mozilla
transport: consider lower pacing rate in congestion avoidance
Neqo currently paces over half the RTT, i.e. 2x the current rate:
https://github.com/mozilla/neqo/blob/207d309e941d3ae9384c191c13463dc81fca4ce9/neqo-transport/src/pace.rs#L19-L27
This is independent of the congestion control phase, i.e. same in slow start and congestion avoidance.
In slow start, we likely want to keep the aggressive pacing rate to keep up with the cwnd doubling.
In congestion avoidance, to prevent overflowing buffers and thus causing packet loss, we might want to reduce the rate to e.g. 1.2x times the rate.
What do others do?
- Linux TCP stack uses 2x in slow start and 1.2x in congestion avoidance.
- quic-go uses 1.25x
//CC @MarcoPolo and @marten-seemann since we discussed this over the week. //CC @mwelzl thanks for the input on Linux.
In slow start, we likely want to keep the aggressive pacing rate to keep up with the cwnd doubling.
Why is this needed? If you always pace at a rate determined by cwnd / rtt, this should not be necessary: During slow start, cwnd will grow, and thereby automatically increase the pacing frequency.
It's just a little too slow in slow start. Yes, the time gaps get smaller, but you also send twice as many packets / RTT. I'm attaching an Excel sheet that shows this for the second RTT (i.e., after IW).
