What are Cookieless Sessions ?

Sessions are identified by a unique identifier that can be read by using the SessionID property. When session state is enabled for an ASP.NET application, each request for a page in the application is examined for a SessionID value sent from the browser. If no SessionID value is supplied, ASP.NET starts a new session and the SessionID value for that session is sent to the browser with the response.

By default, SessionID values are stored in a cookie. However, you can also configure the application to store SessionID values in the URL for a "cookieless" session.

A session is considered active as long as requests continue to be made with the same SessionID value. If the time between requests for a particular session exceeds the specified time-out value in minutes, the session is considered expired. Requests made with an expired SessionID value result in a new session.

By default, the SessionID value is stored in a non-expiring session cookie in the browser. However, you can specify that session identifiers should not be stored in a cookie by setting the cookieless attribute to true in the sessionState section of the Web.config file.

The following example shows a Web.config file that configures an ASP.NET application to use cookieless session identifiers.

ASP.NET maintains cookieless session state by automatically inserting a unique session ID into the page's URL. For example, the following URL has been modified by ASP.NET to include the unique session ID lit3py55t21z5v55vlm25s55:

http://www.example.com/(S(lit3py55t21z5v55vlm25s55))/orderform.aspx 

When ASP.NET sends a page to the browser, it modifies any links in the page that use an application-relative path by embedding a session ID value in the links. (Links with absolute paths are not modified.) Session state is maintained as long as the user clicks links that have been modified in this manner. However, if the client rewrites a URL that is supplied by the application, ASP.NET may not be able to resolve the session ID and associate the request with an existing session. In that case, a new session is started for the request.

The session ID is embedded in the URL after the slash that follows the application name and before any remaining file or virtual directory identifier. This enables ASP.NET to resolve the application name before involving the SessionStateModule in the request.