Instead of returning the “Service Unavailable” message to the client you can terminate the TCP connection. This can be helpful in environments where you have a Load Balancer which ’understands’ a TCP reset instead of “Service Unavailable”. In IIS 7.0 you can set it via the IIS manager UI in the Advanced Properties of the Application Pool.
This option is available in IIS 6.0 as well. But you need to set it using the adsutil.vbs script (or edit the metabase directly)
cscript adsutil.vbs set w3svc/apppools/loadbalancercapabilities 1