Disable KCP congestion control and enable stream mode.

This is some experimenting with parameters to try and provide better
performance with KCP. Without these changes, I was having trouble
getting more than about 40 KB/s. (And it was something latency related,
because I could run at essentially line rate if I ran a local proxy and
bridge instead of relying on public ones.)

Stream mode just gives kcp-go license to coalesce the payloads of
adjacent packets.
https://github.com/xtaci/kcp-go/blob/3bedbae3ba26dfa4356f797bc1a165f7ef9d76c0/kcp.go#L313-L337

The number of packets in flight is constrained to be the minimum of the
local send window (default 32), the remote receive window (default 32),
and the congestion window (uses TCP-like slow start and congestion
avoidance). Or, you can disable the congestion window and then the
constraint is just the minimum of the first two values.
https://github.com/xtaci/kcp-go/blob/3bedbae3ba26dfa4356f797bc1a165f7ef9d76c0/kcp.go#L746-L750
I added some printf statements and saw that the congestion window seemed
to be growing slowly (Seemed like linearly? though I didn't check it
thoroughly. Maybe the ssthresh gets set too low?) and was usually below
10. Disabling the congestion window allows 32 packets in flight in both
directions. Because we are tunnelling through channels that have their
own congestion control (SCTP on the WebRTC link and TCP on the WebSocket
link), we probably don't want any dynamic congestion control in this
inner layer anyway.

1 files changed, 10 insertions, 0 deletions

diff --git a/server/server.go b/server/server.go
index 7db36d1..390788a 100644
--- a/server/server.go
+++ b/server/server.go
@@ -352,6 +352,16 @@ func acceptSessionsKCP(ln *kcp.Listener) error {
 			}
 			return err
 		}
+		// Permit coalescing the payloads of consecutive sends.
+		conn.SetStreamMode(true)
+		// Disable the dynamic congestion window (limit only by the
+		// maximum of local and remote static windows).
+		conn.SetNoDelay(
+			0, // default nodelay
+			0, // default interval
+			0, // default resend
+			1, // nc=1 => congestion window off
+		)
 		go func() {
 			defer conn.Close()
 			err := acceptStreamsKCP(conn)