When a unicast request is received before the first view is received (which sets the channel to connected), and a response to the request is sent, then the response sending throws an exception as the channel hasn't yet been connected (see stack trace below).

Reason:
What happens when a channel is created is:

• JChannel.connect() is called by the user
• As part of this, start() is called in every protocol
• UDP.start() creates the sockets and starts the socket listener which will receive messages from now on and pass them up
• Connect() now sends a JOIN request to the coordinator and receives the JOIN response or become singleton member
• In either case, a new view is installed, which sets the channel to connected

If, between starting the socket listener and setting the channel to connected, a unicast message is received, and it sends a response then you'll get the behavior below.

In NAKACK2, this doesn't happen, as all messages received in the same time frame are queued and replayed when the channel becomes connected.

I could implement the same behavior for UNICAST2, but I'd like to know more about how you got into this situation. What were the steps executed ?

The only way this could happen I can think of is that the coordinator sent a unicast message to P right after it sent the JOIN response to P, e.g. in the view change which includes P. Does this happen in Infinispan code ?

13:29:45,467 ERROR (OOB-1,ISPN,NodeB-50197 [UNICAST2] couldn't deliver OOB message [dst: NodeB-50197, src: NodeA-40526 (3 headers), size=118 bytes, flags=OOB|DONT_BUNDLE]
java.lang.IllegalStateException: channel is not connected
at org.jgroups.blocks.MessageDispatcher$ProtocolAdapter.down(MessageDispatcher.java:621)
at org.jgroups.blocks.RequestCorrelator.handleRequest(RequestCorrelator.java:535)
at org.jgroups.blocks.RequestCorrelator.receiveMessage(RequestCorrelator.java:390)
at org.jgroups.blocks.RequestCorrelator.receive(RequestCorrelator.java:248)
at org.jgroups.blocks.MessageDispatcher$ProtocolAdapter.up(MessageDispatcher.java:604)
at org.jgroups.JChannel.up(JChannel.java:688)
at org.jgroups.stack.ProtocolStack.up(ProtocolStack.java:1020)
at org.jgroups.protocols.FRAG2.up(FRAG2.java:181)
at org.jgroups.protocols.FC.up(FC.java:479)
at org.jgroups.protocols.pbcast.GMS.up(GMS.java:896)
at org.jgroups.protocols.pbcast.STABLE.up(STABLE.java:244)
at org.jgroups.protocols.UNICAST2.handleDataReceived(UNICAST2.java:736)
at org.jgroups.protocols.UNICAST2.up(UNICAST2.java:414)
at org.jgroups.protocols.pbcast.NAKACK2.up(NAKACK2.java:606)
at org.jgroups.protocols.MERGE2.up(MERGE2.java:205)
at org.jgroups.protocols.Discovery.up(Discovery.java:359)
at org.jgroups.protocols.TP.passMessageUp(TP.java:1294)
at org.jgroups.protocols.TP$IncomingPacket.handleMyMessage(TP.java:1857)
at org.jgroups.protocols.TP$IncomingPacket.run(TP.java:1830)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1110)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:603)
at java.lang.Thread.run(Thread.java:722)