= websocket transport
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This transport provides support for SP over websocket using TCP or TLS.
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When using TCP, it is compatible with the libnanomsg legacy transport.
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It also is compatible with mangos (both TCP and TLS).
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TLS support requires the mbedTLS library.
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We set the "protocol" such as "pair.sp.nanomsg.org" in the
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Sec-WebSocket-Protocol field -- the client sets to the the server's
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protocol - i.e. the protocol that the server speaks. For example,
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if the the server is a REP, then a REQ client would send "rep.sp.nanomsg.org".
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The server sends the same value (it's own), per the WebSocket specs. (Note
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that the client's protocol is never sent, but assumed to be complementary
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to the protocol in the Sec-WebSocket-Protocol field.)
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Each SP message is a WebSocket message.
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WebSocket is defined in RFC 6455.
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== Design
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We unfortunately need to implement our own design for this -- the only
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reasonable client library would be libcurl, and there is a dearth of
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suitable server libraries. Since we don't have to support full HTTP, but
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just the initial handshake, this isn't too tragic.
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== Multiple Server Sockets
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In order to support Multiple Server sockets listening on the same port,
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the application must be long lived. We will set up a listener on the
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configured TCP (or TLS) port, and examine the PATH supplied in the GET.
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This will be used to match against the URL requested, and if the URL
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matches we will create the appropriate pipe.
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If no server endpoint at that address can be found, we return an
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HTTP error, and close the socket.
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