Chapter Two

1. Chapter Two Application Layer Prepared by: Dr. Bahjat Qazzaz CS Dept. Sept. 2007

2. Application Layer: Introduction • This layer is the main reason for the existence of computer networks. • Many ingenious and wonderful network applications have been created • Text e-mail, remote access to computers, file transfer, and text chat. • Multimedia Applications (Killer applications or resources hungry applications) such as: video streaming, Internet radio, video conferencing • Instant messages and P2P file sharing.

3. Application Layer: Introduction • Examples of NW applications • Electronic mail • The Web • Instant messaging • Login into a remote computer • P2P file sharing • FTP • Video conferencing …..

4. Application Layer: Introduction • Application’s programs should run on different end systems and communicate with each other over the network. • Example: Web application • Web browser program running in the user’s host • Web server program that runs on web server host

5. Application Layer: Introduction • You need to develop applications that run on multiple machines. • You don’t need to write SW that runs on routers or Ethernet switches.

6. Application Layer: NW application. Paradigm (or architecture) • Client/Server paradigm • P2P architecture • Hybrid of client/server and P2P

7. Application Layer: NW application. Paradigm (or architecture) • Client/Server paradigm • The client is a program running on the user’s host and request a service • The server is a program running on remote host (normally) listening for a request (connection) and responds to the requests • Example: web applications, FTP, … • Clients don’t communicate with each other directly

8. Application Layer: NW application. Paradigm (or architecture) • Client/Server paradigm • The server has a well-known address called IP address. The server is always on, and the client can always request a service • A single server is incapable of keeping up with all the request from clients. So, cluster of servers (hosts) can be implemented. This is referred to as SERVER FARM.

9. Application Layer: NW application. Paradigm (or architecture) • P2P Architecture • There isn’t a server in the center of the application • Instead, pair of hosts (called peers) communicate directly with each other • In the P2P, none of the peer host is required to be always on

10. Application Layer: NW application. Paradigm (or architecture) • Hybrid of client/server and P2P • Participant can communicate bwn each other without passing through intermediate server. • Also, a peer can queries a central server to determine which currently-up (on) peers have a desired file Most NW apps consist of multiple pairs of communicating processes Client & Server

11. Application Layer: Processes communication • Processes on two different end systems communicate with each other by exchanging messages across the computer network. • The client/server paradigm consists of two processes, one of them is labeled client, and the other is labeled server • But how do they exchange messages!!!!!

12. Application Layer: Processes communication • Sockets • A process sends messages into, and receives messages from, network through its socket. • A socket is an interface bwn the application layer and the transport layer, and it is referred to as: • The Application Programming Interface (API).*1 • Addressing processes….follow--

13. Application Layer: Processes communication • Sockets-Addressing processes • For a sending process to identify the receiving process, it has to provide two pieces of information: • The name or address of the host (IP address). The IP address is a 32 bit number that uniquely identifies the host. 1* • An identifier that specifies the receiving process, called Port Number. Such as: 80 for web server. لأن الجهاز ممكن أن يحتوي على أكثر من خادم

14. Application Layer: Processes communication • An application layer protocol defines • The types of messages exchanged (request messages and response messages) • The syntax of the various message types (such as fields in the message and how the fields are delineated) • The semantics of the fields (the meaning of the information in the fields) • Rules for determining when and how a process sends messages and responds to messages

15. Application Layer: services needed • What services does an application need? A network application need the following services from the transport layer • Reliable data transfer *1 • Bandwidth: some app must transfer data at a certain rate, or do different encoding *2 • Timing: Some Interactive real-time applications require tight timing constraints on data delivery. Long delays cause unacceptable results (Internet telephony)

16. Application Layer: services needed • Services provided by the Internet Transport Protocols. TCP & UDP • TCP • Reliable transfer service • Allows client and server to exchange control information before transmission (hand-shaking) • Then connection exist (full-duplex). • It includes congestion-control mechanism (throttles the sender when there is a congestion). *1 • TCP does not • Guarantee minimum transmission rate.TCP regulates that *2 • Provide any delay guarantee *3

17. Application Layer: services needed • Services provided by the Internet Transport Protocols. TCP & UDP • UDP • Unreliable data transfer • No handshaking • Messages may get out of order • Good for loss-tolerant applications • No guarantee of minimum delay or transmission rate