code: func (s *UDPSession) SetMtu(mtu int) bool {
  	if mtu > mtuLimit {
  		return false
  	}
  
  	s.mu.Lock()
  	defer s.mu.Unlock()
  	s.kcp.SetMtu(mtu)
  	return true
  }
docs: // SetMtu sets the maximum transmission unit(not including UDP header)