 A diagram describing a connection using WebSocket | |
| International standard | RFC 6455 |
|---|---|
| Developed by | IETF |
| Industry | Computer science |
| Connector type | TCP |
| Website | https://websockets.spec.whatwg.org/ |
WebSocket is a computercommunications protocol, providing asimultaneous two-way communication channel over a singleTransmission Control Protocol (TCP) connection. The WebSocket protocol was standardized by theIETF asRFC 6455 in 2011. The current specification allowing web applications to use this protocol is known asWebSockets.[1] It is a living standard maintained by theWHATWG and a successor toThe WebSocket API from theW3C.[2]
WebSocket is distinct fromHTTP 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 WebSockethandshake uses theHTTP Upgrade header[3] to change from the HTTP protocol to the WebSocket protocol.
The WebSocket protocol enablesfull-duplex interaction between aweb browser (or otherclient application) and aweb server with lower overhead than half-duplex alternatives such as HTTPpolling, 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 TCPport number 443 (or 80 in the case of unsecured connections), which is beneficial for environments that block non-web Internet connections using afirewall. 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 asComet orAdobe Flash Player.[4]
Most browsers support the protocol, includingGoogle Chrome,Firefox,Microsoft Edge,Internet Explorer,Safari andOpera.[5]
The WebSocket protocol specification definesws (WebSocket) andwss (WebSocket Secure) as two newuniform resource identifier (URI) schemes[6] that are used for unencrypted and encrypted connections respectively. Apart from the scheme name andfragment (i.e.# is not supported), the rest of the URI components are defined to useURI generic syntax.[7]
WebSocket was first referenced as TCPConnection in theHTML5 specification, as a placeholder for a TCP-based socket API.[8] In June 2008, a series of discussions were led byMichael Carter that resulted in the first version of the protocol known as WebSocket.[9]Before WebSocket, port 80 full-duplex communication was attainable usingComet 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 byIan 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 andWHATWG 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, theRFC 6455 was finalized under Ian Fette in December 2011.
RFC 7692 introduced compression extension to WebSocket using theDEFLATE algorithm on a per-message basis.
<!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>
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)
A web application (e.g. web browser) may use theWebSocket interface to connect to a WebSocket server.
| Type | Name[13] | Description |
|---|---|---|
| Constructor | ws = newWebSocket(url [, protocols ]) | Startopening handshake with a WebSocket server.[14]
|
| Method | ws.send(data) | Send data.data must bestring,Blob,ArrayBuffer orArrayBufferView. ThrowInvalidStateError ifws.readyState isWebSocket.CONNECTING. |
ws.close([ code ] [, reason ]) | Startclosing handshake.[15]
| |
| Event | ws.onopen = (event) => {}
| Opening handshake succeeded.event type isEvent. |
ws.onmessage = (event) => {}
| Data received.event type isMessageEvent.event.data contains the data received, of type:[16]
| |
ws.onclose = (event) => {}
| The underlyingTCP connection closed.event type isCloseEvent containing:[17][18][19][20]
Note:
| |
ws.onerror = (event) => {}
| Connection closed due to error.event type isEvent. | |
| Attribute | ws.binaryType | A string indicating the type ofevent.data inws.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 specifyprotocols in theWebSocket 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.[21][22] |
WebSocket.OPEN = 1 | Opening handshake succeeded. The client and server may message each other.[23][24] | |
WebSocket.CLOSING = 2 | Waitingclosing handshake. Eitherws.close() was called or the server sent a Close frame.[25][26] | |
WebSocket.CLOSED = 3 | The underlyingTCP connection is closed.[27][17][18] |
Steps:
The client sends anHTTP request (methodGET,version ≥ 1.1) and the server returns anHTTP response withstatus 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]
Side | Header | Value | Mandatory |
|---|---|---|---|
Request | Origin | Varies | Yes (for browser clients)[29] |
| Host | Varies | Yes | |
| Sec-WebSocket-Version | 13[30] | ||
| Sec-WebSocket-Key | base64-encode(16-byte randomnonce)[31] | ||
Response | Sec-WebSocket-Accept | base64-encode(sha1(Sec-WebSocket-Key +"258EAFA5-E914-47DA-95CA-C5AB0DC85B11"))[32] | |
Both | Connection | Upgrade[33][34] | |
| Upgrade | websocket[35][36] | ||
| Sec-WebSocket-Protocol | The request may contain acomma-separated list of strings (ordered by preference) indicatingapplication-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. | Optional | |
| Sec-WebSocket-Extensions | |||
| Other headers | Varies |
Example request:[38]
GET/chatHTTP/1.1Host:server.example.comUpgrade:websocketConnection:UpgradeSec-WebSocket-Key:x3JJHMbDL1EzLkh9GBhXDw==Sec-WebSocket-Protocol:chat, superchatSec-WebSocket-Version:13Origin:http://example.com
Example response:
HTTP/1.1101Switching ProtocolsUpgrade:websocketConnection:UpgradeSec-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-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 andSec-WebSocket-Accept are intended to prevent acachingproxy from re-sending a previous WebSocket conversation,[39] and does not provide any authentication, privacy, or integrity.
Though some servers accept a shortSec-WebSocket-Key, many modern servers will reject the request with error "invalid Sec-WebSocket-Key header".
After the opening handshake, the client and server can, at any time, send messages to each other, such asdata messages (text or binary) andcontrol 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 largepayload).[41]
FIN = 1 andopcode ≠ 0.FIN = 0 andopcode ≠ 0, followed by zero or more frames withFIN = 0 andopcode = 0, and terminated by a single frame withFIN = 1 andopcode = 0.| Index (in bits) | Field | Size (in bits) | Description | |
|---|---|---|---|---|
| 0 | FIN[42] | 1 |
| |
| 1 | RSV1 | 1 | Must be 0 unless defined by an extension.[43] | |
| 2 | RSV2 | 1 | ||
| 3 | RSV3 | 1 | ||
| 4 | Opcode | 4 | Seeopcodes below. | |
| 8 | Masked[44] | 1 |
| |
| 9 | Payload length[45] | 7, 7+16 or 7+64 | Length of the payload (extension data + application data) in bytes.
| |
| Varies | Masking key | 0 or 32 | Aclient must mask all frames sent to the server. Aserver must not mask any frames sent to the client.[46]Frame masking appliesXOR 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] | |
| Payload | Extension data | Payload length (in bytes) | Must be empty unless defined by an extension. | |
| Application data | Depends on the opcode. | |||
| Frame type | Opcode | Related | Description | Purpose | Fragmentable | Max. payload length | |
|---|---|---|---|---|---|---|---|
| Continuation | 0 | 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 tostartthe closing handshake which may prevent data loss by complementing theTCP 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 integerreason code, optionally followed by a UTF-8 encoded reason message not longer than 123 bytes.[49] | Protocol state | No | 125 bytes[50] |
| Ping | 9 | May be used forlatency measurement,keepalive andheartbeat. 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] | |||||
| Pong | 10 | ||||||
| 11–15 | Reserved | ||||||
| Range[53] | 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. |
A secure version of the WebSocket protocol is implemented in Firefox 6,[55] Safari 6, Google Chrome 14,[56]Opera 12.10 andInternet 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 andSafari 5, as well as the mobile version of Safari iniOS 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]
Protocol Version | Draft date | Internet Explorer | Firefox[64] (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[65][a] | ||||||
| hybi-10, v8 | July 11, 2011 | 7[67][a] | 7 | 14[68] | ||||
| RFC 6455, v13 | December, 2011 | 10[69] | 11 | 11 | 16[70] | 6 | 12.10[71] | 4.4 |
Unlike regular cross-domain HTTP requests, WebSocket requests are not restricted by thesame-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 tocross-site request forgery), which might be possible when the connection is authenticated withcookies 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 ofCable Haunt.
WebSocket protocol client implementations try to detect whether theuser agent is configured to use a proxy when connecting to destination host and port, and if it is, usesHTTP 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 anHTTP 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 ofTransport Layer Security (TLS) in the WebSocket Secure connection ensures that anHTTP 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 noHTTP 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 withreverse proxies and gateways by including eight bytes of key data after the headers, but not advertising that data in aContent-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 aSec-WebSocket-Key header, solving this problem.
TCP connections require a "client" and a "server". Flash Player can create client sockets.
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.