A high degree of stealthiness over a prolonged duration of operation can be defined as Advanced Persistent Threat. Today’s successful targeted attacks use a combination of social engineering, malware, and backdoor activities. Nart Villeneuve and James Bennett (Senior Threat Researcher) fromprovide an ultimate guide for Detecting (APT) AdvancedPersistent Threat activities with Network Traffic Analysis. They also talk about the Challenges during Network-Based Detection i.e Two key factors pose challenges to network-based detection encryption and the cloud.
Source: https://thehackernews.com/2012/10/detecting-advanced-persistent-threat.html