i want to read bytes from vpninterface and then write back to it

Question

i have setup a vpn service and then configure a vpn interface.

 private void setupVPN() {
        try {
            if (vpnInterface == null) {
                Builder builder = new Builder();
                builder.addAddress(VPN_ADDRESS, 32);
                builder.addRoute(VPN_ROUTE, 0);
                builder.setMtu(1499);
                vpnInterface = builder.setSession(getString(R.string.app_name)).setConfigureIntent(pendingIntent).establish();

            }
        } catch (Exception e) {
            Log.e(TAG, "error", e);
            System.exit(0);
        }
    }

i can read bytes from File Descriptor but how to write them back.

 FileChannel vpnInput = new FileInputStream(vpnFileDescriptor).getChannel();
            FileChannel vpnOutput = new FileOutputStream(vpnFileDescriptor).getChannel();
            Thread t = new Thread(new WriteVpnThread(vpnOutput, networkToDeviceQueue));
            t.start();

            try {
                ByteBuffer bufferToNetwork = null;
                while (!Thread.interrupted()) {
                    bufferToNetwork = ByteBufferPool.acquire();
                    int readBytes = vpnInput.read(bufferToNetwork);
                    if (readBytes > 0) {

                       bufferToNetwork.flip();
                    }
                }
            }

do i have to parse them and then write it back or is there any other way?