[Nml-wg] About modelisation of the network description

[Freek Dijkstra]

Aurélien Cedeyn wrote:

> I send you a little document that i made which describes a new object in the 
> NML : the model object. All the description and motivation about this addon 
> are in the attached document.

Aurélien, thank you so much! Very good write down! These are the 
contributions that will help the NML workgroup a lot.

I wholeheartedly agree with what you write, although I use different 
terms, and I don't understand all details of what your propose yet. 
Allow me to ask a bit, so I'll understand.

> The NML goal is to instance modelisations of the real topology. This 
> real topology is too complex regarding the description needs of applications, 
> some informations are not needed.

True, and true. But I think that modelling of the real topology is only 
ONE OF the goals of NML. What you write is that you also like to see NML 
capable of describing "modelisations" of the real topology (I would call 
it abstractions of the real topology). I wholeheartedly agree that that 
should also be another goal of NML.

First a rather academic remark: what exactly is a "real topology" and a 
"modelisation" or "abstracted" topology? Most network engineers, even 
when asked for the "real topology" will describe fibers and devices, but 
still abstract a lot: they often leave out patch panels, and the 
internal workings of devices itself: because they either find it 
irrelevant (decribing patch panels is only relevant if you care about 
inventory management, or power loss details) or because they simply 
don't know the information (few people know how exactly devices work on 
the component level). In short: nearly everything is already a 
"modelisation", although the level of abstraction greatly differs 
between each model, and it is there where the discussion starts.

You propose an object "Model" for the abstraction of networks. So far 
I've seen "network", "domain" (in the perfSONAR schema), 
"abstract_link", "topology_point" (in cNIS), "AdminDomain", and 
"NetworkDomain" (NDL). I'm trying to figure out how these relate.

Your examples seems to imply that there can be multiple Model objects 
for each domain, one for each abstractions. So the first example has a 
model with the layer 2 devices in a network, and a model with the layer 
3 device in the same network. Is that correct, or is it a model 
describing the layer 2 switching capabilities and a model describing the 
layer 3 switching capabilities (rather than the layer 2/3 devices).
The functionality compares to a single AdminDomain in NDL plus two 
SwitchMatrix instantiations (describing respectively the layer 2 and 
layer 3 switching capabilities), although the object relations are 
slightly different.

Your second example is a new thing, which I have not seen before in one 
of the schemas discussed on this list. You seem to describe filtered 
views of a network. That is an interesting idea, but it seems different 
from an "abstracted view" of a network. In my opinion, a filter only 
describes part of the resources, while an abstraction describes the 
functions of all resources, but leaves out implementation details.

Finally, while you did not describe it in text, your picture has a very 
important concept: you want to relate the network description (interco 
information) to other resource descriptions ("cluster information" and 
"frontend information"). I take it that is a similar concept as what 
Cees de Laat is presenting that -in that case RDF- can be used to link 
together different resource descriptions (see e.g. slide 20 presented at 
OGF 20, http://staff.science.uva.nl/~delaat/talks/cdl-2007-05-07.pdf)

These are 3 different examples (describe capabilities, describe filtered 
view, link resources together), I at first did not understand the 
relation and what exactly a "Model" object represents. Am I correct that 
a "Model" simply means "a collection of resources"? If so, do you 
propose that the NML workgroup only describe this "grouping" object, or 
would you suggest to describe also dedicated grouping objects (e.g. also 
a "network" or "domain" object that describe specific groups of resources).

Regards,
Freek

(PS: sorry for the long mail)