



Internet Engineering Task Force (IETF)                           S. Card
Request for Comments: 9434                               A. Wiethuechter
Category: Informational                                    AX Enterprize
ISSN: 2070-1721                                             R. Moskowitz
                                                          HTT Consulting
                                                            S. Zhao, Ed.
                                                                   Intel
                                                               A. Gurtov
                                                    Linköping University
                                                               July 2023


        Drone Remote Identification Protocol (DRIP) Architecture

Abstract

   This document describes an architecture for protocols and services to
   support Unmanned Aircraft System Remote Identification and tracking
   (UAS RID), plus UAS-RID-related communications.  This architecture
   adheres to the requirements listed in the Drone Remote Identification
   Protocol (DRIP) Requirements document (RFC 9153).

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for informational purposes.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Not all documents
   approved by the IESG are candidates for any level of Internet
   Standard; see Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc9434.

Copyright Notice

   Copyright (c) 2023 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   Trust Legal Provisions and are provided without warranty as described
   in the Revised BSD License.

Table of Contents

   1.  Introduction
     1.1.  Overview of UAS RID and Its Standardization
     1.2.  Overview of Types of UAS Remote ID
       1.2.1.  Broadcast RID
       1.2.2.  Network RID
     1.3.  Overview of USS Interoperability
     1.4.  Overview of DRIP Architecture
   2.  Terms and Definitions
     2.1.  Additional Abbreviations
     2.2.  Additional Definitions
   3.  HHIT as the DRIP Entity Identifier
     3.1.  UAS Remote Identifiers Problem Space
     3.2.  HHIT as a Cryptographic Identifier
     3.3.  HHIT as a Trustworthy DRIP Entity Identifier
     3.4.  HHIT for DRIP Identifier Registration and Lookup
   4.  DRIP Identifier Registration and Registries
     4.1.  Public Information Registry
       4.1.1.  Background
       4.1.2.  Public DRIP Identifier Registry
     4.2.  Private Information Registry
       4.2.1.  Background
       4.2.2.  Information Elements
       4.2.3.  Private DRIP Identifier Registry Methods
       4.2.4.  Alternative Private DRIP Registry Methods
   5.  DRIP Identifier Trust
   6.  Harvesting Broadcast Remote ID Messages for UTM Inclusion
     6.1.  The CS-RID Finder
     6.2.  The CS-RID SDSP
   7.  DRIP Contact
   8.  IANA Considerations
   9.  Security Considerations
     9.1.  Private Key Physical Security
     9.2.  Quantum Resistant Cryptography
     9.3.  Denial-of-Service (DoS) Protection
     9.4.  Spoofing and Replay Protection
     9.5.  Timestamps and Time Sources
   10. Privacy and Transparency Considerations
   11. References
     11.1.  Normative References
     11.2.  Informative References
   Appendix A.  Overview of UAS Traffic Management (UTM)
     A.1.  Operation Concept
     A.2.  UAS Service Supplier (USS)
     A.3.  UTM Use Cases for UAS Operations
   Appendix B.  Automatic Dependent Surveillance Broadcast (ADS-B)
   Acknowledgments
   Authors' Addresses

1.  Introduction

   This document describes an architecture for protocols and services to
   support Unmanned Aircraft System Remote Identification and tracking
   (UAS RID), plus UAS-RID-related communications.  The architecture
   takes into account both current (including proposed) regulations and
   non-IETF technical standards.

   The architecture adheres to the requirements listed in the DRIP
   Requirements document [RFC9153] and illustrates how all of them can
   be met, except for GEN-7 QoS, which is left for future work.  The
   requirements document provides an extended introduction to the
   problem space and use cases.  Further, this architecture document
   frames the DRIP Entity Tag (DET) [RFC9374] within the architecture.

1.1.  Overview of UAS RID and Its Standardization

   UAS RID is an application that enables UAS to be identified by UAS
   Traffic Management (UTM), UAS Service Suppliers (USS) (Appendix A),
   and third-party entities, such as law enforcement.  Many
   considerations (e.g., safety and security) dictate that UAS be
   remotely identifiable.

   Civil Aviation Authorities (CAAs) worldwide are mandating UAS RID.
   CAAs currently promulgate performance-based regulations that do not
   specify techniques but rather cite industry consensus technical
   standards as acceptable means of compliance.

   USA Federal Aviation Administration (FAA)
      The FAA published a Notice of Proposed Rule Making [NPRM] in 2019
      and thereafter published a "Final Rule" in 2021 [FAA_RID],
      imposing requirements on UAS manufacturers and operators, both
      commercial and recreational.  The rule states that Automatic
      Dependent Surveillance Broadcast (ADS-B) Out and transponders
      cannot be used to satisfy the UAS RID requirements on UAS to which
      the rule applies (see Appendix B).

   European Union Aviation Safety Agency (EASA)
      In pursuit of the "U-space" concept of a single European airspace
      safely shared by manned and unmanned aircraft, the EASA published
      a [Delegated] regulation in 2019, imposing requirements on UAS
      manufacturers and third-country operators, including but not
      limited to UAS RID requirements.  The same year, the EASA also
      published a regulation [Implementing], laying down detailed rules
      and procedures for UAS operations and operating personnel, which
      then was updated in 2021 [Implementing_update].  A Notice of
      Proposed Amendment [NPA] was published in 2021 to provide more
      information about the development of acceptable means of
      compliance and guidance material to support U-space regulations.

   American Society for Testing and Materials (ASTM)
      ASTM International, Technical Committee F38 (UAS), Subcommittee
      F38.02 (Aircraft Operations), Work Item WK65041 developed an ASTM
      standard [F3411-22a], titled "Standard Specification for Remote ID
      and Tracking".

      ASTM defines one set of UAS RID information and two means, Media
      Access Control (MAC) layer broadcast and IP layer network, of
      communicating it.  If a UAS uses both communication methods, the
      same information must be provided via both means.  [F3411-22a] is
      the technical standard basis of the Means Of Compliance (MOC)
      specified in [F3586-22].  The FAA has accepted [F3586-22] as a MOC
      to the FAA's UAS RID Final Rule [FAA_RID], with some caveats, as
      per [MOC-NOA].  Other CAAs are expected to accept the same or
      other MOCs likewise based on [F3411-22a].

   3rd Generation Partnership Project (3GPP)
      With Release 16, the 3GPP completed the UAS RID requirement study
      [TR-22.825] and proposed a set of use cases in the mobile network
      and services that can be offered based on UAS RID.  The Release 17
      study [TR-23.755] and specification [TS-23.255] focus on enhanced
      UAS service requirements and provide the protocol and application
      architecture support that will be applicable for both 4G and 5G
      networks.  The study of Further Architecture Enhancement for
      Uncrewed Aerial Vehicles (UAV) and Urban Air Mobility (UAM) in
      Release 18 [FS_AEUA] further enhances the communication mechanism
      between UAS and USS/UTM.  The DET in Section 3 may be used as the
      3GPP CAA-level UAS ID for RID purposes.

1.2.  Overview of Types of UAS Remote ID

   This specification introduces two types of UAS Remote IDs as defined
   in ASTM [F3411-22a].

1.2.1.  Broadcast RID

   [F3411-22a] defines a set of UAS RID messages for direct, one-way
   broadcast transmissions from the Unmanned Aircraft (UA) over
   Bluetooth or Wi-Fi.  These are currently defined as MAC layer
   messages.  Internet (or other Wide Area Network) connectivity is only
   needed for UAS registry information lookup by Observers using the
   directly received UAS ID.  Broadcast RID should be functionally
   usable in situations with no Internet connectivity.

   The minimum Broadcast RID data flow is illustrated in Figure 1.

                   +------------------------+
                   | Unmanned Aircraft (UA) |
                   +-----------o------------+
                               |
                               | app messages directly over
                               | one-way RF data link (no IP)
                               |
                               v
             +------------------o-------------------+
             | Observer's device (e.g., smartphone) |
             +--------------------------------------+

                 Figure 1: Minimum Broadcast RID Data Flow

   Broadcast RID provides information only about UA within direct Radio
   Frequency (RF) Line Of Sight (LOS), typically similar to Visual LOS
   (VLOS), with a range up to approximately 1 km.  This information may
   be 'harvested' from received broadcasts and made available via the
   Internet, enabling surveillance of areas too large for local direct
   visual observation or direct RF link-based identification (see
   Section 6).

1.2.2.  Network RID

   [F3411-22a], using the same data dictionary that is the basis of
   Broadcast RID messages, defines a Network Remote Identification (Net-
   RID) data flow as follows.

   *  The information to be reported via UAS RID is generated by the
      UAS.  Typically, some of this data is generated by the UA and some
      by the Ground Control Station (GCS), e.g., their respective
      locations derived from the Global Navigation Satellite System
      (GNSS).

   *  The information is sent by the UAS (UA or GCS) via unspecified
      means to the cognizant Network Remote Identification Service
      Provider (Net-RID SP), typically the USS under which the UAS is
      operating if it is participating in UTM.

   *  The Net-RID SP publishes, via the Discovery and Synchronization
      Service (DSS) over the Internet, that it has operations in various
      4-D airspace volumes (Section 2.2 of [RFC9153]), describing the
      volumes but not the operations.

   *  An Observer's device, which is expected but not specified to be
      based on the Web, queries a Network Remote Identification Display
      Provider (Net-RID DP), typically also a USS, about any operations
      in a specific 4-D airspace volume.

   *  Using fully specified Web-based methods over the Internet, the
      Net-RID DP queries all Net-RID SPs that have operations in volumes
      intersecting that of the Observer's query for details on all such
      operations.

   *  The Net-RID DP aggregates information received from all such Net-
      RID SPs and responds to the Observer's query.

   The minimum Net-RID data flow is illustrated in Figure 2:

    +-------------+     ******************
    |     UA      |     *    Internet    *
    +--o-------o--+     *                *
       |       |        *                *     +------------+
       |       '--------*--(+)-----------*-----o            |
       |                *   |            *     |            |
       |       .--------*--(+)-----------*-----o Net-RID SP |
       |       |        *                *     |            |
       |       |        *         .------*-----o            |
       |       |        *         |      *     +------------+
       |       |        *         |      *
       |       |        *         |      *     +------------+
       |       |        *         '------*-----o            |
       |       |        *                *     | Net-RID DP |
       |       |        *         .------*-----o            |
       |       |        *         |      *     +------------+
       |       |        *         |      *
       |       |        *         |      *     +------------+
    +--o-------o--+     *         '------*-----o Observer's |
    |     GCS     |     *                *     | Device     |
    +-------------+     ******************     +------------+

                    Figure 2: Minimum Net-RID Data Flow

   Command and Control (C2) must flow from the GCS to the UA via some
   path.  Currently (in the year 2023), this is typically a direct RF
   link; however, with increasing Beyond Visual Line Of Sight (BVLOS)
   operations, it is expected to often be a wireless link at either end
   with the Internet between.

   Telemetry (at least the UA's position and heading) flows from the UA
   to the GCS via some path, typically the reverse of the C2 path.
   Thus, UAS RID information pertaining to both the GCS and the UA can
   be sent by whichever has Internet connectivity to the Net-RID SP,
   typically the USS managing the UAS operation.

   The Net-RID SP forwards UAS RID information via the Internet to
   subscribed Net-RID DPs, typically the USS.  Subscribed Net-RID DPs
   then forward RID information via the Internet to subscribed Observer
   devices.  Regulations require and [F3411-22a] describes UAS RID data
   elements that must be transported end to end from the UAS to the
   subscribed Observer devices.

   [F3411-22a] prescribes the protocols between the Net-RID SP, Net-RID
   DP, and DSS.  It also prescribes data elements (in JSON) between the
   Observer and the Net-RID DP.  DRIP could address standardization of
   secure protocols between the UA and the GCS (over direct wireless and
   Internet connection), between the UAS and the Net-RID SP, and/or
   between the Net-RID DP and Observer devices.

   _Neither link-layer protocols nor the use of links (e.g., the link
   often existing between the GCS and the UA) for any purpose other than
   carriage of UAS RID information are in the scope of Network RID
   [F3411-22a]._

1.3.  Overview of USS Interoperability

   With Net-RID, there is direct communication between each UAS and its
   USS.  Multiple USS exchange information with the assistance of a DSS
   so all USS collectively have knowledge about all activities in a 4-D
   airspace.  The interactions among an Observer, multiple UAS, and
   their USS are shown in Figure 3.

                   +------+    +----------+    +------+
                   | UAS1 |    | Observer |    | UAS2 |
                   +---o--+    +-----o----+    +--o---+
                       |             |            |
                 ******|*************|************|******
                 *     |             |            |     *
                 *     |         +---o--+         |     *
                 *     |  .------o USS3 o------.  |     *
                 *     |  |      +--o---+      |  |     *
                 *     |  |         |          |  |     *
                 *   +-o--o-+    +--o--+     +-o--o-+   *
                 *   |      o----o DSS o-----o      |   *
                 *   | USS1 |    +-----+     | USS2 |   *
                 *   |      o----------------o      |   *
                 *   +------+                +------+   *
                 *                                      *
                 *               Internet               *
                 ****************************************

           Figure 3: Interactions Between Observers, UAS, and USS

1.4.  Overview of DRIP Architecture

   Figure 4 illustrates a global UAS RID usage scenario.  Broadcast RID
   links are not shown, as they reach from any UA to any listening
   receiver in range and thus would obscure the intent of the figure.
   Figure 4 shows, as context, some entities and interfaces beyond the
   scope of DRIP (as currently (2023) chartered).  Multiple UAS are
   shown, each with its own UA controlled by its own GCS, potentially
   using the same or different USS, with the UA potentially
   communicating directly with each other (V2V), especially for low-
   latency, safety-related purposes (DAA).

  ***************                                        ***************
  *    UAS1     *                                        *     UAS2    *
  *             *                                        *             *
  * +--------+  *                 DAA/V2V                *  +--------+ *
  * |   UA   o--*----------------------------------------*--o   UA   | *
  * +--o--o--+  *                                        *  +--o--o--+ *
  *    |  |     *   +------+      Lookups     +------+   *     |  |    *
  *    |  |     *   | GPOD o------.    .------o PSOD |   *     |  |    *
  *    |  |     *   +------+      |    |      +------+   *     |  |    *
  *    |  |     *                 |    |                 *     |  |    *
  * C2 |  |     *     V2I      ************     V2I      *     |  | C2 *
  *    |  '-----*--------------*          *--------------*-----'  |    *
  *    |        *              *          *              *        |    *
  *    |        o====Net-RID===*          *====Net-RID===o        |    *
  * +--o--+     *              * Internet *              *     +--o--+ *
  * | GCS o-----*--------------*          *--------------*-----o GCS | *
  * +-----+     * Registration *          * Registration *     +-----+ *
  *             * (and UTM)    *          * (and UTM)    *             *
  ***************              ************              ***************
                                 |  |  |
                  +----------+   |  |  |   +----------+
                  | Public   o---'  |  '---o Private  |
                  | Registry |      |      | Registry |
                  +----------+      |      +----------+
                                 +--o--+
                                 | DNS |
                                 +-----+

  DAA:  Detect And Avoid
  GPOD: General Public Observer Device
  PSOD: Public Safety Observer Device
  V2I:  Vehicle-to-Infrastructure
  V2V:  Vehicle-to-Vehicle

                 Figure 4: Global UAS RID Usage Scenario

      |  Informative note: See [RFC9153] for detailed definitions.

   DRIP is meant to leverage existing Internet resources (standard
   protocols, services, infrastructures, and business models) to meet
   UAS RID and closely related needs.  DRIP will specify how to apply
   IETF standards, complementing [F3411-22a] and other external
   standards, to satisfy UAS RID requirements.

   This document outlines the DRIP architecture in the context of the
   UAS RID architecture.  This includes closing the gaps between the
   CAAs' concepts of operations and [F3411-22a] as it relates to the use
   of Internet technologies and UA-direct RF communications.  Issues
   include, but are not limited to:

   *  the design of trustworthy remote identifiers required by GEN-1
      (Section 3), especially but not exclusively for use as single-use
      session IDs,

   *  mechanisms to leverage the Domain Name System (DNS) [RFC1034] for
      registering and publishing public and private information (see
      Sections 4.1 and 4.2), as required by REG-1 and REG-2,

   *  specific authentication methods and message payload formats to
      enable verification that Broadcast RID messages were sent by the
      claimed sender (Section 5) and that the sender is in the claimed
      DRIP Identity Management Entity (DIME) (see Sections 4 and 5), as
      required by GEN-2,

   *  harvesting Broadcast RID messages for UTM inclusion, with the
      optional DRIP extension of Crowdsourced Remote ID (CS-RID)
      (Section 6), using the DRIP support for gateways required by GEN-5
      [RFC9153],

   *  methods for instantly establishing secure communications between
      an Observer and the pilot of an observed UAS (Section 7), using
      the DRIP support for dynamic contact required by GEN-4 [RFC9153],
      and

   *  privacy in UAS RID messages (personal data protection)
      (Section 10).

   This document should serve as a main point of entry into the set of
   IETF documents addressing the basic DRIP requirements.

2.  Terms 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.

   To encourage comprehension necessary for adoption of DRIP by the
   intended user community, the UAS community's norms are respected
   herein.

   This document uses terms defined in [RFC9153].

   Some of the acronyms have plural forms that remain the same as their
   singular forms, e.g., "UAS" can expand to "Unmanned Aircraft System"
   (singular) or "Unmanned Aircraft Systems" (plural), as appropriate
   for the context.  This usage is consistent with Section 2.2 of
   [RFC9153].

2.1.  Additional Abbreviations

   DET:      DRIP Entity Tag

   EdDSA:    Edwards-curve Digital Signature Algorithm

   HHIT:     Hierarchical HIT

   HI:       Host Identity

   HIP:      Host Identity Protocol

   HIT:      Host Identity Tag

2.2.  Additional Definitions

   This section introduces the terms "Claim", "Evidence", "Endorsement",
   and "Certificate", as used in DRIP.  A DRIP certificate has a
   different context compared with security certificates and Public Key
   Infrastructure used in X.509.

   Claim:
      A claim shares the same definition as a claim in Remote
      ATtestation procedureS (RATS) [RFC9334]; it is a piece of asserted
      information, sometimes in the form of a name/value pair.  It could
      also been seen as a predicate (e.g., "X is Y", "X has property Y",
      and most importantly "X owns Y" or "X is owned by Y").

   Evidence:
      Evidence in DRIP borrows the same definition as in RATS [RFC9334],
      that is, a set of claims.

   Endorsement:
      An Endorsement is inspired from RATS [RFC9334]; it is a secure
      (i.e., signed) statement vouching the integrity and veracity of
      evidence.

   Certificate:
      A certificate in DRIP is an endorsement, strictly over identity
      information, signed by a third party.  This third party should be
      one with no stake in the endorsement over which it is signing.

   DRIP Identity Management Entity (DIME):
      A DIME is an entity that performs functions similar to a domain
      registrar/registry.  A DIME vets Claims and/or Evidence from a
      registrant and delivers back Endorsements and/or Certificates in
      response.  It is a high-level entity in the DRIP registration/
      provisioning process that can hold the role of HHIT Domain
      Authority (HDA), Registered Assigning Authority (RAA), or root of
      trust (typically the HHIT prefix owner or DNS apex owner) for
      DETs.

3.  HHIT as the DRIP Entity Identifier

   This section describes the DRIP architectural approach to meeting the
   basic requirements of a DRIP entity identifier within the external
   technical standard ASTM [F3411-22a] and regulatory constraints.  It
   justifies and explains the use of Hierarchical Host Identity Tags
   (HHITs) [RFC9374] as self-asserting IPv6 addresses suitable as a UAS
   ID type and, more generally, as trustworthy multipurpose remote
   identifiers.

   Self-asserting in this usage means that, given the Host Identity
   (HI), the HHIT Overlay Routable Cryptographic Hash IDentifier
   (ORCHID) construction (see Section 3.5 of [RFC9374]), and a signature
   of the DIME on the HHIT and HI, the HHIT can be verified by the
   receiver as a trusted UAS ID.  The explicit registration hierarchy
   within the HHIT provides registration discovery (managed by a DIME)
   to either yield the HI for third-party (seeking UAS ID endorsement)
   validation or prove that the HHIT and HI have been registered
   uniquely.

3.1.  UAS Remote Identifiers Problem Space

   A DRIP entity identifier needs to be "Trustworthy" (see DRIP
   requirements GEN-1, ID-4, and ID-5 in [RFC9153]).  This means that
   given a sufficient collection of UAS RID messages, an Observer can
   establish that the identifier claimed therein uniquely belongs to the
   claimant.  To satisfy DRIP requirements and maintain important
   security properties, the DRIP identifier should be self-generated by
   the entity it names (e.g., a UAS) and registered (e.g., with a USS;
   see DRIP requirements GEN-3 and ID-2).

   However, Broadcast RID, especially its support for Bluetooth 4,
   imposes severe constraints.  A previous revision of the ASTM UAS RID,
   [F3411-19], allowed a UAS ID of types (1, 2, and 3), each of 20
   bytes.  [F3411-22a] adds type 4, Specific Session ID, for other
   Standards Development Organizations (SDOs) to extend ASTM UAS RID.
   Type 4 uses one byte to index the Specific Session ID subtype,
   leaving 19 bytes (see ID-1 of DRIP Requirements [RFC9153]).  As
   described in Section 3 of [RFC9153], ASTM has allocated Specific
   Session ID subtype 1 to IETF DRIP.

   The maximum ASTM UAS RID Authentication Message payload is 201 bytes
   each for Authentication Types 1, 2, 3, and 4.  [F3411-22a] adds
   Authentication Type 5 for other SDOs (including the IETF) to extend
   ASTM UAS RID with Specific Authentication Methods (SAMs).  With Type
   5, one of the 201 bytes is consumed to index the SAM Type, leaving
   only 200 bytes for DRIP authentication payloads, including one or
   more DRIP entity identifiers and associated authentication data.

3.2.  HHIT as a Cryptographic Identifier

   The only (known to the authors at the time of writing) existing types
   of IP-address-compatible identifiers cryptographically derived from
   the public keys of the identified entities are Cryptographically
   Generated Addresses (CGAs) [RFC3972] and Host Identity Tags (HITs)
   [RFC7401].  CGAs and HITs lack registration/retrieval capability.  To
   provide this, each HHIT embeds plaintext information designating the
   hierarchy within which it is registered, a cryptographic hash of that
   information concatenated with the entity's public key, etc.  Although
   hash collisions may occur, the DIME can detect them and reject
   registration requests rather than issue credentials, e.g., by
   enforcing a First Come First Served policy [RFC8126].  Preimage hash
   attacks are also mitigated through this registration process, locking
   the HHIT to a specific HI.

3.3.  HHIT as a Trustworthy DRIP Entity Identifier

   A Remote UAS ID that can be trustworthy for use in Broadcast RID can
   be built from an asymmetric key pair.  In this method, the UAS ID is
   cryptographically derived directly from the public key.  The proof of
   UAS ID ownership (verifiable endorsement versus mere claim) is
   guaranteed by signing this cryptographic UAS ID with the associated
   private key.  The association between the UAS ID and the private key
   is ensured by cryptographically binding the public key with the UAS
   ID; more specifically, the UAS ID results from the hash of the public
   key.  The public key is designated as the HI, while the UAS ID is
   designated as the HIT.

   By construction, the HIT is statistically unique through the
   mandatory use of cryptographic hash functions with second-preimage
   resistance.  The cryptographically bound addition of the hierarchy
   and a HHIT registration process provide complete, global HHIT
   uniqueness.  This registration forces the attacker to generate the
   same public key rather than a public key that generates the same
   HHIT.  This is in contrast to general IDs (e.g., a Universally Unique
   Identifier (UUID) or device serial number) as the subject in an X.509
   certificate.

   A UA equipped for Broadcast RID MUST be provisioned not only with its
   HHIT but also with the HI public key from which the HHIT was derived
   and the corresponding private key to enable message signature.

   A UAS equipped for DRIP-enhanced Network RID MUST be provisioned
   likewise; the private key resides only in the ultimate source of
   Network RID messages.  If the GCS is the source of the Network RID
   messages, the GCS MUST hold the private key.  If the UA is the source
   of the Network RID messages and they are being relayed by the GCS,
   the UA MUST hold the private key, just as a UA that directly connects
   to the network rather than through its GCS.

   Each Observer device functioning with Internet connectivity MAY be
   provisioned either with public keys of the DRIP identifier root
   registries or certificates for subordinate registries; each Observer
   device that needs to operate without Internet connectivity at any
   time MUST be so provisioned.

   HHITs can also be used throughout the USS/UTM system.  Operators and
   Private Information Registries, as well as other UTM entities, can
   use HHITs for their IDs.  Such HHITs can facilitate DRIP security
   functions, such as those used with HIP, to strongly mutually
   authenticate and encrypt communications.

   A self-endorsement of a HHIT used as a UAS ID can be done in as
   little as 88 bytes when Ed25519 [RFC8032] is used by only including
   the 16-byte HHIT, two 4-byte timestamps, and the 64-byte Ed25519
   signature.

   Ed25519 [RFC8032] is used as the HHIT mandatory-to-implement signing
   algorithm, as GEN-1 and ID-5 [RFC9153] can best be met by restricting
   the HI to 32 bytes.  A larger public key would rule out the offline
   endorsement feature that fits within the 200-byte Authentication
   Message maximum length.  Other algorithms that meet this 32-byte
   constraint can be added as deemed needed.

   A DRIP identifier can be assigned to a UAS as a static HHIT by its
   manufacturer, such as a single HI and derived HHIT encoded as a
   hardware serial number, per [CTA2063A].  Such a static HHIT SHOULD
   only be used to bind one-time-use DRIP identifiers to the unique UA.
   Depending upon implementation, this may leave a HI private key in the
   possession of the manufacturer (see also Section 9).

   In general, Internet access may be needed to validate Endorsements or
   Certificates.  This may be obviated in the most common cases (e.g.,
   endorsement of the UAS ID), even in disconnected environments, by
   prepopulating small caches on Observer devices with DIME public keys
   and a chain of Endorsements or Certificates (tracing a path through
   the DIME tree).  This is assuming all parties on the trust path also
   use HHITs for their identities.

3.4.  HHIT for DRIP Identifier Registration and Lookup

   UAS RID needs a deterministic lookup mechanism that rapidly provides
   actionable information about the identified UA.  Given the size
   constraints imposed by the Bluetooth 4 broadcast media, the UAS ID
   itself needs to be a non-spoofable inquiry input into the lookup.

   A DRIP registration process based on the explicit hierarchy within a
   HHIT provides manageable uniqueness of the HI for the HHIT.  The
   hierarchy is defined in [RFC9374] and consists of 2 levels: an RAA
   and then an HDA.  The registration within this hierarchy is the
   defense against a cryptographic hash second-preimage attack on the
   HHIT (e.g., multiple HIs yielding the same HHIT; see Requirement ID-3
   in [RFC9153]).  The First Come First Served registration policy is
   adequate.

   A lookup of the HHIT into the DIME provides the registered HI for
   HHIT proof of ownership and deterministic access to any other needed
   actionable information based on inquiry access authority (more
   details in Section 4.2).

4.  DRIP Identifier Registration and Registries

   DRIP registries hold both public and private UAS information (see
   PRIV-1 in [RFC9153]) resulting from the DRIP identifier registration
   process.  Given these different uses, and to improve scalability,
   security, and simplicity of administration, the public and private
   information can be stored in different registries.  This section
   introduces the public and private information registries for DRIP
   identifiers.  In this section, for ease of comprehension, the
   registry functions are described (using familiar terminology) without
   detailing their assignment to specific implementing entities (or
   using unfamiliar jargon).  Elsewhere in this document, and in
   forthcoming documents detailing the DRIP registration processes and
   entities, the more specific term "DRIP Identity Management Entity"
   (DIME) will be used.  This DRIP identifier registration process
   satisfies the following DRIP requirements defined in [RFC9153]: GEN-
   3, GEN-4, ID-2, ID-4, ID-6, PRIV-3, PRIV-4, REG-1, REG-2, REG-3, and
   REG-4.

4.1.  Public Information Registry

4.1.1.  Background

   The public information registry provides trustable information, such
   as endorsements of UAS RID ownership and registration with the HDA.
   Optionally, pointers to the registries for the HDA and RAA implicit
   in the UAS RID can be included (e.g., for HDA and RAA HHIT|HI used in
   endorsement signing operations).  This public information will be
   principally used by Observers of Broadcast RID messages.  Data on UAS
   that only use Network RID is available via an Observer's Net-RID DP
   that would directly provide all public registry information.  The
   Net-RID DP is the only source of information for a query on an
   airspace volume.

      |  Note: In the above paragraph, | signifies concatenation of
      |  information, e.g., X | Y is the concatenation of X and Y.

4.1.2.  Public DRIP Identifier Registry

   A DRIP identifier MUST be registered as an Internet domain name (at
   an arbitrary level in the hierarchy, e.g., in .ip6.arpa).  Thus, the
   DNS can provide all the needed public DRIP information.  A
   standardized HHIT Fully Qualified Domain Name (FQDN) can deliver the
   HI via a HIP Resource Record (RR) [RFC8005] and other public
   information (e.g., RAA and HDA PTRs and HIP Rendezvous Servers (RVSs)
   [RFC8004]).  These public information registries can use DNSSEC to
   deliver public information that is not inherently trustable (e.g.,
   everything other than endorsements).

   This DNS entry for the HHIT can also provide a revocation service.
   For example, instead of returning the HI RR, it may return some
   record showing that the HI (and thus HHIT) has been revoked.

4.2.  Private Information Registry

4.2.1.  Background

   The private information required for DRIP identifiers is similar to
   that required for Internet domain name registration.  A DRIP
   identifier solution can leverage existing Internet resources, i.e.,
   registration protocols, infrastructure, and business models, by
   fitting into a UAS ID structure compatible with DNS names.  The HHIT
   hierarchy can provide the needed scalability and management
   structure.  It is expected that the private information registry
   function will be provided by the same organizations that run a USS
   and likely integrated with a USS.  The lookup function may be
   implemented by the Net-RID DPs.

4.2.2.  Information Elements

   When a DET is used as a UA's Session ID, the corresponding
   manufacturer-assigned serial number MUST be stored in a private
   information registry that can be identified uniquely from the DET.
   When a DET is used as either a UA's Session ID or a UA's
   manufacturer-assigned serial number, and the operation is being flown
   under UTM, the corresponding UTM-system-assigned Operational Intent
   Identifier SHOULD be so stored.  Other information MAY be stored as
   such, and often must, to satisfy CAA regulations or USS operator
   policies.

4.2.3.  Private DRIP Identifier Registry Methods

   A DRIP private information registry supports essential registry
   operations (e.g., add, delete, update, and query) using interoperable
   open standard protocols.  It can accomplish this by leveraging
   aspects of the Extensible Provisioning Protocol (EPP) [RFC5730] and
   the Registry Data Access Protocol (RDAP) [RFC7480] [RFC9082]
   [RFC9083].  The DRIP private information registry in which a given
   UAS is registered needs to be findable, starting from the UAS ID,
   using the methods specified in [RFC9224].

4.2.4.  Alternative Private DRIP Registry Methods

   A DRIP private information registry might be an access-controlled DNS
   (e.g., via DNS over TLS).  Additionally, WebFinger [RFC7033] can be
   supported.  These alternative methods may be used by a Net-RID DP
   with specific customers.

5.  DRIP Identifier Trust

   While the DRIP entity identifier is self-asserting, it alone does not
   provide the trustworthiness (i.e., non-repudiation, protection vs.
   spoofing, message integrity protection, scalability, etc.) essential
   to UAS RID, as justified in [RFC9153].  For that, it MUST be
   registered (under DRIP registries) and actively used by the party (in
   most cases the UA).  A sender's identity cannot be proved merely by
   its possessing of a DRIP Entity Tag (DET) and broadcasting it as a
   claim that it belongs to that sender.  Sending data signed using that
   HI's private key proves little, as it is subject to trivial replay
   attacks using previously broadcast messages.  Only sending the DET
   and a signature on novel (i.e., frequently changing and
   unpredictable) data that can be externally validated by the Observer
   (such as a signed Location/Vector message that matches actually
   seeing the UA at the location and time reported in the signed
   message) proves that the observed UA possesses the private key and
   thus the claimed UAS ID.

   The severe constraints of Broadcast RID make it challenging to
   satisfy UAS RID requirements.  From received Broadcast RID messages
   and information that can be looked up using the received UAS ID in
   online registries or local caches, it is possible to establish levels
   of trust in the asserted information and the operator.

   A combination of different DRIP Authentication Messages enables an
   Observer, without Internet connection (offline) or with (online), to
   validate a UAS DRIP ID in real time.  Some messages must contain the
   relevant registration of the UA's DRIP ID in the claimed DIME.  Some
   messages must contain sender signatures over both static (e.g.,
   registration) and dynamically changing (e.g., current UA location)
   data.  Combining these two sets of information, an Observer can piece
   together a chain of trust, including real-time evidence to make a
   determination on the UA's claims.

   This process (combining the DRIP entity identifier, registries, and
   authentication formats for Broadcast RID) can satisfy the following
   DRIP requirements defined in [RFC9153]: GEN-1, GEN-2, GEN-3, ID-2,
   ID-3, ID-4, and ID-5.

6.  Harvesting Broadcast Remote ID Messages for UTM Inclusion

   ASTM anticipated that regulators would require both Broadcast RID and
   Network RID for large UAS but allow UAS RID requirements for small
   UAS to be satisfied with the operator's choice of either Broadcast
   RID or Network RID.  The EASA initially specified Broadcast RID for
   essentially all UAS and is now also considering Network RID.  The FAA
   UAS RID Final Rules [FAA_RID] permit only Broadcast RID for rule
   compliance but still encourage Network RID for complementary
   functionality, especially in support of UTM.

   One opportunity is to enhance the architecture with gateways from
   Broadcast RID to Network RID.  This provides the best of both and
   gives regulators and operators flexibility.  It offers advantages
   over either form of UAS RID alone, i.e., greater fidelity than
   Network RID reporting of [FAA_RID] planned area operations, together
   with surveillance of areas too large for local direct visual
   observation and direct Radio Frequency Line Of Sight (RF-LOS) link-
   based Broadcast RID (e.g., a city or a national forest).

   These gateways could be pre-positioned (e.g., around airports, public
   gatherings, and other sensitive areas) and/or crowdsourced (as
   nothing more than a smartphone with a suitable app is needed).
   Crowdsourcing can be encouraged by quid pro quo, providing CS-RID
   Surveillance Supplemental Data Service Provider (SDSP) outputs only
   to CS-RID Finders.  As Broadcast RID media have a limited range,
   messages claiming sender (typically UA) locations far from a physical
   layer receiver thereof ("Finder" below, typically Observer device)
   should arouse suspicion of possible intent to deceive; a fast and
   computationally inexpensive consistency check can be performed (by
   the Finder or the Surveillance SDSP) on application layer data
   present in the gateway (claimed UA location vs physical receiver
   location), and authorities can be alerted to failed checks.  CS-RID
   SDSPs can use messages with precise date/time/position stamps from
   the gateways to multilaterate UA locations, independent of the
   locations claimed in the messages, which are entirely self-reported
   by the operator in UAS RID and UTM, and thus are subject not only to
   natural time lag and error but also operator misconfiguration or
   intentional deception.

   Multilateration technologies use physical layer information, such as
   precise Time Of Arrival (TOA) of transmissions from mobile
   transmitters at receivers with a priori precisely known locations, to
   estimate the locations of the mobile transmitters.

   Further, gateways with additional sensors (e.g., smartphones with
   cameras) can provide independent information on the UA type and size,
   confirming or refuting those claims made in the UAS RID messages.

   Sections 6.1 and 6.2 define two additional entities that are required
   to provide this Crowdsourced Remote ID (CS-RID).

   This approach satisfies the following DRIP requirements defined in
   [RFC9153]: GEN-5, GEN-11, and REG-1.  As Broadcast messages are
   inherently multicast, GEN-10 is met for local-link multicast to
   multiple Finders (this is how multilateration is possible).

6.1.  The CS-RID Finder

   A CS-RID Finder is the gateway for Broadcast Remote ID Messages into
   UTM.  It performs this gateway function via a CS-RID SDSP.  A CS-RID
   Finder could implement, integrate, or accept outputs from a Broadcast
   RID receiver.  However, it should not depend upon a direct interface
   with a GCS, Net-RID SP, Net-RID DP, or Net-RID client.  It would
   present a new interface to a CS-RID SDSP, similar to but readily
   distinguishable from that which a UAS (UA or GCS) presents to a Net-
   RID SP.

6.2.  The CS-RID SDSP

   A CS-RID SDSP aggregates and processes (e.g., estimates UA locations
   using multilateration when possible) information collected by CS-RID
   Finders.  A CS-RID SDSP should present the same interface to a Net-
   RID SP as it does to a Net-RID DP and to a Net-RID DP as it does to a
   Net-RID SP, but its data source must be readily distinguishable via
   Finders rather than direct from the UAS itself.

7.  DRIP Contact

   One of the ways in which DRIP can enhance [F3411-22a] with
   immediately actionable information is by enabling an Observer to
   instantly initiate secure communications with the UAS remote pilot,
   Pilot In Command, operator, USS under which the operation is being
   flown, or other entity potentially able to furnish further
   information regarding the operation and its intent and/or to
   immediately influence further conduct or termination of the operation
   (e.g., land or otherwise exit an airspace volume).  Such potentially
   distracting communications demand strong "AAA" (Authentication,
   Attestation, Authorization, Access Control, Accounting, Attribution,
   Audit), per applicable policies (e.g., of the cognizant CAA).

   A DRIP entity identifier based on a HHIT, as outlined in Section 3,
   embeds an identifier of the DIME in which it can be found (expected
   typically to be the USS under which the UAS is flying), and the
   procedures outlined in Section 5 enable Observer verification of that
   relationship.  A DRIP entity identifier with suitable records in
   public and private registries, as outlined in Section 5, can enable
   lookup not only of information regarding the UAS but also identities
   of and pointers to information regarding the various associated
   entities (e.g., the USS under which the UAS is flying an operation),
   including means of contacting those associated entities (i.e.,
   locators, typically IP addresses).

   A suitably equipped Observer could initiate a secure communication
   channel, using the DET HI, to a similarly equipped and identified
   entity, i.e., the UA itself, if operating autonomously; the GCS, if
   the UA is remotely piloted and the necessary records have been
   populated in the DNS; the USS; etc.  Assuming secure communication
   setup (e.g., via IPsec or HIP), arbitrary standard higher-layer
   protocols can then be used for Observer to Pilot (O2P) communications
   (e.g., SIP [RFC3261] et seq), Vehicle to Everything (V2X) (or more
   specifically Aircraft to Anything (A2X)) communications (e.g.,
   [MAVLink]), etc.  Certain preconditions are necessary: 1) each party
   needs a currently usable means (typically a DNS) of resolving the
   other party's DRIP entity identifier to a currently usable locator
   (IP address), and 2) there must be currently usable bidirectional IP
   connectivity (not necessarily via the Internet) between the parties.
   One method directly supported by the use of HHITs as DRIP entity
   identifiers is initiation of a HIP Base Exchange (BEX) and Bound End-
   to-End Tunnel (BEET).

   This approach satisfies DRIP requirement GEN-6 Contact, supports
   satisfaction of DRIP requirements GEN-8, GEN-9, PRIV-2, PRIV-5, and
   REG-3 [RFC9153], and is compatible with all other DRIP requirements.

8.  IANA Considerations

   This document has no IANA actions.

9.  Security Considerations

   The size of the public key hash in the HHIT is vulnerable to a
   second-preimage attack.  It is well within current server array
   technology to compute another key pair that hashes to the same HHIT
   (given the current ORCHID construction hash length to fit UAS RID and
   IPv6 address constraints).  Thus, if a receiver were to check HHIT/HI
   pair validity only by verifying that the received HI and associated
   information, when hashed in the ORCHID construction, reproduce the
   received HHIT, an adversary could impersonate a validly registered
   UA.  To defend against this, online receivers should verify the
   received HHIT and received HI with the HDA (typically USS) with which
   the HHIT/HI pair purports to be registered.  Online and offline
   receivers can use a chain of received DRIP link endorsements from a
   root of trust through the RAA and the HDA to the UA, e.g., as
   described in [DRIP-AUTH] and [DRIP-REGISTRIES].

   Compromise of a DIME private key could do widespread harm
   [DRIP-REGISTRIES].  In particular, it would allow bad actors to
   impersonate trusted members of said DIME.  These risks are in
   addition to those involving key management practices and will be
   addressed as part of the DIME process.  All DRIP public keys can be
   found in the DNS, thus they can be revoked in the DNS, and users
   SHOULD check the DNS when available.  Specific key revocation
   procedures are as yet to be determined.

9.1.  Private Key Physical Security

   The security provided by asymmetric cryptographic techniques depends
   upon protection of the private keys.  It may be necessary for the GCS
   to have the key pair to register the HHIT to the USS.  Thus, it may
   be the GCS that generates the key pair and delivers it to the UA,
   making the GCS a part of the key security boundary.  Leakage of the
   private key, from either the UA or the GCS, to the component
   manufacturer is a valid concern, and steps need to be in place to
   ensure safe keeping of the private key.  Since it is possible for the
   UAS RID sender of a small harmless UA (or the entire UA) to be
   carried by a larger dangerous UA as a "false flag", it is out of
   scope to deal with secure storage of the private key.

9.2.  Quantum Resistant Cryptography

   There has been no effort as of yet in DRIP to address post quantum
   computing cryptography.  Small UAS and Broadcast Remote ID
   communications are so constrained that current post quantum computing
   cryptography is not applicable.  Fortunately, since a UA may use a
   unique HHIT for each operation, the attack window can be limited to
   the duration of the operation.  One potential future DRIP use for
   post quantum cryptography is for key pairs that have long usage lives
   but that rarely, if ever, need to be transmitted over bandwidth
   constrained links, such as for serial numbers or operators.  As the
   HHIT contains the ID for the cryptographic suite used in its
   creation, a future post quantum computing safe algorithm that fits
   Remote ID constraints may be readily added.  This is left for future
   work.

9.3.  Denial-of-Service (DoS) Protection

   Remote ID services from the UA use a wireless link in a public space.
   As such, they are open to many forms of RF jamming.  It is trivial
   for an attacker to stop any UA messages from reaching a wireless
   receiver.  Thus, it is pointless to attempt to provide relief from
   DoS attacks, as there is always the ultimate RF jamming attack.
   Also, DoS may be attempted with spoofing/replay attacks; for which,
   see Section 9.4.

9.4.  Spoofing and Replay Protection

   As noted in Section 5, spoofing is combatted by the intrinsic self-
   attesting properties of HHITs, plus their registration.  Also, as
   noted in Section 5, to combat replay attacks, a receiver MUST NOT
   trust any claims nominally received from an observed UA (not even the
   Basic ID message purportedly identifying that UA) until the receiver
   verifies that the private key used to sign those claims is trusted,
   that the sender actually possesses that key, and that the sender
   appears indeed to be that observed UA.  This requires receiving a
   complete chain of endorsement links from a root of trust to the UA's
   leaf DET, plus a message containing suitable nonce-like data signed
   with the private key corresponding to that DET, and verifying all the
   foregoing.  The term "nonce-like" describes data that is readily
   available to the prover and the verifier, changes frequently, is not
   predictable by the prover, and can be checked quickly at low
   computational cost by the verifier; a Location/Vector message is an
   obvious choice.

9.5.  Timestamps and Time Sources

   Section 6 and, more fundamentally, Section 3.3 both require
   timestamps.  In Broadcast RID messages, [F3411-22a] specifies both
   32-bit Unix-style UTC timestamps (seconds since midnight going into
   the 1st day of 2019, rather than 1970) and 16-bit relative timestamps
   (tenths of seconds since the start of the most recent hour or other
   specified event).  [F3411-22a] requires that 16-bit timestamp
   accuracy, relative to the time of applicability of the data being
   timestamped, also be reported, with a worst allowable case of 1.5
   seconds.  [F3411-22a] does not specify the time source, but GNSS is
   generally assumed, as latitude, longitude, and geodetic altitude must
   be reported and most small UAS use GNSS for positioning and
   navigation.

      |  Informative note: For example, to satisfy [FAA_RID], [F3586-22]
      |  specifies tamper protection of the entire RID subsystem and use
      |  of the GPS operated by the US Government.  The GPS has sub-
      |  microsecond accuracy and 1.5-second precision.  In this
      |  example, UA-sourced messages can be assumed to have timestamp
      |  accuracy and precision of 1.5 seconds at worst.

   GCS often have access to cellular LTE or other time sources better
   than the foregoing, and such better time sources would be required to
   support multilateration in Section 6, but such better time sources
   cannot be assumed generally for purposes of security analysis.

10.  Privacy and Transparency Considerations

   Broadcast RID messages can contain personal data (Section 3.2 of
   [RFC6973]), such as the operator ID, and, in most jurisdictions, must
   contain the pilot/GCS location.  The DRIP architectural approach for
   personal data protection is symmetric encryption of the personal data
   using a session key known to the UAS and its USS, as follows.
   Authorized Observers obtain plaintext in either of two ways: 1) an
   Observer can send the UAS ID and the cyphertext to a server that
   offers decryption as a service, and 2) an Observer can send just the
   UAS ID to a server that returns the session key so that the Observer
   can directly, locally decrypt all cyphertext sent by that UA during
   that session (UAS operation).  In either case, the server can be a
   public safety USS, the Observer's own USS, or the UA's USS if the
   latter can be determined (which, under DRIP, can be from the UAS ID
   itself).  Personal data is protected unless the UAS is otherwise
   configured, i.e., as part of DRIP-enhanced RID subsystem
   provisioning, as part of UTM operation authorization, or via
   subsequent authenticated communications from a cognizant authority.
   Personal data protection MUST NOT be used if the UAS loses
   connectivity to its USS; if the UAS loses connectivity, Observers
   nearby likely also won't have connectivity enabling decryption of the
   personal data.  The UAS always has the option to abort the operation
   if personal data protection is disallowed, but if this occurs during
   flight, the UA then MUST broadcast the personal data without
   protection until it lands and is powered off.  Note that normative
   language was used only minimally in this section, as privacy
   protection requires refinement of the DRIP architecture and
   specification of interoperable protocol extensions, which are left
   for future DRIP documents.

11.  References

11.1.  Normative References

   [F3411-22a]
              ASTM International, "Standard Specification for Remote ID
              and Tracking", ASTM F3411-22A, DOI 10.1520/F3411-22A, July
              2022, <https://www.astm.org/f3411-22a.html>.

   [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/info/rfc2119>.

   [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/info/rfc8174>.

   [RFC9153]  Card, S., Ed., Wiethuechter, A., Moskowitz, R., and A.
              Gurtov, "Drone Remote Identification Protocol (DRIP)
              Requirements and Terminology", RFC 9153,
              DOI 10.17487/RFC9153, February 2022,
              <https://www.rfc-editor.org/info/rfc9153>.

   [RFC9374]  Moskowitz, R., Card, S., Wiethuechter, A., and A. Gurtov,
              "DRIP Entity Tag (DET) for Unmanned Aircraft System Remote
              ID (UAS RID)", RFC 9374, DOI 10.17487/RFC9374, March 2023,
              <https://www.rfc-editor.org/info/rfc9374>.

11.2.  Informative References

   [CTA2063A] ANSI, "Small Unmanned Aerial Systems Serial Numbers",
              ANSI/CTA 2063-A, September 2019.

   [Delegated]
              European Union Aviation Safety Agency (EASA), "Commission
              Delegated Regulation (EU) 2019/945 of 12 March 2019 on
              unmanned aircraft systems and on third-country operators
              of unmanned aircraft systems", March 2019,
              <https://eur-lex.europa.eu/eli/reg_del/2019/945/oj>.

   [DRIP-AUTH]
              Wiethuechter, A., Ed., Card, S., and R. Moskowitz, "DRIP
              Entity Tag Authentication Formats & Protocols for
              Broadcast Remote ID", Work in Progress, Internet-Draft,
              draft-ietf-drip-auth-30, 28 March 2023,
              <https://datatracker.ietf.org/doc/html/draft-ietf-drip-
              auth-30>.

   [DRIP-REGISTRIES]
              Wiethuechter, A. and J. Reid, "DRIP Entity Tag (DET)
              Identity Management Architecture", Work in Progress,
              Internet-Draft, draft-ietf-drip-registries-12, 10 July
              2023, <https://datatracker.ietf.org/doc/html/draft-ietf-
              drip-registries-12>.

   [F3411-19] ASTM International, "Standard Specification for Remote ID
              and Tracking", ASTM F3411-19, DOI 10.1520/F3411-19, May
              2022, <https://www.astm.org/f3411-19.html>.

   [F3586-22] ASTM International, "Standard Practice for Remote ID Means
              of Compliance to Federal Aviation Administration
              Regulation 14 CFR Part 89", ASTM F3586-22,
              DOI 10.1520/F3586-22, July 2022,
              <https://www.astm.org/f3586-22.html>.

   [FAA_RID]  United States Federal Aviation Administration (FAA),
              "Remote Identification of Unmanned Aircraft", Federal
              Register, Vol. 86, No. 10, January 2021,
              <https://www.govinfo.gov/content/pkg/FR-2021-01-15/
              pdf/2020-28948.pdf>.

   [FAA_UAS_Concept_Of_Ops]
              United States Federal Aviation Administration (FAA),
              "Unmanned Aircraft System (UAS) Traffic Management (UTM)
              Concept of Operations", v2.0, March 2020,
              <https://www.faa.gov/sites/faa.gov/files/2022-08/
              UTM_ConOps_v2.pdf>.

   [FS_AEUA]  "Study of Further Architecture Enhancement for UAV and
              UAM", S2-2107092, October 2021,
              <https://www.3gpp.org/ftp/tsg_sa/WG2_Arch/
              TSGS2_147E_Electronic_2021-10/Docs/S2-2107092.zip>.

   [Implementing]
              European Union Aviation Safety Agency (EASA), "Commission
              Implementing Regulation (EU) 2019/947 of 24 May 2019 on
              the rules and procedures for the operation of unmanned
              aircraft (Text with EEA relevance.)", May 2019,
              <https://eur-lex.europa.eu/legal-content/EN/
              TXT/?uri=CELEX%3A32019R0947>.

   [Implementing_update]
              European Union Aviation Safety Agency (EASA), "Commission
              Implementing Regulation (EU) 2021/664 of 22 April 2021 on
              a regulatory framework for the U-space (Text with EEA
              relevance)", April 2021, <https://eur-lex.europa.eu/legal-
              content/EN/TXT/?uri=CELEX%3A32021R0664>.

   [LAANC]    United States Federal Aviation Administration (FAA), "Low
              Altitude Authorization and Notification Capability",
              <https://www.faa.gov/
              air_traffic/publications/atpubs/foa_html/
              chap12_section_9.html>.

   [MAVLink]  MAVLink, "Micro Air Vehicle Communication Protocol",
              <http://mavlink.io/>.

   [MOC-NOA]  United States Federal Aviation Administration (FAA),
              "Accepted Means of Compliance; Remote Identification of
              Unmanned Aircraft", Document ID FAA-2022-0859-0001, August
              2022,
              <https://www.regulations.gov/document/FAA-2022-0859-0001>.

   [NPA]      European Union Aviation Safety Agency (EASA), "Notice of
              Proposed Amendment 2021-14: Development of acceptable
              means of compliance and guidance material to support the
              U-space regulation", December 2021,
              <https://www.easa.europa.eu/downloads/134303/en>.

   [NPRM]     United States Federal Aviation Administration (FAA),
              "Remote Identification of Unmanned Aircraft Systems",
              Notice of proposed rulemaking, December 2019,
              <https://www.federalregister.gov/documents/2019/
              12/31/2019-28100/remote-identification-of-unmanned-
              aircraft-systems>.

   [RFC1034]  Mockapetris, P., "Domain names - concepts and facilities",
              STD 13, RFC 1034, DOI 10.17487/RFC1034, November 1987,
              <https://www.rfc-editor.org/info/rfc1034>.

   [RFC3261]  Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
              A., Peterson, J., Sparks, R., Handley, M., and E.
              Schooler, "SIP: Session Initiation Protocol", RFC 3261,
              DOI 10.17487/RFC3261, June 2002,
              <https://www.rfc-editor.org/info/rfc3261>.

   [RFC3972]  Aura, T., "Cryptographically Generated Addresses (CGA)",
              RFC 3972, DOI 10.17487/RFC3972, March 2005,
              <https://www.rfc-editor.org/info/rfc3972>.

   [RFC5730]  Hollenbeck, S., "Extensible Provisioning Protocol (EPP)",
              STD 69, RFC 5730, DOI 10.17487/RFC5730, August 2009,
              <https://www.rfc-editor.org/info/rfc5730>.

   [RFC6973]  Cooper, A., Tschofenig, H., Aboba, B., Peterson, J.,
              Morris, J., Hansen, M., and R. Smith, "Privacy
              Considerations for Internet Protocols", RFC 6973,
              DOI 10.17487/RFC6973, July 2013,
              <https://www.rfc-editor.org/info/rfc6973>.

   [RFC7033]  Jones, P., Salgueiro, G., Jones, M., and J. Smarr,
              "WebFinger", RFC 7033, DOI 10.17487/RFC7033, September
              2013, <https://www.rfc-editor.org/info/rfc7033>.

   [RFC7401]  Moskowitz, R., Ed., Heer, T., Jokela, P., and T.
              Henderson, "Host Identity Protocol Version 2 (HIPv2)",
              RFC 7401, DOI 10.17487/RFC7401, April 2015,
              <https://www.rfc-editor.org/info/rfc7401>.

   [RFC7480]  Newton, A., Ellacott, B., and N. Kong, "HTTP Usage in the
              Registration Data Access Protocol (RDAP)", STD 95,
              RFC 7480, DOI 10.17487/RFC7480, March 2015,
              <https://www.rfc-editor.org/info/rfc7480>.

   [RFC8004]  Laganier, J. and L. Eggert, "Host Identity Protocol (HIP)
              Rendezvous Extension", RFC 8004, DOI 10.17487/RFC8004,
              October 2016, <https://www.rfc-editor.org/info/rfc8004>.

   [RFC8005]  Laganier, J., "Host Identity Protocol (HIP) Domain Name
              System (DNS) Extension", RFC 8005, DOI 10.17487/RFC8005,
              October 2016, <https://www.rfc-editor.org/info/rfc8005>.

   [RFC8032]  Josefsson, S. and I. Liusvaara, "Edwards-Curve Digital
              Signature Algorithm (EdDSA)", RFC 8032,
              DOI 10.17487/RFC8032, January 2017,
              <https://www.rfc-editor.org/info/rfc8032>.

   [RFC8126]  Cotton, M., Leiba, B., and T. Narten, "Guidelines for
              Writing an IANA Considerations Section in RFCs", BCP 26,
              RFC 8126, DOI 10.17487/RFC8126, June 2017,
              <https://www.rfc-editor.org/info/rfc8126>.

   [RFC9082]  Hollenbeck, S. and A. Newton, "Registration Data Access
              Protocol (RDAP) Query Format", STD 95, RFC 9082,
              DOI 10.17487/RFC9082, June 2021,
              <https://www.rfc-editor.org/info/rfc9082>.

   [RFC9083]  Hollenbeck, S. and A. Newton, "JSON Responses for the
              Registration Data Access Protocol (RDAP)", STD 95,
              RFC 9083, DOI 10.17487/RFC9083, June 2021,
              <https://www.rfc-editor.org/info/rfc9083>.

   [RFC9224]  Blanchet, M., "Finding the Authoritative Registration Data
              Access Protocol (RDAP) Service", STD 95, RFC 9224,
              DOI 10.17487/RFC9224, March 2022,
              <https://www.rfc-editor.org/info/rfc9224>.

   [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/info/rfc9334>.

   [TR-22.825]
              3GPP, "Study on Remote Identification of Unmanned Aerial
              Systems (UAS)", Release 16, 3GPP TR 22.825, September
              2018,
              <https://portal.3gpp.org/desktopmodules/Specifications/
              SpecificationDetails.aspx?specificationId=3527>.

   [TR-23.755]
              3GPP, "Study on application layer support for Unmanned
              Aerial Systems (UAS)", Release 17, 3GPP TR 23.755, March
              2021,
              <https://portal.3gpp.org/desktopmodules/Specifications/
              SpecificationDetails.aspx?specificationId=3588>.

   [TS-23.255]
              3GPP, "Application layer support for Uncrewed Aerial
              System (UAS); Functional architecture and information
              flows", Release 17, 3GPP TS 23.255, June 2021,
              <https://portal.3gpp.org/desktopmodules/Specifications/
              SpecificationDetails.aspx?specificationId=3843>.

   [U-Space]  European Organization for the Safety of Air Navigation
              (EUROCONTROL), "U-space Concept of Operations", October
              2019,
              <https://www.sesarju.eu/sites/default/files/documents/u-
              space/CORUS%20ConOps%20vol2.pdf>.

Appendix A.  Overview of UAS Traffic Management (UTM)

A.1.  Operation Concept

   The efforts of the National Aeronautics and Space Administration
   (NASA) and FAA to integrate UAS operations into the national airspace
   system (NAS) led to the development of the concept of UTM and the
   ecosystem around it.  The UTM concept was initially presented in
   2013, and version 2.0 was published in 2020 [FAA_UAS_Concept_Of_Ops].

   The eventual concept refinement, initial prototype implementation,
   and testing were conducted by the joint FAA and NASA UTM research
   transition team.  World efforts took place afterward.  The Single
   European Sky ATM Research (SESAR) started the Concept of Operation
   for EuRopean UTM Systems (CORUS) project to research its UTM
   counterpart concept, namely [U-Space].  This effort is led by the
   European Organization for the Safety of Air Navigation (EUROCONTROL).

   Both NASA and SESAR have published their UTM concepts of operations
   to guide the development of their future air traffic management (ATM)
   system and ensure safe and efficient integration of manned and
   unmanned aircraft into the national airspace.

   UTM comprises UAS operations infrastructure, procedures, and local
   regulation compliance policies to guarantee safe UAS integration and
   operation.  The main functionality of UTM includes, but is not
   limited to, providing means of communication between UAS operators
   and service providers and a platform to facilitate communication
   among UAS service providers.

A.2.  UAS Service Supplier (USS)

   A USS plays an important role to fulfill the key performance
   indicators (KPIs) that UTM has to offer.  Such an entity acts as a
   proxy between UAS operators and UTM service providers.  It provides
   services like real-time UAS traffic monitoring and planning,
   aeronautical data archiving, airspace and violation control,
   interacting with other third-party control entities, etc.  A USS can
   coexist with other USS to build a large service coverage map that can
   load-balance, relay, and share UAS traffic information.

   The FAA works with UAS industry shareholders and promotes the Low
   Altitude Authorization and Notification Capability [LAANC] program,
   which is the first system to realize some of the envisioned
   functionality of UTM.  The LAANC program can automate UAS operational
   intent (flight plan) submissions and applications for airspace
   authorization in real time by checking against multiple aeronautical
   databases, such as airspace classification and operating rules
   associated with it, the FAA UAS facility map, special use airspace,
   Notice to Airmen (NOTAM), and Temporary Flight Restriction (TFR).

A.3.  UTM Use Cases for UAS Operations

   This section illustrates a couple of use case scenarios where UAS
   participation in UTM has significant safety improvement.

   1.  For a UAS participating in UTM and taking off or landing in
       controlled airspace (e.g., Class Bravo, Charlie, Delta, and Echo
       in the United States), the USS under which the UAS is operating
       is responsible for verifying UA registration, authenticating the
       UAS operational intent (flight plan) by checking against a
       designated UAS facility map database, obtaining the air traffic
       control (ATC) authorization, and monitoring the UAS flight path
       in order to maintain safe margins and follow the pre-authorized
       sequence of authorized 4-D volumes (route).

   2.  For a UAS participating in UTM and taking off or landing in
       uncontrolled airspace (e.g., Class Golf in the United States),
       preflight authorization must be obtained from a USS when
       operating BVLOS.  The USS either accepts or rejects the received
       operational intent (flight plan) from the UAS.  An accepted UAS
       operation may, and in some cases must, share its current flight
       data, such as GPS position and altitude, to the USS.  The USS may
       maintain (and provide to authorized requestors) the UAS operation
       status near real time in the short term and may retain at least
       some of it in the longer term, e.g., for overall airspace air
       traffic monitoring.

Appendix B.  Automatic Dependent Surveillance Broadcast (ADS-B)

   ADS-B is the de jure technology used in manned aviation for sharing
   location information, from the aircraft to ground and satellite-based
   systems, designed in the early 2000s.  Broadcast RID is conceptually
   similar to ADS-B but with the receiver target being the general
   public on generally available devices (e.g., smartphones).

   For numerous technical reasons, ADS-B itself is not suitable for low-
   flying, small UAS.  Technical reasons include, but are not limited
   to, the following:

   1.  lack of support for the 1090-MHz ADS-B channel on any consumer
       handheld devices

   2.  Cost, Size, Weight, and Power (CSWaP) requirements of ADS-B
       transponders on CSWaP-constrained UA

   3.  limited bandwidth of both uplink and downlink, which would likely
       be saturated by large numbers of UAS, endangering manned aviation

   Understanding these technical shortcomings, regulators worldwide have
   ruled out the use of ADS-B for the small UAS for which UAS RID and
   DRIP are intended.

Acknowledgments

   The work of the FAA's UAS Identification and Tracking (UAS ID)
   Aviation Rulemaking Committee (ARC) is the foundation of later ASTM
   and IETF DRIP WG efforts.  The work of ASTM F38.02 in balancing the
   interests of diverse stakeholders is essential to the necessary rapid
   and widespread deployment of UAS RID.  Thanks to Alexandre Petrescu,
   Stephan Wenger, Kyle Rose, Roni Even, Thomas Fossati, Valery Smyslov,
   Erik Kline, John Scudder, Murray Kucheraway, Robert Wilton, Roman
   Daniliw, Warren Kumari, Zaheduzzaman Sarker, and Dave Thaler for the
   reviews and helpful positive comments.  Thanks to Laura Welch for her
   assistance in greatly improving this document.  Thanks to Dave Thaler
   for showing our authors how to leverage the RATS model for
   attestation in DRIP.  Thanks to chairs Daniel Migault and Mohamed
   Boucadair for direction of our team of authors and editors, some of
   whom are relative newcomers to writing IETF documents.  Thanks
   especially to Internet Area Director Éric Vyncke for guidance and
   support.

Authors' Addresses

   Stuart W. Card
   AX Enterprize
   4947 Commercial Drive
   Yorkville, NY 13495
   United States of America
   Email: stu.card@axenterprize.com


   Adam Wiethuechter
   AX Enterprize
   4947 Commercial Drive
   Yorkville, NY 13495
   United States of America
   Email: adam.wiethuechter@axenterprize.com


   Robert Moskowitz
   HTT Consulting
   Oak Park, MI 48237
   United States of America
   Email: rgm@labs.htt-consult.com


   Shuai Zhao (editor)
   Intel
   2200 Mission College Blvd.
   Santa Clara,  95054
   United States of America
   Email: shuai.zhao@ieee.org


   Andrei Gurtov
   Linköping University
   IDA
   SE-58183 Linköping
   Sweden
   Email: gurtov@acm.org
