The next-generation gigabit passive optical network (NG-PON) is a promising solution for network growth and expansion. The NG-PON aims to connect more users as fiber's deployment to the home (FTTH) initiative takes place. The network needs urgent attention to its security establishment, which is currently exposed to a degradation attack. During the attack, already available TCP congestion control is invoked to handle collision. The interconnection of TCP in the transport layer and attack in the MAC layer is investigated. During the attack, a reduction of 63% of total throughput is detected. A variation of the number of ONU also determines the competing nature of TCP, which inherently reduces the maximum throughput of 4%.