There is no default value for the Block1 and Block2 Options.  Absence
   of one of these options is equivalent to an option value of 0 with
   respect to the value of NUM and M that could be given in the option,
   i.e., it indicates that the current block is the first and only block
   of the transfer (block number 0, M bit not set).  However, in
   contrast to the explicit value 0, which would indicate an SZX of 0
   and thus a size value of 16 bytes, there is no specific explicit size
   implied by the absence of the option -- the size is left unspecified.
   (As for any uint, the explicit value 0 is efficiently indicated by a
   zero-length option; this, therefore, is different in semantics from
   the absence of the option.)