For example, a first STA may be configured to transmit to a second STA a message including a first value to indicate an available memory size at the first STA at a beginning of a TXOP, and a second value to indicate a maximal length of an A-MPDU transmission during the TXOP; to receive an initial A-MPDU from the second STA during the TXOP, a length of the initial A-MPDU is not longer than the first value; to determine a capacity value based on a current available memory size at the first STA, the capacity value to indicate whether the second STA is to be allowed to send to the first STA a subsequent A-MPDU having a length which is not longer than the second value; and to transmit to the second STA an Ack including a buffer capacity field including the capacity value.