Why does HttpCacheability.Private suppress ETags?

Question

While writing a custom IHttpHandler I came across a behavior that I didn't expect concerning the HttpCachePolicy object.

My handler calculates and sets an entity-tag (using the SetETag method on the HttpCachePolicy associated with the current response object). If I set the cache-control to public using the SetCacheability method everything works like a charm and the server sends along the e-tag header. If I set it to private the e-tag header will be suppressed.

Maybe I just haven't looked hard enough but I haven't seen anything in the HTTP/1.1 spec that would justify this behavior. Why wouldn't you want to send E-Tag to browsers while still prohibiting proxies from storing the data?

using System;
using System.Web;

public class Handler : IHttpHandler {
    public void ProcessRequest (HttpContext ctx) {
        ctx.Response.Cache.SetCacheability(HttpCacheability.Private);
        ctx.Response.Cache.SetETag("\"static\"");
        ctx.Response.ContentType = "text/plain";
        ctx.Response.Write("Hello World");
    }

    public bool IsReusable { get { return true; } }
}

Will return

Cache-Control: private
Content-Type: text/plain; charset=utf-8
Content-Length: 11

But if we change it to public it'll return

Cache-Control: public
Content-Type: text/plain; charset=utf-8
Content-Length: 11
Etag: "static"

I've run this on the ASP.NET development server and IIS6 so far with the same results. Also I'm unable to explicitly set the ETag using

Response.AppendHeader("ETag", "static")

Update: It's possible to append the ETag header manually when running in IIS7, I suspect this is caused by the tight integration between ASP.NET and the IIS7 pipeline.

Clarification: It's a long question but the core question is this: why does ASP.NET do this, how can I get around it and should I?

Update: I'm going to accept Tony's answer since it's essentially correct (go Tony!). I found that if you want to emulate the HttpCacheability.Private fully you can set the cacheability to ServerAndPrivate but you also have call cache.SetOmitVaryStar(true) otherwise the cache will add the Vary: * header to the output and you don't want that. I'll edit that into the answer when I get edit permissions (or if you see this Tony perhaps you could edit your answer to include that call?)

Answer 1

I think you need to use HttpCacheability.ServerAndPrivate

That should give you cache-control: private in the headers and let you set an ETag.

The documentation on that needs to be a bit better.

Edit: Markus found that you also have call cache.SetOmitVaryStar(true) otherwise the cache will add the Vary: * header to the output and you don't want that.