Hi, On 17 Aug 2012, at 14:57, Freek Dijkstra <freek.dijkstra@sara.nl> wrote:
I like NML to be extensible. For example, if some NML publisher likes to add information that benefits monitoring or provisioning, that should be possible.
A NML receiver should silently ignore this added information. However, we also don't want a receiver to just accept anything. If there is some obvious error in the NML, the receiver should still just reply with an error message.
I am not entirely sure that we can reasonably make this distinction. And even if we can, without bloating the current schema into probably twice its current size.
Imagine a receiver that receives some NML line containing a relation.
1. The relation is well-defined. e.g. Node N --hasService--> SwitchingService S 2. The relation is unknown e.g. Node N --hasDynamicService--> SwitchingService S 3. The relation is clearly invalid e.g. Node N --hasService--> Node Y 4. The relation is known, but the meaning is undefined e.g. Port P --hasService--> LabelService L
Should we make a distinction between 3 and 4?
For the purpose of extensibility, I would argue that it is useful to make this distinction.
In fact, the current document makes this distinction. To quote the text on "hasService":
hasService relates a Network Object to a Service. This schema only defines the meaning of: • Port to AdaptationService, relating one server-layer Port to an adaptation function • Port to DeadaptationService, relating one server-layer Port to a deadaptation function • Node or Topology to SwitchingService, describing a switching capability of that Node or Topology.
So in this case "Port P --hasService--> LabelService L" would be syntactical VALID NML but with UNDEFINED meaning. "Node N --hasService--> Node Y" would simply be syntactically INVALID NML.
I am not sure how you make that distinction based on the description given above. For both Port and Node there is a possible "hasService" relation defined, and neither "LabelService" nor "Node" are defined as possible ranges of the hasService relation. Just the fact that "LabelService" is a new kind of object should not make it different.
Regardless if we think this distinction is useful, we should define how a NML receiver should handle the information. I can think of four situation what it should do with it: a) reject the line and reply with an error message b) accept the line and process it (for an known relation) c) silently drop the line d) store the line, even though it is unknown
I'm not sure if (d) is ever a good idea. It means that a receiver will accept a Topology from T, and store all, including unknown statements, and if asked by someone else, it would faithfully replicate the original topology, including all unknown statements.
D is a very well established way of handling things which is also applied to OSPFs extension fields for example. It is an excellent way of easing a new kind of relation into the system without having to rely on a flag-day where everyone switches over. Silently dropping is a really bad idea, which makes diagnostics a nightmare. I am very much in favor of always accepting and storing. Jeroen.