XTLS/Xray-core · error

invalid mKCP TTI:

Error message

invalid mKCP TTI: 

What it means

Thrown by KCPConfig.Build() when the mKCP TTI (transmission time interval, in milliseconds) is outside the accepted 10-1000 range. TTI controls how often mKCP flushes packets; values below 10 ms waste CPU and values above 1000 ms stall the link, so the builder rejects both. The offending value is included in the message.

Source

Thrown at infra/conf/transport_method.go:566

	}
	if c.UpCap != nil {
		config.UplinkCapacity = *c.UpCap
	}
	if c.DownCap != nil {
		config.DownlinkCapacity = *c.DownCap
	}
	if c.CwndMultiplier != nil {
		config.CwndMultiplier = *c.CwndMultiplier
	}
	if c.MaxSendingWindow != nil {
		config.MaxSendingWindow = *c.MaxSendingWindow
	}

	if config.Mtu < 21 {
		return nil, errors.New("Mtu must be at least 21").AtError()
	}
	if config.Tti < 10 || config.Tti > 1000 {
		return nil, errors.New("invalid mKCP TTI: ", c.Tti).AtError()
	}
	if config.CwndMultiplier < 1 {
		return nil, errors.New("CwndMultiplier must be at least 1").AtError()
	}
	if config.GetSendingBufferSize() == 0 {
		return nil, errors.New("MaxSendingWindow must be >= Mtu").AtError()
	}

	return config, nil
}

type GRPCConfig struct {
	Authority           string `json:"authority"`
	ServiceName         string `json:"serviceName"`
	MultiMode           bool   `json:"multiMode"`
	IdleTimeout         int32  `json:"idle_timeout"`
	HealthCheckTimeout  int32  `json:"health_check_timeout"`
	PermitWithoutStream bool   `json:"permit_without_stream"`

View on GitHub (pinned to 7d214f8b09)

Solutions

1. Set tti between 10 and 1000 inclusive (10, 20, 30, 40, 50 are the conventional choices).
2. Remove the tti field to use the default (50).

Example fix

// before
"kcpSettings": { "mtu": 1350, "tti": 5000 }
// after
"kcpSettings": { "mtu": 1350, "tti": 50 }

if kcp.Tti != 0 && (kcp.Tti < 10 || kcp.Tti > 1000) {
    return fmt.Errorf("mKCP tti %d outside 10-1000", kcp.Tti)
}

if _, err := kcpCfg.Build(); err != nil && strings.HasPrefix(err.Error(), "invalid mKCP TTI") {
    return fmt.Errorf("fix tti (10-1000 ms): %w", err)
}

When it happens

Trigger: Setting kcpSettings.tti to something like 5 or 2000 in the JSON config. Note there is no lower-bound protection for the zero value if the field is absent only when the default is applied; an explicitly bad value always trips this.

Common situations: Users copying tti from old docs suggesting aggressive values (e.g. 1ms 'low latency' tweaks), or fat-fingered values like 10000.

Related errors

• Mtu must be at least 21
• CwndMultiplier must be at least 1
• MaxSendingWindow must be >= Mtu
• invalid header
• gecko: invalid min/max packet size

AI-assisted analysis of XTLS/Xray-core@7d214f8b09 (2026-08-15). Data as JSON: /api/errors/84b50380e5760e35. Report an issue: GitHub.