Re: [core] Ben Campbell's Discuss on draft-ietf-core-coap-tcp-tls-08: (with DISCUSS and COMMENT)

[Carsten Bormann <[e-mail]>]

Hi Ben,

thank your for your review.

Before I start working on it in more detail, let me pick this one DISCUSS out because I think it is the most momentous issue that needs to be discussed:

> On May 10, 2017, at 04:27, Ben Campbell <[e-mail]> wrote:
> 
> 2) It seems problematic to encode the transport choice in the URI
> scheme.
> Section 7 says "They are hosted
> in distinct namespaces because each URI scheme implies a distinct
> origin
> server." IIUC, this means any given resource can only be reached over a
> specific transport. That seems to break the idea of cross-transport
> proxies
> as discussed in section 7.
> 
> It also does not seem to fit with a primary motivation for this draft.
> That
> is, one might want to use TCP because of local NAT/FW issues. But if
> there is
> a resource with a "coap" scheme, I cannot switch to TCP when I'm behind
> a
> problematic middlebox, and have an expectation of reaching the same
> resource.


I would rephrase the issue as:
URIs don’t have a good place to put in transport hints.

(The definition of a transport hint in a URI would be information that lets me set up specific transports without creating a separate resource each time the values of that transport hint differ.)

Note that the main use case for the document at hand is one where the client may not be able to reach the server on the “main” transport (CoAP over UDP), so negotiation/upgrade(*) mechanisms are not solving the problem.

The solutions that people are talking about in our domain are about carrying transport alternatives around together.
E.g., see draft-silverajan-core-coap-protocol-negotiation-05.txt, which provides a way to find out about links from a set of links (the resource directory) while specifying a transport type. [It may be instructive to go through previous versions of this document just to see what we also have tried to do.]

What we have right now in draft-ietf-core-coap-tcp-tls is what we think is the least ugly way to solve the problem.
The WG is well aware about its problems.
We’d rather have a mechanism like transport hints, but we did not find a solution that would not need to update the web architecture.

Grüße, Carsten

(*) This is not an “upgrade” in the sense of going to a higher version of something in any case; it is just an alternative transport.