TakahikoKawasaki/nv-websocket-client

README

nv-websocket-client

Overview

High-quality WebSocket client implementation in Java which

complies with RFC 6455 (The WebSocket Protocol),
works on Java SE 1.5+ and Android,
supports all the frame types (continuation, binary, text, close, ping and pong),
provides a method to send a fragmented frame in addition to methods for unfragmented frames,
provides a method to get the underlying raw socket of a WebSocket to configure it,
provides a method for Basic Authentication,
provides a factory class which utilizes javax.net.SocketFactory interface,
provides a rich listener interface to hook WebSocket events,
has fine-grained error codes for fine-grained controllability on errors,
allows to disable validity checks on RSV1/RSV2/RSV3 bits and opcode of frames,
supports HTTP proxy, especially "Secure WebSocket" (wss) through "Secure Proxy" (https),
and supports RFC 7692 (Compression Extensions for WebSocket), also known as permessage-deflate (not enabled by default).

License

Apache License, Version 2.0

Maven

<dependency>
    <groupId>com.neovisionaries</groupId>
    <artifactId>nv-websocket-client</artifactId>
    <version>2.14</version>
</dependency>

Gradle

dependencies {
    compile 'com.neovisionaries:nv-websocket-client:2.14'
}

OSGi

Bundle-SymbolicName: com.neovisionaries.ws.client
Export-Package: com.neovisionaries.ws.client;version="2.14.0"

Source Code

https://github.com/TakahikoKawasaki/nv-websocket-client.git

JavaDoc

https://TakahikoKawasaki.github.io/nv-websocket-client/

Description

Create WebSocketFactory

WebSocketFactory is a factory class that creates WebSocket instances. The first step is to create a WebSocketFactory instance.

// Create a WebSocketFactory instance.
WebSocketFactory factory = new WebSocketFactory();

By default, WebSocketFactory uses SocketFactory.getDefault() for non-secure WebSocket connections (ws:) and SSLSocketFactory.getDefault() for secure WebSocket connections (wss:). You can change this default behavior by using WebSocketFactory.setSocketFactory method, WebSocketFactory.setSSLSocketFactory method and WebSocketFactory.setSSLContext method. Note that you don't have to call a setSSL* method at all if you use the default SSL configuration. Also note that calling setSSLSocketFactory method has no meaning if you have called setSSLContext method. See the description of WebSocketFactory.createSocket(URI) method for details.

The following is an example to set a custom SSL context to a WebSocketFactory instance. (Again, you don't have to call a setSSL* method if you use the default SSL configuration.)

// Create a custom SSL context.
SSLContext context = NaiveSSLContext.getInstance("TLS");

// Set the custom SSL context.
factory.setSSLContext(context);

// Disable manual hostname verification for NaiveSSLContext.
//
// Manual hostname verification has been enabled since the
// version 2.1. Because the verification is executed manually
// after Socket.connect(SocketAddress, int) succeeds, the
// hostname verification is always executed even if you has
// passed an SSLContext which naively accepts any server
// certificate. However, this behavior is not desirable in
// some cases and you may want to disable the hostname
// verification. You can disable the hostname verification
// by calling WebSocketFactory.setVerifyHostname(false).
factory.setVerifyHostname(false);

NaiveSSLContext used in the above example is a factory class to create an SSLContext which naively accepts all certificates without verification. It's enough for testing purposes. When you see an error message "unable to find valid certificate path to requested target" while testing, try NaiveSSLContext.

SNI (Server Name Indication) is supported since version 2.4. To set up server names, call either setServerNames(String[]) method or setServerName(String) method. If your system has SSLParameters.setServerNames(List<SNIServerName>) method, the method is called via reflection. Note that SSLParameters.setServerNames is a relatively new method and it is not available before Java 1.8 and Android 7.0 (API Level 24).

// Set a server name for SNI (Server Name Indication).
factory.setServerName("example.com");

HTTP Proxy

If a WebSocket endpoint needs to be accessed via an HTTP proxy, information about the proxy server has to be set to a WebSocketFactory instance before creating a WebSocket instance. Proxy settings are represented by ProxySettings class. A WebSocketFactory instance has an associated ProxySettings instance and it can be obtained by calling WebSocketFactory.getProxySettings() method.

// Get the associated ProxySettings instance.
ProxySettings settings = factory.getProxySettings();

ProxySettings class has methods to set information about a proxy server such as setHost method and setPort method. The following is an example to set a secure (https) proxy server.

// Set a proxy server.
settings.setServer("https://proxy.example.com");

If credentials are required for authentication at a proxy server, setId method and setPassword method, or setCredentials method can be used to set the credentials. Note that, however, the current implementation supports only Basic Authentication.

// Set credentials for authentication at a proxy server.
settings.setCredentials(id, password);

// Set a server name for SNI (Server Name Indication).
settings.setServerName("example.com");

Create WebSocket

WebSocket class represents a WebSocket. Its instances are created by calling one of createSocket methods of a WebSocketFactory instance. Below is the simplest example to create a WebSocket instance.

// Create a WebSocket. The scheme part can be one of the following:
// 'ws', 'wss', 'http' and 'https' (case-insensitive). The user info
// part, if any, is interpreted as expected. If a raw socket failed
// to be created, an IOException is thrown.
WebSocket ws = new WebSocketFactory().createSocket("ws://localhost/endpoint");

There are two ways to set a timeout value for socket connection. The first way is to call setConnectionTimeout(int timeout) method of WebSocketFactory.

// Create a WebSocket factory and set 5000 milliseconds as a timeout
// value for socket connection.
WebSocketFactory factory = new WebSocketFactory().setConnectionTimeout(5000);

// Create a WebSocket. The timeout value set above is used.
WebSocket ws = factory.createSocket("ws://localhost/endpoint");

The other way is to give a timeout value to a createSocket method.

// Create a WebSocket factory. The timeout value remains 0.
WebSocketFactory factory = new WebSocketFactory();

// Create a WebSocket with a socket connection timeout value.
WebSocket ws = factory.createSocket("ws://localhost/endpoint", 5000);

The timeout value is passed to connect(SocketAddress, int) method of java.net.Socket.

Register Listener

After creating a WebSocket instance, you should call addListener method to register a WebSocketListener that receives WebSocket events. WebSocketAdapter is an empty implementation of WebSocketListener interface.

// Register a listener to receive WebSocket events.
ws.addListener(new WebSocketAdapter() {
    @Override
    public void onTextMessage(WebSocket websocket, String message) throws Exception {
        // Received a text message.
        ......
    }
});

The table below is the list of callback methods defined in WebSocketListener interface.

Method	Description
`handleCallbackError`	Called when an `onXxx()` method threw a `Throwable`.
`onBinaryFrame`	Called when a binary frame was received.
`onBinaryMessage`	Called when a binary message was received.
`onCloseFrame`	Called when a close frame was received.
`onConnected`	Called after the opening handshake succeeded.
`onConnectError`	Called when `connectAsynchronously()` failed.
`onContinuationFrame`	Called when a continuation frame was received.
`onDisconnected`	Called after a WebSocket connection was closed.
`onError`	Called when an error occurred.
`onFrame`	Called when a frame was received.
`onFrameError`	Called when a frame failed to be read.
`onFrameSent`	Called when a frame was sent.
`onFrameUnsent`	Called when a frame was not sent.
`onMessageDecompressionError`	Called when a message failed to be decompressed.
`onMessageError`	Called when a message failed to be constructed.
`onPingFrame`	Called when a ping frame was received.
`onPongFrame`	Called when a pong frame was received.
`onSendError`	Called when an error occurred on sending a frame.
`onSendingFrame`	Called before a frame is sent.
`onSendingHandshake`	Called before an opening handshake is sent.
`onStateChanged`	Called when the state of WebSocket changed.
`onTextFrame`	Called when a text frame was received.
`onTextMessage`	Called when a text message was received.
`onTextMessageError`	Called when a text message failed to be constructed.
`onThreadCreated`	Called after a thread was created.
`onThreadStarted`	Called at the beginning of a thread's run() method.
`onThreadStopping`	Called at the end of a thread's run() method.
`onUnexpectedError`	Called when an uncaught throwable was detected.

Configure WebSocket

Before starting a WebSocket opening handshake with the server, you can configure the WebSocket instance by using the following methods.

METHOD	DESCRIPTION
`addProtocol`	Adds an element to `Sec-WebSocket-Protocol`.
`addExtension`	Adds an element to `Sec-WebSocket-Extensions`.
`addHeader`	Adds an arbitrary HTTP header.
`setUserInfo`	Adds `Authorization` header for Basic Authentication.
`getSocket`	Gets the underlying `Socket` instance to configure it. Note that this may return `null` since version 2.9. Consider using `getConnectedSocket()` as necessary.
`getConnectedSocket`	Establishes and gets the underlying `Socket` instance to configure it. Available since version 2.9.
`setExtended`	Disables validity checks on RSV1/RSV2/RSV3 and opcode.
`setFrameQueueSize`	Set the size of the frame queue for congestion control.
`setMaxPayloadSize`	Set the maximum payload size.
`setMissingCloseFrameAllowed`	Set to whether to allow the server to close the connection without sending a close frame.

Connect To Server

By calling connect() method, connection to the server is established and a WebSocket opening handshake is performed synchronously. If an error occurred during the handshake, a WebSocketException would be thrown. Instead, if the handshake succeeds, the connect() implementation creates threads and starts them to read and write WebSocket frames asynchronously.

try
{
    // Connect to the server and perform an opening handshake.
    // This method blocks until the opening handshake is finished.
    ws.connect();
}
catch (OpeningHandshakeException e)
{
    // A violation against the WebSocket protocol was detected
    // during the opening handshake.
}
catch (HostnameUnverifiedException e)
{
    // The certificate of the peer does not match the expected hostname.
}
catch (WebSocketException e)
{
    // Failed to establish a WebSocket connection.
}

In some cases, connect() method throws OpeningHandshakeException which is a subclass of WebSocketException (since version 1.19). OpeningHandshakeException provides additional methods such as getStatusLine(), getHeaders() and getBody() to access the response from a server. The following snippet is an example to print information that the exception holds.

catch (OpeningHandshakeException e)
{
    // Status line.
    StatusLine sl = e.getStatusLine();
    System.out.println("=== Status Line ===");
    System.out.format("HTTP Version  = %s\n", sl.getHttpVersion());
    System.out.format("Status Code   = %d\n", sl.getStatusCode());
    System.out.format("Reason Phrase = %s\n", sl.getReasonPhrase());

    // HTTP headers.
    Map<String, List<String>> headers = e.getHeaders();
    System.out.println("=== HTTP Headers ===");
    for (Map.Entry<String, List<String>> entry : headers.entrySet())
    {
        // Header name.
        String name = entry.getKey();

        // Values of the header.
        List<String> values = entry.getValue();

        if (values == null || values.size() == 0)
        {
            // Print the name only.
            System.out.println(name);
            continue;
        }

        for (String value : values)
        {
            // Print the name and the value.
            System.out.format("%s: %s\n", name, value);
        }
    }
}

Also, connect() method throws HostnameUnverifiedException which is a subclass of WebSocketException (since version 2.1) when the certificate of the peer does not match the expected hostname.

Connect To Server Asynchronously

The simplest way to call connect() method asynchronously is to use connectAsynchronously() method. The implementation of the method creates a thread and calls connect() method in the thread. When the connect() call failed, onConnectError() of WebSocketListener would be called. Note that onConnectError() is called only when connectAsynchronously() was used and the connect() call executed in the background thread failed. Neither direct synchronous connect() nor connect(ExecutorService) (described below) will trigger the callback method.

// Connect to the server asynchronously.
ws.connectAsynchronously();

Another way to call connect() method asynchronously is to use connect(ExecutorService) method. The method performs a WebSocket opening handshake asynchronously using the given ExecutorService.

// Prepare an ExecutorService.
ExecutorService es = Executors.newSingleThreadExecutor();

// Connect to the server asynchronously.
Future<WebSocket> future = ws.connect(es);

try
{
    // Wait for the opening handshake to complete.
    future.get();
}
catch (ExecutionException e)
{
    if (e.getCause() instanceof WebSocketException)
    {
        ......
    }
}

The implementation of connect(ExecutorService) method creates a Callable<WebSocket> instance by calling connectable() method and passes the instance to submit(Callable) method of the given ExecutorService. What the implementation of call() method of the Callable instance does is just to call the synchronous connect().

Send Frames

WebSocket frames can be sent by sendFrame method. Other sendXxx methods such as sendText are aliases of sendFrame method. All of the sendXxx methods work asynchronously. However, under some conditions, sendXxx methods may block. See Congestion Control for details.

Below are some examples of sendXxx methods. Note that in normal cases, you don't have to call sendClose method and sendPong method (or their variants) explicitly because they are called automatically when appropriate.

// Send a text frame.
ws.sendText("Hello.");

// Send a binary frame.
byte[] binary = ......;
ws.sendBinary(binary);

// Send a ping frame.
ws.sendPing("Are you there?");

If you want to send fragmented frames, you have to know the details of the specification (5.4. Fragmentation). Below is an example to send a text message ("How are you?") which consists of 3 fragmented frames.

// The first frame must be either a text frame or a binary frame.
// And its FIN bit must be cleared.
WebSocketFrame firstFrame = WebSocketFrame
    .createTextFrame("How ")
    .setFin(false);

// Subsequent frames must be continuation frames. The FIN bit of
// all continuation frames except the last one must be cleared.
// Note that the FIN bit of frames returned from
// WebSocketFrame.createContinuationFrame() method is cleared,
// so the example below does not clear the FIN bit explicitly.
WebSocketFrame secondFrame = WebSocketFrame
    .createContinuationFrame("are ");

// The last frame must be a continuation frame with the FIN bit
// set. Note that the FIN bit of frames returned from
// WebSocketFrame.createContinuationFrame methods is cleared,
// so the FIN bit of the last frame must be set explicitly.
WebSocketFrame lastFrame = WebSocketFrame
    .createContinuationFrame("you?")
    .setFin(true);

// Send a text message which consists of 3 frames.
ws.sendFrame(firstFrame)
  .sendFrame(secondFrame)
  .sendFrame(lastFrame);

Alternatively, the same as above can be done like this.

ws.sendText("How ", false)
  .sendContinuation("are ")
  .sendContinuation("you?", true);

Send Ping/Pong Frames Periodically

You can send ping frames periodically by calling setPingInterval method with an interval in milliseconds between ping frames. This method can be called both before and after connect() method. Passing zero stops the periodical sending.

// Send a ping per 60 seconds.
ws.setPingInterval(60 * 1000);

// Stop the periodical sending.
ws.setPingInterval(0);

Likewise, you can send pong frames periodically by calling setPongInterval method. "A Pong frame MAY be sent unsolicited." (RFC 6455, 5.5.3. Pong)

You can customize payload of ping/pong frames that are sent automatically by using setPingPayloadGenerator() and setPongPayloadGenerator() methods. Both methods take an instance of PayloadGenerator interface. The following is an example to use the string representation of the current date as payload of ping frames.

ws.setPingPayloadGenerator(new PayloadGenerator() {
    @Override
    public byte[] generate() {
        // The string representation of the current date.
        return new Date().toString().getBytes();
    }
});

Note that the maximum payload length of control frames (e.g. ping frames) is 125. Therefore, the length of a byte array returned from generate() method must not exceed 125.

You can change the names of the Timers that send ping/pong frames periodically by using setPingSenderName() and setPongSenderName() methods.

// Change the Timers' names.
ws.setPingSenderName("PING_SENDER");
ws.setPongSenderName("PONG_SENDER");

Auto Flush

By default, a frame is automatically flushed to the server immediately after sendFrame method is executed. This automatic flush can be disabled by calling setAutoFlush(false).

// Disable auto-flush.
ws.setAutoFlush(false);

To flush frames manually, call flush() method. Note that this method works asynchronously.

// Flush frames to the server manually.
ws.flush();

Congestion Control

sendXxx methods queue a WebSocketFrame instance to the internal queue. By default, no upper limit is imposed on the queue size, so sendXxx methods do not block. However, this behavior may cause a problem if your WebSocket client application sends too many WebSocket frames in a short time for the WebSocket server to process. In such a case, you may want sendXxx methods to block when many frames are queued.

You can set an upper limit on the internal queue by calling setFrameQueueSize(int) method. As a result, if the number of frames in the queue has reached the upper limit when a sendXxx method is called, the method blocks until the queue gets spaces. The code snippet below is an example to set 5 as the upper limit of the internal frame queue.

// Set 5 as the frame queue size.
ws.setFrameQueueSize(5);

Note that under some conditions, even if the queue is full, sendXxx methods do not block. For example, in the case where the thread to send frames (WritingThread) is going to stop or has already stopped. In addition, method calls to send a control frame (e.g. sendClose() and sendPing()) do not block.

Maximum Payload Size

You can set an upper limit on the payload size of WebSocket frames by calling setMaxPayloadSize(int) method with a positive value. Text, binary and continuation frames whose payload size is bigger than the maximum payload size you have set will be split into multiple frames.

// Set 1024 as the maximum payload size.
ws.setMaxPayloadSize(1024);

Control frames (close, ping and pong frames) are never split as per the specification.

If permessage-deflate extension is enabled and if the payload size of a WebSocket frame after compression does not exceed the maximum payload size, the WebSocket frame is not split even if the payload size before compression exceeds the maximum payload size.

Compression

The permessage-deflate extension (RFC 7692) has been supported since the version 1.17. To enable the extension, call addExtension method with permessage-deflate.

// Enable "permessage-deflate" extension (RFC 7692).
ws.addExtension(WebSocketExtension.PERMESSAGE_DEFLATE);

Missing Close Frame

Some server implementations close a WebSocket connection without sending a close frame to a client in some cases. Strictly speaking, this is a violation against the specification (RFC 6455). However, this library has allowed the behavior by default since the version 1.29. Even if the end of the input stream of a WebSocket connection were reached without a close frame being received, it would trigger neither onError method nor onFrameError method of WebSocketListener. If you want to make a WebSocket instance report an error in the case, pass false to setMissingCloseFrameAllowed method.

// Make this library report an error when the end of the input stream
// of the WebSocket connection is reached before a close frame is read.
ws.setMissingCloseFrameAllowed(false);

Direct Text Message

When a text message was received, onTextMessage(WebSocket, String) is called. The implementation internally converts the byte array of the text message into a String object before calling the listener method. If you want to receive the byte array directly without the string conversion, call setDirectTextMessage(boolean) with true, and onTextMessage(WebSocket, byte[]) will be called instead.

// Receive text messages without string conversion.
ws.setDirectTextMessage(true);

Disconnect WebSocket

Before a WebSocket is closed, a closing handshake is performed. A closing handshake is started (1) when the server sends a close frame to the client or (2) when the client sends a close frame to the server. You can start a closing handshake by calling disconnect() method (or by sending a close frame manually).

// Close the WebSocket connection.
ws.disconnect();

disconnect() method has some variants. If you want to change the close code and the reason phrase of the close frame that this client will send to the server, use a variant method such as disconnect(int, String). disconnect() itself is an alias of disconnect(WebSocketCloseCode.NORMAL, null).

Reconnection

WebSocket.connect() method can be called at most only once regardless of whether the method succeeded or failed. If you want to re-connect to the WebSocket endpoint, you have to create a new WebSocket instance again by calling one of createSocket methods of a WebSocketFactory. You may find recreate() method useful if you want to create a new WebSocket instance that has the same settings as the original instance. Note that, however, settings you made on the raw socket of the original WebSocket instance are not copied.

// Create a new WebSocket instance and connect to the same endpoint.
ws = ws.recreate().connect();

There is a variant of recreate() method that takes a timeout value for socket connection. If you want to use a timeout value that is different from the one used when the existing WebSocket instance was created, use recreate(int timeout) method.

Note that you should not trigger reconnection in onError() method because onError() may be called multiple times due to one error. Instead, onDisconnected() is the right place to trigger reconnection.

Also note that the reason I use an expression of "to trigger reconnection" instead of "to call recreate().connect()" is that I myself won't do it synchronously in WebSocketListener callback methods but will just schedule reconnection or will just go to the top of a kind of application loop that repeats to establish a WebSocket connection until it succeeds.

Error Handling

WebSocketListener has some onXxxError() methods such as onFrameError() and onSendError(). Among such methods, onError() is a special one. It is always called before any other onXxxError() is called. For example, in the implementation of run() method of ReadingThread, Throwable is caught and onError() and onUnexpectedError() are called in this order. The following is the implementation.

@Override
public void run()
{
    try
    {
        main();
    }
    catch (Throwable t)
    {
        // An uncaught throwable was detected in the reading thread.
        WebSocketException cause = new WebSocketException(
            WebSocketError.UNEXPECTED_ERROR_IN_READING_THREAD,
            "An uncaught throwable was detected in the reading thread", t);

        // Notify the listeners.
        ListenerManager manager = mWebSocket.getListenerManager();
        manager.callOnError(cause);
        manager.callOnUnexpectedError(cause);
    }
}

So, you can handle all error cases in onError() method. However, note that onError() may be called multiple times for one error cause, so don't try to trigger reconnection in onError(). Instead, onDisconnected() is the right place to trigger reconnection.

All onXxxError() methods receive a WebSocketException instance as the second argument (the first argument is a WebSocket instance). The exception class provides getError() method which returns a WebSocketError enum entry. Entries in WebSocketError enum are possible causes of errors that may occur in the implementation of this library. The error causes are so granular that they can make it easy for you to find the root cause when an error occurs.

Throwables thrown by implementations of onXxx() callback methods are passed to handleCallbackError() of WebSocketListener.

public void handleCallbackError(WebSocket websocket, Throwable cause) throws Exception {
    // Throwables thrown by onXxx() callback methods come here.
}

Thread Callbacks

Some threads are created internally in the implementation of WebSocket. Known threads are as follows.

Thread Type	Description
`READING_THREAD`	A thread which reads WebSocket frames from the server.
`WRITING_THREAD`	A thread which sends WebSocket frames to the server.
`CONNECT_THREAD`	A thread which calls `WebSocket.connect()` asynchronously.
`FINISH_THREAD`	A thread which does finalization of a `WebSocket` instance.

The following callack methods of WebSocketListener are called according to the life cycle of the threads.

Method	Description
`onThreadCreated()`	Called after a thread was created.
`onThreadStarted()`	Called at the beginning of the thread's `run()` method.
`onThreadStopping()`	Called at the end of the thread's `run()` method.

For example, if you want to change the name of the reading thread, implement onThreadCreated() method like below.

@Override
public void onThreadCreated(WebSocket ws, ThreadType type, Thread thread)
{
    if (type == ThreadType.READING_THREAD)
    {
        thread.setName("READING_THREAD");
    }
}

Sample Application

The following is a sample application that connects to the echo server on websocket.org (ws://echo.websocket.org) and repeats to (1) read a line from the standard input, (2) send the read line to the server and (3) prints the response from the server, until exit is entered. The source code can be downloaded from Gist.

import java.io.*;
import com.neovisionaries.ws.client.*;


public class EchoClient
{
    /**
     * The echo server on websocket.org.
     */
    private static final String SERVER = "ws://echo.websocket.org";

    /**
     * The timeout value in milliseconds for socket connection.
     */
    private static final int TIMEOUT = 5000;


    /**
     * The entry point of this command line application.
     */
    public static void main(String[] args) throws Exception
    {
        // Connect to the echo server.
        WebSocket ws = connect();

        // The standard input via BufferedReader.
        BufferedReader in = getInput();

        // A text read from the standard input.
        String text;

        // Read lines until "exit" is entered.
        while ((text = in.readLine()) != null)
        {
            // If the input string is "exit".
            if (text.equals("exit"))
            {
                // Finish this application.
                break;
            }

            // Send the text to the server.
            ws.sendText(text);
        }

        // Close the WebSocket.
        ws.disconnect();
    }


    /**
     * Connect to the server.
     */
    private static WebSocket connect() throws IOException, WebSocketException
    {
        return new WebSocketFactory()
            .setConnectionTimeout(TIMEOUT)
            .createSocket(SERVER)
            .addListener(new WebSocketAdapter() {
                // A text message arrived from the server.
                public void onTextMessage(WebSocket websocket, String message) {
                    System.out.println(message);
                }
            })
            .addExtension(WebSocketExtension.PERMESSAGE_DEFLATE)
            .connect();
    }


    /**
     * Wrap the standard input with BufferedReader.
     */
    private static BufferedReader getInput() throws IOException
    {
        return new BufferedReader(new InputStreamReader(System.in));
    }
}

Limitations

According to the specification (RFC 6455), the maximum length of the payload part of a frame is (2^63 - 1), but this library cannot treat frames whose payload length is greater than (2^31 - 1).
HTTP response codes other than "101 Switching Protocols" from a WebSocket endpoint are not supported. Note that this means redirection (3xx) is not supported.
Currently, by default, permessage-deflate extension is not enabled.
As for RFC 7692. The current implementation does not support context takeover on the client side. Also, if the agreed size of the sliding window on the client side is less than the maximum size allowed by the specification (32,768) (this happens when a WebSocket server returns cli