Web Service Authentication in ASP.NET (C#)

[Sumit]

Hi guys,

I'm writing a web service and I don't know how I can secure it by
forcing our clients to use a username/password. I've used SalesForce
a
lot and they ask for a username/password once and any subsequent
calls
you don't need to authenticate anymore. How can this be done in
asp.net (C#)?


Ex.I make a call


result = Object.Connect (username, password)
all my other calls I would send result in the method???


Thank you
Sumit