Re: [tae] Transport negotiation

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Bryan Ford wrote:
> Hi Joe,
> 
> I wanted to follow up with you on your comment during my TSVAREA
> presentation, about using DNS as a mechanism for negotiation among
> multiple transports an application may support, since we didn't manage
> to hook up in person afterwards in Minneapolis.  (Note that I'm cc'ing
> this to a couple relevant lists.)
...
> 4. We haven't yet talked about precisely _how_ a DNS-based mechanism
> would handle transport negotiation.  As it turns out, the current DNS
> SRV mechanism of RFC 2872 is precisely wrong for this purpose, because
> it embeds the transport name (e.g., '_tcp' or '_udp') in the name to be
> looked up rather than in the information returned from the DNS lookup. 
> So if the client wants to see if the server supports each of N
> alternative transports, it has to make N separate DNS requests. 

Right. The trick is that there is no such thing as HTTP; it's HTTP over
TCP. What you want is "HTTP by any means"; whether the DNS supports this
is less the issue than the fact that IANA doesn't really define such a
thing.

> Granted, this
> issue could be fixed by defining a new (and incompatible) DNS SRV record
> type that includes only the service in the name and a list of supported
> transports in the (single) response, possibly with a separate (IP
> address or host name, port number) tuple for each - but then we start
> worrying about the effective size limitats of DNS responses...

I'm confused as to how you expect to proceed here. Whether it's an
amorphous SYN or a DNS request, you still need to be able to express
more than a single packet's worth of info, by definition. You talk about
TCP and SCTP, but in reality it's "TCP that supports the options I
require" that matters. I don't see how to negotiate how to negotiate
without starting off with a negotiation - which is what a TCP SYN, e.g., is.

There are several levels of such negotiation that need to happen - IP
version, TCP vs. SCTP vs. UDP etc., the options of the transport, etc.
How do you expect to bootstrap "what transports can you speak" without
some sort of common protocol?

Joe
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