes one
datagram at-a-time, send
response back to client inside
a UDP datagram
 Clients use ephemeral UDP
ports
 Server uses one well-known
UDP port at which all clients’
requests arrive

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Connection-Oriented Concurrent Server
 Requests and responses are streams of data
spanning several segments.
 Parent Server passively opens the well-know
port to listen for incoming connection requests
 Once opened, connections now use ephemeral
ports between one client and one Child Server.

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15.3 Programs and Processes
 Program: code on disk
 Process: a running instance of a
program.
 Process Control Block:
 ProcessID, UserID, Program
Name.
 Where is the data
 Which line will execute next
 Figure shows two processes of
the same program.

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The Process ID & getpid()

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Process Creation & fork()
 By Replication
 One-Parent
needed
 After the fork(),
both parent and
child processes
execute the same
line, the one after
the fork()

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A program with two fork functions

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Return Value of fork()
 In the parent process, fork() returns the processID of the just created
child
 In the child process, fork() simply returns a 0 (which is not a valid
processID)
 This way, each of the two identical replica can detect whether it is
indeed the original parent, or it is the newly created process.

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A program that prints the processIDs of the parent and the
child

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Example of a server program with parent and child processes
 Parent Server listens indefinitely
(to the well-know port) for
connection requests from clients
 Once a client requests a
connection, a child server process
is created to serve this client (on
an ephemeral port)
 The parent server continues to
listen for more clients