C# How to instruct a web server to cancel a request?

[darfur]

I'm writing an application that simplifies the interface to a few websites I use at my job. One of the web servers has a tendency to cause my program to throw a time out exception for a variety of reasons. This is an issue since this web site uses a persistent connection to keep you logged in. So if the request isn't finished and received by my program, the server keeps that open and after two time outs my program just becomes "hosed" and must be restarted. From my troubleshooting, I've found that after two times outs, the server puts roughly a 20 second delay before it returns a new request to you. After three time outs, this delay becomes roughly a minute. This is longer than my program's time out, but using a real web browser I could access the page, but only after a very long delay.

Is there a way to tell the server to abort the request(s) that timed out on the client so that these artificial delays do not happen? I've really had a hard time finding anything online about this. Most search results give me something about closing the request on the client only or SQL connections. I have tried closing the web request (not just the stream) and as you can see in the code below, closing the requests in the exceptions (although I'd think that would happen naturally when the function exited). I'm not really sure what else I could try and do.

If this is not possible, disconnecting my network connection and then reconnecting seems to fix the issue without restarting the program. Would there a way to tell my network adapter to close all connections with the site?

            private String SendRequestTo(String method, Byte[] requestBytes, Uri destination, String refer, NetworkCredential credentials, ThreadSource source)
            {
                HttpWebRequest webRequest = (HttpWebRequest)WebRequest.Create(destination);
                webRequest.Method = method;
                webRequest.Accept = "*/*";
                webRequest.AllowAutoRedirect = false;
                webRequest.UserAgent = "Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:1.8.1.12) Gecko/20080201 Firefox/2.0.0.12";
                webRequest.CookieContainer = new CookieContainer();
                webRequest.ContentLength = requestBytes.Length;
                webRequest.ContentType = "application/x-www-form-urlencoded";
                webRequest.PreAuthenticate = true;
                webRequest.Referer = refer;
                if (source == ThreadSource.WebCT || source == ThreadSource.Directory || source == ThreadSource.DLS || source == ThreadSource.VistaKeepAlive)
                    webRequest.Timeout = 10000;
 
                if (source == ThreadSource.Vista)
                    webRequest.Timeout = 30000;
 
                webRequest.Credentials = credentials;
                webRequest.CookieContainer.Add(cookies.GetCookies(destination));
 
                Stream reqStream = null;
                StreamReader stream = null;
                HttpWebResponse webResponse = null;
 
                try
                {
                    if (method == "post")
                    {
                        reqStream = webRequest.GetRequestStream();
                        reqStream.Write(requestBytes, 0, requestBytes.Length);
                        reqStream.Close();
                    }
 
                    webResponse = (HttpWebResponse)webRequest.GetResponse();
                    if (webRequest.HaveResponse)
                    {
                        foreach (Cookie retCookie in webResponse.Cookies)
                        {
                            bool cookieFound = false;
                            foreach (Cookie oldCookie in cookies.GetCookies(destination))
                            {
                                if (retCookie.Name.Equals(oldCookie.Name))
                                {
                                    oldCookie.Value = retCookie.Value;
                                    cookieFound = true;
                                }
                            }
                            if (!cookieFound)
                                cookies.Add(retCookie);
                        }
 
                        if ((webResponse.StatusCode == HttpStatusCode.Found) || (webResponse.StatusCode == HttpStatusCode.Redirect) || (webResponse.StatusCode == HttpStatusCode.Moved) || (webResponse.StatusCode == HttpStatusCode.MovedPermanently))
                        {
                            WebHeaderCollection headers = webResponse.Headers;
                            return SendRequestTo(method, requestBytes, new Uri(headers["location"]), refer, credentials, source);
                        }
 
                        stream = new StreamReader(webResponse.GetResponseStream());
                        String responseString = stream.ReadToEnd();
                        stream.Close();
                        return responseString;
                    }
                }
                catch (WebException e)
                {
                    try
                    {
                        reqStream.Close();
                    }
                    catch (Exception)
                    {
                    }
 
                    try
                    {
                        stream.Close();
                    }
                    catch (Exception)
                    {
                    }
 
                    try
                    {
                        webResponse.Close();
                    }
                    catch (Exception)
                    {
                    }
 
                    throw new Exception("Exception occured while sending request.", e);
                }
 
                throw new Exception("No response received from host.");
            }