[hybi] Sec-WebSocket-Protocl

Dan Adkins <dadkins@google.com> Fri, 17 June 2011 20:53 UTC

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Date: Fri, 17 Jun 2011 13:53:23 -0700
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From: Dan Adkins <dadkins@google.com>
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Subject: [hybi] Sec-WebSocket-Protocl
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If the client does not specify a subprotocol, is the server allowed to
respond with one?

Section 1.3:
   The |Sec-WebSocket-Protocol| request-header field can be used to
   indicate what subprotocols (application-level protocols layered over
   the WebSocket protocol) are acceptable to the client.  The server
   selects one of the acceptable protocols and echoes that value in its
   handshake to indicate that it has selected that protocol.
...
  Option fields can also be included.  In this version of the protocol,
   the main option field is |Sec-WebSocket-Protocol|, which indicates
   the subprotocol that the server has selected.  Web browsers verify
   that the server included one of the values as was specified in the
   WebSocket client's handshake.  A server that speaks multiple
   subprotocols has to make sure it selects one based on the client's
   handshake and specifies it in its handshake.

>From this, it appears not.  But there's some ambiguity with the
mention of multiple subprotocols.

Section 5.2.2:
       /subprotocol/
          Either a single value or null, representing the subprotocol
          the server is ready to use.  If the server supports multiple
          subprotocols, then the value MUST be derived from the client's
          handshake, specifically by selecting one of the values from
          the "Sec-WebSocket-Protocol" 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, and is not
          a legal value for this field.  The ABNF for the value of this
          header is (token), where the definitions of constructs and
          rules are as given in [RFC2616].

So, we know what the server MUST do if the server supports multiple
subprotocols.  But what if the client didn't include that field?  Is
the server free to respond with the protocol of its choice?  That
seems strange that the server would be allowed to say to the client,
"we're speaking this protocol, like it or not," as the client has no
opportunity to respond.

Maybe I'm reading too much into this, but it does seem like there's a
bit of a hole in the spec as I can't find the answer to my original
question: if the client doesn't specify a subprotocol, can the server
dictate one?

-Dan