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RFC 6455

The WebSocket Protocol

Pages: 71
Proposed Standard
Errata
Updated by:  793683078441
Part 2 of 4 – Pages 14 to 39
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Top   ToC   RFC6455 - Page 14   prevText

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/.
Top   ToC   RFC6455 - Page 15
   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.
Top   ToC   RFC6455 - Page 16
       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.
Top   ToC   RFC6455 - Page 17
       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.
Top   ToC   RFC6455 - Page 18
   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].
Top   ToC   RFC6455 - Page 19
   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.)
Top   ToC   RFC6455 - Page 20
   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.
Top   ToC   RFC6455 - Page 21

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.
Top   ToC   RFC6455 - Page 22
   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.
Top   ToC   RFC6455 - Page 23
       /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
Top   ToC   RFC6455 - Page 24
          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.
Top   ToC   RFC6455 - Page 25
       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
Top   ToC   RFC6455 - Page 26
      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
Top   ToC   RFC6455 - Page 27
   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.
Top   ToC   RFC6455 - Page 28
   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_.
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   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".
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   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.
Top   ToC   RFC6455 - Page 31
    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
Top   ToC   RFC6455 - Page 32
    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.
Top   ToC   RFC6455 - Page 33
   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
Top   ToC   RFC6455 - Page 34
   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.
Top   ToC   RFC6455 - Page 35
   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.
Top   ToC   RFC6455 - Page 36

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.
Top   ToC   RFC6455 - Page 37
   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.
Top   ToC   RFC6455 - Page 38
   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")
Top   ToC   RFC6455 - Page 39
   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)

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