In Figure 2.5, exactly one protocol data unit (PDU) in layer N is encapsulated in a PDU at layer 1N - 12. It is also possible to break one N-level PDU into multiple 1N - 12-level PDUs (segmentation) or to group multiple N-level PDUs into one 1N - 12-level PDU (blocking).

a. In the case of segmentation, is it necessary that each 1N - 12-level segment contain a copy of the N-level header?

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?