



Network Working Group                                          E. Somoza
Internet-Draft                                               Independent
Intended status: Experimental                                29 May 2026
Expires: 30 November 2026


Agent Trust Negotiation: Capability, Delegation, and Provenance Binding
                             for AI Agents
            draft-somoza-dmsc-atn-agent-trust-negotiation-00

Abstract

   This document defines the Agent Trust Negotiation (ATN) protocol.
   ATN sits above whatever mechanism is used to discover an agent
   identity and answers questions that discovery alone cannot: what is
   the agent permitted to do, under whose authority, with what
   provenance, and how do two agents reach a verifiable working
   agreement.

   ATN binds four artifacts to a discovered agent identity:

   1.  A Capability Manifest expressing what the agent is willing to do,
       under what conditions, and with what constraints.

   2.  A Delegation Chain proving authorized principal scope and
       revocation paths.

   3.  A Provenance Attestation proving build-time and runtime
       integrity.

   4.  A Session Receipt produced at handshake completion, recording the
       negotiated scope.

   ATN defines the handshake state machine that consumes these artifacts
   to produce a mutually verified, scope-bounded agent session, and a
   session receipt suitable for transparency log inclusion.

   ATN is discovery-agnostic.  The default binding identifies agents as
   HTTP resources and uses a per-origin well-known URI to bootstrap
   discovery.  A DNS-based binding is also specified for the cross-
   organizational case.  Other bindings can plug into the same abstract
   interface.  ATN composes with existing discovery proposals including
   AID, DNS-AID, and ANS v2.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.



Somoza                  Expires 30 November 2026                [Page 1]

Internet-Draft           agent-trust-negotiation                May 2026


   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on 30 November 2026.

Copyright Notice

   Copyright (c) 2026 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents (https://trustee.ietf.org/
   license-info) in effect on the date of publication of this document.
   Please review these documents carefully, as they describe your rights
   and restrictions with respect to this document.  Code Components
   extracted from this document must include Revised BSD License text as
   described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   5
     1.1.  Scope . . . . . . . . . . . . . . . . . . . . . . . . . .   6
     1.2.  Non-Goals . . . . . . . . . . . . . . . . . . . . . . . .   6
     1.3.  Choice of Venue . . . . . . . . . . . . . . . . . . . . .   7
     1.4.  Lineage and Changes from Prior Drafts . . . . . . . . . .   7
   2.  Conventions and Definitions . . . . . . . . . . . . . . . . .   8
     2.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   8
   3.  Architecture Overview . . . . . . . . . . . . . . . . . . . .   9
   4.  Discovery Binding Framework . . . . . . . . . . . . . . . . .  10
     4.1.  Abstract Binding Interface  . . . . . . . . . . . . . . .  10
     4.2.  HTTP-Resource Binding (Default) . . . . . . . . . . . . .  10
       4.2.1.  Origin Discovery  . . . . . . . . . . . . . . . . . .  11
       4.2.2.  Agent Identification  . . . . . . . . . . . . . . . .  11
       4.2.3.  Per-Agent Resources . . . . . . . . . . . . . . . . .  11
       4.2.4.  Operational Properties  . . . . . . . . . . . . . . .  11
       4.2.5.  Index Document Structure  . . . . . . . . . . . . . .  11
       4.2.6.  Index Document JWS Signature Requirements . . . . . .  12
     4.3.  DNS Binding (Optional)  . . . . . . . . . . . . . . . . .  12
       4.3.1.  TXT Record  . . . . . . . . . . . . . . . . . . . . .  13
       4.3.2.  DNSSEC  . . . . . . . . . . . . . . . . . . . . . . .  13



Somoza                  Expires 30 November 2026                [Page 2]

Internet-Draft           agent-trust-negotiation                May 2026


       4.3.3.  Withdrawal of -00 SVCB ParamKey Registrations . . . .  13
     4.4.  Other Bindings  . . . . . . . . . . . . . . . . . . . . .  13
   5.  Capability Manifest . . . . . . . . . . . . . . . . . . . . .  13
     5.1.  Capability Object Structure . . . . . . . . . . . . . . .  13
     5.2.  Schema Binding  . . . . . . . . . . . . . . . . . . . . .  15
     5.3.  Structure . . . . . . . . . . . . . . . . . . . . . . . .  15
     5.4.  Dimension Semantics . . . . . . . . . . . . . . . . . . .  17
     5.5.  Key Properties  . . . . . . . . . . . . . . . . . . . . .  18
     5.6.  Refusal Semantics . . . . . . . . . . . . . . . . . . . .  18
   6.  Delegation Chain  . . . . . . . . . . . . . . . . . . . . . .  18
     6.1.  Structure . . . . . . . . . . . . . . . . . . . . . . . .  19
     6.2.  Verification  . . . . . . . . . . . . . . . . . . . . . .  19
   7.  Provenance Attestation  . . . . . . . . . . . . . . . . . . .  20
     7.1.  Structure . . . . . . . . . . . . . . . . . . . . . . . .  20
     7.2.  Properties  . . . . . . . . . . . . . . . . . . . . . . .  20
   8.  The Agent Trust Handshake (ATH) . . . . . . . . . . . . . . .  20
     8.1.  State Machine . . . . . . . . . . . . . . . . . . . . . .  20
     8.2.  ATH_HELLO . . . . . . . . . . . . . . . . . . . . . . . .  21
     8.3.  ATH_OFFER . . . . . . . . . . . . . . . . . . . . . . . .  22
     8.4.  ATH_ACCEPT  . . . . . . . . . . . . . . . . . . . . . . .  24
     8.5.  ATH_RECEIPT . . . . . . . . . . . . . . . . . . . . . . .  24
     8.6.  Handshake Limits  . . . . . . . . . . . . . . . . . . . .  25
   9.  Capability Intersection Algebra . . . . . . . . . . . . . . .  25
     9.1.  Identity Rule . . . . . . . . . . . . . . . . . . . . . .  25
     9.2.  Per-Dimension Rules . . . . . . . . . . . . . . . . . . .  25
     9.3.  Refusal Override  . . . . . . . . . . . . . . . . . . . .  26
     9.4.  Worked Example  . . . . . . . . . . . . . . . . . . . . .  26
   10. Mutual Trust Mode . . . . . . . . . . . . . . . . . . . . . .  27
   11. Version Negotiation and Algorithm Agility . . . . . . . . . .  28
     11.1.  Version Negotiation  . . . . . . . . . . . . . . . . . .  28
     11.2.  Algorithm Agility  . . . . . . . . . . . . . . . . . . .  28
     11.3.  Post-Quantum Transition  . . . . . . . . . . . . . . . .  29
   12. Compatibility with Existing Discovery Protocols . . . . . . .  29
     12.1.  With AID AID . . . . . . . . . . . . . . . . . . . . . .  30
     12.2.  With DNS-AID DNS-AID . . . . . . . . . . . . . . . . . .  30
     12.3.  With ANS v2 ANSv2  . . . . . . . . . . . . . . . . . . .  30
     12.4.  Multi-Discovery Tolerance  . . . . . . . . . . . . . . .  30
   13. Relationship to Existing Identity, Authorization, and
           Attestation Frameworks  . . . . . . . . . . . . . . . . .  30
     13.1.  OAuth 2.0 and GNAP . . . . . . . . . . . . . . . . . . .  31
     13.2.  SPIFFE Workload Identity . . . . . . . . . . . . . . . .  31
     13.3.  Mutual TLS . . . . . . . . . . . . . . . . . . . . . . .  32
     13.4.  RATS Remote Attestation  . . . . . . . . . . . . . . . .  32
     13.5.  SCITT Transparency . . . . . . . . . . . . . . . . . . .  33
     13.6.  Composition Summary  . . . . . . . . . . . . . . . . . .  33
     13.7.  When to Use What . . . . . . . . . . . . . . . . . . . .  34
   14. Security Considerations . . . . . . . . . . . . . . . . . . .  35
     14.1.  Threat Model . . . . . . . . . . . . . . . . . . . . . .  35



Somoza                  Expires 30 November 2026                [Page 3]

Internet-Draft           agent-trust-negotiation                May 2026


     14.2.  Artifact Authenticity  . . . . . . . . . . . . . . . . .  35
     14.3.  Cross-Binding Consistency  . . . . . . . . . . . . . . .  35
     14.4.  Enforcement Posture: Evidence, Not Prevention  . . . . .  35
     14.5.  Delegation Forgery . . . . . . . . . . . . . . . . . . .  37
     14.6.  Session Receipt Tampering  . . . . . . . . . . . . . . .  37
     14.7.  Replay . . . . . . . . . . . . . . . . . . . . . . . . .  37
     14.8.  Provenance Attestation Limits  . . . . . . . . . . . . .  37
   15. Privacy Considerations  . . . . . . . . . . . . . . . . . . .  37
     15.1.  Per-Agent Lookups  . . . . . . . . . . . . . . . . . . .  37
     15.2.  DNS Binding Leakage  . . . . . . . . . . . . . . . . . .  37
     15.3.  Negotiation Observability  . . . . . . . . . . . . . . .  38
     15.4.  Witness Aggregation  . . . . . . . . . . . . . . . . . .  38
     15.5.  Minimum Disclosure . . . . . . . . . . . . . . . . . . .  38
     15.6.  SCITT Publication  . . . . . . . . . . . . . . . . . . .  38
   16. Operational Considerations  . . . . . . . . . . . . . . . . .  38
     16.1.  Session Establishment vs Per-Call Latency  . . . . . . .  38
     16.2.  Latency Budget . . . . . . . . . . . . . . . . . . . . .  39
     16.3.  Optimization Techniques  . . . . . . . . . . . . . . . .  40
     16.4.  Session Lifetime . . . . . . . . . . . . . . . . . . . .  40
     16.5.  Deployments Where ATN Is Appropriate . . . . . . . . . .  41
     16.6.  Deployments Where ATN Is Not Appropriate . . . . . . . .  41
   17. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  41
     17.1.  Withdrawal of -00 SVCB ParamKey Registrations  . . . . .  42
     17.2.  Well-Known URI Registration  . . . . . . . . . . . . . .  42
     17.3.  DNS TXT Key Registration . . . . . . . . . . . . . . . .  42
     17.4.  ATN Capability Registry  . . . . . . . . . . . . . . . .  42
     17.5.  ATN Capability Dimension Registries  . . . . . . . . . .  43
     17.6.  ATN Refusal Category Registry  . . . . . . . . . . . . .  45
   18. References  . . . . . . . . . . . . . . . . . . . . . . . . .  46
     18.1.  Normative References . . . . . . . . . . . . . . . . . .  46
     18.2.  Informative References . . . . . . . . . . . . . . . . .  46
   Appendix A.  Complete Example Flow  . . . . . . . . . . . . . . .  47
     A.1.  Step 1: Origin Discovery  . . . . . . . . . . . . . . . .  47
     A.2.  Step 2: Index Document Verification . . . . . . . . . . .  48
     A.3.  Step 3: Artifact Fetch and Verification . . . . . . . . .  48
     A.4.  Step 4: ATH_HELLO . . . . . . . . . . . . . . . . . . . .  48
     A.5.  Step 5: ATH_OFFER . . . . . . . . . . . . . . . . . . . .  49
     A.6.  Step 6: ATH_ACCEPT  . . . . . . . . . . . . . . . . . . .  49
     A.7.  Step 7: ATH_RECEIPT . . . . . . . . . . . . . . . . . . .  50
     A.8.  Step 8: Session and Audit . . . . . . . . . . . . . . . .  50
   Appendix B.  Design Rationale . . . . . . . . . . . . . . . . . .  50
     B.1.  Why a Negotiation Layer . . . . . . . . . . . . . . . . .  51
     B.2.  Why Four Artifacts  . . . . . . . . . . . . . . . . . . .  51
     B.3.  Why Discovery-Agnostic  . . . . . . . . . . . . . . . . .  51
     B.4.  Why Refusals Are First-Class  . . . . . . . . . . . . . .  51
     B.5.  Why Intersection Algebra Is Specified . . . . . . . . . .  51
     B.6.  Why SCITT for Receipts  . . . . . . . . . . . . . . . . .  52
   Appendix C.  Implementation Status  . . . . . . . . . . . . . . .  52



Somoza                  Expires 30 November 2026                [Page 4]

Internet-Draft           agent-trust-negotiation                May 2026


     C.1.  ATN Reference Prototype . . . . . . . . . . . . . . . . .  52
   Appendix D.  Acknowledgments  . . . . . . . . . . . . . . . . . .  53
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  53

1.  Introduction

   Agent-to-agent discovery work in progress at the IETF and adjacent
   venues, including AID [AID], DNS-AID [DNS-AID], and ANS v2 [ANSv2],
   addresses how to locate an agent's endpoint and verify its
   cryptographic identity.  These efforts answer two questions: _where
   is the agent_ and _who is the agent_.

   They do not answer the questions that arise immediately after:

   *  What is the agent permitted to do, and under what constraints?

   *  Under whose authority is the agent operating?

   *  What is the provenance of the agent's software, model, and
      configuration?

   *  How do two agents reach a mutually verifiable working agreement
      before any task begins?

   *  What evidence remains after the session ends, and how is it
      audited?

   This document defines the Agent Trust Negotiation (ATN) protocol to
   address these gaps.  ATN is the layer above discovery.  It assumes
   the discovery layer has located an endpoint and authenticated an
   identity through one of the existing mechanisms.  ATN then negotiates
   the trust relationship.

   The design has four parts:

   1.  Four structured artifacts (Capability Manifest, Delegation Chain,
       Provenance Attestation, Session Receipt) that bind to an agent
       identity regardless of how that identity was discovered.

   2.  A handshake protocol over HTTPS that exchanges and verifies the
       artifacts between two agents.

   3.  A capability intersection algorithm that yields the negotiated
       session scope.

   4.  A receipt format suitable for SCITT-style transparency log
       inclusion.




Somoza                  Expires 30 November 2026                [Page 5]

Internet-Draft           agent-trust-negotiation                May 2026


   ATN deliberately does not introduce a new discovery mechanism.  It
   builds on existing work and adds the negotiation layer that the agent
   ecosystem lacks.

1.1.  Scope

   In scope:

   *  Format and semantics of the four ATN artifacts.

   *  The Agent Trust Handshake (ATH) state machine.

   *  Capability intersection algebra.

   *  An abstract discovery binding interface and two concrete bindings
      (HTTP-Resource and DNS).

   *  IANA registrations for ATN-specific identifiers.

   Out of scope:

   *  Discovery of the agent itself (use [AID], [DNS-AID], [ANSv2], or
      equivalent).

   *  Endpoint authentication (delegated to the discovery layer).

   *  Application protocol (delegated to MCP, A2A, OpenAPI, or
      equivalent).

   *  In-organization service-to-service identity (see [SPIFFE]).

   *  Reputation distribution.

   *  Behavioral attestation.

1.2.  Non-Goals

   *  ATN does NOT define a new DNS record type.

   *  ATN does NOT replace any existing discovery proposal.

   *  ATN does NOT introduce a centralized trust authority.

   *  ATN does NOT enforce policy.  It produces verifiable evidence of
      declared and negotiated policy that downstream systems can audit.






Somoza                  Expires 30 November 2026                [Page 6]

Internet-Draft           agent-trust-negotiation                May 2026


1.3.  Choice of Venue

   ATN's choice of IETF as the venue follows from its dependencies.  The
   discovery substrate options include DNS [RFC4033] and SVCB [RFC9460],
   both IETF standards, as well as HTTP [RFC9110].  The attestation
   architecture is RATS [RFC9334].  The transparency log is SCITT.  The
   signature format is JWS [RFC7515].  The mutual authentication
   transport is TLS [RFC8446].  The authorization frameworks ATN
   composes with are OAuth [RFC6749] and GNAP [RFC9635].

   ATN is the composition of IETF primitives into a peer-to-peer agent
   trust handshake.  Standardizing the composition in any other venue
   would fork the primitive stack and create alignment problems.  The
   natural home for the composition is the same venue that standardized
   the primitives.

1.4.  Lineage and Changes from Prior Drafts

   This document is a renaming of draft-somoza-atn-agent-trust-
   negotiation.  The earlier name was used for the -00 and -01 revisions
   submitted in May 2026.  This revision adopts the DMSC working group
   naming convention (draft-somoza-dmsc-*) for alignment with the DMSC
   suite and to facilitate auto-archiving of DMSC-related documents.
   The technical content is unchanged from draft-somoza-atn-agent-trust-
   negotiation-01.

   The substantive technical changes that were introduced in draft-
   somoza-atn-agent-trust-negotiation-01 (relative to -00) are preserved
   in this document and are summarized below for readers arriving via
   the new name:

   1.  The four artifacts are repositioned as the normative core of ATN,
       independent of any specific discovery substrate.  The original
       framing of ATN as sitting above DNS-based discovery is replaced
       with a discovery-agnostic framing.

   2.  A new HTTP-Resource Binding is specified as the default, using a
       per-origin well-known URI [RFC8615] to bootstrap discovery to a
       signed Index Document.  See Section 4.2.

   3.  The DNS Binding is reduced to a thin TXT record that announces
       ATN support at an origin.  It no longer carries per-agent records
       and no longer carries integrity hints.  See Section 4.3.

   4.  The SVCB ParamKey registrations atn and atn-digest proposed in
       the original -00 are withdrawn.  SVCB ParamKeys are not the
       appropriate mechanism for pointing at a separate document at
       another URL.  See Section 4.3.3.



Somoza                  Expires 30 November 2026                [Page 7]

Internet-Draft           agent-trust-negotiation                May 2026


   5.  The JWS signature requirement on the Index Document is MUST
       regardless of DNSSEC posture.  The Index Document needs offline-
       verifiable provenance for cached, mirrored, and SCITT-published
       copies.  See Section 4.2.6.

   6.  The Introduction's claim that "Recent IETF work has converged on
       DNS-based discovery for AI agents" was removed.  The discovery
       landscape includes multiple proposals at multiple venues; no
       convergence has occurred.

2.  Conventions and Definitions

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in BCP
   14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

2.1.  Terminology

   Agent  An automated software entity that initiates network
      interactions on behalf of, or with delegated authority from, a
      principal, without per-interaction human authorization.

   Initiator  The agent that opens the trust handshake.

   Responder  The agent that answers the trust handshake.

   Origin  As defined in [RFC9110].  The scheme/host/port tuple that
      anchors one or more agents in the HTTP-Resource Binding.

   Capability Manifest  A signed JSON [RFC8259] document declaring what
      an agent is willing to do, under what conditions, and with what
      constraints.

   Delegation Chain  A signed sequence of authorization statements
      binding the agent to a principal, with explicit scope, time
      bounds, and revocation pointers.

   Provenance Attestation  A signed document proving build-time and
      runtime characteristics of the agent.

   Session Receipt  A signed record produced at handshake completion,
      summarizing the negotiated scope and identities, suitable for
      transparency log inclusion.

   Negotiated Scope  The intersection of two agents' capability sets,




Somoza                  Expires 30 November 2026                [Page 8]

Internet-Draft           agent-trust-negotiation                May 2026


      time windows, authorization scopes, and operational constraints,
      as determined by the handshake.

   Index Document  The JWS-signed JSON document fetched during HTTP-
      Resource Binding discovery, listing agents at an origin and
      origin-level capabilities.

3.  Architecture Overview

   ATN defines four roles:

   1.  Agent Operator: publishes the discovery records and the ATN
       artifacts.

   2.  Agent: the runtime entity, in either Initiator or Responder mode
       at any moment.

   3.  Principal: the human, organization, or other agent that
       authorized the agent through the Delegation Chain.

   4.  Witness: an optional third party that can pre-verify ATN
       artifacts and publish endorsements (e.g., via SCITT [SCITT]).

   ATN layers above discovery:

   +----------------------------------------------+
   |  Application Layer (MCP, A2A, OpenAPI, ...)  |
   +----------------------------------------------+
   |  Agent Trust Negotiation (this document)     |
   +----------------------------------------------+
   |  Discovery and Identity                      |
   |  (HTTP-Resource, AID, DNS-AID, ANS v2, ...)  |
   +----------------------------------------------+
   |  HTTP, DNS, DNSSEC, TLS                      |
   +----------------------------------------------+

   The handshake order is:

   1.  Discovery: Initiator resolves the Responder's identity via the
       active discovery binding (HTTP-Resource, DNS, or other).

   2.  Artifact fetch: Initiator retrieves the four ATN artifacts
       referenced by the Index Document.

   3.  Handshake: Initiator opens the ATN handshake.  Both parties
       exchange artifacts and verify signatures.

   4.  Negotiation: Both parties compute the capability intersection.



Somoza                  Expires 30 November 2026                [Page 9]

Internet-Draft           agent-trust-negotiation                May 2026


   5.  Receipt: Both parties produce a signed Session Receipt covering
       the negotiated scope.

   6.  Session: Application traffic proceeds within the negotiated
       scope.

   The ATN core protocol operates on the four artifacts.  How the
   Initiator obtained the Responder's identity and how that identity
   binds to the four artifacts are specified by a binding (see
   Section 4).  The handshake makes no assumptions about the binding
   beyond the abstract interface.

4.  Discovery Binding Framework

   ATN is discovery-agnostic.  The four artifacts and the handshake are
   the normative core; the binding to a discovery substrate is a
   separable concern.  This document specifies two concrete bindings:
   HTTP-Resource Binding (the default, in Section 4.2) and DNS Binding
   (optional, in Section 4.3).  Other bindings can be specified by
   future documents using the abstract interface below.

4.1.  Abstract Binding Interface

   A binding provides two operations:

   1.  Resolve(identifier) -> IndexDocumentURL: given an agent or origin
       identifier in the binding's namespace, return the URL at which
       the Index Document can be fetched.

   2.  Integrity(identifier) -> IntegrityProof: return any integrity
       proof the binding provides for the resolution (e.g., DNSSEC
       signatures for a DNS binding).  Bindings MAY return a null proof
       if no binding-level integrity is provided; the JWS signature on
       the Index Document remains mandatory in that case (see
       Section 4.2.6).

   A binding is conformant if it provides these two operations and if
   the resolved Index Document, when fetched and JWS-verified, contains
   the claimed identifier.

4.2.  HTTP-Resource Binding (Default)

   This is the default binding for HTTP-native agents.  It identifies
   agents as HTTP resources and uses a per-origin well-known URI
   [RFC8615] to bootstrap discovery.






Somoza                  Expires 30 November 2026               [Page 10]

Internet-Draft           agent-trust-negotiation                May 2026


4.2.1.  Origin Discovery

   For an origin https://example.com, the Index Document is fetched
   from:

   https://example.com/.well-known/atn

   This URI is registered in Section 17 per [RFC8615].

4.2.2.  Agent Identification

   An agent is identified by an HTTPS URL of the form:

   https://{origin}/{path}

   The agent URL is the canonical identifier in the HTTP-Resource
   Binding.  Agents at the same origin share one Index Document.

4.2.3.  Per-Agent Resources

   A GET on the agent URL returns a JSON object containing the agent's
   Capability Manifest, Delegation Chain, and Provenance Attestation, or
   URLs from which each can be fetched.  The agent URL MAY also serve as
   the handshake endpoint for the Initiator.

4.2.4.  Operational Properties

   The HTTP-Resource Binding has the following properties relevant to
   operational deployments:

   *  Many agents share one origin and one set of DNS records.

   *  Creating, modifying, or removing an agent requires no DNS changes.

   *  Web infrastructure (caching, load balancing, CDN) handles scale.

   *  Agent lookups do not generate per-agent DNS queries.

4.2.5.  Index Document Structure

   The Index Document at /.well-known/atn is a JSON object with the
   following structure:









Somoza                  Expires 30 November 2026               [Page 11]

Internet-Draft           agent-trust-negotiation                May 2026


   {
     "v": "atn1",
     "origin": "https://example.com",
     "issued_at": "2026-05-15T10:00:00Z",
     "not_after": "2026-08-15T10:00:00Z",
     "origin_capabilities": { "...": "see Capability Manifest" },
     "agents": [
       {
         "id": "https://example.com/agents/alice",
         "manifest_url": "https://example.com/alice/cap.jws",
         "delegation_url": "https://example.com/alice/del.jws",
         "provenance_url": "https://example.com/alice/prov.jws",
         "handshake_endpoint": "https://example.com/alice/hs",
         "key": { "...": "JWK per RFC 7517" }
       }
     ],
     "witness_pointers": [
       "https://scitt.example.org/entries/a1b2c3..."
     ]
   }

   Implementations MAY embed any of the artifact URLs above as inline
   objects instead.

4.2.6.  Index Document JWS Signature Requirements

   The Index Document MUST be signed as a JWS [RFC7515] using JSON
   Flattened Serialization.  The signature is computed over the
   canonicalized JSON object.  The signing key MUST be discoverable by
   the verifier through a trust anchor recognized by the verifier.

   The JWS signature requirement is MUST regardless of whether DNSSEC
   covers the origin's DNS records and regardless of whether the Index
   Document was fetched over TLS.  The Index Document is an artifact
   that needs to be verifiable offline by parties that did not perform
   the original fetch, including SCITT transparency services and audit
   consumers reconstructing sessions long after the original TLS
   connection has closed.

4.3.  DNS Binding (Optional)

   This is an optional binding for cross-organizational discovery, where
   the verifier knows only a domain and needs to locate the ATN-
   supporting origin.







Somoza                  Expires 30 November 2026               [Page 12]

Internet-Draft           agent-trust-negotiation                May 2026


4.3.1.  TXT Record

   A TXT record at _atn.{domain} announces ATN support:

  _atn.example.com.  3600  IN  TXT  "v=atn1; origin=https://example.com"

   The TXT record MUST contain v=atn1 and SHOULD contain an origin key.
   If origin is absent, the origin is https://{domain}.

   The TXT record carries no integrity hints, no per-agent records, and
   no Index Document content.  Its sole purpose is to announce that the
   origin supports ATN and, optionally, which origin to use.

4.3.2.  DNSSEC

   DNSSEC validation of the TXT record [RFC4033] is RECOMMENDED.  JWS
   signing of the resolved Index Document remains MUST regardless of
   DNSSEC posture (see Section 4.2.6).

4.3.3.  Withdrawal of -00 SVCB ParamKey Registrations

   The SVCB ParamKey registrations atn and atn-digest proposed in draft-
   somoza-atn-agent-trust-negotiation-00 are withdrawn.  SVCB ParamKeys
   [RFC9460] are designed to convey service-connection hints (such as
   alpn, port, and ech), not to point at a separate document at another
   URL.  Pointing at the Index Document from DNS is handled instead by
   the thin TXT record in Section 4.3.1.

4.4.  Other Bindings

   Other bindings (URI-based out-of-band, registry-based, embodied-
   agent-specific) can be specified by future documents using the
   abstract binding interface in Section 4.1.  Such bindings are out of
   scope for this document.

5.  Capability Manifest

   The Capability Manifest declares what the agent is willing to do
   under what conditions.  It is a JSON object signed as a JWS.

5.1.  Capability Object Structure

   Each capability is a structured object with explicit dimensions
   covering identification, semantics, scope, and operational bounds.
   The design borrows from the action-resource-condition pattern of
   established authorization frameworks, the schema-binding pattern of
   W3C Verifiable Credentials, and the attenuation principle of
   capability-based security research.



Somoza                  Expires 30 November 2026               [Page 13]

Internet-Draft           agent-trust-negotiation                May 2026


      +=================+==========+================================+
      | Field           | Required | Description                    |
      +=================+==========+================================+
      | id              | Yes      | Capability identifier from the |
      |                 |          | IANA registry, or a vendor-    |
      |                 |          | specific identifier.           |
      +-----------------+----------+--------------------------------+
      | schema          | Yes      | URI plus SHA-256 digest of the |
      |                 |          | formal capability definition.  |
      +-----------------+----------+--------------------------------+
      | actions         | Yes      | List of action verbs scoped to |
      |                 |          | the capability's schema        |
      |                 |          | vocabulary.                    |
      +-----------------+----------+--------------------------------+
      | resources       | Yes      | List of resource patterns the  |
      |                 |          | capability applies to.         |
      +-----------------+----------+--------------------------------+
      | conditions      | No       | Runtime constraints: rate      |
      |                 |          | limits, data residency, time   |
      |                 |          | windows, size limits.          |
      +-----------------+----------+--------------------------------+
      | effects         | Yes      | One of: none, read_only,       |
      |                 |          | idempotent, mutating.          |
      +-----------------+----------+--------------------------------+
      | external_calls  | Yes      | One of: forbidden,             |
      |                 |          | listed_only, free.  Whether    |
      |                 |          | the capability may call out.   |
      +-----------------+----------+--------------------------------+
      | sub_invocations | Yes      | One of: forbidden, same_scope, |
      |                 |          | fresh_handshake_required.      |
      |                 |          | Sub-agent rules.               |
      +-----------------+----------+--------------------------------+
      | persistence     | Yes      | One of: none, session_only,    |
      |                 |          | durable.  Whether state        |
      |                 |          | survives the session.          |
      +-----------------+----------+--------------------------------+
      | resource_bounds | Yes      | Hard ceilings: max_tokens,     |
      |                 |          | max_duration_seconds,          |
      |                 |          | max_cost_usd.                  |
      +-----------------+----------+--------------------------------+
      | preconditions   | No       | Counterparty requirements that |
      |                 |          | must hold before the           |
      |                 |          | capability is exercised.       |
      +-----------------+----------+--------------------------------+

                                  Table 1





Somoza                  Expires 30 November 2026               [Page 14]

Internet-Draft           agent-trust-negotiation                May 2026


   Every dimension uses a controlled vocabulary registered with IANA
   (see Section 17).  Vendor-specific identifiers under the x- prefix
   are permitted for non-standard capabilities and MUST carry a schema
   reference.

5.2.  Schema Binding

   The schema field binds the capability identifier to a formal
   definition.  Two implementations resolving the same id to different
   schema.digest values MUST treat the capabilities as semantically
   distinct, even if the identifier matches.  This eliminates the
   silent-divergence problem that opaque scope strings created in OAuth
   deployments.

   The schema document SHOULD define:

   1.  The semantic meaning of the capability identifier.

   2.  The permitted values in actions.

   3.  The format and semantics of resources patterns.

   4.  Any capability-specific extensions to conditions.

   Schema documents are versioned.  A capability MAY pin a specific
   schema version via the digest.

5.3.  Structure

   {
     "v": "atn-capability-1",
     "agent_id": "<stable-identifier>",
     "issued_at": "2026-05-15T10:00:00Z",
     "valid_until": "2026-08-15T10:00:00Z",
     "capabilities": [
       {
         "id": "data-read",
         "schema": {
           "url": "https://example.com/atn/data-read-v1.json",
           "digest": "sha256:b4c5d6..."
         },
         "actions": ["read", "list"],
         "resources": [
           "dataset:public/*",
           "dataset:internal/research/*"
         ],
         "conditions": {
           "rate_limit": "1000/min",



Somoza                  Expires 30 November 2026               [Page 15]

Internet-Draft           agent-trust-negotiation                May 2026


           "data_residency": ["us", "eu"],
           "time_window": "09:00-17:00 UTC",
           "max_response_size_bytes": 10485760
         },
         "effects": "read_only",
         "external_calls": "forbidden",
         "sub_invocations": "forbidden",
         "persistence": "none",
         "resource_bounds": {
           "max_tokens": 100000,
           "max_duration_seconds": 1800,
           "max_cost_usd": 0.50
         },
         "preconditions": {
           "counterparty_provenance": "required",
           "counterparty_delegation": "required",
           "transport": "tls1.3"
         }
       },
       {
         "id": "task-execute",
         "schema": {
           "url": "https://example.com/atn/task-execute-v1.json",
           "digest": "sha256:c5d6e7..."
         },
         "actions": ["execute"],
         "resources": [
           "task:summarize",
           "task:translate",
           "task:classify"
         ],
         "conditions": {
           "max_session_minutes": 30
         },
         "effects": "idempotent",
         "external_calls": "listed_only",
         "sub_invocations": "same_scope",
         "persistence": "session_only",
         "resource_bounds": {
           "max_tokens": 500000,
           "max_duration_seconds": 1800,
           "max_cost_usd": 2.00
         },
         "preconditions": {
           "human_approval": "required",
           "approval_token_issuer": "https://approvals.example.com"
         }
       }



Somoza                  Expires 30 November 2026               [Page 16]

Internet-Draft           agent-trust-negotiation                May 2026


     ],
     "refusals": [
       {
         "category": "financial_transactions",
         "scope": "all"
       },
       {
         "category": "data_write",
         "scope": "external_systems"
       }
     ]
   }

5.4.  Dimension Semantics

   effects declares the consequence of exercising the capability:

   *  none: no observable consequence (introspection only).

   *  read_only: data is read; no state changes.

   *  idempotent: state may change but repeated execution is equivalent
      to single execution.

   *  mutating: state changes, and the change is not idempotent.

   external_calls declares whether the capability may initiate calls
   outside the agent's own scope:

   *  forbidden: no external calls.

   *  listed_only: external calls are permitted only to destinations
      listed in the schema or conditions.

   *  free: any external call is permitted, bounded only by
      resource_bounds.

   sub_invocations declares the rules for invoking other agents:

   *  forbidden: this capability does not invoke other agents.

   *  same_scope: sub-invocations inherit the present session's
      negotiated scope.

   *  fresh_handshake_required: every sub-invocation requires a separate
      ATN handshake with the sub-agent.

   persistence declares state survivability:



Somoza                  Expires 30 November 2026               [Page 17]

Internet-Draft           agent-trust-negotiation                May 2026


   *  none: no state persists beyond a single request.

   *  session_only: state persists for the session lifetime, then is
      discarded.

   *  durable: state persists beyond the session; the agent MUST
      disclose retention policy in its schema.

   resource_bounds declares hard ceilings on the negotiated scope.
   Conforming implementations MUST terminate the session when any bound
   is exceeded.  The protocol specifies the contract; runtime
   enforcement is the implementation's responsibility, consistent with
   the enforcement posture described in the Security Considerations.

5.5.  Key Properties

   *  The manifest is a declaration of willingness, not an enforcement
      primitive.  Violations are auditable through the Session Receipt
      and transparency log.

   *  The valid_until field forces rotation.  Manifests without expiry
      MUST be rejected.

   *  refusals are first-class.  Explicit "will not" claims provide
      auditable accountability if violated.

   *  Capability id values from the IANA registry have stable semantics;
      vendor-specific identifiers under x- MUST carry a schema
      reference.

   *  Capabilities are deny-by-default.  The negotiated session scope
      contains only what both parties explicitly offered.

5.6.  Refusal Semantics

   Refusals are a deliberate inversion of conventional capability
   advertisement.  Most existing proposals describe what an agent CAN
   do.  ATN requires agents to also describe what they explicitly WILL
   NOT do.  This produces auditable behavioral contracts: a recorded
   refusal that is subsequently violated is direct evidence of
   misbehavior.

6.  Delegation Chain

   The Delegation Chain proves the agent operates under authorized
   principal scope, with explicit time bounds and revocation paths.





Somoza                  Expires 30 November 2026               [Page 18]

Internet-Draft           agent-trust-negotiation                May 2026


6.1.  Structure

   {
     "v": "atn-delegation-1",
     "agent_id": "<stable-identifier>",
     "chain": [
       {
         "issuer": "did:example:organization-root",
         "subject": "did:example:department-ops",
         "scope": ["agent.deploy", "agent.delegate"],
         "issued_at": "2026-01-01T00:00:00Z",
         "valid_until": "2027-01-01T00:00:00Z",
         "revocation": "https://example.com/atn/rev/dept-ops",
         "signature": "<JWS>"
       },
       {
         "issuer": "did:example:department-ops",
         "subject": "agent:<stable-identifier>",
         "scope": [
           "data-read",
           "task-execute:summarize",
           "task-execute:translate"
         ],
         "issued_at": "2026-05-01T00:00:00Z",
         "valid_until": "2026-08-01T00:00:00Z",
         "revocation": "https://example.com/atn/rev/agent",
         "signature": "<JWS>"
       }
     ]
   }

6.2.  Verification

   A relying party MUST:

   1.  Verify each link's signature against the issuer's published key.

   2.  Verify each link's scope is a subset of the parent link's scope.

   3.  Verify all links are within their valid_until window.

   4.  Optionally poll revocation endpoints with rate-limited caching.

   5.  Verify the leaf subject matches the agent's stable identifier
       from discovery.

   If any check fails, the agent MUST NOT be trusted for any capability
   beyond the verifiable subset.



Somoza                  Expires 30 November 2026               [Page 19]

Internet-Draft           agent-trust-negotiation                May 2026


7.  Provenance Attestation

   The Provenance Attestation proves build-time and runtime
   characteristics of the agent.

7.1.  Structure

   {
     "v": "atn-provenance-1",
     "agent_id": "<stable-identifier>",
     "build": {
       "predicateType": "https://slsa.dev/provenance/v1",
       "predicate": { "...": "in-toto SLSA provenance" }
     },
     "model": {
       "model_id": "example-vendor/llm-v1.2.3",
       "model_version_sha256": "9f8e7d6c5b4a...",
       "system_prompt_sha256": "1a2b3c4d5e6f..."
     },
     "runtime": {
       "evidence_type": "RATS",
       "evidence": { "...": "RFC 9334 evidence" },
       "verifier": "https://verifier.example.com/rats"
     },
     "issued_at": "2026-05-15T08:00:00Z",
     "valid_until": "2026-05-15T20:00:00Z"
   }

7.2.  Properties

   *  The Provenance Attestation links three independent supply chains:
      software build (SLSA, in-toto [IN-TOTO]), model identity
      (cryptographic hash of weights or version anchor), and runtime
      environment (RATS evidence [RFC9334]).

   *  All three are optional.  An agent that publishes only build
      provenance is more trustworthy than one that publishes none and
      less trustworthy than one that publishes all three.

   *  Short valid_until windows are encouraged.  The Provenance
      Attestation is the most time-sensitive artifact.

8.  The Agent Trust Handshake (ATH)

8.1.  State Machine






Somoza                  Expires 30 November 2026               [Page 20]

Internet-Draft           agent-trust-negotiation                May 2026


      Initiator                                      Responder
          |                                              |
          |---- ATH_HELLO ------------------------------>|
          |     (initiator artifacts + requested scope)  |
          |                                              |
          |<--- ATH_OFFER -------------------------------|
          |     (responder artifacts + offered scope)    |
          |                                              |
          |---- ATH_ACCEPT ---------------------------- >|
          |     (intersection scope + receipt request)   |
          |                                              |
          |<--- ATH_RECEIPT -----------------------------|
          |     (signed session receipt)                 |
          |                                              |
          === negotiated session begins ===

   All four messages are JSON, signed as JWS, exchanged over HTTPS at
   the Responder's handshake_endpoint.

8.2.  ATH_HELLO

   {
     "v": "ath1",
     "type": "hello",
     "supported_versions": ["ath1"],
     "initiator": {
       "agent_id": "<initiator-id>",
       "artifacts": {
         "capability": { "url": "...", "digest": "..." },
         "delegation": { "url": "...", "digest": "..." },
         "provenance": { "url": "...", "digest": "..." }
       }
     },
     "requested_scope": {
       "capability_ids": ["data-read"],
       "duration_seconds": 1800,
       "purpose": "summarize_research_corpus"
     },
     "nonce": "<random>",
     "timestamp": "..."
   }

   The requested_scope.capability_ids field carries only the identifiers
   of the capabilities the Initiator wishes to negotiate.  The Responder
   consults its own Capability Manifest for the structured definitions
   and computes the intersection (see Section 9).





Somoza                  Expires 30 November 2026               [Page 21]

Internet-Draft           agent-trust-negotiation                May 2026


8.3.  ATH_OFFER

   The Responder verifies the Initiator's artifacts, computes the
   capability intersection, and returns:















































Somoza                  Expires 30 November 2026               [Page 22]

Internet-Draft           agent-trust-negotiation                May 2026


   {
     "v": "ath1",
     "type": "offer",
     "selected_version": "ath1",
     "supported_versions_echo": ["ath1"],
     "responder": {
       "agent_id": "<responder-id>",
       "artifacts": {
         "capability": { "url": "...", "digest": "..." },
         "delegation": { "url": "...", "digest": "..." },
         "provenance": { "url": "...", "digest": "..." }
       }
     },
     "offered_scope": {
       "capabilities": [
         {
           "id": "data-read",
           "schema": {
             "url": "https://example.com/atn/data-read-v1.json",
             "digest": "sha256:b4c5d6..."
           },
           "actions": ["read", "list"],
           "resources": ["dataset:public/*"],
           "conditions": {
             "rate_limit": "500/min",
             "data_residency": ["us"]
           },
           "effects": "read_only",
           "external_calls": "forbidden",
           "sub_invocations": "forbidden",
           "persistence": "none",
           "resource_bounds": {
             "max_tokens": 50000,
             "max_duration_seconds": 1800,
             "max_cost_usd": 0.50
           }
         }
       ],
       "duration_seconds": 1800,
       "purpose": "summarize_research_corpus"
     },
     "nonce": "<random>",
     "in_reply_to_nonce": "<initiator-nonce>",
     "timestamp": "..."
   }






Somoza                  Expires 30 November 2026               [Page 23]

Internet-Draft           agent-trust-negotiation                May 2026


   The offered_scope.capabilities field carries the full structured
   Capability objects after intersection.  Each is the per-dimension
   intersection of the Initiator's requested capability (resolved from
   its Capability Manifest) and the Responder's offered version.

8.4.  ATH_ACCEPT

   If the Initiator agrees with the offered scope:

   {
     "v": "ath1",
     "type": "accept",
     "agreed_scope": { "...": "matches ATH_OFFER offered_scope" },
     "nonce": "<random>",
     "in_reply_to_nonce": "<responder-nonce>",
     "timestamp": "..."
   }

   If the Initiator disagrees, it MAY send ATH_COUNTER with a revised
   request, up to a limit (see Section 8.6).

8.5.  ATH_RECEIPT

   The Responder produces the Session Receipt:

   {
     "v": "ath1",
     "type": "receipt",
     "session_id": "<uuid>",
     "initiator_id": "<initiator-id>",
     "responder_id": "<responder-id>",
     "agreed_scope": { "...": "the intersection result" },
     "artifact_digests": {
       "initiator_capability": "sha256:...",
       "initiator_delegation": "sha256:...",
       "initiator_provenance": "sha256:...",
       "responder_capability": "sha256:...",
       "responder_delegation": "sha256:...",
       "responder_provenance": "sha256:..."
     },
     "scitt_log_pointer": "https://scitt.example.org/entries/...",
     "issued_at": "...",
     "expires_at": "...",
     "signature": "<JWS by responder>"
   }

   Both parties countersign the receipt and MAY publish it to a SCITT
   log [SCITT].



Somoza                  Expires 30 November 2026               [Page 24]

Internet-Draft           agent-trust-negotiation                May 2026


8.6.  Handshake Limits

   *  Total handshake round trips MUST NOT exceed 4.

   *  ATH_COUNTER messages MUST NOT exceed 2 per session.

   *  The handshake MUST complete within 30 seconds.

   *  Sessions exceeding agreed_scope.duration_seconds require a new
      handshake.

9.  Capability Intersection Algebra

   When two agents present capability sets, the negotiated scope is
   computed by intersection.  The algorithm is specified for
   interoperability so that conforming implementations produce identical
   results from identical inputs.

9.1.  Identity Rule

   A capability c is in the intersection only if all of the following
   hold:

   1.  Both Initiator's requested set and Responder's offered set
       include a capability with the same id.

   2.  Both capabilities reference the same schema.url and
       schema.digest.  Identifiers with diverging schemas MUST be
       treated as distinct capabilities and excluded from the
       intersection.

   3.  Neither party has a matching entry in refusals covering c.

9.2.  Per-Dimension Rules

   For each capability c that satisfies the identity rule, the
   intersected capability is computed dimension by dimension as follows.

   actions  List intersection.  If empty, c is dropped.

   resources  Pattern intersection using longest-prefix match.  If
      empty, c is dropped.

   conditions numeric fields (rate_limit, max_response_size_bytes,
   max_session_minutes)  Minimum value of the two.  Tighter bound wins.

   conditions list fields (data_residency, tasks)  List intersection.
      If empty, c is dropped.



Somoza                  Expires 30 November 2026               [Page 25]

Internet-Draft           agent-trust-negotiation                May 2026


   conditions.time_window  Time-window intersection.  If empty, c is
      dropped.

   effects  Most restrictive of the two, where the order from least to
      most restrictive is mutating, idempotent, read_only, none.  Take
      the lower-impact value.

   external_calls  Most restrictive of the two, where the order from
      least to most restrictive is free, listed_only, forbidden.  Take
      the more restrictive value.

   sub_invocations  Most restrictive of the two, where the order from
      least to most restrictive is same_scope, fresh_handshake_required,
      forbidden.  Take the more restrictive value.

   persistence  Most restrictive of the two, where the order from least
      to most persistent is none, session_only, durable.  Take the lower
      value.

   resource_bounds.max_tokens, resource_bounds.max_duration_seconds,
   resource_bounds.max_cost_usd  Minimum value of the two.

   preconditions  Union.  Both parties' preconditions apply to the
      negotiated capability.

9.3.  Refusal Override

   If either party's refusals list contains an entry matching capability
   c by category or by direct id, c is dropped from the intersection
   regardless of any other dimension.  Refusals are absolute.

9.4.  Worked Example

   Initiator requests data-read:

   *  actions: [read, list, search]

   *  resources: [dataset:public/*, dataset:internal/*]

   *  conditions.rate_limit: 1000/min

   *  conditions.data_residency: ["us", "eu", "apac"]

   *  effects: read_only

   *  external_calls: listed_only

   *  resource_bounds.max_cost_usd: 1.00



Somoza                  Expires 30 November 2026               [Page 26]

Internet-Draft           agent-trust-negotiation                May 2026


   Responder offers data-read:

   *  actions: [read, list]

   *  resources: [dataset:public/*]

   *  conditions.rate_limit: 500/min

   *  conditions.data_residency: ["us", "eu"]

   *  effects: read_only

   *  external_calls: forbidden

   *  resource_bounds.max_cost_usd: 0.50

   Negotiated intersection (assuming matching schemas):

   *  actions: [read, list]

   *  resources: [dataset:public/*]

   *  conditions.rate_limit: 500/min

   *  conditions.data_residency: ["us", "eu"]

   *  effects: read_only

   *  external_calls: forbidden

   *  resource_bounds.max_cost_usd: 0.50

   If the schemas had diverged, data-read would be dropped entirely with
   no intersection.  If Initiator's refusals included data-read, the
   capability would be dropped regardless of offer compatibility.

10.  Mutual Trust Mode

   When both agents are sensitive (for example, two agents acting on
   behalf of different organizations), the handshake operates in mutual
   mode:

   *  Both parties MUST present all four artifacts in ATH_HELLO and
      ATH_OFFER.

   *  Both parties MUST verify the counterparty's full chain before
      proceeding.




Somoza                  Expires 30 November 2026               [Page 27]

Internet-Draft           agent-trust-negotiation                May 2026


   *  The Session Receipt MUST be countersigned by both parties.

   In single-sided mode (for example, when one party is a public
   service), only the trusted side must present full artifacts.  The
   other side's identity is still verified at the discovery layer, but
   the trust posture is asymmetric.

11.  Version Negotiation and Algorithm Agility

   ATN is designed for long-term evolution.  Two mechanisms ensure
   forward compatibility: explicit version negotiation in the handshake
   and pluggable cryptographic algorithms in artifacts and messages.

11.1.  Version Negotiation

   The Initiator's ATH_HELLO message advertises every ATN version it
   supports in a supported_versions field.  The Responder's ATH_OFFER
   selects the highest mutually supported version and echoes it in a
   selected_version field.  All subsequent messages in the session use
   the selected version.

   {
     "v": "ath1",
     "type": "hello",
     "supported_versions": ["ath1", "ath2"],
     "initiator": { "...": "..." },
     "requested_scope": { "...": "..." },
     "nonce": "...",
     "timestamp": "..."
   }

   The Responder MUST include the Initiator's supported_versions list in
   the signed envelope of its ATH_OFFER.  This prevents downgrade
   attacks: an on-path adversary that strips advertised versions from
   ATH_HELLO would cause the Responder's signature to cover the stripped
   list, which the Initiator can detect.

   If the Initiator and Responder share no common version, the Responder
   MUST reply with ATH_REJECT carrying error code version_mismatch.  The
   handshake terminates.

11.2.  Algorithm Agility

   ATN does not pin cryptographic algorithms beyond a minimum set
   required for v1 interoperability.  Implementations MAY use any
   algorithm registered for JWS [RFC7515] or COSE.

   Required for ATN v1 conformance:



Somoza                  Expires 30 November 2026               [Page 28]

Internet-Draft           agent-trust-negotiation                May 2026


   1.  Ed25519 signatures over JWS-encoded artifacts and handshake
       messages.

   2.  SHA-256 for digests bound in artifact references.

   Recommended additional algorithms:

   1.  ECDSA P-256 with SHA-256 for environments with ECDSA-only
       hardware tokens.

   2.  ML-DSA (post-quantum signature) for deployments preparing for
       post-quantum transition.

   The Capability Manifest, Delegation Chain, Provenance Attestation,
   and Session Receipt MAY each be signed with a different algorithm.
   The JWS header identifies the algorithm.  Relying parties MUST reject
   artifacts signed with algorithms they do not support.

   Algorithm identifiers are negotiated implicitly via the JWS alg
   header on each artifact.  Explicit negotiation in ATH_HELLO is NOT
   REQUIRED for v1 but MAY be added in future versions.

11.3.  Post-Quantum Transition

   The cryptographic operations in ATN are well-bounded: signature
   verification of artifacts and handshake messages.  There is no key
   agreement step inside ATN itself; TLS handles channel security.
   Migration to post-quantum signatures requires only adopting a PQ-
   capable signature algorithm in JWS, which the IETF JOSE working group
   is standardizing.

   When PQ algorithms are widely deployed, operators SHOULD rotate to a
   hybrid scheme (classical + PQ) during the transition window, then to
   PQ-only.  ATN's version negotiation supports introducing a new minor
   version that prefers PQ signatures without breaking v1 deployments.

12.  Compatibility with Existing Discovery Protocols

   ATN is designed to coexist with existing discovery proposals without
   modification.  Each proposal can be paired with ATN through either
   the HTTP-Resource Binding or the DNS Binding, depending on what the
   proposal provides.









Somoza                  Expires 30 November 2026               [Page 29]

Internet-Draft           agent-trust-negotiation                May 2026


12.1.  With AID [AID]

   AID's TXT record continues to authenticate the agent endpoint via
   PKA.  The TXT record can be paired with ATN's _atn.{domain}
   announcement, with the actual artifact references retrieved from the
   resolved Index Document at /.well-known/atn.  ATN consumes the AID-
   authenticated identity as the input to the handshake.

12.2.  With DNS-AID [DNS-AID]

   DNS-AID's SVCB records resolve an agent endpoint.  ATN composes with
   DNS-AID by publishing a _atn.{domain} TXT record alongside the DNS-
   AID record set, with the artifact references retrieved from the
   resolved Index Document.

12.3.  With ANS v2 [ANSv2]

   ANS v2's Trust Card concept maps directly to a subset of ATN's
   Capability Manifest.  An ANS v2 deployment can publish an ATN Index
   Document where the Capability Manifest cross-references the ANS Trust
   Card.

12.4.  Multi-Discovery Tolerance

   A single agent zone MAY publish records compatible with multiple
   discovery proposals simultaneously.  ATN binding is independent of
   which discovery format is used.

13.  Relationship to Existing Identity, Authorization, and Attestation
     Frameworks

   A reasonable first question for any new trust protocol is whether
   existing frameworks suffice.  ATN composes several mature primitives
   and does not reinvent any of them.  The contribution is the
   composition: a peer-to-peer agent handshake with deterministic scope
   negotiation and an auditable session receipt.  This section addresses
   each adjacent framework and identifies where ATN composes with it.

   The analogy that clarifies the relationship: TLS uses asymmetric
   crypto, symmetric crypto, key exchange, and X.509.  TLS is not
   accused of reinventing those primitives.  TLS is the handshake that
   composes them into a deployable channel-security protocol.  ATN
   occupies the same compositional role for inter-agent trust.








Somoza                  Expires 30 November 2026               [Page 30]

Internet-Draft           agent-trust-negotiation                May 2026


13.1.  OAuth 2.0 and GNAP

   OAuth 2.0 [RFC6749] and GNAP [RFC9635] authenticate and authorize a
   Client to access a Resource Server.  ATN negotiates a mutual
   agreement between two autonomous peers.

   Structural differences:

   1.  Asymmetric vs symmetric.  OAuth and GNAP define Client and
       Resource Server roles.  ATN defines two peers, each presenting
       four artifacts.  There is no "resource server" in agent-to-agent
       collaboration.

   2.  Secret tokens vs publishable artifacts.  OAuth bearer tokens are
       secrets and MUST NOT be logged.  ATN artifacts are designed for
       publication to transparency logs.  Opposite security models for
       the core artifacts.

   3.  No formal scope intersection.  OAuth scopes are evaluated by
       membership at the Resource Server.  ATN specifies a deterministic
       intersection algorithm.

   4.  No refusal semantics.  OAuth represents grants.  ATN represents
       grants and explicit refusals as first-class signed claims.

   Where OAuth and GNAP remain the right tools:

   *  Inside a trust domain with a central authorization server, OAuth
      and GNAP issue access tokens to passive resources.  ATN is
      unnecessary.

   *  Where one party is a passive resource, OAuth-style bearer access
      fits.

   *  Human-to-agent delegation, where a human grants an agent access on
      the human's behalf, fits GNAP.

   ATN composes with OAuth and GNAP.  An agent in an ATN session MAY
   hold OAuth tokens for downstream resource access.  ATN governs the
   inter-agent relationship; OAuth governs subsequent resource calls.

13.2.  SPIFFE Workload Identity

   SPIFFE [SPIFFE] issues SVIDs (SPIFFE Verifiable Identity Documents)
   to workloads within a trust domain.  SPIFFE Federation supports
   cross-domain identity through bilaterally exchanged trust bundles.

   Structural relationship:



Somoza                  Expires 30 November 2026               [Page 31]

Internet-Draft           agent-trust-negotiation                May 2026


   1.  SPIFFE provides identity, not authorization, willingness,
       capability, or provenance.

   2.  SPIFFE is optimized for intra-organizational deployment.  Cross-
       organizational use requires pre-arranged federation.  ATN targets
       cross-organizational interactions where peers may meet for the
       first time without prior trust setup.

   3.  ATN can carry a SPIFFE ID as the stable identifier in its
       artifacts.  The SPIFFE SVID becomes the authentication input; the
       ATN handshake adds capability, delegation, provenance, and
       receipt on top.

   SPIFFE and ATN compose cleanly.  SPIFFE within the trust domain, ATN
   at the boundary.

13.3.  Mutual TLS

   Mutual TLS [RFC8446] authenticates both endpoints at the transport
   layer.  It establishes that the connection is between two specific
   certificates.

   mTLS provides:

   *  Endpoint authentication.

   *  Channel confidentiality and integrity.

   mTLS does not provide:

   *  Capability declaration.

   *  Delegation expression.

   *  Provenance evidence.

   *  Negotiation of scope.

   *  Auditable agreement record.

   ATN runs on top of an mTLS-secured channel where appropriate. mTLS is
   necessary infrastructure for many ATN deployments.  It is not a
   substitute for the negotiation.

13.4.  RATS Remote Attestation

   RATS [RFC9334] defines roles (Attester, Verifier, Relying Party,
   Endorser) and the structure of attestation evidence and results.



Somoza                  Expires 30 November 2026               [Page 32]

Internet-Draft           agent-trust-negotiation                May 2026


   RATS provides:

   *  The architecture for producing and verifying attestation evidence.

   RATS does not provide:

   *  A protocol for combining attestation evidence with authorization
      or capability negotiation.

   *  A specification for what to do with verification results in a peer
      interaction.

   ATN consumes RATS evidence as one axis of its Provenance Attestation.
   The other two axes (build provenance and model identity) come from
   supply-chain attestation frameworks.  RATS is the input; ATN is one
   specified consumer.

13.5.  SCITT Transparency

   SCITT [SCITT] defines an append-only transparency log for signed
   statements with inclusion proofs.

   SCITT provides:

   *  The transparency log infrastructure.

   *  The signed-statement envelope format.

   SCITT does not provide:

   *  The semantics of what to log for a given application.

   *  The receipt format that captures an inter-agent agreement.

   ATN provides the Session Receipt format that becomes the SCITT
   statement for an agent interaction.  SCITT without ATN is a log
   waiting for a record format.  ATN without SCITT is a receipt waiting
   for a transparency layer.

13.6.  Composition Summary

   ATN composes the following primitives per layer:









Somoza                  Expires 30 November 2026               [Page 33]

Internet-Draft           agent-trust-negotiation                May 2026


   +===================+=========================+====================+
   | Concern           | Primitive               | ATN's Role         |
   +===================+=========================+====================+
   | Endpoint          | mTLS [RFC8446]          | Runs above         |
   | authentication    |                         |                    |
   +-------------------+-------------------------+--------------------+
   | Workload identity | SPIFFE [SPIFFE]         | Consumes SVIDs as  |
   | (intra-org)       |                         | stable identifiers |
   +-------------------+-------------------------+--------------------+
   | Cross-org         | HTTP origins [RFC9110], | Discovery          |
   | identity          | DNS [RFC4033], [AID],   | substrate for ATN  |
   |                   | [DNS-AID], [ANSv2]      | artifacts          |
   +-------------------+-------------------------+--------------------+
   | Delegated         | OAuth [RFC6749], GNAP   | Composes with,     |
   | authorization to  | [RFC9635]               | does not replace   |
   | passive resources |                         |                    |
   +-------------------+-------------------------+--------------------+
   | Attestation       | RATS [RFC9334]          | Consumes evidence  |
   | evidence          |                         | in Provenance      |
   |                   |                         | Attestation        |
   +-------------------+-------------------------+--------------------+
   | Public            | SCITT [SCITT]           | Provides the       |
   | auditability      |                         | receipt format for |
   |                   |                         | log entries        |
   +-------------------+-------------------------+--------------------+
   | Peer-to-peer      | (none exists)           | This document      |
   | trust handshake   |                         |                    |
   +-------------------+-------------------------+--------------------+

                                 Table 2

   The contribution of ATN is the bottom row.  The five rows above are
   existing work that ATN composes.

13.7.  When to Use What

   1.  Two workloads inside one trust domain coordinating on a task:
       SPIFFE.

   2.  A client accessing a resource server with delegated user
       authorization: OAuth or GNAP.

   3.  Establishing that you are talking to a specific endpoint: mTLS.

   4.  Producing tamper-evident proof that some software was built and
       deployed as claimed: RATS plus supply-chain attestation.

   5.  Publishing an auditable record of a claim: SCITT.



Somoza                  Expires 30 November 2026               [Page 34]

Internet-Draft           agent-trust-negotiation                May 2026


   6.  Two autonomous agents from different organizations agreeing to
       work together with mutually verified capability, authority,
       provenance, and a signed record of the agreement: ATN.

14.  Security Considerations

14.1.  Threat Model

   ATN assumes:

   *  Adversaries may publish well-formed but malicious ATN artifacts.

   *  Adversaries may compromise an agent and continue to present valid
      pre-compromise artifacts.

   *  Adversaries may attempt to negotiate scope expansions outside the
      published Capability Manifest.

   *  Adversaries may collude across organizational boundaries to
      construct false Delegation Chains.

14.2.  Artifact Authenticity

   Every ATN artifact is signed.  Signatures MUST be verified against
   keys bound to the agent's stable identifier through the active
   binding.  Artifacts whose signatures do not verify, whose valid_until
   has passed, or whose digests do not match the values in the Index
   Document MUST be rejected.

14.3.  Cross-Binding Consistency

   When an agent is reachable through multiple bindings (for example,
   both HTTP-Resource and DNS), the artifacts served through each
   binding MUST be identical.  Inconsistency across bindings MUST cause
   the handshake to fail with binding_mismatch.

14.4.  Enforcement Posture: Evidence, Not Prevention

   A predictable objection to ATN is that auditability is not
   enforcement.  The objection is correct on its terms and misses the
   layer at which ATN operates.  ATN is technical evidence
   infrastructure.  It is not a behavioral enforcement primitive.  No
   protocol layered on top of an autonomous software agent can be one.

   An agent that signs a Capability Manifest declaring it will not
   perform financial transactions, then performs a financial
   transaction, has cryptographically committed to a refusal it then
   violated.  ATN cannot prevent the violation.  ATN produces a Session



Somoza                  Expires 30 November 2026               [Page 35]

Internet-Draft           agent-trust-negotiation                May 2026


   Receipt and a published transparency log entry that, combined with
   the application-layer evidence of the violation, constitute direct
   cryptographic evidence usable for revocation, contract enforcement,
   regulatory complaint, and civil action.

   The complete trust stack for autonomous agents has four layers:

   1.  Technical evidence layer (ATN, SCITT, RATS).  Provides
       cryptographic proof of what was claimed and what was agreed.

   2.  Policy layer (contracts, organizational policy, regulations).
       Specifies what is permitted.

   3.  Consequence layer (revocation, financial penalty, legal action,
       reputational impact).  Imposes cost on violation.

   4.  Operational enforcement layer (sandboxing, capability-based
       execution, hardware-enforced runtime such as TEEs, gating
       proxies).  Prevents violation at runtime.

   ATN is the first layer.  It enables the other three.  It does not
   substitute for them.

   For high-stakes domains (financial transactions, clinical decision
   support, control of physical systems), ATN MUST be deployed with
   operational enforcement (layer 4).  Recommended patterns:

   1.  Run sensitive capabilities inside a TEE that enforces declared
       bounds at hardware level.

   2.  Use a gating proxy that enforces the negotiated scope on every
       outbound call from the agent.

   3.  Use short session lifetimes with frequent re-attestation.

   4.  Combine ATN with object-capability execution where the agent
       runtime denies unauthorized actions by construction, not by
       policy.

   ATN's contribution is to make violations _expensive and
   discoverable_, not impossible.  The analogy is to TCP checksums: they
   do not prevent transmission errors, they detect them so the next
   layer can respond.  Trust at Internet scale has always operated this
   way.  SPF does not prevent spoofed email.  Certificate transparency
   does not prevent mis-issuance.  They make undetected misbehavior
   infeasible.  ATN does the same for inter-agent agreements.





Somoza                  Expires 30 November 2026               [Page 36]

Internet-Draft           agent-trust-negotiation                May 2026


14.5.  Delegation Forgery

   Delegation Chains depend on the cryptographic strength of each
   issuer's signing key.  ATN does not specify the identity verification
   process that issuers use to vouch for subjects.  Operators SHOULD use
   verifiable credentials [W3C-DID] or organizational identity
   infrastructure with established assurance levels.

14.6.  Session Receipt Tampering

   Session Receipts are countersigned and publishable to a SCITT log
   [SCITT].  Tampering after publication is detectable through log
   inclusion proofs.  Operators handling high-value sessions SHOULD
   publish receipts.

14.7.  Replay

   Every handshake message includes a nonce and a timestamp.
   Implementations MUST reject messages with nonces seen within the
   session and with timestamps outside a configurable skew window
   (RECOMMENDED 60 seconds).

14.8.  Provenance Attestation Limits

   Provenance attestations prove what was built and deployed at
   attestation time.  They do not prove the agent is still executing the
   attested code.  Operators SHOULD use short valid_until windows for
   provenance attestations and refresh through RATS verification
   [RFC9334].

15.  Privacy Considerations

15.1.  Per-Agent Lookups

   The HTTP-Resource Binding does not generate per-agent DNS queries.
   Agent lookups occur within a single TLS connection to the origin,
   limiting on-path observers' visibility into which agents are being
   discovered.

15.2.  DNS Binding Leakage

   The DNS Binding's _atn.{domain} TXT lookup leaks the fact that an
   origin supports ATN to on-path DNS observers.  This is a property of
   the binding, not of the core protocol.  Deployments where this
   leakage is unacceptable SHOULD use the HTTP-Resource Binding
   directly.





Somoza                  Expires 30 November 2026               [Page 37]

Internet-Draft           agent-trust-negotiation                May 2026


15.3.  Negotiation Observability

   The handshake exchanges artifacts that may reveal the purpose of the
   session (purpose field) and the identity of the principal (Delegation
   Chain).  Operators SHOULD treat handshake metadata as sensitive.

15.4.  Witness Aggregation

   Witnesses and SCITT log operators see the volume and pattern of agent
   interactions.  Operators SHOULD use multiple witnesses to reduce
   centralized observability.

15.5.  Minimum Disclosure

   ATN supports minimum-disclosure mode where the Initiator presents
   only the artifacts required for the requested scope.  The Responder
   MAY request additional artifacts during the handshake, which the
   Initiator MAY decline (causing the handshake to fail).

15.6.  SCITT Publication

   Session Receipts published to a SCITT Transparency Service are public
   by design.  Implementations MUST NOT include personal data in
   Capability Manifests, Delegation Chains, Provenance Attestations, or
   Session Receipts.  Use of opaque identifiers in place of personal
   identifiers is RECOMMENDED.

16.  Operational Considerations

   ATN introduces non-trivial latency on session establishment.  This
   section quantifies that cost, explains its operational shape, and
   identifies deployments where ATN is appropriate and where it is not.

16.1.  Session Establishment vs Per-Call Latency

   ATN is a session establishment protocol, not a per-request protocol.
   The handshake establishes a negotiated scope.  The scope authorizes
   many subsequent interactions between the two agents.  Per-call
   latency within an established session is zero added by ATN.

   This places ATN in the same operational category as:

   1.  TLS handshake [RFC8446], amortized over the requests in a
       connection.

   2.  OAuth token acquisition [RFC6749], amortized over the token
       lifetime.




Somoza                  Expires 30 November 2026               [Page 38]

Internet-Draft           agent-trust-negotiation                May 2026


   3.  BGP session establishment, amortized over hours of routing
       operation.

   The relevant performance question is the cost of establishment and
   the session lifetime over which it is amortized, not the cost per
   call.

16.2.  Latency Budget

   The following budget is informative.  Implementations and deployments
   will vary based on geography, caching, and network conditions.  The
   figures below assume the HTTP-Resource Binding; the DNS Binding adds
   one DNS lookup on the first connection to an origin.

     +============================+=================+===============+
     | Component                  | Cold (no cache) | Warm (cached, |
     |                            |                 | pooled)       |
     +============================+=================+===============+
     | TLS 1.3 connection to      | 100-200 ms      | 0 (pooled)    |
     | origin                     |                 |               |
     +----------------------------+-----------------+---------------+
     | /.well-known/atn HTTPS GET | 20-50 ms        | 0 (cached by  |
     |                            |                 | digest)       |
     +----------------------------+-----------------+---------------+
     | Three artifact fetches     | 50-100 ms       | 0 (cached by  |
     | (HTTP/2 multiplexed)       |                 | digest)       |
     +----------------------------+-----------------+---------------+
     | Signature verifications    | 1-2 ms          | 1-2 ms        |
     | (4-8 Ed25519)              |                 |               |
     +----------------------------+-----------------+---------------+
     | Provenance validation      | 1-5 ms          | 1 ms          |
     +----------------------------+-----------------+---------------+
     | Two-RTT handshake (HELLO/  | 150-300 ms      | 150-300 ms    |
     | OFFER then ACCEPT/RECEIPT) |                 |               |
     +----------------------------+-----------------+---------------+
     | Receipt signing (Ed25519)  | <1 ms           | <1 ms         |
     +----------------------------+-----------------+---------------+
     | *Total session             | *500-1500 ms*   | *150-300 ms*  |
     | establishment*             |                 |               |
     +----------------------------+-----------------+---------------+
     | Per-call within            | *0 ms added*    | *0 ms added*  |
     | established session        |                 |               |
     +----------------------------+-----------------+---------------+
     | SCITT log submission       | Async, out of   | Async, out of |
     |                            | path            | path          |
     +----------------------------+-----------------+---------------+

                                 Table 3



Somoza                  Expires 30 November 2026               [Page 39]

Internet-Draft           agent-trust-negotiation                May 2026


   Recommended performance targets for conforming implementations:

   *  p50 cold session establishment: <800 ms

   *  p99 cold session establishment: <2000 ms

   *  p50 warm session establishment: <250 ms

   *  p99 warm session establishment: <500 ms

16.3.  Optimization Techniques

   Implementations SHOULD apply the following optimizations to bring
   warm-case latency to the recommended targets:

   1.  Digest-keyed artifact caching.  Artifacts are refetched only when
       the Index Document's published digest changes.  Steady-state
       cache hit rate is typically above 95 percent.

   2.  HTTP/2 or HTTP/3 multiplexing for parallel artifact fetches.

   3.  TLS 1.3 session resumption for repeat handshakes with known
       peers.

   4.  Connection pooling between sessions to the same peer.

   5.  Abbreviated re-handshake.  A repeat ATH_HELLO MAY reference a
       prior session's receipt to extend or renew the same scope in a
       single round trip rather than three.

   6.  Pre-warming.  Agents MAY establish sessions with frequent
       counterparties before application traffic arrives.

   7.  Asynchronous SCITT submission.  Receipt publication to a
       transparency log happens after the session begins and MUST NOT
       block session establishment.

   8.  RATS attestation result caching for the duration of the result's
       validity window.

16.4.  Session Lifetime

   The Session Receipt's expires_at field bounds the session.  Within
   that window, no additional handshake is required for interactions
   covered by the agreed scope.  Implementations SHOULD select session
   lifetimes that amortize establishment cost while bounding exposure to
   revocation events.




Somoza                  Expires 30 November 2026               [Page 40]

Internet-Draft           agent-trust-negotiation                May 2026


   Typical session lifetimes:

   *  Short-lived workflows: 5 to 30 minutes

   *  Sustained collaboration sessions: 1 to 24 hours

   *  Long-running infrastructure peering: up to 7 days

   Session lifetimes longer than 7 days are NOT RECOMMENDED for cross-
   organizational sessions.  The trade-off is between revocation
   responsiveness and handshake amortization.

16.5.  Deployments Where ATN Is Appropriate

   1.  Multi-step agent collaboration where many calls share a session
       context.

   2.  Cross-organizational agent interactions where trust establishment
       must be verifiable and auditable.

   3.  Workflows where the cost of trust failure exceeds the cost of
       establishment latency.

   4.  Agent ecosystems with workflows already operating in the 100 ms
       to multi-second range per step.

16.6.  Deployments Where ATN Is Not Appropriate

   1.  Microsecond-scale latency budgets (high-frequency trading, real-
       time control loops, hardware-in-the-loop systems).

   2.  Stateless per-message protocols where session continuity is not
       meaningful.

   3.  Workflows whose total duration is shorter than the handshake
       establishment cost.

   For these deployments, simpler primitives (mTLS alone, OAuth bearer
   tokens, or no trust layer at all) may be more appropriate.  Operators
   are encouraged to select the trust layer that matches their latency
   budget.

17.  IANA Considerations








Somoza                  Expires 30 November 2026               [Page 41]

Internet-Draft           agent-trust-negotiation                May 2026


17.1.  Withdrawal of -00 SVCB ParamKey Registrations

   The SVCB ParamKey registrations atn and atn-digest proposed in draft-
   somoza-atn-agent-trust-negotiation-00 are withdrawn.  No SVCB
   ParamKey registration is requested by this document.  See
   Section 4.3.3.

17.2.  Well-Known URI Registration

   IANA is requested to register the following well-known URI per
   [RFC8615]:

   *  URI suffix: atn

   *  Change controller: IETF

   *  Specification document: this document

   *  Status: permanent

   *  Related information: returns the ATN Index Document for the origin
      (see Section 4.2.1)

17.3.  DNS TXT Key Registration

   IANA is requested to register the following DNS TXT record key:

   *  Key: atn

   *  Format: v=atn1; origin=https://... (semicolon-separated key=value
      pairs)

   *  Change controller: IETF

   *  Specification document: this document

17.4.  ATN Capability Registry

   Creation of a new registry, "ATN Capabilities," is requested with
   initial entries:











Somoza                  Expires 30 November 2026               [Page 42]

Internet-Draft           agent-trust-negotiation                May 2026


        +================+=====================+==================+
        | Name           | Description         | Reference Schema |
        +================+=====================+==================+
        | data-read      | Read access to a    | TBD              |
        |                | data resource       |                  |
        +----------------+---------------------+------------------+
        | data-write     | Write access to a   | TBD              |
        |                | data resource       |                  |
        +----------------+---------------------+------------------+
        | task-execute   | Execute a named     | TBD              |
        |                | task                |                  |
        +----------------+---------------------+------------------+
        | model-invoke   | Invoke a generative | TBD              |
        |                | model               |                  |
        +----------------+---------------------+------------------+
        | agent-delegate | Delegate work to    | TBD              |
        |                | another agent       |                  |
        +----------------+---------------------+------------------+
        | payment-init   | Initiate a payment  | TBD              |
        |                | flow                |                  |
        +----------------+---------------------+------------------+
        | human-relay    | Surface a request   | TBD              |
        |                | to a human approver |                  |
        +----------------+---------------------+------------------+

                                  Table 4

   Registration policy: Specification Required with Designated Expert
   review.  Each registered identifier MUST include a reference to a
   published capability schema that defines the semantics of actions,
   resources patterns, and any capability-specific extensions to
   conditions.

   Vendor-specific identifiers prefixed with x- are permitted without
   IANA registration but MUST carry a schema reference in the capability
   object.

17.5.  ATN Capability Dimension Registries

   Four new registries are requested to govern the controlled
   vocabularies used in capability dimensions:

   "ATN Capability Effects":








Somoza                  Expires 30 November 2026               [Page 43]

Internet-Draft           agent-trust-negotiation                May 2026


     +============+=================================================+
     | Name       | Description                                     |
     +============+=================================================+
     | none       | No observable consequence (introspection only). |
     +------------+-------------------------------------------------+
     | read_only  | Data is read; no state changes.                 |
     +------------+-------------------------------------------------+
     | idempotent | State may change; repeated execution equivalent |
     |            | to single.                                      |
     +------------+-------------------------------------------------+
     | mutating   | State changes; not idempotent.                  |
     +------------+-------------------------------------------------+

                                 Table 5

   "ATN External Call Modes":

              +=============+==============================+
              | Name        | Description                  |
              +=============+==============================+
              | forbidden   | No external calls permitted. |
              +-------------+------------------------------+
              | listed_only | External calls permitted     |
              |             | only to listed destinations. |
              +-------------+------------------------------+
              | free        | Any external call permitted  |
              |             | within resource_bounds.      |
              +-------------+------------------------------+

                                 Table 6

   "ATN Sub-Invocation Modes":

       +==========================+===============================+
       | Name                     | Description                   |
       +==========================+===============================+
       | forbidden                | No sub-agent invocations.     |
       +--------------------------+-------------------------------+
       | same_scope               | Sub-invocations inherit the   |
       |                          | present negotiated scope.     |
       +--------------------------+-------------------------------+
       | fresh_handshake_required | Every sub-invocation requires |
       |                          | a separate ATN handshake.     |
       +--------------------------+-------------------------------+

                                 Table 7

   "ATN Persistence Modes":



Somoza                  Expires 30 November 2026               [Page 44]

Internet-Draft           agent-trust-negotiation                May 2026


       +==============+============================================+
       | Name         | Description                                |
       +==============+============================================+
       | none         | No state persists beyond a single request. |
       +--------------+--------------------------------------------+
       | session_only | State persists for the session lifetime,   |
       |              | then discarded.                            |
       +--------------+--------------------------------------------+
       | durable      | State persists beyond the session;         |
       |              | retention policy required.                 |
       +--------------+--------------------------------------------+

                                  Table 8

   Registration policy for all four registries: Specification Required
   with Designated Expert review.  The controlled vocabularies are
   deliberately small and stable; expansion requires consensus to avoid
   the semantic-drift problem that affected OAuth scopes.

17.6.  ATN Refusal Category Registry

   Creation of a new registry, "ATN Refusal Categories," is requested
   with initial entries:

   +========================+==========================================+
   | Name                   | Description                              |
   +========================+==========================================+
   | financial_transactions | Any movement of funds                    |
   +------------------------+------------------------------------------+
   | personal_data          | Processing of personal identifiers       |
   +------------------------+------------------------------------------+
   | weaponization          | Use in weapons systems                   |
   +------------------------+------------------------------------------+
   | mass_persuasion        | Generation of mass-distributed content   |
   +------------------------+------------------------------------------+
   | child_safety           | Content involving minors                 |
   +------------------------+------------------------------------------+
   | medical_advice         | Clinical recommendations                 |
   +------------------------+------------------------------------------+
   | legal_advice           | Legal recommendations binding on         |
   |                        | parties                                  |
   +------------------------+------------------------------------------+

                                  Table 9

   Registration policy: Specification Required with Designated Expert
   review.




Somoza                  Expires 30 November 2026               [Page 45]

Internet-Draft           agent-trust-negotiation                May 2026


18.  References

18.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/rfc/rfc2119>.

   [RFC4033]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
              Rose, "DNS Security Introduction and Requirements",
              RFC 4033, DOI 10.17487/RFC4033, March 2005,
              <https://www.rfc-editor.org/rfc/rfc4033>.

   [RFC7515]  Jones, M., Bradley, J., and N. Sakimura, "JSON Web
              Signature (JWS)", RFC 7515, DOI 10.17487/RFC7515, May
              2015, <https://www.rfc-editor.org/rfc/rfc7515>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/rfc/rfc8174>.

   [RFC8259]  Bray, T., Ed., "The JavaScript Object Notation (JSON) Data
              Interchange Format", STD 90, RFC 8259,
              DOI 10.17487/RFC8259, December 2017,
              <https://www.rfc-editor.org/rfc/rfc8259>.

   [RFC8446]  Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
              <https://www.rfc-editor.org/rfc/rfc8446>.

   [RFC8615]  Nottingham, M., "Well-Known Uniform Resource Identifiers
              (URIs)", RFC 8615, DOI 10.17487/RFC8615, May 2019,
              <https://www.rfc-editor.org/rfc/rfc8615>.

   [RFC9110]  Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
              Ed., "HTTP Semantics", STD 97, RFC 9110,
              DOI 10.17487/RFC9110, June 2022,
              <https://www.rfc-editor.org/rfc/rfc9110>.

   [RFC9460]  Schwartz, B., Bishop, M., and E. Nygren, "Service Binding
              and Parameter Specification via the DNS (SVCB and HTTPS
              Resource Records)", RFC 9460, DOI 10.17487/RFC9460,
              November 2023, <https://www.rfc-editor.org/rfc/rfc9460>.

18.2.  Informative References





Somoza                  Expires 30 November 2026               [Page 46]

Internet-Draft           agent-trust-negotiation                May 2026


   [AID]      Nemethi, B., "Agent Identity and Discovery (AID)", 2026,
              <https://datatracker.ietf.org/doc/draft-nemethi-aid-agent-
              identity-discovery/>.

   [ANSv2]    Narajala, K., "Agent Name Service v2", 2026,
              <https://datatracker.ietf.org/doc/draft-narajala-courtney-
              ansv2/>.

   [DNS-AID]  Mozley, J., Williams, N., Sarikaya, B., and R. Schott,
              "DNS for AI Discovery", 2026,
              <https://datatracker.ietf.org/doc/draft-mozleywilliams-
              dnsop-dnsaid/>.

   [IN-TOTO]  in-toto / CNCF, "in-toto Attestation Framework", 2024,
              <https://in-toto.io/>.

   [RFC6749]  Hardt, D., Ed., "The OAuth 2.0 Authorization Framework",
              RFC 6749, DOI 10.17487/RFC6749, October 2012,
              <https://www.rfc-editor.org/rfc/rfc6749>.

   [RFC9334]  Birkholz, H., Thaler, D., Richardson, M., Smith, N., and
              W. Pan, "Remote ATtestation procedureS (RATS)
              Architecture", RFC 9334, DOI 10.17487/RFC9334, January
              2023, <https://www.rfc-editor.org/rfc/rfc9334>.

   [RFC9635]  Richer, J., Ed. and F. Imbault, "Grant Negotiation and
              Authorization Protocol (GNAP)", RFC 9635,
              DOI 10.17487/RFC9635, October 2024,
              <https://www.rfc-editor.org/rfc/rfc9635>.

   [SCITT]    IETF SCITT WG, "Supply Chain Integrity, Transparency, and
              Trust (SCITT) Architecture", 2025,
              <https://datatracker.ietf.org/wg/scitt/about/>.

   [SPIFFE]   SPIFFE / CNCF, "SPIFFE: Secure Production Identity
              Framework for Everyone", 2024,
              <https://github.com/spiffe/spiffe>.

   [W3C-DID]  W3C, "Decentralized Identifiers (DIDs) v1.0", 2022,
              <https://www.w3.org/TR/did-core/>.

Appendix A.  Complete Example Flow

   This appendix walks through a full ATN handshake between two agents
   at research.org and publisher.com using the HTTP-Resource Binding.

A.1.  Step 1: Origin Discovery




Somoza                  Expires 30 November 2026               [Page 47]

Internet-Draft           agent-trust-negotiation                May 2026


   GET https://publisher.com/.well-known/atn

   Returns the JWS-signed Index Document listing the agents at the
   origin and their artifact URLs.

A.2.  Step 2: Index Document Verification

   The Initiator verifies the JWS signature on the Index Document
   against a trust anchor recognized by the verifier.  The signature is
   required regardless of DNSSEC posture.

A.3.  Step 3: Artifact Fetch and Verification

   The Initiator selects the target agent from the Index Document's
   agents array and fetches:

   *  capability.jws

   *  delegation.jws

   *  provenance.jws

   Verifies each signature, and confirms valid_until is in the future.

A.4.  Step 4: ATH_HELLO

   The Initiator (research.org agent) sends ATH_HELLO to the Responder's
   handshake_endpoint:

   {
     "v": "ath1",
     "type": "hello",
     "supported_versions": ["ath1"],
     "initiator": {
       "agent_id": "https://research.org/agents/initiator",
       "artifacts": { "...": "capability/delegation/provenance refs" }
     },
     "requested_scope": {
       "capability_ids": ["data-read"],
       "duration_seconds": 600,
       "purpose": "academic_research_summarization"
     },
     "nonce": "n-init-001",
     "timestamp": "2026-05-27T14:00:00Z"
   }






Somoza                  Expires 30 November 2026               [Page 48]

Internet-Draft           agent-trust-negotiation                May 2026


A.5.  Step 5: ATH_OFFER

   The Responder verifies the Initiator's artifacts, computes the
   intersection, and returns:

   {
     "v": "ath1",
     "type": "offer",
     "selected_version": "ath1",
     "supported_versions_echo": ["ath1"],
     "responder": { "...": "responder agent_id and artifact refs" },
     "offered_scope": {
       "capabilities": [
         {
           "id": "data-read",
           "schema": {
             "url": "https://example.com/atn/data-read-v1.json",
             "digest": "sha256:b4c5d6..."
           },
           "actions": ["read", "list"],
           "resources": ["dataset:public/*"],
           "conditions": {
             "rate_limit": "300/min",
             "data_residency": ["us", "eu"]
           },
           "effects": "read_only",
           "external_calls": "forbidden",
           "sub_invocations": "forbidden",
           "persistence": "none",
           "resource_bounds": {
             "max_tokens": 50000,
             "max_duration_seconds": 600,
             "max_cost_usd": 0.30
           }
         }
       ],
       "duration_seconds": 600,
       "purpose": "academic_research_summarization"
     },
     "nonce": "n-resp-001",
     "in_reply_to_nonce": "n-init-001",
     "timestamp": "2026-05-27T14:00:01Z"
   }

A.6.  Step 6: ATH_ACCEPT






Somoza                  Expires 30 November 2026               [Page 49]

Internet-Draft           agent-trust-negotiation                May 2026


   {
     "v": "ath1",
     "type": "accept",
     "agreed_scope": { "...": "matches ATH_OFFER" },
     "nonce": "n-init-002",
     "in_reply_to_nonce": "n-resp-001",
     "timestamp": "2026-05-27T14:00:02Z"
   }

A.7.  Step 7: ATH_RECEIPT

   {
     "v": "ath1",
     "type": "receipt",
     "session_id": "5b7c-...",
     "initiator_id": "https://research.org/agents/initiator",
     "responder_id": "https://publisher.com/agents/responder",
     "agreed_scope": { ... },
     "artifact_digests": { ... },
     "scitt_log_pointer": "https://scitt.research.org/entries/789xyz",
     "issued_at": "2026-05-27T14:00:03Z",
     "expires_at": "2026-05-27T14:10:03Z",
     "signature": "<JWS by responder>"
   }

   The Initiator countersigns.  The session begins.

A.8.  Step 8: Session and Audit

   The agents transact within the agreed scope.  The Session Receipt is
   published to SCITT.  Any downstream audit can:

   1.  Retrieve the receipt.

   2.  Verify all artifact digests against the agents' original
       publications.

   3.  Compare actual session traffic (where logged) against the agreed
       scope.

   4.  Flag any deviation.

Appendix B.  Design Rationale








Somoza                  Expires 30 November 2026               [Page 50]

Internet-Draft           agent-trust-negotiation                May 2026


B.1.  Why a Negotiation Layer

   Discovery answers "where" and "who."  Application protocols (MCP,
   A2A) answer "how."  Nothing in the current stack answers "what may we
   do together, and what evidence will remain."  ATN fills that gap with
   a two-round-trip handshake.

B.2.  Why Four Artifacts

   Each artifact answers a distinct question:

   *  Capability Manifest: what is this agent willing to do?

   *  Delegation Chain: who authorized it?

   *  Provenance Attestation: what is it actually running?

   *  Session Receipt: what did we agree?

   Collapsing them into one document obscures the separation of
   concerns.  Splitting further fragments the trust evaluation.  Four is
   the minimum.

B.3.  Why Discovery-Agnostic

   Discovery proposals for AI agents are still evolving across multiple
   venues and approaches.  Binding ATN tightly to any one discovery
   substrate would couple the trust layer's adoption curve to that
   substrate's adoption curve.  The four artifacts have value regardless
   of how the agent identity was resolved.  Specifying an abstract
   binding interface, with HTTP-Resource as the default and DNS as one
   option, lets ATN compose with whatever discovery substrate a
   deployment uses.

B.4.  Why Refusals Are First-Class

   Capability advertisement alone creates a moral hazard: agents have an
   incentive to advertise broadly.  Mandatory refusals create symmetric
   accountability: an agent that explicitly refused a category and then
   violated the refusal has produced cryptographic evidence against
   itself.

B.5.  Why Intersection Algebra Is Specified

   If two implementations compute intersections differently,
   interoperability fails.  The algebra is small, specified, and
   testable.  The cost is one paragraph of spec; the benefit is
   deterministic handshake outcomes.



Somoza                  Expires 30 November 2026               [Page 51]

Internet-Draft           agent-trust-negotiation                May 2026


B.6.  Why SCITT for Receipts

   Session Receipts gain value through public auditability.  SCITT
   [SCITT] provides the transparency infrastructure.  Receipts published
   to SCITT cannot be tampered with after the fact without detection.

Appendix C.  Implementation Status

   This section records the status of known implementations of the
   protocol defined by this specification at the time of posting of this
   Internet-Draft, per the guidelines of RFC 7942.  The description of
   implementations in this section is intended to assist the IETF in its
   decision processes in progressing drafts to RFCs.  Note to RFC
   Editor: this section may be removed prior to final publication.

C.1.  ATN Reference Prototype

   *  Organization: Independent (E.  Somoza).

   *  Implementation Name: atn-prototype.

   *  Description: A TypeScript prototype that demonstrates the four-
      message handshake, the capability intersection algebra, Ed25519
      signing and verification of artifacts and messages, and the
      digest-bound artifact reference model.

   *  License: CC0 1.0 Universal (public domain).

   *  Maturity: prototype.  Not production-ready.  Not security-audited.

   *  Coverage: Capability Manifest types, Delegation Chain types,
      Provenance Attestation types, Session Receipt, ATH state machine
      (HELLO, OFFER, ACCEPT, RECEIPT), capability intersection algebra
      per Section 9, version negotiation with downgrade protection.  The
      .well-known/atn HTTP-Resource Binding and SCITT submission are
      stubbed in -01 pending implementation alignment.

   *  Verified behavior: end-to-end handshake between two synthetic
      agents with mismatched capability constraints produces a signed
      Session Receipt with negotiated scope exactly matching the
      algorithm in Section 9.  Numeric constraints take minimums, list
      constraints take intersections, refusals override.

   *  Repository: https://github.com/e-somoza/atn-prototype







Somoza                  Expires 30 November 2026               [Page 52]

Internet-Draft           agent-trust-negotiation                May 2026


Appendix D.  Acknowledgments

   This document builds on the substantial work of the agent discovery
   community, including the authors of AID, DNS-AID, ANS v2, and related
   proposals.  ATN is intended to complement, not compete with, that
   work.

   The author thanks Ben Schwartz and Aijun Wang for substantial review
   feedback on draft-somoza-atn-agent-trust-negotiation-00 that shaped
   the architectural changes in this revision: the move to a discovery-
   agnostic core, the introduction of the HTTP-Resource Binding as the
   default, the reduction of the DNS Binding to a thin TXT record, and
   the withdrawal of the proposed SVCB ParamKey registrations.

Author's Address

   Enrique Somoza
   Independent
   United States of America
   Email: enrique@somoza.co































Somoza                  Expires 30 November 2026               [Page 53]
