9.3 Web sockets

9.3.1 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.

9.3.2 The WebSocket interface

enum BinaryType { "blob", "arraybuffer" };
[Constructor(DOMString url, optional (DOMString or DOMString[]) protocols), Exposed=Window,Worker]
interface WebSocket : EventTarget {
  readonly attribute DOMString 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 bufferedAmount;

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

  // messaging
           attribute EventHandler onmessage;
           attribute BinaryType binaryType;
  void send(DOMString data);
  void send(Blob data);
  void send(ArrayBuffer data);
  void send(ArrayBufferView data);
};

The WebSocket(url, protocols) constructor takes one or two arguments. The first argument, url, specifies the URL to which to connect. The second, protocols, if present, 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 must all be strings that match the requirements for elements that comprise the value of Sec-WebSocket-Protocol fields as defined by the WebSocket protocol specification. [WSP]

When the WebSocket() constructor is invoked, the UA must run these steps:

  1. Parse a WebSocket URL's components from the url argument, to obtain host, port, resource name, and secure. If this fails, throw a SyntaxError exception and abort these steps. [WSP]

  2. If secure is false but the origin specified by the entry settings object has a scheme component that is itself a secure protocol, e.g. HTTPS, then throw a SecurityError exception and abort these steps.

  3. If port is a port to which the user agent is configured to block access, then throw a SecurityError exception and abort these steps. (User agents typically block access to well-known ports like SMTP.)

    Access to ports 80 and 443 should not be blocked, including the unlikely cases when secure is false but port is 443 or secure is true but port is 80.

  4. If protocols is absent, let protocols be an empty array.

    Otherwise, if protocols is present and a string, let protocols instead be an array consisting of just that string.

  5. 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 specification, then throw a SyntaxError exception and abort these steps. [WSP]

  6. Let origin be the ASCII serialization of the origin specified by the entry settings object, converted to ASCII lowercase.

  7. Return a new WebSocket object, but continue these steps asynchronously.

  8. Let the new object's client-specified protocols be the values (if any) given in protocols.

  9. Establish a WebSocket connection given the set (host, port, resource name, secure), along with the protocols list, an empty list for the extensions, and origin. The headers to send appropriate cookies must be a Cookie header whose value is the cookie-string computed from the user's cookie store and the URL url; for these purposes this is not a "non-HTTP" API. [WSP] [COOKIES]

    When the user agent validates the server's response during the "establish a WebSocket connection" algorithm, if the status code received from the server is not 101 (e.g. it is a redirect), the user agent must fail the WebSocket connection.

    Following HTTP procedures here 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.

    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 attribute must return the result of resolving the URL that was passed to the constructor, with the URL character encoding set to UTF-8. (It doesn't matter what it is resolved relative to, since we already know it is an absolute URL.)

The readyState attribute represents the state of the connection. 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.

When the object is created its readyState must be set to CONNECTING (0).

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

The extensions attribute returns the extensions selected by the server, if any. (Currently this will only ever be the empty string.)

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

The protocol attribute 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.

The close() method must run the following steps:

  1. If the method's first argument is present but is neither an integer equal to 1000 nor an integer in the range 3000 to 4999, throw an InvalidAccessError exception and abort these steps.

  2. If the method's second argument is present, then run these substeps:

    1. Let raw reason be the method's second argument.

    2. Let Unicode reason be the result of converting raw reason to a sequence of Unicode characters.

    3. Let reason be the result of encoding Unicode reason as UTF-8.

    4. If reason is longer than 123 bytes, then throw a SyntaxError exception and abort these steps. [ENCODING]

  3. Run the first matching steps from the following list:

    If the readyState attribute is in the CLOSING (2) or CLOSED (3) state

    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 the readyState attribute's value 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 the readyState attribute's value to CLOSING (2). [WSP]

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

    If the second argument is also present, then reason must be provided in the Close message after the status code. [ENCODING] [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 the readyState attribute's value 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 attribute must 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 asynchronously with script execution.) This does not include framing overhead incurred by the protocol, or buffering done by the operating system or network hardware. If the 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).

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.


When a WebSocket object is created, its binaryType IDL attribute must be set to the string "blob". On getting, it must return the last value it was set to. On setting, the user agent must set the IDL attribute to the new value.

This attribute allows authors to control how binary data is exposed to scripts. By setting the attribute to "blob", binary data is returned in Blob form; by setting it to "arraybuffer", it is returned in ArrayBuffer form. User agents can use this as a hint for how to handle incoming binary data: if the attribute is set to "blob", it is safe to spool it to disk, and if it is set to "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 transmits data using the connection. If the readyState attribute is CONNECTING, it must throw an InvalidStateError exception. Otherwise, the user agent must run the appropriate set of steps from the following list:

If the argument is a string

Let data be the result of converting the data argument to a sequence of Unicode characters. 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 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 the argument 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 the argument is an ArrayBuffer 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 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] [ECMA262]

If the argument is an object that matches the ArrayBufferView type definition

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] [ECMA262]


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

9.3.3 Feedback from the protocol

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

  1. If the WebSocket object's client-specified protocols was not an empty list, but the subprotocol in use is the null value, then fail the WebSocket connection, set the readyState attribute's value to CLOSING (2), and abort these steps. [WSP]

  2. Change the readyState attribute's value to OPEN (1).

  3. Change the extensions attribute's value to the extensions in use, if is not the null value. [WSP]

  4. Change the protocol attribute's value to the subprotocol in use, if is not the null value. [WSP]

  5. Act as if the user agent had received a set-cookie-string consisting of the cookies set during the server's opening handshake, for the URL url given to the WebSocket() constructor. [COOKIES] [ENCODING] [WSP]

  6. Fire a simple event named open at the WebSocket object.


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]

  1. If the readyState attribute's value is not OPEN (1), then abort these steps.

  2. Let event be a newly created trusted event that uses the MessageEvent interface, with the event type message, which does not bubble, is not cancelable, and has no default action.

  3. Initialize event's origin attribute to the Unicode serialization of the origin of the URL that was passed to the WebSocket object's constructor.

  4. If type indicates that the data is Text, then initialize event's data attribute to data.

    If type indicates that the data is Binary, and binaryType is set to "blob", then initialize event's data attribute to a new Blob object that represents data as its raw data. [FILEAPI]

    If type indicates that the data is Binary, and binaryType is set to "arraybuffer", then initialize event's data attribute to a new ArrayBuffer object whose contents are data. [ECMA262]

  5. Dispatch event at the WebSocket object.

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 binaryType attribute was set to "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 binaryType 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 readyState attribute's value to CLOSING (2). (If the close() method was called, the readyState attribute's value 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:

  1. Change the readyState attribute's value to CLOSED (3).

  2. If the user agent was required to fail the WebSocket connection, or if the the WebSocket connection was closed after being flagged as full, fire a simple event named error at the WebSocket object. [WSP]

  3. Create a trusted event that uses the CloseEvent interface, with the event type close, which does not bubble, is not cancelable, has no default action, whose wasClean attribute is initialized to true if the connection closed cleanly and false otherwise, whose code attribute is initialized to the WebSocket connection close code, and whose reason attribute is initialized to the result of applying the UTF-8 decoder to the WebSocket connection close reason, and dispatch the event at the WebSocket object. [WSP]

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

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

9.3.4 Ping and Pong frames

The WebSocket protocol specification 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.

9.3.5 Parsing WebSocket URLs

The steps to parse a WebSocket URL's components from a string url are as follows. These steps return either a host, a port, a resource name, and a secure flag, or they fail.

  1. If the url string is not an absolute URL, then fail this algorithm.

  2. Resolve the url string, with the URL character encoding set to UTF-8. [ENCODING]

    It doesn't matter what it is resolved relative to, since we already know it is an absolute URL at this point.

  3. If the resulting parsed URL does not have a scheme component whose value is either "ws" or "wss", then fail this algorithm.

  4. If the resulting parsed URL has a non-null fragment component, then fail this algorithm.

  5. If the scheme component of the resulting parsed URL is "ws", set secure to false; otherwise, the scheme component is "wss", set secure to true.

  6. Let host be the value of the resulting parsed URL's host component.

  7. If the resulting parsed URL has a port component that is not the empty string, then let port be that component's value; otherwise, there is no explicit port.

  8. If there is no explicit port, then: if secure is false, let port be 80, otherwise let port be 443.

  9. Let resource name be the value of the resulting parsed URL's path component (which might be empty).

  10. If resource name is the empty string, set it to a single character U+002F SOLIDUS (/).

  11. If the resulting parsed URL has a non-null query component, then append a single U+003F QUESTION MARK character (?) to resource name, followed by the value of the query component.

  12. Return host, port, resource name, and secure.

9.3.6 The CloseEvent interfaces

[Constructor(DOMString type, optional CloseEventInit eventInitDict), Exposed=Window,Worker]
interface CloseEvent : Event {
  readonly attribute boolean wasClean;
  readonly attribute unsigned short code;
  readonly attribute DOMString reason;
};

dictionary CloseEventInit : EventInit {
  boolean wasClean;
  unsigned short code;
  DOMString reason;
};

The wasClean attribute must return the value it was initialized to. When the object is created, this attribute must be initialized to false. It represents whether the connection closed cleanly or not.

The code attribute must return the value it was initialized to. When the object is created, this attribute must be initialized to zero. It represents the WebSocket connection close code provided by the server.

The reason attribute must return the value it was initialized to. When the object is created, this attribute must be initialized to empty string. It represents the WebSocket connection close reason provided by the server.

9.3.7 Garbage collection

A WebSocket object whose readyState attribute's value was set to CONNECTING (0) as of the last time the event loop started 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 readyState attribute's value 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 readyState attribute's value 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.