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### Control Frames

Control frames are small (less than 128 bytes) messages entirely contained in an individual WebSocket frame. They may be sent at any time by either peer on an established connection, and can appear in between continuation frames for a message. There are three types of control frames: ping, pong, and close.

A sent ping indicates a request that the sender wants to receive a pong. A pong is a response to a ping. Pongs may be sent unsolicited, at any time. One use for an unsolicited pong is to inform the remote peer that the session is still active after a long period of inactivity. A close frame indicates that the remote peer wishes to close the WebSocket connection. The connection is considered gracefully closed when each side has sent and received a close frame.

During read operations, Beast automatically reads and processes control frames. If a control callback is registered, the callback is notified of the incoming control frame. The implementation will respond to pings automatically. The receipt of a close frame initiates the WebSocket close procedure, eventually resulting in the error code `error::closed` being delivered to the caller in a subsequent read operation, assuming no other error takes place.

A consequence of this automatic behavior is that caller-initiated read operations can cause socket writes. However, these writes will not compete with caller-initiated write operations. For the purposes of correctness with respect to the stream invariants, caller-initiated read operations still only count as a read. This means that callers can have a simultaneously active read, write, and ping/pong operation in progress, while the implementation also automatically handles control frames.

##### Control Callback

Ping, pong, and close messages are control frames which may be sent at any time by either peer on an established WebSocket connection. They are sent using the functions `ping`, `pong`. and `close`. To be notified of control frames, callers may register a control callback using `control_callback`. The object provided with this option should be callable with the following signature:

    ws.control_callback(
        [](frame_type kind, string_view payload)
        {
            // Do something with the payload
            boost::ignore_unused(kind, payload);
        });

When a control callback is registered, it will be invoked for all pings, pongs, and close frames received through either synchronous read functions or asynchronous read functions. The type of frame and payload text are passed as parameters to the control callback. If the frame is a close frame, the close reason may be obtained by calling `reason`.

Unlike regular completion handlers used in calls to asynchronous initiation functions, the control callback only needs to be set once. The callback is not reset after being called. The same callback is used for both synchronous and asynchronous reads. The callback is passive; in order to be called, a stream read operation must be active.

| Note
When an asynchronous read function receives a control frame, the control callback is invoked in the same manner as that used to invoke the final completion handler of the corresponding read function.

##### Close Frames

The WebSocket protocol defines a procedure and control message for initiating a close of the session. In this procedure, a host requests the close by sending a close frame. To request a close use a close function such as `close` or `async_close`:

    ws.close(close_code::normal);

The close function will send a close frame, read and discard incoming message data until receiving a close frame, and then shut down the underlying connection before returning.

When a close frame is received by during a read operation, the implementation will automatically respond with a close frame and then shut down the underlying connection before returning. In this case, the read operation will complete with the code `error::closed`. This indicates to the caller that the connection has been closed cleanly.

| Important
To receive the `error::closed` error, a read operation is required.

##### Auto-fragment

To ensure timely delivery of control frames, large outgoing messages can be broken up into smaller sized frames. The automatic fragment option turns on this feature, and the write buffer size option determines the maximum size of the fragments:

    ws.auto_fragment(true);
    ws.write_buffer_bytes(16384);