July 2008 doc IEEE 802 11 080766 r

July 2008 doc. : IEEE 802. 11 -08/0766 r 0 Group Block Acknowledgements for Multicast Traffic Date: 2008 -07 -07 Authors: Submission 1 Alex Ashley, NDS Ltd

July 2008 doc. : IEEE 802. 11 -08/0766 r 0 Abstract This presentation describes a way to extend the “leader based” approach to any number of reporting STAs This approach is based upon re-using the existing block -ACK mechanisms It has the further advantage of not requiring one (or more) ACKs per multicast frame Submission 2 Alex Ashley, NDS Ltd

July 2008 doc. : IEEE 802. 11 -08/0766 r 0 Leader Based Multicast • One receiving device is elected as a “leader” for a set of group addresses • Leader sends back an ACK if it successfully receives a group addressed frame • AP uses these ACKs (or lack of) to retransmit packets not received by the leader to all members of the group Submission 3 Alex Ashley, NDS Ltd

July 2008 doc. : IEEE 802. 11 -08/0766 r 0 Leader Based Multicast • Assumes errors are correlated such that if leader successfully received frame, all other stations also received it correctly – Random & dynamic nature of WM does not fit well with this assumption • Requires AP to send one frame, wait for an acknowledgment (or time out) before sending the next frame – If LB multicast not enabled, the AP can burst multiple frames – This means power saving STAs have to stay awake for longer Submission 4 Alex Ashley, NDS Ltd

July 2008 doc. : IEEE 802. 11 -08/0766 r 0 Block ACK Setup • To start block ACK, a request is sent – Request contains first sequence number to start using block ACK and a buffer size. Buffer size is max number of frames that the transmitter will keep for possible retransmission. • Recipient replies with Addba. Response – Response says if request has been accepted and a buffer size. Buffer size in Addba. Response defines max number of frames that either the transmitter or receiver needs to keep while the block ACK is active – Receiver creates a re-order buffer for frames of this BA agreement Submission 5 Alex Ashley, NDS Ltd

July 2008 doc. : IEEE 802. 11 -08/0766 r 0 Block ACK • • Submission Once block ACK is active, sending device can send frames with the ACK policy set to BLOCK_ACK – Frames with this policy set are not acknowledged immediately Sending device sends a Block. Ack. Request frame – Requests information on the frames that have been received successfully – Block. Ack. Request must be sent at least as often as the buffer size to ensure that it can retransmit lost frames before the receiver’s buffer overflows Recipient sends a Block. Ack frame – Contains a map of received sequence (fragment) numbers – Sending device can use this Block. Ack frame to discover which frames have been received by the destination and which ones need to be retransmitted. – When a Block. Ack. Request is received, the receiver can empty its receive buffer of all the oldest frames in its buffer that make a continuous set of sequence numbers. Buffer overflow in the receiver – If, after an MPDU is received, the receive buffer is full, the complete MSDU with the earliest sequence number shall be indicated to the MAC client using the MAUNIDATA. indication primitive 6 Alex Ashley, NDS Ltd

July 2008 doc. : IEEE 802. 11 -08/0766 r 0 Block ACK for Multicast STA 2 STA 1 AP 1 STA 4 STA 3 Submission 7 Alex Ashley, NDS Ltd

July 2008 doc. : IEEE 802. 11 -08/0766 r 0 Block ACK for Multicast 1 1 STA 2 STA 1 AP 2 1 STA 4 1 STA 3 Submission 8 Alex Ashley, NDS Ltd

July 2008 doc. : IEEE 802. 11 -08/0766 r 0 Block ACK for Multicast 1 2 STA 1 AP 3 1 2 STA 4 1 2 STA 3 Submission 9 Alex Ashley, NDS Ltd

July 2008 doc. : IEEE 802. 11 -08/0766 r 0 Block ACK for Multicast 1 2 3 STA 2 STA 1 BA 3 R= BA AP 1 2 STA 4 1 2 STA 3 Submission 10 Alex Ashley, NDS Ltd

July 2008 doc. : IEEE 802. 11 -08/0766 r 0 Block ACK for Multicast 1 2 3 STA 2 STA 1 AP 4 1 2 STA 3 Submission 11 Alex Ashley, NDS Ltd

July 2008 doc. : IEEE 802. 11 -08/0766 r 0 Block ACK for Multicast 1 2 3 4 STA 2 STA 1 AP 5 1 2 4 STA 4 1 2 4 STA 3 Submission 12 Alex Ashley, NDS Ltd

July 2008 doc. : IEEE 802. 11 -08/0766 r 0 Block ACK for Multicast 1 2 3 5 1 2 3 4 5 STA 1 =5 STA 2 R BA BA AP 1 2 4 5 STA 4 1 2 4 5 STA 3 Submission 13 Alex Ashley, NDS Ltd

July 2008 doc. : IEEE 802. 11 -08/0766 r 0 Block ACK for Multicast 1 2 3 5 1 2 3 4 5 STA 2 STA 1 AP 4 1 2 4 5 STA 3 Submission 14 Alex Ashley, NDS Ltd

July 2008 doc. : IEEE 802. 11 -08/0766 r 0 Block ACK for Multicast 1 2 3 4 5 STA 2 STA 1 AP 6 1 2 4 5 STA 4 1 2 4 5 STA 3 Submission 15 Alex Ashley, NDS Ltd

July 2008 doc. : IEEE 802. 11 -08/0766 r 0 Block ACK for Multicast 1 2 3 4 5 6 STA 2 STA 1 AP 4 5 6 R= 6 1 2 4 5 BA 1 2 STA 4 BA STA 3 Submission 16 Alex Ashley, NDS Ltd

July 2008 doc. : IEEE 802. 11 -08/0766 r 0 Block ACK for Multicast 1 2 3 4 5 6 STA 2 STA 1 AP 3 1 2 4 5 6 STA 4 1 2 4 5 STA 3 Submission 17 Alex Ashley, NDS Ltd

July 2008 doc. : IEEE 802. 11 -08/0766 r 0 Block ACK for Multicast 1 2 3 4 5 6 STA 2 STA 1 AP 6 1 2 3 4 5 6 STA 4 1 2 3 4 5 STA 3 Submission 18 Alex Ashley, NDS Ltd

July 2008 doc. : IEEE 802. 11 -08/0766 r 0 Block ACK for Multicast 1 2 3 4 5 6 STA 2 STA 1 AP 1 2 3 4 5 6 STA 4 1 2 3 4 5 6 STA 3 Submission 19 Alex Ashley, NDS Ltd

July 2008 doc. : IEEE 802. 11 -08/0766 r 0 Block ACK for Multicast - Setup • AP needs to know which stations are listening to which multicast groups – E. g. snoop on IGMP messages sent by STAs, send out IGMP query messages • AP selects ‘n’ stations from the ‘m’ listening to the multicast group to become block ACK recipients – where 0<n<=m – For each of the ‘m’ stations, the access point sends Addba. Requests – From the resulting Addba. Response frames, AP knows minimum buffer size that can satisfy all ‘m’ stations. Submission 20 Alex Ashley, NDS Ltd

July 2008 doc. : IEEE 802. 11 -08/0766 r 0 Block ACK for Multicast - Setup • AP calculates number of group addressed frames to send before sending a Block. Ack. Request: – Block. Ack. Request. Frequency * n <= Buffer. Size (eqn 1) • By satisfying equation 1, each of the ‘n’ stations will be asked at least once their reception status before its receive buffer becomes full* – This allows at worst case one retransmission of each frame • An improvement can be made by reducing Block. Ack. Request. Frequency: – Block. Ack. Request. Frequency * n * retries <= Buffer. Size (eqn 2) * Assuming all STAs use immediate block ACK Submission 21 Alex Ashley, NDS Ltd

July 2008 doc. : IEEE 802. 11 -08/0766 r 0 Block ACK for Multicast - Operation • After transmitting Block. Ack. Request. Frequency group addressed frames, the AP sends a Block. Ack. Request to a STA from of its list of ‘n’ – Normally first in list, however AP does not select a STA that was the source of any of the packets in its block ACK buffer • The sender of the frame will not have this frame in its receive buffer, so a Block. Ack. Request would erroneously look like a failed reception – Selected STA moves to the back of the list • Typically will not receive another Block. Ack. Request until all other ‘n’ stations have been sent a Block. Ack. Request. • Extra Block. Ack. Requests may need to be sent to STAs not in ‘n’ to stop Block ACK agreement timeout Submission 22 Alex Ashley, NDS Ltd

July 2008 doc. : IEEE 802. 11 -08/0766 r 0 Block ACK for Multicast - Operation • When frames are retransmitted, all STAs listening to this multicast/broadcast will attempt to receive the frame – If reception errors between stations are correlated, the retransmission caused by processing one Block. Ack may provide frames lost by many stations – This technique allows improved reliability even when ‘n’ is less than ‘m’ – This is also why Block. Ack. Requests are staggered between stations, as opposed to sending them at the same time or one after the other. • The Block ACK setup means that there is a delay of up to Buffer. Size frames between a frame first being received and being passed up the software stack – If a lost frame is retransmitted during this delay it will be placed in its correct position in the receive buffer. Submission 23 Alex Ashley, NDS Ltd

July 2008 doc. : IEEE 802. 11 -08/0766 r 0 Legacy Issues • Retransmissions need to be hidden from legacy STAs that do not have active block ACK agreement – They will not have setup a re-order buffer and will probably pass up all received frames • Block ACK requires a TID – For multicast, this needs to be a TID not used by any associated STA (apart from the source of the traffic) • Assumes legacy STA can handle block ACK on group addressed frames – Suggest adding a flag to the Add. Block. Ack. Request to signal group block ACK (E. g. using reserved B 0 of parameter set) – Capability signalling in an IE also a recommended Submission 24 Alex Ashley, NDS Ltd

July 2008 doc. : IEEE 802. 11 -08/0766 r 0 Conclusions • Technique built upon existing 802. 11 technology – Can re-use existing non-AP STA block-ACK implementation – AP needs to keep buffer of sent frames and send periodic Block. Ack. Req frames and schedule retransmits when Block. Ack received • Any number of group members can be used for reception reports • Does not require an ACK on every transmitted frame Submission 25 Alex Ashley, NDS Ltd

July 2008 doc. : IEEE 802. 11 -08/0766 r 0 References 1. “Leader based Multicast. ” Yongho SEOK, Diego DUJOVNE, Thierry TURLETTI, Emily Qi, Menzo Wentink IEEE 802. 11 -07/2128 r 0 Submission 26 Alex Ashley, NDS Ltd