How i can retrieve the callers identity from the server side

i hosted my service as windows authentication by default it will authenticate with logged in user but on the server side i am not able to get the callers identity all i can get is application pool identity, any guiedence how to get the callers identity /?

FYI, if i pass like this i can able to get callers identity on server side not sure i am doing something wrong or not..

clientcredentials.windows.clientcredential.username=username

clientcredentials.windows.clientcredential.domain=domainname

clientcredentials.windows.clientcredential.password=password