private void sendPart(int session, FileReference ref, int partId, byte [] buf) {
            Request request = new Request(FileReceiver.RECEIVE_PART_METHOD);
            request.parameters().add(new StringValue(ref.value()));
            request.parameters().add(new Int32Value(session));
            request.parameters().add(new Int32Value(partId));
            request.parameters().add(new DataValue(buf));
            invokeRpcIfValidConnection(request);
            if (request.isError()) {
                throw new IllegalArgumentException("Failed delivering part of reference '" + ref.value() + "' to " +
                                                           target.toString() + " with error: '" + request.errorMessage() + "'.");
            } else {
                if (request.returnValues().get(0).asInt32() != 0) {
                    throw new IllegalArgumentException("Unknown error from target '" + target.toString() + "' during rpc call " + request.methodName());
                }
            }
        }
Returns true if empty sentinel config can be returned when a request from a host that is
not part of an application asks for sentinel config
configserver/src/main/java/com/yahoo/vespa/config/server/rpc/RpcServer.java