Data Encapsulation

network protocols define the shape of data being transmitted

Network protocols have two main parts: addressing and encapsulation. Addressing describes when and where data is sent, and encapsulation handles how that data is packaged for interpretation, comparable to the envelope of a letter.

At each layer of the OSI reference model, two computer systems must support the same networking protocol in order to exchange data. This is known as same layer interaction. To transmit or receive data, each layer provides services that enables communication with the layer above and below that of itself, which is called adjacent layer interaction.

Encapsulation

Encapsulation is the process in which, as the data packet goes up and down the different layers of the OSI reference model (except the first layer, the physical layer) , headers are attached to the packet, creating a chunk of data called a protocol data unit (or PDU).

For example, data generated by an application includes its own header on the packet. Then the TCP header would be attached at the transport layer, then the IP header at the network layer, and so on and so forth. When the packet reaches the receiving node, the reverse happens (decapsulation), all the packets are peeled back until they hit the physical layer, from when then the original data can be read and processed by the end system.