GZIP Compression causing web page expiration

Question

I have implemented GZIP compression on a few of my ASP.NET pages, using a class that inherits from System.Web.UI.Page, and implementing the OnLoad method to do the compression, like so:

    protected override void OnLoad(EventArgs e)
    {
        base.OnLoad(e);

        if (Internet.Browser.IsGZIPSupported())
        {
            base.Response.Filter = new GZipStream(base.Response.Filter, CompressionMode.Compress, true);
            base.Response.AppendHeader("Content-encoding", "gzip");
            base.Response.AppendHeader("Vary", "Content-encoding");
        }
        else if (Internet.Browser.IsDeflateSupported())
        {
            base.Response.Filter = new DeflateStream(base.Response.Filter, CompressionMode.Compress, true);
            base.Response.AppendHeader("Content-encoding", "deflate");
            base.Response.AppendHeader("Vary", "Content-encoding");
        }
    }

The IsGZIPSupported method just determines whether the browser supports GZIP, looking at the Accept-encoding request header, and the browser's user agent (IE5-6 are excluded from GZIP compression). However, with this code, I am getting the web page has expired message in IE, when I postback from the page and try to use the back button. Setting the cache control to private seems to fix the problem:

base.Response.Cache.SetCacheability(HttpCacheability.Private);

But I am not sure why, or whether this will cause other problems. I haven't set any caching for any other pages in the site, and the site is running on an intranet with only a dozen concurrent users, so performance isn't a big issue at the moment.

Answer 1

See this article on Vary header and WinInet/MSIE

It seems you should be sending Vary: Accept-Encoding instead of Vary: Content-Encoding, as the response will vary depending on the request header.