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 RFC6455 in 2011. The current specification allowing web applications to use this protocol is known as WebSockets.[1] It is a living standard maintained by the WHATWG and a successor to The WebSocket API from the W3C.[2]
WebSocket is distinct from HTTP used to serve most webpages. Although they are different, RFC6455 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[3] 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.[4]
The WebSocket protocol specification defines ws (WebSocket) and wss (WebSocket Secure) as two new uniform resource identifier (URI) schemes[6] 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.[7]
History
WebSocket was first referenced as TCPConnection in the HTML5 specification, as a placeholder for a TCP-based socket API.[8] In June 2008, a series of discussions were led by Michael Carter that resulted in the first version of the protocol known as WebSocket.[9]
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,[10] 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.[11] 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.[12]
After the protocol was shipped and enabled by default in multiple browsers, the RFC6455 was finalized under Ian Fette in December 2011.
RFC7692 introduced compression extension to WebSocket using the DEFLATE algorithm on a per-message basis.
Client example
<!DOCTYPE html><script>// Connect to serverws=newWebSocket("ws://127.0.0.1/scoreboard")// Local server// ws = new WebSocket("wss://game.example.com/scoreboard") // Remote serverws.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
fromsocketimportsocketfrombase64importb64encodefromhashlibimportsha1MAGIC=b"258EAFA5-E914-47DA-95CA-C5AB0DC85B11"# Create socket and listen at port 80ws=socket()ws.bind(("",80))ws.listen()conn,addr=ws.accept()# Parse requestforlineinconn.recv(4096).split(b"\r\n"):ifline.startswith(b"Sec-WebSocket-Key"):nonce=line.split(b":")[1].strip()# Format responseresponse=f"""\HTTP/1.1 101 Switching ProtocolsUpgrade: websocketConnection: UpgradeSec-WebSocket-Accept: {b64encode(sha1(nonce+MAGIC).digest()).decode()}"""conn.send(response.replace("\n","\r\n").encode())whileTrue:# decode messages from the clientheader=conn.recv(2)FIN=bool(header[0]&0x80)# bit 0assertFIN==1,"We only support unfragmented messages"opcode=header[0]&0xf# bits 4-7assertopcode==1oropcode==2,"We only support data messages"masked=bool(header[1]&0x80)# bit 8assertmasked,"The client must mask all frames"payload_size=header[1]&0x7f# bits 9-15assertpayload_size<=125,"We only support small messages"masking_key=conn.recv(4)payload=bytearray(conn.recv(payload_size))foriinrange(payload_size):payload[i]=payload[i]^masking_key[i%4]print(payload)
Web API
A web application (e.g. web browser) may use the WebSocket interface to connect to a WebSocket server.
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.
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.
The client sends an HTTP request (methodGET, 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.[28]
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.[37] If the client sends this header, the server response must be one of the values from the list.
In HTTP each line ends in \r\n and the last line is empty.
# Calculate Sec-WebSocket-Accept using Sec-WebSocket-Keyfrombase64importb64encodefromhashlibimportsha1fromosimporturandom# key = b64encode(urandom(16)) # Client should do thiskey=b"x3JJHMbDL1EzLkh9GBhXDw=="# Value in example request abovemagic=b"258EAFA5-E914-47DA-95CA-C5AB0DC85B11"# Protocol constantprint(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 cachingproxy from re-sending a previous WebSocket conversation,[39] 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.[40] It also enables multiplexing several streams simultaneously (e.g. to avoid monopolizing a socket for a single large payload).[41]
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.
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.[46]
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:[47]
for i = 0 to payload_length - 1
payload[i] = payload[i] xor masking_key[i modulo 4]
Identifies an intermediate frame of a fragmented message.
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 startthe closing handshake which may prevent data loss by complementing the TCP closing handshake.[48] 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.[49]
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.[51][52]
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,[55] Safari 6, Google Chrome 14,[56]Opera 12.10 and Internet Explorer 10.[57] A detailed protocol test suite report[58] 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.[59] The BlackBerry Browser in OS7 implements WebSockets.[60] Because of vulnerabilities, it was disabled in Firefox 4 and 5,[61] and Opera 11.[62]
Using browser developer tools, developers can inspect the WebSocket handshake as well as the WebSocket frames.[63]
Nginx has supported WebSockets since 2013, implemented in version 1.3.13[72] including acting as a reverse proxy and load balancer of WebSocket applications.[73]
lighttpd has supported WebSockets since 2017, implemented in lighttpd 1.4.46.[77] 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.[78]
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.[79] 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.[80]
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.[81] This data was not forwarded by all intermediates, which could lead to protocol failure. More recent drafts (e.g., hybi-09[82]) put the key data in a Sec-WebSocket-Key header, solving this problem.
^ abGecko-based browsers versions 6–10 implement the WebSocket object as "MozWebSocket",[66] requiring extra code to integrate with existing WebSocket-enabled code.
^"Adobe Flash Platform – Sockets". help.adobe.com. Archived from the original on 2021-04-18. Retrieved 2021-07-28. TCP connections require a "client" and a "server". Flash Player can create client sockets.
^"Main Goal of WebSocket protocol". IETF. Archived from the original on 22 April 2016. Retrieved 25 July 2015. The computation [...] is meant to prevent a caching intermediary from providing a WS-client with a cached WS-server reply without actual interaction with the WS-server.
