This section describes the definition and objective for port pre-allocation and incremental allocation.
Port pre-allocation and incremental allocation are enhanced technologies based on NAT. Port pre-allocation indicates that the CGN device pre-allocates port resources to the Customer Premise Equipment (CPE). If the CGN receives the first flow of user traffic from the CPE, the CGN pre-allocates port resources for the CPE (including a public IP address and the related port range). Subsequent user traffic selects a port from the pre-allocated port range. Incremental allocation indicates that if the port resources pre-allocated by the CPE for the first time are used up, more port resources can be allocated.
When port pre-allocation and incremental allocation are not used, a dynamic method is used to allocate a port for each flow. The ports are randomly selected from the available port range in the NAT address pool. This method increases the usage of public IP addresses, but generates a log for each flow. Therefore, lots of logs are generated at service peaks. When the syslog server traces addresses, it traces the source address of each flow, bringing too much load to the CGN device and syslog server.
NAT with port pre-allocation and incremental allocation logs only when a port range is assigned and recycled, which generates fewer logs. The syslog server parses logs to obtain information such as users (CPE) source address, helping resolve address exhaustion.