P3P是什么 P3P（Platform for Privacy Preferences）是W3C公布的一项隐私保护推荐标准，以为用户提供隐私保护。   P3P标准的构想是：Web 站点的隐私策略应该告之访问者该站点所收集的信息类型、信息将提供给哪些人、信息将被保留多少时间及其使用信息的方式，如站点应做诸如 “本网站将监测您所访问的页面以提高站点的使用率”或“本网站将尽可能为您提供更合适的广告”等申明。访问支持P3P网站的用户有权查看站点隐私报告，然 后决定是否接受cookie 或是否使用该网站。   利用P3P实现跨域 有别于JS跨域、IFRAME跨域等的常用处理办法，通过发送P3P头信息而实现的跨域。   PHP 使用P3P协议 header(‘P3P: CP="CURa ADMa DEVa PSAo PSDo OUR BUS UNI PUR INT DEM STA PRE COM NAV OTC NOI DSP COR"‘); 　　 JS 使用P3P协议 xmlhttp.setRequestHeader( "P3P" , ‘CP="CURa ADMa DEVa PSAo PSDo OUR BUS UNI PUR INT DEM STA PRE COM NAV OTC NOI DSP COR"‘ ); 　　 ASP.NET 使用P3P协议 HttpContext.Current.Response.AddHeader("p3p", "CP="IDC DSP COR ADM DEVi TAIi PSA PSD IVAi IVDi CONi HIS OUR IND CNT"") 　　 JSP 使用P3P协议 response.addHeader("P3P", "CP=IDC DSP COR ADM DEVi TAIi PSA PSD IVAi IVDi CONi HIS OUR IND CNT");   策略说明 (http://www.w3.org/2002/04/P3Pv1-header.html) compact-policy-field         =   `CP="` compact-policy `"` compact-policy                = compact-token *(" " compact-token) compact-token                = compact-access           |                                           compact-disputes         |                                           compact-remedies         |                                           compact-non-identifiable |                                           compact-purpose          |                                           compact-recipient        |                                           compact-retention        |                                           compact-categories       |                                           compact-test   compact-access           = "NOI" | "ALL" | "CAO" | "IDC" | "OTI" | "NON"  compact-disputes            = "DSP"  compact-remedies          = "COR" | "MON" | "LAW"  compact-non-identifiable = "NID" compact-purpose           = "CUR"        | "ADM" [creq] | "DEV" [creq] | "TAI" [creq] |                                         "PSA" [creq] | "PSD" [creq] | "IVA" [creq] | "IVD" [creq] |                                          "CON" [creq] | "HIS" [creq] | "TEL" [creq] | "OTP" [creq]   creq                              = "a" | "i" | "o"   compact-recipient        = "OUR" | "DEL" [creq] | "SAM" [creq] | "UNR" [creq] |                                          "PUB" [creq] | "OTR" [creq]   compact-retention          = "NOR" | "STP" | "LEG" | "BUS" | "IND"  compact-category           = "PHY" | "ONL" | "UNI" | "PUR" | "FIN" | "COM" |                                            "NAV" | "INT" | "DEM" | "CNT" | "STA" | "POL" |                                           "HEA" | "PRE" | "LOC" | "GOV" | "OTC"   compact-test                  = "TST"   简洁策略 常用的简洁策略的 P3P头为 -   P3P : CP=CAO PSA OUR 最简洁的写法是 P3P:CP=.
compact-access(访问):  CAO - contact-and-other  允许第三方cookie的读写)
compact-purpose(目的):  PSA -  pseudo-analysis .目的就是做身份验证、分析
compact-recipient(受体):  OUR - ours 声明使用相关信息的人是谁,ours 第三方自己
    浏览器支持情况 浏览器 默认允许第三方Cookie 是否支持P3P 禁止第三方Cookie后,配置P3P简明策略头的效果 IE6 否 是 HTTP可读写Cookie JS可读Cookie 首次读到P3P头,JS无写Cookie权限.第二次才OK (第二次.直接Cache.也不行.除非第一次非Cache并读到p3p头.后面我会提到解决方案.) 应当避免JS的写操作 IE7-IE9 否 是 HTTP、JS,可随意读写. FireFox 是 否 HTTP、JS都不可读写 Chrome 是 部分支持,趋势-否 趋势为HTTP、JS可读不可写. Safari 否 否 HTTP、JS可读不可写
可以借助借助Post提交表单,实现写操作. Opera 是 否 JS可读写 HTTP可读不可写.   P3P策略      相关资料：http://www.w3.org/P3P/