Acknowledgement of transmissions in a wireless local area network

A method for acknowledging a data unit, the method comprising:

• receiving, at a first communication device, one or more aggregate medium access control (MAC) data units from respective one or more second communication devices, respective ones of the one or more aggregate MAC data units including respective sets of multiple MAC data units from respective ones of the one or more second communication devices;
• generating, at the first communication device, one or more acknowledgement information fields corresponding to the one or more second communication devices, including generating a first acknowledgement information field corresponding to a particular second communication device of the one or more second communication devices, wherein the first acknowledgement information field includes
• (i) a length indication that indicates a length of an acknowledgement field, the length of the acknowledgement field being selected from a subset of predetermined lengths, among a set of predetermined lengths, the subset including multiple predetermined lengths that do not exceed a buffer size determined based on an acknowledgement setup procedure previously conducted between the first communication device and the particular second communication device, and
• (ii) the acknowledgment field of the indicated length, wherein the acknowledgement field includes respective acknowledgement information for at least some of the multiple MAC data units;
• generating, at the first communication device, an acknowledgement data unit to include the one or more one or more acknowledgement information fields;
• and
• transmitting the acknowledgment data unit from the first communication device to the one or more second communication devices.