1. Internet Applications & Programming
Chapter 3
Internet Applications & Programming
© Bobby Hoggard, Department of Computer Science, East Carolina University
These slides may not be used or duplicated without permission

2. Internet Applications
The Internet is diverse with services
None of the services is part of the Internet structure
The Internet provides a common communication medium
New services can be created

3. Two Communication Means
Stream
Message
Connection Oriented
1-to-1 Communication
Send Sequence of Bytes
Any Length Transfer
Widely Used
TCP
Connectionless
Many-to-Many
Send Individual Message
Limited to 64K
Multimedia Applications
UDP

4. Messages (UDP) Connection-less: Very little set-up involved
A packet sent represents and entire message as it was originally sent
No guarantee that messages sent will be received
Messages can be lost
Messages can be duplicated
Messages can be delivered out of order
Faster: no error checking involved
Examples of uses include: DNS (domain name lookup) VOIP (voice over IP) Some Games

5. Streams (TCP) Connection Oriented Guaranteed delivery of packets
Two applications must agree on having a connection created
Both applications can send and receive
After communication is finished, connection is terminated
Guaranteed delivery of packets
Packets are guaranteed to be in the same order sent
Slower: due to the error checking involved
Examples of uses include: HTTP (web service) FTP (file transfer) SMTP ( service)

6. Streams (TCP) Uses a client / server model
Client: application which requests a connection to be made
Server: application which waits for connection requests

7. Clients vs. Servers Client Server Starts second
Must know the server to which it will connect
Initiates contact when needed
Can send & receive
When finished, the client shuts down
Starts first
Does not know which client will communicate
Passively waits for long periods of time for a connection to be made
Can send & receive
When finished, waits for another connection

8. Multiple Clients
It is possible (and very common) to have multiple clients operating on one machine
All clients can communicate with the same server
All clients can communicate with different servers

9. Multiple Servers
It is possible to have multiple servers operating on one machine
More efficient use of the machine Makes use of idle time between connections
Can be problematic if the servers are busy

10. Server Identification
Servers are identified by a 2-part identification number
Computer Identifier
Service Identifier
IP Address
32-bit number
Port Number
16-bit number
Domain Name
Translator

11. Server Identification
Each service on the server computer has a unique port number When the server starts it registers its port number with the operating system This is what allows the OS to send requests to the correct program when multiple servers are on the same machine Two commonly used port numbers are: port 80 = web browsing port 25 = service

12. Server Identification
Web
Server
Client
Computer
Server
Computer 1
Request
IP Address
Port Number
Server
Request
IP Address
Port Number
Server
Computer 2

13. Client/Server Interaction
Start Get Server Name Translate To IP Address Specify Port Number Contact Server Stop
Start Register Port # with OS Wait For Client Interact With Client Finished

14. Concurrent Server Processing
Server uses a thread of control for each client Will have a main thread and one thread for each client it is processing The main thread waits for connections and handles the connection set up Once the connection has been made, a client thread is created to continue the communication, while the main thread continues to wait for more requests

15. Servers Can Be Clients! Web Browser Client Computer Request For
ASPX page
Web
Server
Request For
Data
Database
Server
HTML page
Data
CLIENT
SERVER
CLIENT
SERVER