///|
/// 解析 UDP 头：src port 0 / dst port 2 / len 4（含 8 字节头）
fn parse_udp(data : Bytes, off : Int, ip_payload : Int) -> (Int, Int, Int)? {
  guard ip_payload >= 8 else { return None }
  let sport = u16(data, off)
  let dport = u16(data, off + 2)
  let udp_len = u16(data, off + 4)
  guard udp_len >= 8 && udp_len <= ip_payload else { return None }
  Some((sport, dport, udp_len - 8))
}