I have been working on Lync 2013 split domain topology and thought of sharing peer-peer VOIP call flow between an on-premise user and online user.
Let’s say , you have a Lync 2013 split domain topology. User-A is provisioned in on-premise Lync 2013 pool and user-B is part of office 365 Lync online. Please refer figure-1; (Arrows highlighted in green are sip signaling traffic and red is media path )
• User-A and User-B will try to sign-in (first time ), Lync client will locate the SRV record published in local DNS and send a registration request to on-premise FE server.
• User-A will sign-in instantly (after authentication) and User-B will be redirected to office 365 for registration and sign-in. Both users eventually sign-in successfully.
• User-A try to initiate a VOIP call to User-B.
• On-premise FE pool will try to find out user-B’s status. But User-B is not part of on-premise pool , it will forward the request to office 365 Lync online.
• Office 365 Lync online server will send an invite to User-B.
• User-A and User-B will start media negotiation and share the IP address and port details for media connectivity.
• Lync client will learn both clients are located in same network during negotiation.
• Lync client will take best path for media route which is local.
• Sip signaling will continue to follow the same path.