Subprotocol Registry#

Every Net packet carries a 16-bit subprotocol_id in its header. The id tells the receiver how to interpret the payload — plain event, causal event, snapshot, daemon migration message, NAT-traversal probe, and so on. This page is the canonical list of assigned ids, the ranges reserved for future use, and the rules that govern how the space evolves.

ID space#

The space is 16 bits — 65,536 IDs. The substrate has carved out the first 0x2200 for its own use; vendors get most of the rest; the top 0x1000 is for experiments that don't need a permanent assignment.

The opaque-forwarding guarantee#

Packets carrying an unregistered subprotocol_id are forwarded by intermediate nodes without modification or decryption. This is the rule that makes incremental subprotocol deployment possible: a new subprotocol can roll out to producers and consumers before every node in the mesh understands it, and the in-between nodes will pass the packets through.

The guarantee applies to forwarding only — the destination must understand the subprotocol to do anything with the packet beyond delivering it. The capability layer is the typical way to know in advance whether a destination supports a subprotocol; nodes advertise subprotocol:0x<id> tags for every protocol they handle.

Version negotiation#

Each subprotocol has a major/minor version. When two peers establish a session, they exchange SubprotocolManifests listing what they support; the result is a NegotiatedSet of subprotocols both peers handle at compatible versions:

pub struct SubprotocolDescriptor {
    pub id: u16,
    pub name: String,
    pub version: SubprotocolVersion,
    pub min_compatible: SubprotocolVersion,
    pub handler_present: bool,
}
 
pub struct SubprotocolVersion {
    pub major: u8,
    pub minor: u8,
}

Compatibility check: both peers' version must satisfy the other's min_compatible. Subprotocols not in the negotiated set fall back to opaque forwarding — they can still travel through the mesh, they just can't be interpreted by the peer that didn't sign up for them.

The negotiation itself rides on subprotocol 0x0600.

SUBPROTOCOL_REDEX dispatch codes#

The RedEX replication subprotocol (0x0E00) partitions its payload by a single-byte dispatch code immediately after the subprotocol id:

SyncNack carries one of four typed error codes:

There's deliberately no LeaderElection code in the replication subprotocol. Leader election is a deterministic function over each node's locally-known state (proximity-graph RTT, replica-set membership, NodeId ordering) — no election message rides on the wire. The capability-tag layer is what peers observe; the result of elect() drives a transition_to call that announces or withdraws the channel's capability tag.

Capability advertisement#

Nodes advertise the subprotocols they handle through the capability graph. SubprotocolRegistry::enrich_capabilities() adds a tag subprotocol:0x<id> for each handled subprotocol to the node's CapabilitySet. Other nodes use these tags to find peers for capability-routed operations — a daemon migration request looks for nodes advertising subprotocol:0x0500, a partition-recovery handshake looks for subprotocol:0x0801.

let caps = subprotocol_registry.enrich_capabilities(CapabilitySet::new());
// caps now carries: "subprotocol:0x0400", "subprotocol:0x0500", ...
 
let filter = SubprotocolRegistry::capability_filter_for(0x0500);
let migration_targets = capability_index.query(&filter);

Adding a new subprotocol#

The process is the same whether you're adding to core or to a vendor extension:

1. Pick an id. Core subprotocols go into 0x0001..=0x21FF — by convention each new subsystem claims a 0xXY00 base and uses the low byte for variants (e.g. 0x0B00 / 0x0B01 / 0x0B02 for the stream-window family). Vendor extensions go into 0x2200..=0xEFFF. Experimental work goes into 0xF000..=0xFFFF.
2. Define a descriptor. Name, version, min_compatible. The version pair tracks breaking changes (major) and backward-compatible changes (minor).
3. Register a handler. SubprotocolRegistry::register(descriptor); the dispatch surface picks up the registration.
4. Enrich capabilities. Once the handler is registered, enrich_capabilities() will add the subprotocol:0x<id> tag automatically.
5. Advertise the descriptor. Other peers see the new subprotocol on session establishment via the manifest exchange.

Forward compatibility is on by default — peers that don't understand the new subprotocol fall back to opaque forwarding, so deployment is rolling, not coordinated.