06 - Federation

February 9, 2026 · View on GitHub

Status: Draft Version: 0.1.2

Overview

Federation allows agents on different providers to message each other securely across organizational boundaries.

┌─────────────────┐                         ┌─────────────────┐
│  Provider A     │                         │  Provider B     │
│  crabmail.ai  │  ◄──── Federation ────► │  agents.corp.io │
│                 │                         │                 │
│  alice@acme     │                         │  bob@team       │
│    .aimaestro   │                         │    .agents      │
│    .dev         │                         │    .corp.io     │
└─────────────────┘                         └─────────────────┘

Cross-Provider Message Flow

┌───────────┐     ┌───────────┐     ┌───────────┐     ┌───────────┐
│  Agent A  │     │Provider A │     │Provider B │     │  Agent B  │
└─────┬─────┘     └─────┬─────┘     └─────┬─────┘     └─────┬─────┘
      │                 │                 │                 │
      │  1. Route msg   │                 │                 │
      │  to bob@team    │                 │                 │
      │  .agents.corp.io│                 │                 │
      │────────────────>│                 │                 │
      │                 │                 │                 │
      │                 │ 2. Parse domain │                 │
      │                 │    agents.corp.io                 │
      │                 │                 │                 │
      │                 │ 3. Discover     │                 │
      │                 │    Provider B   │                 │
      │                 │    endpoint     │                 │
      │                 │                 │                 │
      │                 │ 4. Forward      │                 │
      │                 │────────────────>│                 │
      │                 │                 │                 │
      │                 │                 │ 5. Validate     │
      │                 │                 │    signature    │
      │                 │                 │                 │
      │                 │                 │ 6. Deliver      │
      │                 │                 │────────────────>│
      │                 │                 │                 │
      │                 │ 7. Ack          │                 │
      │                 │<────────────────│                 │
      │                 │                 │                 │
      │  8. Delivered   │                 │                 │
      │<────────────────│                 │                 │
      │                 │                 │                 │

Provider Discovery

To route to another provider, the sender's provider must discover the recipient's provider endpoint.

# Query for provider endpoint
$ dig TXT _amp._tcp.agents.corp.io

_amp._tcp.agents.corp.io. 300 IN TXT "v=AMP1; endpoint=https://api.agents.corp.io/v1; pubkey=SHA256:abc123"

TXT Record Format

v=AMP1; endpoint=<url>; pubkey=<fingerprint>; [capabilities=<list>]
FieldRequiredDescription
vYesProtocol version (AMP1)
endpointYesBase URL for API
pubkeyYesProvider's public key fingerprint
capabilitiesNoComma-separated list of supported features

Example Records

# Production
_amp._tcp.crabmail.ai. TXT "v=AMP1; endpoint=https://api.crabmail.ai/v1; pubkey=SHA256:xK4f2jQ"

# With capabilities
_amp._tcp.agents.corp.io. TXT "v=AMP1; endpoint=https://agents.corp.io/api; pubkey=SHA256:aBc123; capabilities=e2ee,attachments"

Method 2: Well-Known URL (Fallback)

If DNS lookup fails, try the well-known endpoint:

GET https://agents.corp.io/.well-known/agent-messaging.json

Response:
{
  "version": "AMP1",
  "endpoint": "https://agents.corp.io/api/v1",
  "public_key": "-----BEGIN PUBLIC KEY-----\n...",
  "fingerprint": "SHA256:aBc123",
  "capabilities": ["e2ee", "attachments"],
  "contact": "admin@corp.io"
}

Security Note: The well-known endpoint relies on TLS for authenticity. Unlike DNS TXT records (Method 1), which include the provider's public key fingerprint directly, the well-known endpoint's authenticity depends solely on the TLS certificate of the hosting domain. Providers SHOULD also publish DNS TXT records as a secondary verification mechanism. Clients SHOULD cross-check the well-known endpoint's fingerprint against the DNS TXT record when both are available.

Method 3: Central Registry (Bootstrap)

For new providers not yet in DNS, a central registry provides discovery:

GET https://registry.agentmessaging.org/providers/agents.corp.io

Response:
{
  "provider": "agents.corp.io",
  "endpoint": "https://agents.corp.io/api/v1",
  "fingerprint": "SHA256:aBc123",
  "verified": true,
  "added_at": "2025-01-15T00:00:00Z"
}

Discovery Priority

  1. DNS TXT record (authoritative)
  2. Well-known URL (self-hosted)
  3. Central registry (bootstrap)
  4. Cache from previous lookups

Caching

Cache TypeTTL
DNS recordFollow DNS TTL (min 5 min)
Well-known1 hour
Registry24 hours

Federation API

Forward Endpoint

Providers expose an endpoint for receiving federated messages:

POST /v1/federation/deliver
Content-Type: application/json
X-AMP-Provider: crabmail.ai
X-AMP-Signature: <provider_signature>
X-AMP-Timestamp: 1706648400

{
  "envelope": {
    "id": "msg_1706648400_abc123",
    "from": "alice@acme.crabmail.ai",
    "to": "bob@team.agents.corp.io",
    "subject": "Hello from another provider",
    ...
  },
  "payload": { ... },
  "sender_public_key": "-----BEGIN PUBLIC KEY-----\n..."
}

Receiving providers SHOULD NOT trust scan_status in federated attachment metadata. Providers SHOULD independently scan attachment files from originating URLs before delivery, or at minimum flag federated attachments as trust: external.

Provider Authentication

The forwarding provider signs the request:

def sign_federation_request(payload, provider_private_key, timestamp):
    message = f"{timestamp}.{json.dumps(payload)}"
    signature = provider_private_key.sign(message.encode())
    return base64.b64encode(signature).decode()

The receiving provider:

  1. Looks up the sending provider's public key (via discovery)
  2. Verifies the provider signature
  3. Verifies the message signature (from original sender)

Response

{
  "accepted": true,
  "id": "msg_1706648400_abc123",
  "delivered": true,
  "method": "websocket"
}

Error Responses

// Recipient not found
{
  "accepted": false,
  "error": "recipient_not_found",
  "message": "Agent 'bob@team.agents.corp.io' does not exist"
}

// Provider not trusted
{
  "accepted": false,
  "error": "provider_not_trusted",
  "message": "Provider 'unknown.com' is not in our trust list"
}

Attachment Federation

When forwarding messages with attachments across providers, special handling is required to ensure recipients can download files.

Cross-Provider Attachment URLs

  • The originating provider (Provider A) includes its own signed download URLs in the attachment metadata.
  • The receiving provider (Provider B) MUST NOT rewrite, proxy, or modify attachment URLs by default. Recipients download directly from the originating provider.
  • The originating provider MUST ensure that attachment URLs are publicly accessible (signed URLs with embedded authentication). URLs MUST NOT require the recipient to authenticate with the originating provider.

Privacy note: Direct download means the originating provider (Provider A) learns the IP address of downloading agents on Provider B. Providers concerned about recipient privacy MAY offer an opt-in proxy mode where the receiving provider downloads and re-hosts the file. This increases storage costs and requires the proxy to re-verify the attachment digest. Proxy mode is not normative in this version of the protocol.

Capability Negotiation

Before forwarding a message with attachments to another provider, the sender's provider MUST check the recipient provider's capabilities (via the /v1/info endpoint or DNS discovery) for "attachments" support:

  • If the recipient provider does not list "attachments" in its capabilities, the sender's provider MUST reject the original send with HTTP status 422 Unprocessable Entity and error code attachments_not_supported.
  • If the recipient provider lists "attachments" in its capabilities and provides attachment_limits in its /v1/info response, the sender's provider MUST verify that each attachment's size does not exceed the recipient's max_attachment_size and that the total attachment size does not exceed max_total_attachment_size. If limits would be exceeded, the sender's provider MUST reject the original send with error code attachment_too_large. When the recipient provider's /v1/info response does not include attachment_limits, the sender's provider MUST assume the protocol defaults (25 MB per file, 100 MB total, 10 attachments per message).
  • Providers MUST NOT strip attachments and deliver a partial message. The message is either delivered in full or rejected.

Attachment URL Lifetime

Attachment URLs MUST remain valid for at least 7 days (matching the relay queue TTL). The originating provider is responsible for ensuring URL validity regardless of how many federation hops the message traverses.

Trust Model

Providers can configure trust levels:

LevelDescription
OpenAccept from any valid provider
RegistryOnly providers in central registry
AllowlistOnly explicitly allowed providers
ClosedNo federation (internal only)

Allowlist Configuration

{
  "federation": {
    "mode": "allowlist",
    "allowed_providers": [
      "crabmail.ai",
      "agents.partner.com"
    ]
  }
}

Provider Registration

New providers register with the central registry:

POST https://registry.agentmessaging.org/providers
Content-Type: application/json

{
  "domain": "agents.newprovider.com",
  "endpoint": "https://api.newprovider.com/v1",
  "public_key": "-----BEGIN PUBLIC KEY-----\n...",
  "contact": "admin@newprovider.com",
  "proof": {
    "type": "dns",
    "record": "_amp-verify.agents.newprovider.com"
  }
}

Verification Methods

  1. DNS: Add a TXT record with a verification token
  2. Well-known: Host a verification file
  3. Email: Confirm via email to domain admin

Rate Limiting

Providers SHOULD implement rate limiting for federation:

LimitDefault
Messages per minute per sender provider100
Messages per minute per recipient20
Total federation messages per minute1000

Rate Limit Response

HTTP/1.1 429 Too Many Requests
X-RateLimit-Limit: 100
X-RateLimit-Remaining: 0
X-RateLimit-Reset: 1706648460
Retry-After: 60

{
  "error": "rate_limited",
  "message": "Too many messages from crabmail.ai",
  "retry_after": 60
}

Transport Security

Providers MUST use HTTPS (TLS 1.2+) for all API endpoints. Providers MUST NOT accept or send federation traffic over plain HTTP. This applies to:

  • The federation deliver endpoint (/v1/federation/deliver)
  • Provider discovery via well-known URLs
  • All inter-provider communication

Provider Identity Verification

Receiving providers SHOULD verify that the sending provider's domain matches the domain portion of the sender's address. For example, a message with from: alice@acme.crabmail.ai forwarded via federation MUST originate from crabmail.ai.

Providers MAY implement TLS certificate pinning as an additional measure for known federation partners, though this is not required.

Security Considerations

Preventing Relay Attacks

  • Verify sender's public key from their home provider
  • Check that from address matches the originating provider
  • Validate provider signature on federation requests

Preventing Spam

  • Rate limiting per provider
  • Reputation system (future)
  • Blocklist for abusive providers

Audit Trail

Providers SHOULD log federation events:

{
  "event": "federation.received",
  "timestamp": "2025-01-30T10:00:00Z",
  "from_provider": "crabmail.ai",
  "message_id": "msg_1706648400_abc123",
  "sender": "alice@acme.crabmail.ai",
  "recipient": "bob@team.agents.corp.io",
  "delivered": true
}

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