DesPDUBytes is the desired PDU size in bytes. This is not necessarily the same as the minimum necessary PDU size. This value may be made smaller by intervening ST agents so long as it is not made smaller than LimitOnPDUBytes. The *PDUBytes limits measure the size of the PDUs of next-higher protocol layer, i.e., the user information contained in a data packet. An ST agent must account for both the ST Header (including possible IP encapsulation) and any local network headers and trailers when comparing a network's MTU with *PDUBytes. In an ACCEPT message, the value of this field will be no larger than the MTU of the path to the specified target.