@Override
    public void attach(NetworkOwner owner) {
        if (this.owner != null) {
            throw new IllegalStateException("Network is already attached to another owner.");
        }
        this.owner = owner;

        sendAdapters.put(new Version(6,149), new RPCSendV2(this));
    }
The network uses a cache of RPC targets (see {@link RPCTargetPool}) that allows it to save time by reusing open
connections. It works by keeping a set of the most recently used targets open. Calling this method forces all
unused connections to close immediately.
messagebus/src/main/java/com/yahoo/messagebus/network/rpc/RPCNetwork.java