In a wireless local area network system, a receiving device may receive a Block Acknowledgment Request (BAR) frame including information related to a link for which an ACK is requested, and transmit a Block ACK (BA) frame on the basis of the BAR frame. The receiving device is a Multi-Link Device (MLD), which supports communication over multiple links. Each link of the multiple links can be configured based on 2.4 GHz band, 5 GHz band, and/or 6 GHz band defined in a conventional wireless local area network system.