/**
     * Receives a response from the remote endpoint.
     *
     * @param response the response object to populate
     * @throws IOException if an I/O error occurs
     */
    protected abstract void doRecv(Response response) throws IOException;

    /**
     * Skips an unexpected or unrecognized message.
     *
     * @throws IOException if an I/O error occurs
     */

    /**
     * Receive a response from the transport
     *
     * @param response the response object to populate
     * @throws IOException if an I/O error occurs
     */
    protected abstract void doRecv(Response response) throws IOException;

    /**
     * Skip a response with the given key
     *
     * @param key the key of the response to skip
     * @throws IOException if an I/O error occurs
     */

        /* Unless key returned is null or invalid Transport.loop() always
         * calls doRecv() after and no one else but the transport thread
         * should call doRecv(). Therefore it is ok to expect that the data
         * in sbuf will be preserved for copying into BUF in doRecv().
         */

        return key;
    }

    @Override