Hi Freek;
My main point of criticism is that I do not (yet) understand why this protocol was developed; as a naive reader it seems to highly overlap with what has been done with WSDL. Unfortunately, I'm not an expert on either, so I might miss the point. WSDL was developed as a generic request-response protocol framework, and so is "Extensible Protocol for NMC".
I still like the work, for example the idea on metadata is good. I just recommend to (a) build more on existing frameworks (e.g. specify that the protocol MUST use webservices instead of SHOULD, and refer to WS-whatever for authentication extension) and (b) make it more clear what was added.
Thank you for these comments, I will record them on the TODO lists. Regarding the issue of 'why nmc' instead of something like WSDL, the idea was sprung from the original use case of encoding measurements. The rationale at the time was if we were using an encoding scheme for the measurements, wrapping a similarly constructed control structure around each for communication was not a far stretch and may even make the job of parsing and interpreting the information easier.
While I'm not very familiar with RELAX-NG, I think it is a good way forward, as it can be translated to XSD, and hopefully, this means there can be automated syntax check for messages. Has this been done so far?
In practice services don't implement a strict schema check since it can be expensive, but the tools exist for both major implementations to do this.
Last, the wording could be better in some places, especially when it comes to the RFC 2119 words (MUST, SHOULD, MAY). Two examples: "MAY or MAY NOT" (page 6), and my favourite: "Data MUST contain information, especially if it was empty in the initial request". Beside the obviousness of this statement (what else then information?) and the unclarity where "it" refers to, it seems that "SHOULD" was intended here, not MUST (why else the "especially"?).
It would be very helpful if you could go through and correct these mistakes as you see them - particularly since you have experience in writing documents of this nature. May I mark you down as willing to do this task? Thanks; -jason