Lost session in iframe child page.

we have a pag (parent.aspx in domain abc.com) with one iframe which will display a page (child.aspx) from domain def.com. Inside child.aspx, we will store variable into session. However, we notice lost session in child.aspx. 
do I need add following lines to child.aspx? Will change fix issue in different version browser? 
protected override void OnInit(EventArgs e)
        {
            Response.AddHeader("p3p", "CP=\"IDC DSP COR ADM DEVi TATi PSA PSD IVAi IVDi CONi HIS OUR IND CNT\"");
        } 
 
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