code: func AddTCPOptionPadding(options []byte, offset int) int {
  	paddingToAdd := -offset & 3
  	// Now add any padding bytes that might be required to quad align the
  	// options.
  	for i := offset; i < offset+paddingToAdd; i++ {
  		options[i] = TCPOptionNOP
  	}
  	return paddingToAdd
  }
docs: // AddTCPOptionPadding adds the required number of TCPOptionNOP to quad align
  // the option buffer. It adds padding bytes after the offset specified and
  // returns the number of padding bytes added. The passed in options slice
  // must have space for the padding bytes.