how can I get IDs of all current sessions?
To get the session id, do this:
// In a user control or page
string sessionId = this.Session.SessionID;
// In a normal class, running in a asp.net app.
string sessionId = System.Web.HttpContext.Current.Session.SessionID;
You should not need to:
In a cookieless scenario, the session id is created when you access the Session object for the first time. This shouldn't matter much, because the moment you access the SessionID property, the session object is accessed.
For more info, look into this:
http://msdn.microsoft.com/en-us/library/system.web.sessionstate.httpsessionstate.sessionid.aspx
Note: The msdn examples have been written by monkeys.