application layer protocols

computer network

1. Overview Process network applications and application-layer protocols Components of web application Application-layer protocols Addressing User agents

2. Process Processes are communicating in a network. A process can be thought of as a program that is running within an end system. Processes on two different end systems communicate with each other by exchanging messages across the computer network. A sending process creates and sends messages into the network; A receiving process receives these messages and possibly responds by sending messages back.

3. Process… Networking applications have application-layer protocols that define the format and order of the messages exchanged between processes, as well as the actions taken on the transmission or receipt of a message.

4. Network applications and application-layer protocols An application-layer protocol is a piece of a network application. The Web is a network application that allows users to obtain "documents" from Web servers on demand.

5. Components of web application A standard for document formats (i.e., HTML) Web browsers (e.g., Netscape Navigator and Internet Explorer) Web servers (e.g., Apache, Microsoft and Netscape servers), An application-layer protocol(e.g., HTTP, FTP) HTTP defines how messages are passed between browser and Web server.

6. Application-layer protocol Defines how an application's processes, running on different end systems, pass messages to each other. The types of messages exchanged, e.g., request messages and response messages; The syntax of the various message types, i.e., the fields in the message and how the fields are delineated; The semantics of the fields, i.e., the meaning of the information in the fields; The rules for determining when and how a process sends messages and responds to messages.

7. Clients and Servers The client side in one end system communicates with the server side in another end system. For example, a Web browser implements the client side of HTTP and a Web server implements the server side of HTTP

8. Processes Communicating Across a Network An application involves two processes in two different hosts communicating with each other over a network. The two processes communicate with each other by sending and receiving messages through their sockets . A process's socket can be thought of as the process's door A process sends messages into, and receives message from, the network through its socket.

9. Socket A socket is the interface between the application layer and the transport layer within a host. It is also referred to as the API (application programmers interface) between the application and the network, since the socket is the programming interface with which networked applications are built in the Internet

10. Addressing Processes In order for a process on one host to send a message to a process on another host, the sending process must identify the receiving process. To identify the receiving process, one must typically specify two pieces of information: the name or address of the host machine, and an identifier that specifies the identity of the receiving process on the destination host.

11. Addressing Processes… In Internet applications, the destination host is specified by its IP address. T he IP address is a 32-bit quantity that uniquely identifies the end-system. Since the IP address of any end system connected to the public Internet must be globally unique.

12. Addressing Processes… To identify the destination process to direct the message to the appropriate process on the destination system, a receive-side port number is used. Popular application-layer protocols have been assigned specific port numbers. For example, a Web server process (which uses the HTTP protocol) is identified by port number 80. A mail server (using the SMTP) protocol is identified by port number 25.

13. User Agents The user agent is an interface between the user and the network application. For example, for Web application, the user agent is a browser such as Netscape Navigator or Microsoft Explorer. The browser allows a user to view Web pages, to navigate in the Web, to provide input to forms, to interact with Java applets, etc. The browser also implements the client side of the HTTP protocol. Thus, when activated, the browser is a process that, along with providing an interface to the user, sends messages into a socket.

14. What Services Does an Application Need? We can broadly classify an application's service requirements along three dimensions: data loss, bandwidth, and timing.

15. Services Provided by the Internet Transport Protocols TCP Services Connection-oriented service Reliable transport service: Congestion Control UDP Services Connectionless No guarantee No congestion control