Telecommunication networks based on the IP protocol face many security threats. One of the most important threats is abnormal traffic, which can cause serious impact on the secure and steady operation of telecommunication networks. Abnormal traffic attacks consume large quantities of network resources and easily lead to network unsteadiness and link blockage. Moreover, abnormal traffic attacks have increasingly been aimed at achieving certain business objectives, and are a great challenge to telecommunication operators. Therefore, detecting and controlling abnormal traffic effectively has become an urgent task for telecommunication operators. Supplement 18 to ITU-T X.1205 series of Recommendations identifies abnormal traffic detection technologies and control measures for IP-based telecommunication networks. The aim of this Supplement is to provide telecommunication operators with a comprehensive guideline for monitoring, detecting and controlling abnormal IP traffic.