From Authentication Tools for Joomla! (JAuthTools)

HTTP SSO

HTTP SSO is usually reliant on another system to handle the actual user identification procedure, typically the web server. Web servers are usually very good at doing this and are much more efficient at doing so.

Active Directory Troubleshooting

If you're using Active Directory, be aware that the domain will typically be prefixed onto the provided username (e.g. DOMAIN\username). Because of this you will normally need to put your domain down level name with a slash at the end into the username replacement setting of the SSO - HTTP plugin.

This will also impact on the username that is handed to the user source. So depending on how your user source is configured, SSO may not work properly because if you are connecting to AD you will need a service account to find the user's details as you won't have their account to begin with.