Section 3, along with a list of /protocols/ and /extensions/ to be used. Additionally, if the client is a web browser, it supplies /origin/.
Clients running in controlled environments, e.g., browsers on mobile handsets tied to specific carriers, MAY offload the management of the connection to another agent on the network. In such a situation, the client for the purposes of this specification is considered to include both the handset software and any such agents. When the client is to _Establish a WebSocket Connection_ given a set of (/host/, /port/, /resource name/, and /secure/ flag), along with a list of /protocols/ and /extensions/ to be used, and an /origin/ in the case of web browsers, it MUST open a connection, send an opening handshake, and read the server's handshake in response. The exact requirements of how the connection should be opened, what should be sent in the opening handshake, and how the server's response should be interpreted are as follows in this section. In the following text, we will use terms from Section 3, such as "/host/" and "/secure/ flag" as defined in that section. 1. The components of the WebSocket URI passed into this algorithm (/host/, /port/, /resource name/, and /secure/ flag) MUST be valid according to the specification of WebSocket URIs specified in Section 3. If any of the components are invalid, the client MUST _Fail the WebSocket Connection_ and abort these steps. 2. If the client already has a WebSocket connection to the remote host (IP address) identified by /host/ and port /port/ pair, even if the remote host is known by another name, the client MUST wait until that connection has been established or for that connection to have failed. There MUST be no more than one connection in a CONNECTING state. If multiple connections to the same IP address are attempted simultaneously, the client MUST serialize them so that there is no more than one connection at a time running through the following steps. If the client cannot determine the IP address of the remote host (for example, because all communication is being done through a proxy server that performs DNS queries itself), then the client MUST assume for the purposes of this step that each host name refers to a distinct remote host, and instead the client SHOULD limit the total number of simultaneous pending connections to a reasonably low number (e.g., the client might allow simultaneous pending connections to a.example.com and b.example.com, but if thirty simultaneous connections to a single host are requested, that may not be allowed). For example, in a web browser context, the client needs to consider the number of tabs the user has open in setting a limit to the number of simultaneous pending connections.
NOTE: This makes it harder for a script to perform a denial-of- service attack by just opening a large number of WebSocket connections to a remote host. A server can further reduce the load on itself when attacked by pausing before closing the connection, as that will reduce the rate at which the client reconnects. NOTE: There is no limit to the number of established WebSocket connections a client can have with a single remote host. Servers can refuse to accept connections from hosts/IP addresses with an excessive number of existing connections or disconnect resource- hogging connections when suffering high load. 3. _Proxy Usage_: If the client is configured to use a proxy when using the WebSocket Protocol to connect to host /host/ and port /port/, then the client SHOULD connect to that proxy and ask it to open a TCP connection to the host given by /host/ and the port given by /port/. EXAMPLE: For example, if the client uses an HTTP proxy for all traffic, then if it was to try to connect to port 80 on server example.com, it might send the following lines to the proxy server: CONNECT example.com:80 HTTP/1.1 Host: example.com If there was a password, the connection might look like: CONNECT example.com:80 HTTP/1.1 Host: example.com Proxy-authorization: Basic ZWRuYW1vZGU6bm9jYXBlcyE= If the client is not configured to use a proxy, then a direct TCP connection SHOULD be opened to the host given by /host/ and the port given by /port/. NOTE: Implementations that do not expose explicit UI for selecting a proxy for WebSocket connections separate from other proxies are encouraged to use a SOCKS5 [RFC1928] proxy for WebSocket connections, if available, or failing that, to prefer the proxy configured for HTTPS connections over the proxy configured for HTTP connections. For the purpose of proxy autoconfiguration scripts, the URI to pass the function MUST be constructed from /host/, /port/, /resource name/, and the /secure/ flag using the definition of a WebSocket URI as given in Section 3.
NOTE: The WebSocket Protocol can be identified in proxy autoconfiguration scripts from the scheme ("ws" for unencrypted connections and "wss" for encrypted connections). 4. If the connection could not be opened, either because a direct connection failed or because any proxy used returned an error, then the client MUST _Fail the WebSocket Connection_ and abort the connection attempt. 5. If /secure/ is true, the client MUST perform a TLS handshake over the connection after opening the connection and before sending the handshake data [RFC2818]. If this fails (e.g., the server's certificate could not be verified), then the client MUST _Fail the WebSocket Connection_ and abort the connection. Otherwise, all further communication on this channel MUST run through the encrypted tunnel [RFC5246]. Clients MUST use the Server Name Indication extension in the TLS handshake [RFC6066]. Once a connection to the server has been established (including a connection via a proxy or over a TLS-encrypted tunnel), the client MUST send an opening handshake to the server. The handshake consists of an HTTP Upgrade request, along with a list of required and optional header fields. The requirements for this handshake are as follows. 1. The handshake MUST be a valid HTTP request as specified by [RFC2616]. 2. The method of the request MUST be GET, and the HTTP version MUST be at least 1.1. For example, if the WebSocket URI is "ws://example.com/chat", the first line sent should be "GET /chat HTTP/1.1". 3. The "Request-URI" part of the request MUST match the /resource name/ defined in Section 3 (a relative URI) or be an absolute http/https URI that, when parsed, has a /resource name/, /host/, and /port/ that match the corresponding ws/wss URI. 4. The request MUST contain a |Host| header field whose value contains /host/ plus optionally ":" followed by /port/ (when not using the default port). 5. The request MUST contain an |Upgrade| header field whose value MUST include the "websocket" keyword.
6. The request MUST contain a |Connection| header field whose value MUST include the "Upgrade" token. 7. The request MUST include a header field with the name |Sec-WebSocket-Key|. The value of this header field MUST be a nonce consisting of a randomly selected 16-byte value that has been base64-encoded (see Section 4 of [RFC4648]). The nonce MUST be selected randomly for each connection. NOTE: As an example, if the randomly selected value was the sequence of bytes 0x01 0x02 0x03 0x04 0x05 0x06 0x07 0x08 0x09 0x0a 0x0b 0x0c 0x0d 0x0e 0x0f 0x10, the value of the header field would be "AQIDBAUGBwgJCgsMDQ4PEC==" 8. The request MUST include a header field with the name |Origin| [RFC6454] if the request is coming from a browser client. If the connection is from a non-browser client, the request MAY include this header field if the semantics of that client match the use-case described here for browser clients. The value of this header field is the ASCII serialization of origin of the context in which the code establishing the connection is running. See [RFC6454] for the details of how this header field value is constructed. As an example, if code downloaded from www.example.com attempts to establish a connection to ww2.example.com, the value of the header field would be "http://www.example.com". 9. The request MUST include a header field with the name |Sec-WebSocket-Version|. The value of this header field MUST be 13. NOTE: Although draft versions of this document (-09, -10, -11, and -12) were posted (they were mostly comprised of editorial changes and clarifications and not changes to the wire protocol), values 9, 10, 11, and 12 were not used as valid values for Sec-WebSocket-Version. These values were reserved in the IANA registry but were not and will not be used. 10. The request MAY include a header field with the name |Sec-WebSocket-Protocol|. If present, this value indicates one or more comma-separated subprotocol the client wishes to speak, ordered by preference. The elements that comprise this value MUST be non-empty strings with characters in the range U+0021 to U+007E not including separator characters as defined in [RFC2616] and MUST all be unique strings. The ABNF for the value of this header field is 1#token, where the definitions of constructs and rules are as given in [RFC2616].
11. The request MAY include a header field with the name |Sec-WebSocket-Extensions|. If present, this value indicates the protocol-level extension(s) the client wishes to speak. The interpretation and format of this header field is described in Section 9.1. 12. The request MAY include any other header fields, for example, cookies [RFC6265] and/or authentication-related header fields such as the |Authorization| header field [RFC2616], which are processed according to documents that define them. Once the client's opening handshake has been sent, the client MUST wait for a response from the server before sending any further data. The client MUST validate the server's response as follows: 1. If the status code received from the server is not 101, the client handles the response per HTTP [RFC2616] procedures. In particular, the client might perform authentication if it receives a 401 status code; the server might redirect the client using a 3xx status code (but clients are not required to follow them), etc. Otherwise, proceed as follows. 2. If the response lacks an |Upgrade| header field or the |Upgrade| header field contains a value that is not an ASCII case- insensitive match for the value "websocket", the client MUST _Fail the WebSocket Connection_. 3. If the response lacks a |Connection| header field or the |Connection| header field doesn't contain a token that is an ASCII case-insensitive match for the value "Upgrade", the client MUST _Fail the WebSocket Connection_. 4. If the response lacks a |Sec-WebSocket-Accept| header field or the |Sec-WebSocket-Accept| contains a value other than the base64-encoded SHA-1 of the concatenation of the |Sec-WebSocket- Key| (as a string, not base64-decoded) with the string "258EAFA5- E914-47DA-95CA-C5AB0DC85B11" but ignoring any leading and trailing whitespace, the client MUST _Fail the WebSocket Connection_. 5. If the response includes a |Sec-WebSocket-Extensions| header field and this header field indicates the use of an extension that was not present in the client's handshake (the server has indicated an extension not requested by the client), the client MUST _Fail the WebSocket Connection_. (The parsing of this header field to determine which extensions are requested is discussed in Section 9.1.)
6. If the response includes a |Sec-WebSocket-Protocol| header field and this header field indicates the use of a subprotocol that was not present in the client's handshake (the server has indicated a subprotocol not requested by the client), the client MUST _Fail the WebSocket Connection_. If the server's response does not conform to the requirements for the server's handshake as defined in this section and in Section 4.2.2, the client MUST _Fail the WebSocket Connection_. Please note that according to [RFC2616], all header field names in both HTTP requests and HTTP responses are case-insensitive. If the server's response is validated as provided for above, it is said that _The WebSocket Connection is Established_ and that the WebSocket Connection is in the OPEN state. The _Extensions In Use_ is defined to be a (possibly empty) string, the value of which is equal to the value of the |Sec-WebSocket-Extensions| header field supplied by the server's handshake or the null value if that header field was not present in the server's handshake. The _Subprotocol In Use_ is defined to be the value of the |Sec-WebSocket-Protocol| header field in the server's handshake or the null value if that header field was not present in the server's handshake. Additionally, if any header fields in the server's handshake indicate that cookies should be set (as defined by [RFC6265]), these cookies are referred to as _Cookies Set During the Server's Opening Handshake_.
RFC2616], the order of the header fields is not important), including but not limited to any violations of the ABNF grammar specified for the components of the handshake, the server MUST stop processing the client's handshake and return an HTTP response with an appropriate error code (such as 400 Bad Request). 1. An HTTP/1.1 or higher GET request, including a "Request-URI" [RFC2616] that should be interpreted as a /resource name/ defined in Section 3 (or an absolute HTTP/HTTPS URI containing the /resource name/). 2. A |Host| header field containing the server's authority. 3. An |Upgrade| header field containing the value "websocket", treated as an ASCII case-insensitive value. 4. A |Connection| header field that includes the token "Upgrade", treated as an ASCII case-insensitive value. 5. A |Sec-WebSocket-Key| header field with a base64-encoded (see Section 4 of [RFC4648]) value that, when decoded, is 16 bytes in length. 6. A |Sec-WebSocket-Version| header field, with a value of 13. 7. Optionally, an |Origin| header field. This header field is sent by all browser clients. A connection attempt lacking this header field SHOULD NOT be interpreted as coming from a browser client. 8. Optionally, a |Sec-WebSocket-Protocol| header field, with a list of values indicating which protocols the client would like to speak, ordered by preference. 9. Optionally, a |Sec-WebSocket-Extensions| header field, with a list of values indicating which extensions the client would like to speak. The interpretation of this header field is discussed in Section 9.1.
10. Optionally, other header fields, such as those used to send cookies or request authentication to a server. Unknown header fields are ignored, as per [RFC2616]. RFC5246]. 2. The server can perform additional client authentication, for example, by returning a 401 status code with the corresponding |WWW-Authenticate| header field as described in [RFC2616]. 3. The server MAY redirect the client using a 3xx status code [RFC2616]. Note that this step can happen together with, before, or after the optional authentication step described above. 4. Establish the following information: /origin/ The |Origin| header field in the client's handshake indicates the origin of the script establishing the connection. The origin is serialized to ASCII and converted to lowercase. The server MAY use this information as part of a determination of whether to accept the incoming connection. If the server does not validate the origin, it will accept connections from anywhere. If the server does not wish to accept this connection, it MUST return an appropriate HTTP error code (e.g., 403 Forbidden) and abort the WebSocket handshake described in this section. For more detail, refer to Section 10. /key/ The |Sec-WebSocket-Key| header field in the client's handshake includes a base64-encoded value that, if decoded, is 16 bytes in length. This (encoded) value is used in the creation of the server's handshake to indicate an acceptance of the connection. It is not necessary for the server to base64- decode the |Sec-WebSocket-Key| value.
/version/ The |Sec-WebSocket-Version| header field in the client's handshake includes the version of the WebSocket Protocol with which the client is attempting to communicate. If this version does not match a version understood by the server, the server MUST abort the WebSocket handshake described in this section and instead send an appropriate HTTP error code (such as 426 Upgrade Required) and a |Sec-WebSocket-Version| header field indicating the version(s) the server is capable of understanding. /resource name/ An identifier for the service provided by the server. If the server provides multiple services, then the value should be derived from the resource name given in the client's handshake in the "Request-URI" [RFC2616] of the GET method. If the requested service is not available, the server MUST send an appropriate HTTP error code (such as 404 Not Found) and abort the WebSocket handshake. /subprotocol/ Either a single value representing the subprotocol the server is ready to use or null. The value chosen MUST be derived from the client's handshake, specifically by selecting one of the values from the |Sec-WebSocket-Protocol| field that the server is willing to use for this connection (if any). If the client's handshake did not contain such a header field or if the server does not agree to any of the client's requested subprotocols, the only acceptable value is null. The absence of such a field is equivalent to the null value (meaning that if the server does not wish to agree to one of the suggested subprotocols, it MUST NOT send back a |Sec-WebSocket-Protocol| header field in its response). The empty string is not the same as the null value for these purposes and is not a legal value for this field. The ABNF for the value of this header field is (token), where the definitions of constructs and rules are as given in [RFC2616]. /extensions/ A (possibly empty) list representing the protocol-level extensions the server is ready to use. If the server supports multiple extensions, then the value MUST be derived from the client's handshake, specifically by selecting one or more of the values from the |Sec-WebSocket-Extensions| field. The absence of such a field is equivalent to the null value. The empty string is not the same as the null value for these
purposes. Extensions not listed by the client MUST NOT be listed. The method by which these values should be selected and interpreted is discussed in Section 9.1. 5. If the server chooses to accept the incoming connection, it MUST reply with a valid HTTP response indicating the following. 1. A Status-Line with a 101 response code as per RFC 2616 [RFC2616]. Such a response could look like "HTTP/1.1 101 Switching Protocols". 2. An |Upgrade| header field with value "websocket" as per RFC 2616 [RFC2616]. 3. A |Connection| header field with value "Upgrade". 4. A |Sec-WebSocket-Accept| header field. The value of this header field is constructed by concatenating /key/, defined above in step 4 in Section 4.2.2, with the string "258EAFA5- E914-47DA-95CA-C5AB0DC85B11", taking the SHA-1 hash of this concatenated value to obtain a 20-byte value and base64- encoding (see Section 4 of [RFC4648]) this 20-byte hash. The ABNF [RFC2616] of this header field is defined as follows: Sec-WebSocket-Accept = base64-value-non-empty base64-value-non-empty = (1*base64-data [ base64-padding ]) | base64-padding base64-data = 4base64-character base64-padding = (2base64-character "==") | (3base64-character "=") base64-character = ALPHA | DIGIT | "+" | "/" NOTE: As an example, if the value of the |Sec-WebSocket-Key| header field in the client's handshake were "dGhlIHNhbXBsZSBub25jZQ==", the server would append the string "258EAFA5-E914-47DA-95CA-C5AB0DC85B11" to form the string "dGhlIHNhbXBsZSBub25jZQ==258EAFA5-E914-47DA-95CA- C5AB0DC85B11". The server would then take the SHA-1 hash of this string, giving the value 0xb3 0x7a 0x4f 0x2c 0xc0 0x62 0x4f 0x16 0x90 0xf6 0x46 0x06 0xcf 0x38 0x59 0x45 0xb2 0xbe 0xc4 0xea. This value is then base64-encoded, to give the value "s3pPLMBiTxaQ9kYGzzhZRbK+xOo=", which would be returned in the |Sec-WebSocket-Accept| header field. 5. Optionally, a |Sec-WebSocket-Protocol| header field, with a value /subprotocol/ as defined in step 4 in Section 4.2.2.
6. Optionally, a |Sec-WebSocket-Extensions| header field, with a value /extensions/ as defined in step 4 in Section 4.2.2. If multiple extensions are to be used, they can all be listed in a single |Sec-WebSocket-Extensions| header field or split between multiple instances of the |Sec-WebSocket-Extensions| header field. This completes the server's handshake. If the server finishes these steps without aborting the WebSocket handshake, the server considers the WebSocket connection to be established and that the WebSocket connection is in the OPEN state. At this point, the server may begin sending (and receiving) data. Section 2.1 of [RFC2616], including the "implied *LWS rule". Note that the following ABNF conventions are used in this section. Some names of the rules correspond to names of the corresponding header fields. Such rules express values of the corresponding header fields, for example, the Sec-WebSocket-Key ABNF rule describes syntax of the |Sec-WebSocket-Key| header field value. ABNF rules with the "-Client" suffix in the name are only used in requests sent by the client to the server; ABNF rules with the "-Server" suffix in the name are only used in responses sent by the server to the client. For example, the ABNF rule Sec-WebSocket-Protocol-Client describes syntax of the |Sec-WebSocket-Protocol| header field value sent by the client to the server. The following new header fields can be sent during the handshake from the client to the server: Sec-WebSocket-Key = base64-value-non-empty Sec-WebSocket-Extensions = extension-list Sec-WebSocket-Protocol-Client = 1#token Sec-WebSocket-Version-Client = version base64-value-non-empty = (1*base64-data [ base64-padding ]) | base64-padding base64-data = 4base64-character base64-padding = (2base64-character "==") | (3base64-character "=") base64-character = ALPHA | DIGIT | "+" | "/" extension-list = 1#extension extension = extension-token *( ";" extension-param ) extension-token = registered-token registered-token = token
extension-param = token [ "=" (token | quoted-string) ] ; When using the quoted-string syntax variant, the value ; after quoted-string unescaping MUST conform to the ; 'token' ABNF. NZDIGIT = "1" | "2" | "3" | "4" | "5" | "6" | "7" | "8" | "9" version = DIGIT | (NZDIGIT DIGIT) | ("1" DIGIT DIGIT) | ("2" DIGIT DIGIT) ; Limited to 0-255 range, with no leading zeros The following new header fields can be sent during the handshake from the server to the client: Sec-WebSocket-Extensions = extension-list Sec-WebSocket-Accept = base64-value-non-empty Sec-WebSocket-Protocol-Server = token Sec-WebSocket-Version-Server = 1#version
The response from the server might look as follows: HTTP/1.1 400 Bad Request ... Sec-WebSocket-Version: 13, 8, 7 Note that the last response from the server might also look like: HTTP/1.1 400 Bad Request ... Sec-WebSocket-Version: 13 Sec-WebSocket-Version: 8, 7 The client now repeats the handshake that conforms to version 13: GET /chat HTTP/1.1 Host: server.example.com Upgrade: websocket Connection: Upgrade ... Sec-WebSocket-Version: 13 Section 10.3, a client MUST mask all frames that it sends to the server (see Section 5.3 for further details). (Note that masking is done whether or not the WebSocket Protocol is running over TLS.) The server MUST close the connection upon receiving a frame that is not masked. In this case, a server MAY send a Close frame with a status code of 1002 (protocol error) as defined in Section 7.4.1. A server MUST NOT mask any frames that it sends to the client. A client MUST close a connection if it detects a masked frame. In this case, it MAY use the status code 1002 (protocol error) as defined in Section 7.4.1. (These rules might be relaxed in a future specification.) The base framing protocol defines a frame type with an opcode, a payload length, and designated locations for "Extension data" and "Application data", which together define the "Payload data". Certain bits and opcodes are reserved for future expansion of the protocol.
A data frame MAY be transmitted by either the client or the server at any time after opening handshake completion and before that endpoint has sent a Close frame (Section 5.5.1). RFC5234] given in detail in this section. (Note that, unlike in other sections of this document, the ABNF in this section is operating on groups of bits. The length of each group of bits is indicated in a comment. When encoded on the wire, the most significant bit is the leftmost in the ABNF). A high-level overview of the framing is given in the following figure. In a case of conflict between the figure below and the ABNF specified later in this section, the figure is authoritative. 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-------+-+-------------+-------------------------------+ |F|R|R|R| opcode|M| Payload len | Extended payload length | |I|S|S|S| (4) |A| (7) | (16/64) | |N|V|V|V| |S| | (if payload len==126/127) | | |1|2|3| |K| | | +-+-+-+-+-------+-+-------------+ - - - - - - - - - - - - - - - + | Extended payload length continued, if payload len == 127 | + - - - - - - - - - - - - - - - +-------------------------------+ | |Masking-key, if MASK set to 1 | +-------------------------------+-------------------------------+ | Masking-key (continued) | Payload Data | +-------------------------------- - - - - - - - - - - - - - - - + : Payload Data continued ... : + - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - + | Payload Data continued ... | +---------------------------------------------------------------+ FIN: 1 bit Indicates that this is the final fragment in a message. The first fragment MAY also be the final fragment. RSV1, RSV2, RSV3: 1 bit each MUST be 0 unless an extension is negotiated that defines meanings for non-zero values. If a nonzero value is received and none of the negotiated extensions defines the meaning of such a nonzero value, the receiving endpoint MUST _Fail the WebSocket Connection_.
Opcode: 4 bits Defines the interpretation of the "Payload data". If an unknown opcode is received, the receiving endpoint MUST _Fail the WebSocket Connection_. The following values are defined. * %x0 denotes a continuation frame * %x1 denotes a text frame * %x2 denotes a binary frame * %x3-7 are reserved for further non-control frames * %x8 denotes a connection close * %x9 denotes a ping * %xA denotes a pong * %xB-F are reserved for further control frames Mask: 1 bit Defines whether the "Payload data" is masked. If set to 1, a masking key is present in masking-key, and this is used to unmask the "Payload data" as per Section 5.3. All frames sent from client to server have this bit set to 1. Payload length: 7 bits, 7+16 bits, or 7+64 bits The length of the "Payload data", in bytes: if 0-125, that is the payload length. If 126, the following 2 bytes interpreted as a 16-bit unsigned integer are the payload length. If 127, the following 8 bytes interpreted as a 64-bit unsigned integer (the most significant bit MUST be 0) are the payload length. Multibyte length quantities are expressed in network byte order. Note that in all cases, the minimal number of bytes MUST be used to encode the length, for example, the length of a 124-byte-long string can't be encoded as the sequence 126, 0, 124. The payload length is the length of the "Extension data" + the length of the "Application data". The length of the "Extension data" may be zero, in which case the payload length is the length of the "Application data".
Masking-key: 0 or 4 bytes All frames sent from the client to the server are masked by a 32-bit value that is contained within the frame. This field is present if the mask bit is set to 1 and is absent if the mask bit is set to 0. See Section 5.3 for further information on client- to-server masking. Payload data: (x+y) bytes The "Payload data" is defined as "Extension data" concatenated with "Application data". Extension data: x bytes The "Extension data" is 0 bytes unless an extension has been negotiated. Any extension MUST specify the length of the "Extension data", or how that length may be calculated, and how the extension use MUST be negotiated during the opening handshake. If present, the "Extension data" is included in the total payload length. Application data: y bytes Arbitrary "Application data", taking up the remainder of the frame after any "Extension data". The length of the "Application data" is equal to the payload length minus the length of the "Extension data". The base framing protocol is formally defined by the following ABNF [RFC5234]. It is important to note that the representation of this data is binary, not ASCII characters. As such, a field with a length of 1 bit that takes values %x0 / %x1 is represented as a single bit whose value is 0 or 1, not a full byte (octet) that stands for the characters "0" or "1" in the ASCII encoding. A field with a length of 4 bits with values between %x0-F again is represented by 4 bits, again NOT by an ASCII character or full byte (octet) with these values. [RFC5234] does not specify a character encoding: "Rules resolve into a string of terminal values, sometimes called characters. In ABNF, a character is merely a non-negative integer. In certain contexts, a specific mapping (encoding) of values into a character set (such as ASCII) will be specified." Here, the specified encoding is a binary encoding where each terminal value is encoded in the specified number of bits, which varies for each field.
ws-frame = frame-fin ; 1 bit in length frame-rsv1 ; 1 bit in length frame-rsv2 ; 1 bit in length frame-rsv3 ; 1 bit in length frame-opcode ; 4 bits in length frame-masked ; 1 bit in length frame-payload-length ; either 7, 7+16, ; or 7+64 bits in ; length [ frame-masking-key ] ; 32 bits in length frame-payload-data ; n*8 bits in ; length, where ; n >= 0 frame-fin = %x0 ; more frames of this message follow / %x1 ; final frame of this message ; 1 bit in length frame-rsv1 = %x0 / %x1 ; 1 bit in length, MUST be 0 unless ; negotiated otherwise frame-rsv2 = %x0 / %x1 ; 1 bit in length, MUST be 0 unless ; negotiated otherwise frame-rsv3 = %x0 / %x1 ; 1 bit in length, MUST be 0 unless ; negotiated otherwise frame-opcode = frame-opcode-non-control / frame-opcode-control / frame-opcode-cont frame-opcode-cont = %x0 ; frame continuation frame-opcode-non-control= %x1 ; text frame / %x2 ; binary frame / %x3-7 ; 4 bits in length, ; reserved for further non-control frames frame-opcode-control = %x8 ; connection close / %x9 ; ping / %xA ; pong / %xB-F ; reserved for further control ; frames ; 4 bits in length
frame-masked = %x0 ; frame is not masked, no frame-masking-key / %x1 ; frame is masked, frame-masking-key present ; 1 bit in length frame-payload-length = ( %x00-7D ) / ( %x7E frame-payload-length-16 ) / ( %x7F frame-payload-length-63 ) ; 7, 7+16, or 7+64 bits in length, ; respectively frame-payload-length-16 = %x0000-FFFF ; 16 bits in length frame-payload-length-63 = %x0000000000000000-7FFFFFFFFFFFFFFF ; 64 bits in length frame-masking-key = 4( %x00-FF ) ; present only if frame-masked is 1 ; 32 bits in length frame-payload-data = (frame-masked-extension-data frame-masked-application-data) ; when frame-masked is 1 / (frame-unmasked-extension-data frame-unmasked-application-data) ; when frame-masked is 0 frame-masked-extension-data = *( %x00-FF ) ; reserved for future extensibility ; n*8 bits in length, where n >= 0 frame-masked-application-data = *( %x00-FF ) ; n*8 bits in length, where n >= 0 frame-unmasked-extension-data = *( %x00-FF ) ; reserved for future extensibility ; n*8 bits in length, where n >= 0 frame-unmasked-application-data = *( %x00-FF ) ; n*8 bits in length, where n >= 0 Section 5.2.
The masking key is contained completely within the frame, as defined in Section 5.2 as frame-masking-key. It is used to mask the "Payload data" defined in the same section as frame-payload-data, which includes "Extension data" and "Application data". The masking key is a 32-bit value chosen at random by the client. When preparing a masked frame, the client MUST pick a fresh masking key from the set of allowed 32-bit values. The masking key needs to be unpredictable; thus, the masking key MUST be derived from a strong source of entropy, and the masking key for a given frame MUST NOT make it simple for a server/proxy to predict the masking key for a subsequent frame. The unpredictability of the masking key is essential to prevent authors of malicious applications from selecting the bytes that appear on the wire. RFC 4086 [RFC4086] discusses what entails a suitable source of entropy for security-sensitive applications. The masking does not affect the length of the "Payload data". To convert masked data into unmasked data, or vice versa, the following algorithm is applied. The same algorithm applies regardless of the direction of the translation, e.g., the same steps are applied to mask the data as to unmask the data. Octet i of the transformed data ("transformed-octet-i") is the XOR of octet i of the original data ("original-octet-i") with octet at index i modulo 4 of the masking key ("masking-key-octet-j"): j = i MOD 4 transformed-octet-i = original-octet-i XOR masking-key-octet-j The payload length, indicated in the framing as frame-payload-length, does NOT include the length of the masking key. It is the length of the "Payload data", e.g., the number of bytes following the masking key.
to split the message into smaller fragments to better share the output channel. (Note that the multiplexing extension is not described in this document.) Unless specified otherwise by an extension, frames have no semantic meaning. An intermediary might coalesce and/or split frames, if no extensions were negotiated by the client and the server or if some extensions were negotiated, but the intermediary understood all the extensions negotiated and knows how to coalesce and/or split frames in the presence of these extensions. One implication of this is that in absence of extensions, senders and receivers must not depend on the presence of specific frame boundaries. The following rules apply to fragmentation: o An unfragmented message consists of a single frame with the FIN bit set (Section 5.2) and an opcode other than 0. o A fragmented message consists of a single frame with the FIN bit clear and an opcode other than 0, followed by zero or more frames with the FIN bit clear and the opcode set to 0, and terminated by a single frame with the FIN bit set and an opcode of 0. A fragmented message is conceptually equivalent to a single larger message whose payload is equal to the concatenation of the payloads of the fragments in order; however, in the presence of extensions, this may not hold true as the extension defines the interpretation of the "Extension data" present. For instance, "Extension data" may only be present at the beginning of the first fragment and apply to subsequent fragments, or there may be "Extension data" present in each of the fragments that applies only to that particular fragment. In the absence of "Extension data", the following example demonstrates how fragmentation works. EXAMPLE: For a text message sent as three fragments, the first fragment would have an opcode of 0x1 and a FIN bit clear, the second fragment would have an opcode of 0x0 and a FIN bit clear, and the third fragment would have an opcode of 0x0 and a FIN bit that is set. o Control frames (see Section 5.5) MAY be injected in the middle of a fragmented message. Control frames themselves MUST NOT be fragmented. o Message fragments MUST be delivered to the recipient in the order sent by the sender.
o The fragments of one message MUST NOT be interleaved between the fragments of another message unless an extension has been negotiated that can interpret the interleaving. o An endpoint MUST be capable of handling control frames in the middle of a fragmented message. o A sender MAY create fragments of any size for non-control messages. o Clients and servers MUST support receiving both fragmented and unfragmented messages. o As control frames cannot be fragmented, an intermediary MUST NOT attempt to change the fragmentation of a control frame. o An intermediary MUST NOT change the fragmentation of a message if any reserved bit values are used and the meaning of these values is not known to the intermediary. o An intermediary MUST NOT change the fragmentation of any message in the context of a connection where extensions have been negotiated and the intermediary is not aware of the semantics of the negotiated extensions. Similarly, an intermediary that didn't see the WebSocket handshake (and wasn't notified about its content) that resulted in a WebSocket connection MUST NOT change the fragmentation of any message of such connection. o As a consequence of these rules, all fragments of a message are of the same type, as set by the first fragment's opcode. Since control frames cannot be fragmented, the type for all fragments in a message MUST be either text, binary, or one of the reserved opcodes. NOTE: If control frames could not be interjected, the latency of a ping, for example, would be very long if behind a large message. Hence, the requirement of handling control frames in the middle of a fragmented message. IMPLEMENTATION NOTE: In the absence of any extension, a receiver doesn't have to buffer the whole frame in order to process it. For example, if a streaming API is used, a part of a frame can be delivered to the application. However, note that this assumption might not hold true for all future WebSocket extensions.
Section 7.4. Following the 2-byte integer, the body MAY contain UTF-8-encoded data with value /reason/, the interpretation of which is not defined by this specification. This data is not necessarily human readable but may be useful for debugging or passing information relevant to the script that opened the connection. As the data is not guaranteed to be human readable, clients MUST NOT show it to end users. Close frames sent from client to server must be masked as per Section 5.3. The application MUST NOT send any more data frames after sending a Close frame. If an endpoint receives a Close frame and did not previously send a Close frame, the endpoint MUST send a Close frame in response. (When sending a Close frame in response, the endpoint typically echos the status code it received.) It SHOULD do so as soon as practical. An endpoint MAY delay sending a Close frame until its current message is sent (for instance, if the majority of a fragmented message is already sent, an endpoint MAY send the remaining fragments before sending a Close frame). However, there is no guarantee that the endpoint that has already sent a Close frame will continue to process data.
After both sending and receiving a Close message, an endpoint considers the WebSocket connection closed and MUST close the underlying TCP connection. The server MUST close the underlying TCP connection immediately; the client SHOULD wait for the server to close the connection but MAY close the connection at any time after sending and receiving a Close message, e.g., if it has not received a TCP Close from the server in a reasonable time period. If a client and server both send a Close message at the same time, both endpoints will have sent and received a Close message and should consider the WebSocket connection closed and close the underlying TCP connection. Section 5.5.3. An endpoint MAY send a Ping frame any time after the connection is established and before the connection is closed. NOTE: A Ping frame may serve either as a keepalive or as a means to verify that the remote endpoint is still responsive. Section 5.5.2 details requirements that apply to both Ping and Pong frames. A Pong frame sent in response to a Ping frame must have identical "Application data" as found in the message body of the Ping frame being replied to. If an endpoint receives a Ping frame and has not yet sent Pong frame(s) in response to previous Ping frame(s), the endpoint MAY elect to send a Pong frame for only the most recently processed Ping frame.
A Pong frame MAY be sent unsolicited. This serves as a unidirectional heartbeat. A response to an unsolicited Pong frame is not expected. Section 8.1. Binary The "Payload data" is arbitrary binary data whose interpretation is solely up to the application layer.
o Unmasked Ping request and masked Ping response * 0x89 0x05 0x48 0x65 0x6c 0x6c 0x6f (contains a body of "Hello", but the contents of the body are arbitrary) * 0x8a 0x85 0x37 0xfa 0x21 0x3d 0x7f 0x9f 0x4d 0x51 0x58 (contains a body of "Hello", matching the body of the ping) o 256 bytes binary message in a single unmasked frame * 0x82 0x7E 0x0100 [256 bytes of binary data] o 64KiB binary message in a single unmasked frame * 0x82 0x7F 0x0000000000010000 [65536 bytes of binary data] Section 9.1. Below are some anticipated uses of extensions. This list is neither complete nor prescriptive. o "Extension data" may be placed in the "Payload data" before the "Application data". o Reserved bits can be allocated for per-frame needs. o Reserved opcode values can be defined. o Reserved bits can be allocated to the opcode field if more opcode values are needed. o A reserved bit or an "extension" opcode can be defined that allocates additional bits out of the "Payload data" to define larger opcodes or more per-frame bits.