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. 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. 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. My proposal is to: 1. The relation is well-defined. Requirement b: The receiver SHOULD accept and process the NML. 2. The relation is unknown Requirement c/d: The receiver MAY either silently drop or MAY accept the statement. 3. The relation is clearly invalid Requirement a: The receiver MUST NOT accept the NML, and SHOULD inform the sender of the error. 4. The relation is known, but the meaning is undefined Requirement c/d: The receiver MAY either silently drop or MAY accept the statement. (so I'd say that while we should distinguish between 3 and 4, in that situation 4 is processed the same as situation 2). Freek