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Understanding the Lockout Period for PPPoE Subscriber Session Lockout

Duration of PPPoE Subscriber Session Lockout Period

The duration of the lockout period is based on a default or configured lockout time and the number of consecutive short-cycle (short-lived) events that occur repeatedly for the same subscriber. When you include the short-cycle-protection statement to configure PPPoE subscriber session lockout on a PPPoE underlying interface, you can use the default lockout time range of 1 through 300 seconds (5 minutes), or you can override the default lockout period by configuring a nondefault lockout time in the range 1 through 86,400 seconds (24 hours).

The lockout time penalty applied by the router for each short-cycle event differs depending on the event. For example, some short-cycle events represent normal subscriber behavior, such as a PPPoE subscriber logging in once per hour to check e-mail and logging out shortly thereafter. The router does not noticeably penalize a subscriber for these types of events.

By contrast, other short-cycle events are the result of repeated attempts to log in to the router for reasons such as an incorrectly typed password, customer premises equipment (CPE) that performs repeated auto-retries, or malicious attempts to access the Internet illegally. For these types of short-cycle events, the router applies a lockout time penalty that starts with a short time interval and increases exponentially. In these instances, the initial lockout time is short enough to avoid noticeably penalizing a subscriber who, for example, types a password incorrectly several times before entering the correct one.

For example, using the default lockout time range of 1 through 300 seconds, the increasing lockout period on the router is: 1 second, 2 seconds, 4 seconds, 8 seconds, 16 seconds, 32 seconds, 64 seconds, 128 seconds, 256 seconds, and finally, 300 seconds (5 minutes).

How the Router Determines the PPPoE Subscriber Session Lockout Period

• The lockout period is derived from the following formula:

  (minimum lockout time) * (2 ^ n-1)

  where n represents the number of consecutive short-cycle events for the same subscriber. The router identifies a PPPoE subscriber session by its MAC source address, which should be unique on the underlying PPPoE interface.

• The router increments the value of n when the time between short-cycle events is either within 15 minutes or the maximum lockout time, whichever is greater.
• When the time between short-cycle events is greater than either 15 minutes or the maximum lockout time, the value of n reverts to 1. This condition is referred to as a lockout grace period.
• The lockout period never exceeds the maximum configured lockout time.

  For example, for a configured (nondefault) lockout time in the range 20 through 120 seconds, the increasing lockout period on the router is: 20 seconds, 40 seconds, 80 seconds, and finally, 120 seconds (2 minutes).

• A short-cycle event is detected, partially or completely created, and terminated by the router within 150 seconds. The router tracks the time between short-cycle events to determine whether to increase the lockout time for a subsequent short-cycle event for the same subscriber.

  Note: When the calculated lockout time is equal to or exceeds the maximum lockout time, the router uses the maximum lockout time value until the time to the next short-cycle event exceeds the greater of 15 minutes or the maximum lockout time value. At that point, the lockout time reverts to the minimum lockout time value.

• The minimum lockout time value cannot exceed the maximum lockout time value.

  When the minimum and maximum lockout time values are equal, the lockout time becomes fixed at that value.