4) (4 points) In protocol layer structure, one protocol data unit (PDU) in layer N is encapsulated in a PDU at layer (N1) and a (N1)-header is added. It is also possible to break one N-level PDU into multiple ( N-1)-level PDUs (segmentation) or to group multiple N-level PDUs into one (N-1)-level PDU (blocking). a) In the case of segmentation, is it necessary that each (N1)-level segment contain a copy of the N-level header? Please explain your answer. b) In the case of blocking, is it necessary that each N-level PDU retain its own header, or can the data be consolidated into a single N-level PDU with a single N-level header? Please explain your