code: protected void physicalCloseNotification() {
        if (TraceComponent.isAnyTracingEnabled() && tc.isEntryEnabled()) SibTr.entry(this, tc, "physicalCloseNotification");
        synchronized(connectionClosedLock) {
           connectionClosed = true;
        }
        if (TraceComponent.isAnyTracingEnabled() && tc.isEntryEnabled()) SibTr.exit(this, tc, "physicalCloseNotification");
     }
docs: Register notification that the physical underlying connection has been closed.