WebSocket

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 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, 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 was finalized under Ian Fette in December 2011.

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

Web API

A web application (e.g. web browser) may use the WebSocket interface to maintain bidirectional communications with a WebSocket server.

Client example

Interface

Protocol

Steps:
# Opening handshake: HTTP request + HTTP response.
# Exchange frame-based messages: application data, ping and pong messages.
# Closing handshake: request + response ''Close'' frames.

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. HTTP and WebSocket clients can connect to a server using the same port because the handshake is compatible with HTTP. Sending additional HTTP headers (that are not in the table below) is allowed. HTTP headers may be sent in any order. After the ''Switching Protocols'' HTTP response, the opening handshake is complete, the HTTP protocol stops being used, and communication switches to a binary frame-based protocol.

The following Python code generates a random Sec-WebSocket-Key.

import os, base64
print(base64.b64encode(os.urandom(16)))

Example request:

GET /chat HTTP/1.1
Host: server.example.com
Upgrade: websocket
Connection: Upgrade
Sec-WebSocket-Key: dGhlIHNhbXBsZSBub25jZQ

Origin: http://example.com
Sec-WebSocket-Protocol: chat, superchat
Sec-WebSocket-Version: 13

Example response:

HTTP/1.1 101 Switching Protocols
Upgrade: websocket
Connection: Upgrade
Sec-WebSocket-Accept: s3pPLMBiTxaQ9kYGzzhZRbK+xOo=
Sec-WebSocket-Protocol: chat

The following Python code calculates Sec-WebSocket-Accept using Sec-WebSocket-Key from the example above.

import base64, hashlib
Sec_WebSocket_Key = b"dGhlIHNhbXBsZSBub25jZQ

"
MAGIC = b"258EAFA5-E914-47DA-95CA-C5AB0DC85B11"
print(base64.b64encode(hashlib.sha1(Sec_WebSocket_Key + MAGIC).digest()))

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 data messages (text or binary) and control messages (''Close'', ''Ping'', ''Pong'') to each other. A message is composed of one frame if not fragmented or at least two frames if fragmented.

Fragmentation splits a message 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 one frame with FIN = 1 and opcode ≠ 0.
* A fragmented message consists of one frame with FIN = 0 and opcode ≠ 0, followed by zero or more frames with FIN = 0 and opcode = 0, and terminated by one frame with FIN = 1 and opcode = 0.

Frame structure

Opcodes

Status codes

Server implementation example

In Python.

from socket import socket
from base64 import b64encode
from hashlib import sha1
import struct

MAGIC = b"258EAFA5-E914-47DA-95CA-C5AB0DC85B11"

# Create socket and listen (on all network interfaces) 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"):
Sec_WebSocket_Key = line.split(b":") strip()

# Format response
response = f"""\
HTTP/1.1 101 Switching Protocols
Upgrade: websocket
Connection: Upgrade
Sec-WebSocket-Accept:

"""

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 % 4 conn.send(struct.pack("BB", 0x80 , opcode, payload_size) + payload) # echo message
print("Received", "text" if opcode

1 else "binary", "message", payload)

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.

Server implementations

* Nginx has supported WebSockets since 2013, implemented in version 1.3.13 including acting as a reverse proxy and load balancer of WebSocket applications.

* 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.

ASP.NET Core have support for WebSockets using the middleware.

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

* Comparison of WebSocket implementations
* Network socket
* Push technology
* XMLHttpRequest
* Server-sent events
* WebRTC
* HTTP/2
* Internet protocol suite
* BOSH

Notes

References

External links

IETF Hypertext-Bidirectional (HyBi) working group
** The WebSocket protocol – Proposed Standard published by the IETF HyBi Working Group
*
The WebSocket protocol
– Internet-Draft published by the IETF HyBi Working Group
*
The WebSocket protocol
– Original protocol proposal by Ian Hickson

The WebSocket API
– W3C Working Draft specification of the API

The WebSocket API
– W3C Candidate Recommendation specification of the API

WebSocket.org
WebSocket demos, loopback tests, general information and community

{{Web interfaces

Web standards
Application layer protocols
HTML5
Internet terminology
Network socket
Real-time web
Web development
2011 in computing