2012-02-20 07:12:35

by Yingang Fu

Subject: AP send null-data packet?

hi, all

It is wired that my AP send the station a null-date packet.
As i know that this packet could only been sent by the station.
Under which case the AP will send out this type of packet?

Thanks

2012-02-20 11:11:07

by Mohammed Shafi

Subject: Re: AP send null-data packet?

> Yes, this is true, it happens when the station sends a PS-Poll but we
> timed out the frame in the meantime -- should be uncommon. In the uAPSD
> case it's more common, if nothing is pending and the station sends a
> trigger frame.

thanks!

>
> In any case, it's perfectly normal for an AP to send null data frames.

--
thanks,
shafi

2012-02-20 11:05:26

by Johannes Berg

Subject: Re: AP send null-data packet?

On Mon, 2012-02-20 at 16:33 +0530, Mohammed Shafi wrote:
> > Don't know much about other AP software. But hostapd sends them
> > to check if a station is still in range and active.
> >
> > quote from hostapd.conf:
> >
> > # Station inactivity limit
> > #
> > # If a station does not send anything in ap_max_inactivity seconds, an
> > # empty data frame is sent to it in order to verify whether it is
> > # still in range. If this frame is not ACKed, the station will be
> > # disassociated and then deauthenticated. This feature is used to
> > # clear station table of old entries when the STAs move out of the
> > # range.
>
> i think also in this path in mac80211
> sta_notify (driver call back) for AP mode
> ieee80211_sta_block_awake
> sta_unblock ->
> ieee80211_sta_ps_deliver_poll_response ->
> ieee80211_sta_ps_deliver_response ->
> ieee80211_send_null_response
>
> also in UAPSD path. i don't know much about the technical details

Yes, this is true, it happens when the station sends a PS-Poll but we
timed out the frame in the meantime -- should be uncommon. In the uAPSD
case it's more common, if nothing is pending and the station sends a
trigger frame.

In any case, it's perfectly normal for an AP to send null data frames.

johannes

2012-02-20 09:48:07

by Christian Lamparter

Subject: Re: AP send null-data packet?

On Monday, February 20, 2012 08:12:34 AM Yingang Fu wrote:
> It is wired that my AP send the station a null-date packet.
> As i know that this packet could only been sent by the station.
> Under which case the AP will send out this type of packet?

Don't know much about other AP software. But hostapd sends them
to check if a station is still in range and active.

quote from hostapd.conf:

# Station inactivity limit
#
# If a station does not send anything in ap_max_inactivity seconds, an
# empty data frame is sent to it in order to verify whether it is
# still in range. If this frame is not ACKed, the station will be
# disassociated and then deauthenticated. This feature is used to
# clear station table of old entries when the STAs move out of the
# range.

Regards,
Chr

2012-02-20 11:03:36

by Mohammed Shafi

Subject: Re: AP send null-data packet?

> Don't know much about other AP software. But hostapd sends them
> to check if a station is still in range and active.
>
> quote from hostapd.conf:
>
> # Station inactivity limit
> #
> # If a station does not send anything in ap_max_inactivity seconds, an
> # empty data frame is sent to it in order to verify whether it is
> # still in range. If this frame is not ACKed, the station will be
> # disassociated and then deauthenticated. This feature is used to
> # clear station table of old entries when the STAs move out of the
> # range.

i think also in this path in mac80211
sta_notify (driver call back) for AP mode
ieee80211_sta_block_awake
sta_unblock ->
ieee80211_sta_ps_deliver_poll_response ->
ieee80211_sta_ps_deliver_response ->
ieee80211_send_null_response

also in UAPSD path. i don't know much about the technical details

--
thanks,
shafi