Preventing duplicate sources from clients served by a network address port translator

Preventing duplicate sources on a protocol connection that uses network addresses, protocols and port numbers to identify connections that include port number translation. In response to an inbound IPsec packet from a remote source dient, a determination is made as to whether or not a port number is available within a range of port numbers that comply with a security association governing the connection. If so, an available port number is assigned to the connection, thereby avoiding a possibility of a duplicate source. If a port number is not available, the packet is rejected.