 request response model. When an element connected to a network wants to communicate with another element in the network, it raises a request message identifying itself as the source and specifying the destination to which the message needs to be sent. Upon receiving the service request, the destination element returns a response message to the source. In a client-server model, there will be one or more clients connected to a server. The management application sends a request to the agents to perform a service like getting management data, setting the data, or notifying when an event occurs. The agents send a response message to the management applica- tion on completing a service request. The response message can be synchro- nous or asynchronous message. Socket Communication Services: There are different types of services that can be offered in the application layer. These make use of the client-server paradigm for message exchange. Some of the popular protocol-based services are: - Services using simple mail transfer protocol (SMTP): The SMTP service allows transfer of message between two users on the Internet via elec- tronic mail. - Services using file transfer protocol (FTP): The FTP service allows the exchange of files between two elements. Services using hypertext transfer protocol (HTTP): The HTTP transfer protocol facilitates the use of the World Wide Web. Addressing: The client message includes the address of the server as the desti- nation address and its own address as the source address for communication. Similarly for messages from the server, the server address is used as the source address and the address of the client is the destination address. The application layer has its own addressing format, which is different from the address for- mat used by the underlying layers. For example, electronic mail service has a host address format like editor@crc.com and the world wide web service has a host address format like www.crcpress.com. The application program uses an alias name as the address rather 