NNRP/1 Schema / Profile Registry

Typed payloads should not grow by endlessly appending new public enums. The registry separates "what the public layer freezes" from "how specific payloads extend the protocol".

The problem without a registry vs the structure with one

Without a registry, every new payload family forces changes to the public layer:

With a registry, the public layer is frozen and new payload families plug in through the registry:

Where the registry appears in the protocol exchange

Public layer vs Registry responsibilities

Public layer (shared by all profiles)	Registry / Profile private
connection / session / operation semantics	Profile logical identity and version
Flow control, budget, priority, error boundaries	How descriptor fields are interpreted
Cache / lease lifecycle for schema objects	Payload body format
schema_mismatch / schema_miss error types	Business-specific parameters and tool call bodies

Best practices

Pre-install large schemas: Schema definitions larger than a few hundred bytes should be pre-installed via CACHE_PUT at session start rather than inlined with every frame. Install once; all subsequent frames use schema_ref.

Keep version identifiers stable: schema_id + schema_version is the logical identity. schema_hash is only a consistency check. Do not use hash as identity — different receivers may compute different hashes for the same schema, causing false mismatch errors.

Record the negotiated result: The active_schema_id and active_schema_version fields in SESSION_OPEN_ACK are the ground truth for the current session's schema. Record them on the host side. When a mismatch occurs, check against this record before retrying.

Install before switching: Before the current schema is invalidated, pre-install the new version with CACHE_PUT and wait for CACHE_ACK(ready=true) before switching references in subsequent frames. This avoids a window where the old schema is gone but the new one is not yet ready.

Extend with the registry, not the public header: To support a new payload family, use the profile/schema registry extension path rather than adding constants to the public enum. Implementations that do not support the new family can then safely skip it rather than failing with a parse error.

Boundaries with other pages

1. Fixed layout of the descriptor itself — see "Typed Payload Descriptors".
2. tensor / token specific field definitions — see the standard profile pages.
3. How cache and leases work — see the previous page "Cache Capabilities and Leases".
4. Expressing backpressure and priority across multiple sessions — see the next page "Flow Control and Priority".