[Nml-wg] Summary of today (Mon,) discussion

[Freek Dijkstra]

Paola Grosso wrote:

> In essence, we have tried to define the basic classes that go into the 
> NML-WG schema, and began to define each class in more detail.
> We will use this as starting point for tomorrow discussion.

This work was presumably done after the formal meeting?
My compliments!!!! You seem to have been doing some real substantial work!


I have a few random comments (well, not truly random).

slide 2: Why this specific list of NDL basic classes? I personally would 
not say Service is a basic class, while Layer, AdaptationFunction and 
AdaptationProperty are.

slide 4: Nice. I never imagined paths and domains as similar concepts.
Paths are hard. It should be possible to refer to a path other than the 
explicit ordered sequence of links (i.e. but simply by its name). A path 
can also be an ordered sequence of other paths! Image a path through a 
network, another path through another network. Those in series form a 
new path as well. This is a useful to describe connections through 
domains without knowing the details. Note that this description of a 
path equals that of a "tandem connection" in ITU-T G.805 terminology, 
not of a "network connection" (which is a special form of a tandem 
connection). Paths as described here can only be used for "horizontal" 
grouping (on the same layer), as opposed to "vertical" grouping 
involving multiple layers -- see my comments at slide 7.

slide 4: Another term I've seen is technology domain. (which is slightly 
different from the "Layer" concept, which I like to use). This is 
arguable distinct  from a VLAN domain (while a technology domain is only 
a group of the same technology, a VLAN domain is a group of the same 
interface with the same technology AND the same label).
-> Please refer to the Stitching Framework ([1] page 6): they have a 
nice list of definitions for all these things.

slide 5: I recommend to distinguish between a device (the physical 
thing) and its (switching) capabilities. That would allow us to describe 
the (switching) capabilities of a domain without describing all details 
of each physical device (which IMHO is a scalability nightmare). I like 
to list that concept here.

slide 6: Are we talking about logical or physical interfaces? I strongly 
argue for logical interfaces.

slide 6: NOT every interface belongs to a node: it may also be useful to 
describe an interface of a domain, without telling the details of which 
devices it is connected to.
(I'll knowingly skip other problems like the description of a port in a 
patch panel, which may not be considered a device -- for our purpose 
it's not relevant, although if you want to use this schema for inventory 
management, it may be important.)

slide 6: I like a certain hierarchy in interfaces classes, from generic 
to technology specific. However, I think that using the OSI layer at the 
intermediate step is not a good idea. I prefer a "circuit switched 
interface" and a "packet switched" interface as intermediate, or no 
intermediate step. Also, I really oppose the term "Ethernet interface": 
I think Ethernet deserve two interface subclasses: one for the 
MAC-sublayer, and a different one for the VLAN-'sublayer'. But I'm 
willing to compromise, especially if we could have an easy mapping from 
our 'interfaces' / layers to GMPLS encodings.

slide 6: I strongly recommend to put as MUCH information in the generic 
Interface class as possible, and only rely on subclassing if all else 
fails. This is important to keep the schema as technology-agnostic as 
possible.
Do we really need to subclass Interfaces? Do we really need to subclass 
Devices? Do we really need to subclass Links? etc. etc. Perhaps the 
answer is yes to two of these questions, or perhaps to all three. But I 
like to remind people that in NDL we did NOT have to subclass ANY of the 
these! Still, we are able to find paths through nearly all technologies, 
including fiber, WDM, TDM, and VLANs. The only thing I needed subclasses 
for are for Ethernet MTU size, and for fiber physical properties 
(cladding of the fiber: multimode/singlemode, etc.).

slide 6: Note that rather than subclassing interfaces, subclassing link, 
subclassing switch matrices (/devices) all to technology-specific 
subclasses, I tried to come up with the notion of some sort of mix-in 
class. E.g. WDMNetworkElement, with a WDMInterface a subclass of both 
Interface and WDMNetworkElement, and a WDMDevice a subclass of both 
Device and WDMNetworkElemenet, etc. My experience: while this idea 
sounded nice, it didn't really work in practice. But again, do we really 
need all subclasses?

slide 7: I have a little preference for a generic type BroadcastSegment, 
with special case subclass Link. I have no strong preference for 
unidirectional/bidirectional links, but would suggest to make it easy to 
somehow group two unidirectional links to a bidirectional links. This is 
only easy since in the GLIF community, people are used to talking about 
bidirectional links.

slide 7: I am inclined to rant about the hierarchy of links here. I 
really, really like to see a clear distinction between HORIZONTAL 
grouping of links: a path as a series of links (G.805 terminology: a 
tandem connection is a series of link connections and/or subnetwork 
connections), and VERTICAL grouping of links: a layer /n/ link is a "sum 
of" adaptations and a layer /n-1/ path. (G.805 terminology: a link 
connection at a server layer is composed an adaptation source and sink 
with a netwerk connection on the client layer; where a network 
connection is a *terminated* tandem connection). The logic between these 
two groupings is distinct, and I really like us to adopt the G.805 
concepts (though I'm fine sticking to the current terminology).
-> Please refer to the concept of VERTICAL hierarchies in NDL (using 
adaptation functions) [2] and refer to the concept of HORIZONTAL 
hierarchies in cNIS [3].

slide 8: what is a node? Is a device a node? Is a domain a node? If a 
domain can be a node, the second bullet is not true anymore.

slide 9: I would recommend to the working group not to define services 
for the time being. That concept is part of the control plane, while I 
think our focus should be on the data plane for now. That said, by all 
means do include it if there are good counter arguments to do so. The 
same argument more or less holds for defining Locations: I'm not opposed 
to defining them (we did in NDL), but I much rather refer to work of 
other specialized groups, rather than reinventing the wheel here.

slide 11: I very much applaud the choice for UML diagrams, with both an 
RDF and XML schema. The use of UML prevents me from suggesting some 
Really Neat Features of RDF, but for interoperability it would be a Good 
Thing if I'm restricted in that sense ;-). That said, I'll gladly battle 
for the use of opaque URI's as identifiers for all class instances!

Regards,
Freek

[1] 
http://www.geant2.net/upload/pdf/GN2-07-066v5-DJ3-5-3-Report_on_Testing_of_Technology_Stitching.pdf
[2] http://www.science.uva.nl/research/sne/ndl/?c=12-Layer-Schema
[3] I got a file cnis_erdv0.14.png, but couldn't find a public reference 
yet.