Extension model
Vendors ship fast, the core stays stable, and proven ideas graduate. Four tiers and one graduation rule keep those goals from fighting each other.
Four tiers
Extensions are organized into four tiers so that innovation never destabilizes the core:
- Tier 1: Core. TSC-owned. The minimum every conforming implementation must support, with two-year deprecation windows on breaking changes.
- Tier 2: Augments. Vendor or agency additions in their own namespace. They add nodes; they never modify core. A consumer that doesn’t load them simply ignores the extra fields.
- Tier 3: Deviations. TSC-reviewed constraint refinements: “in our jurisdiction the rule is this, not the maximum-permissive set,” for example stricter MUTCD timing.
- Tier 4: Proprietary. Vendor-internal modules on a separate subject prefix, outside conformance. Keep the IP you want; keep it out of the standard.
The graduation rule
The core is what the industry has converged on, and nothing lands by fiat. When three independent organizations (at least one an operator) ship the same augment wire-compatibly, and the TSC passes a graduation motion, that augment folds into the next minor revision of core, with attribution preserved.
Namespaces & compatibility
Because augments live in their own YANG namespaces and only add nodes, they are backward-compatible by construction: an older consumer ignores fields it doesn’t know. Deviations tighten constraints rather than loosen them, so data valid under a deviation is always valid under core. This is what lets vendors move quickly without ever being able to surprise a conforming consumer.