> QUIC congestion control has been written based on many years of TCP experience, so it is little surprise that the two have mechanisms that bear resemblance. It’s based on the CWND (congestion window, the limit of how many bytes you can send to the network) and the SSTHRESH (slow start threshold, sets a limit when slow start will stop).
https://blog.cloudflare.com/cubic-and-hystart-support-in-qui...
Per stream and per connection flow control windows, which kind of indicate how much data the peer may send on a given connection before it gets a window update. Those windows also indicate how much the server is willing to store in its receive buffers, since the updates are likely sent when those buffers are drained.
A congestion window, which indicates how many low-level packets and data in them can be in-flight without being acknowledged. Those also account for retransmissions, and packets which do not necessarily contain stream data.