Abstract: Congestion has been considered as one of the basic important issue in packet switched network [1][9]. Congestion control refers to the mechanisms and techniques to control the congestion and keep the load below the capacity. This paper provides an overview of category provided by congestion control .It also includes how TCP uses congestion control to avoid congestion or alleviate congestion in network. Computer networks have experienced an explosive growth over the past few years and with that growth have come severe congestion problems. This paper also concentrates on avoidance of congestion. This scheme allows a network to operate in the region of low delay and high throughput. In this paper, a survey on various mechanisms of congestion control and avoidance has been done.