MaxConnectionReceiveWindow must be at least 16384

Error message

MaxConnectionReceiveWindow must be at least 16384

What it means

The 16384-byte minimum for the connection-level maximum receive window (finalMask.quicParams.maxConnectionReceiveWindow). Positive values below 16 KiB fail validation; 0 is the default. This is the last of the four window floor checks.

Source

Thrown at infra/conf/transport_internet.go:270

			default:
				return nil, errors.New("unknown congestion control: ", c.FinalMask.QuicParams.Congestion, ", valid values: reno, bbr, brutal, force-brutal")
			}

			if (c.FinalMask.QuicParams.UdpHop.Interval.From != 0 && c.FinalMask.QuicParams.UdpHop.Interval.From < 5) || (c.FinalMask.QuicParams.UdpHop.Interval.To != 0 && c.FinalMask.QuicParams.UdpHop.Interval.To < 5) {
				return nil, errors.New("Interval must be at least 5")
			}

			if c.FinalMask.QuicParams.InitStreamReceiveWindow > 0 && c.FinalMask.QuicParams.InitStreamReceiveWindow < 16384 {
				return nil, errors.New("InitStreamReceiveWindow must be at least 16384")
			}
			if c.FinalMask.QuicParams.MaxStreamReceiveWindow > 0 && c.FinalMask.QuicParams.MaxStreamReceiveWindow < 16384 {
				return nil, errors.New("MaxStreamReceiveWindow must be at least 16384")
			}
			if c.FinalMask.QuicParams.InitConnectionReceiveWindow > 0 && c.FinalMask.QuicParams.InitConnectionReceiveWindow < 16384 {
				return nil, errors.New("InitConnectionReceiveWindow must be at least 16384")
			}
			if c.FinalMask.QuicParams.MaxConnectionReceiveWindow > 0 && c.FinalMask.QuicParams.MaxConnectionReceiveWindow < 16384 {
				return nil, errors.New("MaxConnectionReceiveWindow must be at least 16384")
			}
			if c.FinalMask.QuicParams.MaxIdleTimeout != 0 && (c.FinalMask.QuicParams.MaxIdleTimeout < 4 || c.FinalMask.QuicParams.MaxIdleTimeout > 120) {
				return nil, errors.New("MaxIdleTimeout must be between 4 and 120")
			}
			if c.FinalMask.QuicParams.KeepAlivePeriod != 0 && (c.FinalMask.QuicParams.KeepAlivePeriod < 2 || c.FinalMask.QuicParams.KeepAlivePeriod > 60) {
				return nil, errors.New("KeepAlivePeriod must be between 2 and 60")
			}
			if c.FinalMask.QuicParams.MaxIncomingStreams != 0 && c.FinalMask.QuicParams.MaxIncomingStreams < 8 {
				return nil, errors.New("MaxIncomingStreams must be at least 8")
			}

			if c.FinalMask.QuicParams.Debug {
				os.Setenv("HYSTERIA_BBR_DEBUG", "true")
				os.Setenv("HYSTERIA_BRUTAL_DEBUG", "true")
			}

			config.QuicParams = &internet.QuicParams{
				Congestion: c.FinalMask.QuicParams.Congestion,

View on GitHub (pinned to 7d214f8b09)

Solutions

Set maxConnectionReceiveWindow >= 16384 bytes (e.g. 8388608).
Or omit it for the default.
Verify max >= init for both stream and connection windows.

Example fix

// before
"quicParams": { "maxConnectionReceiveWindow": 8192 }
// after
"quicParams": { "maxConnectionReceiveWindow": 8388608 }

Defensive patterns

Strategy: validation

Validate before calling

func connectionWindowsOK(qp map[string]any) bool {
    return windowFloorOK(qp["maxConnectionReceiveWindow"], 16384) &&
        windowFloorOK(qp["initConnectionReceiveWindow"], 16384)
}

Prevention

Tune the four window knobs as a set; do not shrink max below init.
Use byte literals (e.g. 8388608) in configs to avoid unit ambiguity.
Re-run config validation after any QUIC tuning change.

When it happens

Trigger: "quicParams": { "maxConnectionReceiveWindow": 8192 }.

Common situations: Memory-tuning that crosses the protocol floor; inconsistent window sets where max < init; stale values from older configs.

Related errors

AI-assisted analysis of XTLS/Xray-core@7d214f8b09 (2026-08-15). Data as JSON: /api/errors/bde8e368c9f6fb48. Report an issue: GitHub.