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Could somebody clear this up for me (I've seem to forgotten it): I know you can run a HTTP and SPDY "versions" of your webpage in parallel. But how does the browser know which protocol to use (if it supports both)? Does it initially make a HTTP request and the server responds with some header that let's it know it can use SPDY or what?

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The server tells the client it can handle SPDY through an Alternate-Protocol header. The client then makes subsequent requests over SPDY.

http://dev.chromium.org/spdy/spdy-protocol/spdy-protocol-dra...

OK thanks. That cleared things up :) Too bad that this means that the first request (which is usually the most expensive one) is always done trough HTTP :/

Not really. The HTTPS requests can choose SPDY using the Next-Protocol-Negotiation(NPN).

Alternate protocol header would matter only to HTTP requests to know this server is capable of supporting SPDY. However, as per the latest post by Opera Labs[1], all 3 major browsers supporting SPDY doesn't handles this header correctly. Also, it's not mentioned in SPDY draft 3.

[1] http://dev.opera.com/articles/view/opera-spdy-build/

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