Cast on EON (Every Other Needle) with waste yarn and knit a few rows, and then knit 1 row ravel cord. Knit required depth of rib at MT, and 1 row MT+2, followed by depth of rib again. Bring all needles into work.
Hang first row of knitting onto empty needles and knit 1 row.
Note: mock rib is not as stretchy as real rib.