Regarding ACK frames
Last Post: August 29, 2006:
I have a query;
Any wireless frame that is sent from a source will wait for ACK from the destination. if it does not receive any ACK in the SIFS amount of period it waited the source will resend the packet.
My question here is - IS THERE ANY FRAMES THAT WOULD NOT REQUIRE A ACK? IF SO COULD SOME ONE TELL WHAT ARE THOS FRAMES?
The short answer is that data and management frames directed to a unicast receiver address need to be acknowledged by that receiver. All other received frames go unacknowledged.
IEEE P802.11REVma/D5.2-2006 section 9.2 DCF
"The basic medium access protocol is a DCF that allows for automatic medium sharing between compatible PHYs through the use of CSMA/CA and a random backoff time following a busy medium condition. In addition, all directed traffic uses immediate positive acknowledgment (ACK frame) where retransmission is scheduled by the sender if no ACK is received.
9.2.7 Broadcast and multicast MPDU transfer procedure
"In the absence of a PCF, when broadcast or multicast MPDUs are transferred from a STA with the ToDS bit clear, only the basic access procedure shall be used. Regardless of the length of the frame, no RTS/CTS exchange shall be used. In addition, no ACK shall be transmitted by any of the recipients of the frame. Any broadcast or multicast MPDUs transferred from a STA with a ToDS bit set shall, in addition to conforming to the basic access procedure of CSMA/CA, obey the rules for RTS/CTS exchange and the ACK procedure, because the MPDU is directed to the AP. The broadcast/multicast message shall be distributed into the BSS. The STA originating the message shall receive the message as a broadcast/multicast message. Therefore, all STAs shall filter out broadcast/multicast messages that contain their address as the source address. Broadcast and multicast MSDUs shall be propagated throughout the ESS.
"There is no MAC-level recovery on broadcast or multicast frames, except for those frames sent with the ToDS bit set. As a result, the reliability of this traffic is reduced, relative to the reliability of directed traffic, due to the increased probability of lost frames from interference, collisions, or time-varying channel properties.
9.2.8 ACK procedure
"An ACK frame shall be generated as shown in the frame exchange sequences listed in 9.12.
"Upon successful reception of a frame of a type that requires acknowledgment with the ToDS bit set, an AP shall generate an ACK frame. An ACK frame shall be transmitted by the destination STA that is not an AP, when it successfully receives a unicast frame of a type that requires acknowledgment, but not if it receives a broadcast or multicast frame of such type. After a successful reception of a frame requiring acknowledgment, transmission of the ACK frame shall commence after a SIFS period, without regard to the busy/idle state of the medium."
I hope this helps. Thanks. /criss
Thanks for the reply.
Does a deauth frame also require Ack. In other words, if an AP sends a deauth frame to a client, the client will still send a Ack to the AP and will them remove itself from the network? Is this true.
By the standard all unicast traffic should receive an ACK. However, manufacturers more often than not implement deauth and disassociation frames without and ACK because they are notifications not requests. This frees the media for other traffic. If you run a capture of one of the two you may or may not see an ACK, most likely you will not.
A deauthentication frame is a MAC Management Protocol Data Unit (MMPDU). A deauthentication frame may be directed to one receiver or multicast to many receivers. A directed MMPDU requires an ACK and should be retransmitted if the ACK is not properly received.
I find no discussion in the IEEE 802.11 standard regarding what a deauthenticating station should do if a directed deauthentication frame exchange fails after retransmission. I reason that one station considers the relationship over while the other station considers it continuing.
Authentication is a relationship between a pair of stations that defines what frame types may be exchanged. If one station transmits a disallowed frame type to a second station, that second station is required to send a deauthentication frame back to the first. This is a natural cure for a previously failed directed deauthentication frame exchange. This also cures the situation following a multicast deauthentication frame exchange that is received by some but not all members of the BSS.
Vendors are known to ignore or violate the standard at times, especially when it simply doesn't seem to matter. Not bothering to acknowledge directed deauthentication frames and not bothering to retransmit for lack of an ACK may be two of those times.
The short answer again is that data and management frames (and not control frames) transmitted to a unicast receiver address need to be acknowledged by that receiver. All other received frames go unacknowledged.
I hope this helps. Thanks. /criss