Tuesday 1 July 2014

Which parameter defines wireless client max idle timeout ?

Question : Once the wireless client is connected (authenticated and associated) to Access Point, if no data is passed between Client and AP, after how much time the connection will break or which parameter defines it?

Answer : This parameter is known as BSS Max Idle Period and is defined in 802.11 standard, 802.11-2012.pdf as:

The BSS Max Idle Period element contains the time period a non-AP STA can refrain from transmitting frames to the AP before the AP disassociates the STA due to inactivity.

The Max Idle Period field indicates the time period during which a STA can refrain from transmitting frames to its associated AP without being disassociated. The Max Idle Period field is a 16-bit unsigned integer. The time period is specified in units of 1000 TUs. The value of 0 is reserved. A non-AP STA is considered inactive if the AP has not received a data frame or management frame of a frame exchange sequence initiated by the STA for a time period equal to or greater than the time specified by the Max Idle Period field value.

The Idle Options field indicates the options associated with the BSS Idle capability.

The Protected Keep-Alive Required bit set to 1 indicates that the STA sends an RSN protected frame to the AP to reset the Idle Timer at the AP for the STA, as defined in 10.23.12. If the Protected Keep-Alive  Required bit is 0, the STA sends either an unprotected or a protected frame to the AP to reset the Idle Timer  at the AP.

The BSS Max Idle Period element is included in Association Response frames and  Re-association Response frames.

For hostapd we can define it using parameter  ap_max_inactivity=300

# 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.
# The station can associate again with the AP if it is still in range;
# this inactivity poll is just used as a nicer way of verifying
# inactivity; i.e., client will not report broken connection because
# disassociation frame is not sent immediately without first polling
# the STA with a data frame.
# default: 300 (i.e., 5 minutes)

In Cisco Wireless LAN controller , this field is "User idle Timeout"

The User Idle Timeout: When a user is idle without any communication with the LAP for the amount of time set as User Idle Timeout, the client is de-authenticated by the WLC. The client has to reauthenticate and  reassociate to the WLC. It is used in situations where a client can drop out from its associated LAP without notifying the LAP. This can occur if the battery goes dead on the client or the client associates move away.