A PDP context remains open until it is terminated. To terminate the PDP context an MS sends a
Deactivate PDP context message to the SGSN, which then sends a
Delete PDP Context message to the GGSN. When the SGSN receives a PDP context deletion acknowledgment from the GGSN, the SGSN confirms to the MS the PDP context deactivation. The PDP can be terminated by the SGSN or GGSN as well with a slight variation of the order of the messages passed.
When the PDP Context is terminated, the tunnel it was using is deleted as well. If this is not completed in a timely manner, it is possible for someone else to start using the tunnel before it is deleted. This hijacking will result in the original customer being over billed for the extra usage. Anti-overbilling helps prevent this.