How is network congestion detected in TCP?

Question

I am trying to learn about Congestion Control in TCP and in this video (at 1:56), he says if there is no ack received, it assumes network congestion and cuts back the cwnd, no where he mentions about re transmitting the packet.

This article on the other hand mentions that if there is no ack received within the expiry of the timer, the packet is simply re transmitted and doesn't mention about cutting back the cwnd. My question is, in this case, how is congestion really detected if the packet gets successfully delivered after re transmission?

Answer 1

How is congestion really detected?

Using a retransmission timer.

5.3 Transmission Timeout

Because every TCP network has its own characteristics, the delay between sending a segment and receiving an acknowledgement varies. Different methods are available for calculating this Transmission Timeout and will depend on the stack. TCP maintains a retransmission timer for each connection. This retransmission timer is used when TCP expects to receive an acknowledgment from the other end. Once data is sent, TCP monitors this Retransmission Time-Out (RTO) and also a Round Trip Time (RTT). If an ACK is not received by the time the RTO expires, TCP retransmits the data using an exponentially increasing value for the RTO. This doubling is called an Exponential Back-Off The RTO is calculated as a linear function of the RTT and its value changes over time with changes in routing and traffic load. Typically RTT+4*mean deviation.

Source Transfer Control Protocol, 3-way handshake, TCP sliding window