A method performed by a STA may comprise transmitting a first negotiation message to an AP and receiving a second negotiation message from the AP. The first negotiation message and the second negotiation message may negotiate a block acknowledgement agreement. The method may further comprise receiving a data unit, from the AP, the data unit having a duration field indicating a duration of a block-ACK frame. The duration of the block-ACK frame may be determined based on the negotiated block ACK agreement. A block-ACK frame may be transmitted within the duration indicated by the duration field.