This INTEGER, four or eight, selects the size in bytes of the Initial Vector used by this IPSec Transform Set's cipher, per its description. An admin sets this to match the block-cipher mode the chosen encryption transform expects, since a mismatched IV size can prevent the transform set from negotiating correctly with a peer. For example, an admin configuring a transform set around a cipher needing a larger IV would set this to 'eight' rather than the smaller four-byte option.