code: func chksum(p []byte, csum []byte) {
  	cklen := len(p)
  	s := uint32(0)
  	for i := 0; i < (cklen - 1); i += 2 {
  		s += uint32(p[i+1])<<8 | uint32(p[i])
  	}
  	if cklen&1 == 1 {
  		s += uint32(p[cklen-1])
  	}
  	s = (s >> 16) + (s & 0xffff)
  	s = s + (s >> 16)
  	s = ^s
  
  	csum[0] = uint8(s & 0xff)
  	csum[1] = uint8(s >> 8)
  }
docs: // compute's 1's complement checksum