Define are headers and trailers, and how are they added and removed?

Answer

Header &Trailer

            A header is attached in the front side or the side which is forward and trailer is at end of each packet, these are control bit not the actual data. Control information means the parity bit or other error detection or correction bits.

How are they added and removed

The process starts from application layer and moves from layer to layer in descending, sequential order. At each layer a header can be added to the data unit. At layer 2 a trailer is added as well. When the formatted data unit passes through the physical layer (layer 1), it changes into electromagnetic signal and transported along the physical link. Upon reaching the destination, the signal passes into layer 1 and is transformed back into digital form. The data unit then moves back up through the layers. As each block of data reaches the next higher layer, the header and trailer attached to it at the corresponding sending layer are removed, and actions appropriate to that layer are taken. By the time it reaches the application layer (layer 5), the message is again in a form appropriate to the application and is made available to the recipient.