WebSockets Standard

1. Introduction

This section is non-normative.

To enable web applications to maintain bidirectional communications with server-side processes, this specification introduces the WebSocket interface.

This interface does not allow for raw access to the underlying network. For example, this interface could not be used to implement an IRC client without proxying messages through a custom server.

2. WebSocket protocol alterations

This section replaces part of the WebSocket protocol opening handshake client requirement to integrate it with algorithms defined in Fetch. This way CSP, cookies, HSTS, and other Fetch-related protocols are handled in a single location. Ideally the RFC would be updated with this language, but it is never that easy. The WebSocket API, defined below, uses this language. [WSP] [FETCH]

The way this works is by replacing The WebSocket Protocol’s "establish a WebSocket connection" algorithm with a new one that integrates with Fetch. "Establish a WebSocket connection" consists of three algorithms: setting up a connection, creating and transmiting a handshake request, and validating the handshake response. That layering is different from Fetch, which first creates a handshake, then sets up a connection and transmits the handshake, and finally validates the response. Keep that in mind while reading these alterations.

2.1. Connections

To obtain a WebSocket connection, given a url, run these steps:

Let host be url’s host.
Let port be url’s port.
Let resource name be U+002F (/), followed by the strings in url’s path (including empty strings), if any, separated from each other by U+002F (/).
If url’s query is non-empty, append U+003F (?), followed by url’s query, to resource name.
Let secure be false, if url’s scheme is "http"; otherwise true.
Follow the requirements stated in step 2 to 5, inclusive, of the first set of steps in section 4.1 of The WebSocket Protocol to establish a WebSocket connection, passing host, port, resource name and secure. [WSP]
If that established a connection, return it, and return failure otherwise.

Although structured a little differently, carrying different properties, and therefore not shareable, a WebSocket connection is very close to identical to an "ordinary" connection.

2.2. Opening handshake

To establish a WebSocket connection, given a url, protocols, and client, run these steps:

Let requestURL be a copy of url, with its scheme set to "http", if url’s scheme is "ws"; otherwise to "https".

This change of scheme is essential to integrate well with fetching. E.g., HSTS would not work without it. There is no real reason for WebSocket to have distinct schemes, it’s a legacy artefact. [HSTS]
Let request be a new request, whose URL is requestURL, client is client, service-workers mode is "none", referrer is "no-referrer", mode is "websocket", credentials mode is "include", cache mode is "no-store" , and redirect mode is "error".
Append (`Upgrade`, `websocket`) to request’s header list.
Append (`Connection`, `Upgrade`) to request’s header list.
Let keyValue be a nonce consisting of a randomly selected 16-byte value that has been forgiving-base64-encoded and isomorphic encoded.

If the randomly selected value was the byte sequence 0x01 0x02 0x03 0x04 0x05 0x06 0x07 0x08 0x09 0x0a 0x0b 0x0c 0x0d 0x0e 0x0f 0x10, keyValue would be forgiving-base64-encoded to "AQIDBAUGBwgJCgsMDQ4PEC==" and isomorphic encoded to `AQIDBAUGBwgJCgsMDQ4PEC==`.
Append (`Sec-WebSocket-Key`, keyValue) to request’s header list.
Append (`Sec-WebSocket-Version`, `13`) to request’s header list.
For each protocol in protocols, combine (`Sec-WebSocket-Protocol`, protocol) in request’s header list.
Let permessageDeflate be a user-agent defined "permessage-deflate" extension header value. [WSP]

`permessage-deflate; client_max_window_bits`
Append (`Sec-WebSocket-Extensions`, permessageDeflate) to request’s header list.
Fetch request with useParallelQueue set to true, and processResponse given response being these steps:
1. If response is a network error or its status is not 101, fail the WebSocket connection.
2. If protocols is not the empty list and extracting header list values given `Sec-WebSocket-Protocol` and response’s header list results in null, failure, or the empty byte sequence, then fail the WebSocket connection.
  
  This is different from the check on this header defined by The WebSocket Protocol. That only covers a subprotocol not requested by the client. This covers a subprotocol requested by the client, but not acknowledged by the server.
3. Follow the requirements stated step 2 to step 6, inclusive, of the last set of steps in section 4.1 of The WebSocket Protocol to validate response. This either results in fail the WebSocket connection or the WebSocket connection is established.

Fail the WebSocket connection and the WebSocket connection is established are defined by The WebSocket Protocol. [WSP]

The reason redirects are not followed and this handshake is generally restricted is because it could introduce serious security problems in a web browser context. For example, consider a host with a WebSocket server at one path and an open HTTP redirector at another. Suddenly, any script that can be given a particular WebSocket URL can be tricked into communicating to (and potentially sharing secrets with) any host on the internet, even if the script checks that the URL has the right hostname.

3. The `WebSocket` interface

3.1. Interface definition

The Web IDL definition for the WebSocket class is given as follows:

enum BinaryType { "blob", "arraybuffer" };

[Exposed=(Window,Worker)]
interface WebSocket : EventTarget {
  constructor(USVString url, optional (DOMString or sequence<DOMString>) protocols = []);
  readonly attribute USVString url;

  // ready state
  const unsigned short CONNECTING = 0;
  const unsigned short OPEN = 1;
  const unsigned short CLOSING = 2;
  const unsigned short CLOSED = 3;
  readonly attribute unsigned short readyState;
  readonly attribute unsigned long long bufferedAmount;

  // networking
  attribute EventHandler onopen;
  attribute EventHandler onerror;
  attribute EventHandler onclose;
  readonly attribute DOMString extensions;
  readonly attribute DOMString protocol;
  undefined close(optional [Clamp] unsigned short code, optional USVString reason);

  // messaging
  attribute EventHandler onmessage;
  attribute BinaryType binaryType;
  undefined send((BufferSource or Blob or USVString) data);
};

Each WebSocket object has an associated url, which is a URL record.

Each WebSocket object has an associated binary type, which is a BinaryType. Initially it must be "blob".

Each WebSocket object has an associated ready state, which is a number representing the state of the connection. Initially it must be CONNECTING (0). It can have the following values:

CONNECTING (numeric value 0): The connection has not yet been established.
OPEN (numeric value 1): The WebSocket connection is established and communication is possible.
CLOSING (numeric value 2): The connection is going through the closing handshake, or the close() method has been invoked.
CLOSED (numeric value 3): The connection has been closed or could not be opened.

socket = new WebSocket(url [, protocols ])

Creates a new WebSocket object, immediately establishing the associated WebSocket connection.

url is a string giving the URL over which the connection is established. Only "ws", "wss", "http", and "https" schemes are allowed; others will cause a "SyntaxError" DOMException. URLs with fragments will always cause such an exception.

protocols is either a string or an array of strings. If it is a string, it is equivalent to an array consisting of just that string; if it is omitted, it is equivalent to the empty array. Each string in the array is a subprotocol name. The connection will only be established if the server reports that it has selected one of these subprotocols. The subprotocol names have to match the requirements for elements that comprise the value of `Sec-WebSocket-Protocol` fields as defined by The WebSocket protocol. [WSP]

socket.send(data)

Transmits data using the WebSocket connection. data can be a string, a Blob, an ArrayBuffer, or an ArrayBufferView.

socket.close([ code ] [, reason ])

Closes the WebSocket connection, optionally using code as the WebSocket connection close code and reason as the WebSocket connection close reason.

socket.url

Returns the URL that was used to establish the WebSocket connection.

socket.readyState

Returns the state of the WebSocket connection. It can have the values described above.

socket.bufferedAmount

Returns the number of bytes of application data (UTF-8 text and binary data) that have been queued using send() but not yet been transmitted to the network.

If the WebSocket connection is closed, this attribute’s value will only increase with each call to the send() method. (The number does not reset to zero once the connection closes.)

socket.extensions

Returns the extensions selected by the server, if any.

socket.protocol

Returns the subprotocol selected by the server, if any. It can be used in conjunction with the array form of the constructor’s second argument to perform subprotocol negotiation.

socket.binaryType

Returns a string that indicates how binary data from socket is exposed to scripts:

"blob": Binary data is returned in Blob form.
"arraybuffer": Binary data is returned in ArrayBuffer form.

The default is "blob".

socket.binaryType = value

Changes how binary data is returned.

The

new
 WebSocket(url, protocols)

constructor steps are:

Let baseURL be this's relevant settings object's API base URL.
Let urlRecord be the result of applying the URL parser to url with baseURL.
If urlRecord is failure, then throw a "SyntaxError" DOMException.
If urlRecord’s scheme is "http", then set urlRecord’s scheme to "ws".
Otherwise, if urlRecord’s scheme is "https", set urlRecord’s scheme to "wss".
If urlRecord’s scheme is not "ws" or "wss", then throw a "SyntaxError" DOMException.
If urlRecord’s fragment is non-null, then throw a "SyntaxError" DOMException.
If protocols is a string, set protocols to a sequence consisting of just that string.
If any of the values in protocols occur more than once or otherwise fail to match the requirements for elements that comprise the value of `Sec-WebSocket-Protocol` fields as defined by The WebSocket protocol, then throw a "SyntaxError" DOMException. [WSP]
Set this's url to urlRecord.
Let client be this's relevant settings object.
Run this step in parallel:
1. Establish a WebSocket connection given urlRecord, protocols, and client. [FETCH]
  
  If the establish a WebSocket connection algorithm fails, it triggers the fail the WebSocket connection algorithm, which then invokes the close the WebSocket connection algorithm, which then establishes that the WebSocket connection is closed, which fires the close event as described below.

The url getter steps are to return this's url, serialized.

The readyState getter steps are to return this's ready state.

The extensions attribute must initially return the empty string. After the WebSocket connection is established, its value might change, as defined below.

The protocol attribute must initially return the empty string. After the WebSocket connection is established, its value might change, as defined below.

The close(code, reason) method steps are:

If code is present, but is neither an integer equal to 1000 nor an integer in the range 3000 to 4999, inclusive, throw an "InvalidAccessError" DOMException.
If reason is present, then run these substeps:
1. Let reasonBytes be the result of encoding reason.
2. If reasonBytes is longer than 123 bytes, then throw a "SyntaxError" DOMException.
Run the first matching steps from the following list:

If this's ready state is CLOSING (2) or CLOSED (3)

Do nothing.

The connection is already closing or is already closed. If it has not already, a close event will eventually fire as described below.

If the WebSocket connection is not yet established [WSP]

Fail the WebSocket connection and set this's ready state to CLOSING (2). [WSP]

The fail the WebSocket connection algorithm invokes the close the WebSocket connection algorithm, which then establishes that the WebSocket connection is closed, which fires the close event as described below.

If the WebSocket closing handshake has not yet been started [WSP]

Start the WebSocket closing handshake and set this's ready state to CLOSING (2). [WSP]

If neither code nor reason is present, the WebSocket Close message must not have a body.

The WebSocket Protocol erroneously states that the status code is required for the start the WebSocket closing handshake algorithm.

If code is present, then the status code to use in the WebSocket Close message must be the integer given by code. [WSP]

If reason is also present, then reasonBytes must be provided in the Close message after the status code. [WSP]

The start the WebSocket closing handshake algorithm eventually invokes the close the WebSocket connection algorithm, which then establishes that the WebSocket connection is closed, which fires the close event as described below.

Otherwise

Set this's ready state to CLOSING (2).

The WebSocket closing handshake is started, and will eventually invoke the close the WebSocket connection algorithm, which will establish that the WebSocket connection is closed, and thus the close event will fire, as described below.

The close() method does not discard previously sent messages before starting the WebSocket closing handshake — even if, in practice, the user agent is still busy sending those messages, the handshake will only start after the messages are sent.

The bufferedAmount getter steps are to return the number of bytes of application data (UTF-8 text and binary data) that have been queued using send() but that, as of the last time the event loop reached step 1, had not yet been transmitted to the network. (This thus includes any text sent during the execution of the current task, regardless of whether the user agent is able to transmit text in the background in parallel with script execution.) This does not include framing overhead incurred by the protocol, or buffering done by the operating system or network hardware.

In this simple example, the bufferedAmount attribute is used to ensure that updates are sent either at the rate of one update every 50ms, if the network can handle that rate, or at whatever rate the network can handle, if that is too fast.

var socket = new WebSocket('ws://game.example.com:12010/updates');
socket.onopen = function () {
  setInterval(function() {
    if (socket.bufferedAmount == 0)
      socket.send(getUpdateData());
  }, 50);
};

The bufferedAmount attribute can also be used to saturate the network without sending the data at a higher rate than the network can handle, though this requires more careful monitoring of the value of the attribute over time.

The binaryType getter steps are to return this's binary type.

The binaryType setter steps are to set this's binary type to the given value.

User agents can use the binary type as a hint for how to handle incoming binary data: if it is "blob", it is safe to spool it to disk, and if it is "arraybuffer", it is likely more efficient to keep the data in memory. Naturally, user agents are encouraged to use more subtle heuristics to decide whether to keep incoming data in memory or not, e.g. based on how big the data is or how common it is for a script to change the attribute at the last minute. This latter aspect is important in particular because it is quite possible for the attribute to be changed after the user agent has received the data but before the user agent has fired the event for it.

The send(data) method steps are:

If this's ready state is CONNECTING, then throw an "InvalidStateError" DOMException.
Run the appropriate set of steps from the following list:

If data is a string

If the WebSocket connection is established and the WebSocket closing handshake has not yet started, then the user agent must send a WebSocket Message comprised of the data argument using a text frame opcode; if the data cannot be sent, e.g. because it would need to be buffered but the buffer is full, the user agent must flag the WebSocket as full and then close the WebSocket connection. Any invocation of this method with a string argument that does not throw an exception must increase the bufferedAmount attribute by the number of bytes needed to express the argument as UTF-8. [UNICODE] [ENCODING] [WSP]

If data is a Blob object

If the WebSocket connection is established, and the WebSocket closing handshake has not yet started, then the user agent must send a WebSocket Message comprised of data using a binary frame opcode; if the data cannot be sent, e.g. because it would need to be buffered but the buffer is full, the user agent must flag the WebSocket as full and then close the WebSocket connection. The data to be sent is the raw data represented by the Blob object. Any invocation of this method with a Blob argument that does not throw an exception must increase the bufferedAmount attribute by the size of the Blob object’s raw data, in bytes. [WSP] [FILEAPI]

If data is an ArrayBuffer

If the WebSocket connection is established, and the WebSocket closing handshake has not yet started, then the user agent must send a WebSocket Message comprised of data using a binary frame opcode; if the data cannot be sent, e.g. because it would need to be buffered but the buffer is full, the user agent must flag the WebSocket as full and then close the WebSocket connection. The data to be sent is the data stored in the buffer described by the ArrayBuffer object. Any invocation of this method with an ArrayBuffer argument that does not throw an exception must increase the bufferedAmount attribute by the length of the ArrayBuffer in bytes. [WSP]

If data is an ArrayBufferView

If the WebSocket connection is established, and the WebSocket closing handshake has not yet started, then the user agent must send a WebSocket Message comprised of data using a binary frame opcode; if the data cannot be sent, e.g. because it would need to be buffered but the buffer is full, the user agent must flag the WebSocket as full and then close the WebSocket connection. The data to be sent is the data stored in the section of the buffer described by the ArrayBuffer object that data references. Any invocation of this method with this kind of argument that does not throw an exception must increase the bufferedAmount attribute by the length of data’s buffer in bytes. [WSP]

The following are the event handlers (and their corresponding event handler event types) that must be supported, as event handler IDL attributes, by all objects implementing the WebSocket interface:

Event handler	Event handler event type
`onopen`	`open`
`onmessage`	`message`
`onerror`	`error`
`onclose`	`close`

4. Feedback from the protocol

When the WebSocket connection is established, the user agent must queue a task to run these steps:

Change the ready state to OPEN (1).
Change the extensions attribute’s value to the extensions in use, if it is not the null value. [WSP]
Change the protocol attribute’s value to the subprotocol in use, if it is not the null value. [WSP]
Fire an event named open at the WebSocket object.

Since the algorithm above is queued as a task, there is no race condition between the WebSocket connection being established and the script setting up an event listener for the open event.

When a WebSocket message has been received with type type and data data, the user agent must queue a task to follow these steps: [WSP]

If ready state is not OPEN (1), then return.
Let dataForEvent be determined by switching on type and binary type:

type indicates that the data is Text

a new DOMString containing data

type indicates that the data is Binary and binary type is "blob"

a new Blob object, created in the relevant Realm of the WebSocket object, that represents data as its raw data [FILEAPI]

type indicates that the data is Binary and binary type is "arraybuffer"

a new ArrayBuffer object, created in the relevant Realm of the WebSocket object, whose contents are data
Fire an event named message at the WebSocket object, using MessageEvent, with the origin attribute initialized to the serialization of the WebSocket object’s url's origin, and the data attribute initialized to dataForEvent.

User agents are encouraged to check if they can perform the above steps efficiently before they run the task, picking tasks from other task queues while they prepare the buffers if not. For example, if the binary type is "blob" when the data arrived, and the user agent spooled all the data to disk, but just before running the above task for this particular message the script switched binary type to "arraybuffer", the user agent would want to page the data back to RAM before running this task so as to avoid stalling the main thread while it created the ArrayBuffer object.

Here is an example of how to define a handler for the message event in the case of text frames:

mysocket.onmessage = function (event) {
  if (event.data == 'on') {
    turnLampOn();
  } else if (event.data == 'off') {
    turnLampOff();
  }
};

The protocol here is a trivial one, with the server just sending "on" or "off" messages.

When the WebSocket closing handshake is started, the user agent must queue a task to change the ready state to CLOSING (2). (If the close() method was called, the ready state will already be set to CLOSING (2) when this task runs.) [WSP]

When the WebSocket connection is closed, possibly cleanly, the user agent must queue a task to run the following substeps:

Change the ready state to CLOSED (3).
If the user agent was required to fail the WebSocket connection, or if the WebSocket connection was closed after being flagged as full, fire an event named error at the WebSocket object. [WSP]
Fire an event named close at the WebSocket object, using CloseEvent, with the wasClean attribute initialized to true if the connection closed cleanly and false otherwise, the code attribute initialized to the WebSocket connection close code, and the reason attribute initialized to the result of applying UTF-8 decode without BOM to the WebSocket connection close reason. [WSP]

User agents must not convey any failure information to scripts in a way that would allow a script to distinguish the following situations:

A server whose host name could not be resolved.
A server to which packets could not successfully be routed.
A server that refused the connection on the specified port.
A server that failed to correctly perform a TLS handshake (e.g., the server certificate can’t be verified).
A server that did not complete the opening handshake (e.g. because it was not a WebSocket server).
A WebSocket server that sent a correct opening handshake, but that specified options that caused the client to drop the connection (e.g. the server specified a subprotocol that the client did not offer).
A WebSocket server that abruptly closed the connection after successfully completing the opening handshake.

In all of these cases, the WebSocket connection close code would be 1006, as required by WebSocket Protocol. [WSP]

Allowing a script to distinguish these cases would allow a script to probe the user’s local network in preparation for an attack.

In particular, this means the code 1015 is not used by the user agent (unless the server erroneously uses it in its close frame, of course).

The task source for all tasks queued in this section is the WebSocket task source.

5. Ping and Pong frames

The WebSocket protocol defines Ping and Pong frames that can be used for keep-alive, heart-beats, network status probing, latency instrumentation, and so forth. These are not currently exposed in the API.

User agents may send ping and unsolicited pong frames as desired, for example in an attempt to maintain local network NAT mappings, to detect failed connections, or to display latency metrics to the user. User agents must not use pings or unsolicited pongs to aid the server; it is assumed that servers will solicit pongs whenever appropriate for the server’s needs.

6. The `CloseEvent` interface

WebSocket objects use the CloseEvent interface for their close events:

[Exposed=(Window,Worker)]
interface CloseEvent : Event {
  constructor(DOMString type, optional CloseEventInit eventInitDict = {});

  readonly attribute boolean wasClean;
  readonly attribute unsigned short code;
  readonly attribute USVString reason;
};

dictionary CloseEventInit : EventInit {
  boolean wasClean = false;
  unsigned short code = 0;
  USVString reason = "";
};

event . wasClean: Returns true if the connection closed cleanly; false otherwise.
event . code: Returns the WebSocket connection close code provided by the server.
event . reason: Returns the WebSocket connection close reason provided by the server.

The wasClean attribute must return the value it was initialized to. It represents whether the connection closed cleanly or not.

The code attribute must return the value it was initialized to. It represents the WebSocket connection close code provided by the server.

The reason attribute must return the value it was initialized to. It represents the WebSocket connection close reason provided by the server.

7. Garbage collection

A WebSocket object whose ready state was set to CONNECTING (0) as of the last time the event loop reached step 1 must not be garbage collected if there are any event listeners registered for open events, message events, error events, or close events.

A WebSocket object whose ready state was set to OPEN (1) as of the last time the event loop reached step 1 must not be garbage collected if there are any event listeners registered for message events, error, or close events.

A WebSocket object whose ready state was set to CLOSING (2) as of the last time the event loop reached step 1 must not be garbage collected if there are any event listeners registered for error or close events.

A WebSocket object with an established connection that has data queued to be transmitted to the network must not be garbage collected. [WSP]

If a WebSocket object is garbage collected while its connection is still open, the user agent must start the WebSocket closing handshake, with no status code for the Close message. [WSP]

If a user agent is to make disappear a WebSocket object (this happens when a Document object goes away), the user agent must follow the first appropriate set of steps from the following list:

If the WebSocket connection is not yet established [WSP]: Fail the WebSocket connection. [WSP]
If the WebSocket closing handshake has not yet been started [WSP]: Start the WebSocket closing handshake, with the status code to use in the WebSocket Close message being 1001. [WSP]
Otherwise: Do nothing.

Acknowledgments

Until the creation of this standard in 2021, the text here was maintained in the HTML Standard and Fetch Standard. Thanks to all of the contributors to those repositories who helped develop the specification, especially Ian Hickson and Anne van Kesteren as the respective original authors.

Thanks to devsnek and 平野裕 (Yutaka Hirano) for their contributions after the creation of the WebSockets Standard.

This standard is written by Adam Rice (Google, ricea@chromium.org).

Intellectual property rights

Copyright © WHATWG (Apple, Google, Mozilla, Microsoft). This work is licensed under a Creative Commons Attribution 4.0 International License. To the extent portions of it are incorporated into source code, such portions in the source code are licensed under the BSD 3-Clause License instead.

This is the Living Standard. Those interested in the patent-review version should view the Living Standard Review Draft.

WebSockets

Abstract

1. Introduction

2. WebSocket protocol alterations

2.1. Connections

2.2. Opening handshake

3. The `WebSocket` interface

3.1. Interface definition

4. Feedback from the protocol

5. Ping and Pong frames

6. The `CloseEvent` interface

7. Garbage collection

Acknowledgments

Intellectual property rights

Index

Terms defined by this specification

Terms defined by reference

References

Normative References

IDL Index

WebSockets

Abstract

1. Introduction

2. WebSocket protocol alterations

2.1. Connections

2.2. Opening handshake

3. The WebSocket interface

3.1. Interface definition

4. Feedback from the protocol

5. Ping and Pong frames

6. The CloseEvent interface

7. Garbage collection

Acknowledgments

Intellectual property rights

Index

Terms defined by this specification

Terms defined by reference

References

Normative References

IDL Index

3. The `WebSocket` interface

6. The `CloseEvent` interface