As described above, a synchronized SLD is stored locally by the Source Gateway. Because the functionality in an IPS is rarely changed, SLDs are rarely updated. For this reason, it is more efficient for the Source Gateway to store a copy of the Destination IPS’s SLD than to ask the Destination Gateway for the SLD on a payment-by-payment basis. The SLD synchronization process described earlier ensures that all the local copies of SLD are up to date.