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.