RFC 6455
The WebSocket Protocol
4. Opening Handshake
4.1. Client Requirements
To _Establish a WebSocket Connection_, a client opens a connection and sends a handshake as defined in this section. A connection is defined to initially be in a CONNECTING state. A client will need to supply a /host/, /port/, /resource name/, and a /secure/ flag, which are the components of a WebSocket URI as discussed in 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_.
4.2. Server-Side Requirements
Servers MAY offload the management of the connection to other agents
on the network, for example, load balancers and reverse proxies. In
such a situation, the server for the purposes of this specification
is considered to include all parts of the server-side infrastructure
from the first device to terminate the TCP connection all the way to
the server that processes requests and sends responses.
EXAMPLE: A data center might have a server that responds to WebSocket
requests with an appropriate handshake and then passes the connection
to another server to actually process the data frames. For the
purposes of this specification, the "server" is the combination of
both computers.
4.2.1. Reading the Client's Opening Handshake
When a client starts a WebSocket connection, it sends its part of the opening handshake. The server must parse at least part of this handshake in order to obtain the necessary information to generate the server part of the handshake. The client's opening handshake consists of the following parts. If the server, while reading the handshake, finds that the client did not send a handshake that matches the description below (note that as per [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].
4.2.2. Sending the Server's Opening Handshake
When a client establishes a WebSocket connection to a server, the
server MUST complete the following steps to accept the connection and
send the server's opening handshake.
1. If the connection is happening on an HTTPS (HTTP-over-TLS) port,
perform a TLS handshake over the connection. If this fails
(e.g., the client indicated a host name in the extended client
hello "server_name" extension that the server does not host),
then close the connection; otherwise, all further communication
for the connection (including the server's handshake) MUST run
through the encrypted tunnel [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.
4.3. Collected ABNF for New Header Fields Used in Handshake
This section is using ABNF syntax/rules from 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
4.4. Supporting Multiple Versions of WebSocket Protocol
This section provides some guidance on supporting multiple versions
of the WebSocket Protocol in clients and servers.
Using the WebSocket version advertisement capability (the
|Sec-WebSocket-Version| header field), a client can initially request
the version of the WebSocket Protocol that it prefers (which doesn't
necessarily have to be the latest supported by the client). If the
server supports the requested version and the handshake message is
otherwise valid, the server will accept that version. If the server
doesn't support the requested version, it MUST respond with a
|Sec-WebSocket-Version| header field (or multiple
|Sec-WebSocket-Version| header fields) containing all versions it is
willing to use. At this point, if the client supports one of the
advertised versions, it can repeat the WebSocket handshake using a
new version value.
The following example demonstrates version negotiation described
above:
GET /chat HTTP/1.1
Host: server.example.com
Upgrade: websocket
Connection: Upgrade
...
Sec-WebSocket-Version: 25
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
5. Data Framing
5.1. Overview
In the WebSocket Protocol, data is transmitted using a sequence of
frames. To avoid confusing network intermediaries (such as
intercepting proxies) and for security reasons that are further
discussed in 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).5.2. Base Framing Protocol
This wire format for the data transfer part is described by the ABNF [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
5.3. Client-to-Server Masking
A masked frame MUST have the field frame-masked set to 1, as defined
in 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.5.4. Fragmentation
The primary purpose of fragmentation is to allow sending a message that is of unknown size when the message is started without having to buffer that message. If messages couldn't be fragmented, then an endpoint would have to buffer the entire message so its length could be counted before the first byte is sent. With fragmentation, a server or intermediary may choose a reasonable size buffer and, when the buffer is full, write a fragment to the network. A secondary use-case for fragmentation is for multiplexing, where it is not desirable for a large message on one logical channel to monopolize the output channel, so the multiplexing needs to be free
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.
5.5. Control Frames
Control frames are identified by opcodes where the most significant bit of the opcode is 1. Currently defined opcodes for control frames include 0x8 (Close), 0x9 (Ping), and 0xA (Pong). Opcodes 0xB-0xF are reserved for further control frames yet to be defined. Control frames are used to communicate state about the WebSocket. Control frames can be interjected in the middle of a fragmented message. All control frames MUST have a payload length of 125 bytes or less and MUST NOT be fragmented.5.5.1. Close
The Close frame contains an opcode of 0x8. The Close frame MAY contain a body (the "Application data" portion of the frame) that indicates a reason for closing, such as an endpoint shutting down, an endpoint having received a frame too large, or an endpoint having received a frame that does not conform to the format expected by the endpoint. If there is a body, the first two bytes of the body MUST be a 2-byte unsigned integer (in network byte order) representing a status code with value /code/ defined in 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.5.5.2. Ping
The Ping frame contains an opcode of 0x9. A Ping frame MAY include "Application data". Upon receipt of a Ping frame, an endpoint MUST send a Pong frame in response, unless it already received a Close frame. It SHOULD respond with Pong frame as soon as is practical. Pong frames are discussed in 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.5.5.3. Pong
The Pong frame contains an opcode of 0xA. 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.5.6. Data Frames
Data frames (e.g., non-control frames) are identified by opcodes where the most significant bit of the opcode is 0. Currently defined opcodes for data frames include 0x1 (Text), 0x2 (Binary). Opcodes 0x3-0x7 are reserved for further non-control frames yet to be defined. Data frames carry application-layer and/or extension-layer data. The opcode determines the interpretation of the data: Text The "Payload data" is text data encoded as UTF-8. Note that a particular text frame might include a partial UTF-8 sequence; however, the whole message MUST contain valid UTF-8. Invalid UTF-8 in reassembled messages is handled as described in Section 8.1. Binary The "Payload data" is arbitrary binary data whose interpretation is solely up to the application layer.5.7. Examples
o A single-frame unmasked text message * 0x81 0x05 0x48 0x65 0x6c 0x6c 0x6f (contains "Hello") o A single-frame masked text message * 0x81 0x85 0x37 0xfa 0x21 0x3d 0x7f 0x9f 0x4d 0x51 0x58 (contains "Hello") o A fragmented unmasked text message * 0x01 0x03 0x48 0x65 0x6c (contains "Hel") * 0x80 0x02 0x6c 0x6f (contains "lo")
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]
5.8. Extensibility
The protocol is designed to allow for extensions, which will add
capabilities to the base protocol. The endpoints of a connection
MUST negotiate the use of any extensions during the opening
handshake. This specification provides opcodes 0x3 through 0x7 and
0xB through 0xF, the "Extension data" field, and the frame-rsv1,
frame-rsv2, and frame-rsv3 bits of the frame header for use by
extensions. The negotiation of extensions is discussed in further
detail in 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.
(page 39 continued on part 3)
