# Non-standard MQTT options

## Maximum Client Id Size

Set the maximum size for client ids, MQTT v3.1 specifies a limit of 23 characters.

```
max_client_id_size = 23
```

This option default to `23`.

## Maximum Topic Depth

Usually, you'll configure permissions on your topic structures using ACLs. In addition to that, `topic_max_depth` sets a global maximum value for topic levels. This protects the broker from clients subscribing to arbitrary deep topic levels.

```
topic_max_depth = 20
```

The default value for `topic_max_depth` is 10. As an example, this value will allow topics like `a/b/c/d/e/f/g/h/i/k`, that is 10 levels. A client running into the topic depth limit will be disconnected and an error will be logged.

## Persistent Client Expiration

This option allows persistent clients (those with `clean_session` set to `false`) to be removed if they do not reconnect within a certain time frame.

{% hint style="warning" %}
This is a non-standard option. As far as the MQTT specification is concerned, persistent clients are persisted forever.
{% endhint %}

The expiration period should be an integer followed by one of `h`, `d`, `w`, `m`, `y` for hour, day, week, month, and year; or `never`:

```
persistent_client_expiration = 1w
```

This option defaults to `never`.

## Message Size Limit

Limit the maximum publish payload size in bytes that VerneMQ allows. Messages that exceed this size won't be accepted.

```
max_message_size = 0
```

Defaults to `0`, which means that all valid messages are accepted. MQTT specification imposes a maximum payload size of 268435455 bytes.