|- SCHC ACK Header -|- W=0 -|      |- W=1 -|...
+------+------+-----+-------+------+-------+...
|RuleID|W=b'00|C=b'0|Bitmap |W=b'01|Bitmap |...
+------+------+-----+-------+------+-------+...
|6 bits|2 bits|1 bit|12 bits|2 bits|12 bits|...

            ...       |- W=2 -|      |- W=3 -|
            ...+------+-------+------+-------+---+
            ...|W=b'10|Bitmap |W=b'11|Bitmap |b'0|
            ...+------+-------+------+-------+---+
            ...|2 bits|12 bits|2 bits|12 bits|