[ekr____]

A few points of technical clarification might help here.

1. The reason for a relatively small initial congestion window (cwnd) is to avoid situations where a lot of connections start up and collectively exceed the capacity of the network, causing congestion collapse. Instead, you start slow and then gradually ramp up, as you learn the available capacity. Slow start started in TCP but it's in QUIC too. Initial windows actually used to be a lot smaller and TCP only moved up to its current 10 packet initial window (IW10) after a bunch of experimentation that determined it was safe.

2. The congestion window is actually a property of the sender, not the receiver. The receiver advertises the size of their flow control window, but that's about the buffer, not the sending rate (see section 7 of RFC 9002 for the discussion of slow start in QUIC). So in this case, the server controls cwnd, no matter what the client advertises (though the server isn't allowed to exceed the client's advertised flow control window).

3. QUIC and TCP behave fairly similarly in terms of the broad strokes of rate control. As I noted above, QUIC also uses Slow Start. The amplification limit you mention is a separate limit from initial cwnd, which is intended to avoid blind amplification attacks, because, unlike TCP, QUIC servers can start sending data immediately upon receiving the first packet, so you don't know that the IP address wasn't forged. However, even if the peer's IP is authenticated, that doesn't mean it's safe to use an arbitarily large initial cwnd.