The ietf draft confused me by saying spdy runs over tcp, instead of saying it runs over tls over tcp. Probably I am misreading it --- I'm out of practice at reading rfcs, and I expect it would have been all over HN if they were no longer mandating tls --- but I didn't find any language in the current draft that says it runs over tls.
Upon closer review, you're right in that it's not very clear that SPDY requires TLS. I did find this in section 5, Security Considerations:
5.3. Cross-Protocol Attacks
By utilizing TLS, we believe that SPDY introduces no new cross-
protocol attacks. TLS encrypts the contents of all transmission
(except the handshake itself), making it difficult for attackers to
control the data which could be used in a cross-protocol attack.
Also interesting is that the language in the Overview makes it seem as though TCP itself is not a requirement and that SPDY can be used over any reliable transport layer such as TCP.
RFCs generally (or at least "modern RFCs when I was paying attention", which was about a decade ago) don't like to over-specify. e.g. RFC 2068 (HTTP/1.1) includes this paragraph:
HTTP communication usually takes place over TCP/IP connections. The default port is TCP 80, but other ports can be used. This does not preclude HTTP from being implemented on top of any other protocol on the Internet, or on other networks. HTTP only presumes a reliable transport; any protocol that provides such guarantees can be used; the mapping of the HTTP/1.1 request and response structures onto the transport data units of the protocol in question is outside the scope of this specification.
This makes clear that although you can use HTTP/1.1 over, say Unix domain sockets, and it would still work, and you could still call it HTTP/1.1, they expect it to be mostly used over TCP/IP port 80.
The language in the SPDY overview is probably the same sort of thing: making clear that they expect most usage to be over TCP, but allowing you to use something else if you really want to.
The TLS issue is probably intended to be the same thing: there's no reason why I shouldn't be able to use SPDY directly over TCP if the connection's between machines in my datacentre. But they don't make it explicit that they expect SPDY to run over TLS in normal use. Perhaps it's careless editing.
