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(Redirected from WebSockets) Computer network protocol
WebSocket
A diagram describing a connection using WebSocket
International standardRFC 6455
Developed byIETF
IndustryComputer science
Connector typeTCP
Websitehttps://websockets.spec.whatwg.org/

WebSocket is a computer communications protocol, providing a simultaneous two-way communication channel over a single Transmission Control Protocol (TCP) connection. The WebSocket protocol was standardized by the IETF as RFC 6455 in 2011. The current specification allowing web applications to use this protocol is known as WebSockets. It is a living standard maintained by the WHATWG and a successor to The WebSocket API from the W3C.

WebSocket is distinct from HTTP used to serve most webpages. Although they are different, RFC 6455 states that WebSocket "is designed to work over HTTP ports 443 and 80 as well as to support HTTP proxies and intermediaries", thus making it compatible with HTTP. To achieve compatibility, the WebSocket handshake uses the HTTP Upgrade header to change from the HTTP protocol to the WebSocket protocol.

The WebSocket protocol enables full-duplex interaction between a web browser (or other client application) and a web server with lower overhead than half-duplex alternatives such as HTTP polling, facilitating real-time data transfer from and to the server. This is made possible by providing a standardized way for the server to send content to the client without being first requested by the client, and allowing messages to be passed back and forth while keeping the connection open. In this way, a two-way ongoing conversation can take place between the client and the server. The communications are usually done over TCP port number 443 (or 80 in the case of unsecured connections), which is beneficial for environments that block non-web Internet connections using a firewall. Additionally, WebSocket enables streams of messages on top of TCP. TCP alone deals with streams of bytes with no inherent concept of a message. Similar two-way browser–server communications have been achieved in non-standardized ways using stopgap technologies such as Comet or Adobe Flash Player.

Most browsers support the protocol, including Google Chrome, Firefox, Microsoft Edge, Internet Explorer, Safari and Opera.

The WebSocket protocol specification defines ws (WebSocket) and wss (WebSocket Secure) as two new uniform resource identifier (URI) schemes that are used for unencrypted and encrypted connections respectively. Apart from the scheme name and fragment (i.e. # is not supported), the rest of the URI components are defined to use URI generic syntax.

History

WebSocket was first referenced as TCPConnection in the HTML5 specification, as a placeholder for a TCP-based socket API. In June 2008, a series of discussions were led by Michael Carter that resulted in the first version of the protocol known as WebSocket. Before WebSocket, port 80 full-duplex communication was attainable using Comet channels; however, Comet implementation is nontrivial, and due to the TCP handshake and HTTP header overhead, it is inefficient for small messages. The WebSocket protocol aims to solve these problems without compromising the security assumptions of the web. The name "WebSocket" was coined by Ian Hickson and Michael Carter shortly thereafter through collaboration on the #whatwg IRC chat room, and subsequently authored for inclusion in the HTML5 specification by Ian Hickson. In December 2009, Google Chrome 4 was the first browser to ship full support for the standard, with WebSocket enabled by default. Development of the WebSocket protocol was subsequently moved from the W3C and WHATWG group to the IETF in February 2010, and authored for two revisions under Ian Hickson.

After the protocol was shipped and enabled by default in multiple browsers, the RFC 6455 was finalized under Ian Fette in December 2011.

RFC 7692 introduced compression extension to WebSocket using the DEFLATE algorithm on a per-message basis.

Client example

<!DOCTYPE html>
<script>
// Connect to server
ws = new WebSocket("ws://127.0.0.1/scoreboard") // Local server
// ws = new WebSocket("wss://game.example.com/scoreboard") // Remote server
ws.onopen = () => {
    console.log("Connection opened")
    ws.send("Hi server, please send me the score of yesterday's game")
}
ws.onmessage = (event) => {
    console.log("Data received", event.data)
    ws.close() // We got the score so we don't need the connection anymore
}
ws.onclose = (event) => {
    console.log("Connection closed", event.code, event.reason, event.wasClean)
}
ws.onerror = () => {
    console.log("Connection closed due to error")
}
</script>

Server example

from socket import socket
from base64 import b64encode
from hashlib import sha1
MAGIC = b"258EAFA5-E914-47DA-95CA-C5AB0DC85B11"
# Create socket and listen at port 80
ws = socket()
ws.bind(("", 80))
ws.listen()
conn, addr = ws.accept()
# Parse request
for line in conn.recv(4096).split(b"\r\n"):
    if line.startswith(b"Sec-WebSocket-Key"):
        nonce = line.split(b":").strip()
# Format response
response = f"""\
HTTP/1.1 101 Switching Protocols
Upgrade: websocket
Connection: Upgrade
Sec-WebSocket-Accept: {b64encode(sha1(nonce + MAGIC).digest()).decode()}
"""
conn.send(response.replace("\n", "\r\n").encode())
while True: # decode messages from the client
    header = conn.recv(2)
    FIN = bool(header & 0x80) # bit 0
    assert FIN == 1, "We only support unfragmented messages"
    opcode = header & 0xf # bits 4-7
    assert opcode == 1 or opcode == 2, "We only support data messages"
    masked = bool(header & 0x80) # bit 8
    assert masked, "The client must mask all frames"
    payload_size = header & 0x7f # bits 9-15
    assert payload_size <= 125, "We only support small messages"
    masking_key = conn.recv(4)
    payload = bytearray(conn.recv(payload_size))
    for i in range(payload_size):
        payload = payload ^ masking_key
    print(payload)

Web API

A web application (e.g. web browser) may use the WebSocket interface to connect to a WebSocket server.

WebSocket web API specification
Type Name Description
Constructor ws = new WebSocket(url ) Start opening handshake with a WebSocket server.
  • url: A string containing:
    • Scheme: must be ws, wss, http or https.
    • Server hostname.
    • Optional server port: If not specified, 80 is used for ws/http and 443 for wss/https.
    • Optional path.
    • No fragment. There must not be any fragment, otherwise SyntaxError is thrown.
  • Optional protocols: A string or an array of strings used as the value for the Sec-WebSocket-Protocol header.
Method ws.send(data) Send data. data must be string, Blob, ArrayBuffer or ArrayBufferView. Throw InvalidStateError if ws.readyState is WebSocket.CONNECTING.
ws.close( ) Start closing handshake.
  • Optional code: If specified, must be 1000 (normal closure) or in the range 3000 to 4999 (application-defined), otherwise InvalidAccessError is thrown. If not specified, 1000 is used.
  • Optional reason: If specified, must be a string not longer than 123 bytes (UTF-8), otherwise SyntaxError is thrown. If not specified, an empty string is used.
Event ws.onopen = (event) => {}

ws.addEventListener("open", (event) => {})

Opening handshake succeeded. event type is Event.
ws.onmessage = (event) => {}

ws.addEventListener("message", (event) => {})

Data received. event type is MessageEvent. event.data contains the data received, of type:
  • String for text.
  • Blob or ArrayBuffer for binary (see ws.binaryType).
ws.onclose = (event) => {}

ws.addEventListener("close", (event) => {})

The underlying TCP connection closed. event type is CloseEvent containing:
  • event.code: status code (integer).
  • event.reason: reason for closing (string).
  • event.wasClean: true if the TCP connection was closed after the closing handshake was completed; false otherwise.

Note:

  • If the received Close frame contains a payload: the payload data contains event.code and event.reason.
  • If the received Close frame contains no payload: event.code is 1005 (no code received) and event.reason is an empty string.
  • If no Close frame was received: event.code is 1006 (connection closed abnormally) and event.reason is an empty string.
ws.onerror = (event) => {}

ws.addEventListener("error", (event) => {})

Connection closed due to error. event type is Event.
Attribute ws.binaryType A string indicating the type of event.data in ws.onmessage when binary data is received. Initially set to "blob" (Blob object). May be changed to "arraybuffer" (ArrayBuffer object).
Read-only attribute ws.url The URL given to the WebSocket constructor.
ws.bufferedAmount The number of bytes waiting to be transmitted.
ws.protocol The protocol accepted by the server, or an empty string if the client did not specify protocols in the WebSocket constructor.
ws.extensions The extensions accepted by the server.
ws.readyState The connection state. It is one of the constants below.
Constant WebSocket.CONNECTING = 0 Waiting opening handshake.
WebSocket.OPEN = 1 Opening handshake succeeded. The client and server may message each other.
WebSocket.CLOSING = 2 Waiting closing handshake. Either ws.close() was called or the server sent a Close frame.
WebSocket.CLOSED = 3 The underlying TCP connection is closed.

Protocol

Steps:

  1. Opening handshake (HTTP request + HTTP response) to establish a connection.
  2. Data messages to transfer application data.
  3. Closing handshake (two Close frames) to close the connection.

Opening handshake

The client sends an HTTP request (method GET, version ≥ 1.1) and the server returns an HTTP response with status code 101 (Switching Protocols) on success. This means a WebSocket server can use the same port as HTTP (80) and HTTPS (443) because the handshake is compatible with HTTP.

HTTP headers
Side Header Value Mandatory
Request Origin Varies Yes (for browser clients)
Host Varies Yes
Sec-WebSocket-Version 13
Sec-WebSocket-Key base64-encode(16-byte random nonce)
Response Sec-WebSocket-Accept base64-encode(sha1(Sec-WebSocket-Key + "258EAFA5-E914-47DA-95CA-C5AB0DC85B11"))
Both Connection Upgrade
Upgrade websocket
Sec-WebSocket-Protocol The request may contain a comma-separated list of strings (ordered by preference) indicating application-level protocols (built on top of WebSocket data messages) the client wishes to use. If the client sends this header, the server response must be one of the values from the list. Optional
Sec-WebSocket-Extensions
Other headers Varies

Example request:

GET /chat HTTP/1.1
Host: server.example.com
Upgrade: websocket
Connection: Upgrade
Sec-WebSocket-Key: x3JJHMbDL1EzLkh9GBhXDw==
Sec-WebSocket-Protocol: chat, superchat
Sec-WebSocket-Version: 13
Origin: http://example.com

Example response: 

HTTP/1.1 101 Switching Protocols
Upgrade: websocket
Connection: Upgrade
Sec-WebSocket-Accept: HSmrc0sMlYUkAGmm5OPpG2HaGWk=
Sec-WebSocket-Protocol: chat

In HTTP each line ends in \r\n and the last line is empty.

# Calculate Sec-WebSocket-Accept using Sec-WebSocket-Key
from base64 import b64encode
from hashlib import sha1
from os import urandom
# key = b64encode(urandom(16)) # Client should do this
key = b"x3JJHMbDL1EzLkh9GBhXDw==" # Value in example request above
magic = b"258EAFA5-E914-47DA-95CA-C5AB0DC85B11" # Protocol constant
print(b64encode(sha1(key + magic).digest()))
# Output: HSmrc0sMlYUkAGmm5OPpG2HaGWk=

Once the connection is established, communication switches to a binary frame-based protocol which does not conform to the HTTP protocol.

Sec-WebSocket-Key and Sec-WebSocket-Accept are intended to prevent a caching proxy from re-sending a previous WebSocket conversation, and does not provide any authentication, privacy, or integrity.

Though some servers accept a short Sec-WebSocket-Key, many modern servers will reject the request with error "invalid Sec-WebSocket-Key header".

Frame-based message

After the opening handshake, the client and server can, at any time, send messages to each other, such as data messages (text or binary) and control messages (close, ping, pong). A message is composed of one or more frames.

Fragmentation allows a message to be split into two or more frames. It enables sending messages with initial data available but complete length unknown. Without fragmentation, the whole message must be sent in one frame, so the complete length is needed before the first byte can be sent, which requires a buffer. It also enables multiplexing several streams simultaneously (e.g. to avoid monopolizing a socket for a single large payload).

  • An unfragmented message consists of a single frame with FIN = 1 and opcode ≠ 0.
  • A fragmented message consists of a single frame with FIN = 0 and opcode ≠ 0, followed by zero or more frames with FIN = 0 and opcode = 0, and terminated by a single frame with FIN = 1 and opcode = 0.

Frame structure

Index
(in bits) 		Field Size
(in bits) 		Description
0 FIN 1
  • 1 = final frame of a message.
  • 0 = message is fragmented and this is not the final frame.
1 RSV1 1 Must be 0 unless defined by an extension.
2 RSV2 1
3 RSV3 1
4 Opcode 4 See opcodes below.
8 Masked 1
  • 1 = frame is masked and masking key is present.
  • 0 = frame is not masked and masking key is not present.
See client-to-server masking below.
9 Payload length 7, 7+16 or 7+64 Length of the payload (extension data + application data) in bytes.
  • 0–125 = This is the payload length.
  • 126 = The following 16 bits are the payload length.
  • 127 = The following 64 bits (MSB must be 0) are the payload length.
Endianness is big-endian. Signedness is unsigned. The minimum number of bits must be used to encode the length.
Varies Masking key 0 or 32 A client must mask all frames sent to the server. A server must not mask any frames sent to the client. Frame masking applies XOR between the masking key (a four-byte random nonce) and the payload data. The following algorithm is used to mask/unmask a frame:
for i = 0 to payload_length - 1
    payload = payload xor masking_key
Payload Extension data Payload length (in bytes) Must be empty unless defined by an extension.
Application data Depends on the opcode.

Opcodes

Frame type Opcode Related

Web API

Description Purpose Fragmentable Max. payload length
Continuation 0 Identifies an intermediate frame of a fragmented message. ⁠ 2 63 − 1 {\displaystyle 2^{63}-1} ⁠ bytes
Data frame Text 1 send(), onmessage UTF-8 encoded application text. Application data Yes
Binary 2 Application binary data.
3–7 Reserved
Control frame Close 8 close(), onclose A Close frame is sent to start the closing handshake which may prevent data loss by complementing the TCP closing handshake. No frame can be sent after a Close frame. If a Close frame is received and no prior Close frame was sent, a response Close frame with the same payload must be sent. The payload is optional, but if present, it must start with a two-byte big-endian unsigned integer reason code, optionally followed by a UTF-8 encoded reason message not longer than 123 bytes. Protocol state No 125 bytes
Ping 9 May be used for latency measurement, keepalive and heartbeat. Both sides can initiate a ping (with any payload). Whoever receives it must immediately send back a pong with the same payload. A pong should be ignored if no prior ping was sent.
Pong 10
11–15 Reserved

Status codes

Range Allowed in Close frame Code

Description
0–999 No Unused
1000–2999 (Protocol) Yes 1000 Normal closure.
1001 Going away (e.g. browser tab closed).
1002 Protocol error.
1003 Unsupported data (e.g. endpoint only understands text but received binary).
No 1004 Reserved for future usage
1005 No code received.
1006 Connection closed abnormally (closing handshake did not occur).
Yes 1007 Invalid payload data (e.g. non UTF-8 data in a text message).
1008 Policy violated.
1009 Message too big.
1010 Unsupported extension. The client should write the extensions it expected the server to support in the payload.
1011 Internal server error.
No 1015 TLS handshake failure.
3000–3999 Yes Used by libraries, frameworks and applications.
4000–4999 Private use.

Browser support

A secure version of the WebSocket protocol is implemented in Firefox 6, Safari 6, Google Chrome 14, Opera 12.10 and Internet Explorer 10. A detailed protocol test suite report lists the conformance of those browsers to specific protocol aspects.

An older, less secure version of the protocol was implemented in Opera 11 and Safari 5, as well as the mobile version of Safari in iOS 4.2. The BlackBerry Browser in OS7 implements WebSockets. Because of vulnerabilities, it was disabled in Firefox 4 and 5, and Opera 11. Using browser developer tools, developers can inspect the WebSocket handshake as well as the WebSocket frames.

ProtocolVersion Draft date Internet Explorer Firefox
(PC) 		Firefox
(Android) 		Chrome
(PC, Mobile) 		Safari
(Mac, iOS) 		Opera
(PC, Mobile) 		Android Browser
hixie-75 February 4, 2010 4 5.0.0
hixie-76
hybi-00 		May 6, 2010
May 23, 2010 		4.0
(disabled) 		6 5.0.1 11.00
(disabled)
hybi-07, v7 April 22, 2011 6
hybi-10, v8 July 11, 2011 7 7 14
RFC 6455, v13 December, 2011 10 11 11 16 6 12.10 4.4

Server implementations

  • Apache HTTP Server has supported WebSockets since July, 2013, implemented in version 2.4.5
  • Internet Information Services added support for WebSockets in version 8 which was released with Windows Server 2012.
  • lighttpd has supported WebSockets since 2017, implemented in lighttpd 1.4.46. lighttpd mod_proxy can act as a reverse proxy and load balancer of WebSocket applications. lighttpd mod_wstunnel can act as a WebSocket endpoint to transmit arbitrary data, including in JSON format, to a backend application. lighttpd supports WebSockets over HTTP/2 since 2022, implemented in lighttpd 1.4.65.

Security considerations

Unlike regular cross-domain HTTP requests, WebSocket requests are not restricted by the same-origin policy. Therefore, WebSocket servers must validate the "Origin" header against the expected origins during connection establishment, to avoid cross-site WebSocket hijacking attacks (similar to cross-site request forgery), which might be possible when the connection is authenticated with cookies or HTTP authentication. It is better to use tokens or similar protection mechanisms to authenticate the WebSocket connection when sensitive (private) data is being transferred over the WebSocket. A live example of vulnerability was seen in 2020 in the form of Cable Haunt.

Proxy traversal

WebSocket protocol client implementations try to detect whether the user agent is configured to use a proxy when connecting to destination host and port, and if it is, uses HTTP CONNECT method to set up a persistent tunnel.

While the WebSocket protocol itself is unaware of proxy servers and firewalls, it features an HTTP-compatible handshake, thus allowing HTTP servers to share their default HTTP and HTTPS ports (80 and 443 respectively) with a WebSocket gateway or server. The WebSocket protocol defines a ws:// and wss:// prefix to indicate a WebSocket and a WebSocket Secure connection respectively. Both schemes use an HTTP upgrade mechanism to upgrade to the WebSocket protocol. Some proxy servers are transparent and work fine with WebSocket; others will prevent WebSocket from working correctly, causing the connection to fail. In some cases, additional proxy-server configuration may be required, and certain proxy servers may need to be upgraded to support WebSocket.

If unencrypted WebSocket traffic flows through an explicit or a transparent proxy server without WebSockets support, the connection will likely fail.

If an encrypted WebSocket connection is used, then the use of Transport Layer Security (TLS) in the WebSocket Secure connection ensures that an HTTP CONNECT command is issued when the browser is configured to use an explicit proxy server. This sets up a tunnel, which provides low-level end-to-end TCP communication through the HTTP proxy, between the WebSocket Secure client and the WebSocket server. In the case of transparent proxy servers, the browser is unaware of the proxy server, so no HTTP CONNECT is sent. However, since the wire traffic is encrypted, intermediate transparent proxy servers may simply allow the encrypted traffic through, so there is a much better chance that the WebSocket connection will succeed if WebSocket Secure is used. Using encryption is not free of resource cost, but often provides the highest success rate, since it would be travelling through a secure tunnel.

A mid-2010 draft (version hixie-76) broke compatibility with reverse proxies and gateways by including eight bytes of key data after the headers, but not advertising that data in a Content-Length: 8 header. This data was not forwarded by all intermediates, which could lead to protocol failure. More recent drafts (e.g., hybi-09) put the key data in a Sec-WebSocket-Key header, solving this problem.

See also

Notes

  1. ^ Gecko-based browsers versions 6–10 implement the WebSocket object as "MozWebSocket", requiring extra code to integrate with existing WebSocket-enabled code.

References

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  30. Client requirement 9. p. 18. doi:10.17487/RFC6455. RFC 6455.
  31. Client requirement 7. p. 18. doi:10.17487/RFC6455. RFC 6455.
  32. Server step 5.4. p. 24. doi:10.17487/RFC6455. RFC 6455.
  33. Client requirement 6. p. 18. doi:10.17487/RFC6455. RFC 6455.
  34. Server step 5.3. p. 24. doi:10.17487/RFC6455. RFC 6455.
  35. Client requirement 5. p. 17. doi:10.17487/RFC6455. RFC 6455.
  36. Server step 5.2. p. 24. doi:10.17487/RFC6455. RFC 6455.
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  42. FIN. p. 28. doi:10.17487/RFC6455. RFC 6455.
  43. RSV1, RSV2, RSV3. p. 28. doi:10.17487/RFC6455. RFC 6455.
  44. Mask. p. 29. doi:10.17487/RFC6455. RFC 6455.
  45. Payload length. p. 29. doi:10.17487/RFC6455. RFC 6455.
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  47. Client-to-Server Masking. sec. 5.3. doi:10.17487/RFC6455. RFC 6455.
  48. Closing Handshake. sec. 1.4. doi:10.17487/RFC6455. RFC 6455.
  49. Close. sec. 5.5.1. doi:10.17487/RFC6455. RFC 6455.
  50. Ping. sec. 5.5.2. doi:10.17487/RFC6455. RFC 6455.
  51. Pong. sec. 5.5.3. doi:10.17487/RFC6455. RFC 6455.
  52. Reserved Status Code Ranges. sec. 7.4.2. doi:10.17487/RFC6455. RFC 6455.
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