T3 is Oracle WebLogic Server’s proprietary RMI protocol. Basically, t3 protocol is a protocol that sits on top of Java RMI. There is some documentation available.
RMI communications in WebLogic Server use the T3 protocol to transport data between WebLogic Server and other Java programs, including clients and other WebLogic Server instances.
If you need to control T3 access through a firewall, you may use the protocol filter “T3”.
Determining Connection Availability in Cluster
Any two Java programs with a valid T3 connection—such as two server instances, or a server instance and a Java client—use periodic point-to-point “heartbeats” to announce and determine continued availability. Each end point periodically issues a heartbeat to the peer, and similarly, determines that the peer is still available based on continued receipt of heartbeats from the peer.
The frequency with which a server instance issues heartbeats is determined by the heartbeat interval, which by default is 60 seconds.
The number of missed heartbeats from a peer that a server instance waits before deciding the peer is unavailable is determined by the heartbeat period, which by default, is 4. Hence, each server instance waits up to 240 seconds, or 4 minutes, with no messages—either heartbeats or other communication—from a peer before deciding that the peer is unreachable.
Changing timeout defaults is not recommended.