Authentication on DomainService

I have a Silverlight Business Application, three project on it:
1. SLApp (Silverlight Application).
2. SLDataServices (ADO.NET Entity Framework, Domain Service, Metadata, Extensions for metadata).
3. SLWeb (ASp.NET WebSite with ASP.NET Membership enabled and working).
 
When I run the project a Login Page appears, you type user and pass. After that you logged on, so the Silverlight app loads (I pass user & pass parameter) and login in silverlight app too.
 
When the Ribbon Menu loads (via DomainDataSource) I see the error on the RequiredAuthentication Atributte on my DomainService in SLDataServices project. When I breakpointed on this line of code I can see that I have a cookie empty and the ServiceContext setted to null.
 
I'm using an Oracle Database on this project.
 
How can I obtain the user authenticated on my DomainService?
 
Thanks in advance.