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When the Internet started, the network layer was designed to provide a connection less service, in which the network layer protocol treats each packet independently, with each packet having no relationship to any other packet. The packets in a message may or may not travel the same path to their destination. When the Internet started, it was decided to make the network layer a connection less service to make it simple. The network layer is only responsible for delivery of packets from the source to the destination.
In a connection-oriented service, there is a relation between all packets belonging to a message. Before all data grams in a message can be sent, a virtual connection should be set up to define the path for the data grams. After connection setup, the data grams can follow the same path. In this type of service, not only must the packet contain the source and destination addresses, it must also contain a flow label, a virtual circuit identifier that defines the virtual path the packet should follow. Figure shows the concept of connection-oriented service.
To create a connection-oriented service, a three-phase process is used: setup, data transfer, and tear down. In the setup phase, the source and destination addresses of the sender and receiver is used to make table entries for the connection-oriented service. In the tear down phase, the source and destination inform the router to delete the corresponding entries. Data transfer occurs between these two phases. In the setup phase, a router creates an entry for a virtual circuit. For example, suppose source A needs to create a virtual circuit to destination B. Two auxiliary packets need to be exchanged between the sender and the receiver: the request packet and the acknowledgment packet.
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