[Serusers] Proxy to Proxy between different realm/domain

Hi all, Need you experts' help on SIP design. I've setup a testbed composed of two SER Registrar/Proxies in two different realm/domain(s) as below; <--Internal Private(192.168.x.0----><----External Public(x.x.x.0)---------> UA1<-----------------> SER1 <---------------------> SER2 <------------> UA2 Mydomain.org Registrar/Proxy Theirdomain.org Registrar/Proxy What is the right way to pass the user credential of UA1 to SER2 so that "INVITE" to UA2 will be accepted at SER2, knowing that UA1 user has its login ID/password to SER2 but it is locally "REGISTER"ed at SER1 in private domain? I guess I am having trouble understanding working mechanism of "inter" SIP realm or domain. Any comments will be appreciated. John Kim email: johnk(a)koreanet.us johnk(a)netcomusa.us