DARTC Protocol Specification v0.2

Distributed Agent Real-Time Communication

Status: Implementation Draft Date: May 13, 2026 Maintainer: GemmaPod Project License: MIT

1. Summary

DARTC is the real-time transport layer for portable AI agents. It defines a signed, topic-multiplexed message envelope over WebRTC DataChannels, with a WebSocket relay fallback when peer-to-peer connectivity fails.

DARTC is designed to carry A2A messages without replacing A2A:

• A2A is the semantic layer: Agent Cards, messages, tasks, artifacts, and agent capability discovery.
• DARTC is the real-time binding: low-latency peer-to-peer delivery, streaming, topic multiplexing, acknowledgements, and session policy.
• MCP remains the tool/data access layer behind an agent runtime.

In GemmaPod, the signed pod manifest is the initial trust anchor. DARTC messages then provide session-level integrity, topic routing, replay defense, and a clean path to A2A-compatible agent-to-agent interaction.

2. Current A2A Alignment

DARTC v0.2 targets the current Agent2Agent model:

• Agent discovery uses an AgentCard.
• Interaction payloads use Message, Task, Artifact, and Part objects.
• Streaming should be modeled as task/message updates and stream closure, not as a custom final task status.
• A2A protocol bindings may include HTTP+JSON, JSON-RPC, gRPC, and this DARTC binding.

DARTC topics beginning with a2a. MUST carry A2A-shaped objects in the a2a field. DARTC-specific streaming metadata belongs in the dartc field.

3. Envelope

Every DARTC frame is one JSON object.

type DartcEnvelope<TPayload = unknown, TA2A = unknown> = {
  version: "0.2";
  msg_id: string;
  from: string;
  to: string;
  topic: string;
  timestamp: number;
  signature: string;
  a2a?: TA2A;
  dartc?: {
    stream?: boolean;
    chunk_id?: number;
    is_final?: boolean;
    priority?: "low" | "normal" | "high";
    requires_ack?: boolean;
    ack_for?: string;
  };
  payload?: TPayload;
};

Rules:

• version is the DARTC envelope version.
• msg_id SHOULD be UUIDv7. Implementations MAY accept UUIDv4 during early development.
• from and to are agent/session identifiers. to = "*" means broadcast within the current connection.
• topic controls routing and policy.
• timestamp is Unix epoch milliseconds.
• signature is base64 Ed25519 over the canonical JSON representation of the envelope with signature omitted.
• payload is for non-A2A topics or DARTC binding-specific data.

4. Canonicalization and Signatures

DARTC signatures cover the entire envelope except the signature field.

Implementations MUST:

1. Remove signature.
2. Canonicalize JSON by sorting object keys recursively.
3. UTF-8 encode the canonical JSON string.
4. Sign or verify those bytes with Ed25519.

GemmaPod implementation detail:

• Packed pods do not contain the owner private key.
• Visitor/browser sessions generate an ephemeral session key.
• The origin daemon may sign responses with an origin session key or an owner key if configured.
• The signed pod manifest remains the authority for pod identity, system prompt, model preference, transport policy, and tool allow-list.

5. Topics

Reserved DARTC topics:

GemmaPod topics:

A2A topics:

Application topics may use any non-reserved prefix, such as orders, negotiate, calendar, or support.

6. Session Handshake

After the WebRTC DataChannel opens, both peers SHOULD exchange dartc.hello.

{
  "version": "0.2",
  "msg_id": "018f2f42-...",
  "from": "visitor:session-pubkey",
  "to": "pod:raj-card:origin",
  "topic": "dartc.hello",
  "timestamp": 1747070000000,
  "dartc": { "requires_ack": true },
  "payload": {
    "role": "visitor",
    "pod_id": "raj-card",
    "agent_id": "visitor:session-pubkey",
    "protocol_versions": { "dartc": "0.2", "a2a": "0.2.2" },
    "supported_topics": ["gemmapod.chat.*", "a2a.discovery", "dartc.*"],
    "signedManifestB64": "..."
  },
  "signature": "..."
}

The origin daemon MUST verify:

• The DARTC envelope signature.
• timestamp is within the accepted clock skew window.
• msg_id has not been seen before on this session.
• The signed manifest is valid.
• The manifest pod id or WebRTC pod id matches the registered origin pod id.
• The owner pubkey matches OWNER_PUBKEY when configured.
• The requested topics are allowed by local origin policy.

7. GemmaPod Chat Binding

gemmapod.chat.request payload:

type GemmaPodChatRequest = {
  request_id: string;
  model?: string;
  messages: Array<{ role: "system" | "user" | "assistant"; content: string }>;
  signedManifestB64?: string;
};

gemmapod.chat.delta payload:

type GemmaPodChatDelta = {
  request_id: string;
  delta: string;
};

gemmapod.chat.done payload:

type GemmaPodChatDone = {
  request_id: string;
};

dartc.error payload:

type DartcErrorPayload = {
  code: string;
  message: string;
  request_id?: string;
  fatal?: boolean;
};

8. A2A Binding

For topics beginning with a2a., the a2a field carries the A2A object or operation payload. The DARTC envelope provides delivery metadata only.

Example Agent Card exchange:

{
  "version": "0.2",
  "topic": "a2a.discovery",
  "from": "pod:raj-card:origin",
  "to": "visitor:session-pubkey",
  "a2a": {
    "kind": "AgentCard",
    "card": {
      "name": "Raj Card",
      "description": "A portable AI business card.",
      "capabilities": { "streaming": true },
      "skills": []
    }
  },
  "dartc": { "stream": false },
  "payload": {
    "binding": "dartc",
    "topics": ["a2a.discovery", "a2a.message", "a2a.task"]
  },
  "signature": "..."
}

9. Backpressure and Chunking

v0.2 uses ordered reliable WebRTC DataChannels by default.

Implementations SHOULD:

• Keep individual JSON frames below 64 KiB.
• Use chunk_id for ordered stream chunks.
• Set is_final only for DARTC stream completion metadata. A2A task completion still follows A2A task state semantics.
• Use requires_ack for high-value control messages, not for every text delta.

10. Relay Fallback

DARTC is transport-neutral above the envelope. When WebRTC cannot connect, GemmaPod Cloud MAY relay the same signed envelopes over WebSocket.

Relay servers MUST NOT need to decrypt or mutate DARTC payloads. They MAY route by to, topic, and pod/session metadata.

11. Initial Implementation Milestones

1. Shared @gemmapod/dartc package: envelope types, canonicalization, signing adapters, topic helpers, ack/error helpers, tests.
2. Generic byte signing/verification exports in gemmapod-core.
3. Browser shim sends dartc.hello and gemmapod.chat.request over the existing WebRTC DataChannel.
4. Origin daemon accepts both legacy chat frames and DARTC envelopes, verifies DARTC signatures, and streams signed DARTC deltas back.
5. Add A2A Agent Card generation from the signed pod manifest.
6. Add WebSocket relay fallback using the same DARTC envelope.

12. Open Questions

• Should DARTC standardize CBOR framing after JSON v0.2 is stable?
• Should topic-level encryption use sender keys, shared session keys, or an MLS style group protocol?
• Should Agent Cards declare DARTC as an A2A extension or a formal protocol binding?
• What should the minimum replay cache window be for browser-origin sessions?