code: func PseudoHeaderChecksum(protocol TransportProtocolNumber, srcAddr net.IP, dstAddr net.IP) uint16 {
  	xsum := Checksum([]byte(srcAddr), 0)
  	xsum = Checksum([]byte(dstAddr), xsum)
  	return Checksum([]byte{0, uint8(protocol)}, xsum)
  }
docs: // PseudoHeaderChecksum calculates the pseudo-header checksum for the
  // given destination protocol and network address, ignoring the length
  // field. Pseudo-headers are needed by transport layers when calculating
  // their own checksum.