©The McGraw-Hill Companies, Inc., 2000© Adapted for use at JMU by Mohamed Aboutabl, 2003Mohamed Aboutabl1 1 Chapter 15 Application Layer and Client-Server Model  CLIENT-SERVER MODEL  CONCURRENCY  PROCESSES

©The McGraw-Hill Companies, Inc., 2000© Adapted for use at JMU by Mohamed Aboutabl, 2003Mohamed Aboutabl2 2 Comparison between OSI and TCP/IP

©The McGraw-Hill Companies, Inc., 2000© Adapted for use at JMU by Mohamed Aboutabl, 2003Mohamed Aboutabl 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.

©The McGraw-Hill Companies, Inc., 2000© Adapted for use at JMU by Mohamed Aboutabl, 2003Mohamed Aboutabl4 4 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.

©The McGraw-Hill Companies, Inc., 2000© Adapted for use at JMU by Mohamed Aboutabl, 2003Mohamed Aboutabl 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

©The McGraw-Hill Companies, Inc., 2000© Adapted for use at JMU by Mohamed Aboutabl, 2003Mohamed Aboutabl6 6 Server types Transport Protocol  Operation Mode

©The McGraw-Hill Companies, Inc., 2000© Adapted for use at JMU by Mohamed Aboutabl, 2003Mohamed Aboutabl7 7 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

©The McGraw-Hill Companies, Inc., 2000© Adapted for use at JMU by Mohamed Aboutabl, 2003Mohamed Aboutabl8 8 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.

©The McGraw-Hill Companies, Inc., 2000© Adapted for use at JMU by Mohamed Aboutabl, 2003Mohamed Aboutabl 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.

©The McGraw-Hill Companies, Inc., 2000© Adapted for use at JMU by Mohamed Aboutabl, 2003Mohamed Aboutabl10 10 The Process ID & getpid()

©The McGraw-Hill Companies, Inc., 2000© Adapted for use at JMU by Mohamed Aboutabl, 2003Mohamed Aboutabl11 11 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()

©The McGraw-Hill Companies, Inc., 2000© Adapted for use at JMU by Mohamed Aboutabl, 2003Mohamed Aboutabl12 12 A program with two fork functions

©The McGraw-Hill Companies, Inc., 2000© Adapted for use at JMU by Mohamed Aboutabl, 2003Mohamed Aboutabl13 13 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.

©The McGraw-Hill Companies, Inc., 2000© Adapted for use at JMU by Mohamed Aboutabl, 2003Mohamed Aboutabl14 14 A program that prints the processIDs of the parent and the child

©The McGraw-Hill Companies, Inc., 2000© Adapted for use at JMU by Mohamed Aboutabl, 2003Mohamed Aboutabl15 15 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