You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: mtu.go
    path of file: ./repos/sliver/vendor/tailscale.com/net/tstun
    
    the code of the file until where you have to start completion: // Copyright (c) Tailscale Inc & AUTHORS
// SPDX-License-Identifier: BSD-3-Clause

package tstun

import (
	"tailscale.com/envknob"
)

// The MTU (Maximum Transmission Unit) of a network interface is the largest
// packet that can be sent or received through that interface, including all
// headers above the link layer (e.g. IP headers, UDP headers, Wireguard
// headers, etc.). We have to think about several different values of MTU:
//
// Wire MTU: Th
    
func DefaultTUNMTU() TUNMTU {
	if m, ok := envknob.LookupUintSized("TS_DEBUG_MTU", 10, 32); ok {
		return min(TUNMTU(m), maxTUNMTU)
	}

	debugPMTUD, _ := envknob.LookupBool("TS_DEBUG_ENABLE_PMTUD")
	if debugPMTUD {
		// TODO: While we are just probing MTU but not generating PTB,
		// this has to continue to return the safe MTU. When we add the
		// code to generate PTB, this will be:
		//
		// return WireToTUNMTU(maxProbedWireMTU)
		return safeTUNMTU
	}

	return safeTUNMTU
}