<P> The above process can be likened to the mail system in which a letter (SDU) is placed in an envelope on which is written an address (addressing and control information) making it a PDU . The sending post office might look only at the post code and place the letter in a mail bag so that the address on the envelope can no longer be seen, making it now an SDU . The mail bag is labelled with the destination post code and so becomes a PDU, until it is combined with other bags in a crate, when it is now an SDU, and the crate is labelled with the region to which all the bags are to be sent, making the crate a PDU . When the crate reaches the destination matching its label, it is opened and the bags (SDUs) removed only to become PDUs when someone reads the code of the destination post office . The letters themselves are SDUs when the bags are opened but become PDUs when the address is read for final delivery . When the addressee finally opens the envelope, the top - level SDU, the letter itself, emerges . </P> <Table> <Tr> <Td> </Td> <Td> This section needs expansion . You can help by adding to it . (July 2017) </Td> </Tr> </Table> <Tr> <Td> </Td> <Td> This section needs expansion . You can help by adding to it . (July 2017) </Td> </Tr> <P> Transport protocol data unit (TPDU) is a message encapsulation format where there are several bytes of routing header added to the front of the payload message . </P>

Protocol data unit (pdu) for the application layer